head-twitch response

The HTR is a rapid, rhythmic side-to-side or rotational head movement that intermittently occurs in mice and rats in association with serotonin 5-HT_2A receptor activation. In mice, each individual head movement lasts about 10{{nbsp}}milliseconds and each HTR consists of 5 to 11{{nbsp}}individual head movements. The HTR is spontaneous and irregularly occurring over the drug's duration. Head twitches also occur naturally in rodents but occur at low frequencies and are only rarely observed in non-stimulated animals. Drugs inducing the HTR cause the frequency to increase by many orders of magnitude above the spontaneous rate. Within a 10-minute period, between 4 and 68{{nbsp}}head twitches have been observed following administration of DOI, depending on the dose, enantiomer, and rodent species and strain. The head twitches produced by HTR-inducing drugs are identical to spontaneous head twitches and to touch-induced head twitches (also known as the pinna reflex).

File:Head-twitch response induced by a psychedelic drug in mice.webm.]]

In rats, the HTR is also sometimes known instead as wet dog shakes (WDS). This is because the response in rats can involve more of the whole body instead of just the head shaking and can resemble the shaking of dogs coming out of water. On account of the preceding, the test has also been referred to as the head-twitch response/wet dog shake (HTR/WDS) test.{{cite journal | vauthors = Wojtas A, Gołembiowska K | title = Molecular and Medical Aspects of Psychedelics | journal = Int J Mol Sci | volume = 25 | issue = 1 | date = December 2023 | page = 241 | pmid = 38203411 | pmc = 10778977 | doi = 10.3390/ijms25010241 | doi-access = free | url = | quote = The 5-HT2A receptor is a key player in inducing the psychedelic experience in humans and its activation is considered as a proxy for hallucinogenic effect in animal models [4]. The head twitch response/wet dog shake (HTR/WDS) test is based on this mechanism, exhibiting significant construct validity. The assay quantifies rapid, rhythmic head movements observed in rodents post-administration of psychedelic 5-HT2A receptor agonists [33]. While some false positives have been identified, such as fenfluramine, p-chloroamphetamine, and 5-hydroxytryptophan, the test predominantly exhibits specificity for 5-HT2A receptor agonists [15]. Furthermore, the HTR assay seems to be highly sensitive to 5-HT2A receptor agonists known to produce psychedelic effects in humans, proving its predictive validity. This is evidenced by the fact that the non-psychedelic 5-HT2A receptor agonist, namely lisuride, does not evoke the head twitch phenomenon [33]. Unfortunately, the face validity of the assay is poor, as humans do not exhibit head-twitching behavior after administration of psychoactive drugs. }}

Serotonin 5-HT_2A receptor agonists show an inverted U-shaped dose–response curve for induction of the HTR in terms of its frequency.{{cite journal | vauthors = Fantegrossi WE, Murnane KS, Reissig CJ | title = The behavioral pharmacology of hallucinogens | journal = Biochem Pharmacol | volume = 75 | issue = 1 | pages = 17–33 | date = January 2008 | pmid = 17977517 | pmc = 2247373 | doi = 10.1016/j.bcp.2007.07.018 | url = }} Tolerance rapidly develops to the induction of the HTR with many but not all serotonergic psychedelics.{{cite journal | vauthors = Smith DA, Bailey JM, Williams D, Fantegrossi WE | title = Tolerance and cross-tolerance to head twitch behavior elicited by phenethylamine- and tryptamine-derived hallucinogens in mice | journal = J Pharmacol Exp Ther | volume = 351 | issue = 3 | pages = 485–491 | date = December 2014 | pmid = 25271256 | pmc = 4309922 | doi = 10.1124/jpet.114.219337 | url = }} More specifically, tolerance has been observed with LSD, DOB, DOI, 2C-T-7, 25CN-NBOH, and 5-MeO-AMT, but not with DPT or DiPT.{{cite journal | vauthors = Buchborn T, Schröder H, Dieterich DC, Grecksch G, Höllt V | title = Tolerance to LSD and DOB induced shaking behaviour: differential adaptations of frontocortical 5-HT(2A) and glutamate receptor binding sites | journal = Behav Brain Res | volume = 281 | issue = | pages = 62–68 | date = March 2015 | pmid = 25513973 | doi = 10.1016/j.bbr.2014.12.014 | url = }}{{cite journal | vauthors = Abiero A, Botanas CJ, Sayson LV, Custodio RJ, de la Peña JB, Kim M, Lee HJ, Seo JW, Ryu IS, Chang CM, Yang JS, Lee YS, Jang CG, Kim HJ, Cheong JH | title = 5-Methoxy-α-methyltryptamine (5-MeO-AMT), a tryptamine derivative, induces head-twitch responses in mice through the activation of serotonin receptor 2a in the prefrontal cortex | journal = Behav Brain Res | volume = 359 | issue = | pages = 828–835 | date = February 2019 | pmid = 30053461 | doi = 10.1016/j.bbr.2018.07.020 | url = }}{{cite journal | vauthors = Buchborn T, Lyons T, Knöpfel T | title = Tolerance and Tachyphylaxis to Head Twitches Induced by the 5-HT2A Agonist 25CN-NBOH in Mice | journal = Front Pharmacol | volume = 9 | issue = | pages = 17 | date = 2018 | pmid = 29467649 | pmc = 5808243 | doi = 10.3389/fphar.2018.00017 | doi-access = free | url = }} Development of tolerance to the HTR and other serotonin 5-HT_2A receptor agonist effects in animals parallels the rapid development of tolerance to the hallucinogenic effects of many psychedelics in humans, including LSD, DOM, psilocybin, and mescaline, among others.{{cite journal | vauthors = Nichols DE | title = Hallucinogens | journal = Pharmacol Ther | volume = 101 | issue = 2 | pages = 131–181 | date = February 2004 | pmid = 14761703 | doi = 10.1016/j.pharmthera.2003.11.002 | url = | quote = The earliest hypothesis that hallucinogenic drugs acted specifically at 5-HT2 receptor subtypes was proposed by Glennon et al. (1983c) based on drug discrimination studies in rats showing that the 5-HT2 antagonists ketanserin and pirenperone blocked the discriminative stimulus effects of phenethylamine and tryptamine hallucinogens, including LSD (Colpaert et al., 1982; Leysen et al., 1982; Colpaert & Janssen, 1983). Earlier studies (Browne & Ho, 1975; Winter, 1975) had also shown that the discriminative stimulus of mescaline was blocked by 5-HT antagonists that later were recognized to block 5-HT2 receptors. }} Conversely, similarly to the HTR with DPT and DiPT, tolerance does not appear to develop to the psychedelic effects of DMT,{{cite journal | vauthors = Strassman RJ, Qualls CR, Berg LM | title = Differential tolerance to biological and subjective effects of four closely spaced doses of N,N-dimethyltryptamine in humans | journal = Biol Psychiatry | volume = 39 | issue = 9 | pages = 784–795 | date = May 1996 | pmid = 8731519 | doi = 10.1016/0006-3223(95)00200-6 | url = }} ayahuasca (which contains DMT),{{cite journal | vauthors = Dos Santos RG, Hallak JE | title = Ayahuasca: pharmacology, safety, and therapeutic effects | journal = CNS Spectr | volume = 30| issue = 1| pages = e2 | date = November 2024 | pmid = 39564645 | doi = 10.1017/S109285292400213X | url = }}{{cite journal | vauthors = Dos Santos RG, Grasa E, Valle M, Ballester MR, Bouso JC, Nomdedéu JF, Homs R, Barbanoj MJ, Riba J | title = Pharmacology of ayahuasca administered in two repeated doses | journal = Psychopharmacology (Berl) | volume = 219 | issue = 4 | pages = 1039–1053 | date = February 2012 | pmid = 21842159 | doi = 10.1007/s00213-011-2434-x | url = }} or 5-MeO-DMT in humans.{{cite journal | vauthors = Ermakova AO, Dunbar F, Rucker J, Johnson MW | title = A narrative synthesis of research with 5-MeO-DMT | journal = J Psychopharmacol | volume = 36 | issue = 3 | pages = 273–294 | date = March 2022 | pmid = 34666554 | doi = 10.1177/02698811211050543 | pmc = 8902691 | url = }}{{cite journal | vauthors = Blough BE, Landavazo A, Decker AM, Partilla JS, Baumann MH, Rothman RB | title = Interaction of psychoactive tryptamines with biogenic amine transporters and serotonin receptor subtypes | journal = Psychopharmacology (Berl) | volume = 231 | issue = 21 | pages = 4135–4144 | date = October 2014 | pmid = 24800892 | pmc = 4194234 | doi = 10.1007/s00213-014-3557-7 | url = }}{{cite journal | vauthors = Reckweg JT, Uthaug MV, Szabo A, Davis AK, Lancelotta R, Mason NL, Ramaekers JG | title = The clinical pharmacology and potential therapeutic applications of 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT) | journal = J Neurochem | volume = 162 | issue = 1 | pages = 128–146 | date = July 2022 | pmid = 35149998 | pmc = 9314805 | doi = 10.1111/jnc.15587 | url = }}{{cite journal | vauthors = Dourron HM, Nichols CD, Simonsson O, Bradley M, Carhart-Harris R, Hendricks PS | title = 5-MeO-DMT: An atypical psychedelic with unique pharmacology, phenomenology & risk? | journal = Psychopharmacology (Berl) | volume = | issue = | pages = | date = December 2023 | pmid = 38072874 | doi = 10.1007/s00213-023-06517-1 | url = }} Time-dependent supersensitivity to the HTR in animals has also been reported, for instance with DOI.

The effective doses (ED₅₀) of numerous serotonergic psychedelics in producing the HTR have been reviewed as well as correlated with human psychedelic doses.

HTR-like behaviors are also induced by psychedelics in other animal species, for instance cats and stump-tailed macaque monkeys.{{cite journal | vauthors = Schlemmer RF, Davis JM | title = A primate model for the study of hallucinogens | journal = Pharmacol Biochem Behav | volume = 24 | issue = 2 | pages = 381–392 | date = February 1986 | pmid = 3952128 | doi = 10.1016/0091-3057(86)90368-0 | url = }} Other related behaviors to head twitches induced by serotonergic agents in animals include ear scratching in mice, limb jerks or flicks in cats, head bobs in rabbits, and body scratches. However, other behaviors induced by psychedelics may not be as reliable as the HTR. In addition, ear scratches appear to be mediated primarily by activation of the serotonin 5-HT_2C receptor rather than by activation of the serotonin 5-HT_2A receptor. On the other hand, psychedelic-induced head bobs in rabbits appear to be mediated specifically by central serotonin 5-HT_2A receptor activation.{{Cite journal | last = Nichols | first = David E. | date = 2016 | title = Psychedelics | journal = Pharmacological Reviews | language = en | volume = 68 | issue = 2 | pages = 264–355 | doi = 10.1124/pr.115.011478 | issn = 0031-6997 | pmc = 4813425 | pmid = 26841800 | url = https://pmc.ncbi.nlm.nih.gov/articles/PMC4813425/pdf/pr.115.011478.pdf}}

The HTR method is reliable and simple to perform in that it simply involves direct behavioral observation following drug administration. No animal training or expensive equipment are required. The HTR can be measured in a single animal or a group of animals and can be observed in real-time or via video-recording and later observation.

DOI is the most commonly used psychedelic to induce the HTR. DOI and other psychedelics show a biphasic or inverted U-shaped dose–response curve in terms of HTR induction. For example, no HTR is observed at 0.1{{nbsp}}mg/kg DOI, maximal HTR is observed at 1 to 10{{nbsp}}mg/kg, and lesser HTR is observed at 3 to 20{{nbsp}}mg/kg in rodents. The doses can vary depending on the rodent species and strain. Hence, based on the preceding, proper drug dosing is important for induction of the HTR.

A drawback of the HTR assay is that manual observation can be very laborious and time-consuming. More recently however, semi- and fully-automated forms of the assay, notably allowing for the possibility of high-throughput screening, have been developed.{{cite journal | vauthors = Halberstadt AL, Geyer MA | title = Characterization of the head-twitch response induced by hallucinogens in mice: detection of the behavior based on the dynamics of head movement | journal = Psychopharmacology (Berl) | volume = 227 | issue = 4 | pages = 727–739 | date = June 2013 | pmid = 23407781 | pmc = 3866102 | doi = 10.1007/s00213-013-3006-z | url = | quote = Although most 5-HT2A agonists induce the HTR in mice and rats, one notable exception is the non-hallucinogenic LSD analog lisuride (Gerber et al., 1985; González-Maeso et al., 2003, 2007). It was recently proposed that the behavioral differences between LSD and lisuride may be due to 5-HT2A functional selectivity, whereby lisuride activates the 5-HT2A receptor but does not recruit the specific signaling mechanisms necessary to induce the HTR and provoke hallucinogenesis (González-Maeso et al., 2007). Alternatively, as we have discussed previously (Halberstadt and Geyer, 2010), lisuride is a weak 5-HT2A partial agonist (Cussac et al., 2008), and it is possible that lisuride does not activate the receptor with sufficient efficacy to induce the HTR. The fact that lisuride has been found to induce the HTR in the least shrew (Cryptotis parva), a non-rodent species that is reportedly highly sensitive to 5-HT2A agonists (Darmani, 1994), is consistent with the latter hypothesis. }}{{cite journal | vauthors = de la Fuente Revenga M, Shin JM, Vohra HZ, Hideshima KS, Schneck M, Poklis JL, González-Maeso J | title = Fully automated head-twitch detection system for the study of 5-HT2A receptor pharmacology in vivo | journal = Sci Rep | volume = 9 | issue = 1 | pages = 14247 | date = October 2019 | pmid = 31582824 | pmc = 6776537 | doi = 10.1038/s41598-019-49913-4 | url = | quote = To our knowledge, the first study reporting mouse HTR behavior induced by the psychedelic drug LSD was published by Keller and Umbreit in 195636. However, it was not until mid-2000s with our previous observations suggesting the potential use of HTR as a mouse behavioral proxy of human psychedelic potential5 that this particular behavior became routinely used in molecular and pharmacological studies related to 5-HT2AR-dependent behavioral events. }}{{cite journal | vauthors = de la Fuente Revenga M, Vohra HZ, González-Maeso J | title = Automated quantification of head-twitch response in mice via ear tag reporter coupled with biphasic detection | journal = J Neurosci Methods | volume = 334 | issue = | pages = 108595 | date = January 2020 | pmid = 31954738 | pmc = 7363508 | doi = 10.1016/j.jneumeth.2020.108595 | url = }}{{cite journal | vauthors = Halberstadt AL | title = Automated detection of the head-twitch response using wavelet scalograms and a deep convolutional neural network | journal = Sci Rep | volume = 10 | issue = 1 | pages = 8344 | date = May 2020 | pmid = 32433580 | pmc = 7239849 | doi = 10.1038/s41598-020-65264-x | bibcode = 2020NatSR..10.8344H | url = }}{{cite journal | vauthors = Glatfelter GC, Chojnacki MR, McGriff SA, Wang T, Baumann MH | title = Automated Computer Software Assessment of 5-Hydroxytryptamine 2A Receptor-Mediated Head Twitch Responses from Video Recordings of Mice | journal = ACS Pharmacol Transl Sci | volume = 5 | issue = 5 | pages = 321–330 | date = May 2022 | pmid = 35592434 | pmc = 9112414 | doi = 10.1021/acsptsci.1c00237 | url = }}{{cite book | vauthors = Jaster AM, González-Maeso J | title = Schizophrenia | chapter = Automated Detection of Psychedelic-Induced Head-Twitch Response in Mice | series = Methods Mol Biol | volume = 2687 | pages = 65–76 | date = 2023 | publisher = Springer | pmid = 37464163 | doi = 10.1007/978-1-0716-3307-6_6 | isbn = 978-1-0716-3306-9 | chapter-url = }}{{cite journal | vauthors = Cyrano E, Popik P | title = Assessing the effects of 5-HT2A and 5-HT5A receptor antagonists on DOI-induced head-twitch response in male rats using marker-less deep learning algorithms | journal = Pharmacol Rep | volume = 77| issue = 1| pages = 135–144| date = November 2024 | pmid = 39602080 | doi = 10.1007/s43440-024-00679-1 | url = | doi-access = free | pmc = 11743402 }}

{{See also|Psychedelic drug#Mechanism of action}}

The HTR produced by serotonergic psychedelics, which act as non-selective serotonin receptor agonists, appears to be mediated specifically by agonism of the serotonin 5-HT_2A receptor. Selective and non-selective serotonin 5-HT_2A antagonists, like volinanserin (M100907), can block production of the HTR by serotonergic psychedelics. Similarly, the HTR of psychedelics is absent in serotonin 5-HT_2A receptor knockout mice. Restoration of the serotonin 5-HT_2A receptor to cortical neurons in these knockout mice can restore the HTR.{{cite journal | vauthors = González-Maeso J, Weisstaub NV, Zhou M, Chan P, Ivic L, Ang R, Lira A, Bradley-Moore M, Ge Y, Zhou Q, Sealfon SC, Gingrich JA | title = Hallucinogens recruit specific cortical 5-HT(2A) receptor-mediated signaling pathways to affect behavior | journal = Neuron | volume = 53 | issue = 3 | pages = 439–452 | date = February 2007 | pmid = 17270739 | doi = 10.1016/j.neuron.2007.01.008 | url = }} The intracellular signaling cascade activated by the serotonin 5-HT_2A to produce the HTR appears to be the G_q pathway.{{cite journal | vauthors = Barksdale BR, Doss MK, Fonzo GA, Nemeroff CB | title = The mechanistic divide in psychedelic neuroscience: An unbridgeable gap? | journal = Neurotherapeutics | volume = 21 | issue = 2 | pages = e00322 | date = March 2024 | pmid = 38278658 | doi = 10.1016/j.neurot.2024.e00322 | url = | pmc = 10963929 | quote = The 5-HT2A receptor is a metabotropic serotonin receptor. When a psychedelic compound binds to this receptor, it activates Gq-like G proteins. This activation leads to a second messenger cascade that includes the hydrolysis of phosphatidylinositol-4,5-bisphosphate, leading to intracellular Ca2+ release by inositol trisphosphate and the activation of protein kinase C by diacylglycerol [18]. Gq-biased downstream signaling cascades appear to underlie differences in subjective effects between psychedelic vs. non-psychedelic 5-HT2A agonists (e.g., lisuride, ergotamine). Specifically, preferential Gq protein signaling recruitment downstream of 5-HT2A receptor activation was recently demonstrated to mediate the HTR in rodents, an effect that was dissociable from 5-HT2A β-arrestin 2-induced tachyphylaxis and receptor down-regulation [29]. However, the comparability of 5-HT2A Gq recruitment by classical psychedelics in humans and animals remains to be elucidated, as does comparability of psychedelic-induced β-arrestin 2 translocation and effects on tachyphylaxis/receptor down-regulation. Additional signaling pathways, e.g., arachidonic acid release, may be involved as well [19,30–32]. }}{{cite journal | vauthors = Glennon RA, Dukat M | title = 1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane (DOI): From an Obscure to Pivotal Member of the DOX Family of Serotonergic Psychedelic Agents - A Review | journal = ACS Pharmacology & Translational Science | volume = 7 | issue = 6 | pages = 1722–1745 | date = June 2024 | pmid = 38898956 | doi = 10.1021/acsptsci.4c00157 | quote = Although the specific signaling cascades mediating the HTR have not been conclusively identified, Gq and β-arrestin2 have been implicated. Recent studies with different existing and novel agents, including DOI, found that the HTR was correlated with Gq efficacy but not with β-arrestin2 recruitment.114 | doi-access = free | pmc = 11184610 }}{{cite journal | vauthors = Gumpper RH, Nichols DE | title = Chemistry/structural biology of psychedelic drugs and their receptor(s) | journal = Br J Pharmacol | volume = | issue = | pages = | date = October 2024 | pmid = 39354889 | doi = 10.1111/bph.17361 | url = | quote = Roth and his collaborators were the first to identify 5-HT2A receptors highly expressed on apical dendrites of cortical pyramidal cells (Willins et al., 1997). Subsequent activation of these receptors increases neuronal excitability (Aghajanian & Marek, 1997) through the canonical Gq signalling cascade (Roth & Gumpper, 2023). This increase in cortical neuron excitability is presumed to be the mechanism of action for many psychedelic compounds, and McCorvy has postulated that there is a threshold for Gq activation at which hallucinations are produced (Wallach et al., 2023). However, it has also been shown that 5HT2A receptor- mediated β-arrestin 2 signalling is important to produce mouse behaviour suggestive of hallucinogenic activity through the use of β-arrestin 2 genetic knock-out models (Rodriguiz et al., 2021). There is also some evidence that 5-HT2A receptors can couple to the Gi/o family (Kim et al., 2020; Wright et al., 2024); however, the physiological relevance of these assays utilizing modified sensors needs to be validated. }}{{cite journal | vauthors = Wallach J, Cao AB, Calkins MM, Heim AJ, Lanham JK, Bonniwell EM, Hennessey JJ, Bock HA, Anderson EI, Sherwood AM, Morris H, de Klein R, Klein AK, Cuccurazzu B, Gamrat J, Fannana T, Zauhar R, Halberstadt AL, McCorvy JD | title = Identification of 5-HT2A receptor signaling pathways associated with psychedelic potential | journal = Nat Commun | volume = 14 | issue = 1 | pages = 8221 | date = December 2023 | pmid = 38102107 | pmc = 10724237 | doi = 10.1038/s41467-023-44016-1 | quote = Although 5-HT2A receptor activation mediates psychedelic effects, prototypical psychedelics activate both 5-HT2A-Gq/11 and β-arrestin2 transducers, making their respective roles unclear. To elucidate this, we develop a series of 5-HT2A-selective ligands with varying Gq efficacies, including β-arrestin-biased ligands. We show that 5-HT2A-Gq but not 5-HT2A-β-arrestin2 recruitment efficacy predicts psychedelic potential, assessed using head-twitch response (HTR) magnitude in male mice. We further show that disrupting Gq-PLC signaling attenuates the HTR and a threshold level of Gq activation is required to induce psychedelic-like effects, consistent with the fact that certain 5-HT2A partial agonists (e.g., lisuride) are non-psychedelic. }} However, the cascades have not been conclusively determined, and other pathways, such as the G_s{{cite journal | vauthors = Liu X, Zhu H, Gao H, Tian X, Tan B, Su R | title = Gs signaling pathway distinguishes hallucinogenic and nonhallucinogenic 5-HT2AR agonists induced head twitch response in mice | journal = Biochem Biophys Res Commun | volume = 598 | issue = | pages = 20–25 | date = April 2022 | pmid = 35149433 | doi = 10.1016/j.bbrc.2022.01.113 | url = }}{{cite journal | vauthors = Gao H, Liu X, Xie L, Tan B, Su R | title = Modulation of DOM-Induced Head-Twitch Response by mGluR2 Agonist/Inverse Agonist is Associated with 5-HT2AR-Mediated Gs Signaling Pathway | journal = Neurochem Res | volume = 49 | issue = 3 | pages = 636–648 | date = March 2024 | pmid = 37989895 | doi = 10.1007/s11064-023-04055-y | url = }} and β-arrestin2 pathways, have also been implicated in other studies.

Activation of serotonin 5-HT_2A receptors in the medial prefrontal cortex (mPFC), with layer V pyramidal neurons especially implicated and with subsequent release of glutamate in this area, may be the origin of the HTR.{{cite book | vauthors = Marek GJ, Schoepp DD | chapter = Cortical influences of serotonin and glutamate on layer V pyramidal neurons | title = 5-HT Interaction with Other Neurotransmitters: Experimental Evidence and Therapeutic Relevance - Part B - Cortical influences of serotonin and glutamate on layer V pyramidal neurons | journal = Prog Brain Res | series = Progress in Brain Research | volume = 261 | issue = | pages = 341–378 | date = 2021 | pmid = 33785135 | doi = 10.1016/bs.pbr.2020.11.002 | isbn = 978-0-444-64258-5}}{{cite journal | vauthors = Willins DL, Meltzer HY | title = Direct injection of 5-HT2A receptor agonists into the medial prefrontal cortex produces a head-twitch response in rats | journal = The Journal of Pharmacology and Experimental Therapeutics | volume = 282 | issue = 2 | pages = 699–706 | date = August 1997 | doi = 10.1016/S0022-3565(24)36840-5 | pmid = 9262333 }} However, other brain areas have also been independently implicated. Serotonin 5-HT_2A and metabotropic glutamate mGlu₂ receptor heterodimeric complexes may or may not be important for induction of the HTR by psychedelics, with research findings in this area being conflicting.{{cite journal | last1=López-Giménez | first1=Juan F. | last2=González-Maeso | first2=Javier | title=Behavioral Neurobiology of Psychedelic Drugs: Hallucinogens and Serotonin 5-HT2A Receptor-Mediated Signaling Pathways | journal=Current Topics in Behavioral Neurosciences | publisher=Springer Berlin Heidelberg | publication-place=Berlin, Heidelberg | volume=36 | date=2017 | isbn=978-3-662-55878-2 | pmid=28677096 | pmc=5756147 | doi=10.1007/7854_2017_478 | doi-access=free | pages=45–73 | quote=The role of the 5-HT2A receptor in the mechanism of action of hallucinogens was first proposed by Richard Glennon, Milt Titeler and their teams (Glennon et al. 1984, 1986). However, it was not until the development of 5-HT2A knockout mice in 2003 that the fundamental role of 5-HT2A receptor-dependent signaling in the cellular and behavioral effects of hallucinogens was verified conclusively (Gonzalez-Maeso et al. 2003, 2007). }}{{cite journal | vauthors = Moreno JL, Muguruza C, Umali A, Mortillo S, Holloway T, Pilar-Cuéllar F, Mocci G, Seto J, Callado LF, Neve RL, Milligan G, Sealfon SC, López-Giménez JF, Meana JJ, Benson DL, González-Maeso J | title = Identification of three residues essential for 5-hydroxytryptamine 2A-metabotropic glutamate 2 (5-HT2A·mGlu2) receptor heteromerization and its psychoactive behavioral function | journal = J Biol Chem | volume = 287 | issue = 53 | pages = 44301–44319 | date = December 2012 | pmid = 23129762 | pmc = 3531745 | doi = 10.1074/jbc.M112.413161 | doi-access = free | url = }}{{cite journal | vauthors = Moreno JL, Holloway T, Albizu L, Sealfon SC, González-Maeso J | title = Metabotropic glutamate mGlu2 receptor is necessary for the pharmacological and behavioral effects induced by hallucinogenic 5-HT2A receptor agonists | journal = Neurosci Lett | volume = 493 | issue = 3 | pages = 76–79 | date = April 2011 | pmid = 21276828 | pmc = 3064746 | doi = 10.1016/j.neulet.2011.01.046 | url = }}

The HTR is said to resemble a strong pinna reflex involving the whole head. The pinna reflex can be elicited by tactile stimulation, for example stimulation of the ear by a fine hair. In the case of the HTR however, the reflex occurs without tactile stimulation. The HTR induced by the serotonin precursor 5-hydroxytryptophan (5-HTP) has been found to be sensitive to environmental interference by background noise and can be prevented by local anesthesia of the pinna (outer part of the ear).{{cite journal | vauthors = Boulton CS, Handley SL | title = Factors modifying the head-twitch response to 5-hydroxytryptophan | journal = Psychopharmacologia | volume = 31 | issue = 3 | pages = 205–214 | date = July 1973 | pmid = 4542469 | doi = 10.1007/BF00422511 | url = }} These findings suggest that the HTR might be due specifically to disturbances of auditory sensory processing, although more research is needed to confirm this possibility.

The reasons for the biphasic or inverted U-shaped dose–response curve with psychedelics are unknown. However, activation of serotonin 5-HT_2C and 5-HT_1A receptors at higher doses appears to at least partly be involved.{{cite journal | vauthors = Fantegrossi WE, Simoneau J, Cohen MS, Zimmerman SM, Henson CM, Rice KC, Woods JH | title = Interaction of 5-HT2A and 5-HT2C receptors in R(-)-2,5-dimethoxy-4-iodoamphetamine-elicited head twitch behavior in mice | journal = J Pharmacol Exp Ther | volume = 335 | issue = 3 | pages = 728–734 | date = December 2010 | pmid = 20858706 | pmc = 2993545 | doi = 10.1124/jpet.110.172247 | url = }}{{cite journal | vauthors = Zhu H, Wang L, Wang X, Yao Y, Zhou P, Su R | title = 5-hydroxytryptamine 2C/1A receptors modulate the biphasic dose response of the head twitch response and locomotor activity induced by DOM in mice | journal = Psychopharmacology (Berl) | volume = 241 | issue = 11 | pages = 2315–2330 | date = November 2024 | pmid = 38916640 | doi = 10.1007/s00213-024-06635-4 | url = }}

Tolerance and tachyphylaxis to the HTR and/or other effects of serotonergic psychedelics may be mediated by serotonin 5-HT_2A receptor downregulation.{{cite journal | vauthors = de la Fuente Revenga M, Jaster AM, McGinn J, Silva G, Saha S, González-Maeso J | title = Tolerance and Cross-Tolerance among Psychedelic and Nonpsychedelic 5-HT2A Receptor Agonists in Mice | journal = ACS Chem Neurosci | volume = 13 | issue = 16 | pages = 2436–2448 | date = August 2022 | pmid = 35900876 | pmc = 10411500 | doi = 10.1021/acschemneuro.2c00170 | url = }}{{cite journal | vauthors = Smith RL, Barrett RJ, Sanders-Bush E | title = Mechanism of tolerance development to 2,5-dimethoxy-4-iodoamphetamine in rats: down-regulation of the 5-HT2A, but not 5-HT2C, receptor | journal = Psychopharmacology (Berl) | volume = 144 | issue = 3 | pages = 248–254 | date = June 1999 | pmid = 10435391 | doi = 10.1007/s002130051000 | url = }} LSD, psilocin, DOM, DOI, and DOB have all been found to reduce the density of brain serotonin 5-HT_2A receptors in animals in vivo and/or to desensitize the receptor in transfected cell lines, and this downregulation has been found to recover very slowly. LSD has also been specifically shown to reduce brain serotonin 5-HT_2A receptor signaling in animals. Conversely however, DMT, which is not associated with tolerance development in humans, did not desensitize the serotonin 5-HT_2A receptor in cell lines.{{cite journal | vauthors = Smith RL, Canton H, Barrett RJ, Sanders-Bush E | title = Agonist properties of N,N-dimethyltryptamine at serotonin 5-HT2A and 5-HT2C receptors | journal = Pharmacol Biochem Behav | volume = 61 | issue = 3 | pages = 323–330 | date = November 1998 | pmid = 9768567 | doi = 10.1016/s0091-3057(98)00110-5 | url = }} Activation of the serotonin 5-HT_2A receptor β-arrestin2 pathway may mediate serotonin 5-HT_2A receptor internalization and tolerance. However, findings are conflicting, as β-arrestin2 knockout mice still showed tolerance to the HTR induced by DOI. It is also notable that, in contrast to most G protein-coupled receptors (GPCRs), serotonin 5-HT_2A receptor downregulation has been found to occur in response to both agonists and antagonists of the receptor.{{cite journal | vauthors = Gray JA, Roth BL | title = Paradoxical trafficking and regulation of 5-HT(2A) receptors by agonists and antagonists | journal = Brain Res Bull | volume = 56 | issue = 5 | pages = 441–451 | date = November 2001 | pmid = 11750789 | doi = 10.1016/s0361-9230(01)00623-2 | url = https://escholarship.org/uc/item/86r04058}} Besides serotonin 5-HT_2A receptor downregulation, tolerance to psychedelics may also develop due to adaptations in downstream glutamate receptors.

Head twitches do not occur with psychedelics in humans{{cite journal | vauthors = Fordyce BA, Roth BL | title = Making Sense of Psychedelics in the CNS | journal = Int J Neuropsychopharmacol | volume = 27 | issue = 2 | pages = | date = February 2024 | pmid = 38289825 | pmc = 10888522 | doi = 10.1093/ijnp/pyae007 | url = | quote = In 1983, Richard Glennon and colleagues identified psychedelic-induced behaviors by mescaline and LSD that were subsequently blocked by antagonists ketanserin and pirenperone in rats (Glennon et al., 1983a). From this, serotonin (5-HT) 5-HT2 receptors were suspected as primary contributors to these affects.}} and head twitches lack face validity as an animal behavioral proxy of psychedelic effects.{{cite journal | vauthors = Hanks JB, González-Maeso J | title = Animal models of serotonergic psychedelics | journal = ACS Chem Neurosci | volume = 4 | issue = 1 | pages = 33–42 | date = January 2013 | pmid = 23336043 | pmc = 3547517 | doi = 10.1021/cn300138m}} In any case, it has been said that head twitches might be a behavioral response to sensory disturbances during hallucinogenic experiences. On the other hand, many drugs that are not hallucinogenic in humans also induce the HTR. Despite the preceding limitations, the assay has strong predictive validity for hallucinogenic effects of serotonin 5-HT_2A receptor agonists in humans. There is a strong correlation between the capacity of serotonergic psychedelics to induce head twitches in rodents and their reported potency in inducing hallucinogenic effects in humans.{{cite journal | vauthors = Halberstadt AL, Chatha M, Klein AK, Wallach J, Brandt SD | title = Correlation between the potency of hallucinogens in the mouse head-twitch response assay and their behavioral and subjective effects in other species | journal = Neuropharmacology | volume = 167 | issue = | pages = 107933 | date = May 2020 | pmid = 31917152 | doi = 10.1016/j.neuropharm.2019.107933 | pmc = 9191653}} The HTR is easily quantifiable and there is high agreement in counts between independent observers. In addition, there is a low level of within-subject and between-subject variability in induction of the HTR in animals.

There are few or no known examples of serotonergic psychedelics with hallucinogenic effects in humans that do not produce the HTR in animals.{{cite journal | vauthors = Gumpper RH, Roth BL | title = Psychedelics: preclinical insights provide directions for future research | journal = Neuropsychopharmacology | volume = 49 | issue = 1 | pages = 119–127 | date = January 2024 | pmid = 36932180 | doi = 10.1038/s41386-023-01567-7 | pmc = 10700551 | url = }} One of the only known instances, the LSD prodrug ALD-52 (1-acetyl-LSD),{{cite journal | vauthors = Halberstadt AL, Chatha M, Klein AK, McCorvy JD, Meyer MR, Wagmann L, Stratford A, Brandt SD | title = Pharmacological and biotransformation studies of 1-acyl-substituted derivatives of d-lysergic acid diethylamide (LSD) | journal = Neuropharmacology | volume = 172 | issue = | pages = 107856 | date = August 2020 | pmid = 31756337 | pmc = 9191647 | doi = 10.1016/j.neuropharm.2019.107856 | url = }}{{cite journal | vauthors = Elliott SP, Holdbrook T, Brandt SD | title = Prodrugs of New Psychoactive Substances (NPS): A New Challenge | journal = J Forensic Sci | volume = 65 | issue = 3 | pages = 913–920 | date = May 2020 | pmid = 31943218 | doi = 10.1111/1556-4029.14268 | url = https://researchonline.ljmu.ac.uk/id/eprint/11926/1/JOFS-19-562.R1_accepted_uncorrected.pdf}} could be explained by species differences in metabolism. Other possible exceptions, including various 2C psychedelics like 2C-B, 2C-I, and 2C-D, as well as the phenylpiperazine TFMPP, may be explained by these agents having relatively low intrinsic activity at the serotonin 5-HT_2A receptor and by species differences in sensitivity to HTR elicitation by serotonin 5-HT_2A receptor partial agonists (mice being more sensitive than rats). Dimethyltryptamine (DMT) shows effects on the HTR in mice that are highly strain-dependent, including producing an HTR comparable to other psychedelics, producing an HTR that is much weaker than that of other psychedelics, or producing no HTR at all.{{cite journal | vauthors = Cameron LP, Olson DE | title = Dark Classics in Chemical Neuroscience: N, N-Dimethyltryptamine (DMT) | journal = ACS Chem Neurosci | volume = 9 | issue = 10 | pages = 2344–2357 | date = October 2018 | pmid = 30036036 | doi = 10.1021/acschemneuro.8b00101 | url = | quote = With the exception of a few reports of unconditioned responses41,88−90 and a seminal study demonstrating that DMT increases the activity of pargyline pretreated rodents,88 very little is known about the effects of DMT on rodent behavior. In C57BL/6 mice, DMT produces a 5-HT2Adependent head-twitch response (HTR)41a characteristic behavioral phenotype of serotonergic psychedelics.91 However, this behavioral response is highly dependent on the mouse strain employed. In C57BL/6 mice, DMT was shown to produce far fewer head twitches per unit time than the structurally related tryptamine psychedelic N,N-diisopropyltryptamine or the structurally dissimilar phenethylamine psychedelic DOI.41 However, in 129S6/SvEv mice, DMT produced a HTR comparable to DOI,92 while in NIH Swiss mice, DMT did not produce a HTR.93 }}{{cite journal | vauthors = Carbonaro TM, Gatch MB | title = Neuropharmacology of N,N-dimethyltryptamine | journal = Brain Res Bull | volume = 126 | issue = Pt 1 | pages = 74–88 | date = September 2016 | pmid = 27126737 | pmc = 5048497 | doi = 10.1016/j.brainresbull.2016.04.016 | url = | quote = Like other classic psychedelics, DMT does induce this head twitch response in C57Bl/6 mice, which is blocked by 5-HT2A inverse agonist, MDL100907 (Carbonaro et al., 2015). However, the overall number of head twitches induced by DMT is much smaller compared to most other psychedelic compounds. DMT failed to produce this head twitch response in Swiss Webster mice (Fantegrossi et al., 2006) These discrepancies may be due to the rapid degradation of DMT or other peculiarities specific to DMT. }} These conflicting results may be due to rapid metabolism of DMT and/or other peculiarities of DMT in different species.

{{See also|5-Hydroxytryptophan#Psychedelic effects|Serotonin releasing agent#Psychedelic-like effects}}

The HTR can be non-specific and can have false positives, with head twitches also produced by some drugs that do not act through serotonin 5-HT₂ receptors. Examples of these agents include NMDA receptor antagonists like phencyclidine (PCP), certain benzodiazepines and Z-drugs like estazolam, triazolam, and zopiclone, α₂-adrenergic receptor antagonists like yohimbine, muscarinic acetylcholine receptor antagonists like atropine and scopolamine, serotonin 5-HT_1A receptor antagonists like WAY-100635 and UH-301, and CB₁ receptor antagonists like rimonabant. In the cases of benzodiazepines, rimonabant, and serotonin 5-HT_1A receptor antagonists however, this effect appeared to be mediated by indirect or direct activation of serotonin 5-HT_2A receptors. A number of other drugs, including the acetylcholine receptor agonist carbachol, opioids, and thyrotropin-releasing hormone (TRH) among others, have also been reported to induce the HTR.{{cite journal | last1=Handley | first1=Sheila L. | last2=Singh | first2=Lakhbir | title=Neurotransmitters and shaking behaviour — more than a 'gut-bath' for the brain? | journal=Trends in Pharmacological Sciences | volume=7 | date=1986 | doi=10.1016/0165-6147(86)90371-8 | pages=324–328}}

Drugs such as the serotonin precursors tryptophan and 5-hydroxytryptophan (5-HTP), serotonin releasing agents (SRAs) like fenfluramine and para-chloroamphetamine (PCA), and other agents like 1-methylpsilocin and 3,4-dimethoxyphenethylamine (DMPEA) stimulate serotonin receptors and can produce head twitches, but are not known to be hallucinogenic in humans.{{Cite book |last1=Shulgin |first1=Alexander |title=PiHKAL: A Chemical Love Story |last2=Shulgin |first2=Ann |publisher=Transform Press |year=1991 |isbn=978-0-9630096-0-9 |edition=1st |pages=614–616 |language=en}} However, at least in the case of 5-HTP, this could be just be due to the very high doses required. It is notable in this regard that hallucinations are reported in a subset of cases of serotonin syndrome, although it is unclear at the present time whether these hallucinations are psychedelic in nature or are of a different etiology.{{cite journal | vauthors = Mills KC | title = Serotonin syndrome. A clinical update | journal = Crit Care Clin | volume = 13 | issue = 4 | pages = 763–783 | date = October 1997 | pmid = 9330840 | doi = 10.1016/s0749-0704(05)70368-7 | url = }} While the SRA and mixed entactogen and psychedelic MDA likewise induces the HTR, findings are mixed and conflicting for the SRA and less hallucinogenic MDMA.{{cite thesis | last=Dunlap | first=Lee E. | title=Development of Non-Hallucinogenic Psychoplastogens | publisher=University of California, Davis | date=2022 | url=https://escholarship.org/uc/item/5qr3w0gm | access-date=18 November 2024 | quote=Finally, since R-MDMA is known to partially substitute for LSD in animal models we decided to test both compounds in the head twitch response assay (HTR) (FIG 3.3C).3 The HTR is a well-validated mouse model for predicting the hallucinogenic potential of test drugs. Serotonergic psychedelics will cause a rapid back and forth head movement in mice. The potency measured in the HTR assay has been shown to correlate very well with the human potencies of psychedelics.18 Neither R-MDMA or LED produced any head twitches at all doses tested, suggesting that neither has high hallucinogenic potential.}} The SRA dexfenfluramine produces wet dog shakes in rats, whereas the serotonin reuptake inhibitor fluoxetine has little or no effect on wet dog shakes.{{cite journal | vauthors = Higgins GA, Fletcher PJ | title = Therapeutic Potential of 5-HT2C Receptor Agonists for Addictive Disorders | journal = ACS Chem Neurosci | volume = 6 | issue = 7 | pages = 1071–1088 | date = July 2015 | pmid = 25870913 | doi = 10.1021/acschemneuro.5b00025 | url = }} Amphetamine as well as para-hydroxyamphetamine (given intracerebroventricularly) can also elicit the HTR at sufficiently high doses.{{cite journal | vauthors = Taylor WA, Sulser F | title = Effects of amphetamine and its hydroxylated metabolites on central noradrenergic mechanisms | journal = J Pharmacol Exp Ther | volume = 185 | issue = 3 | pages = 620–632 | date = June 1973 | pmid = 4712653 | doi = 10.1016/S0022-3565(25)29545-3| url = }}{{cite journal | vauthors = Tadano T, Satoh S, Kisara K | title = Head-twitches induced by p-hydroxyamphetamine in mice | journal = Jpn J Pharmacol | volume = 41 | issue = 4 | pages = 519–523 | date = August 1986 | pmid = 3022049 | doi = 10.1254/jjp.41.519 | url = | doi-access = free }}

The preceding findings collectively suggest that while the HTR can be a useful indicator as to whether a compound is likely to display hallucinogenic activity in humans, the induction of the HTR does not necessarily mean that a compound will be hallucinogenic. In relation to this, caution should be exercised when interpreting such results.{{cite journal | vauthors = Canal CE, Morgan D | title = Head-twitch response in rodents induced by the hallucinogen 2,5-dimethoxy-4-iodoamphetamine: a comprehensive history, a re-evaluation of mechanisms, and its utility as a model | journal = Drug Testing and Analysis | volume = 4 | issue = 7–8 | pages = 556–576 | date = July 2012 | pmid = 22517680 | pmc = 3722587 | doi = 10.1002/dta.1333 }}

Some serotonin 5-HT_2A receptor agonists, including lisuride, 2-bromo-LSD (bromolysergide; BOL-148), ergotamine, 6-fluoro-DET, 6-MeO-DMT, Ariadne, zalsupindole (DLX-001; AAZ-A-154), ITI-1549, 25N-N1-Nap, and IHCH-7086 among others, are either non-hallucinogenic or are thought to be non-hallucinogenic in spite of activating the serotonin 5-HT_2A receptor. The HTR is among the only animal behavioral tests that can reliably distinguish between hallucinogenic and non-hallucinogenic serotonin 5-HT_2A receptor agonists. Although lisuride and other non-hallucinogenic serotonin 5-HT_2A receptor agonists do not produce the HTR in rodents, lisuride does produce the HTR in the least shrew, a non-rodent species that is said to be highly sensitive to serotonin 5-HT_2A receptor agonists.{{cite journal | vauthors = Darmani NA, Mock OB, Towns LC, Gerdes CF | title = The head-twitch response in the least shrew (Cryptotis parva) is a 5-HT2- and not a 5-HT1C-mediated phenomenon | journal = Pharmacol Biochem Behav | volume = 48 | issue = 2 | pages = 383–396 | date = June 1994 | pmid = 8090805 | doi = 10.1016/0091-3057(94)90542-8 | url = }} In any case, it is thought that partial agonism with sufficiently low efficacy for specific intracellular signaling pathways underlies the lack of HTR and psychedelic effects with non-hallucinogenic serotonin 5-HT_2A receptor agonists.{{cite journal | vauthors = Duan W, Cao D, Wang S, Cheng J | title = Serotonin 2A Receptor (5-HT2AR) Agonists: Psychedelics and Non-Hallucinogenic Analogues as Emerging Antidepressants | journal = Chem Rev | volume = 124 | issue = 1 | pages = 124–163 | date = January 2024 | pmid = 38033123 | doi = 10.1021/acs.chemrev.3c00375 | url = }} However, other findings suggest that ergotamine may be non-hallucinogenic due to inability to efficiently cross the blood–brain barrier and peripheral selectivity, while lisuride and 2-bromo-LSD may actually be variably hallucinogenic at sufficiently high doses.{{cite journal | vauthors = Kehler J, Lindskov MS | title = Are the LSD-analogs lisuride and ergotamine examples of non-hallucinogenic serotonin 5-HT2A receptor agonists? | journal = Journal of Psychopharmacology | pages = 2698811251330741 | date = May 2025 | pmid = 40322975 | doi = 10.1177/02698811251330741 }}

Serotonin administered by intracerebroventricular injection at high doses produces the HTR in animals. However, serotonin itself has been considered to be non-hallucinogenic in humans.{{cite journal | vauthors = Sapienza J | title=The Key Role of Intracellular 5-HT2A Receptors: A Turning Point in Psychedelic Research? | journal=Psychoactives | volume=2 | issue=4 | date=13 October 2023 | issn=2813-1851 | doi=10.3390/psychoactives2040018 | doi-access=free | pages=287–293}}{{cite journal | vauthors = Vargas MV, Dunlap LE, Dong C, Carter SJ, Tombari RJ, Jami SA, Cameron LP, Patel SD, Hennessey JJ, Saeger HN, McCorvy JD, Gray JA, Tian L, Olson DE | title = Psychedelics promote neuroplasticity through the activation of intracellular 5-HT2A receptors | journal = Science | volume = 379 | issue = 6633 | pages = 700–706 | date = February 2023 | pmid = 36795823 | pmc = 10108900 | doi = 10.1126/science.adf0435 | bibcode = 2023Sci...379..700V | url = }}{{cite book | vauthors = Schmid CL, Bohn LM | title=5-HT2A Receptors in the Central Nervous System | chapter=βArrestins: Ligand-Directed Regulators of 5-HT2A Receptor Trafficking and Signaling Events | publisher=Springer International Publishing | publication-place=Cham | date=2018 | isbn=978-3-319-70472-2 | doi=10.1007/978-3-319-70474-6_2 | pages=31–55 | quote=Centrally expressed receptors were implicated by the fact that the systemic injection of serotonin, which is not brain penetrant, does not induce the head twitch response [109, 110], yet head twitches are induced by the direct injection of serotonin into the intracerebroventricular (i.c.v.) space [66, 120, 121].}}{{cite book | vauthors = Kozlenkov A, González-Maeso J | title=The Neuroscience of Hallucinations | chapter=Animal Models and Hallucinogenic Drugs | publisher=Springer New York | publication-place=New York, NY | date=2013 | isbn=978-1-4614-4120-5 | doi=10.1007/978-1-4614-4121-2_14 | pages=253–277}}{{cite journal | vauthors = Schmid CL, Bohn LM | title = Serotonin, but not N-methyltryptamines, activates the serotonin 2A receptor via a β-arrestin2/Src/Akt signaling complex in vivo | journal = J Neurosci | volume = 30 | issue = 40 | pages = 13513–24 | date = October 2010 | pmid = 20926677 | pmc = 3001293 | doi = 10.1523/JNEUROSCI.1665-10.2010 | url = | quote = Serotonin and 5-hydroxy-L-tryptophan (5-HTP) induce a head-twitch response in wild-type (WT) mice that is a behavioral proxy for 5-HT2AR activation. The response in β-arrestin2 knock-out (βarr2-KO) mice is greatly attenuated until the doses are elevated, at which point, βarr2-KO mice display a head-twitch response that can exceed that of WT mice. Direct administration of N-methyltryptamines also produces a greater response in βarr2-KO mice. Moreover, the inhibition of N-methyltransferase blocks 5-HTP-induced head twitches in βarr2-KO mice, indicating that N-methyltryptamines, rather than serotonin, primarily mediate this response. }} This would be in accordance with the lack of inherent psychedelic effects with serotonin releasing agents, serotonin reuptake inhibitors, and serotonin precursors in humans. The HTR with high doses of serotonin in animals appears to be mediated by formation of more lipophilic N-methylated psychedelic metabolites of serotonin, like bufotenin (N,N-dimethylserotonin).

While the serotonin 5-HT_2A receptor mediates the HTR, other serotonin receptors, including the serotonin 5-HT_1A and 5-HT_2C receptors, appear to modulate the serotonin 5-HT_2A receptor-induced HTR.{{cite journal | vauthors = Carbonaro TM, Eshleman AJ, Forster MJ, Cheng K, Rice KC, Gatch MB | title = The role of 5-HT2A, 5-HT 2C and mGlu2 receptors in the behavioral effects of tryptamine hallucinogens N,N-dimethyltryptamine and N,N-diisopropyltryptamine in rats and mice | journal = Psychopharmacology | volume = 232 | issue = 1 | pages = 275–284 | date = January 2015 | pmid = 24985890 | pmc = 4282596 | doi = 10.1007/s00213-014-3658-3 }} Serotonin 5-HT_1A receptor agonists like 8-OH-DPAT suppress the HTR, while serotonin 5-HT_1A receptor antagonists can augment it.{{cite journal | vauthors = Shahar O, Botvinnik A, Esh-Zuntz N, Brownstien M, Wolf R, Lotan A, Wolf G, Lerer B, Lifschytz T | title = Role of 5-HT2A, 5-HT2C, 5-HT1A and TAAR1 Receptors in the Head Twitch Response Induced by 5-Hydroxytryptophan and Psilocybin: Translational Implications | journal = Int J Mol Sci | volume = 23 | issue = 22 | date = November 2022 | page = 14148 | pmid = 36430623 | pmc = 9698447 | doi = 10.3390/ijms232214148 | doi-access = free | url = }}{{cite journal | vauthors = Glatfelter GC, Clark AA, Cavalco NG, Landavazo A, Partilla JS, Naeem M, Golen JA, Chadeayne AR, Manke DR, Blough BE, McCorvy JD, Baumann MH | title = Serotonin 1A Receptors Modulate Serotonin 2A Receptor-Mediated Behavioral Effects of 5-Methoxy-N,N-dimethyltryptamine Analogs in Mice | journal = ACS Chem Neurosci | volume = 15| issue = 24| pages = 4458–4477| date = December 2024 | pmid = 39636099 | doi = 10.1021/acschemneuro.4c00513 | url = }} In addition, LSM-775, which is a weakly hallucinogenic psychedelic in humans, does not induce the HTR in animals unless the serotonin 5-HT_1A receptor is blocked with WAY-100635, suggesting that serotonin 5-HT_1A receptor activation masks its psychedelic-like effects.{{cite book | last1=Halberstadt | first1=Adam L. | last2=Nichols | first2=David E. | title=Handbook of Behavioral Neuroscience | chapter=Serotonin and serotonin receptors in hallucinogen action | publisher=Elsevier | volume=31 | date=2020 | isbn=978-0-444-64125-0 | doi=10.1016/b978-0-444-64125-0.00043-8 | pages=843–863}} The serotonin 5-HT_1A receptor agonist buspirone has been reported to suppress the hallucinogenic effects of serotonergic psychedelics in humans, while the serotonin 5-HT_1A receptor antagonist pindolol has been reported to markedly potentiate them.{{Cite journal |vauthors=Halman A, Kong G, Sarris J, Perkins D |date=January 2024 |title=Drug-drug interactions involving classic psychedelics: A systematic review |journal=J Psychopharmacol |volume=38 |issue=1 |pages=3–18 |doi=10.1177/02698811231211219 |pmc=10851641 |pmid=37982394}}{{cite journal | vauthors = Brandt SD, Kavanagh PV, Twamley B, Westphal F, Elliott SP, Wallach J, Stratford A, Klein LM, McCorvy JD, Nichols DE, Halberstadt AL | title = Return of the lysergamides. Part IV: Analytical and pharmacological characterization of lysergic acid morpholide (LSM-775) | journal = Drug Test Anal | volume = 10 | issue = 2 | pages = 310–322 | date = February 2018 | pmid = 28585392 | pmc = 6230476 | doi = 10.1002/dta.2222 | url = | quote = Additionally, pretreatment with the 5‐HT1A agonist buspirone (20 mg p.o.) markedly attenuates the visual effects of psilocybin in human volunteers.59 Although buspirone failed to completely block the hallucinogenic effects of psilocybin, the limited inhibition is not necessarily surprising because buspirone is a low efficacy 5‐HT1A partial agonist.60 The level of 5‐HT1A activation produced by buspirone may not be sufficient to completely counteract the stimulation of 5‐HT2A receptors by psilocin (the active metabolite of psilocybin). Another consideration is that psilocin acts as a 5‐HT1A agonist.30 If 5‐HT1A activation by psilocin buffers its hallucinogenic effects similar to DMT58 then competition between psilocin and a weaker partial agonist such as buspirone would limit attenuation of the hallucinogenic response.}}{{cite journal | vauthors = Pokorny T, Preller KH, Kraehenmann R, Vollenweider FX | title = Modulatory effect of the 5-HT1A agonist buspirone and the mixed non-hallucinogenic 5-HT1A/2A agonist ergotamine on psilocybin-induced psychedelic experience | journal = Eur Neuropsychopharmacol | volume = 26 | issue = 4 | pages = 756–766 | date = April 2016 | pmid = 26875114 | doi = 10.1016/j.euroneuro.2016.01.005 | url = }}{{cite journal | vauthors = Strassman RJ | title = Human psychopharmacology of N,N-dimethyltryptamine | journal = Behav Brain Res | volume = 73 | issue = 1–2 | pages = 121–124 | date = 1996 | pmid = 8788488 | doi = 10.1016/0166-4328(96)00081-2 | url = }} However, paradoxically, whereas the serotonin 5-HT_1A receptor full agonist 8-OH-DPAT suppresses the HTR induced by 5-hydroxytryptophan (5-HTP) or DOI, buspirone, a serotonin 5-HT_1A receptor partial agonist, has been shown to enhance the HTR induced by 5-HTP plus pargyline.{{cite journal | vauthors = Haberzettl R, Bert B, Fink H, Fox MA | title = Animal models of the serotonin syndrome: a systematic review | journal = Behav Brain Res | volume = 256 | issue = | pages = 328–345 | date = November 2013 | pmid = 24004848 | doi = 10.1016/j.bbr.2013.08.045 | url = | quote = As reported in rats, studies have also demonstrated a functional relationship between 5-HT1A and 5-HT2A receptors in mice [95], [133], [186], [192]. For example, 8-OH-DPAT has been shown to attenuate head twitches induced by 5-HTP or DOI [100], [133], [162], while the partial 5-HT1A agonist buspirone has been shown to increase head twitches induced by 5-HTP plus pargyline [190].| doi-access = free }}{{cite journal | vauthors = Kitamura Y, Nagatani T, Watanabe T | title = Buspirone enhances head twitch behavior in mice | journal = Eur J Pharmacol | volume = 253 | issue = 3 | pages = 297–301 | date = March 1994 | pmid = 8200425 | doi = 10.1016/0014-2999(94)90206-2 | url = }} The possible influence of serotonin 5-HT_2B receptor signaling on the HTR has been little-studied and is largely unknown.

Serotonin 5-HT_2C receptor agonists, for instance Ro 60-0175, CP-809,101, and meta-chlorophenylpiperazine (mCPP), have been reported to suppress the HTR, while serotonin 5-HT_2C receptor antagonists, like SB-242084, have been reported to potentiate the HTR. However, in some studies, serotonin 5-HT_2C receptor inactivation, by antagonism with SB-242084 or SB-206553 or by receptor knockout, has been reported to diminish the HTR. The reasons for these contradictory findings are unclear. In any case, animal strain differences have been suggested. In addition, the influence of serotonin 5-HT_2C receptor signaling on the HTR may be bimodal, with a more recent study finding that the serotonin 5-HT_2C receptor antagonist RS-102221 enhanced the HTR at lower doses but inhibited it at higher doses.

A number of other drugs have also been found to modulate the HTR. Monoamine oxidase inhibitors (MAOIs) like harmine, iproniazid, pargyline, clorgyline, and tranylcypromine have been found to potentiate the HTR induced by serotonergic psychedelics and other serotonergic agents without inducing the HTR on their own. This is the case even with psychedelics that are not themselves monoamine oxidase (MAO) substrates, indicating that the potentiation is not simply due to inhibition of their metabolism. In contrast to MAOIs, serotonin reuptake inhibitors (SRIs), including citalopram, fluoxetine, fluvoxamine, and imipramine, do not affect the HTR induced by DOI. Conversely, serotonin transporter (SERT) knockout greatly reduces or even eliminates the psychedelic-induced HTR.{{cite journal | vauthors = Gattuso JJ, Wilson C, Li S, Hannan AJ, Renoir T | title = Mice Lacking the Serotonin Transporter do not Respond to the Behavioural Effects of Psilocybin | journal = Eur J Pharmacol | volume = 991| issue = | pages = 177304 | date = January 2025 | pmid = 39864573 | doi = 10.1016/j.ejphar.2025.177304 | url = }} This may be due to elevated serotonin levels and decreased serotonin 5-HT_2A receptor expression. Unlike SRIs, chronic administration of serotonin–norepinephrine reuptake inhibitors (SNRIs) has been found to decrease the DOI-induced HTR. The anticonvulsant phenytoin potentiates the HTR. NMDA receptor antagonists like phencyclidine (PCP), ketamine, and dizocilpine (MK-801) have been found to enhance the DOI-induced HTR as well.

A variety of other agents, including the β-adrenergic receptor agonist clenbuterol, AMPA receptor antagonists like tezampanel (LY-293558), metabotropic glutamate mGlu₂ and mGlu₃ receptor agonists like eglumegad and LY-379268, antipsychotics like haloperidol, antihistamines, μ-opioid receptor agonists like morphine, methadone, and pethidine,{{cite journal | vauthors = Salinsky LM, Merritt CR, Zamora JC, Giacomini JL, Anastasio NC, Cunningham KA | title = μ-opioid receptor agonists and psychedelics: pharmacological opportunities and challenges | journal = Front Pharmacol | volume = 14 | issue = | pages = 1239159 | date = 2023 | pmid = 37886127 | pmc = 10598667 | doi = 10.3389/fphar.2023.1239159 | doi-access = free | url = }} adenosine A₁ receptor agonists like N⁶-cyclopentyladenosine, and the TAAR1 antagonist EPPTB, have been reported to inhibit the HTR induced by serotonergic psychedelics and/or other serotonergic agents in animals.{{cite journal | vauthors = Halberstadt AL | title = Recent advances in the neuropsychopharmacology of serotonergic hallucinogens | journal = Behav Brain Res | volume = 277 | issue = | pages = 99–120 | date = January 2015 | pmid = 25036425 | pmc = 4642895 | doi = 10.1016/j.bbr.2014.07.016 | url = | quote = The HTR induced by hallucinogens and other 5-HT agonists is closely linked to 5-HT2A activation. It was proposed in 1982 that the mescaline-induced HTR is mediated by the 5-HT2A receptor, based on the fact that the relative potency of 5-HT antagonists to block the behavior is correlated (r = 0.875) with their 5-HT2A affinity (Leysen et al., 1982). }} Conversely, the metabotropic glutamate mGlu₂ and mGlu₃ receptor antagonist LY-341495 has been found to potentiate the psychedelic-induced HTR.

Serotonin depletion has been found to potentiate the HTR. This appears to be related to increased postsynaptic serotonin 5-HT₂ receptors.Heal, D. J., Metz, A., & Philpot, J. (1985). 5, 7-dihydroxytryptamine lesioning increases both 5-HT-2 mediated behavior and cortical 5-HT-2 receptor number in mice. Brit. J. Pharmacol., 85, 311P. https://scholar.google.com/scholar?cluster=6981396242978161484

The HTR was first described as an effect induced by LSD, independently by Winter and Flataker and by Keller and Umbreit, in 1956.{{cite book | vauthors = Halberstadt AL, Geyer MA | title = Behavioral Neurobiology of Psychedelic Drugs: Effect of Hallucinogens on Unconditioned Behavior | chapter = Effect of Hallucinogens on Unconditioned Behavior | journal = Curr Top Behav Neurosci | series = Current Topics in Behavioral Neurosciences | volume = 36 | issue = | pages = 159–199 | date = 2018 | pmid = 28224459 | pmc = 5787039 | doi = 10.1007/7854_2016_466 | isbn = 978-3-662-55878-2}}{{cite journal | vauthors = McClure-Begley TD, Roth BL | title = The promises and perils of psychedelic pharmacology for psychiatry | journal = Nat Rev Drug Discov | volume = 21 | issue = 6 | pages = 463–473 | date = June 2022 | pmid = 35301459 | doi = 10.1038/s41573-022-00421-7 | url = https://cdr.lib.unc.edu/downloads/jw827p78p}}{{cite journal | vauthors = Winter CA, Flataker L | title = Effects of lysergic acid diethylamide upon performance of trained rats | journal = Proc Soc Exp Biol Med | volume = 92 | issue = 2 | pages = 285–289 | date = June 1956 | pmid = 13350323 | doi = 10.3181/00379727-92-22453 | url = }}{{cite journal | vauthors = Keller DL, Umbreit WW | title = Permanent alteration of behavior in mice by chemical and psychological means | journal = Science | volume = 124 | issue = 3225 | pages = 723–724 | date = October 1956 | pmid = 13371313 | doi = 10.1126/science.124.3225.723 | url = }} Subsequently, it was described as an effect of large doses of 5-HTP, by Corne, Pickering, and Warner, in 1963.{{cite journal | vauthors = Corne SJ, Pickering RW, Warner BT | title = A method for assessing the effects of drugs on the central actions of 5-hydroxytryptamine | journal = Br J Pharmacol Chemother | volume = 20 | issue = 1 | pages = 106–120 | date = February 1963 | pmid = 14023050 | pmc = 1703746 | doi = 10.1111/j.1476-5381.1963.tb01302.x | url = }} At first, the HTR was just a pharmacological curiosity and was not used as a tool in scientific research. In 1967 however, Corne and Pickering demonstrated the influence of a wide range of drugs on the HTR and proposed the HTR as a behavioral predictor of hallucinogenic effects in humans.{{cite journal | vauthors = Corne SJ, Pickering RW | title = A possible correlation between drug-induced hallucinations in man and a behavioural response in mice | journal = Psychopharmacologia | volume = 11 | issue = 1 | pages = 65–78 | date = 1967 | pmid = 5302272 | doi = 10.1007/BF00401509 | url = }} The reliability of the HTR for identifying psychedelics is said to have been established by the mid-1970s.{{cite book | last=Glennon | first=Richard A. | title=Animal Models of Drug Addiction | chapter=Animal Models for Assessing Hallucinogenic Agents | publisher=Humana Press | publication-place=New Jersey | volume=24 | date=23 October 1992 | isbn=978-0-89603-217-0 | doi=10.1385/0-89603-217-5:345 | pages=345–382 | quote=The head-twitch response has subsequently been shown to be one of the most reliable models for identifying hallucinogenic agents (Silva and Calil, 1975; Calil, 1978). It is not, however, without serious drawbacks. For example, certain nonhallucinogens (e.g., 5-HTP) are very potent in pro ducing head-twitch. Quipazine (Malick et al., 1977), 5-HT (e.g., Mawson and Wittington, 1970), amphetamine (Taylor and Sulser, 1973), and intraventricularly administered para-hydroxyamphetamine (Tadano et al., 1986) also elicit head-twitch. [...] 5-HTP-induced head-twitch in rodents can be antagonized by a wide variety of 5-HT antagonists, including cinanserin, cyproheptadine, metergoline, methysergide, and mianserin (Corne et al., 1963; Bedard and Pycock, 1977; Ortmann et al., 1982; Colpaert and Janssen, 1983; Green, 1984; Lucki et al., 1984; Gerber et al., 1985). [...}}{{cite journal | vauthors = Silva MT, Calil HM | title = Screening hallucinogenic drugs: systematic study of three behavioral tests | journal = Psychopharmacologia | volume = 42 | issue = 2 | pages = 163–171 | date = May 1975 | pmid = 1080571 | doi = 10.1007/BF00429548 | url = https://www.researchgate.net/publication/22073683}} However, it has been said that the HTR test did not become widely used in studying serotonin 5-HT_2A receptor activation until the mid-2000s.

Studies published in the 1960s and 1970s had shown that serotonin receptor antagonists, such as cinanserin, methysergide, and cyproheptadine, blocked the hallucinogen-like effects of psychedelics in animals.{{cite journal | vauthors = Browne RG, Ho BT | title = Role of serotonin in the discriminative stimulus properties of mescaline | journal = Pharmacol Biochem Behav | volume = 3 | issue = 3 | pages = 429–435 | date = 1975 | pmid = 125425 | doi = 10.1016/0091-3057(75)90052-0 | url = }}{{cite journal | vauthors = Winter JC | title = Blockade of the stimulus properties of mescaline by a serotonin antagonist | journal = Arch Int Pharmacodyn Ther | volume = 214 | issue = 2 | pages = 250–253 | date = April 1975 | pmid = 1156033 | doi = | url = }} Mediation of the HTR specifically via serotonin 5-HT₂ receptor agonism was first proposed by Peroutka, Lebovitz, and Snyder in 1981,{{cite journal | vauthors = Peroutka SJ, Lebovitz RM, Snyder SH | title = Two distinct central serotonin receptors with different physiological functions | journal = Science | volume = 212 | issue = 4496 | pages = 827–829 | date = May 1981 | pmid = 7221567 | doi = 10.1126/science.7221567 | bibcode = 1981Sci...212..827P | url = | quote = Drug affinities for serotonin 1 receptors labeled by [3H]serotonin do not correlate with head twitch blockade (Fig. IA). By contrast, drug potencies in blocking induced head twitches closely correlate with affinities for serotonin 2 receptors labeled by [3H]spiroperidol (r = .98, P < .001).}} followed by supporting studies by Ortmann and colleagues in 1982{{cite journal | vauthors = Ortmann R, Bischoff S, Radeke E, Buech O, Delini-Stula A | title = Correlations between different measures of antiserotonin activity of drugs. Study with neuroleptics and serotonin receptor blockers | journal = Naunyn Schmiedebergs Arch Pharmacol | volume = 321 | issue = 4 | pages = 265–270 | date = December 1982 | pmid = 6132341 | doi = 10.1007/BF00498511 | url = }} and Leysen and colleagues also in 1982.{{cite journal | vauthors = Leysen JE, Niemegeers CJ, Van Nueten JM, Laduron PM | title = [3H]Ketanserin (R 41 468), a selective 3H-ligand for serotonin2 receptor binding sites. Binding properties, brain distribution, and functional role | journal = Mol Pharmacol | volume = 21 | issue = 2 | pages = 301–314 | date = March 1982 | pmid = 7099138 | doi = 10.1016/S0026-895X(25)14623-3| url = https://molpharm.aspetjournals.org/article/S0026-895X(25)14623-3/abstract| url-access = subscription }} Richard Glennon and colleagues further supported mediation of the hallucinogen-like effects of psychedelics by serotonin 5-HT₂ receptor agonism with subsequent studies, for instance employing drug discrimination, in 1983 and thereafter.{{cite journal | vauthors = Glennon RA, Young R, Rosecrans JA | title = Antagonism of the effects of the hallucinogen DOM and the purported 5-HT agonist quipazine by 5-HT2 antagonists | journal = Eur J Pharmacol | volume = 91 | issue = 2–3 | pages = 189–196 | date = July 1983 | pmid = 6617740 | doi = 10.1016/0014-2999(83)90464-8 | url = }}{{cite journal | vauthors = Glennon RA, Titeler M, McKenney JD | title = Evidence for 5-HT2 involvement in the mechanism of action of hallucinogenic agents | journal = Life Sci | volume = 35 | issue = 25 | pages = 2505–2511 | date = December 1984 | pmid = 6513725 | doi = 10.1016/0024-3205(84)90436-3 | url = }}{{cite journal | vauthors = Titeler M, Lyon RA, Glennon RA | title = Radioligand binding evidence implicates the brain 5-HT2 receptor as a site of action for LSD and phenylisopropylamine hallucinogens | journal = Psychopharmacology (Berl) | volume = 94 | issue = 2 | pages = 213–216 | date = 1988 | pmid = 3127847 | doi = 10.1007/BF00176847 | url = }} However, the role of the serotonin 5-HT_2A receptor in the mediation of psychedelic-like effects, including the HTR, was not conclusively validated until studies with serotonin 5-HT_2A receptor knockout mice were published in 2003.{{cite journal | vauthors = González-Maeso J, Yuen T, Ebersole BJ, Wurmbach E, Lira A, Zhou M, Weisstaub N, Hen R, Gingrich JA, Sealfon SC | title = Transcriptome fingerprints distinguish hallucinogenic and nonhallucinogenic 5-hydroxytryptamine 2A receptor agonist effects in mouse somatosensory cortex | journal = J Neurosci | volume = 23 | issue = 26 | pages = 8836–8843 | date = October 2003 | pmid = 14523084 | pmc = 6740401 | doi = 10.1523/JNEUROSCI.23-26-08836.2003 | url = }} It was found in 1985 that the non-hallucinogenic serotonin receptor agonist lisuride did not produce the HTR in animals and could antagonize the HTR induced by other drugs, leading to the suggestion that it was a low-efficacy partial agonist of the serotonin 5-HT₂ receptors.{{cite journal | vauthors = Gerber R, Barbaz BJ, Martin LL, Neale R, Williams M, Liebman JM | title = Antagonism of L-5-hydroxytryptophan-induced head twitching in rats by lisuride: a mixed 5-hydroxytryptamine agonist-antagonist? | journal = Neurosci Lett | volume = 60 | issue = 2 | pages = 207–213 | date = September 1985 | pmid = 2997677 | doi = 10.1016/0304-3940(85)90245-9 | url = }}

Automation of the HTR assay was first described by Adam Halberstadt and colleagues in 2013. They developed a semi-automated assay using magnetometer-based detection. de la Fuente Revenga and colleagues developed a fully automated HTR test based on Halberstadt's work and published their system in 2019. Additional automated HTR systems, including ones employing deep learning techniques, were developed in 2020 and thereafter.

The only other behavioral paradigms for assessment of psychedelic-like effects in animals at present are drug discrimination (DD) and, to a lesser extent, prepulse inhibition (PPI) and time perception. However, the HTR is far less costly and time-consuming than drug discrimination and hence has become much more popular in recent years. Other paradigms for assessing psychedelic-associated effects have also been studied but have not shown satisfactory consistency for general use. In the 2020s, a test of psychedelic-induced visual distortions in animals was published.{{cite journal | last1=Vejmola | first1=Čestmír | last2=Šíchová | first2=Klára | last3=Syrová | first3=Kateřina | last4=Janečková | first4=Lucie | last5=Koudelka | first5=Vlastimil | last6=Tesař | first6=Michael | last7=Nikolič | first7=Marek | last8=Viktorinová | first8=Michaela | last9=Tylš | first9=Filip | last10=Korčák | first10=Jakub | last11=Viktorin | first11=Vojtěch | last12=Kelemen | first12=Eduard | last13=Nekovářová | first13=Tereza | last14=Brunovský | first14=Martin | last15=Horáček | first15=Jiří | last16=Kuchař | first16=Martin | last17=Páleníček | first17=Tomáš | title=Cross-Species Evidence for Psilocin-induced Visual Distortions: Apparent Motion is Perceived by Both Humans and Rats | journal=Biological Psychiatry Global Open Science | date=2025 | doi=10.1016/j.bpsgos.2025.100524 | doi-access=free | page=100524}}{{cite journal | last=Vejmola | first=č. | last2=Syrová | first2=K. | last3=Šíchová | first3=K. | last4=Vlastimil | first4=K. | last5=Klučková | first5=T. | last6=Kelemen | first6=E. | last7=Páleníček | first7=T. | title=Rats hallucinate in a similar manner to humans after psilocin | journal=Neuroscience Applied | volume=1 | date=2022 | doi=10.1016/j.nsa.2022.100950 | doi-access=free | page=100950}} This study marked the first evidence of psychedelic-induced visual distortions in animals.

{{Reflist}}

• [https://psychedelicreview.com/psychedelic-or-not-of-mice-5-ht2a-agonists-and-head-shakes/ Psychedelic or Not: Of Mice, 5-HT2A Agonists, and Head Shakes (February 1, 2022) - Nate Seltenrich - Psychedelic Science Review (PSR)]
• [https://psychedelicalpha.com/news/opinions-do-mice-hallucinate-do-humans-head-twitch Do Mice Hallucinate? Do Humans Head-Twitch? (February 24, 2023) - Psychedelic Alpha]
• [https://www.youtube.com/watch?v=ROcQ3AybFks LSD & The Head Twitch Response - Hamilton Morris (with Jason Wallach) - YouTube]
• [https://www.youtube.com/watch?v=wyuFBmlQS_s&t=21m42s Getting High On HIV Medication (short segment on head-twitch response) - Hamilton Morris - VICE - YouTube]
• [https://www.youtube.com/watch?v=8UDHZkV50QE The Search for Non-Hallucinogenic Psychedelics: Uncovering the Head Twitch Response - Mind & Matter Podcast]
• [https://www.youtube.com/watch?v=T-yeEEUVi-U Head-twitch response (psilocybin) - Alex Kwan - YouTube]

Category:Animal testing techniques

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