Autapse
{{Short description|Chemical or electrical synapse from a neuron onto itself}}
An autapse is a chemical or electrical synapse from a neuron onto itself.{{Cite journal|last1=Seung|first1=H. Sebastian|last2=Lee|first2=Daniel D.|last3=Reis|first3=Ben Y.|last4=Tank|first4=David W.|date=2000-09-01|title=The Autapse: A Simple Illustration of Short-Term Analog Memory Storage by Tuned Synaptic Feedback|journal=Journal of Computational Neuroscience|language=en|volume=9|issue=2|pages=171–185|doi=10.1023/A:1008971908649|pmid=11030520|s2cid=547421|issn=0929-5313}} It can also be described as a synapse formed by the axon of a neuron on its own dendrites, in vivo or in vitro.
History
The term "autapse" was first coined in 1972 by Van der Loos and Glaser, who observed them in Golgi preparations of the rabbit occipital cortex while originally conducting a quantitative analysis of neocortex circuitry.{{Cite journal|last1=Van der Loos|first1=H.|last2=Glaser|first2=E. M.|date=1972-12-24|title=Autapses in neocortex cerebri: synapses between a pyramidal cell's axon and its own dendrites|journal=Brain Research|volume=48|pages=355–360|issn=0006-8993|pmid=4645210|doi=10.1016/0006-8993(72)90189-8}} Also in the 1970s, autapses have been described in dog and rat cerebral cortex,{{Cite book|title=Neurons and Interneuronal Connections of the Central Visual System {{!}} SpringerLink|last=Shkol’nik-Yarros|first=Ekaterina G.|language=en-gb|doi=10.1007/978-1-4684-0715-0|year = 1971|isbn = 978-1-4684-0717-4|s2cid=37317913}}{{Cite journal|date=1980-02-10|title=Medium spiny neuron projection from the rat striatum: An intracellular horseradish peroxidase study|journal=Brain Research|volume=183|issue=2|pages=253–263|doi=10.1016/0006-8993(80)90462-X|pmid=7353139|issn=0006-8993 | last1 = Preston | first1 = R.J. | last2 = Bishop | first2 = G.A. | last3 = Kitai | first3 = S.T.|s2cid=1827091}}{{Cite journal|last1=Peters|first1=A.|last2=Proskauer|first2=C. C.|date=April 1980|title=Synaptic relationships between a multipolar stellate cell and a pyramidal neuron in the rat visual cortex. A combined Golgi-electron microscope study|journal=Journal of Neurocytology|volume=9|issue=2|pages=163–183|issn=0300-4864|pmid=6160209|doi=10.1007/bf01205156|s2cid=34203892}} monkey neostriatum,{{Cite journal|last1=DiFiglia|first1=M.|last2=Pasik|first2=P.|last3=Pasik|first3=T.|date=1976-09-17|title=A Golgi study of neuronal types in the neostriatum of monkeys|journal=Brain Research|volume=114|issue=2|pages=245–256|issn=0006-8993|pmid=822916|doi=10.1016/0006-8993(76)90669-7|s2cid=40311354}} and cat spinal cord.{{Cite journal|last1=Scheibel|first1=M.E.|last2=Scheibel|first2=A.B.|date=1971|title=Inhibition and the Renshaw Cell A Structural Critique; pp. 73–93|journal=Brain, Behavior and Evolution|language=en|volume=4|issue=1|pages=73–93|doi=10.1159/000125425|issn=0006-8977}}
In 2000, they were first modeled as supporting persistence in recurrent neural networks. In 2004, they were modeled as demonstrating oscillatory behavior, which was absent in the same model neuron without autapse. More specifically, the neuron oscillated between high firing rates and firing suppression, reflecting the spike bursting behavior typically found in cerebral neurons. In 2009, autapses were, for the first time, associated with sustained activation. This proposed a possible function for excitatory autapses within a neural circuit. In 2014, electrical autapses were shown to generate stable target and spiral waves in a neural model network. This indicated that they played a significant role in stimulating and regulating the collective behavior of neurons in the network. In 2016, a model of resonance was offered.
Autapses have been used to simulate "same cell" conditions to help researchers make quantitative comparisons, such as studying how N-methyl-D-aspartate receptor (NMDAR) antagonists affect synaptic versus extrasynaptic NMDARs.{{Cite journal|last1=Xia|first1=Peng|last2=Chen|first2=Huei-sheng Vincent|last3=Zhang|first3=Dongxian|last4=Lipton|first4=Stuart A.|date=2010-08-18|title=Memantine Preferentially Blocks Extrasynaptic over Synaptic NMDA Receptor Currents in Hippocampal Autapses|journal=Journal of Neuroscience|language=en|volume=30|issue=33|pages=11246–11250|doi=10.1523/JNEUROSCI.2488-10.2010|issn=0270-6474|pmid=20720132|pmc=2932667}}
Formation
Recently, it has been proposed that autapses could possibly form as a result of neuronal signal transmission blockage, such as in cases of axonal injury induced by poisoning or impeding ion channels.{{Cite journal|last1=Wang|first1=Chunni|last2=Guo|first2=Shengli|last3=Xu|first3=Ying|last4=Ma|first4=Jun|last5=Tang|first5=Jun|last6=Alzahrani|first6=Faris|last7=Hobiny|first7=Aatef|date=2017|title=Formation of Autapse Connected to Neuron and Its Biological Function|journal=Complexity|language=en|volume=2017|pages=1–9|doi=10.1155/2017/5436737|issn=1076-2787|doi-access=free}} Dendrites from the soma in addition to an auxiliary axon may develop to form an autapse to help remediate the neuron's signal transmission.
Structure and function
Autapses can be either glutamate-releasing (excitatory) or GABA-releasing (inhibitory), just like their traditional synapse counterparts.{{Cite journal|last1=Ikeda|first1=Kaori|last2=Bekkers|first2=John M.|date=2006-05-09|title=Autapses|journal=Current Biology|volume=16|issue=9|pages=R308|doi=10.1016/j.cub.2006.03.085|issn=0960-9822|pmid=16682332|doi-access=free|bibcode=2006CBio...16.R308I }} Similarly, autapses can be electrical or chemical by nature.{{Cite journal|last1=Yilmaz|first1=Ergin|last2=Ozer|first2=Mahmut|last3=Baysal|first3=Veli|last4=Perc|first4=Matjaž|date=2016-08-02|title=Autapse-induced multiple coherence resonance in single neurons and neuronal networks|journal=Scientific Reports|language=En|volume=6|issue=1|doi=10.1038/srep30914|pmid=27480120|pmc=4969620|issn=2045-2322|page=30914|bibcode=2016NatSR...630914Y}}
Broadly speaking, negative feedback in autapses tends to inhibit excitable neurons whereas positive feedback can stimulate quiescent neurons.{{Cite journal|last1=Qin|first1=Huixin|last2=Wu|first2=Ying|last3=Wang|first3=Chunni|last4=Ma|first4=Jun|title=Emitting waves from defects in network with autapses|journal=Communications in Nonlinear Science and Numerical Simulation|language=en|volume=23|issue=1–3|pages=164–174|doi=10.1016/j.cnsns.2014.11.008|year=2015|bibcode=2015CNSNS..23..164Q}}
Although the stimulation of inhibitory autapses did not induce hyperpolarizing inhibitory post-synaptic potentials in interneurons of layer V of neocortical slices, they have been shown to impact excitability.{{Cite journal|last1=Bacci|first1=Alberto|last2=Huguenard|first2=John R.|last3=Prince|first3=David A.|date=2003-02-01|title=Functional autaptic neurotransmission in fast-spiking interneurons: a novel form of feedback inhibition in the neocortex|journal=The Journal of Neuroscience|volume=23|issue=3|pages=859–866|issn=1529-2401|pmid=12574414|doi=10.1523/JNEUROSCI.23-03-00859.2003|pmc=6741939|doi-access=free}} Upon using a GABA-antagonist to block autapses, the likelihood of an immediate subsequent second depolarization step increased following a first depolarization step. This suggests that autapses act by suppressing the second of two closely timed depolarization steps and therefore, they may provide feedback inhibition onto these cells. This mechanism may also potentially explain shunting inhibition.
In cell culture, autapses have been shown to contribute to the prolonged activation of B31/B32 neurons, which significantly contribute food-response behavior in Aplysia. This suggests that autapses may play a role in mediating positive feedback. The B31/B32 autapse was unable to play a role in initiating the neuron's activity, although it is believed to have helped sustain the neuron's depolarized state. The extent to which autapses maintain depolarization remains unclear, particularly since other components of the neural circuit (i.e. B63 neurons) are also capable of providing strong synaptic input throughout the depolarization. Additionally, it has been suggested that autapses provide B31/B32 neurons with the ability to quickly repolarize. Bekkers (2009) has proposed that specifically blocking the contribution of autapses and then assessing the differences with or without blocked autapses could better illuminate the function of autapses.{{Cite journal|last=Bekkers|first=John M.|title=Synaptic Transmission: Excitatory Autapses Find a Function?|journal=Current Biology|volume=19|issue=7|pages=R296–R298|doi=10.1016/j.cub.2009.02.010|pmid=19368875|year=2009|s2cid=15821336|doi-access=free|bibcode=2009CBio...19.R296B }}
Hindmarsh–Rose (HR) model neurons have demonstrated chaotic, regular spiking, quiescent, and periodic patterns of burst firing without autapses.{{Cite journal|last1=Wang|first1=Hengtong|last2=Ma|first2=Jun|last3=Chen|first3=Yueling|last4=Chen|first4=Yong|title=Effect of an autapse on the firing pattern transition in a bursting neuron|journal=Communications in Nonlinear Science and Numerical Simulation|volume=19|issue=9|pages=3242–3254|doi=10.1016/j.cnsns.2014.02.018|year=2014|bibcode=2014CNSNS..19.3242W}} Upon the introduction of an electrical autapse, the periodic state switches to the chaotic state and displays an alternating behavior that increases in frequency with a greater autaptic intensity and time delay. On the other hand, excitatory chemical autapses enhanced the overall chaotic state. The chaotic state was reduced and suppressed in the neurons with inhibitory chemical autapses. In HR model neurons without autapses, the pattern of firing altered from quiescent to periodic and then to chaotic as DC current was increased. Generally, HR model neurons with autapses have the ability to swap into any firing pattern, regardless of the prior firing pattern.
Location
Neurons from several brain regions, such as the neocortex, substantia nigra, and hippocampus have been found to contain autapses.{{Cite journal|last1=Park|first1=Melburn R.|last2=Lighthall|first2=James W.|last3=Kitai|first3=Stephen T.|title=Recurrent inhibition in the rat neostriatum|journal=Brain Research|volume=194|issue=2|pages=359–369|doi=10.1016/0006-8993(80)91217-2|pmid=7388619|year=1980|s2cid=29451737}}{{Cite journal|last1=Karabelas|first1=Athanasios B.|last2=Purrura|first2=Dominick P.|title=Evidence for autapses in the substantia nigra|journal=Brain Research|volume=200|issue=2|pages=467–473|doi=10.1016/0006-8993(80)90935-x|pmid=6158366|year=1980|s2cid=35517474}}{{Cite journal|last1=Cobb|first1=S.R|last2=Halasy|first2=K|last3=Vida|first3=I|last4=Nyı́ri|first4=G|last5=Tamás|first5=G|last6=Buhl|first6=E.H|last7=Somogyi|first7=P|title=Synaptic effects of identified interneurons innervating both interneurons and pyramidal cells in the rat hippocampus|journal=Neuroscience|volume=79|issue=3|pages=629–648|doi=10.1016/s0306-4522(97)00055-9|year=1997|pmid=9219929|s2cid=15479304}}
Autapses have been observed to be relatively more abundant in GABAergic basket and dendrite-targeting cells of the cat visual cortex compared to spiny stellate, double bouquet, and pyramidal cells, suggesting that the degree of neuron self-innervation is cell-specific.{{Cite journal|last1=Tamás|first1=G.|last2=Buhl|first2=E. H.|last3=Somogyi|first3=P.|date=1997-08-15|title=Massive autaptic self-innervation of GABAergic neurons in cat visual cortex|journal=The Journal of Neuroscience|volume=17|issue=16|pages=6352–6364|issn=0270-6474|pmid=9236244|doi=10.1523/JNEUROSCI.17-16-06352.1997|pmc=6568358|doi-access=free}} Additionally, dendrite-targeting cell autapses were, on average, further from the soma compared to basket cell autapses.
80% of layer V pyramidal neurons in developing rat neocortices contained autaptic connections, which were located more so on basal dendrites and apical oblique dendrites rather than main apical dendrites.{{Cite journal|last1=Lübke|first1=J.|last2=Markram|first2=H.|last3=Frotscher|first3=M.|last4=Sakmann|first4=B.|date=1996-05-15|title=Frequency and dendritic distribution of autapses established by layer 5 pyramidal neurons in the developing rat neocortex: comparison with synaptic innervation of adjacent neurons of the same class|journal=The Journal of Neuroscience|volume=16|issue=10|pages=3209–3218|issn=0270-6474|pmid=8627359|doi=10.1523/JNEUROSCI.16-10-03209.1996|pmc=6579140|doi-access=free}} The dendritic positions of synaptic connections of the same cell type were similar to those of autapses, suggesting that autaptic and synaptic networks share a common mechanism of formation.
Disease implications
In the 1990s, paroxysmal depolarizing shift-type interictal epileptiform discharges has been suggested to be primarily dependent on autaptic activity for solitary excitatory hippocampal rat neurons grown in microculture.{{Cite journal|last=Segal|first=M. M.|date=October 1994|title=Endogenous bursts underlie seizurelike activity in solitary excitatory hippocampal neurons in microcultures|journal=Journal of Neurophysiology|volume=72|issue=4|pages=1874–1884|issn=0022-3077|pmid=7823106|doi=10.1152/jn.1994.72.4.1874}}
More recently, in human neocortical tissues of patients with intractable epilepsy, the GABAergic output autapses of fast-spiking (FS) neurons have been shown to have stronger asynchronous release (AR) compared to both non-epileptic tissue and other types of synapses involving FS neurons.{{Cite journal|last1=Jiang|first1=Man|last2=Zhu|first2=Jie|last3=Liu|first3=Yaping|last4=Yang|first4=Mingpo|last5=Tian|first5=Cuiping|last6=Jiang|first6=Shan|last7=Wang|first7=Yonghong|last8=Guo|first8=Hui|last9=Wang|first9=Kaiyan|date=2012-05-08|title=Enhancement of Asynchronous Release from Fast-Spiking Interneuron in Human and Rat Epileptic Neocortex|journal=PLOS Biology|volume=10|issue=5|pages=e1001324|doi=10.1371/journal.pbio.1001324|pmid=22589699|pmc=3348166|issn=1545-7885 |doi-access=free }} The study found similar results using a rat model as well. An increase in residual Ca2+ concentration in addition to the action potential amplitude in FS neurons was suggested to cause this increase in AR of epileptic tissue. Anti-epileptic drugs could potentially target this AR of GABA that seems to rampantly occur at FS neuron autapses.
Effects of drugs
Using a glia-conditioned medium to treat glia-free purified rat retinal ganglion microcultures has been shown to significantly increase the number of autapses per neuron compared to a control.{{Cite journal|last1=Nägler|first1=Karl|last2=Mauch|first2=Daniela H|last3=Pfrieger|first3=Frank W|date=2001-06-15|title=Glia-derived signals induce synapse formation in neurones of the rat central nervous system|journal=The Journal of Physiology|volume=533|issue=Pt 3|pages=665–679|doi=10.1111/j.1469-7793.2001.00665.x|issn=0022-3751|pmc=2278670|pmid=11410625}} This suggests that glia-derived soluble, proteinase K-sensitive factors induce autapse formation in rat retinal ganglion cells.
References
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Category:Cellular neuroscience