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Sensitized by a sea slug: site-specific short-term and general long-term sensitization in Aplysia following Navanax attack. Neurobiol Learn Mem 2021; 187:107542. [PMID: 34748927 DOI: 10.1016/j.nlm.2021.107542] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2021] [Revised: 10/13/2021] [Accepted: 10/22/2021] [Indexed: 11/23/2022]
Abstract
Neurobiological studies of the model species, Aplysia californica (Mollusca, Gastropoda, Euopisthobranchia), have helped advance our knowledge of the neural bases of different forms of learning, including sensitization, a non-associative increase in withdrawal behaviors in response to mild innocuous stimuli However, our understanding of the natural context for this learning has lagged behind the mechanistic studies. Because previous studies of sensitization used electric shock, or other artificial stimulus to produce sensitization, they left unaddressed the question of what stimuli in nature might cause sensitization, until our laboratory demonstrated short and long-term sensitization after predatory attack by spiny lobsters. In the present study, we tested for sensitization after attack by a very different predator, the predacious sea-slug, Navanax inermis (Mollusca, Gastropoda, Euopisthobranchia). Unlike the biting and prodding action of lobster attack, Navanax uses a rapid strike that sucks and squeezes its prey in an attempt to swallow it whole. We found that Navanax attack to the head of Aplysia caused strong immediate sensitization of head withdrawal, and weaker, delayed, sensitization of tail-mantle withdrawal. By contrast, attack to the tail of Aplysia resulted in no sensitization of either reflex. We also developed an artificial attack stimulus that allowed us to mimick a more consistently strong attack. This artificial attack produced stronger but qualitatively similar sensitization: Strong immediate sensitization of head withdrawal and weaker sensitization of tail-mantle withdrawal after head attack, immediate sensitization in tail-mantle withdrawal, but no sensitization of head withdrawal after tail attack. We conclude that Navanax attack causes robust site-specific sensitization (enhanced sensitization near the site of attack), and weaker general sensitization (sensitization of responses to stimuli distal to the attack site). We also tested for long-term sensitization (lasting longer than 24 hours) after temporally-spaced delivery of four natural Navanax attacks to the head of subject Aplysia. Surprisingly, these head attacks, any one of which strongly sensitizes head withdrawal in the short term, failed to sensitize head-withdrawal in the long term. Paradoxically, these repeated head attacks produced long-term sensitization in tail-mantle withdrawal. These experiments and observations confirm that Navanax attack causes short, and long-term sensitization of withdrawal reflexes of Aplysia. Together with the observation of sensitization after lobster attack, they strongly support the premise that sensitization in Aplysia is an adaptive response to sub-lethal predator attack. They also add site-specific sensitization to the list of naturally induced learning phenotypes, as well as paradoxical long-term sensitization of tail-mantle withdrawal (but not head withdrawal) after multiple head attacks.
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Rosiles T, Nguyen M, Duron M, Garcia A, Garcia G, Gordon H, Juarez L, Calin-Jageman IE, Calin-Jageman RJ. Registered Report: Transcriptional Analysis of Savings Memory Suggests Forgetting is Due to Retrieval Failure. eNeuro 2020; 7:ENEURO.0313-19.2020. [PMID: 32928882 PMCID: PMC7665899 DOI: 10.1523/eneuro.0313-19.2020] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/10/2020] [Revised: 07/18/2020] [Accepted: 07/30/2020] [Indexed: 11/21/2022] Open
Abstract
There is fundamental debate about the nature of forgetting: some have argued that it represents the decay of the memory trace, others that the memory trace persists but becomes inaccessible because of retrieval failure. These different accounts of forgetting lead to different predictions about savings memory, the rapid re-learning of seemingly forgotten information. If forgetting is because of decay, then savings requires re-encoding and should thus involve the same mechanisms as initial learning. If forgetting is because of retrieval failure, then savings should be mechanistically distinct from encoding. In this registered report, we conducted a preregistered and rigorous test between these accounts of forgetting. Specifically, we used microarray to characterize the transcriptional correlates of a new memory (1 d after training), a forgotten memory (8 d after training), and a savings memory (8 d after training but with a reminder on day 7 to evoke a long-term savings memory) for sensitization in Aplysia californica (n = 8 samples/group). We found that the reactivation of sensitization during savings does not involve a substantial transcriptional response. Thus, savings is transcriptionally distinct relative to a newer (1-d-old) memory, with no coregulated transcripts, negligible similarity in regulation-ranked ordering of transcripts, and a negligible correlation in training-induced changes in gene expression (r = 0.04 95% confidence interval (CI) [-0.12, 0.20]). Overall, our results suggest that forgetting of sensitization memory represents retrieval failure.
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Affiliation(s)
- Tania Rosiles
- Neuroscience Program, Dominican University, River Forest, Illinois 60305
| | - Melissa Nguyen
- Neuroscience Program, Dominican University, River Forest, Illinois 60305
| | - Monica Duron
- Neuroscience Program, Dominican University, River Forest, Illinois 60305
| | - Annette Garcia
- Neuroscience Program, Dominican University, River Forest, Illinois 60305
| | - George Garcia
- Neuroscience Program, Dominican University, River Forest, Illinois 60305
| | - Hannah Gordon
- Neuroscience Program, Dominican University, River Forest, Illinois 60305
| | - Lorena Juarez
- Neuroscience Program, Dominican University, River Forest, Illinois 60305
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Patel U, Perez L, Farrell S, Steck D, Jacob A, Rosiles T, Krause E, Nguyen M, Calin-Jageman RJ, Calin-Jageman IE. Transcriptional changes before and after forgetting of a long-term sensitization memory in Aplysia californica. Neurobiol Learn Mem 2018; 155:474-485. [PMID: 30243850 PMCID: PMC6365195 DOI: 10.1016/j.nlm.2018.09.007] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/02/2018] [Revised: 09/11/2018] [Accepted: 09/19/2018] [Indexed: 01/06/2023]
Abstract
Most long-term memories are forgotten, becoming progressively less likely to be recalled. Still, some memory fragments may persist, as savings memory (easier relearning) can be detected long after recall has become impossible. What happens to a memory trace during forgetting that makes it inaccessible for recall and yet still effective to spark easier re-learning? We are addressing this question by tracking the transcriptional changes that accompany learning and then forgetting of a long-term sensitization memory in the tail-elicited siphon withdrawal reflex of Aplysia californica. First, we tracked savings memory. We found that even though recall of sensitization fades completely within 1 week of training, savings memory is still detectable at 2 weeks post training. Next, we tracked the time-course of regulation of 11 transcripts we previously identified as potentially being regulated after recall has become impossible. Remarkably, 3 transcripts still show strong regulation 2 weeks after training and an additional 4 are regulated for at least 1 week. These long-lasting changes in gene expression always begin early in the memory process, within 1 day of training. We present a synthesis of our results tracking gene expression changes accompanying sensitization and provide a testable model of how sensitization memory is forgotten.
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Affiliation(s)
- Ushma Patel
- Neuroscience Program, Dominican University, 7900 West Division Street, River Forest, IL 60305, United States
| | - Leticia Perez
- Neuroscience Program, Dominican University, 7900 West Division Street, River Forest, IL 60305, United States
| | - Steven Farrell
- Neuroscience Program, Dominican University, 7900 West Division Street, River Forest, IL 60305, United States
| | - Derek Steck
- Neuroscience Program, Dominican University, 7900 West Division Street, River Forest, IL 60305, United States
| | - Athira Jacob
- Neuroscience Program, Dominican University, 7900 West Division Street, River Forest, IL 60305, United States
| | - Tania Rosiles
- Neuroscience Program, Dominican University, 7900 West Division Street, River Forest, IL 60305, United States
| | - Everett Krause
- Neuroscience Program, Dominican University, 7900 West Division Street, River Forest, IL 60305, United States
| | - Melissa Nguyen
- Neuroscience Program, Dominican University, 7900 West Division Street, River Forest, IL 60305, United States
| | - Robert J Calin-Jageman
- Neuroscience Program, Dominican University, 7900 West Division Street, River Forest, IL 60305, United States
| | - Irina E Calin-Jageman
- Neuroscience Program, Dominican University, 7900 West Division Street, River Forest, IL 60305, United States.
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Transcriptional analysis of a whole-body form of long-term habituation in Aplysia californica. ACTA ACUST UNITED AC 2014; 22:11-23. [PMID: 25512573 PMCID: PMC4274328 DOI: 10.1101/lm.036970.114] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
Habituation is the simplest form of learning, but we know little about the transcriptional mechanisms that encode long-term habituation memory. A key obstacle is that habituation is relatively stimulus-specific and is thus encoded in small sets of neurons, providing poor signal/noise ratios for transcriptional analysis. To overcome this obstacle, we have developed a protocol for producing whole-body long-term habituation of the siphon-withdrawal reflex (SWR) of Aplysia californica. Specifically, we constructed a computer-controlled brushing apparatus to apply low-intensity tactile stimulation over the entire dorsal surface of Aplysia at regular intervals. We found that 3 d of training (10 rounds of stimulation/day; each round = 15 min brushing at a 10-sec ISI; 15-min rest between rounds) produces habituation with several characteristics favorable for mechanistic investigation. First, habituation is widespread, with SWR durations reduced whether the reflex is evoked by tactile stimulation to the head, tail, or the siphon. Second, long-term habituation is sensitive to the pattern of training, occurring only when brushing sessions are spaced out over 3 d rather than massed into a single session. Using a custom-designed microarray and quantitative PCR, we show that long-term habituation produces long-term up-regulation of an apparent Aplysia homolog of cornichon, a protein important for glutamate receptor trafficking. Our training paradigm provides a promising starting point for characterizing the transcriptional mechanisms of long-term habituation memory.
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Herdegen S, Conte C, Kamal S, Calin-Jageman RJ, Calin-Jageman IE. Immediate and persistent transcriptional correlates of long-term sensitization training at different CNS loci in Aplysia californica. PLoS One 2014; 9:e114481. [PMID: 25486125 PMCID: PMC4259342 DOI: 10.1371/journal.pone.0114481] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/02/2014] [Accepted: 11/10/2014] [Indexed: 11/18/2022] Open
Abstract
Repeated noxious stimulation produces long-term sensitization of defensive withdrawal reflexes in Aplysia californica, a form of long-term memory that requires changes in both transcription and translation. Previous work has identified 10 transcripts which are rapidly up-regulated after long-term sensitization training in the pleural ganglia. Here we use quantitative PCR to begin examining how these transcriptional changes are expressed in different CNS loci related to defensive withdrawal reflexes at 1 and 24 hours after long-term sensitization training. Specifically, we sample from a) the sensory wedge of the pleural ganglia, which exclusively contains the VC nociceptor cell bodies that help mediate input to defensive withdrawal circuits, b) the remaining pleural ganglia, which contain withdrawal interneurons, and c) the pedal ganglia, which contain many motor neurons. Results from the VC cluster show different temporal patterns of regulation: 1) rapid but transient up-regulation of Aplysia homologs of C/EBP, C/EBPγ, and CREB1, 2) delayed but sustained up-regulation of BiP, Tolloid/BMP-1, and sensorin, 3) rapid and sustained up-regulation of Egr, GlyT2, VPS36, and an uncharacterized protein (LOC101862095), and 4) an unexpected lack of regulation of Aplysia homologs of calmodulin (CaM) and reductase-related protein (RRP). Changes in the remaining pleural ganglia mirror those found in the VC cluster at 1 hour but with an attenuated level of regulation. Because these samples had almost no expression of the VC-specific transcript sensorin, our data suggests that sensitization training likely induces transcriptional changes in either defensive withdrawal interneurons or neurons unrelated to defensive withdrawal. In the pedal ganglia, we observed only a rapid but transient increase in Egr expression, indicating that long-term sensitization training is likely to induce transcriptional changes in motor neurons but raising the possibility of different transcriptional endpoints in this cell type.
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Affiliation(s)
- Samantha Herdegen
- Neuroscience Program, Dominican University, River Forest, Illinois, United States of America
| | - Catherine Conte
- Neuroscience Program, Dominican University, River Forest, Illinois, United States of America
| | - Saman Kamal
- Neuroscience Program, Dominican University, River Forest, Illinois, United States of America
| | - Robert J. Calin-Jageman
- Neuroscience Program, Dominican University, River Forest, Illinois, United States of America
- * E-mail: (RC-J); (IC-J)
| | - Irina E. Calin-Jageman
- Neuroscience Program, Dominican University, River Forest, Illinois, United States of America
- * E-mail: (RC-J); (IC-J)
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Herdegen S, Holmes G, Cyriac A, Calin-Jageman IE, Calin-Jageman RJ. Characterization of the rapid transcriptional response to long-term sensitization training in Aplysia californica. Neurobiol Learn Mem 2014; 116:27-35. [PMID: 25117657 DOI: 10.1016/j.nlm.2014.07.009] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/09/2014] [Revised: 07/17/2014] [Accepted: 07/24/2014] [Indexed: 11/24/2022]
Abstract
We used a custom-designed microarray and quantitative PCR to characterize the rapid transcriptional response to long-term sensitization training in the marine mollusk Aplysia californica. Aplysia were exposed to repeated noxious shocks to one side of the body, a procedure known to induce a long-lasting, transcription-dependent increase in reflex responsiveness that is restricted to the side of training. One hour after training, pleural ganglia from the trained and untrained sides of the body were harvested; these ganglia contain the sensory nociceptors which help mediate the expression of long-term sensitization memory. Microarray analysis from 8 biological replicates suggests that long-term sensitization training rapidly regulates at least 81 transcripts. We used qPCR to test a subset of these transcripts and found that 83% were confirmed in the same samples, and 86% of these were again confirmed in an independent sample. Thus, our new microarray design shows strong convergent and predictive validity for analyzing the transcriptional correlates of memory in Aplysia. Fully validated transcripts include some previously identified as regulated in this paradigm (ApC/EBP and ApEgr) but also include novel findings. Specifically, we show that long-term sensitization training rapidly up-regulates the expression of transcripts which may encode Aplysia homologs of a C/EBPγ transcription factor, a glycine transporter (GlyT2), and a vacuolar-protein-sorting-associated protein (VPS36).
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Affiliation(s)
- Samantha Herdegen
- Neuroscience Program, Dominican University, River Forest, IL, United States
| | - Geraldine Holmes
- Neuroscience Program, Dominican University, River Forest, IL, United States
| | - Ashly Cyriac
- Neuroscience Program, Dominican University, River Forest, IL, United States
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Philips GT, Sherff CM, Menges SA, Carew TJ. The tail-elicited tail withdrawal reflex of Aplysia is mediated centrally at tail sensory-motor synapses and exhibits sensitization across multiple temporal domains. Learn Mem 2011; 18:272-82. [PMID: 21450911 DOI: 10.1101/lm.2125311] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
Abstract
The defensive withdrawal reflexes of Aplysia californica have provided powerful behavioral systems for studying the cellular and molecular basis of memory formation. Among these reflexes the tail-elicited tail withdrawal reflex (T-TWR) has been especially useful. In vitro studies examining the monosynaptic circuit for the T-TWR, the tail sensory-motor (SN-MN) synapses, have identified the induction requirements and molecular basis of different temporal phases of synaptic facilitation that underlie sensitization in this system. They have also permitted more recent studies elucidating the role of synaptic and nuclear signaling during synaptic facilitation. Here we report the development of a novel, compartmentalized semi-intact T-TWR preparation that allows examination of the unique contributions of processing in the SN somatic compartment (the pleural ganglion) and the SN-MN synaptic compartment (the pedal ganglion) during the induction of sensitization. Using this preparation we find that the T-TWR is mediated entirely by central connections in the synaptic compartment. Moreover, the reflex is stably expressed for at least 24 h, and can be modified by tail shocks that induce sensitization across multiple temporal domains, as well as direct application of the modulatory neurotransmitter serotonin. This preparation now provides an experimentally powerful system in which to directly examine the unique and combined roles of synaptic and nuclear signaling in different temporal domains of memory formation.
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Affiliation(s)
- Gary T Philips
- Department of Neurobiology and Behavior, Center for the Neurobiology of Learning and Memory, University of California Irvine, California 92697-4550, USA
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Bedini C, Geppetti L. A morphological study on the amount and origin of axons in the pleuroabdominal connectives of Aplysia fasciata. ACTA ACUST UNITED AC 2009. [DOI: 10.1080/11250000009356288] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022]
Affiliation(s)
- Celina Bedini
- a Dipartimento di Etologia, Ecologia, Evoluzione , Università di Pisa , via Volta 6, Pisa, I‐56126, Italy
| | - Laura Geppetti
- a Dipartimento di Etologia, Ecologia, Evoluzione , Università di Pisa , via Volta 6, Pisa, I‐56126, Italy
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Barbas D, DesGroseillers L, Castellucci VF, Carew TJ, Marinesco S. Multiple serotonergic mechanisms contributing to sensitization in aplysia: evidence of diverse serotonin receptor subtypes. Learn Mem 2003; 10:373-86. [PMID: 14557610 PMCID: PMC218003 DOI: 10.1101/lm.66103] [Citation(s) in RCA: 89] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023]
Abstract
The neurotransmitter serotonin (5-HT) plays an important role in memory encoding in Aplysia. Early evidence showed that during sensitization, 5-HT activates a cyclic AMP-protein kinase A (cAMP-PKA)-dependent pathway within specific sensory neurons (SNs), which increases their excitability and facilitates synaptic transmission onto their follower motor neurons (MNs). However, recent data suggest that serotonergic modulation during sensitization is more complex and diverse. The neuronal circuits mediating defensive reflexes contain a number of interneurons that respond to 5-HT in ways opposite to those of the SNs, showing a decrease in excitability and/or synaptic depression. Moreover, in addition to acting through a cAMP-PKA pathway within SNs, 5-HT is also capable of activating a variety of other protein kinases such as protein kinase C, extracellular signal-regulated kinases, and tyrosine kinases. This diversity of 5-HT responses during sensitization suggests the presence of multiple 5-HT receptor subtypes within the Aplysia central nervous system. Four 5-HT receptors have been cloned and characterized to date. Although several others probably remain to be characterized in molecular terms, especially the Gs-coupled 5-HT receptor capable of activating cAMP-PKA pathways, the multiplicity of serotonergic mechanisms recruited into action during learning in Aplysia can now be addressed from a molecular point of view.
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Affiliation(s)
- Demian Barbas
- Département de biochimie, Université de Montréal, Québec H3C 3J7, Canada
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Abstract
Biophysical, biochemical, and morphological studies have implicated sensory neurons as key sites of plasticity in the formation and retention of the memory of long-term sensitization in Aplysia californica. This study examined the effects of different sensitization training protocols on the structure of sensory neurons mediating the tail-siphon withdrawal reflex. A 4 d training period produced a robust localized outgrowth in these sensory neurons observed 24 hr after the end of training. These changes are consistent with previous results in siphon sensory neurons (Bailey and Chen, 1988a). In contrast, 1 d of sensitization training, which has been shown to effectively induce long-term behavioral sensitization and synaptic facilitation (Frost et al., 1985; Cleary et al., 1998), is not associated with morphological changes in tail sensory neurons at either 24 hr or 4 d after training. Similarly, a single treatment with the growth factor TGF-beta, which also induced facilitation, did not alter sensory neuron morphology. The different effectiveness of the two protocols was not simply a reflection of the number of stimuli presented, because a 1 d massed training protocol did not produce sensitization 24 hr after training, nor did it induce neuronal outgrowth. These results suggest that extensive sensitization training is required to induce neuronal outgrowth in tail sensory neurons, indicating that the memory of long-term sensitization induced by 1 d of training is mechanistically different from that induced by 4 d of training. Moreover, the induction of a form of long-term sensitization associated with neuronal outgrowth does not appear to be a function of the amount of stimulation but does appear to be dependent on the temporal spacing of the stimulation over multiple days.
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Pivovarov AS, Drozdova EI, Moskvitin AA. Generalized post-tetanic changes in excitatory postsynaptic and acetylcholine-evoked currents in neurons in the common snail. NEUROSCIENCE AND BEHAVIORAL PHYSIOLOGY 2001; 31:55-60. [PMID: 11265814 DOI: 10.1023/a:1026674214042] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
Changes in excitatory postsynaptic currents (EPSC) and influx currents induced by local iontophoretic application of acetylcholine (ACh currents) to the surfaces of the bodies of identified neurons LPa3 and RPa3 in the common snail after heteroreceptor tetanic stimulation were studied. The effects of rhythmic application of acetylcholine (0.5-1.0 times per sec for 10-40 sec) on the magnitude of EPSC were analyzed, along with the effects of tetanic stimulation of the nerves (n. intestinalis, n. pallialis dexter, n. pallialis sinister; 1-5 Hz for 1-2 min) on the amplitudes of the ACh current and the magnitudes of EPSC evoked by stimulation of another nerve. The following changes in conditioning responses were found over 1-1.5 h after tetanization: rhythmic application of ACh to the dorsal surface of the cell body resulted in potentiation of EPSC, and tetanic orthodromic stimulation potentiated the ACh current and induced heterosynaptic depression of EPSC. These post-tetanic changes were not local--they developed in those chemoreceptor zones of the membrane which were not activated by the preceding conditioning rhythmic stimulation. It is concluded that generalized post-tetanic changes in neuron responses are induced by activation of its subsynaptic and extrasynaptic mediator chemoreceptors.
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Prescott SA. Interactions between Depression and Facilitation within Neural Networks: Updating the Dual-Process Theory of Plasticity. Learn Mem 1998. [DOI: 10.1101/lm.5.6.446] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
Abstract
Repetitive stimulation often results in habituation of the elicited response. However, if the stimulus is sufficiently strong, habituation may be preceded by transient sensitization or even replaced by enduring sensitization. In 1970, Groves and Thompson formulated the dual-process theory of plasticity to explain these characteristic behavioral changes on the basis of competition between decremental plasticity (depression) and incremental plasticity (facilitation) occurring within the neural network. Data from both vertebrate and invertebrate systems are reviewed and indicate that the effects of depression and facilitation are not exclusively additive but, rather, that those processes interact in a complex manner. Serial ordering of induction of learning, in which a depressing locus precedes the modulatory system responsible for inducing facilitation, causes the facilitation to wane. The parallel and/or serial expression of depression and waning facilitation within the stimulus–response pathway culminates in the behavioral changes that characterize dual-process learning. A mathematical model is presented to formally express and extend understanding of the interactions between depression and facilitation.
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Abstract
Although in vitro analyses of long-term changes in the sensorimotor connection of Aplysia have been used extensively to understand long-term sensitization, relatively little is known about the ways in which the connection is modified by learning in vivo. Moreover, sites other than the sensory neurons might be modified as well. In this paper, several different biophysical properties of sensory neurons, motor neurons, and LPl17, an identified interneuron, were examined. Membrane properties of sensory neurons, which were expressed as increased excitability and increased spike afterdepolarization, were affected by the training. The biophysical properties of motor neurons also were affected by training, resulting in hyperpolarization of the resting membrane potential and a decrease in spike threshold. These results suggest that motor neurons are potential loci for storage of the memory in sensitization. The strength of the connection between sensory and motor neurons was affected by the training, although the connection between LPl17 and the motor neuron was unaffected. Biophysical properties of LPl17 were unaffected by training. The results emphasize the importance of plasticity at sensory-motor synapses and are consistent with the idea that there are multiple sites of plasticity distributed throughout the nervous system.
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Heterosynaptic facilitation of tail sensory neuron synaptic transmission during habituation in tail-induced tail and siphon withdrawal reflexes of Aplysia. J Neurosci 1996. [PMID: 8756425 DOI: 10.1523/jneurosci.16-16-04933.1996] [Citation(s) in RCA: 39] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
In cellular studies of habituation, such as in the gill and siphon withdrawal reflex to tactile stimulation of the siphon of Aplysia, a mechanism that has emerged as an explanation for response decrement during habituation is homosynaptic depression at sensory neurons mediating the behavioral response. We have examined the contribution of homosynaptic depression to habituation in sensory neurons that contribute to two reflex behaviors in Aplysia, tail withdrawal and siphon withdrawal, both elicited by threshold-level tail stimulation. In a companion paper (this issue), we reported that repeated tail stimulation, identical to that producing habituation in siphon withdrawal in freely moving animals, also produces habituation in reduced preparations. In this paper, we extend these behavioral findings by showing that in reduced preparations, identical tail stimulation also produces habituation of the tail withdrawal reflex. In addition, our cellular experiments show that (1) identified sensory and motor neurons in both reflex systems respond to identical repeated tail stimulation; in sensory neurons it produces a progressive decrease in spike number and increase in spike latency, and in motor neurons it produces progressive decrement in complex EPSPs and spike output. (2) Homosynaptic depression of the tail sensory neuron to tail motor neuron synapse does occur when the sensory neurons are activated repetitively by intracellular current. (3) Homosynaptic depression at this synapse does not occur when the sensory neurons are activated repetitively by threshold-level tail stimuli that elicit the behavioral reflex and cause habituation; rather, the sensory neurons exhibit significant heterosynaptic facilitation. Thus, in these reflexes, habituation is not accompanied by homosynaptic depression at the sensory neurons, suggesting that the plasticity underlying habituation occurs primarily at interneuronal sites.
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