Behavioral neurobiology of learning in terrestrial snails

DNA methylation inhibition participates in the anterograde amnesia key mechanism through the suppression of the transcription of genes involved in memory formation in grape snails

The study of the amnesia mechanisms is of both theoretical and practical importance. The mechanisms of anterograde amnesia are the least studied, due to the lack of an experimental model that allows studying this amnesia type molecular and cellular mechanisms. Previously, we found that conditional food aversion memory reconsolidation impairment in snails by NMDA glutamate receptor antagonists led to the amnesia induction, in the late stages of which (>10 days) repeated training did not cause long-term memory formation. In the same animals, long-term memory aversion to a new food type was formed. We characterized this amnesia as specific anterograde amnesia. In the present work we studied the role of epigenetic DNA methylation processes as well as protein and mRNA synthesis in the mechanisms of anterograde amnesia and memory recovery. DNMT methyltransferase inhibitors (iDNMT: zebularine, RG108 (N-Phthalyl-1-tryptophan), and 5-AZA (5-Aza-2'-deoxycytidine)) were used to alter DNA methylation. It was found that in amnesic animals the iDNMT administration before or after shortened repeated training led to the rapid long-term conditional food aversion formation (Ebbinghaus saving effect). This result suggests that amnestic animals retain a latent memory, which is the basis for accelerated memory formation during repeated training. Protein synthesis inhibitors administration (cycloheximide) before or immediately after repeated training or administration of RNA synthesis inhibitor (actinomycin D) after repeated training prevented memory formation under iDNMT action. The earlier protein synthesis inhibitor effect suggests that the proteins required for memory formation are translated from the pre-existing, translationally repressed mRNAs. Thus, we have shown for the first time that the anterograde amnesia key mechanism is DNMT-dependent suppression of the transcription of genes involved in memory mechanisms. Inhibition of DNMT during repeated training reversed these genes expression blockade, opening access to them by transcription factors synthesized during training from the pre-existing mRNAs.

Protein synthesis inhibitor administration before a reminder caused recovery from amnesia induced by memory reconsolidation impairment with NMDA glutamate receptor antagonist

Memory recovery in amnestic animals is one of the most poorly studied processes. In this paper, we examine the role of protein synthesis and a reminder in the mechanisms of amnesia and memory recovery in grape snails trained to conditioned food aversion. Amnesia was induced by the impairment of memory reconsolidation using NMDA (N-methyl d-aspartate) glutamate receptor antagonists. In an early stage of amnesia (day 3), injections of protein synthesis inhibitors into animals combined with a reminder by a conditioned stimulus (CS) led to the recovery of aversive reactions to its presentation. Two types of changes in reactions to CS were revealed. In most animals, a persistent recovery of memory retrieval was found that lasted for at least 10 days. In other snails, aversive responses to CS persisted for 24 h. Isolated injections of inhibitors, injections of inhibitors and a reminder by the learning environment (without presenting a CS), usage of a differentiating stimulus instead of a CS, or inhibitor injections after the reminder did not affect the development of amnesia. The administration of protein synthesis inhibitors and a reminder in the late period after amnesia induction (10 days) did not affect its development or caused a short-term memory recovery. We suggest that amnesia is an active process that develops over time. The reminder induces the reactivation of the amnesia process dependent on protein synthesis, while the administration of protein synthesis inhibitors leads to the impairment of amnesia reactivation and recovery of the state formed before amnesia induction (i.e., recovery of conditioned food aversion memory).

Histone deacetylase inhibitors rescue the impaired memory in terrestrial snails

It is becoming increasingly clear that the long-term plasticity can be regulated via histone modifications. Many studies demonstrated the role of histone acetylation in acquisition, maintenance, and extinction of long-term memory. Nonetheless, the role of histone acetylation in memory reinstatement following its disruption by antimnemonic treatments was not studied in details. In terrestrial snails, we examined effects of the histone deacetylases inhibitors (HDACi) sodium butyrate (NaB) and trichostatin A (TSA) on reinstatement of the context fear memory impaired by antimnemonic agents such as protein synthesis blocker anisomycin (ANI) + reminding or a specific inhibitor of protein-kinase Mζ, zeta inhibitory peptide (ZIP). It was observed that both NaB and TSA applications restored the ANI-impaired context memory regardless of memory reactivation, while a combination of NaB or TSA plus memory reactivation (or additional training) was necessary for the effective reinstatement of the ZIP-impaired memory. Additionally, NaB injections significantly facilitated development of long-term memory in animals with weak memory, while no effect was observed in animals with strong memory. The data obtained confirmed the assumption that histone acetylation is a critical regulatory component of memory development and reinstatement.

Increase in serotonin precursor levels reinstates the context memory during reconsolidation

In the present study, we tested possible ways of modification of the context long-term memory using the reconsolidation as a tool. Recently, using a depletion of the serotonin content, it was shown that the reinforcing neurotransmitter serotonin is necessary for successful repeated reconsolidation of context memory in terrestrial snails Helix lucorum (Balaban et al. in Sci Rep 6:36933, 2016), and in the present study, we investigated effects of serotonin increase in memory maintenance by injection of the serotonin precursor 5-hydroxytryptophan (5-HTP). We studied reinstatement of the context memory after its impairment during reconsolidation with a protein synthesis blocker anisomycin (ANI) or with a specific inhibitor of protein-kinase Mζ (ZIP). It was observed that applications of 5-HTP alone, known to increase the release of serotonin, or reactivation of memory alone did not restore the ZIP- or ANI-impaired context memory, while combination of the 5-HTP + reactivation of memory effectively reinstated the context memory. The data obtained confirmed the assumption that serotonin/reinforcing transmitter is a part of successful reconsolidation necessary for memory maintenance, demonstrated possible ways of long-term memory regulation during the reconsolidation process.

Experiments with Snails Add to Our Knowledge about the Role of aPKC Subfamily Kinases in Learning

Protein kinase Mζ is considered important for memory formation and maintenance in different species, including invertebrates. PKMζ participates in multiple molecular pathways in neurons, regulating translation initiation rate, AMPA receptors turnover, synaptic scaffolding assembly, and other processes. Here, for the first time, we established the sequence of mRNA encoding PKMζ homolog in land snail Helix lucorum. We annotated important features of this mRNA: domains, putative capping sites, translation starts, and splicing sites. We discovered that this mRNA has at least two isoforms, and one of them lacks sequence encoding C1 domain. C1 deletion may be unique for snail because it has not been previously found in other species. We performed behavioral experiments with snails, measured expression levels of identified isoforms, and confirmed that their expression correlates with one type of learning.

Peculiarities of Participation of DNA Methyltransferases in the Mechanisms of Storage, Impairment, and Recovery of Conditioned Food Aversion Memory

We studied the participation of DNA-methylation processes in the mechanisms of memory storage and reconsolidation, amnesia induction, and in recovery of the conditioned food aversion memory in edible snails. It was found that daily injections of DNA methyltransferases inhibitor over 3 days combined with a reminder of a conditioned food stimulus did not affect memory storage. The administration of DNA methyltransferase inhibitors did not suppress induction of amnesia caused the NMDA receptor antagonist/reminder. Injections of DNA methyltransferase inhibitors combined with the reminder led to memory recovery in 3 days after amnesia induction. Thus, DNA methyltransferase inhibitors in the same doses did not affect storage and reconsolidation of memory, as well as the mechanisms of amnesia induction. At the same time, injections of inhibitors led to memory recovery, apparently, due to disruption of reactivation and amnesia development.

Using invertebrate model organisms for neuroscience research and training: an opportunity for Africa

Africa is faced with an increasing underrepresentation of her research progress in many fields of science including neuroscience. This underrepresentation stems from the very low investments directed towards research by African governments as these are thought to be high-priced. Scientists and researchers within the continent are left to compete highly for the very limited research grants or choose to fund research from their personal purse. Therefore, presenting a need for all possible strategies to make science and research approaches more affordable in Africa. This paper presents one of such strategy, which advocates the use of invertebrate animal models for neuroscience research in place of the commonly used vertebrate models. Invertebrates are cheaper, more available and easy to handle options and their use is on the rise, even in the developed societies of the world. Here, we investigate the current state of invertebrate neuroscience research in Africa looking at countries and institutions conducting neuroscience research with invertebrates and their publication output. We discuss the factors which impede invertebrate neuroscience research in Africa like lack of research infrastructure and adequate expert scientists and conclude by suggesting solutions to these challenges.

Responses of Withdrawal Interneurons to Serotonin Applications in Naïve and Learned Snails Are Different

Long-term changes in membrane potential after associative training were described previously in identified premotor interneurons for withdrawal of the terrestrial snail Helix. Serotonin was shown to be a major transmitter involved in triggering the long-term changes in mollusks. In the present study we compared the changes in electrophysiological characteristics of identifiable premotor interneurons for withdrawal in response to bath applications of serotonin (5-HT) or serotonin precursor 5-hydroxytryptophan (5-HTP) in preparations from naïve, neurotoxin-injected or associatively trained snails. It was found that 5-HT or 5-HTP applications caused a significant decrease of membrane potential in premotor interneurons of naïve snails, associatively trained snails and snails with impaired serotonergic system by injection of a selective neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) 1 week before the experiments. Applications of 5-HT or 5-HTP did not cause significant changes in the action potential (AP) threshold potential of these neurons in naïve snails. Conversely, applications of 5-HT or 5-HTP to the premotor interneurons of previously trained or 5,7-DHT-injected snails caused a significant increase in the firing threshold potential in spite of a depolarizing shift of the resting membrane potential. Results demonstrate that responsiveness of premotor interneurons to extracellularly applied 5-HT or 5-HTP changes for days after the associative training or serotonin depletion. Similarity of the effects in trained and 5,7-DHT-injected animals may be due to massive release of serotonin elicited by 5,7-DHT injection. Our results suggest that serotonin release due to aversive conditionining or elicited by the neurotoxin administration triggers similar changes in resting membrane potential and AP threshold in response to bath applications of 5-HT or its precursor 5-HTP.

The Sensitivity of the Crayfish Reward System to Mammalian Drugs of Abuse

The idea that addiction occurs when the brain is not able to differentiate whether specific reward circuits were triggered by adaptive natural rewards or falsely activated by addictive drugs exist in several models of drug addiction. The suitability of crayfish (Orconectes rusticus) for drug addiction research arises from developmental variation of growth, life span, reproduction, behavior and some quantitative traits, especially among isogenic mates reared in the same environment. This broad spectrum of traits makes it easier to analyze the effect of mammalian drugs of abuse in shaping behavioral phenotype. Moreover, the broad behavioral repertoire allows the investigation of self-reinforcing circuitries involving appetitive and exploratory motor behavior, while the step-wise alteration of the phenotype by metamorphosis allows accurate longitudinal analysis of different behavioral states. This paper reviews a series of recent experimental findings that evidence the suitability of crayfish as an invertebrate model system for the study of drug addiction. Results from these studies reveal that unconditioned exposure to mammalian drugs of abuse produces a variety of stereotyped behaviors. Moreover, if presented in the context of novelty, drugs directly stimulate exploration and appetitive motor patterns along with molecular processes for drug conditioned reward. Findings from these studies indicate the existence of drug sensitive circuitry in crayfish that facilitates exploratory behavior and appetitive motor patterns via increased incentive salience of environmental stimuli or by increasing exploratory motor patterns. This work demonstrates the potential of crayfish as a model system for research into the neural mechanisms of addiction, by contributing an evolutionary, comparative context to our understanding of natural reward as an important life-sustaining process.

Adaptive Changes in the Vestibular System of Land Snail to a 30-Day Spaceflight and Readaptation on Return to Earth

The vestibular system receives a permanent influence from gravity and reflexively controls equilibrium. If we assume gravity has remained constant during the species' evolution, will its sensory system adapt to abrupt loss of that force? We address this question in the land snail Helix lucorum exposed to 30 days of near weightlessness aboard the Bion-M1 satellite, and studied geotactic behavior of postflight snails, differential gene expressions in statocyst transcriptome, and electrophysiological responses of mechanoreceptors to applied tilts. Each approach revealed plastic changes in the snail's vestibular system assumed in response to spaceflight. Absence of light during the mission also affected statocyst physiology, as revealed by comparison to dark-conditioned control groups. Readaptation to normal tilt responses occurred at ~20 h following return to Earth. Despite the permanence of gravity, the snail responded in a compensatory manner to its loss and readapted once gravity was restored.

Impairment of the serotonergic neurons underlying reinforcement elicits extinction of the repeatedly reactivated context memory

We analyzed changes in the activity of individually identifiable neurons involved in the networks underlying feeding and withdrawal behaviors in snails before, during, and after aversive learning in vitro. Responses to food in the “reinforcing” serotonergic neurons involved in withdrawal changed significantly after training, implying that these serotonergic cells participate in the reactivation of memory and are involved in the reconsolidation process. In behavioral experiments it was shown that impairment of the functioning of the serotonergic system with the selective neurotoxin 5,7-DiHT did not change the memory, when tested once, but resulted in a complete extinction of the contextual memory after repeated reactivation of memory. Conversely, the cued memory to a specific type of food was significantly reduced but still present. Thus, we conclude that it is only for the context memory, that participation of the “reinforcing” serotonergic neurons in memory retrieval may be the gate condition for the choice between extinction/reconsolidation.

Modulation of defensive reflex conditioning in snails by serotonin

Highlights Daily injection of serotonin before a training session accelerated defensive reflex conditioning in snails.Daily injection of 5-hydroxytryptophan before a training session in snails with a deficiency of serotonin induced by the "neurotoxic" analog of serotonin 5,7-dihydroxytryptamine, restored the ability of snails to learn.After injection of the "neurotoxic" analogs of serotonin 5,6- and 5,7-dihydroxytryptamine as well as serotonin, depolarization of the membrane and decrease of the threshold potential of premotor interneurons was observed. We studied the role of serotonin in the mechanisms of learning in terrestrial snails. To produce a serotonin deficit, the "neurotoxic" analogs of serotonin, 5,6- or 5,7-dihydroxytryptamine (5,6/5,7-DHT) were used. Injection of 5,6/5,7-DHT was found to disrupt defensive reflex conditioning. Within 2 weeks of neurotoxin application, the ability to learn had recovered. Daily injection of serotonin before a training session accelerated defensive reflex conditioning and daily injections of 5-HTP in snails with a deficiency of serotonin induced by 5,7-DHT restored the snail's ability to learn. We discovered that injections of the neurotoxins 5,6/5,7-DHT as well as serotonin, caused a decrease in the resting and threshold potentials of the premotor interneurons LPa3 and RPa3.

Nitric oxide is necessary for labilization of a consolidated context memory during reconsolidation in terrestrial snails

Nitric oxide (NO) is known to be involved in associative memory formation. We investigated the influence of blocking NO function on the reconsolidation of context memory in terrestrial snails (Helix lucorum L.). After a 10 day session of electric shocks in one context only, context memory in snails was observed in test sessions as the significant difference of amplitudes of withdrawal responses to tactile stimuli in two different contexts. After a 1 day rest, a session of 'reminding' was performed, preceded by injection in different groups of the snails with either vehicle or combination of the protein synthesis blocker anisomycin (ANI) with one of the following drugs: the NO scavenger carboxy-PTIO, the NO-synthase inhibitors N-omega-nitro-L-arginin, nitroindazole and NG-nitro-L-arginine methyl ester hydrochloride, or the NO donor S-nitroso-N-acetyl-DL-penicillamine. Testing the context memory at different time intervals after the reminder under ANI injection showed that the context memory was impaired at 24 h and later, whereas the reminder under combined injection of ANI and each of the NO-synthase inhibitors used or the NO scavenger showed no impairment of long-term context memory. Injection of the NO donor S-nitroso-N-acetyl-DL-penicillamine with or without reminder had no effect on context memory. The results obtained demonstrated that NO is necessary for labilization of a consolidated context memory.

Nitric oxide is necessary for long-term facilitation of synaptic responses and for development of context memory in terrestrial snails

Correlated electrophysiological and behavioral experiments in the snail Helix lucorum were conducted to investigate the contribution of nitric oxide (NO) to synaptic plasticity during withdrawal reflex and aversive context memory development. Time, stimulation frequency and number of tetani/electrical shocks were determined in vitro and in vivo. In isolated brain preparations, nerve tetanization accompanied by bath application of serotonin induced long-term facilitation (LTF) of the excitatory postsynaptic potential (EPSP) in withdrawal interneurons. Bathing with either the NO-synthase inhibitor N-omega-nitro-L-arginin (L-NNA) or the NO-scavenger 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl 3-oxide (PTIO) before the tetanization prevented tetanus-induced long-term increase of EPSP. Withdrawal interneurons are key elements in the network underlying aversive behavior, with LTF considered the basis for aversive learning. We hypothesized that L-NNA injections in free-behaving snails could influence aversive learning. Snails were trained for 1 or 5days to remember the context in which they were shocked. In one-day training experiments, the snails received 5 electrical shocks in one context. Different groups of snails were sham-injected or L-NNA-injected before or after training. After training, the sham-injected groups demonstrated a significant increase in behavioral responses compared to the L-NNA-injected groups. On the following day, only sham-injected snails demonstrated altered behavioral responses, but no associative context differences were observed. These results correlated with the electrophysiological results. In another series of experiments, the snails received electrical shocks for 5days. Testing on the second day after training demonstrated that the sham-injected group maintained selective aversive context memory, whereas the L-NNA-injected snails were not different between the two contexts. Together these results demonstrated that inhibition of NO synthesis prevents memory formation and influences synaptic plasticity in the withdrawal interneurons that underlie the behavioral changes. This suggests that NO influences the behavior via regulation of synaptic plasticity.

Drug-sensitive reward in crayfish: an invertebrate model system for the study of SEEKING, reward, addiction, and withdrawal

In mammals, rewarding properties of drugs depend on their capacity to activate appetitive motivational states. With the underlying mechanisms strongly conserved in evolution, invertebrates have recently emerged as a powerful new model in addiction research. In crayfish natural reward has proven surprisingly sensitive to human drugs of abuse, opening an unlikely avenue of research into the basic biological mechanisms of drug addiction. In a series of studies we first examined the presence of natural reward systems in crayfish, then characterized its sensitivity to a wide range of human drugs of abuse. A conditioned place preference (CPP) paradigm was used to demonstrate that crayfish seek out those environments that had previously been paired with the psychostimulants cocaine and amphetamine, and the opioid morphine. The administration of amphetamine exerted its effects at a number of sites, including the stimulation of circuits for active exploratory behaviors (i.e., SEEKING). A further study examined morphine-induced reward, extinction and reinstatement in crayfish. Repeated intra-circulatory infusions of morphine served as a reward when paired with distinct visual or tactile cues. Morphine-induced CPP was extinguished after repeated saline injections. Following this extinction phase, morphine-experienced crayfish were once again challenged with the drug. The priming injections of morphine reinstated CPP at all tested doses, suggesting that morphine-induced CPP is unrelenting. In an exploration of drug-associated behavioral sensitization in crayfish we concurrently mapped measures of locomotion and rewarding properties of morphine. Single and repeated intra-circulatory infusions of morphine resulted in persistent locomotory sensitization, even 5 days following the infusion. Moreover, a single dose of morphine was sufficient to induce long-term behavioral sensitization. CPP for morphine and context-dependent cues could not be disrupted over a drug free period of 5 days. This work demonstrates that crayfish offer a comparative and complementary approach in addiction research. Serving as an invertebrate animal model for the exposure to mammalian drugs of abuse, modularly organized and experimentally accessible nervous systems render crayfish uniquely suited for studying (1) the basic biological mechanisms of drug effects, (2) to explore how the appetitive/seeking disposition is implemented in a simple neural system, and (3) how such a disposition is related to the rewarding action of drugs of abuse. This work aimed to contribute an evolutionary, comparative context to our understanding of a key component in learning, and of natural reward as an important life-sustaining process.

Changes in frequency of spontaneous oscillations in procerebrum correlate to behavioural choice in terrestrial snails

The aim of our study was to understand functional significance of spontaneous oscillations of local field potential in the olfactory brain lobe of terrestrial snail, the procerebrum (PC). We compared changes in frequency of oscillations in semi-intact preparations from snails trained to percept the same conditioned odor as positive (associated with food reinforcement) or negative (associated with noxious reinforcement). In vivo recordings in freely behaving naïve snails showed a significant decrease of spontaneous PC oscillations frequency during a stage of tentacle withdrawal to odor presentation. In in vitro preparations from naïve snails, a similar decrease in frequency of the PC oscillations to odor presentation was observed. Changes in frequency of the oscillations to cineole presentations in the “aversive” group of snails (demonstrating withdrawal) were much more pronounced than in naïve snails. No significant difference in responses to 5% and 20% cineole was noted. Changes in the spontaneous oscillations frequency in the snails trained to respond with positive reaction (approach) to cineole depended on the concentration of the applied odor, and these responses were qualitatively similar to responses of other groups during the first 10 s of responses to odor, but significantly different (increase in PC oscillations frequency) from the responses of the aversively trained and naïve snails in the interval 11–30 s, which corresponds to the end of the tentacle withdrawal and timing of decision making (approach or escape) in the free behaving snails. Obtained results suggest that frequency of the PC lobe spontaneous oscillations correlate to the choice of behavior in snails: withdrawal (decrease in frequency) or approach (increase in frequency) to the source of odor.

Antibodies to calcium-binding S100B protein block the conditioning of long-term sensitization in the terrestrial snail

The effects of antibodies to calcium-binding S100B protein diluted to 10(-12) (LAS100B) on the long-term sensitization in the Helix lucorum snail (neurobiological model of the anxious-depressive state) were evaluated. The administration of LAS100B prior to conditioning of long-term sensitization in the terrestrial snail 10 min prior to the first electric stimulus) prevents strengthening of the defensive reaction of withdrawing the ommatophores (eye tentacles) and the defensive reaction of closing the pneumostome. This effect is termed "protective", as it prevents the conditioning of long-term sensitization. At the same time, snails given an injection of saline developed long-term sensitization with a significant strengthening of the defensive reactions of withdrawing the ommatophores and closing the pneumostome. When LAS100B was administered before long-term sensitization in advance, the membrane and threshold potentials of premotor interneurons, which regulate defensive behaviour, decreased to a significantly lesser extent compared to the long-term sensitization arm. It is possible that the "protective" effect is linked to the mechanisms of maintaining the membrane potential and changes in extra- and intracellular balance of calcium-binding S100B protein.

Impairment of context memory by beta-amyloid peptide in terrestrial snail

We examined influence of the β-amyloid peptide (βAP) (25–35) neurotoxic fragment on Helix lucorum food-aversion learning. Testing with aversively conditioned carrot showed that 2, 5 and 14 days after training the βAP-injected group responded in a significantly larger number of cases and with a significantly smaller latency than the sham-injected control group. The results demonstrate that the AP partially impairs the learning process. In an attempt to specify what component of memory is impaired we compared responses in a context in which the snails were aversively trained, and in a neutral context. It was found that the sham-injected learned snails significantly less frequently took the aversively conditioned food in the context in which the snails were shocked, while the βAP-injected snails remembered the aversive context 2 days after associative training, but were not able to distinguish two contexts 5, and 14 days after training. In a separate series of experiments a specific context was associated with electric shock, and changes in general responsiveness were tested in two contexts several days later. It was found that the βAP-injected snails significantly increased withdrawal responses in all tested contexts, while the sham-injected control animals selectively increased responsiveness only in the context in which they were reinforced with electric shocks. These results demonstrate that the βAP (25–35) interferes with the learning process, and may play a significant role in behavioral plasticity and memory by selectively impairing only one component of memory – the context memory.

Cellular mechanisms of behavioral plasticity in simple nervous systems

Review of our own experimental studies of the cellular mechanisms of learning in the nervous systems of gastropod mollusks, along with published results, allows identification of a number of the principles of operation of nervous systems, which are important for descriptions of learning and memory processes: 1) the main plastic changes on learning occur at the level of interneurons; 2) learning results in selective alteration of the efficiency of particular synaptic inputs of command neurons; 3) reinforcement is not linked with neuron activity in the receptor-sensory neuron-interneuron-motoneuron-effector reflex arc, but is mediated by neurons which modulate this circuit, this involving a single neuron in some simple cases; 4) modulator neuron activity is required for the acquisition of plastic modifications to defensive behavior (including associative modifications) but is not necessary for the reproduction of acquired responses to a conditioned stimulus. At the same time, modulator neurons (comprising the reinforcement neuron system) are required for reproduction of contextual associative responses; and 5) changes resulting from learning occur at at least two independent loci in the nervous system.

The conditioned reflex: detectors and command neurons

The conditioned reflex is characterized by plasticity supporting bilateral selective connections between its input and output. In simple nervous systems, input stimuli are represented by selective detectors connected to command neurons by plastic synapses whose activity increases on learning and decreases on extinction. The process of associative learning occurs when excitation of the detector and the command neuron coincide. Short-term memory in a plastic synapse is associated with phosphorylation of postsynaptic receptor molecules and does not require protein synthesis. Long-term memory is associated with early gene expression, structural genes, and protein synthesis. The simple "detector-command neuron" association has increased in complexity during evolution. At the input, pre-detector interneurons activating a specific detector converge on the command neuron: the command neuron determines the selectivity of the mechanisms of conditioned reflexes for complex stimuli. The output mechanism has also become more complex: command neurons have become more specialized and premotor interneurons have appeared between them and motor neurons, excitation of premotor neurons being passed to groups of motor neurons responsible for the configuration of the behavioral act. Conditioned reflexes combining more complex signals at the input with more flexible results at the output allow a diversity of behavioral acts.

Reconsolidation of a context long-term memory in the terrestrial snail requires protein synthesis

We investigated the influence of the protein synthesis blocker anisomycin on contextual memory in the terrestrial snail Helix. Prior to the training session, the behavioral responses in two contexts were similar. Two days after a session of electric shocks (5 d) in one context only, the context conditioning was observed as the significant difference of behavioral response amplitudes in two contexts. On the day following testing of context learning, a session of "reminding" was performed, immediately after which the snails were injected with anisomycin or vehicle. Testing of long-term context memory has shown that only anisomycin injections impaired the context conditioning. In control series, the snails were injected after the training session with anisomycin/saline without reminding, and no impairment of the long-term context memory was observed, while injection of anisomycin during the training session completely abolished the long-term memory. No effects of anisomycin on the short-term memory were observed. Surprisingly, injection of anisomycin after the reminding combined with reinforcing stimuli elicited no effect on the context memory. Differences between single-trial and multisession learning are discussed.

Modulation of impaired cholinesterase activity in experimental diabetes: effect of Gymnema montanum leaf extract

We reported that a leaf extract (GLEt) obtained from an anti-diabetic plant, Gymnema montanum, an endangered species endemic to India, has anti-peroxidative and antioxidant effects on diabetic brain tissue in rats. Here we examined the effect of the extract on the activity of reduced brain and retinal acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) in streptozotocin (STZ)-induced diabetic male Wistar rats. Diabetic rats received GLEt orally (200 mg/kg bwt/d) for 12 wk, and changes in blood glucose, plasma insulin, the lipid peroxidation marker thiobarbituric acid-reactive substance (TBARS), and AChE and BChE activity were measured. The results confirmed prior reports that hyperglycemia significantly enhances TBARS levels in brain and retinal tissue and decreases AChE and BChE activity. Treatment with GLEt significantly reversed the impairment in enzymatic activity in addition to reducing the level of TBARS, suggesting that GLEt protects against the adverse effect of lipid peroxidation on brain and retinal cholinesterases. We suggest that GLEt could be useful for preventing the cholinergic neural and retinal complications of hyperglycemia in diabetes.

Serotonergic immunoreactivity in the pedal ganglia of the pulmonate snail Megalobulimus abbreviatus after thermal stimulus: A semi-quantitative analysis

Using an immunohistochemical procedure and optical densitometry, the distribution of neurons containing serotonin (5-HT) was investigated in the pedal ganglia of Megalobulimus abbreviatus after thermal "non-functional stimulus" (22 degrees C) and stressful thermal conditions (50 degrees C). The animals were sacrificed at different times (3 h, 6 h and 24 h) following these stimuli. In control animals, the results showed the location of these serotonergic immunoreactive elements (5HT-ir) in this ganglion to be similar to those shown in other studies, where the anterior region of ventral sections showed the largest number of 5HT-ir neurons. In the anterior neurons, significant differences (p < 0.01) were observed between the groups of animals stimulated at 50 degrees C and 22 degrees C and sacrificed after 6 h. In the medial neurons, significant differences (p < 0.05) were observed between the control group and the groups of animals stimulated at 50 degrees C and sacrificed after 6 and 24 h. Neuropilar area 1 showed differences (p < 0.01) in 5HT-ir between the control group and the groups of animals stimulated at 50 degrees C and sacrificed after 3 and 24 h. Neuropilar area 2 showed a significant difference (p < 0.05) between the groups of animals stimulated at 22 degrees C and sacrificed after 3 and 24 h. These results suggest the involvement of 5-HT in the nociceptive circuit of M. abbreviatus, mainly that of the medial neurons and neuropilar area 1, which showed increases in 5HT-ir after thermal aversive stimuli. These results could be helpful in drawing cellular homologies with other gastropods.

The terrestrial Gastropoda Megalobulimus abbreviatus as a useful model for nociceptive experiments: effects of morphine and naloxone on thermal avoidance behavior

We describe the behavior of the snail Megalobulimus abbreviatus upon receiving thermal stimuli and the effects of pretreatment with morphine and naloxone on behavior after a thermal stimulus, in order to establish a useful model for nociceptive experiments. Snails submitted to non-functional (22 degrees C) and non-thermal hot-plate stress (30 degrees C) only displayed exploratory behavior. However, the animals submitted to a thermal stimulus (50 degrees C) displayed biphasic avoidance behavior. Latency was measured from the time the animal was placed on the hot plate to the time when the animal lifted the head-foot complex 1 cm from the substrate, indicating aversive thermal behavior. Other animals were pretreated with morphine (5, 10, 20 mg/kg) or naloxone (2.5, 5.0, 7.5 mg/kg) 15 min prior to receiving a thermal stimulus (50 degrees C; N = 9 in each group). The results (means +/- SD) showed an extremely significant difference in response latency between the group treated with 20 mg/kg morphine (63.18 +/- 14.47 s) and the other experimental groups (P < 0.001). With 2.5 mg/kg (16.26 +/- 3.19 s), 5.0 mg/kg (11.53 +/- 1.64 s) and 7.5 mg/kg naloxone (7.38 +/- 1.6 s), there was a significant, not dose-dependent decrease in latency compared to the control (33.44 +/- 8.53 s) and saline groups (29.1 +/- 9.91 s). No statistically significant difference was found between the naloxone-treated groups. With naloxone plus morphine, there was a significant decrease in latency when compared to all other groups (minimum 64% in the saline group and maximum 83.2% decrease in the morphine group). These results provide evidence of the involvement of endogenous opioid peptides in the control of thermal withdrawal behavior in this snail, and reveal a stereotyped and reproducible avoidance behavior for this snail species, which could be studied in other pharmacological and neurophysiological studies.

In vitro formation and activity-dependent plasticity of synapses between Helix neurons involved in the neural control of feeding and withdrawal behaviors

Short-term activity-dependent synaptic plasticity has a fundamental role in short-term memory and information processing in the nervous system. Although the neuronal circuitry controlling different behaviors of land snails of the genus Helix has been characterized in some detail, little is known about the activity-dependent plasticity of synapses between identified neurons regulating specific behavioral acts. In order to study homosynaptic activity-dependent plasticity of behaviorally relevant Helix synapses independently of heterosynaptic influences, we sought to reconstruct them in cell culture. To this aim, we first investigated in culture the factors regulating synapse formation between Helix neurons, and then we studied the short-term plasticity of in vitro-reconstructed monosynaptic connections involved in the neural control of salivary secretion and whole-body withdrawal. We found that independently of extrinsic factors, cell-cell interactions are seemingly sufficient to trigger the formation of electrical and chemical synapses, although mostly inappropriate--in their type or association--with respect to the in vivo synaptic connectivity. The presence of ganglia-derived factors in the culture medium was required for the in vitro reestablishment of the appropriate in vivo-like connectivity, by reducing the occurrence of electrical connections and promoting the formation of chemical excitatory synapses, while apparently not influencing the formation of inhibitory connections. These heat-labile factors modulated electrical and chemical synaptogenesis through distinct protein tyrosine kinase signal transduction pathways. Taking advantage of in vitro-reconstructed synapses, we have found that feeding interneuron-efferent neuron synapses and mechanosensory neuron-withdrawal interneuron synapses display multiple forms of short-term enhancement-like facilitation, augmentation and posttetanic potentiation as well as homosynaptic depression. These forms of plasticity are thought to be relevant in the regulation of Helix feeding and withdrawal behaviors by inducing dramatic activity-dependent changes in the strength of input and output synapses of high-order interneurons with a crucial role in the control of Helix behavioral hierarchy.

Postsynaptic calcium contributes to reinforcement in a three-neuron network exhibiting associative plasticity

We show that activation of a single serotonergic cell is sufficient to trigger long-term associative enhancement of synaptic input to the withdrawal interneuron in a simple network consisting of three interconnected identified cells in the nervous system of terrestrial snail Helix. 1,2-bis (2-aminophenoxy) Ethane-N,N,N',N'-tetraacetic acid (BAPTA) injection in the postsynaptic neuron abolishes the pairing-specific enhancement of synaptic input. Activation of a single modulatory cell that we used to reinforce the synaptic input induced an increase of the intracellular [Ca2+] in the ipsilateral withdrawal interneuron without any changes of its membrane potential or input resistance. Similar changes in intracellular [Ca2+] were observed in the same withdrawal interneuron under bath application of 10(-5) m serotonin. Responses to repeated glutamate applications to the soma of synaptically isolated withdrawal interneurons increased after 10 min of serotonin or thapsigargin bath application, but were absent in conditions of preliminary BAPTA intracellular injection, significantly decreased under heparin injection. Thus, activity of a single modulatory cell may mediate reinforcement via an increase of [Ca2+] in the postsynaptic cell in a simple network consisting of neurons with defined behavioural roles.

Selective blockade of gene expression in a single identified snail neuron

In the present study, the applicability of antisense morpholino oligos for loss-of-function experiments in neurobiology was investigated. The identified withdrawal interneurons of the parietal ganglia expressing helix command neuron-specific 2 (HCS2) gene were pressure injected with HCS2 antisense or control morpholino oligo solution at a final concentration 1-4 microM. No toxic or side effects for the neural functioning were noted immediately or several hours after injection. The changes in the concentration of HCS2-encoded protein in neurons after injection were monitored by two methods, Western blotting and immunostaining of the brain. The amount of the peptide immunoreactive with the HCS2 antibody started to decline in the injected cells at day 2 post-injection, decreased four- to five-fold at day 4, and stayed at this low level thereafter. Similar results obtained by both methods suggest significant selective blockade of production of the HCS2-encoded peptide. In contrast, no substantial decrease of the HCS2-encoded polypeptide was observed after injection with control oligos. Due to the high stability of the morpholino oligos in the cell, they represent a highly efficient tool for a specific long-term blockade of gene expression in molluscan neurons.

Synaptic contact between mechanosensory neuron and withdrawal interneuron in terrestrial snail is mediated by L-glutamate-like transmitter

The properties of the monosynaptic input from mechanosensory neurons to withdrawal interneurons were examined in Helix lucorum. The instantaneous I-V relation of the excitatory postsynaptic current in withdrawal interneurons was nonlinear, having a plateau region between -40 and -60 mV. On application of the blocker of vertebrate N-methyl-D-aspartate (NMDA) receptors AP5, or reduction of the Mg(2+) concentration, the current-voltage relation became more linear, suggesting that Mg(2+) may partially block the ion channel underlying the EPSC at voltages ranging from -40 to around -60 mV and the involvement of NMDA-like receptors. DNQX and 6-cyano-7-nitroquinoxaline-2,3-dione, which are known to block the glutamate non-NMDA receptors in mammals, significantly depress in a dose-dependent manner the actions of the natural transmitter. Exogenous L-glutamate applications mimicked the action of the mechanosensory neuron transmitter.

Cellular mechanisms of behavioral plasticity in terrestrial snail

Functional organization of networks underlying withdrawal, feeding, and respiration in terrestrial gastropod snail Helix are described. Tracking the changes during non-associative and associative modifications of behavior, analysis of plasticity mechanisms in identified neurons involved in these networks allowed to formulate several conceptual principles which are not widely accepted. The review will present data underlying the following principles: 1. Command neuron concept can be applied only to all-or-none behavior. 2. Habituation is an active down-regulation process opposite to up-regulating sensitization. All long-term behavioral changes at least in part are associative. 3. Reinforcement is a motivational state mediated by neuromodulatory neurons and can be produced by activity of a single modulatory neuron. 4. Non-addressed ('soft-wired') neuromodulatory influences are necessary for acquisition of memory, while retention of memory depends mostly on 'hard-wired' local changes in synaptic connectivity. 5. Retrieval of declarative (sensory) and procedural (motor) memory involves different functional classes of neurons.

Identification of mechanoafferent neurons in terrestrial snail: response properties and synaptic connections

In this study, we describe the putative mechanosensory neurons, which are involved in the control of avoidance behavior of the terrestrial snail Helix lucorum. These neurons, which were termed pleural ventrolateral (PlVL) neurons, mediated part of the withdrawal response of the animal via activation of the withdrawal interneurons. Between 15 and 30 pleural mechanosensory neurons were located on the ventrolateral side of each pleural ganglion. Intracellular injection of neurobiotin revealed that all PlVL neurons sent their axons into the skin nerves. The PlVL neurons had no spontaneous spike activity or fast synaptic potentials. In the reduced "CNS-foot" preparations, mechanical stimulation of the skin covering the dorsal surface of the foot elicited spikes in the PlVL neurons without any noticeable prepotential activity. Mechanical stimulus-induced action potentials in these cells persisted in the presence of high-Mg(2+)/zero-Ca(2+) saline. Each neuron had oval-shaped receptive field 5-20 mm in length located on the dorsal surface of the foot. Partial overlapping of the receptive fields of different neurons was observed. Intracellular stimulation of the PlVL neurons produced excitatory inputs to the parietal and pleural withdrawal interneurons, which are known to control avoidance behavior. The excitatory postsynaptic potentials (EPSPs) in the withdrawal interneurons were induced in 1:1 ratio to the PlVL neuron spikes, and spike-EPSP latency was short and highly stable. These EPSPs also persisted in the high-Mg(2+)/high-Ca(2+) saline, suggesting monosynaptic connections. All these data suggest that PlVL cells were the primary mechanosensory neurons.

Can Invertebrates Suffer? or, How Robust is Argument-By-Analogy?

It is a popular notion that, compared to vertebrates, invertebrates have a reduced capacity to experience suffering. This is usually based on arguments that invertebrates show only simple forms of learning, have little memory capacity, do not show behavioural responses to stimuli that would cause ‘higher’ vertebrates to exhibit responses indicative of pain, and have differences in their physiology that would preclude the capacity for suffering. But, how convincing is this ‘evidence’ of a reduced capacity to suffer? Suffering is a negative mental state - a private experience - and, as such, it cannot be measured directly. When assessing the capacity of an animal to experience suffering, we often compare the similarity of its responses with those of ‘higher’ animals, conceptualized in the principle of argument-by-analogy. By closely examining the responses of invertebrates, it can be seen that they often behave in a strikingly analogous manner to vertebrates. In this paper, I discuss published studies that show that invertebrates such as cockroaches, flies and slugs have short- and long-term memory; have age effects on memory; have complex spatial, associative and social learning; perform appropriately in preference tests and consumer demand studies; exhibit behavioural and physiological responses indicative of pain; and, apparently, experience learned helplessness. The similarity of these responses to those of vertebrates may indicate a level of consciousness or suffering that is not normally attributed to invertebrates. This indicates that we should either be more cautious when using argument-by-analogy, or remain open-minded to the possibility that invertebrates are capable of suffering in a similar way to vertebrates.

A single serotonergic modulatory cell can mediate reinforcement in the withdrawal network of the terrestrial snail

A cluster of 40 serotonergic cells in the rostral part of pedal ganglia of the terrestrial snail Helix lucorum was shown previously to participate in the modulation of withdrawal behavior and to be necessary during the acquisition of aversive withdrawal conditioning in intact snails. Local extracellular stimulation of the serotonergic cells paired with a test stimulus elicited a pairing-specific increase (the difference between paired and explicitly unpaired sessions was significant, p <.01) of synaptic responses to test stimulation in the premotor interneurons involved in withdrawal. This result suggested participation of serotonergic cells in mediating the reinforcement in the withdrawal network. Intracellular stimulation of only one identified Pd4 cell from the pedal group of serotonergic neurons paired with a test stimulus also significantly increased (the difference between paired and explicitly unpaired sessions was significant, p <.05) synaptic responses to paired nerve stimulation in same premotor interneurons involved in withdrawal. Morphological investigation of a cluster of pedal serotonergic neurons showed that only the Pd4 cell had branches in the parietal ganglia neuropile where the synapses of premotor withdrawal interneurons and of presynaptic neurons are located. The data suggest that a single serotonergic cell can mediate the reinforcement in the withdrawal network of the terrestrial snail. Patterns of responses of the Pd4 cells to tactile and chemical stimuli conform to the suggestion.

Optical recording of odor-evoked responses in the olfactory brain of the naïve and aversively trained terrestrial snails

Regular spontaneous oscillations were recorded both electro- and optophysiologically using a voltage-sensitive absorption dye in the olfactory part of the brain (procerebral lobe of the cerebral ganglia) of the gastropod mollusk Helix lucorum. Odor application caused transient changes in procerebral oscillations, and an odor-evoked potential was recorded in the procerebrum (PC). The wave of evoked potential originated near the place of olfactory nerve entrance into the PC and propagated via the procerebral neuropile toward the cell body layer. The spread of the odor-evoked potential corresponded roughly to the neuropile area, whereas the spontaneous oscillations were recorded in the cell body layer of the PC and were not observed in the neuropile. Evoked potential did not produce additional events intercalated into the ongoing spontaneous oscillations. Changes in parameters of spontaneous oscillations to the repeated presentations of the same odor were variable. To estimate the role of spontaneous oscillations in odor encoding, we trained the snail to avoid cineole, using paired presentations of cineole and electric shock. Elaboration of conditioned aversion to cineole applications resulted in distinct pairing-specific changes in behavior of the snails and procerebral activity. Responses to odor (cineole) applications were not different in amplitude or frequency of spontaneous oscillations in control and trained snails, whereas ratio of amplitudes of the same oscillation wave in proximal and distal regions of the procerebrum was significantly different in control and aversively trained snails, reflecting changes in neural firing in certain areas of the olfactory lobe.

Sites of Plasticity in the Neural Circuit Mediating Tentacle Withdrawal in the Snail Helix aspersa: Implications for Behavioral Change and Learning Kinetics

The tentacle withdrawal reflex of the snail Helix aspersa exhibits a complex combination of habituation and sensitization consistent with the dual-process theory of plasticity. Habituation, sensitization, or a combination of both were elicited by varying stimulation parameters and lesion condition. Analysis of response plasticity shows that the late phase of the response is selectively enhanced by sensitization, whereas all phases are decreased by habituation. Previous data have shown that tentacle withdrawal is mediated conjointly by parallel monosynaptic and polysynaptic pathways. The former mediates the early phase, whereas the latter mediates the late phase of the response. Plastic loci were identified by stimulating and recording at different points within the neural circuit, in combination with selective lesions. Results indicate that depression occurs at an upstream locus, before circuit divergence, and is therefore expressed in all pathways, whereas facilitation requires downstream facilitatory neurons and is selectively expressed in polysynaptic pathways. Differential expression of plasticity between pathways helps explain the behavioral manifestation of depression and facilitation. A simple mathematical model is used to show how serial positioning of depression and facilitation can explain the kinetics of dual-process learning. These results illustrate how the position of cellular plasticity in the network affects behavioral change and how forms of plasticity can interact to determine the kinetics of the net changes.

Nitric oxide-containing neurons in the nervous ganglia of Helix aspersa during rest and activity: immunocytochemical and enzyme histochemical detection

Nitric oxide synthase (NOS) immunoreactivity and staining for nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-diaphorase) activity are two cytochemical markers for nitric oxide (NO)-containing neurons. The authors examined the changes in the distribution of NOS immunolabeling and NADPH-diaphorase reactivity in the cerebral and buccal ganglia of the terrestrial snail Helix aspersa during resting and active phases. During inactivity and after 1 day of activity, in the mesocerebrum and metacerebrum of the snails, there were several reactive neurons for both markers; after 7 days of activity, the number of reactive neurons was lower. Opposite results were obtained in the buccal ganglia, in which increased staining and numbers of reactive neurons were present in the active snails (after 1 day and 7 days of activity). Although the staining patterns for the two reactions were similar, colocalization was not always observed. The comparison between inactive and active animals provided a more precise survey of NOS-containing neurons in the snail cerebral ganglia than previously described. Moreover, it suggested that not only is NO involved in distinct nervous circuits, but, as a ubiquitous molecule, it also plays a role in neuroprotection and neuropeptide release.

Interactions between Depression and Facilitation within Neural Networks: Updating the Dual-Process Theory of Plasticity

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.

Neural circuit mediating tentacle withdrawal in Helix aspersa, with specific reference to the competence of the motor neuron C3

The tentacle withdrawal reflex in the terrestrial snail Helix aspersa involves bending and retraction of the tentacles. When elicited by mechanical stimulation of the tentacle, the reflex is mediated by the conjoint action of the central and peripheral nervous systems. The neural circuit underlying the stimulus-response pathways was studied in vitro using a combination of morphological and physiological techniques. Sensory input caused by stimulation of the nose (situated at the superior tentacle's tip) first passes into the tentacle ganglion. Motor fibers are likely excited in the tentacle ganglion to form a peripheral stimulus-response pathway. While still in the tentacle ganglion, the excitation caused by a brief stimulus is transformed into a prolonged neuronal discharge. This modified signal travels, via the olfactory nerve, to the cerebral ganglion where it excites the giant motor neuron C3 along with numerous smaller motor neurons. Afferent input to C3 also arrives from several other sources. The afferent convergence is followed by a marked divergence of C3's output. C3 innervates the muscles mediating both tentacle retraction and tentacle bending through multiple cerebral nerves. Thus C3's pattern of effector innervation allows this single cell to elicit and coordinate both components of the tentacle withdrawal reflex. Lesion experiments indicate that C3 is responsible for 85% of the central contribution to tentacle retraction, though C3 is actually sufficient to mediate maximal muscle contraction as evidenced by intracellular stimulation. In addition to C3, three groups of putative central motor neurons were identified through nerve backfills and nerve recordings. The additional motor neurons mediating tentacle retraction are important for maximizing the rate of muscle contraction, whereas those mediating tentacle bending are likely more important for nondefensive behaviors. These neurons are arranged in parallel with C3, but unlike C3, each of these neurons innervates only a single effector or portion thereof. Given C3's direct innervation of multiple effectors and its sufficiency to evoke strong responses in those effectors, we conclude that C3 is paramount in eliciting and coordinating tentacle withdrawal.

Vector coding and neuronal maps

A model of vector coding is proposed. An excitation vector is generated in an ensemble of neurons, which has simultaneous actions on the map of selective detectors (selectors), creating a local excitation maximum which represents the input stimulus. Vector coding is also proposed as an explanation for associative learning and memory. Responses to the input in this model are determined by the excitation vectors triggered by command neurons in ensembles of premotor neurons.

Aplysia hemolymph promotes neurite outgrowth and synaptogenesis of identified Helix neurons in cell culture

Hemolymph of adult Aplysia californica significantly affects neurite outgrowth of identified neurons of the land snail Helix pomatia. The metacerebral giant cell (MGC) and the motoneuron C3 from the cerebral ganglion and the neuron B2 from the buccal ganglion of H. pomatia were isolated by enzymatic and mechanical dissociation and plated onto poly-L-lysine-coated dishes either containing culture medium conditioned by Helix ganglia, or pre-treated with Aplysia hemolymph. To determine the extent of neuronal growth we measured the neurite elongation and the neuritic field of cultured neurons at different time points. Aplysia hemolymph enhances the extent and rate of linear outgrowth and the branching domain of Helix neurons. With the hemolymph treatment the MGC neuron more consistently forms specific chemical synapses with its follower cell B2, and these connections are more effective than those established in the presence of the conditioned medium.

The cerebral neurons of Helix aspersa during hibernation. Changes in the cytochemical detection of calmodulin, cytoskeletal components and phosphatases

Some markers of the intracellular systems that regulate neuronal activity and morphology were analyzed in the cerebral ganglion of hibernating snails (Helix aspersa), in comparison with active animals. The immunocytochemical expression of a calcium-binding protein, i.e. calmodulin, and some cytoskeletal components, i.e. 200 kDa phosphorylated neurofilament protein (pNFH), microtubule associated protein 2 (MAP2) and alpha-tubulin were analyzed by the use of a panel of antibodies raised against mammal antigens. Moreover, by enzymatic reactions the Ca(2+)-ATPase and alkaline phosphatase (AIPase) activities were demonstrated. In comparison with the active phase, the hibernation induced an increase in the immunopositivity for calmodulin in all the neurons. The increase may be linked to unmasking of immunoreactive epitopes due to conformational changes of the protein, which in turn may be a consequence of a reduction or absence of binding with calcium ions or of a real increase in the amount of calmodulin in the somata of neurons. In any event, both the hypotheses indicate that neurons have decreased or suppressed the Ca(2+)-dependent mechanisms as also shown by the lower Ca(2+)-ATPase activity. Nevertheless, the AIPase activity, which was localized in the epineural sheat, was not significantly changed during hibernation and this supports that some metabolic activities are preserved in the hibernated animals. Changes in the immunopositivity for cytoskeletal components were found. There was an increase in the epitopes recognized by the mammalian pNF antibody, that concerned both the positivity of the entire cytoplasm of some clusters of metacerebral neurons and the intensity of the reaction. This would be aimed to improve the stability of the somata and primary neurites. Moreover, the decrease of alpha-tubulin and MAP2 immunopositivity, suggests that a disassembly of microtubules have occurred. The findings indicate that the transport of vesicles in the axons is slowed down during hibernation. In fact, research in progress show that the patterns of neurotransmission and neuromodulation are also deeply modified.