Pedal serotonergic neurons modulate the synaptic input of withdrawal interneurons ofHelix

Storage and erasure of behavioural experiences at the single neuron level

Although predictions from the past about the future have been of major interest to current neuroscience, how past and present behavioral experience interacts at the level of a single neuron remains largely unknown. Using the pond snail Lymnaea stagnalis we found that recent experience of terrestrial locomotion (exercise) results in a long-term increase in the firing rate of serotonergic pedal (PeA) neurons. Isolation from the CNS preserved the "memory" about previous motor activity in the neurons even after the animals rested for two hours in deep water after the exercise. In contrast, in the CNS, no difference in the firing rate between the control and "exercise-rested" (ER) neurons was seen. ER snails, when placed again on a surface to exercise, nevertheless showed faster locomotor arousal. The difference in the firing rate between the control and ER isolated neurons disappeared when the neurons were placed in the microenvironment of their home ganglia. It is likely that an increased content of dopamine in the CNS masks an increased excitation of PeA neurons after rest: the dopamine receptor antagonist sulpiride produced sustained excitation in PeA neurons from ER snails but not in the control. Therefore, our data suggest the involvement of two mechanisms in the interplay of past and present experiences at the cellular level: intrinsic neuronal changes in the biophysical properties of the cell membrane and extrinsic modulatory environment of the ganglia.

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.

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 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.

Neural control of olfaction and tentacle movements by serotonin and dopamine in terrestrial snail

We investigated the role of serotonin (5HT) and dopamine (DA) in the regulation of olfactory system function and odor-evoked tentacle movements in the snail Helix. Preparations of the posterior tentacle (including sensory pad, tentacular ganglion and olfactory nerve) or central ganglia with attached posterior tentacles were exposed to cineole odorant and the evoked responses were affected by prior application of 5HT or DA or their precursors 5-hydroxytryptophan (5HTP) and L: -DOPA, respectively. 5HT applications decreased cineole-evoked responses recorded in the olfactory nerve and hyperpolarized the identified tentacle retractor muscle motoneuron MtC3, while DA applications led to the opposite changes. 5HTP and L: -DOPA modified MtC3 activity comparable to 5HT and DA action. DA was also found to decrease the amplitude of spontaneous local field potential oscillations in the procerebrum, a central olfactory structure. In vivo studies demonstrated that injection of 5HTP in freely moving snails reduced the tentacle withdrawal response to aversive ethyl acetate odorant, whereas the injection of L: -DOPA increased responses to "neutral" cineole and aversive ethyl acetate odorants. Our data suggest that 5HT and DA affect the peripheral (sensory epithelium and tentacular ganglion), the central (procerebrum), and the single motor neuron (withdrawal motoneuron MtC3) level of the snail's nervous system.

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.

Role of nociceptin/orphanin FQ and the pseudopeptide [Phe1Psi(CH2NH)Gly2]-nociceptin(1-13)-NH2 and their interaction with classic opioids in the modulation of thermonociception in the land snail Helix aspersa

The role in nociception of nociceptin/orphanin FQ (N/OFQ) and its receptor, the opioid receptor-like 1 (NOP), remains unclear because this peptide has been implicated in both suppression and enhancement of nociception. The present work characterises the effects of N/OFQ and the NOP receptor antagonist, the pseudopeptide [Phe(1)Psi(CH(2)NH)Gly(2)]-nociceptin(1-13)-NH(2) (Phe(1)Psi), on thermonociception in the snail Helix aspersa using the hot plate assay. Additionally, the possible interaction of each of these compounds with morphine or dynorphin A(1-17) and naloxone was studied. Compounds were administered into the hemocoel cavity of H. aspersa and the latency to the aversive withdrawal behaviour recorded. Dose-response and time course curves were done. N/OFQ and naloxone produced a similar dose-dependent pronociceptive effect; however, N/OFQ reached its peak effect earlier and was 30 times more potent than naloxone. [Phe(1)Psi(CH(2)NH)Gly(2)]-nociceptin(1-13)-NH(2) and the opioid agonists, morphine and dynorphin A(1-17) produced antinociception with a similar efficacy, but [Phe(1)Psi(CH(2)NH)Gly(2)]-nociceptin(1-13)-NH(2) reached its peak effect more rapidly and lasted longer than that of dynorphin A(1-17) and morphine. [Phe(1)Psi(CH(2)NH)Gly(2)]-nociceptin(1-13)-NH(2) was 50 times less potent than dynorphin A(1-17), but 30 times more potent than morphine. N/OFQ significantly reduced morphine and dynorphin A(1-17)-induced antinociception. Combined administration of low doses of [Phe(1)Psi(CH(2)NH)Gly(2)]-nociceptin(1-13)-NH(2) and morphine or dynorphin A(1-17) produced a potent antinociceptive effect. Sub-effective doses of naloxone and N/OFQ also synergised to produce pronociception. Data suggest that these two opioid classes regulate nociception through parallel systems. The H. aspersa model appears as a valuable experimental preparation to continue the study of these opioid receptor systems.

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.

Neuroanatomical, immunocytochemical, and physiological studies of the pharyngeal retractor muscle and its putative regulatory neurons playing a role in withdrawal and feeding in the snail, Helix pomatia

We describe the neurons regulating two separate functions of the pharyngeal retractor muscle (PRM), namely sustained contraction during body withdrawal and rhythmic phasic contractions during feeding, in the snail, Helix pomatia. The distribution of central neurons innervating the PRM is organized into two main units; one in the buccal-cerebral ganglion complex, the other in the subesophageal ganglion complex. Serotonin- (5-HT-), FMRFamide- (FMRFa-), and tyrosine-hydroxylase-immunostained neurons are present among the PRM neurons that densely innervate the PRM. 5HT both decreases and increases the amplitude of the electrically evoked contraction between concentrations of 0.1 microM and 1 microM. Dopamine (DA) only decreases the amplitude of contraction at a 1-microM threshold concentration. In contrast, FMRFa increases the amplitude of the contraction and slightly elevates the tone of the PRM but requires a higher threshold (10 microM). Assay by high-performance liquid chromatography of 5HT and DA in the PRM has shown that the 5HT level decreases during locomotion but increases during feeding, whereas the DA level increases during locomotion but slightly decreases during feeding. Thus, different segments of the PRM are innervated by neurons from different loci within the central nervous system. The segments of the PRM distal to the pharynx are innervated from loci of the subesophageal ganglion complex suggesting that they mediate withdrawal. The proximal segment of the PRM is innervated from cerebral and buccal loci indicating that these neurons mediate the feeding rhythm produced by buccal and cerebral feeding central pattern generators to induce rhythmic phasic contractions in the PRM during feeding.

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.

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.

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.

Up- and down-regulation of Helix command-specific 2 (HCS2) gene expression in the nervous system of terrestrial snail Helix lucorum

A novel gene named Helix command-specific 2 (HCS2) was shown to be expressed predominantly in four giant parietal interneurons involved in withdrawal behavior of the terrestrial snail Helix lucorum L. and several single neurons in other ganglia. Decrease in spontaneous electrophysiological activity of neurons in the isolated CNS by 24h incubation in saline with elevated Mg(2+) concentration significantly decreased the number of HCS2-expressing neurons. Five short-term serotonin applications (each of 10microM), during a 24h incubation of the nervous system in saline induced expression of the HCS2 gene in many cells in cerebral, parietal, pleural and pedal ganglia. Dopamine applications under similar conditions were not effective. Application of anisomycin or cycloheximide, known to block protein synthesis, did not prevent the induction of HCS2 expression under serotonin influence. Skin injury elicited a significant increase in the number of HCS2-expressing cells 24h later in pleural and cerebral ganglia. Incubation of the isolated nervous system preparations for three days in culture medium elicited close to a maximum increase in number of HCS2-expressing cells. Elevation of the normal Mg(2+) concentration in the culture medium significantly decreased the number of cells demonstrating HCS2 expression. Application of the cAMP activator forskolin (10microM) increased the expression under Mg(2+), indicating that cAMP was involved in the up-regulation of HCS2. Application of thapsigargin (10microM), known to release Ca(2+) from intracellular stores, was also effective in increasing expression, suggesting participation of Ca(2+) in regulation of HCS2 expression. Cellular groups expressing the HCS2 gene under different conditions seem to be functionally related since it was demonstrated earlier that some neurons constituting these clusters are involved in the withdrawal behavior and the response of the organism to stress stimuli. From these results we suggest that the HCS2 pattern of expression can be down-regulated by a decrease in synaptic activity in the nervous system, and up-regulated by external noxious inputs, as well as the application of neurotransmitters and second messengers known to be involved in the withdrawal behavior and maintenance of isolated ganglia in culture medium. When up-regulated, the HCS2 expression appears, at least in part in neurons, to be involved in the withdrawal behavior.

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.

Two Modulatory Inputs Exert Reciprocal Reinforcing Effects on Synaptic Input of Premotor Interneurons for Withdrawal in Terrestrial Snails

A cluster of serotonergic cells in the rostral part of pedal ganglia of the terrestrial snail Helix lucorum was shown previously to participate in modulation of withdrawal behavior, and to be necessary for elaboration of aversive withdrawal conditioning in intact snails. In the present experiments local extracellular stimulation of the serotonergic cells elicited a pairing-specific increase (difference between paired and explicitly unpaired sessions was significant, P < 0.01) of synaptic responses in the premotor interneurons involved in withdrawal to paired nerve stimulation. Intracellular stimulation of only one Pd4 cell from the pedal group of serotonergic neurons increased (P < 0.05) synaptic responses to contingent test nerve stimulation significantly in the same premotor interneurons for 2–3 hr.

Mesocerebral cells are known to participate in male sexual behavior, and their extracellular stimulation was shown previously to suppress the amplitude of synaptic responses in withdrawal interneurons. Local extracellular stimulation of the mesocerebral cells elicited a pairing-specific decrease (P < 0.01) of synaptic responses to contingent test nerve stimulation in the premotor interneurons involved in withdrawal for 2–3 hr. Paired application of met-enkephaline (10−6m, some mesocerebral cells are enkephaline-like immunoreactive) also selectively decreased synaptic responses to contingent nerve stimulation in the premotor interneurons for hours. Thus, two modulatory inputs exert pairing-specific effects that influence the same synaptic connection in opposite directions, which may underlie the long-term up- and down-regulation of behavioral responses.

Effects of serotonin levels on postsynaptically induced potentiation of snail neuron responses

Studies on identified neurons in the common snail were performed to investigate potentiation of EPSP arising after intracellular tetanization of the post-synaptic neuron. These experiments showed that high-frequency intracellular tetanization of a command neuron leads to biphasic long-term increases in the amplitude of synaptic responses to test stimulation. The role of serotonin in forming potentiation was studied. It was suggested that the presence of particular serotonin concentrations in the intercellular fluid is required for forming the second phase of the increase in synaptic responses, while the first (transient) phase is insensitive to CNS serotonin levels.

Excitability increase in withdrawal interneurons after conditioning in snail

Membrane mechanisms of conditioning of the defensive reflex in the snails Helix pomatia and H. lucorum were investigated. Tapping on the shell was used as a conditioned stimulus, which under normal conditions produces no defensive reaction. A light blow of air into the pneumostome, called the defensive closure reaction, was used as an unconditioned stimulus. When a combination of conditioned and unconditioned stimuli were presented with 2-4 min interval, the reflex developed over a period of 3 days. The separate conditioned and unconditioned stimuli presented randomly were used as an active control. The electrical characteristics of identified interneurons involved in this defensive behavior were then measured in an isolated preparation. There was shown to be a decrease in the threshold of action potential generation from 20.5 to 16.3 mV and depolarizing shift of membrane potential from -62.1 to -57.0 mV. The electrical characteristics of withdrawal interneurons of active control snails did not differ from those in intact animals. All results show an increase in excitability of withdrawal interneurons after associative learning.

Characterization of a cDNA clone encoding pedal peptide in the terrestrial snail

We report the isolation of a Helix lucorum cDNA clone encoding a precursor of neuropeptides that are closely related to Aplysia and Tritonia pedal peptides (Pep). The predicted propeptide contains 20 copies of the two variants of Helix Pep interspersed with Lys-Arg endopeptidase cleavage sites. Northern blot hybridization revealed multiple Pep-hybridizing species in the Helix CNS RNA. The Pep gene was expressed by several identified serotonergic neurones in pedal and cerebral ganglia, groups of sensory neurones in procerebrum, peripheral neurones in olfactory bulb, mantle and foot, and group of neurones in pedal ganglia presumably involved in locomotion control. Pep mRNA was detected in several neurones at the early stages of nervous system development.

Reinforcement concept in investigations on simple nervous systems

An analysis of the applicability of the concept of reinforcement to the studies of learning in simple nervous systems of invertebrates is made. Analysis of the literature data and my own results suggests that reinforcement cannot be regarded as an independent behavioral phenomenon. A description of reinforcement as a state of the nervous system which precedes long-term changes of behavior is given. Using the example of aversive conditioning to food in gastropod snails it is shown that a state of the network that can be correlated with the state of reinforcement can be elicited in the simple nervous system by activation of serotonergic pedal cells modulating avoidance behavior of the animal. The conclusion is made that with certain limitations the reinforcement concept can be used in studies on simple nervous systems.

Dependence of synaptic facilitation postsynaptically induced in snail neurones on season and serotonin level

The problem of stability of long-lasting synaptic facilitation postsynaptically induced by intracellular current pulses without concomitant presynaptic activation was addressed. A short (15-20 min) phase of synaptic facilitation induced by intracellular tetanization in identified snail neurones was stable and present in all experiments, while a long-term phase (lasting > or = 50 min) was observed only some experiments. Data analysis revealed dependence of the long-term phase on season. Dependence of the long-term phase of facilitation on serotonin concentration in hemolymph, which is known to change with season, was shown using the selective neurotoxin 5,7-dihydroxytriptamine. Dependence of habituation rate in the same synaptic connection on season and serotonin level was shown.

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.