O. A. Maksimova

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.

Positive and negative brain zones in the snail

Fine wire electrodes were surgically implanted in two regions of the brain of the snail Helix aspersa. To receive electrical stimulation of the brain, a tethered snail was required to displace the end of a rod. Self‐stimulation delivered to the parietal ganglion resulted in non‐repetition of the operant response, whereas self‐stimulation delivered to the mesocerebrum resulted in an increase in response frequency. The reinforcing effect of local extracellular stimulation of two brain zones was investigated in a semi‐intact preparation of a closely related species with an identical cellular map (H.lucorum). It was found that mesocerebral stimulation increased the frequency of the reinforced spontaneous movement, but decreased the frequency of the same movement if its absence was reinforced. These results allow us to attribute positive reinforcing effects to this brain area, which is involved in the control of sexual behaviour. Different results were obtained by contingent stimulation of the rostral part of the parietal ganglia, where giant cells controlling avoidance behaviour are located. Stimulation of this zone resulted in a decrease in the frequency of the ongoing spontaneous movements. These findings make possible intracellular investigations of the mechanisms of positive and negative reinforcement.

Behavioral plasticity in a snail and its neural mechanisms.

This paper reviews the behavioral and neural mechanisms of habituation, sensitization, environmental condition, and food-aversion conditioning in the snail helix. The possible participation of motivational systems and changes in plasticity during development are discussed.

Decrease of effectivity of ‘competing’ synaptic input requires protein synthesis

We describe that tetanus-induced long-term increase of the response amplitude in one input to an identified neuron is accompanied by a decrease of synaptic responses in another, non-tetanized input. Induction of the decrease of responses in non-tetanized inputs was prevented by bath application of protein synthesis blockers (anisomycin or cycloheximide). The decrease of responses also did not occur in experiments when tetanization was substituted by short-term serotonin applications or extracellular stimulation of serotonergic cells known to potentiate investigated synaptic inputs. The results suggest that the heterosynaptically evoked decrease of synaptic response amplitude may be due to involvement of postsynaptic protein synthesis-dependent mechanisms.

Reinforcing effect of stimulation of the mesocerebral region of the brain of the edible snail

Two regions of the brain of the edible snail were stimulated. The spontaneous movements, either opening or closing, of the opening of the mantle cavity served as the signal for the stimulation. The stimulation of the region of the mesocerebrum of the edible snail in a semi-intact preparation may serve as a positive reinforcement of intercurrent behavior, while stimulation of the rostral portion of the parietal ganglia may serve as a negative reinforcement. Depending upon whether the movement itself or its absence is reinforced, the change in the intercurrent behavior may change sign.

Differences in responses of identified neurons to chemostimuli in satiated and hungry grape snails

Conclusions1.The reactions of command neurons of avoidance behavior to the presentation of food differ significantly between hungry and satiated snails if the hemolymph is substituted by salt solution.2.Introduction to hungry snails of the hemolymph from satiated animals significantly changes the responses of the command neurons of avoidance behavior to food, the responses becoming similar to those of neurons of satiated snails.3.The avoidance reactions to the reinforced form of food, appearing after development of the avoidance conditioned reflex to food, are similar to the avoidance reactions to food in satiated animals.