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 acquis ition of aversive withdrawal conditioning in intact snails. Local extracell ular stimulation of the serotonergic cells paired with a test stimulus elic ited a pairing-specific increase (the difference between paired and explici tly unpaired sessions was significant, p <.01) of synaptic responses to tes t stimulation in the premotor interneurons involved in withdrawal. This res ult suggested participation of serotonergic cells in mediating the reinforc ement in the withdrawal network. Intracellular stimulation of only one iden tified Pd4 cell from the pedal group of serotonergic neurons paired with a test stimulus also significantly increased (the difference between paired a nd explicitly unpaired sessions was significant, p <.05) synaptic responses to paired nerve stimulation in same premotor interneurons involved in with drawal. Morphological investigation of a cluster of pedal serotonergic neur ons showed that only the Pd4 cell had branches in the parietal ganglia neur opile where the synapses of premotor withdrawal interneurons and of presyna ptic 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 stimu li conform to the suggestion. (C) 2001 Academic Press.