Two modulatory inputs exert reciprocal reinforcing effects on synaptic input of premotor interneurons for withdrawn in terrestrial snails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 mod ulation of withdrawal behavior, and to be necessary for elaboration of aver sive withdrawal conditioning in intact snails. In the present experiments l ocal extracellular stimulation of the serotonergic cells elicited a pairing -specific increase (difference between paired and explicitly unpaired sessi ons was significant, P < 0.01) of synaptic responses in the premotor intern eurons involved in withdrawal to paired nerve stimulation. Intracellular st imulation of only one Pd4 cell from the pedal group of serotonergic neurons increased (P < 0.05) synaptic responses to contingent test nerve stimulati on significantly in the same premotor interneurons for 2-3 hr. Mesocerebral cells are known to participate in male sexual behavior, and th eir extracellular stimulation was shown previously to suppress the amplitud e of synaptic responses in withdrawal interneurons. Local extracellular sti mulation of the mesocerebral cells elicited a pairing-specific decrease (P < 0.01) of synaptic responses to contingent test nerve stimulation in the p remotor interneurons involved in withdrawal for 2-3 hr. Paired application of met-enkephaline (10(-6) M, some mesocerebral cells are enkephaline-like immunoreactive) also selectively decreased synaptic responses to contingent nerve stimulation in the premotor interneurons for hours. Thus, two modula tory inputs exert pairing-specific effects that influence the same synaptic connection in opposite directions, which may underlie the long-term up- an d down-regulation of behavioral responses.