DIFFERENTIAL RESPONSES OF APLYSIA SIPHON MOTOR-NEURONS AND INTERNEURONS TO TAIL AND MANTLE STIMULI - IMPLICATIONS FOR BEHAVIORAL-RESPONSE SPECIFICITY

1. Tail shock and mantle shock elicit different forms of siphon respon ses in Aplysia (flaring and backward bending vs. constriction and forw ard bending, respectively). Moreover, training with these two uncondit ioned stimuli (USs) in US-alone or classical conditioning paradigms di fferentially modifies the direction of the response to a siphon tap su bsequently presented. As a first step toward addressing neural mechani sms underlying this response specificity, we systematically mapped the central siphon withdrawal circuit to determine which motor neurons an d interneurons are differentially engaged by, and potentially modified by, tail and mantle USs. We utilized semi-intact preparations consist ing of the intact mantle organs (including the gill and siphon), the t ail, and the abdominal and circumesophageal ganglia. USs were delivere d either cutaneously through silver wires implanted in the tail and ma ntle or via suction electrodes to the tail and branchial nerves. 2. We found that one class of central siphon motor neurons, the LF(SB) cell s, was preferentially activated by tail USs, whereas other siphon moto r neurons, the LB(S) cells and RD(S) cells, were preferentially activa ted by mantle USs. These motor neurons thus appear to be the final com mon path for the differential siphon movements to these USs. In additi on, because activation of these cells can elicit neuromuscular facilit ation and thereby enhance siphon movements, this differential activati on may contribute to behavioral response specificity by imposing a spe cific response bias. 3. L29 interneurons, which both mediate and modul ate the siphon withdrawal response, responded preferentially and exhib ited synaptic facilitation selectively in response to tail shock USs. In contrast, L34 and the interneuron Il network did not show different ial activation. Facilitation at L29-LF(SB) connections following train ing with tail shock may contribute to tail-directed siphon responses t o siphon tap and may thus be an additional mechanism contributing to b ehavioral response specificity. Possibly, facilitation at other L29 co nnections could also enhance its modulatory capabilities. 4. The gener ation of specific response topographies thus appears to involve the co ordinate regulation of diverse neuronal elements and multiple mechanis ms, which may contribute to different aspects of learning.