Classical conditioning of feeding in Aplysia: II. Neurophysiological correlates

Feeding behavior in Aplysia californica can be classically conditioned usin g tactile stimulation of the lips as conditional stimulus (CS) and food as unconditional stimulus (US) [Lechner et al., 2000 (companion paper)]. Condi tioning resulted in an increase in the number of CS-evoked bites that persi sted for at least 24 hr after training. In this study, neurophysiological c orrelates of classical conditioning training were identified and characteri zed in an in vitro preparation of the cerebral and buccal ganglia. Stimulat ion of a lip nerve (AT(4)), which mediates mechanosensory information, resu lted in a greater number of buccal motor patterns (BMPs) in ganglia isolate d from animals that had received paired training than in ganglia from contr ol animals. The majority of the evoked BMPs were classified as ingestion-li ke patterns. Intracellular recordings from pattern-initiating neuron B31/32 revealed that stimulation of AT(4) evoked greater excitatory input in B31/ 32 in preparations from animals that had received paired training than from control animals. In contrast, excitatory input to buccal neuron B4/5 in re sponse to stimulation of AT(4) was not significantly increased by paired tr aining. Moreover, correlates of classical conditioning were specific to sti mulation of AT(4). The number of spontaneously occurring BMPs and the intri nsic properties of two buccal neurons (B4/5 and B31/32) did not differ betw een groups. These results suggest that appetitive classical conditioning of feeding resulted in the pairing-specific strengthening of the polysynaptic pathway between afferent fibers and pattern-initiating neurons of the bucc al central pattern generator.