Multiple forms of facilitation produced by aversive tentacular stimuli in cerebral ganglion sensory neurons of Aplysia.

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Abstract
Aversive tentacular stimuli can produce both nonassociative and associative modification of head-waving behavior of Aplysia. Sensory neurons (the J/K cluster SNs) in the cerebral ganglion of Aplysia constitute an afferent pathway for aversive stimuli of the anterior tentacles. We used intracellular recording to examine plasticity in these neurons, particularly side-specific or site-specific alterations, which may be involved in mediating aspects of the learning induced by tentacle shock. The results of these experiments indicate that the J/K SNs exhibit several forms of plasticity: (1) Post-tetanic potentiation (PTP); (2) heterosynaptic facilitation; (3) activity-dependent synaptic facilitation; and (4) side-specific spike broadening. Furthermore, by activating SNs directly to produce PTP and producing heterosynaptic facilitation with tentacular stimuli that were either inside or outside the receptive field of individual SNs, it was possible to dissect and analyze the differential contribution of intrinsic SN activity and heterosynaptic modulation to activity-dependent facilitation induced by behaviorally relevant stimuli. Collectively, these data raise the possibility that plasticity in primary afferent SNs may be involved in US processing during learning induced by tentacle stimulation.