Multiple sensory neuronal correlates of site-specific sensitization in Aplysia.

Noxious stimulation of a restricted site on the skin of Aplysia (training) causes site-specific sensitization of the tail-withdrawal reflex that is associated with several sensory correlates that are evident both 10 min and 2 hr after training. First, extracellularly recorded afferent activity evoked by test stimulation of the trained site increases, indicating peripheral sensory changes. Second, central sensory alterations are manifested by tail sensory neurons within the pleural VC cluster that innervate the trained site and are activated during training. These mechanosensory/nociceptive cells display a number of differences from unactivated tail sensory neurons innervating other sites: slow depolarization of the soma observed immediately after training, decrease in soma action potential threshold, and enhancement of monosynaptic EPSPs to identified motor neurons. Noxious stimulation of a more extensive region also produces site-specific sensitization of the tail-withdrawal reflex and site-specific enhancement of EPSP amplitude measured 1 d after training. This training produced a novel cellular correlate of behavioral enhancement in Aplysia--regenerative bursting responses (2-35 spikes) in response to brief depolarization of the sensory neuron soma. The changes in peripheral and central excitability appear similar to changes associated with mammalian models of primary hyperalgesia. Site-specific enhancement of nociceptive signaling also occurs during aversive associative conditioning in a noxious unconditioned stimulus (US) pathway. These site-specific changes involve activity-dependent extrinsic modulation (ADEM) of the VC sensory neurons, suggesting a close relationship to changes underlying associative conditioning in conditioned stimulus (CS) pathways in Aplysia.