Neuronal mechanisms of site-specific nociceptive sensitization in the common snail.

Comparative studies of the electrophysiological effects of site-specific nociceptive sensitization were performed on LPl1 and RPl1 command neurons in the common snail, after placing concentrated quinine solution (10%) on the animal's head or mantle ridge. Greater synaptic facilitation was observed in command neuron responses to tactile test stimuli applied to the same part of the body as the sensitizing stimulus, as compared with the level of synaptic facilitation in responses to tactile stimulation of other parts of the body. Synaptic facilitation of responses of neurons LPl1 and RPl1 to tactile stimulation of the head appeared 1 h after sensitizing stimulation and consisted of two phases: the first phase was characterized by a peak-like increase in the area of slow EPSP (sEPSP) and lasted 1 h; the second phase was characterized by maintenance of a relatively stable level of facilitation of sEPSP, which lasted to the end of the observation period (2-3 h). Synaptic facilitation of neuron responses to tactile stimulation of the mantle ridge appeared 40-60 min after facilitation of responses to test stimulation of the snail's head, and was characterized by maintenance of a relatively stable level of sEPSP facilitation. It is suggested that the specificity of synaptic facilitation occurring in snail defensive behavior command neurons during the period of long-term nociceptive sensitization is associated with the genetic regulation of the various synaptic "inputs" to neurons.