Serotonin imitates several of the neuronal effects of nociceptive sensitization in the common snail.

Experiments on defensive behavior command neurons in common snails showed that synaptic facilitation in the responses of nerve cells to sensory stimulation occurs 50-60 min after the onset of application of serotonin (10 microM) to the CNS. The properties of neuron electrogenic membranes (membrane potential, membrane excitability) did not change after exposure to serotonin. Along with synaptic facilitation, serotonin (100 microM) increased the excitability and produced minimal depolarization of the membranes of command neurons. Serotonin had selective effects on the reactions of neurons to different sensory stimuli: facilitation of neuron responses to tactile stimulation of the head lasted 1 h, while responses to application of dilute quinine solution lasted 2-3 h; serotonin facilitated neuron responses to tactile stimulation only of the snail's head, and did not alter the responses to stimulation of the foot or the mantle ridge. The time course of the electrophysiological effects of serotonin coincided with changes in bound calcium (Cab) levels in command neurons. This set of serotonin-induced neurophysiological effects is similar to the effects resulting from the development of nociceptive sensitization. It is suggested that serotonin is involved in the mechanisms of transient changes and consolidation of long-term plastic rearrangements in command neurons which underlie sensitization.