Participation of cardioactive peptides in habituation and sensitization of the synaptic input of command neurons of snail defense behavior.

The influence of serotonin, a small cardioactive peptide (SCPb) and FMRFamide on the time course of the responses of defense behavior command neurons in response to rhythmic stimulation of the intestinal nerve was studied in a preparation of the isolated common snail nervous system. It was found that the application of serotonin and SCPb induces an increase in the absolute magnitude of the synaptic response of the neurons under investigation; however, testing of the time course of the responses against the background of the action of these substances reveals the absence of sensitization and an increase in the rate of habituation to the rhythmic stimulus. The effect of the action of FMRFamide was entirely opposite to the effect of serotonin and SCPb, in both the influence on the amplitude of the single response and on the rate of habituation of the responses of the neurons. The data obtained point to the independence of the subcellular mechanisms of sensitization and habituation.