[The participation of cardioactive peptides in the processes of habituation and sensitization of the synaptic input to the command neurons in snail defensive behavior].

Small cardioactive peptide (SCPb) application in a concentration of 5 x 10(-8) M in the saline results in a significant increase of the amplitude of the summary excitatory postsynaptic potential elicited by the intestinal nerve stimulation in the command neurons (CN) for withdrawal reactions. Serotonin (5-HT) exerts comparable effects in a concentration of 10(-6) M. FMRFamide application (10(-6) M) significantly decreases the amplitude of the synaptic response to a single stimulus. Habituation rate of synaptic responses to the rhythmic (0.1 cps) stimulation in CN is significantly increased by serotonin application. SCPb acts in a similar fashion but in a lower concentration. FMRFamide application in a concentration of 10(-6) M affects in the opposite way the habituation rate of synaptic responses to rhythmic stimulation and in some cases evokes sensitization of responses.