Neuronal correlates of the serotonin-dependent behavior of the pteropod mollusc Clione limacina.

A pharmacological model of hunting behavior, previously developed by the authors on the basis of the utilization of 5-hydroxytryptophan (5-HTP), a precursor of serotonin synthesis (5-HT), was employed in order to determine its cytochemical and neurophysiological correlates. In the presence of baseline behavior, 5-HT-containing neurons were not detected by the glyoxylate method, but they were readily identified in the buccal, cerebral, and pedal (the greatest number) ganglia of the CNS and in the peripheral organs during hunting behavior induced by an injection of 5-HTP (100-200 micrograms/g). The disappearance of pauses in the initial unsustained fictive swimming, an increase in the frequency of the rhythm of swimming and in the generation of spikes in each cycle, as well as a change in the phasing of the locomotor cycle, the neurophysiological correlates of "fast" locomotion in the presence of 5-HTP-induced hunting behavior, were recorded in isolated pedal ganglia in the presence of 5-HTP (100-200 micrograms/ml). The activity of 5-HT-neurons of the pedal ganglia, which was initially synchronized with the episodes of swimming, increases against the background of 5-HTP and becomes continuous. The effects of exogenous and endogenous 5-HT on the generator of locomotion are identical, and are induced by the direct action of the mediator on the membrane of the locomotor neurons. The hypothesis of the participation of 5-HT in the monoaminergic control of behavior is discussed.