PREPULSE INHIBITION OF THE TRITONIA ESCAPE SWIM

Presenting a weak stimulus just before a strong, startle stimulus redu ces the amplitude of the ensuing startle response in humans and other vertebrates. This phenomenon, termed ''prepulse inhibition'' (PPI), ap pears to function to reduce distraction while processing sensory input . To date, no detailed neural mechanism has been described for PPI. He re we demonstrate PPI in the marine mollusk Tritonia diomedea, which h as a nervous system highly suitable for cellular analyses. We found th at a 100 msec vibrotactile prepulse prevented the animal's escape swim response to a closely following 1 sec tail shock. This inhibition was highly transient, with a significant effect lasting just 2.5 sec. The se findings indicate that the Tritonia escape swim response undergoes a form of PPI phenomenologically similar to that observed in vertebrat es. Further tests showed that the vibrotactile stimulus had no inhibit ory effect if applied after tail shock, while the animal was preparing to swim, but it acted to terminate swims once they were actively unde r way. As a first step toward a cellular analysis of PPI, we recorded from neurons of the swim circuit in a semi-intact preparation and foun d that the vibrotactile stimulus used in the behavioral experiments al so prevented the tail shock-elicited swim motor program. These results represent the first explicit demonstration of PPI in an invertebrate and establish Tritonia as a model system for analyzing its physiologic al basis.