Differences in responses of premotor interneurons to serotonin and 5-hydroxytryptophan, a precursor for serotonin synthesis, in naive and trained snails

Abstract

This study investigated the responses of premotor interneurons LPa3 and RPa3 of the snails to applications of serotonin and 5-hydroxytryptophan (5-HTP), a precursor for serotonin synthesis, as ingredients of the broth that bathes the central nervous system of naive snails and snails after the formation of long-term sensitization. Measurements of the electrical characteristics have shown that a membrane potential of interneurons LPa3 and RPa3 in naive snails was significantly depolarized (3.7 mV) in the presence of serotonin, while the threshold potential was increased (unsupported hypothesis). The similar pattern was observed in the presence of 5-hydroxytryptophan: true depolarization to 3.1 mV occurred in interneurons until reaching the unreliable rise in the threshold potential inducing an action potential. It was found that application of serotonin causes a significant decrease in the membrane potential of interneurons LPa3 and RPa3 of trained snails (depolarization to 4.6 mV) and the unreliable increase in the threshold potential of premotor interneurons (0.9 mV). In contrast, application of 5-hydroxytryptophan causes an unreliable increase (2.5 mV) of the membrane potential and also an unreliable increase (0.8 mV) in the threshold potential.