Levels of serotonin in the hemolymph of Aplysia are modulated by light/dark cycles and sensitization training

Serotonin (5-hydroxytryptamine, 5-HT) modulates the behavior and physiology of both vertebrate and invertebrate animals. Effects of injections of 5-HT and the morphology of the serotonergic system of Aplysia indicate that 5-H T may have a humoral, in addition to a neurotransmitter, role. To study pos sible humoral roles of 5-HT, we measured 5-HT in the hemolymph. The concent ration of 5-HT in the hemolymph was similar to 18 nM, a value close to prev iously reported thresholds for eliciting physiological responses. The conce ntration of 5-HT in the hemolymph expressed a diurnal rhythm. In addition, electrical stimulation that leads to long-term sensitization significantly increased levels of 5-HT in the hemolymph during training, 1.5 hr after tra ining, and 24 hr after training. Moreover, levels of 5-HT in the hemolymph were significantly correlated with the magnitude of sensitization. The half -life of an increase in 5-HT in the hemolymph was similar to 0.5 hr. Theref ore, the persistent increase of 5-HT in the hemolymph 24 hr after sensitiza tion training indicates that training caused a long-lasting increase in the release of 5-HT. This long-lasting increase in 5-HT in the hemolymph was b locked by treatment with an inhibitor of protein synthesis during training. Based on the levels of 5-HT in the hemolymph and its regulation by environ mental events, we propose that 5-HT has a humoral role in regulation of the behavioral state of Aplysia. In support of this hypothesis, we found that increasing levels of 5-HT in the hemolymph led to significant alterations i n feeding behavior. Increasing levels of 5-HT during the daytime when they were normally low increased the latency to assume feeding posture from dayt ime to nighttime values.