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-HT may have a humoral, in addition to a neurotransmitter, role. To study possible humoral roles of 5-HT, we measured 5-HT in the hemolymph. The concentration of 5-HT in the hemolymph was approximately 18 nM, a value close to previously reported thresholds for eliciting physiological responses. The concentration 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 training, 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 approximately 0.5 hr. Therefore, the persistent increase of 5-HT in the hemolymph 24 hr after sensitization 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 blocked by treatment with an inhibitor of protein synthesis during training. Based on the levels of 5-HT in the hemolymph and its regulation by environmental 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 in feeding behavior. Increasing levels of 5-HT during the daytime when they were normally low increased the latency to assume feeding posture from daytime to nighttime values.