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.
Levenson
J
J
Department of Biology and Biochemistry, University of Houston, Houston, Texas 77204-5513, USA.
Byrne
J H
JH
Eskin
A
A
D000818:Animals; D001048:Aplysia / Q000502:physiology*; D002940:Circadian Rhythm / Q000502:physiology*; D003213:Conditioning, Psychological; D003624:Darkness; D004558:Electric Stimulation; D006458:Hemolymph / Q000737:chemistry / Q000502:physiology*; D008027:Light; D017440:Photoperiod; D012701:Serotonin / Q000032:analysis / Q000378:metabolism*
D012701:Serotonin