Regulation of the circadian clock in the Aplysia eye: mimicry of neural action by serotonin.

In Aplysia, activity of efferent fibers in the optic nerve can influence the circadian clock in the eye. In addition, serotonin (5-HT) is present in the eye and influences the function of the clock. Accordingly, we hypothesized that 5-HT is the transmitter of the optic efferents, and tested the prediction that exogenous 5-HT would mimic the action of the optic efferents on the clock. We also tested the prediction that the action of the efferents would be prevented by blocking synaptic secretions with high Mg2+, low Ca2+ (HMLC). Activity of the optic efferents enhances clock resetting in response to the onset of darkness. We used this neural enhancement as a measure of neural action on the eye clock. We found that HMLC blocked neural enhancement. Serotonin enhanced resetting to the same extent as efferent activity. Enhancement by 5-HT did not summate with neural enhancement. High concentrations of 5-HT ([5-HT] greater than or equal to 6 X 10(-4) did not cause enhancement. High concentrations of 5-HT also blocked neural enhancement. Enhancement by 5-HT depended on the phase of drug application, much as neural enhancement depends on the phase of neural activity. Enhancement by 5-HT depended on day length in a manner similar to the enhancement by neural activity. HMLC did not block the facilitatory action of 5-HT. Several other transmitters that may be active in the eye did not enhance resetting.(ABSTRACT TRUNCATED AT 250 WORDS)