Journal of Neuroscience, Vol 6, 14-21, Copyright © 1986 by Society for Neuroscience
Regulation of the circadian clock in the Aplysia eye: mimicry of neural action by serotonin
JJ Nadakavukaren, ME Lickey and WP Jordan
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)