ORCADIAN OSCILLATORS AND PHOTORECEPTORS IN THE GASTROPOD,APLYSIA*

Abstract— The sea slug, Aplysia, is a useful model system for research on the neurophysiology of circadian integration. The animal contains several circadian oscillators and several photoreceptors. Each eye contains a circadian oscillator as well as photoreceptors. The ocular oscillators can be entrained by extraocular photoreceptors as well as their own ocular photoreceptors. The abdominal ganglion probably contains another oscillator but it has been much more difficult to manipulate in the laboratory than have the oscillators in the eyes. There is also a circadian rhythm in overt behavioral activity. This rhythm is controlled in part by extraocular oscillators and extraocular photoreceptors and in part by the eyes. In exerting their influence on the behavioral rhythm, the eyes appear to act in the capacity of oscillators and not merely as photoreceptors. Although neurons in the retina have neurosecretory morphology, the entire influence of the eyes on the behavioral rhythm appears to be mediated by nerve signals which travel in the optic nerve. As yet there is no evidence to suggest that any two oscillators in Aplysia are internally coupled. There is also no evidence yet for hormonal coupling between photoreceptors and oscillators or between oscillators and rhythmic outputs.