Circadian Rhythm of Output from Neurones in the Eye of Aplysia I. Effects of Deuterium Oxide and Temperature

1. The compound action potential (CAP) output from the isolated eye of Aplysia, in darkness at constant temperature, exhibits a circadian rhythm of CAP frequency and a circadian rhythm of CAP amplitude when recorded in culture medium for up to two weeks. 2. Deuterated culture medium lengthened the period of the CAP frequency rhythm linearly from 26-7 h in normal culture medium to 33-7 h in 50% DaO culture medium. In 60 % DaO culture medium, the rhythm of CAP frequency was abolished after a single cycle. 3. Lowering temperature had a direct inhibitory effect on the CAP generating mechanism, so that CAP production was virtually abolished below 7 °C. 4. Lowering temperature decreased the amplitude and increased the period of the rhythm, and in eyes maintained at 9-5 °C the rhythm damped out after several cycles. Between 15 and 22-5 °C there was almost complete temperature compensation (Q10 1-07). 5. It is suggested that the CAP generating cells of the eye may be similar in mechanism to central endogenous bursters in Aplysia and other gastropods, and that the circadian rhythm of CAP frequency and CAP burst frequency may be due to a clock-controlled change in frequency of a membrane potential oscillation in the electrotonically coupled secondary neurones.