Photoresponsiveness of Aplysia Eye is Modulated by the Ocular Circadian Pacemaker and Serotonin

The eye of the sea hare, Aplysia, contains a circadian pacemaker that controls rhythmic behaviors of the animal. This report shows that the pacemaker controls the photoresponsiveness of the eye as well. The electro-retinogram (ERG) of the isolated eye-optic nerve preparation, evoked by brief green light pulses in otherwise dark conditions, was recorded regularly, while the circadian rhythm of compound action potential activity was continuously recorded from the optic nerve. The waveform of the ERG changed systematically and rhythmically during the circadian cycle. One wave component of the ERG was prominent during the subjective night phase of the rhythm when the compound action potential frequency was minimal; and it was inconspicuous during the subjective day phase of the rhythm when the compound action potential frequency was maximal. Because eyes attached to the central nervous system and isolated eyes both exhibited the same rhythmic ERG changes, the circadian pacemaker in the eye is responsible for modulation of the ERG. Addition of serotonin, a putative efferent transmitter, to the bathing saline induced the ERG wave component characteristic of the subjective night phase of the rhythm. The threshold serotonin concentration was 10-7 M, and serotonin had a long lasting effect.