In vitro resetting of the circadian clock in the Aplysia eye. III. Location of photoreceptors that signal continuous light to continuous darkness.

As shown in the previous two papers (Prichard, R.G., and M.E. Lickey (1981 a, b) J. Neurosci. 1: 835-839; 840-845), the circadian rhythm of compound action potentials (CAPs) from the Aplysia eye can be reset in vitro by single transitions from continuous light to continuous darkness (LL/DD). The magnitude of the reset depends on the duration of LL that precedes the LL/DD. When LL duration is 18, 21, or 24 hr, the magnitude of the reset is different if the eye is neurally attached to the brain than if it is neurally detached. The brain gives rise to optic efferent fibers that terminate in the retina. In the present paper, we explore how the optic efferents contribute to resetting by asking whether LL/DD is sensed by the photoreceptors known to exist in the brain or those in the eye. Eye-brain preparations were placed in a special recording chamber in which the eye and brain could be isolated photically from each other while one optic nerve remained intact. The LL/DD then was delivered to the eyes and to the brain at different times. During the separate illumination, neural communication between the eyes and brain was prevented by reversible nerve blockade. The magnitude of the resulting resets always corresponded to the duration of LL that was delivered to the eyes. Photoreceptors in the brain did not contribute effectively to resetting. Also, resetting was not disturbed by nerve blockade that began before LL/DD and lasted for more than 6 hr after LL/DD. This implies that information about the timing of LL/DD was stored in the eye for more than 6 hr before it was expressed in the form of a reset. We conclude that the efferent fibers in the optic nerve do not instruct the eye about the magnitude of the reset. Instead, they activate or modulate circadian functions that are latent in the eye itself.