In vitro resetting of the circadian clock in the Aplysia eye. II. The critical period for optic nerve activity.

When constant light is 18, 21, or 24 hr, a constant light to constant darkness (LL/DD) transition in vitro results in a phase difference (delta psi) between an eye that is attached to the cerebral ganglion and its partner that is detached by cutting its optic nerve just prior to the LL/DD. This delta psi develops during the first 24 hr after LL/DD as the result of efferent activity in the optic nerve of the attached eye. By cutting the nerve of the original attached eye at various times after LL/DD, we determined when the phase resetting nerve activity occurs. This experiment was carried out following LL = 18, 21, and 24 hr. Optic nerve activity at the time of and for several hours after LL/DD did not produce nerve-dependent resetting (NDR). Instead, there was a restricted critical period, 2 to 3 hr long during which the effective nerve activity occurred. Following 24 hr of LL, a 2-hr window of optic nerve activity during the critical period could produce NDR, but a 2-hr window outside of the critical period was ineffective. The resetting effect of nerve activity was produced suddenly as if NDR were an all-or-nothing event. Following 18, 21, and 24 hr of LL, the critical period occurred 10, 7, and 4 hr, respectively, after LL/DD. In each case, this was about 28 hr after the last dawn of the light cycles to which the animal had been exposed prior to LL. We conclude that the critical period is timed by a mechanism that is not reset by LL/DD.