Conditioned fear attenuates light-induced suppression of melatonin release in rats.

Male rats were given 5 min of intermittent footshock, or were not shocked, for 3 or 5 consecutive days in a novel context at the midpoint of the dark phase of a 12:12-h light:dark cycle. Six days later, animals were reexposed to the context without footshock and received either a 5-min light pulse or were not disturbed. Reexposure to the context significantly increased plasma corticosterone in animals previously shocked there. Prior context-shock pairings significantly attenuated the suppression of melatonin by light, but did not affect basal levels of melatonin. These results suggest that the circuitry underlying the suppression of melatonin by light can be modified by changes in emotional state produced by aversive conditioning.