Circadian modulation of passive avoidance is not eliminated in arrhythmic hamsters with suprachiasmatic nucleus lesions.

The expression of passive avoidance (PA) learning in rats displays a daily or circadian rhythm in that optimal performance is displayed when the time of testing matches the time of training. Lesions of the suprachiasmatic nucleus (SCN) were later shown to abolish this rhythm. Using golden hamsters, we have since demonstrated similar rhythms of performance in a conditioned place avoidance (CPA) task but unlike the PA results in rats, the rhythmic expression of CPA was maintained in arrhythmic hamsters with lesions of the SCN. We determined whether PA performance in hamsters is dependent on the SCN (as in the rat) or independent (as in the hamster CPA). Performance on the PA task was rhythmic in intact control animals with optimal performance occurring when training and testing time matched and significantly diminished at both 6h before and 6h after training time. SCN-lesions, verified by the loss of behavioral circadian rhythms, had no effect on the rhythmic expression. Therefore, time of day modulation of PA performance in the hamster does not depend on the SCN circadian clock.