GABA(B) receptor activation in the suprachiasmatic nucleus of diurnal and nocturnal rodents.

Diurnal (day-active) and nocturnal (night-active) animals have very different daily activity patterns. We recently demonstrated that the suprachiasmatic nucleus (SCN) responds to GABAergic stimulation differently in diurnal and nocturnal animals. Specifically, GABAA receptor activation with muscimol during the subjective day causes phase delays in diurnal grass rats while producing phase advances in nocturnal hamsters. The aim of the following experiments was to determine if diurnal and nocturnal animals differ in their response to GABAB receptor activation in the SCN. Baclofen, a GABAB receptor agonist, was microinjected into the SCN region of grass rats or hamsters under free-running conditions and phase alterations were analyzed. Changes in phase were not detected after baclofen treatment during the subjective day in either grass rats or hamsters. During the night, however, GABAB receptor activation significantly decreased the ability of light to induce phase delays in grass rats. Taken together with previous data from our laboratory, these results demonstrate that, in both hamsters and grass rats, GABAB receptor activation in the SCN significantly affects circadian phase during the night, but not during the day. Copyright 2004 Elsevier Inc.