Rhythms in Fos expression in brain areas related to the sleep-wake cycle in the diurnal Arvicanthis niloticus

Most mammals show daily rhythms in sleep and wakefulness controlled by the primary circadian pacemaker, the suprachiasmatic nucleus (SCN). Regardless of whether a species is diurnal or nocturnal, neural activity in the SCN an d expression of the immediate-early gene product Fos increases during the l ight phase of the cycle. This study investigated daily patterns of Fos expr ession in brain areas outside the SCN in the diurnal rodent Arvicanthis nil oticus. We specifically focused on regions related to sleep and arousal in animals kept on a 12:12-h light-dark cycle and killed at 1 and 5 h after bo th lights-on and lights-off. The ventrolateral preoptic area (VLPO), which contained cells immunopositive for galanin, showed a rhythm in Fos expressi on with a peak at zeitgeber time (ZT) 17 (with lights-on at ZT 0). Fos expr ession in the paraventricular thalamic nucleus (PVT) increased during the m orning (ZT 1) but not the evening activity peak of these animals. No rhythm in Fos expression was found in the centromedial thalamic nucleus (CMT), bu t Fos expression in the CMT and PVT was positively correlated. A rhythm in Fos expression in the Ventral tuberomammillary nucleus (VTM) was 180 degree s out of phase with the rhythm in the VLPO. Furthermore, Fos production in histamine-immunoreactive neurons of the VTM cells increased at the light-da rk transitions when A. niloticus show peaks of activity The difference in t he timing of the sleep-wake cycle in diurnal and nocturnal mammals may be d ue to changes in the daily pattern of activity in brain regions important i n sleep and wakefulness such as the VLPO and the VTM.