DAILY RHYTHMS IN FOS ACTIVITY IN THE RAT VENTROLATERAL PREOPTIC AREA AND MIDLINE THALAMIC NUCLEI

The present experiment investigated the expression of the nuclear phos phoprotein Fos over the 24-h light-dark cycle in regions of the rat br ain related to sleep and vigilance, including the ventrolateral preopt ic area (VLPO), the paraventricular thalamic nucleus (PVT), and the ce ntral medial thalamic nucleus (CMT). Immunocytochemistry for Fos, an i mmediate-early gene product used as an index of neuronal activity, was carried out on brain sections from rats perfused at zeitgeber time (Z T) 1, ZT 5, ZT 12.5, and ZT 17 (lights on ZT 0-ZT 12). The number of F os-immunopositive (Fos(+)) cells in the VLPO was elevated at ZT 5 and 12.5 (i.e., during or just after the rest phase of the cycle). Fos+ ce ll number increased at ZT 17 and ZT 1 in the PVT and CMT, 180 degrees out of phase with the VLPO. A positive correlation was found between t he numbers of Fos(+) cells in the PVT and CMT, and Fos expression in e ach thalamic nucleus was negatively correlated with VLPO Fos; cell num ber. The VLPO, PVT, and CMT may integrate circadian and homeostatic in fluences to regulate the sleep-wake cycle.