Effects of daytime and nighttime stress on Fos-like immunoreactivity in the paraventricular nucleus of the hypothalamus, the habenula, and the posterior paraventricular nucleus of the thalamus.

Circadian effects on basal and stress-induced Fos-like immunoreactivity (IR) in the paraventricular nucleus of the hypothalamus (PVN), the habenula (Hab) and the posterior paraventricular nucleus of the thalamus (PVN-Thal) were examined. Stress induced a significant increase in the number of Fos-like IR cells within all 3 brain regions. In the Hab, expression was localized specifically to the medial region of the lateral Hab. No differences between the effects of daytime vs nighttime stress on numbers of Fos-like IR cells in the PVN and PVP-Thal were observed. Significantly fewer Fos-like IR cells were observed, however, in the lateral habenula of nighttime vs daytime non-stressed controls, resulting in a significantly greater percentage increase in Fos-like IR in the lateral habenula following nighttime vs daytime stress.