| Abstract: | The dorsal and median mesencephalic raphe nuclei provide a robust projection to the hypothalamic suprachiasmatic nucleus, the site of a putative neuronal circadian pacemaker. Although it has been suggested that the raphe may play a role in the circadian timing system, this role has not yet been specified. In the present report, we examined the circadian activity patterns of rats with large midbrain lesions aimed at the median and dorsal raphe nuclei under conditions of light-dark entrainment, and while free-running in constant light and constant darkness. The results indicate that midbrain raphe lesions may interface with the expression of free-running circadian activity rhythms. |
