Circadian rhythms of norepinephrine in the rat suprachiasmatic nucleus

Norepinephrine (NE) contents in the suprachiasmatic nucleus (SCN) in rats kept under light-dark (LD) or constant dark (DD) conditions displayed significant variations over one day with a peak during the subjective day and a trough during the subjective night. These results indicate that rhythmic variation of NE in the SCN is driven by an endogenous pacemaker and independent of external light. In agreement with these observations, immunocytochemical study localized tyrosine hydroxylase immunopositive fibers in the medial part of the SCN. Unlike serotonin, histamine and neuropeptide Y projections to the ventrolateral SCN, NE innervation may represent the general activity level of the brainstem, from where these fibers arise.