Abstract
In mammals, circadian rhythms of locomotor activity and many other behavioral and physiological functions are controlled by an endogenous pacemaker located in the hypothalamic suprachiasmatic nucleus (SCN). One of the SCN's afferents is a dense serotonergic input from the mesencephalic raphe complex. Previous work from this laboratory demonstrated that systemic administrations of the serotonin agonist quipazine mimic the effects of light on the circadian system of rats, i.e. they induce photic-like phase shifts of the circadian activity rhythm as well as c-Fos expression in the SCN. In contrast, no such effect has been demonstrated so far in the isolated rat SCN slice preparation. In this study we demonstrate that local injections of quipazine (0.5 microg/kg) into the region of the SCN induce photic-like effects similar to those induced by systemic injections. These findings suggest a role for 5-HT in the transmission of photic information to the rat circadian system through a direct action at the level of the SCN.
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