Subcellular distribution of 5-HT_1B and 5-HT₇ receptors in the mouse suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN), a circadian oscillator, receives glutamatergic afferents from the retina and serotonergic (5-HT) afferents from the median raphe. 5-HT_1B and 5-HT₇ receptor agonists inhibit the effects of light on SCN circadian activity, Electron microscopic (EM) immunocytochemical procedures were used to determine the subcellular localization of 5-HT_1B and 5-HT₇ receptors in the SCN. 5-HT_1B receptor immunostaining was associated with the plasma membrane of thin unmyelinated axons, preterminal axons, and terminals of optic and nonoptic origin, 5-HT_1B receptor immunostaining in terminals was almost never observed at the synaptic active zone. To a much lesser extent, 5-HT_1B immunoreaction product was noted in dendrites and somata of SCN neurons. 5-HT₇ receptor immunoreactivity in γ-aminobutyric acid (GABA), vasoactive intestinal polypeptide (VIP), and vasopressin (VP) neuronal elements in the SCN was examined by using double-label procedures. 5-HT₇ receptor immunoreaction product was often observed in GABA-, VIP-, and VP-immunoreactive dendrites as postsynaptic receptors and in axonal terminals as presynaptic receptors. 5-HT₇ receptor immunoreactivity in terminals and dendrites was often associated with the plasma membrane but very seldom at the active zone. In GABA-, VIP-, and VP-immunoreactive perikarya, 5-HT₇ receptor immunoreaction product was distributed throughout the cytoplasm often in association with the endoplasmic reticulum and the Golgi complex. The distribution of 5-HT_1B receptors in presynaptic afferent terminals and postsynaptic SCN processes, as well as the distribution of 5-HT₇ receptors in both pre- and postsynaptic GABA, VIP, and VP SCN processes, suggests that serotonin plays a significant role in the regulation of circadian rhythms by modulating SCN synaptic activity.