Subcellular distribution of 5‐HT1b and 5‐HT7 receptors in the mouse suprachiasmatic nucleus

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Abstract
The suprachiasmatic nucleus (SCN), a circadian oscillator, receives glutamatergic afferents from the retina and serotonergic (5‐HT) afferents from the median raphe. 5‐HT1B 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‐HT1B and 5‐HT7 receptors in the SCN. 5‐HT1B receptor immunostaining was associated with the plasma membrane of thin unmyelinated axons, preterminal axons, and terminals of optic and nonoptic origin. 5‐HT1B receptor immunostaining in terminals was almost never observed at the synaptic active zone. To a much lesser extent, 5‐HT1B 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‐HT1B 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. J. Comp. Neurol. 432:371–388, 2001.