Abstract

The distribution of vasopressin-, vasoactive intestinal polypeptide-, somatostatin-, avian pancreatic polypeptide-, 5-hydroxytryptamine- and glutamic acid decar☐ylase-like immunoreactivity was analyzed in the suprachiasmatic nuclei of male and female golden hamsters. Vasopressin. Vasopressinlike immunoreactivity is localized within neurons, dendrites and axons throughout the rostrocaudal extent of the suprachiasmatic nuclei. Immunoreactive perikarya are restricted to the dorsomedial aspect of each nucleus and occur in highest numbers within the intermediate two-thirds of the rostrocaudal axis. Axons containing vasopressin-like immunoreactivity form a dense plexus in the dorsomedial suprachiasmatic nuclei and in a vertical column at the lateral aspect of each nucleus. Somatostatin. Somatostatin-like immunoreactivity is also contained in neurons in the dorsomedial aspect of the suprachiasmatic nuclei and in thin varicose axons distributed throughout the suprachiasmatic nuclei in a pattern similar to that of vasopressin-immunoreactive axons. Vasoactive intestinal polypeptide. Vasoactive intestinal polypeptide-immunoreactive neurons are concentrated in the ventrolateral portion of each nucleus and occur almost exclusively within the intermediate two-thirds of the rostrocaudal axis. An extremely dense plexus of varicose axons exhibiting vasoactive intestinal polypeptide-like immunoreactivity extends throughout the suprachiasmatic nuclei and passes out of the dorsal aspect of each nucleus into the periventricular and anterior hypothalamic areas. Avian pancreatic polypeptide. Avian pancreatic polypeptide-like immunoreactivity is restricted to axons which arborize within the ventrolateral aspect of each nucleus. These fibers extend throughout the rostrocaudal extent of each nucleus and partially overlap the terminal field of retinal afférents. Glutamic acid decar☐ylase. A very dense plexus of axonal varicosities exhibiting glutamic acid decar☐ylase-like immunoreactivity fills both the dorsomedial and ventrolateral portions of the suprachiasmatic nuclei throughout the rostrocaudal extent of each nucleus. Lightly stained immunoreactive perikarya also occur throughout the suprachiasmatic nuclei. 5-Hydroxytryptamine. 5-Hydroxytryptamine-like immunoreactivity is restricted to axons which form a plexus in the ventromedial portion of each nucleus that is most dense in the intermediate two-thirds of the rostrocaudal axis. These findings demonstrate that the suprachiasmatic nuclei of the golden hamster hypothalamus are composed of distinct subdivisions which are distinguished on the basis of cell morphology, segregation of immunohistochemically distinct neuron populations and the differential distribution of afferents. The data also provide evidence of large populations of immunohistochemically distinct local circuit neurons within each nucleus which appear to couple suprachiasmatic nuclei subfields. The organization of the hamster suprachiasmatic nuclei therefore permits selective afferent input to specific subpopulations of neurons of the suprachiasmatic nuclei and also provides a mechanism whereby neuronal activity in the suprachiasmatic nuclei may be integrated and synchronized in the generation and entrainment of circadian rhythms.

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