Distribution of orexin neurons in the adult rat brain

Orexin (ORX)-A and -B are recently identified neuropeptides, which are spec ifically localized in neurons within and around the lateral hypothalamic ar ea (LHA) and dorsomedial hypothalamic nucleus (DMH), the regions classicall y implicated in feeding behavior. Here, we report a further study of the di stribution of ORX-containing neurons in the adult rat brain to provide a ge neral overview of the ORX neuronal system. Immunohistochemical study using anti-ORX antiserum showed ORX-immunoreactive (ir) neurons specifically loca lized within the hypothalamus, including the perifornical nucleus, LHA, DMH , and posterior hypothalamic area. ORX-ir axons and their varicose terminal s showed a widespread distribution throughout the adult rat brain. ORX-ir n erve terminals were observed throughout the hypothalamus, including the arc uate nucleus and paraventricular hypothalamic nucleus, regions implicated i n the regulation of feeding behavior. We also observed strong staining of O RX-ir varicose terminals in areas outside the hypothalamus, including the c erebral cortex, medial groups of the thalamus, circumventricular organs (su bfornical organ and area postrema), Limbic system (hippocampus, amygdala, a nd indusium griseum), and brain stem (locus coeruleus and raphe nuclei). Th ese results indicate that the ORX system provides a link between the hypoth alamus and other brain regions, and that ORX-containing LHA and DMH neurons play important roles in integrating the complex physiology underlying feed ing behavior. (C) 1999 Elsevier Science B.V. All rights reserved.