Orexin A-like immunoreactivity in the rat brain

Distribution of orexin-A-like immunoreactivity (ORX-LI) in rat brains was i nvestigated with the use of a rabbit polyclonal antibody against the full l ength peptide orexin A. Virtually all the ORX-LI cell bodies were observed in the lateral hypothalamus at the level of median eminence. The large majo rity of ORX-LI neurons appeared spherical or fusiform, 20-30 mu m in diamet er and issued two to five cell processes with few secondary branchings. Num erous ORX-LI fibers were observed in subregions of the hypothalamus. ORX-LI cell processes were sparsely distributed in the cortex, hippocampus and th alamus. Many varicose ORX-LI cell processes were situated close to the 3rd and lateral ventricles, some of which appeared to be protruding into the lu men. As a corollary, orexin A may be released into the ventricles and inter act with neurons in distant targets, in addition to influencing the activit y of neurons with which ORX-LI axons make synaptic contacts. (C) 1999 Elsev ier Science Ireland Ltd. All rights reserved.