Distribution of hypocretin-(orexin) immunoreactivity in the central nervous system of Syrian hamsters (Mesocricetus auratus)

The hypocretins are peptides synthesized in neurons of the hypothalamus. Re cent studies have suggested a role for these peptides in the regulation of sleep, feeding, and endocrine regulation. The distribution of hypocretin-im munoreactive cell bodies and fibers has been extensively described in rats, but not in other species. This study was designed to examine the distribut ion of hypocretin immunoreactivity in Syrian hamsters, as important differe nces in neuropeptide distribution between rats and hamsters have previously been demonstrated. Immunoreactive cell bodies were found primarily in the lateral hypothalamic area and the perifornical area, although a few hypocre tin-positive cells were also located in the dorsomedial hypothalamus and th e retrochiasmatic area. Fibers were distributed throughout the brain in a p attern similar to that seen in rats. The densest projections were found in the paraventricular nucleus of the thalamus, locus coeruleus, dorsal raphe, and lateroanterior hypothalamus. The innervation of the anterior hypothala mus may be of particular interest as similar cluster of immunoreactivity do es not appear to be present in rats. Moderate levels of immunoreactivity co uld be seen throughout the hypothalamus, the lateral septum, bed nucleus of the stria terminalis, A5 noradrenergic area, and the midline thalamic nucl ei. Hypocretin-immunoreactive fibers are present in all lamina of the spina l cord, with the greatest axon densities in lamina 1 and 10. The widespread distribution of hypocretin suggests its involvement in a wide variety of p hysiological and behavioral processes. Our results in hamsters indicate tha t the organization of the hypocretin system is strongly conserved across sp ecies, suggesting an important role for the peptide and its projections. (C ) 2001 Elsevier Science B.V. All rights reserved.