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. Recent studies have suggested a role for these peptides in the regulation of sleep, feeding, and endocrine regulation. The distribution of hypocretin-immunoreactive cell bodies and fibers has been extensively described in rats, but not in other species. This study was designed to examine the distribution of hypocretin immunoreactivity in Syrian hamsters, as important differences 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 hypocretin-positive cells were also located in the dorsomedial hypothalamus and the retrochiasmatic area. Fibers were distributed throughout the brain in a pattern 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 hypothalamus may be of particular interest as similar cluster of immunoreactivity does not appear to be present in rats. Moderate levels of immunoreactivity could be seen throughout the hypothalamus, the lateral septum, bed nucleus of the stria terminalis, A5 noradrenergic area, and the midline thalamic nuclei. Hypocretin-immunoreactive fibers are present in all lamina of the spinal cord, with the greatest axon densities in lamina 1 and 10. The widespread distribution of hypocretin suggests its involvement in a wide variety of physiological and behavioral processes. Our results in hamsters indicate that the organization of the hypocretin system is strongly conserved across species, suggesting an important role for the peptide and its projections.