Distribution of orexin/hypocretin immunoreactivity in the brain of a male songbird, the house finch, Carpodacus mexicanus.

Previous research has shown orexin/hypocretin immunoreactive (orexin-ir) neurons in domesticated Galliformes. However, these findings may not be representative of other birds and these studies did not include a distribution of orexin-ir projections throughout the brain. The present study was carried out in a wild-caught passerine, the house finch, Carpodacus mexicanus, and includes a detailed description of orexin-ir neurons and their projections. Orexin A and B-ir neurons were located in a single population centered on the paraventricular nucleus of the hypothalamus extending into the lateral hypothalamic area, consistent with other studies in birds. Orexin A and B-ir fibers were similarly visible across the brain, with the highest density within the preoptic area, hypothalamus and thalamus. Orexin-ir projections extended from the paraventricular nucleus rostrally to the preoptic area, laterally towards the medial striatum, nidopallium, and dorsally along the lateral ventricle towards the mesopallium. Caudally, the highest densities of orexin-ir fibers were found along the third ventricle. The periaqueductal grey, substantia nigra pars compacta and the locus coeruleus also showed a high density of orexin-ir fibers. This study showed a detailed fiber distribution previously unreported in birds and showed that orexin-ir neurons were located in similar areas regardless of phylogeny or domestication in birds. The apparently conserved neural distribution of orexins suggests that these peptides play similar roles among birds. The widespread distribution of the projections in brain areas serving various roles indicates the potential involvement of these peptides in multiple behavioral and physiological functions.