Distribution of orexin-A immunoreactive neurons and their terminal networks in the brain of the rock hyrax, Procavia capensis

Abstract

The present study describes the distribution of orexin-A immunoreactive neurons and terminal networks in relation to the previously described catecholaminergic, cholinergic and serotonergic systems within the brain of the rock hyrax, Procavia capensis. Adult female rock hyrax brains were sectioned and immunohistochemically stained with an antibody to orexin-A. The staining revealed that the neurons were mainly located within the hypothalamus as with other mammals. The orexinergic terminal network distribution also resembled the typical mammalian plan. High-density orexinergic terminal networks were located within regions of the diencephalon (e.g. paraventricular nuclei), midbrain (e.g. serotonergic nuclei) and pons (locus coeruleus), while medium density orexinergic terminal networks were evident in the telencephalic (e.g. basal forebrain), diencephalic (e.g. hypothalamus), midbrain (e.g. periaqueductal gray matter), pontine (e.g. serotonergic nuclei) and medullary regions (e.g. serotonergic and catecholaminergic nuclei). Although the distribution of the orexinergic terminal networks was typically mammalian, the rock hyrax did show one atypical feature, the presence of a high-density orexinergic terminal network within the anterodorsal nucleus of the dorsal thalamus (AD). The dense orexinergic innervation of the AD nucleus has only been reported previously in the Nile grass rat, Arvicanthis niloticus and Syrian hamster, Mesocricetus auratus, both diurnal mammals. It is possible that orexinergic innervation of the AD nucleus might be a unique feature associated with diurnal mammals. It was also noted that the dense orexinergic innervation of the AD nucleus coincided with previously identified cholinergic neurons and terminal networks in this particular nucleus of the rock hyrax brain. It is possible that this dense orexinergic innervation of the AD nucleus in the brain of the rock hyrax may act in concert with the cholinergic neurons and/or the cholinergic axonal terminals, which in turn may influence arousal states and motivational processing.