Immunohistochemical localization of orexins (hypocretins) in the brain of reptiles and its relation to monoaminergic systems

With the aim of gaining more insight into the evolution of the orexinergic systems in the brain of vertebrates we have conducted a comparative analysis of the distribution of orexin-immunoreactive cell bodies and fibers in two reptiles, the lizard Gekko gecko and the turtle Pseudemys scripta elegans. In both species most immunoreactive neurons were found in the periventricular hypothalamic nucleus and in the infundibular hypothalamus. Only in the gecko, orexinergic cell bodies were present in the dorsolateral hypothalamic nucleus and the periventricular preoptic nucleus. Fiber labeling was observed in all main brain subdivisions but was more abundant in regions such as the septum, preoptic area, suprachiasmatic nucleus, lateral hypothalamic area and median eminence. Less conspicuous was the innervation of the olfactory bulbs, pallial regions, habenula, dorsomedial and dorsolateral thalamic nuclei, torus semicircularis and spinal cord. Double immunohistofluorescence techniques were applied for the simultaneous detection of the orexinergic systems and the catecholaminergic or serotoninergic systems in the brain of reptiles. Actual colocalization of orexins and catecholamines or serotonin in the same neurons was not observed. However, orexinergic innervation was found in dopaminergic, noradrenergic and serotoninergic cell groups, such as the substantia nigra and ventral tegmental area in the midbrain tegmentum, the locus coeruleus, the nucleus of the solitary tract and the raphe nuclei.The comparison of the distribution of orexin-immunoreactive neurons and fibers found in reptiles with those reported for other vertebrates reveals a strong resemblance but also notable variations. In addition, the relation between the orexinergic and monoaminergic systems observed in the brain of reptiles seems to be a shared feature among vertebrates.