Orexin-A immunoreactive cells and fibers in the central nervous system of the axolotl brain and their association with tyrosine hydroxylase and serotonin immunoreactive somata

Orexin-A-like immunoreactivity in the axolotl brain was investigated by immunohistochemistry. Immunoreactive somata formed a single group in the hypothalamus, but were distributed beyond several nuclei, namely, the ventral aspect of the nucleus preopticus posterior, dorsal aspect of the nucleus suprachiasmaticus and anterior aspect of the pars ventralis hypothalami. Immunoreactive fibers were distributed throughout the brain from the olfactory bulb to the spinal cord except the cerebellum. The densest immunoreactive fibers were seen in the medial forebrain bundle and caudal lateral forebrain bundle. The largest number of immunoreactive puncta were seen in the mesencephalic tectum in addition to the hypothalamus. Immunoelectron microscopic analysis revealed the presence of synaptoid connections of immunoreactive fibers on neuronal somata in the tectum. The function of the mesencephalic system in the urodele seems to be sensory integration, suggesting that the orexin-A nervous system is associated with the modulation of sensory inputs. Orexin-A immunoreactive puncta were also observed on catecholaminergic and serotonergic somata. In view of the restricted somatic distribution in the hypothalamus, wide distribution of fibers throughout the central nervous system (CNS), and intimate association with monoaminergic somata, the orexin nervous system in the axolotl CNS is similar to those of other vertebrates, suggesting that this system is essential for brain functions throughout vertebrates.