Overlapping Distributions of Parvalbumin Immunoreactivity and Projections of Orexin Neurons in Sleep-Wake Related Brain Areas in the Mouse

The hypothalamic peptide orexin has a prominent role in arousal-related processes. Parvalbumin (PV) positive neurons, as one of the most important subtypes of γ-aminobutyric acid (GABA) interneurons, which also participate in sleep-wake regulation. To broaden the understanding of the neural control and connection between these two types of neurons in the sleep associated nuclei, a double-label immunohistochemical method was used to investigate the overlap between PV immunopositive distributions and projections of orexin neurons in mouse brain. In the basal forebrain, overlap was observed in the nucleus accumbens shell (AcbSh), the nucleus of the horizontal limb of the diagonal band (HDB), magnocellular preoptic nucleus (MCPO), substantia innominata (SI), but avoided the nucleus accumbens core (AcbC). In the diencephalon, this dual innervation extended in the direction near the edge of the 3rd ventricle (3V) to the ventrolateral preoptic nucleus (VLPO), although the density of PV neurons there was somewhat lighter. Orexin immunoreactive neurons in several lateral hypothalamic areas (LH) were embedded within dense clusters of PV processes when the buttons of orexin were in obvious contact with PV immunoreactivity. In the remainder of hypothalamus, mainly including the ventral tuberomammillary nucleus (VTM), there were few fine orexin fibers and buttons contact with PV soma. Intermingled orexin and PV immunoreactivity were observed in the ventral tegmental area (VTA), and sparse overlap was observed in the ventrolateral periaqueductal gray (VLPAG) and laterodorsal tegmental nucleus (LDT) in the brainstem. Thus, these two different types of neurons, originating in different parts of the brain, together target several brain regions and brainstem monoaminergic nuclei involved in the roles of motivation, stress, locomotion, especially in the regulation of sleep.