A double-label immunohistochemical study was carried out to investigate overlap between dopamine-beta-hydroxylase (DbetaH)-immunopositive projections and the projections of hypothalamic neurons containing the arousal- and feeding-related peptide, orexin/hypocretin (HCRT), in rat brain. Numerous intermingled HCRT-immunopositive and DbetaH-immunopositive fibers were seen in a ventrally situated corridor extending from the hypothalamus to deep layers of the infralimbic cortex. Both fiber types avoided the nucleus accumbens core, caudate putamen, and the globus pallidus. In the diencephalon, overlap was observed in several hypothalamic areas, including the perifornical, dorsomedial, and paraventricular nuclei, as well as in the paraventricular thalamic nucleus. Intermingled HCRT-containing and DPH-containing fibers extended from the hypothalamus into areas within the medial and central amygdala, terminating at the medial border of the lateral subdivision of the central nucleus of the amygdala. Dense overlap between the two fiber types was also observed in the periaqueductal gray, particularly in the vicinity of the dorsal raphe, as well as (to a lesser extent) in the ventral tegmental area, the retrorubral field, and the pedunculopontine tegmental nucleus. Hypocretin-containing cell bodies, located in the perifornical and lateral hypothalamus, were embedded within a dense plexus of DPH-immunopositive fibers and boutons, with numerous cases of apparent contacts of DPH-containing boutons onto HCRT-immunopositive soma and dendrites. HCRT-containing fibers were observed amid the noradrenergic cells of the locus coeruleus, and in the vicinity of the A1, A2, and A5 cell groups. Hence, the projections of these two arousal-related systems, originating in distinctly different parts of the brain, jointly target several forebrain regions and brainstem monoaminergic nuclei involved in regulating core motivational processes. (C) 2003 Wiley-Liss, Inc.
