Orexins, orexigenic hypothalamic peptides, interact with autonomic, neuroendocrine and neuroregulatory systems

We determined the immunohistochemical distributions of orexin-A and orexin- B, hypothalamic peptides that function in the regulation of feeding behavio r and energy homeostasis. Orexin-A and -B neurons were restricted to the la teral and posterior hypothalamus, whereas both orexin-A and -B nerve fibers projected widely into the olfactory bulb, cerebral cortex, thalamus, hypot halamus, and brainstem, Dense populations of orexin containing fibers were present in the paraventricular thalamic nucleus, central gray, raphe nuclei , and locus coeruleus, Moderate numbers of these fibers were found in the o lfactory bulb, insular, infralimbic and prelimbic cortex, amygdala, ventral , and dorsolateral parts of the suprachiasmatic nucleus, paraventricular nu cleus except the lateral magnocellular division, arcuate nucleus, supramamm illary nucleus, nucleus of the solitary tract, and dorsal motor nucleus of the vagus, Small numbers of orexin fibers were present in the perirhinal, m otor and sensory cortex, hippocampus, and supraoptic nucleus, and a very sm all number in the lateral magnocellular division of the paraventricular nuc leus. Intracerebroventricular injections of orexins induced c-fos expressio n in the paraventricular thalamic nucleus, locus coeruleus, arcuate nucleus , central gray, raphe nuclei, nucleus of the solitary tract, dorsal motor n ucleus of the vagus, suprachiasmatic nucleus, supraoptic nucleus, and parav entricular nucleus except the lateral magnocellular division. The unique ne uronal distribution of orexins and their functional activation of neural ci rcuits suggest specific complex roles of the peptides in autonomic and neur oendocrine control.