| Neurons expressing the orexin (hypocretin) peptides project widely across the central nervous system from a single point of origin in the lateral hypothalamus. As such, the central orexin system has been implicated in a wide variety of physiological functions, including feeding behavior, energy balance, and autonomic tone. Specifically, orexin peptides are thought to increase the activity of the sympathetic nervous system. While there are two known receptors for the orexin peptides, the orexin 1 receptor and orexin 2 receptor, little is known about the distribution in the brain. Using in situ hybridization histochemistry we mapped the distribution of both orexin receptor mRNA throughout the brain and spinal cord of the rat. Furthermore, we performed dual-labeling and retrograde tracing studies to determine the expression of orexin receptor mRNA on neurons expressing cholinergic and monoaminergic markers in areas classically implicated in autonomic control, and on neurons that project to the thoracic spinal cord. Using c-fos protein expression as a surrogate for neuronal activation, we assessed the activation of spinally projecting orexin neurons in response to insulin-induced hypoglycemia. Based on the results of these studies, orexin neurons may represent a subpopulation of neurons that respond to hypoglycemic conditions and directly engage sympathetic preganglionic neurons in the spinal cord. |
