Hypothalamic orexin A-immunoreactive neurons project to the rat dorsal medulla.

Retrograde tract tracing combined with immunohistochemical techniques were used to identify the origin of orexin A-immunoreactive (OrA-ir) fibers in the rat medulla. One to 5 days following injection of the fluorescent dye Fluorogold into the dorsal medulla, labeled neurons were found in the lateral half of the lateral hypothalamus, paraventricular, perifornical, dorsomedial, dorsal and posterior hypothalamic nuclei. Labeling the same sections with OrA antisera revealed a concentration of OrA-ir neurons in the perifornical and dorsomedial regions of the tuberal hypothalamus. A maximum of 10% of Fluorogold-labeled hypothalamic neurons were OrA-ir and 15% of OrA-ir hypothalamic neurons contained Fluorogold. Our results demonstrate that a fraction of OrA-ir neurons in the tuberal hypothalamus project to areas of the medulla that are involved in autonomic functions.