Initially recognized for their importance in control of appetite, orexins (also called hypocretins) are neuropeptides that are also involved in regulating sleep, arousal, and cardiovascular function. Loss of orexin appears to be the primary cause of narcolepsy. Cells expressing the orexins are restricted to a discrete region of the hypothalamus, but their terminal projections are widely distributed throughout the brain. With the diversity of function and broad distribution of orexin terminals, it is not known whether the orexin cells constitute a homogeneous population. Because orexins produce neuroexcitatory effects, we hypothesized that orexin-containing neurons are glutamatergic. In the present study we used digoxigenin-labeled cRNA probes for the vesicular glutamate transporters, VGLUT1 and VGLUT2, for in situ hybridization studies in combination with immunohistochemical detection of orexin cell bodies in the hypothalamus. In general, cells in the hypothalamus expressed low levels of the vesicular glutamate transporters relative to other areas of the forebrain, such as the cortex and thalamus. Light labeling for VGLUT2 mRNA was detected in about 50% of the orexin-immunoreactive neurons, and a much smaller percentage (approximately 13%) of orexin-immunoreactive cells was found to express VGLUT1. Despite the fact that intense labeling for GAD67 mRNA was found in a large number of cells throughout the hypothalamus, none of the orexin-immunoreactive cells was found to be GABAergic. These findings, showing that many of the orexin neurons are glutamatergic, are consistent with the neuroexcitatory effects of orexin but suggest that another neurochemical phenotype may define the remaining subset of orexin neurons.
Copyright 2003 Wiley-Liss, Inc.