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© 2001 by the American Diabetes Association, Inc.
Hypoglycemia Activates Orexin Neurons and Selectively Increases Hypothalamic Orexin-B Levels
Responses Inhibited by Feeding and Possibly Mediated by the Nucleus of the Solitary Tract
From the Diabetes and Endocrinology Research Group (X.J.C., G.W.), University of Liverpool, Liverpool; and the Divisions of Neuroscience (M.L.E., R.A.L.) and Vascular Biology (C.A.L., J.R.S.A., S.W.), SmithKline Beecham Pharmaceuticals, Harlow, U.K.
Address correspondence and reprint requests to Dr. Xue J Cai, Diabetes and Endocrinology Research Unit, University of Liverpool, Liverpool L69 3GA, U.K. E-mail: xjcai{at}liv.ac.uk .
Orexins are novel appetite-stimulating peptides expressed in the lateral hypothalamic area (LHA), and their expression is stimulated by hypoglycemia in fasted rats. We investigated activation of orexin and other neurons during insulin-induced hypoglycemia using the immediate early gene product Fos. Insulin (50 U/kg) lowered plasma glucose by >50% after 5 h and stimulated feeding sixfold compared with saline-injected controls. Hypoglycemic rats allowed to feed and normoglycemic controls both showed sparse Fos-positive (Fos+) neurons in the LHA and the paraventricular nucleus (PVN) and arcuate nucleus (ARC) and showed none in the nucleus of the solitary tract (NTS), which relays visceral feeding signals to the LHA. In the LHA, total numbers of Fos+ neurons were comparable in fed hypoglycemic and control groups (60 ± 6 vs. 52 ± 4 cells/mm2, P > 0.05), as were Fos+ neurons immunoreactive for orexin (1.4 ± 0.4 vs. 0.6 ± 0.4 cells/mm2, P > 0.05). By contrast, hypoglycemic rats that were fasted showed significantly more Fos+ nuclei in the LHA (96 ± 10 cells/mm2, P < 0.05, vs. both other groups) and Fos+ orexin neurons (8.4 ± 3.3 cells/mm2, P < 0.001, vs. both other groups). They also showed two- to threefold more Fos+ nuclei (P < 0.001) in the PVN and ARC than both fed hypoglycemic rats and controls and showed strikingly abundant Fos+ neurons in the NTS and dorsal motor nucleus of the vagus. In parallel studies, whole hypothalamic orexin-A levels were not changed in hypoglycemic rats, whether fasted or freely fed, whereas orexin-B levels were 10-fold higher in hypoglycemic fasted rats than in control and hypoglycemic fed groups. These data support our hypothesis that orexin neurons are stimulated by falling glucose levels but are readily inhibited by signals related to nutrient ingestion and suggest that they may functionally link with neuronal activity in the NTS. Orexin-A and -B may play specific roles in behavioral or neuroendocrine responses to hypoglycemia.
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