CO2 activates orexin-containing neurons in mice

Accumulating evidence shows possible participation of orexin, a hypothalamic neuropeptide also known as hypocretin, in regulation of breathing. In this study we examined whether hypercapnic stimulation in vivo activates orexin-containing neurons in the hypothalamus. A hypercapnic gas mixture Of 10% CO2, 21% O-2, and residual N-2 or room air was inhaled to awake and freely moving mice for 3 h. Their brains were examined by double immunohistochemical staining for orexin and c-Fos. We found that inhalation of CO2 significantly increased c-Fos expression in the orexin-containing cells that were located in the perifornical region and the dorsomedial hypothalamus (15.8 +/- 1.6% for room air vs. 28.4 +/- 2.7% for CO2, p = 0.011) but not in the lateral hypothalamic area (12.3 +/- 1.2% for room air vs. 12.4 +/- 3.0% for CO2). Although current methodology cannot differentiate between direct effect Of CO2 on the orexin-containing neurons and indirect one through other neurons, this is the first report showing that inhalation of CO2 did activate the orexin-containing neurons in vivo. (C) 2009 Elsevier B.V. All rights reserved.