Orexins: effects on behavior and localisation of orexin receptor 2 messenger ribonucleic acid in the rat brainstem

The orexins are neuropeptides originally reported to be involved in the sti mulation of food intake. However, analysis of orexin immunoreactive fibres have revealed the densest innervation in brain sites involved in arousal an d sleep-wake control, notably the noradrenergic locus coeruleus, an area th at also expresses orexin receptor 1 (OX,R) messenger RNA (mRNA). We report here that, in the rat, a single intracerebroventricular injection of orexin A (1 and 3 nmol) or orexin R (3 nmol), during the early light phase, did n ot increase food intake over the first 4 h postinjection. However, the freq uency of active behaviors such as grooming, rearing, burrowing and locomoti on increased. Feeding behavior and food intake subsequently decreased over the following 20 h (4-24 h postinjection period) in the orexin A 3 nmol inj ected group whilst the frequency of inactive behavior (still or asleep) in this group increased. Using riboprobes, we performed in situ hybridization histochemistry to map the distribution of orexin receptor 2 (OX2R) mRNA wit hin the rat brainstem. We report here, for the first time, the presence of OX2R mRNA in the nucleus of the solitary tract and the lateral reticular fi eld (LRt), The LRt is a brainstem site that, amongst other functions, is im plicated in attention and wakefulness. This distribution of (OXR)-R-2 and t he effects on behavior support recent reports that the orexins might modula te central nervous system arousal and sleep-wake mechanisms rather than exc lusively being involved in the control of food intake. (C) 2001 Elsevier Sc ience B.V. All rights reserved.