Diurnal variation in orexin A immunoreactivity and prepro-orexin mRNA in the rat central nervous system

Orexins are a family of neuropeptides originally believed to be im portent mediators of food intake. The wide distribution of orexins and their recept ors, however, has suggested other regulatory functions for these peptides i ncluding involvement in sleep and arousal mechanisms. In this study, we hav e demonstrated diurnal variation in orexin A immunoreactivity in the pens, from where locus coeruleus noradrenergic neurones innervate other brain are as to stimulate arousal, and in the preoptic/anterior hypothalamic region, an area implicated in the regulation of sleep and circadian rhythms. Orexin A immunoreactivity decreased by 50% in the preoptic/anterior hypothalamus from 09:00 to 21:00 h (P < 0.0001), whilst in the pens, it increased by ove r 30% from 09:00 to 01:00 h (P = 0.02). Prepro-orexin mRNA also displayed d iurnal variation. This further suggests that orexins are involved in the re gulation of the sleep/wake cycle. (C) 2000 Elsevier Science Ireland Ltd. Al l rights reserved.