Selective stimulation of orexin receptor type 2 promotes wakefulness in freely behaving rats

Orexins A and B are a pair of neuropeptides implicated in the regulation of feeding and arousal behavior mediated through two orexin receptors type 1 and type 2. We have determined the arousal effects of newly developed selective orexin receptor type 2 agonist, [Ala11]orexin-B, on the sleep-wake cycle in rats. The effects of third ventricle intracerebroventricular (ICV) infusion of the novel orexin receptor type 2 selective agonist, [Ala11]orexin-B, on the sleep-wake cycle were investigated. ICV infusion of [Ala11]orexin-B (1, 10 and 40 nmol) during the light period (11:00-16:00) dose-dependently resulted in a significant increase in wake duration by 46.9% (n = 5, P < 0.05), 159.2% (n = 4, P < 0.01) and 163.6% (n = 7, P < 0.01)), respectively, and a significant decrease in rapid eye movement (REM) (P < 0.01) and non-REM sleep (P < 0.01) for all doses. In contrast, ICV infusion of orexin B at the same doses (1, 10 and 40 nmol) caused a 16.6% (n = 6, non-significant), 99.8% (n = 6, P < 0.05) and 72.0% (n = 4, P < 0.05) increase in wakefulness, respectively. Moreover, orexin-A, which exerts its effects through orexin receptor type 1 and orexin receptor type 2 with similar potency, resulted in a significant increase in wakefulness duration by 17.1% (n = 6, P < 0.05), 184.0% (n = 6, P < 0.01) and 228.6% (n = 6, P < 0.01) at doses of 0.1, 1 and 10 nmol, respectively. Further, the enhancement of wakefulness was accompanied by a marked reduction in REM and non-REM sleep. These findings suggest that orexin receptor type 2 plays an important role in the modulation of sleep-wake state and behavioral responses.