Opposite effects of sleep rebound on orexin OX1 and OX2 receptor expression in rat brain

Orexins (hypocretins) have been implicated in the regulation of the normal sleep-wake cycle, in sensorimotor programming, and in other homeostatic and neuroregulatory processes. The present study examined the effects of sleep deprivation (SD) and sleep recovery on the expression of orexin 1 receptors (OX1R) and orexin 2 receptors (OX2R) throughout the brain. Rats were sacrificed either immediately after 96 h of sleep deprivation (SD group) or after SD followed by 24 h of sleep recovery (Rebound group). Prepro-orexin mRNA showed a non-significant increase in the SD group relative to controls, but a pronounced and significant increase in the Rebound group (+88%, P < 0.007). Similarly, sleep deprivation produced no effect on OX1R or OX2R mRNA levels. However, in the Rebound group, OX1R mRNA levels increased significantly, compared to either control or SD groups, in 37 of 92 brain regions analyzed, with particularly strong effects in the amygdala and hypothalamus. Changes in OX2R mRNA levels were also seen only in the sleep Rebound group, but they were fewer in number (10 out of 86 regions), were in the direction of decreased rather than increased expression, and were predominantly confined to cerebral cortical areas. These observations indicate that some factor associated with sleep recovery, possibly the compensatory increase in REM sleep, has strong effects on the orexin system at the mRNA level. They further indicate that OX1 and OX2 receptors are affected in opposite way and that the former are more vulnerable to these effects than the latter.