Histamine-1 receptor is not required as a downstream effector of orexin-2 receptor in maintenance of basal sleep/wake states

Aim: The effect of orexin on wakefulness has been suggested to be largely mediated by activation of histaminergic neurones in the tuberomammillary nucleus (TMN) via orexin receptor-2 (OX₂R). However, orexin receptors in other regions of the brain might also play important roles in maintenance of wakefulness. To dissect the role of the histaminergic system as a downstream mediator of the orexin system in the regulation of sleep/wake states without compensation by the orexin receptor-1 (OX₁R) mediated pathways, we analysed the phenotype of Histamine-1 receptor (H₁R) and OX ₁R double-deficient (H₁R^-/-;OX ₁R^-/-) mice. These mice lack OX₁R-mediated pathways in addition to deficiency of H₁R, which is thought to be the most important system in downstream of OX₂R. Methods: We used H ₁R deficient (H₁R^-/-) mice, H₁R ^-/-;OX₁R^-/- mice, OX₁R and OX ₂R double-deficient (OX₁R^-/-;OX ₂R^-/-) mice, and wild type controls. Rapid eye movement (REM) sleep, non-REM (NREM) sleep and awake states were determined by polygraphic electroencephalographic/electromyographic recording. Results: No abnormality in sleep/wake states was observed in H₁R^-/- mice, consistent with previous studies. H₁R^-/-;OX ₁R^-/- mice also showed a sleep/wake phenotype comparable to that of wild type mice, while OX₁R^-/-; OX ₂R^-/- mice showed severe fragmentation of sleep/wake states. Conclusion: Our observations showed that regulation of the sleep/wake states is completely achieved by OX₂R-expressing neurones without involving H₁R-mediated pathways. The maintenance of basal physiological sleep/wake states is fully achieved without both H₁ and OX₁ receptors. Downstream pathways of OX₂R other than the histaminergic system might play an important role in the maintenance of sleep/wake states.