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

Abstract

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 (OX(2)R). 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 (OX(1)R) mediated pathways, we analysed the phenotype of Histamine-1 receptor (H(1)R) and OX(1)R double-deficient (H(1)R(-/-);OX(1)R(-/-)) mice. These mice lack OX(1)R-mediated pathways in addition to deficiency of H(1)R, which is thought to be the most important system in downstream of OX(2)R.

Methods: We used H(1)R deficient (H(1)R(-/-)) mice, H(1)R(-/-);OX(1)R(-/-) mice, OX(1)R and OX(2)R double-deficient (OX(1)R(-/-);OX(2)R(-/-)) 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 H(1)R(-/-) mice, consistent with previous studies. H(1)R(-/-);OX(1)R(-/-) mice also showed a sleep/wake phenotype comparable to that of wild type mice, while OX(1)R(-/-); OX(2)R(-/-) mice showed severe fragmentation of sleep/wake states.

Conclusion: Our observations showed that regulation of the sleep/wake states is completely achieved by OX(2)R-expressing neurones without involving H(1)R-mediated pathways. The maintenance of basal physiological sleep/wake states is fully achieved without both H(1) and OX(1) receptors. Downstream pathways of OX(2)R other than the histaminergic system might play an important role in the maintenance of sleep/wake states.

Figure 1

Representative 12 h dark period (20:00 hours to 08:00 hours) hypnograms for wild type mice (WT), H₁R^−/− mice, H₁R^−/−; OX₁R^−/− mice and OX₁R^−/−;OX₂R^−/− mice. The height of the horizontal line above baseline indicates the vigilance state of the mouse at the time. W, wakefulness; NR, non-rapid eye movement (REM) sleep; R, REM sleep. There were no significant differences in sleep/wake phenotypes between wild type mice, H₁R^−/− mice, and H₁R^−/−;OX₁R^−/− mice. In contrast, OX₁R^−/−;OX₂R^−/− mice showed severe fragmentation of sleep/wake states, along with frequent direct transitions from wakefulness to REM sleep (mark with arrowhead). Hypnograms were obtained by simultaneous EEG/EMG recording as described previously (Chemelli et al. 1999).

Figure 2

(a) Total time (min, mean ± SEM) spent in each state in control mice (wild type) (n = 5), H₁R^−/− mice (n = 4), H₁R^−/−; OX₁R^−/− mice (n = 4) and OX₁R^−/−;OX₂R^−/− mice (n = 5). W, awake; NR, non-rapid eye movement (REM) sleep; R, REM sleep. The graphs summarize the data recorded during the 12 h dark period. (B) Episode duration (s ± SEM) spent in each state in control mice, H₁R^−/− mice, H₁R^−/−;OX₁R^−/− mice and OX₁R^−/−;OX₂R^−/− mice. W, awake; NR, non-REM sleep; R, REM sleep. OX₁R^−/−; OX₂R^−/− mice show significantly shorter wakefulness and NREM sleep episodes during dark period. Mice with other genotypes did not show any abnormality in sleep/wake states. *P < 0.001. The graphs summarize the data recorded during the 12 h dark period.

Figure 3

Hourly analysis of sleep/wake amounts in control mice (n = 5), H₁R^−/− mice (n = 4), H₁R^−/−;OX₁R^−/− mice (n = 4) and OX₁R^−/−;OX₂R^−/− mice (n = 5). The shaded areas represent the 12 h dark period. There are no significant differences between control mice, H₁R^−/− mice, H₁R^−/−;OX₁R^−/− mice, while OX₁R^−/−;OX₂R^−/− mice show abnormal circadian distribution of rapid eye movement (REM) sleep.