Introduction: Orexin/Hypocretin deficiency causes the neurological disease narcolepsy, with excessive sleepiness, state fragmentation, emergence of rapid-eye-movement sleep (REMS)-related states into wake, and cataplexy. Several mouse models phenocopy narcolepsy remarkably well by inactivation of the Hypocretin (Hcrt) gene, or ablation of Hypocretin neurons. In dogs, several breeds harboring mutations in hypocretin receptor type-2 (HcrtR2) exhibit severe narcolepsy. The role of HcrtR2 in narcolepsy has been a longstanding conundrum. Whether the canine model represents the human disease has never been clarified, although the central role of hypocretin in narcolepsy is undisputed. Whether HcrtR2-deficient mice display cataplexy is of major preclinical importance for the understanding of the disease, but remains unclear.
Methods: Here we report generation of mice where HcrtR1 or 2 is inactivated and replaced by Gfp (HcrtR1Gfp and HcrtR2Gfp), or both receptors are ablated (HcrtR1&2Gfp/Gfp). The mice were recorded with EEG/EMG/video polysomnography for 96h including 48h of baseline, 6h sleep deprivation, 42h of recovery, followed by a chocolate exposure challenge in dark phase.
Results: We found that neither HcrtR1Gfp/Gfp, or HcrtR2Gfp/Gfp mice, display cataplexy, even after exposure to a powerful excitatory stimulus (chocolate) (n=9 HcrtR1Gfp/Gfp, n=11 HcrtR2Gfp/Gfp). Both lines however display sleep attacks (short episode of delta-dominated sleep without complete loss of muscle tone) during the active phase, with or without chocolate, and appear sleepy, with shorter time awake in dark periods. HcrtR1Gfp/Gfp deficient mice show moreover a pronounced increase in NREM sleep slow-delta activity in light phase, and NREM sleep fragmentation. Cataplexy (sudden loss of muscle tone after a prolonged episode of intense purposeful activity) is only observed in doubly ablated mice (n=5±2 cataplexies per 12 h dark phase, n=9 mice), a similar incidence as Ox-Ataxin-3 Tg mice but slightly inferior to the number observed in Hcrt-KO mice. Cataplexy in these mice displays the delta-theta EEG signature we priorly described in cataplexy and REMS of Hcrt-KO mice.
Conclusions: Our findings suggest that cataplexy protection is mediated by an interplay of Hypocretin Receptor 1 and 2 signaling. The data have preclinical relevance for the treatment of narcolepsy and other hypersomnias, and the use of the newly developed selective HCRTR1 and HCRTR2 agonists in these patients.