Hypocretin Receptor Expression in Canine and Murine Narcolepsy Models and in Hypocretin-Ligand Deficient Human Narcolepsy

NARCOLEPSY IS A DISABLING SLEEP DISORDER CHARACTERIZED BY EXCESSIVE DAYTIME SLEEPINESS (EDS), SLEEP PARALYSIS, HYPNAGOGIC HALLUCINATIONS, and cataplexy.1,2 Narcolepsy is currently treated with amphetamine-like stimulants (or modafinil for EDS) and antidepressants for cataplexy and REM sleep related symptoms.1 These treatments are purely symptom-oriented and are often problematic due to their limited effectiveness and undesirable side effects.1 Recent studies in animals and humans revealed that a deficit in hypocretin (hcrt, also known as orexin) transmission is the major pathophysiology. Doberman pinschers with autosomal recessive narcolepsy possess null mutations in the gene encoding hcrt receptor2 (hcrtR2),3 one of two hypocretin receptors. Sporadic cases of canine narcolepsy show undetectable levels of the brain and cerebrospinal fluid (CSF) hcrt neuropeptide.4 Preprohypocretin gene knockout mice as well as orexin/ataxin-3-transgenic mice (TG mice) with postnatal death of hcrt-producing neurons by hcrt neuron-specific expression of a truncated Machado-Joseph disease gene product (ataxin-3), also exhibit the narcolepsy phenotype.5,6 In humans, genetic mutation in hcrt-related genes is rare, but a large majority of narcolepsy-cataplexy is found to be associated with the loss of hcrt production in the brain and CSF, possibly due to the postnatal cell death of hypocretin neurons.7–11 Since most human narcolepsy-cataplexy is caused by the loss of the hcrt peptide, hcrt replacement is a promising future therapeutic option. In this regard, the result that central administration of hctr-1 rescues the sleep abnormalities and cataplexy in orexin/ataxin-3-TG mice is very encouraging.12 The potential efficacy of hypocretin replacement therapies in human narcolepsy will however, require the integrity of hypocretin receptors. Functional hcrtRs may disappear gradually following the loss of hypocretin in narcolepsy. In previous studies, the effect of hcrt replacement was only examined in 14- to 15-week-old hypocretin cell deficient narcoleptic mice; whether longer-term changes in the responsiveness to hypocretin occur is unknown. Additionally, the etiological mechanisms of hypocretin cell death in human narcolepsy is currently unknown, and consequent hcrtR changes may well be different from those found in orexin/ataxin-3-TG mice. Thannickal et al. suggested that the loss of hypocretin function in human narcolepsy results from a cytotoxic or immunologically mediated attack focused on hcrtR2.13 If this is the case, then a loss of function of hcrtR2 might also exist in human narcolepsy, and patients may not respond to hcrt replacement, as previously shown in hcrtR2-mutated narcoleptic dogs.14 In the present study, we evaluated the influences of long-term hcrt deficiency on hcrtR1 and hcrtR2 expression in brain samples from canine and murine models of narcolepsy, as well as from a limited number of human narcolepsy-cataplexy patients. The evaluation in humans is critical, since the results may predict responsiveness to the hcrt replacement therapy and the potential usefulness of cell transplantation or gene-based therapies.