Study objectives: Whether the cause of daytime sleepiness in narcolepsy type 1 (NT1) is a direct consequence of the loss of orexin (ORX) neurons or whether low orexin reduces the efficacy of the monoaminergic systems to promote wakefulness is unclear. The neurobiology underlying sleepiness in other central hypersomnolence disorders, narcolepsy type 2 (NT2), and idiopathic hypersomnia (IH), is currently unknown.
Methods: Eleven biogenic amines including the monoaminergic neurotransmitters and their metabolites and five trace amines were measured in the cerebrospinal fluid (CSF) of 94 drug-free subjects evaluated at the French National Reference Center for Narcolepsy: 39 NT1(orexin-deficient) patients, 31 patients with objective sleepiness non orexin-deficient (NT2 and IH), and 24 patients without objective sleepiness.
Results: Three trace amines were undetectable in the sample: tryptamine, octopamine, and 3-iodothyronamine. No significant differences were found among the three groups for quantified monoamines and their metabolites in crude and adjusted models; however, CSF 5-hydroxyindoleacetic acid (5-HIAA) levels tended to increase in NT1 compared to other patients after adjustment. Most of the biomarkers were not associated with ORX-A levels, clinical or neurophysiological parameters, but a few biomarkers (e.g. 3-methoxy-4-hydroxyphenylglycol and norepinephrine) correlated with daytime sleepiness and high rapid eye movement (REM) sleep propensity.
Conclusions: We found no striking differences among CSF monoamines, their metabolites and trace amine levels, and few associations between them and key clinical or neurophysiological parameters in NT1, NT2/IH, and patients without objective sleepiness. Although mostly negative, these findings are a significant contribution to our understanding of the neurobiology of hypersomnolence in these disorders that remain mysterious and deserve further exploration.
Keywords: central disorders of hypersomnolence; cerebrospinal fluid; hypersomnia; hypocretin/orexin; monoamine; narcolepsy; sleepiness.