The
hypocretin/
orexin system regulates arousal through central nervous system mechanisms and plays an important role in sleep, wakefulness and energy homeostasis. It is unclear whether
hypocretin peptides are also present in blood due to difficulties in measuring reliable and reproducible levels of the
peptides in blood samples. Lack of
hypocretin signalling causes the
sleep disorder narcolepsy type 1, and low concentration of cerebrospinal fluid
hypocretin-1/
orexin-A peptide is a hallmark of the disease. This measurement has high diagnostic value, but performing a lumbar puncture is not without discomfort and possible complications for the patient. A blood-based test to assess
hypocretin-1 deficiency would therefore be of obvious benefit. We here demonstrate that heating plasma or serum samples to 65°C for 30 min at pH 8 significantly increases
hypocretin-1 immunoreactivity enabling stable and reproducible measurement of
hypocretin-1 in blood samples. Specificity of the signal was verified by high-performance liquid chromatography and by measuring blood samples from mice lacking
hypocretin. Unspecific background signal in the assay was high. Using our method, we show that
hypocretin-1 immunoreactivity in blood samples from
narcolepsy type 1 patients does not differ from the levels detected in control samples. The data presented here suggest that
hypocretin-1 is present in the blood stream in the low picograms per millilitres range and that peripheral
hypocretin-1 concentrations are unchanged in
narcolepsy type 1.