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The Number of Hypothalamic Hypocretin (Orexin) Neurons Is Not Affected in Prader-Willi Syndrome

Netherlands Institute for Brain Research (R.F., R.B., U.A.U., D.F.S.), Meibergdreef 33, 1105 AZ Amsterdam ZO, The Netherlands; and Leiden University Medical Centre (R.F., G.J.L.), Albinusdreef 2, 2300 RC Leiden, The Netherlands

Address all correspondence and requests for reprints to: Rolf Fronczek, Leiden University Medical Centre, J3R-151, Postbus 9600, 2300 RC Leiden, The Netherlands. E-mail: r.fronczek{at}lumc.nl.

Context: Narcoleptic patients with cataplexy have a general loss of hypocretin (orexin) in the lateral hypothalamus, possibly due to an autoimmune-mediated degeneration of the hypocretin neurons. In addition to excessive daytime sleepiness, Prader-Willi syndrome (PWS) patients may show narcolepsy-like symptoms, such as sleep-onset rapid eye movement sleep and cataplexy, independent of obesity-related sleep disturbances, which suggests a disorder of the hypocretin neurons.

Objective: We hypothesized that the narcolepsy-like symptoms in PWS are caused by a decline in the number of hypocretin neurons.

Design: We estimated the number of hypocretin neurons in postmortem hypothalami using immunocytochemistry and an image analysis system.

Setting: This study was conducted at the Netherlands Institute for Brain Research.

Patients: Eight PWS adults, three PWS infants, and 11 controls were studied.

Main Outcome Measure: The total number of hypocretin neurons in the lateral hypothalamus was measured.

Results: There was no significant difference in the total number of hypocretin-containing neurons among the seven PWS patients (in whom sufficient hypothalamic material was available to quantify total cell number) and seven age-matched controls, either in adults or in infants. A significant decline with age was found in adult PWS patients (r = –0.9; P = 0.037).

Conclusions: We conclude that a decrease in the number of hypocretin neurons does not play a major role in the occurrence of narcolepsy-like symptoms in PWS.

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