Reduced number of hypocretin neurons in human narcolepsy

Abstract

Murine and canine narcolepsy can be caused by mutations of the hypocretin (Hcrt) (orexin) precursor or Hcrt receptor genes. In contrast to these animal models, most human narcolepsy is not familial, is discordant in identical twins, and has not been linked to mutations of the Hcrt system. Thus, the cause of human narcolepsy remains unknown. Here we show that human narcoleptics have an 85%-95% reduction in the number of Hcrt neurons. Melanin-concentrating hormone (MCH) neurons, which are intermixed with Hcrt cells in the normal brain, are not reduced in number, indicating that cell loss is relatively specific for Hcrt neurons. The presence of gliosis in the hypocretin cell region is consistent with a degenerative process being the cause of the Hcrt cell loss in narcolepsy.

Figure 1.

Distribution of Hypocretin-Labeled Somas in Normal and Narcoleptic Subjects Normal is subject CG, and narcoleptic is subject NA. Cell counts are listed below each section. On average, the narcoleptics had only 7% of the Hcrt cells seen in normals.

Figure 2.

Distribution of Cells in Perifornical and Dorsomedial Hypothalamic Regions of Normal and Narcoleptic Humans Plotted in Figure 1 All cells in figure are from the levels indicated in (A) and (B). (C and D) Low power photomicrographs (calibration [Cal.] = 100 μm) covering regions outlined in gray at the top shows many Hcrt somas and axons in the normal and few in the narcoleptic (arrows in narcoleptic indicate Hcrt somas verified as immunolabeled at higher magnification). Higher power photomicrographs (Cal. = 25μm) show Hcrt cells and axons in normal (E and G) and narcoleptic (F and H) subjects. Note the reduced density of axonal staining in the background of the narcoleptic as compared to that seen in the normal.

Figure 3.

The Number of Hcrt Cells Is Decreased in Narcoleptics

Figure 4.

Melanin-Concentrating Hormone Neurons in Normal and Narcoleptic Subjects Normal is subject CK, and narcoleptic is subject NA. Narcoleptics have normal numbers of MCH neurons despite the loss of 93% of Hcrt cells in the same region. Cal. = 250 μm.

Figure 5.

Glial Fibrillary Acidic Protein–Labeled Astrocytes in the Periventricular/Perifornical Region of a Normal and Narcoleptic Brain Low power photomicrograph is at the top, and a higher magnification of area marked with plus signs is at the bottom. Elevated GFAP staining in the hypothalamus of narcoleptics is consistent with a degenerative cause of cell loss. Top, Cal. = 500 μm; bottom, Cal. = 50 μm.

Figure 6.

Pattern of Gliosis in the Hypothalamus and Thalamus of Narcoleptics and Controls The number of GFAP labeled astrocytes is significantly increased in the hypothalamus of narcoleptics, the region containing Hcrt cells, but not in the thalamus.