Greatly increased numbers of histamine cells in human narcolepsy with cataplexy

Abstract

Objective: To determine whether histamine cells are altered in human narcolepsy with cataplexy and in animal models of this disease.

Methods: Immunohistochemistry for histidine decarboxylase (HDC) and quantitative microscopy were used to detect histamine cells in human narcoleptics, hypocretin (Hcrt) receptor-2 mutant dogs, and 3 mouse narcolepsy models: Hcrt (orexin) knockouts, ataxin-3-orexin, and doxycycline-controlled-diphtheria-toxin-A-orexin.

Results: We found an average 64% increase in the number of histamine neurons in human narcolepsy with cataplexy, with no overlap between narcoleptics and controls. However, we did not see altered numbers of HDC cells in any of the animal models of narcolepsy.

Interpretation: Changes in histamine cell numbers are not required for the major symptoms of narcolepsy, because all animal models have these symptoms. The histamine cell changes we saw in humans did not occur in the 4 animal models of Hcrt dysfunction we examined. Therefore, the loss of Hcrt receptor-2, of the Hcrt peptide, or of Hcrt cells is not sufficient to produce these changes. We speculate that the increased histamine cell numbers we see in human narcolepsy may instead be related to the process causing the human disorder. Although research has focused on possible antigens within the Hcrt cells that might trigger their autoimmune destruction, the present findings suggest that the triggering events of human narcolepsy may involve a proliferation of histamine-containing cells. We discuss this and other explanations of the difference between human narcoleptics and animal models of narcolepsy, including therapeutic drug use and species differences.

FIGURE 1:

Number of histamine neurons in human controls and in narcoleptics with cataplexy, arranged as in the Table. [Color figure can be viewed in the online issue, which is available at www.annalsofneurology.org.]

FIGURE 2:

Distribution of histamine cells in narcoleptic and control. The number of cells at each level is indicated in a representative control (Patient 6) and narcoleptic (Patient 11). Fx = fornix; MM = mammillary body; Opt = optic tract. Scale bars = 50μm. Note the increased intensity of histidine decarboxylase staining seen in the narcoleptic brain processed identically to the normal control. [Color figure can be viewed in the online issue, which is available at www.annalsofneurology.org.]

FIGURE 3:

The (A) cell number and (B) size of histidine decarboxylase neurons in the tuberomammillary nucleus (TMN). The total number of histamine neurons is significantly increased in the mTMN subregion (***p = 0.006). There is no difference in the cell size in all subregions. cTMN = caudal part of TMN; mTMN = middle part of TMN; rTMN = rostral part of TMN. [Color figure can be viewed in the online issue, which is available at www.annalsofneurology.org.]

FIGURE 4:

Histidine decarboxylase (HDC) staining (A) in a wild-type mouse and (B) in an HDC knockout (KO) mouse. Arrows indicate HDC-labeled cells. Lack of staining in KO demonstrates the specificity of the antibody and staining procedures we used. Scale bars = 50μm (A and B) and 12.5μm (inset). fx = fornix. [Color figure can be viewed in the online issue, which is available at www.annalsofneurology.org.]