Alteration of the serotonergic nervous system in fatal familial insomnia.

Fatal familial insomnia (FFI) is a unique hereditary prion disease with characteristic disturbances of sleep. We studied the serotonergic system in 8 FFI-affected subjects by immunohistochemistry for the serotonin-synthesizing enzyme, tryptophan hydroxylase (TH). Quantification of neurons in median raphe nuclei showed no total neuronal loss in FFI but a substantial increase of TH+ neurons (approximately 62%) in FFI subjects compared with controls. Our data indicate an alteration of the serotonergic system that might represent the functional substrate of some typical symptoms of FFI.