Morphometry of the dorsal raphe nucleus serotonergic neurons in suicide victims

Background: The serotonin deficiency hypothesis of suicide has been important heuristically. Few studies have directly examined the brainstem dorsal raphe nucleus (DRN) serotonin neurons. We determined the number and morphometry of DRN serotonergic neurons in suicide victims (n = 7) compared to controls (n = 6). Methods: Brainstems were collected at autopsy, fixed and cryoprotected. Tissue was sectioned, stained for Nissl and processed with an antiserum that cross-reacts with tryptophan hydroxylase. All DRN neurons were identified, counted and analyzed every 1000 μm. Neuron morphometry was characterized by soma area (μm2), sphericity, perimeter, length and density (neurons per mm3). Results: Neuron number and density was higher in suicide victims (1,780 ± 127 neurons/mm3) than controls (1,349 ± 68). The DRN volume did not differ between groups (66 ± 9 mm3 for controls vs. 67 ± 5 mm3 for suicides). Mean neuronal area and sphericity did not differ between suicides and controls. The total number and the density of DRN neurons did not correlate with age. Conclusions: The finding of an increased number of neurons indicates that impaired serotonergic transmission found in association with serious suicide attempts is not due to fewer neurons.