Morphometry of dorsal raphe nucleus serotonergic neurons in alcoholism

Background: Reduced serotonergic function is hypothesized in alcohol abuse and dependence. Serotonergic innervation of the cortex arises predominantly from the dorsal raphe nucleus (DRN). We sought to determine the number and morphometric characteristics of DRN serotonergic neurons postmortem in alcoholic individuals (n=9; age: 16-66years; 8M:1F) compared with psychiatrically normal, nonalcoholic controls (n=6; age: 17-74 years; 4M:2F). Methods: Brainstems were collected at autopsy, fixed and cryoprotected. Alcohol dependence or abuse was determined by psychological autopsy (DSM-IV), the presence of liver fatty changes or cirrhosis and/or high blood alcohol level. Tissue was sectioned at 50 mu m (-25 degrees C). A series of 1:10 sections was immunoreacted with antiserum to tryptophan hydroxylase (TPH), the rate-limiting enzyme in the biosynthesis of serotonin. The total number of TPH-immunoreactive (IR) DRN neurons was determined by stereology. Neuron morphometry indices were determined using a video-based imaging system attached to a microscope. We identified TPH-IR neurons every 1,000 mu m in each brainstem and measured neuron area, total cross sectional neuron area, and the total area and density of immunolabeled processes. Results: Dorsal raphe nucleus neuron number (controls: 80,386 +/- 10,238; alcoholic individuals: 85,884 +/- 12,478) was not different between groups but TPH-IR was greater in alcoholic individuals throughout the rostrocaudal extent of the DRN. The volume of the DRN was 66 +/- 9 mm(3) in controls and 55 +/- 5 mm(3) in alcoholic individuals (p > 0.05). The average size of DRN neurons did not differ between groups (353 +/- 12 mu m(2) for controls vs 360 +/- 15 mu m(2) for alcoholic subjects). However, the area occupied by neuron processes (area of processes/DRN area) was 2.2-fold greater in alcoholic individuals compared with controls (p < 0.05). Conclusions: The increased area occupied by neuron processes in alcoholic individuals may represent sprouting and, together with greater TPH-IR, be a compensatory response to impaired serotonergic transmission or cumulative effects of alcohol on the serotonin system.