SEROTONERGIC NEURONS IN THE ALCOHOL-PREFERRING RATS

Previously, we have shown that the serotonin (5-HT) content and fiber density in a number of terminal brain regions have been found to be de creased in the selectively bred, alcohol preferring (P) rats than in t he alcohol nonpreferring (NP) rats. In this study, we further report t hat, compared with NP rats, there were fewer 5-HT-immunostained (5-HT- IM) neurons in the major ascending raphe nuclei of the P rats. Among t he three major groups of 5-HT neurons responsible for the majority of ascending projections to forebrain, dorsal raphe (B-7), median raphe ( B-8), and B-9, there were fewer 5-HT-IM neurons in the median and dors al raphe (not including nucleus oralis) of P rats, compared with NP ra ts (unpaired Student's test). No difference was observed in the B-9 gr oup. When the animals were treated with pargyline and L-tryptophan to enhance the 5-HT in the neurons, the number of 5-HT-IM neurons increas ed in both lines of rats. However, the difference in the number of 5-H T neurons between the rat lines remained. The intensity of 5-HT-IM was also found to be lower in the dorsal raphe neurons of the untreated P than in the untreated NP rats. The decreased 5-HT-IM was supported by high performance liquid chromatography measurement of 5-HT content, w hich also indicated that 5-HT content of the dorsal raphe was lower in the P than in the NP rats. These results indicate that lower 5-HT con tent and fewer 5-HT-IM neurons in the raphe account for the reduced de nsity of detectable 5-HT-IM fibers in the terminal brain regions in th e P rats, and that 5-HT neuronal transmission is reduced in the alcoho l preferring rats.