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 decreased in the selectively bred, alcohol preferring (P) rats than in the alcohol nonpreferring (NP) rats. In this study, we further report that, 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 the 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 dorsal raphe (not including nucleus oralis) of P rats, compared with NP rats (unpaired Student's test). No difference was observed in the B-9 group. 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 increased in both lines of rats. However, the difference in the number of 5-HT 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, which 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 content and fewer 5-HT-IM neurons in the raphe account for the reduced density of detectable 5-HT-IM fibers in the terminal brain regions in the P rats, and that 5-HT neuronal transmission is reduced in the alcohol preferring rats.