Chronic fluoxetine in tests of anxiety in rat lines selectively bred for differential 5-HT1A receptor function

Selective breeding for high and low sensitivity to the hypothermic response induced by the 5-HT1A receptor agonist 8- OH-DPAT has established two lines of rat (HDS and LDS, respectively) whose behavior differs in a model of depression and in the social interaction test of anxiety. The HDS line has a higher level of anxiety and, furthermore, does not display the usual anxiogenic response to intrahippocampal administration of 8-OH-DPAT. It was therefore hypothesized that this line of rat might be a useful model of high trait anxiety with a susceptibility to depression. We thus investigated whether chronic treatment with fluoxetine would result in an anxiolytic effect in the social interaction test in the LDS and HDS lines of rat. In both lines, acute fluoxetine (10 mg/kg) produced an anxiogenic effect in the social interaction test; when rats were tested 24 h after 14 days of fluoxetine treatment there were no anxiolytic effects in either line. In the social interaction test, chronic fluoxetine treatment did not change either the anxiogenic effect of 8-OH-DPAT (100 ng) injected bilaterally into the dorsal hippocampus in the LDS line or the lack of response in the HDS line. In the elevated plus-maze, chronic fluoxetine treatment resulted in a significant anxiogenic effect in the HDS line, but was without effect in the LDS line. Intrahippocampal 8-OH-DPAT was without effect in the plus-maze in either line. These results suggest that chronic treatment with fluoxetine did not modify the hippocampal 5-HT1A receptor in either line. The anxiogenic effects observed in the plus-maze in the HDS line after chronic fluoxetine might relate to line differences in 5-HT1A receptors in other brain regions.