Anxiety but not arousal increases 5-HT release in the rat ventral hippocampus in vivo

Abstract

Central serotonin (5-HT) is involved in the aetiology of depression and anxiety-disorders. Exposure of rats to tests of anxiety increases cortical and hippocampal 5-HT release determined by in vivo microdialysis. To discriminate whether this increase is caused by aversive conditions of a test for anxiety or by a unconditioned stressor evoking mainly arousal, we investigated the effects of an unconditioned acoustic stimulus and exposure to the elevated plus maze (X-maze), respectively, on 5 HT release in the ventral hippocampus compared to 5-HT release in the home cage and in a non-aversive unfamiliar environment in freely moving rats. Our results showed distinct pattern of 5-HT release in the ventral hippocampus depending on the stimulus used. Exposure to the X-maze for 20 minutes accompanied by an ‘anxious’ behaviour in the rats and increased extracellular 5-HT to 165% of basal release. Exposure to a less aversive ‘deactivated’ plus maze (115%) or to white noise for 20 minutes in the familiar surroundings of the home cage (98%) did not change hippocampal 5-HT release significantly, despite similar behavioural activation indicated by increased locomotor activity. While both, X-maze and white noise, may model anxiety and stress to a certain extent, it seems that the X-maze is more aversive. The results suggest a close relationship between anxiety-related behaviour, but not between arousal/non-specific behavioural activation, and 5-HT release in the ventral hippocampus.