Effect of conditioned fear stress on serotonin metabolism in the rat brain

The effects of electric foot-shock stress (EFS) and conditioned fear stress (CFS) on serotonin [5-hydroxytryptamine (5-HT)] metabolism in seven various brain regions of the rat were studied by measuring tryptophan, 5-HT and 5-hydroxyindoleacetic acid (5-HIAA). EFS for 30 min increased tryptophan levels in almost all brain regions but did not change 5-HT levels in any regions. It increased 5-HIAA levels in the medial prefrontal cortex (mPFC), nucleus accumbens, and amygdala. CFS (exposure to an environment paired previously with foot-shock) increased defecation and induced freezing behavior. It failed to change tryptophan and 5-HT levels in any brain regions but increased 5-HIAA level only in the mPFC. In contrast to EFS, which increased 5-HT metabolism in several other brain regions, increased metabolism of 5-HT was especially marked in the mPFC after CFS, regarded as psychological stress.