Indole-pyruvic acid, a tryptophan ketoanalogue, antagonizes the endocrine but not the behavioral effects of repeated stress in a model of depression

Abstract

Increased glucocorticoid secretion is frequent in mood disorders and is normalized by long-term antidepressant therapy. Many antidepressants act by increasing central serotonin transmission. We investigated the effects of a serotonin precursor, indole-pyruvic acid (IPA), in an animal model of depression based on repeated exposure to unpredictable stress. Rats were divided in groups, and IPA (20 mg/kg), the tricyclic antidepressant imipramine (IMI) (5 mg/kg), or vehicle was administered daily during 3 weeks of repeated exposure to various stressors according to the procedure described by Katz et al [Katz RJ, Roth KA, Carroll BJ (1981): Neurosci Biobehav Rev 5:247–251]. After treatment, rats were evaluated for stress-induced exploratory behavior and killed 24 hr later. Serum corticosterone levels and glucocorticoid receptor (GR) immunoreactivity (IR) in the nuclei of neurons located in the hippocampal subregion CA1 were also measured. Rats exposed to repeated stress showed a lower exploratory behavior score ( p < 0.01), higher basal corticosterone levels ( p < 0.01), and stronger GR IR in the hippocampus ( p < 0.05) than control rats. All of these effects were antagonized by IMI treatment. IPA administration did not affect the behavioral response induced by repeated stress ( p < 0.01) but normalized serum corticosterone levels. In addition, IPA treatment produced a decrease in GR IR ( p < 0.05 versus control group) that was not modified by exposure to repeated stress. Differences in the drug effects suggest that (1) the dose of IPA used for treatment was not sufficient to produce behavior effects; (2) a side product of IPA in vivo transformation, kynurenic acid, may have affected glutamate transmission, interacting with the behavioral outcomes; (3) the serotonin precursor IPA and the uptake blocker IMI may have produced different adaptive changes in hippocampal serotonin transmission, as indicated by nuclear GR IR measurements.