Interaction of 5-HT and HPA axis in depression and treatment-resistant depression

Psychoendocrinology studies of depressed patients focus on the disregulation of the hypothalamic-pituitary-adrenal (HPA) axis. Abnormalities in the HPA axis have been noted in depressed patients. Numerous data have demonstrated the existence of reciprocal interactions between the central serotonin (5-HT) system and HPA axis. These interactions are of particular relevance when considering pathological conditions, such as depression, in which modifications of both the 5-HT system and HPA axis have been evidenced. In our laboratory, we examined the effects of adrenocorticotropic hormone (ACTH) on the immobilization of rats in the forced swim test and on the wet-dog shakes induced by the DOI, 5-HT₂ receptor agonist with the administration of imipramine and lithium. The reduction of immobility, induced by the chronic administration of imipramine for 15 days, was blocked by treatment with ACTH for 14 days. And, chronic ACTH treatment for 14 days increased the wet-dog shake response. This effect of ACTH was not inhibited by a 14-day administration of imipramine. Accordingly, the chronic treatment of rats with ACTH may prove to be an effective model for antidepressant-treatment-resistant depression. We believe that behavioral pharmacological and molecular biological research into the interaction between the 5-HT and HPA axis will elucidate the pathogenesis of depression or antidepressant-treatment-resistant depression and the mechanism of antidepressants action.