Depressive-like profile induced by MCH microinjections into the dorsal raphe nucleus evaluated in the forced swim test.

Antagonism of the melanin-concentrating hormone (MCH) receptor 1 (MCH-R1) has been recently shown to have antidepressant-like profile in rats. However, the mechanisms by which the MCHergic system participates in the modulation of emotional states are still to be determined. In the present study we confirmed the presence of MCHergic fibers within the dorsal raphe nucleus (DRN), a serotonergic nucleus involved in the physiopathology of major depression. We also assessed the effects of the administration of MCH and anti-MCH antibody (immunoneutralization) into the DRN using the forced swim test in rats, an animal model to screen antidepressant drugs. We found that a low dose of MCH (50 ng) evoked a depressive-like behavior indicated by a significant increase in the immobility time as well as a decrease in climbing behavior. Furthermore, the depressive-like response was prevented by pretreatment with fluoxetine. Consistent with these results, the immunoneutralization of MCH produced an antidepressant-like effect. By means of the open field test we discarded that these effects were related to unspecific changes in motor activity. Our results suggest that the MCHergic neurons are involved in the regulation of emotional behaviors through the modulation of the serotonergic neuronal activity within the DRN. In addition, the present results are in agreement with previous reports showing that antagonism of the MCHergic system may be a novel therapeutic strategy for the treatment of depressive disorders.