The median raphe nucleus participates in the depressive-like behavior induced by MCH: Differences with the dorsal raphe nucleus

• We investigated a putative MCHergic modulation on the median raphe nucleus (MnR).
• Behavioral changes induced by MCH intra-MnR were determined.
• A large density of MCHergic fibers was detected in MnR.
• MCH directly microinjected into MnR induced a depressive-like effect.
• MnR participates in the MCHergic control of affective-related behavioral responses.

An emerging body of evidence involves the hypothalamic neuropeptide melanin-concentrating hormone (MCH) in the regulation of emotional states. We have reported a pro-depressive effect induced by MCH after its microinjection into the dorsal raphe nucleus (DR) evaluated in the forced swimming test (FST) in rats. Here we extended this study to the median raphe nucleus (MnR). Firstly, the presence of MCH-containing fibers in the rat MnR was analyzed by means of immunohistochemistry. Secondly, the behavioral effect induced by the microinjection of MCH into the MnR was assessed using the FST. Morphological results showed a large density of MCHergic fibers within the MnR. Behavioral results indicated that 100 ng of MCH (but not 50 ng) significantly increased the immobility time and decreased the swimming time, demonstrating a depressive-like effect. In contrast, climbing behavior was not significantly affected. Present findings revealed that the MnR neurons participate in the MCHergic control of affective-related behavioral responses. However, the behavioral patterns induced by MCH in the MnR and DR were different. This could be explained by anatomical and physiological differences between both nuclei.