Effects of agonists of melanocortin receptors MC3Rand MC4R on components of sexual behavior of male rats reared in condition of chronic social isolation

BACKGROUND: Melanocortins, including -melanocyte stimulating hormone (MSH), MSH, MSH, and adrenocorticotropic hormone, are products of the polypeptide proopiomelanocortin. These products bind to five different melanocortin receptors (MC1R, MC2R, MC3R, MC4R, MC5R) with different affinities. Because MC3R and MC4R are the major types expressed in the brain, these two receptors can mediate the behavioral effects of melanocortins. AIM: To clear the role of MC4R in sexual behavior of male rats after social isolation from relatives. MATERIALS AND METHODS: In the present study, 40 naive male Wistar rats divided into 4 groups, 3 of which were grown in complete social and partial sensory isolation, received saline, PT-141 (MC3R/MC4R agonist) in a dose 0.3 g intraperitoneally, and THIQ (MC4R agonist) in a dose 0.1 g intranasally. Behavioral effects were registered in a cage with unreachable reinforcement in which the female in the estrus phase, separated by a transparent perforated barrier, was kept for 10 minutes under red light. Blood samples were taken 30 minutes after administration from the tail vein. Testosterone concentrations were measured by ELISA. RESULTS: Social isolation had no significant effect on latent time before trying to reach a female but decreased the time spent near the cage with a female. Isolated animals did not show genital grooming acts during the experiment. PT-141 decreased the latent time to approach the female, increased the time spent near the female cage, and stimulated genital grooming. THIQ decreased the latent time to reach a female and stimulated genital grooming. At the same time, THIQ had no effect on the time spent near the female cage. Social isolation reduced testosterone levels more than twice compared to controls. The administration of PT-141 and THIQ had no effect on testosterone concentration. CONCLUSIONS: The results confirm the depressing effect of chronic social isolation stress on the sexual motivation of male rats and demonstrate different roles of melanocortin MC3R/MC4R receptors in the realization of sexual behavior.