Objective:?To explore the mechanism by which Melanotan-II (MT-II) improves social deficits in a Shank3 gene-deficient autism model. Methods:?A Shank3-deficient rat model (n=20) and a control group (n=20) were established by microinjecting Shank3-interfering lentivirus or empty lentivirus, respectively, into the right lateral ventricle of neonatal rats. The Shank3-deficient rats were randomly divided into two groups: the Shank3+Saline (Sh3-Sal) group (n=9) and the Shank3+MT-II (Sh3-MT-II) group (n=9). Similarly, the control rats were divided into the Control+Saline (V-Sal) group (n=9) and the Control+MT-II (V-MT-II) group (n=9). On day 28, the V-MT-II and Sh3-MT-II groups received intraperitoneal (i.p.) injections of MT-II (3.3 mg/kg), while the V-Sal and Sh3-Sal groups received i.p. saline (3.3 ml/kg). Behavioral changes were assessed using the open field test, grooming behavior analysis, the three-chamber social test, and the Morris water maze test. The mRNA and protein expression levels of hypothalamic oxytocin (OXT), oxytocin receptor (OXTR), and melanocortin receptor 4 (MC4R) were detected by reverse transcription polymerase chain reaction (RT-PCR) and Western blot, respectively. Results:?In the three-chamber social test, compared to the time spent with stranger rat 1, the Sh3-Sal group showed no significant social preference (P > 0.05). In contrast, after MT-II intervention, the Sh3-MT-II group spent significantly more time with stranger rat 2 (P < 0.01). In the Morris water maze test, the Sh3-Sal group exhibited significant learning and memory impairments compared to the V-Sal group (P < 0.05). MT-II intervention significantly improved the learning and memory performance of the Sh3-MT-II group (P < 0.01). The open field and grooming tests revealed that compared to the V-Sal group, the Sh3-Sal group spent significantly more time in the peripheral zone of the open field and exhibited increased grooming behavior (P < 0.01). However, MT-II intervention did not significantly alter the center time or self-grooming behavior compared to the Sh3-Sal group (P > 0.05). RT-PCR analysis showed that the mRNA expression levels of OXT, OXTR, and MC4R in the Sh3-MT-II group were significantly higher than those in the Sh3-Sal group (P < 0.05). Western blot analysis indicated that hypothalamic OXT protein expression was significantly increased in the Sh3-MT-II group compared to the Sh3-Sal group (P < 0.05). Compared to the V-Sal group, hypothalamic SHANK3 protein expression was significantly decreased in both the Sh3-Sal and Sh3-MT-II groups (P < 0.05), while the protein expression levels of OXTR and MC4R showed no significant changes (P > 0.05). Conclusion:?The melanocortin receptor agonist MT-II may ameliorate social deficits in Shank3-deficient autistic rats by activating the hypothalamic OXT system. This finding suggests that targeting the OXT/MC4R pathway could be a potential therapeutic strategy for social deficits in autism spectrum disorder.