Multiple pharmacological, behavioral, and immunohistological c-Fos mapping approaches were used to characterize the effects of μ-opioid receptor agonists on social behavior and investigate the mechanisms in naive mice and autism spectrum disorder–like (ASD-like) mouse models, such as prenatally valproic acid–treated mice and Fmr1-KO mice. The gene discussed is FMR1; the disease is autism spectrum disorder.