Therapeutic Potential of Low-Dose Opioids Through Brain Region-Specific Neural Activation for Autism Spectrum Disorder

Background Social behaviors are essential for the health and well-being of individuals, and deficits in these behaviors are key features of neuropsychiatric disorders such as autism spectrum disorder (ASD). The opioid system, which includes the μ-opioid receptor (MOR), plays an important role in regulating a range of brain functions, including social behaviors and pain perception. However, its therapeutic potential for improving social functioning in ASD remains unclear. Aims & Objectives In this study, we aimed to elucidate the pharmacological effects of MOR agonists, morphine and buprenorphine, on social behavior in a dose-dependent manner and to explore the underlying neural mechanisms in both naive mice and ASD-like mice induced by prenatal exposure to valproic acid (VPA). Method All experiments were performed in accordance with the NIH Guide for the Care and Use of Laboratory Animals. C57BL/6J wild-type mice were used for experiments under naive conditions, and CD-1 mice were used for the VPA-induced ASD-like mouse model, in which 500 mg/kg VPA was administered intraperitoneally on embryonic day 12.5. Results We examined the effects of morphine on social behavior in naive mice using the single-chamber social interaction test (SIT). Low-dose morphine (0.03 mg/kg) significantly increased the time spent in the interaction zone, whereas high-dose morphine (5 mg/kg) significantly decreased the time. We investigated brain regions involved in the differential effects of morphine on social behavior using c-Fos immunostaining. Both low and high doses of morphine activated the nucleus accumbens (NAc) and medial prefrontal cortex (mPFC), brain areas that have a high density of MORs and are critical for social behavior. High-dose morphine also activated the ventral tegmental area (VTA) and dorsomedial periaqueductal gray (dmPAG), regions involved in addiction and analgesia, while low-dose morphine did not. Notably, activation of the dmPAG has been reported to suppress social behavior, and local administration of morphine to the dmPAG blocked the increased social behavior induced by systemic administration of low-dose morphine. We then investigated the effects of a wide range of morphine doses in the reciprocal SIT in VPA model. Low doses of morphine (0.03 and 0.1 mg/kg), but not high doses (0.3, 1, or 3 mg/kg), improved reduced social behavior without analgesic effects. In addition, buprenorphine (3 μg/kg, but not 30 μg/kg), a μ-opioid receptor partial agonist with less abuse liability and a well-established safety profile, also improved social behavior deficits in the VPA model. Furthermore, similar to morphine, buprenorphine activated the NAc and mPFC at both low and high doses but activated the VTA and dmPAG only at the high dose. Discussion & Conclusions These results show that MOR agonists promote social behavior by activating the NAc and mPFC at low doses in both naive mice and the ASD model. However, the effects on social behavior are abolished by activation of the dmPAG at high doses. Thus, there are dose-dependent differences in the pharmacological effects of opioids on social behavior, and it is not necessary to maximally stimulate the MORs to promote social behavior, but only partial stimulation of the MORs is sufficient.