Wang Xiao-Fei, Barbier Elisabeth, Chiu Yi-Ting, He Yi, Zhan Jia, Bi Guo-Hua, Zhang Hai-Ying, Feng Bo, Liu-Chen Lee-Yuan, Wang Jia Bei, Xi Zheng-Xiong
Molecular Targets and Medications Discovery Branch, National Institute on Drug Abuse, Intramural Research Program, Baltimore, Maryland 21224, Beijing Institute of Pharmacology and Toxicology, Beijing 100850, China.
Department of Pharmaceutical Sciences, School of Pharmacy, University of Maryland, Baltimore, Maryland 21201.
J Neurosci. 2016 Oct 5;36(40):10392-10403. doi: 10.1523/JNEUROSCI.0603-16.2016.
The etiology and pathophysiology underlying opioid tolerance and dependence are still unknown. Because mu opioid receptor (MOR) plays an essential role in opioid action, many vulnerability-related studies have focused on single nucleotide polymorphisms of MOR, particularly on A118G. In this study, we found that a single-point mutation at the MOR T394 phosphorylation site could be another important susceptive factor in the development of opioid tolerance and dependence in mice. T394A mutation, in which a threonine at 394 was replaced by an alanine, did not alter agonist binding to MOR and opioid analgesia, but resulted in loss of etorphine-induced MOR internalization in spinal dorsal horn neurons and opioid analgesic tolerance induced by either morphine or etorphine. In addition, this mutation also caused an increase in intravenous heroin self-administration and in nucleus accumbens dopamine response to heroin. These findings suggest that T394 phosphorylation following MOR activation causes MOR internalization and desensitization, which subsequently contributes to the development of tolerance in both opioid analgesia and opioid reward. Accordingly, T394A mutation blocks opioid tolerance and leads to an increase in brain dopamine response to opioids and in opioid-taking behavior. Thus, the T394 may serve as a new drug target for modulating opioid tolerance and the development of opioid abuse and addiction.
The mechanisms underlying opioid tolerance and susceptibility to opioid addiction remain unclear. The present studies demonstrate that a single-point mutation at the T394 phosphorylation site in the C-terminal of mu opioid receptor (MOR) results in loss of opioid tolerance and enhanced vulnerability to heroin self-administration. These findings suggest that modulation of the MOR-T394 phosphorylation or dephosphorylation status may have therapeutic potential in management of pain, opioid tolerance, and opioid abuse and addiction. Accordingly, MOR-T394 mutation or polymorphisms could be a risk factor in developing opioid abuse and addiction and therefore be used as a new biomarker in prediction and prevention of opioid abuse and addiction.
阿片类药物耐受性和依赖性的病因及病理生理学仍不清楚。由于μ阿片受体（MOR）在阿片类药物作用中起关键作用，许多与易感性相关的研究都集中在MOR的单核苷酸多态性上，尤其是A118G。在本研究中，我们发现MOR T394磷酸化位点的单点突变可能是小鼠阿片类药物耐受性和依赖性发展中的另一个重要易感因素。T394A突变，即394位的苏氨酸被丙氨酸取代，并未改变激动剂与MOR的结合及阿片类药物镇痛作用，但导致脊髓背角神经元中埃托啡诱导的MOR内化丧失以及吗啡或埃托啡诱导的阿片类药物镇痛耐受性丧失。此外，该突变还导致静脉注射海洛因自我给药增加以及伏隔核多巴胺对海洛因的反应增强。这些发现表明，MOR激活后T394磷酸化导致MOR内化和脱敏，这随后促成了阿片类药物镇痛和阿片类药物奖赏耐受性的发展。因此，T394A突变阻断阿片类药物耐受性，并导致大脑多巴胺对阿片类药物的反应增加以及阿片类药物摄取行为增加。因此，T394可能作为调节阿片类药物耐受性以及阿片类药物滥用和成瘾发展的新药物靶点。
阿片类药物耐受性及对阿片类药物成瘾易感性的潜在机制仍不清楚。目前的研究表明，μ阿片受体（MOR）C末端T394磷酸化位点的单点突变导致阿片类药物耐受性丧失以及对海洛因自我给药的易感性增强。这些发现表明，调节MOR - T394磷酸化或去磷酸化状态可能在疼痛管理、阿片类药物耐受性以及阿片类药物滥用和成瘾方面具有治疗潜力。因此，MOR - T394突变或多态性可能是阿片类药物滥用和成瘾发展的危险因素，因此可作为预测和预防阿片类药物滥用和成瘾的新生物标志物。
