Effects of facilitated desensitization of µ-opioid receptors on analgesic and respiratory depressive tolerance development in vivo

阿片类药物是治疗剧烈疼痛最有效的药物，但其长期临床应用受到止痛耐受性和依赖性的发展的限制。奇怪的是，其他阿片类药物的作用，如呼吸抑制或便秘，几乎没有耐受性。耐受性形成的第一个也是最关键的生化步骤是激动剂诱导µ-阿片受体(MOP)的C端Ser/Thr残基的磷酸化，导致该受体的脱敏。使用一种新开发的技术来显示大脑中磷酸化的MOP受体，我们意外地观察到，在阿片类药物暴露后，MOP的磷酸化并不是无处不在的。事实上，在MOP的表达和磷酸化之间发现了显著的不匹配，在大脑中一些特别富含MOP的区域，如伏隔核和纹状体，可检测到的MOP的磷酸化水平非常低。这导致了一种假说，即在这些脑区，特别是G蛋白偶联受体激酶(GRK)中“缺乏激酶”。我们推测，呼吸抑制或便秘的耐受性有限可能是由于缺乏GRKs的共表达导致MOP脱敏失败所致。为了验证这一假说，我们建议建立内源性MOP受体与GRK2或GRK5共表达的小鼠模型，已知这两种受体以不同的方式促进高效或低效阿片类药物诱导的MOP磷酸化。我们推测，在这些小鼠中，所有的MOP受体都将经历激动剂诱导的磷酸化，从而脱敏，不仅导致对止痛作用的耐受性增加，而且在反复阿片类药物暴露后还会导致呼吸抑制和便秘。易化的MOP脱敏也可能减少阿片类药物在这些小鼠中的成瘾特性。增加对阿片类药物的止痛和呼吸抑制作用的耐受性在临床上是可取的，因为阿片类药物剂量的增加不会受到潜在致命呼吸停止的限制。拟议的项目旨在提供重要的数据来支持这样的临床策略，并可能刺激对作为一种新的治疗概念的调节GRK表达或活性的药物的探索。

Opioids are the most effective drugs for treating severe pain, but their long-term clinical use is limited by the development of analgesic tolerance and dependence. Curiously, other opioid effects such as respiratory depression or constipation develop little or no tolerance. The first and most critical biochemical step in tolerance development is agonist-induced phosphorylation of the C-terminal Ser/Thr residues of the µ-opioid receptor (MOP), leading to desensitization of the receptor. Using a newly developed technique to visualize phosphorylated MOP receptors in the brain, we made the unexpected observation that MOP phosphorylation does not occur ubiquitously after opioid exposure. In fact, a significant mismatch between MOP expression and phosphorylation was found, with detectable MOP phosphorylation strikingly low in some brain regions particularly rich in MOP expression, such as the nucleus accumbens and striatum. This led to the hypothesis of "lack of kinases" in these brain regions, particularly G protein-coupled receptor kinases (GRK). We hypothesize that limited tolerance development in respiratory depression or constipation may be due to a failure of MOP desensitization because of the lack of co-expression of GRKs. To test this hypothesis, we propose to generate mouse models that co-express endogenous MOP receptors with either GRK2 or GRK5, which are known to contribute in different ways to MOP phosphorylation induced by high- or low-efficacy opioids. We hypothesize that in these mice, all MOP receptors will undergo agonist-induced phosphorylation and thus desensitization, leading to increased tolerance not only to analgesic effects but also to respiratory depression and constipation after repeated opioid exposure. It is also possible that facilitated MOP desensitization reduces the addictive properties of opioids in these mice. Increased tolerance to analgesic and respiratory depressant effects of opioids would be clinically desirable, as opioid dose escalation would not be limited by potentially fatal respiratory arrest. The proposed project aims to provide important data to support such a clinical strategy and may stimulate the exploration of drugs that modulate GRK expression or activity as a novel therapeutic concept.