Postoperative shivering and cold hypersensitivity are major side effects of acute and chronic opioid treatments respectively. TRPM8 is a cold and menthol-sensitive channel found in a subset of dorsal root ganglion (DRG) nociceptors. Deletion or inhibition of the TRPM8 channel was found to prevent the cold hyperalgesia induced by chronic administration of morphine. Here, we examined the mechanisms by which morphine was able to promote cold hypersensitivity in DRG neurons and transfected HEK cells. Mice daily injected with morphine for 5 days developed cold hyperalgesia. Treatment with morphine did not alter the expressions of cold sensitive TREK-1, TRAAK and TRPM8 in DRGs. However, TRPM8-expressing DRG neurons isolated from morphine-treated mice exhibited hyperexcitability. Sustained morphine treatment in vitro sensitized TRPM8 responsiveness to cold or menthol and reduced activation-evoked desensitization of the channel. Blocking phospholipase C (PLC) as well as protein kinase C beta (PKCβ), but not protein kinase A (PKA) or Rho-associated protein kinase (ROCK), restored channel desensitization. Identification of two PKC phosphorylation consensus sites, S1040 and S1041, in the TRPM8 and their site-directed mutation were able to prevent the MOR-induced reduction in TRPM8 desensitization. Our results show that activation of MOR by morphine 1) promotes hyperexcitability of TRPM8-expressing neurons and 2) induces a PKCβ-mediated reduction of TRPM8 desensitization. This MOR-PKCβ dependent modulation of TRPM8 may underlie the onset of cold hyperalgesia caused by repeated administration of morphine. Our findings point to TRPM8 channel and PKCβ as important targets for opioid-induced cold hypersensitivity.

中文翻译：

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慢性吗啡通过MOR-PKCβ信号传导调节TRPM8通道。

术后颤抖和感冒超敏反应分别是急性和慢性阿片类药物治疗的主要副作用。TRPM8是在背根神经节（DRG）伤害感受器的子集中发现的一种对薄荷醇敏感的冷通道。发现删除或抑制TRPM8通道可防止长期服用吗啡引起的冷痛觉过敏。在这里，我们检查了吗啡能够促进DRG神经元和转染的HEK细胞冷超敏反应的机制。每天注射吗啡5天的小鼠会出现冷痛觉过敏。吗啡治疗并未改变DRGs中冷敏感的TREK-1，TRAAK和TRPM8的表达。但是，从吗啡治疗的小鼠中分离出表达TRPM8的DRG神经元表现出超兴奋性。体外吗啡持续治疗可使TRPM8对感冒或薄荷醇的反应敏感，并减少了通道引起的激活诱发的脱敏。阻断磷脂酶C（PLC）以及蛋白激酶C beta（PKCβ），但不阻断蛋白激酶A（PKA）或Rho相关蛋白激酶（ROCK），可恢复通道脱敏。在TRPM8中鉴定两个PKC磷酸化共有位点S1040和S1041及其定点突变能够防止MOR诱导的TRPM8脱敏减少。我们的结果表明吗啡对MOR的激活1）促进表达TRPM8的神经元的过度兴奋性，2）诱导PKCβ介导的TRPM8脱敏减少。TRPM8的这种MOR-PKCβ依赖性调节可能是由于反复服用吗啡引起的冷痛觉过敏的起因。