Fox Philip D, Hentges Shane T
Department of Biomedical Sciences, Colorado State University, Fort Collins, Colorado 80523.
Department of Biomedical Sciences, Colorado State University, Fort Collins, Colorado 80523
J Neurosci. 2017 Sep 6;37(36):8667-8677. doi: 10.1523/JNEUROSCI.1030-17.2017. Epub 2017 Aug 7.
Activation of somatic μ-opioid receptors (MORs) in hypothalamic proopiomelanocortin (POMC) neurons leads to the activation of G-protein-coupled inward rectifier potassium (GIRK) channels and hyperpolarization, but in response to continued signaling MORs undergo acute desensitization resulting in robust reduction in the peak GIRK current after minutes of agonist exposure. We hypothesized that the attenuation of the GIRK current would lead to a recovery of neuronal excitability whereby desensitization of the receptor would lead to a new steady state of POMC neuron activity reflecting the sustained GIRK current observed after the initial decline from peak with continued agonist exposure. However, electrophysiologic recordings and GCaMP6f Ca imaging in POMC neurons in mouse brain slices indicate that maximal inhibition of cellular activity by these measures can be maintained after the GIRK current declines. Blockade of the GIRK current by Ba or Tertiapin-Q did not disrupt the sustained inhibition of Ca transients in the continued presence of agonist, indicating the activation of an effector other than GIRK channels. Use of an irreversible MOR antagonist and Furchgott analysis revealed a low receptor reserve for the activation of GIRK channels but a >90% receptor reserve for the inhibition of Ca events. Altogether, the data show that somatodendritic MORs in POMC neurons inhibit neuronal activity through at least two effectors with distinct levels of receptor reserve and that differentially reflect receptor desensitization. Thus, in POMC cells, the decline in the GIRK current during prolonged MOR agonist exposure does not reflect an increase in cellular activity as expected. Desensitization of the μ-opioid receptor (MOR) is thought to underlie the development of cellular tolerance to opiate therapy. The present studies focused on MOR desensitization in hypothalamic proopiomelanocortin (POMC) neurons as these neurons produce the endogenous opioid β-endorphin and are heavily regulated by opioids. Prolonged activation of somatic MORs in POMC neurons robustly inhibited action potential firing and Ca activity despite desensitization of the MOR and reduced activation of a potassium current over the same time course. The data show that somatic MORs in POMC neurons couple to multiple effectors that have differential sensitivity to desensitization of the receptor. Thus, in these cells, the cellular consequence of MOR desensitization cannot be defined by the activity of a single effector system.
下丘脑促阿片黑素细胞皮质素（POMC）神经元中体细胞膜μ-阿片受体（MORs）的激活会导致G蛋白偶联内向整流钾（GIRK）通道的激活和超极化，但在持续信号作用下，MORs会发生急性脱敏，导致激动剂暴露数分钟后GIRK电流峰值大幅降低。我们推测，GIRK电流的衰减会导致神经元兴奋性的恢复，即受体脱敏会导致POMC神经元活动达到新的稳态，这反映了在持续激动剂暴露下，GIRK电流从峰值开始最初下降后所观察到的持续状态。然而，对小鼠脑片POMC神经元的电生理记录和GCaMP6f钙成像表明，在GIRK电流下降后，这些测量方法对细胞活动的最大抑制作用仍可维持。在持续存在激动剂的情况下，用钡或替尔泊肽-Q阻断GIRK电流并不会破坏对钙瞬变的持续抑制，这表明除GIRK通道外，还有其他效应器被激活。使用不可逆的MOR拮抗剂和弗奇戈特分析显示，激活GIRK通道的受体储备较低，但抑制钙事件的受体储备>90%。总之，数据表明，POMC神经元中的树突体MORs通过至少两种对受体脱敏具有不同敏感性的效应器来抑制神经元活动，并且不同程度地反映受体脱敏。因此，在POMC细胞中，长时间MOR激动剂暴露期间GIRK电流的下降并不像预期的那样反映细胞活动的增加。μ-阿片受体（MOR）的脱敏被认为是对阿片类药物治疗产生细胞耐受性的基础。目前的研究集中在下丘脑促阿片黑素细胞皮质素（POMC）神经元中的MOR脱敏，因为这些神经元会产生内源性阿片β-内啡肽，并且受到阿片类药物的严格调控。尽管MOR脱敏且钾电流在同一时间过程中激活减少，但POMC神经元中体细胞膜MOR的长时间激活仍能强烈抑制动作电位发放和钙活动。数据表明，POMC神经元中的体细胞膜MOR与多种对受体脱敏具有不同敏感性的效应器偶联。因此，在这些细胞中，MOR脱敏的细胞后果不能由单一效应器系统的活动来定义。
