Bagley Elena E, Chieng Billy C H, Christie MacDonald J, Connor Mark
Pain Management Research Institute, E25, Kolling Institute, University of Sydney at Royal North Shore Hospital, St Leonards, Australia.
Br J Pharmacol. 2005 Sep;146(1):68-76. doi: 10.1038/sj.bjp.0706315.
The midbrain periaqueductal gray (PAG) is a major site of opioid analgesic action, and a significant site of cellular adaptations to chronic morphine treatment (CMT). We examined mu-opioid receptor (MOP) regulation of voltage-gated calcium channel currents (I(Ca)) and G-protein-activated K channel currents (GIRK) in PAG neurons from CMT mice. Mice were injected s.c. with 300 mg kg(-1) of morphine base in a slow release emulsion three times over 5 days, or with emulsion alone (vehicles). This protocol produced significant tolerance to the antinociceptive effects of morphine in a test of thermal nociception. Voltage clamp recordings were made of I(Ca) in acutely isolated PAG neurons and GIRK in PAG slices. The MOP agonist DAMGO (Tyr-D-Ala-Gly-N-Me-Phe-Gly-ol enkephalin) inhibited I(Ca) in neurons from CMT mice (230 nM) with a similar potency to vehicle (150 nM), but with a reduced maximal effectiveness (37% inhibition in vehicle neurons, 27% in CMT neurons). Inhibition of I(Ca) by the GABA(B) agonist baclofen was not altered by CMT. Met-enkephalin-activated GIRK currents recorded in PAG slices were significantly smaller in neurons from CMT mice than vehicles, while GIRK currents activated by baclofen were unaltered. These data demonstrate that CMT-induced antinociceptive tolerance is accompanied by homologous reduction in the effectiveness of MOP agonists to inhibit I(Ca) and activate GIRK. Thus, a reduction in MOP number and/or functional coupling to G proteins accompanies the characteristic cellular adaptations to CMT previously described in PAG neurons.
中脑导水管周围灰质（PAG）是阿片类镇痛作用的主要部位，也是细胞对慢性吗啡治疗（CMT）产生适应性变化的重要部位。我们研究了CMT小鼠PAG神经元中μ-阿片受体（MOP）对电压门控钙通道电流（I(Ca)）和G蛋白激活的钾通道电流（GIRK）的调节作用。小鼠在5天内分3次皮下注射300 mg kg(-1)的吗啡碱缓释乳剂，或仅注射乳剂（对照）。该方案在热痛觉测试中使小鼠对吗啡的镇痛作用产生了显著耐受性。对急性分离的PAG神经元的I(Ca)和PAG脑片的GIRK进行电压钳记录。MOP激动剂DAMGO（酪氨酸-D-丙氨酸-甘氨酸-N-甲基苯丙氨酸-甘氨酰胺脑啡肽）抑制CMT小鼠神经元（230 nM）的I(Ca)，其效力与对照（150 nM）相似，但最大效应降低（对照神经元中抑制37%，CMT神经元中抑制27%）。CMT对GABA(B)激动剂巴氯芬抑制I(Ca)的作用无影响。在PAG脑片中记录到的甲硫氨酸脑啡肽激活的GIRK电流在CMT小鼠神经元中明显小于对照，而巴氯芬激活的GIRK电流未改变。这些数据表明，CMT诱导的镇痛耐受性伴随着MOP激动剂抑制I(Ca)和激活GIRK的效力同源性降低。因此，MOP数量的减少和/或与G蛋白的功能偶联减少伴随着PAG神经元中先前描述的对CMT的特征性细胞适应性变化。
