Messer W S, Ellerbrock B, Price M, Hoss W
Department of Medicinal Chemistry, College of Pharmacy, University of Toledo, OH 43606.
Biochem Pharmacol. 1989 Mar 1;38(5):837-50. doi: 10.1016/0006-2952(89)90239-6.
The binding of four muscarinic receptor agonists to regions of rat brain was examined through quantitative autoradiographic techniques. Oxotremorine, arecoline, pilocarpine and bethanechol were chosen based on their different potencies and efficacies in muscarinic second messenger systems. Overall, the order of potency for inhibition of [3H]-l-quinuclidinyl benzilate ([3H]-l-QNB) binding to rat brain slices was oxotremorine greater than pilocarpine = arecoline much greater than bethanechol. Regional assays of agonist potency indicated that all agonists were more selective for brainstem and thalamic regions than for hippocampal and cortical regions. The high selectivity of agonists for areas such as the paraventricular thalamus and the superior colliculus, which also display low affinity for pirenzepine, suggests that muscarinic agonists bind with higher affinity to M2 receptors. Of the four agonists examined, pilocarpine displayed the lowest selectivity for M2 receptors in that IC50 values for pilocarpine were only 3-fold higher in the hippocampal and striatal regions (e.g. CA3: 40.6 +/- 9.4 microM) than in thalamic and brainstem regions (e.g. paraventricular thalamus: 14.9 +/- 6.2 microM). Oxotremorine was 8-fold more potent in the brainstem and thalamus, while arecoline and bethanechol were, respectively, 19- and 100-fold more selective for brainstem and thalamic receptors. Scatchard analyses revealed heterogeneous binding profiles for some agonists within single brain regions, suggesting that multiple agonist sites exist even within regions of predominantly M1 or M2 receptors. For example, arecoline displayed curved Scatchard plots within the external layers of the cerebral cortex, layer CA1 of the hippocampus (predominantly M1 subtype), and the paraventricular thalamus (predominantly M2 subtype). The ability of agonists to recognize multiple sites within a single region may reflect the ability to recognize receptors coupled or uncoupled to second messenger systems through G-proteins.
通过定量放射自显影技术研究了四种毒蕈碱受体激动剂与大鼠脑区的结合情况。基于它们在毒蕈碱第二信使系统中不同的效价和效能，选择了氧化震颤素、槟榔碱、毛果芸香碱和氨甲酰甲胆碱。总体而言，抑制[3H]-1-喹核醇基苯甲酸酯（[3H]-1-QNB）与大鼠脑切片结合的效价顺序为：氧化震颤素＞毛果芸香碱 = 槟榔碱＞氨甲酰甲胆碱。激动剂效价的区域分析表明，所有激动剂对脑干和丘脑区域的选择性高于海马和皮质区域。激动剂对室旁丘脑和上丘等区域具有高选择性，这些区域对哌仑西平也显示出低亲和力，这表明毒蕈碱激动剂与M2受体的结合亲和力更高。在所研究的四种激动剂中，毛果芸香碱对M2受体的选择性最低，因为毛果芸香碱在海马和纹状体区域（如CA3：40.6±9.4微摩尔）的IC50值仅比丘脑和脑干区域（如室旁丘脑：14.9±6.2微摩尔）高3倍。氧化震颤素在脑干和丘脑中的效力高8倍，而槟榔碱和氨甲酰甲胆碱对脑干和丘脑受体的选择性分别高19倍和100倍。Scatchard分析揭示了单个脑区内某些激动剂的异质结合曲线，表明即使在主要为M1或M2受体的区域内也存在多个激动剂位点。例如，槟榔碱在大脑皮质外层、海马CA1层（主要为M1亚型）和室旁丘脑（主要为M2亚型）内显示出曲线型Scatchard图。激动剂识别单个区域内多个位点的能力可能反映了其识别通过G蛋白与第二信使系统偶联或未偶联的受体的能力。
