Wallace M A, Claro E
Department of Biochemistry, University of Tennessee, Memphis.
J Pharmacol Exp Ther. 1990 Dec;255(3):1296-300.
Stimulation of phosphoinositide-specific phospholipase C (PLC) by muscarinic cholinergic and serotoninergic agonists was measured in rat brain cortical membranes by using exogenously supplied substrates. Serotonin, tryptamine, 5-fluorotryptamine and 5-methyltryptamine stimulated PLC with EC50 values of 1.7, 11.2, 15.0, and 29.4 microM, respectively. Maximal PLC stimulation by serotoninergic agonists, which were all equally efficacious, was about 30% of that attained by carbachol. Ketanserin blocked serotoninergic but not cholinergic activation of PLC, whereas, conversely, atropine blocked the latter but not the former response. The rank order of potency for muscarinic agonists was oxotremorine-M greater than pilocarpine = arecoline greater than carbachol = bethanecol. Unlike the case with tissue slices, all of these muscarinic agonists exhibited full efficacy in this assay of PLC stimulation. Activation of PLC by the neurotransmitters or their analogs was dependent on the addition of guanosine 3'-O-thiotriphosphate (GTP gamma S). Stimulation of PLC by GTP gamma S alone or in combination with 5-methyltryptamine had an apparent EC50 of about 0.4 microM. However, when carbachol or other muscarinic agonists were used, the EC50 for GTP gamma S was significantly lower. We have previously shown that dopamine working through D1 receptors inhibits the PLC response to carbachol by preventing this shift in the apparent EC50 for GTP gamma S. Dopamine did not have a similar effect on 5-methyltryptamine stimulation of PLC. The results indicate that the postreceptor mechanisms of PLC activation are distinct for muscarinic as opposed to serotoninergic agonists in brain cortex.
通过使用外源性提供的底物，在大鼠脑皮质膜中测量了毒蕈碱胆碱能和5-羟色胺能激动剂对磷酸肌醇特异性磷脂酶C（PLC）的刺激作用。5-羟色胺、色胺、5-氟色胺和5-甲基色胺刺激PLC的EC50值分别为1.7、11.2、15.0和29.4微摩尔。所有效力相同的5-羟色胺能激动剂对PLC的最大刺激作用约为卡巴胆碱所达到的最大刺激作用的30%。酮色林阻断5-羟色胺能对PLC的激活，但不阻断胆碱能激活，而阿托品则相反，阻断后者但不阻断前者的反应。毒蕈碱激动剂的效价顺序为：氧化震颤素-M＞毛果芸香碱 = 槟榔碱＞卡巴胆碱 = 氨甲酰甲胆碱。与组织切片的情况不同，所有这些毒蕈碱激动剂在该PLC刺激试验中均表现出完全效力。神经递质或其类似物对PLC的激活依赖于添加鸟苷3'-O-硫代三磷酸（GTPγS）。单独的GTPγS或与5-甲基色胺联合使用对PLC的刺激作用的表观EC50约为0.4微摩尔。然而，当使用卡巴胆碱或其他毒蕈碱激动剂时，GTPγS的EC50显著降低。我们先前已经表明，通过D1受体起作用的多巴胺通过阻止GTPγS表观EC50的这种变化来抑制PLC对卡巴胆碱的反应。多巴胺对5-甲基色胺刺激PLC没有类似作用。结果表明，在脑皮质中，与5-羟色胺能激动剂相反，PLC激活的受体后机制对于毒蕈碱激动剂是不同的。
