Mode of action and physiological significance of endogenous cannabinoids as a retrograde messenger at central synapses

大麻的活性成分（Δ^9-四氢大麻酚）通过与广泛分布于中枢神经系统的CB1大麻素受体结合，发挥各种精神运动作用。内源性大麻素（eCB）的两个候选物（即CB1受体的内源性配体）是anandamide和2-花生四烯醇甘油。CB1受体存在于中枢神经元的突触前位点，大麻素与该受体的结合导致突触前末端神经递质释放减少。然而，在本研究项目开始时，对神经元的什么刺激可以产生ecb以及ecb在脑功能中起什么作用在很大程度上是未知的。我们利用电生理方法研究了脑脊液在突触传递调节中的作用，得到了以下结果。在海马神经元和小脑浦肯野细胞中，去极化和由此引起的细胞内Ca^<2+>浓度升高导致eCBs的产生，eCBs逆行激活突触前CB1受体并诱导神经递质释放的短暂抑制。gq偶联受体（包括I组代谢性谷氨酸受体和M_1/M_3毒蕈碱乙酰胆碱受体）的激活也诱导了eb介导的神经递质释放的逆行抑制。此外，我们发现，在培养的海马神经元中，弱去极化和M_1/M_3毒蕈碱乙酰胆碱受体的轻度激活有效地诱导了ecb介导的逆行抑制，而单独的任何刺激都不会引起可检测到的抑制。我们发现这种协同作用归因于磷脂酶Css1 （PLCss1）的特性，该酶被Gq/11的一个亚基激活，并且对细胞内Ca^<2+>的生理范围敏感。因此，PLCss1可以作为胆碱能传入活动（即突触前活动）和突触后去极化的重合检测器。

The active component of marijuana (Δ^9-tetrahydrocannabinol) exerts various psychomotor actions through binding to the CB1 cannabinoid receptor that is distributed widely in the central nervous system. Two candidates for endogenous cannabinoid (eCB) (i.e.,endogenous ligands for the CB1 receptor) are anandamide and 2-arachidonoylglycerol. The CB1 receptor is present at presynaptic sites of central neurons and the binding of cannabinoids to this receptor results in reduction of neurotransmitter release from presynaptic terminals. However, at the beginning of the present research project, it was largely unknown what stimuli to neurons could produce eCBs and what roles eCBs played in brain functions. We have examined roles of eCBs in modulation of synaptic transmission by using electrophysiological methods and have obtained the following results.In hippocampal neurons and cerebellar Purkinje cells, depolarization and resultant elevation of intracellular Ca^<2+> concentration causes production of eCBs that retrogradely activates presynapric CB1 receptors and induces transient suppression of neurotransmitter release. Activation of Gq-coupled receptors including group I metabotropic glutamate receptors and M_1/M_3 muscarinic acetylcholine receptors also induces eCB-mediated rretrograde suppression of neurotransmitter release. Furthermore, we have found that, in cutured hippocampal neurons, weak depolarization and mild activation of M_1/M_3 muscarinic acetylcholine receptors effectively induce eCB-mediated retrograde suppression, whereas either stimulus alone causes no detectable suppression. We have disclosed that this synergism is attributable to the property of phospholipase Css1 (PLCss1) that is activated by a subunit of Gq/11 and also sensitive to physiological range of intracellular Ca^<2+>. Therefore, PLCss1 can function as a coincidence detector of cholinergic afferent activity (i.e.,presynaptic activity) and postsynaptic depolarization.