2-Arachidonoylglycerol elicits neuroprotective effects on excitotoxically lesioned dentate gyrus granule cells via abnormal-cannabidiol-sensitive receptors on microglial cells.

Endocannabinoids like 2-arachidonoylglycerol (2-AG) exert neuroprotective effects after brain injuries. According to current concepts, these neuroprotective effects are due to interactions between 2-AG and cannabinoid (CB)1 receptors on neurons. Moreover, 2-AG modulates migration and proliferation of microglial cells which are rapidly activated after brain lesion. This effect is mediated via CB2- and abnormal-cannabidiol (abn-CBD)-sensitive receptors. In the present study, we investigated whether the abn-CBD-sensitive receptor on microglial cells contributes to 2-AG-mediated neuroprotection in organotypic hippocampal slice cultures (OHSCs) after excitotoxic lesion induced by NMDA (50 microM) application for 4 h. This lesion caused neuronal damage and accumulation of microglial cells within the granule cell layer. To analyze the role of abn-CBD-sensitive receptors for neuroprotection and microglial cell accumulation, two agonists of the abn-CBD-sensitive receptor, abn-CBD or 2-AG, two antagonists, 1,3-dimethoxy-5-methyl-2-[(1R,6R)-3-methyl-6-(1-methylethenyl)-2-cyclohexen1-yl]-benzene (O-1918) or cannabidiol (CBD), and the CB1 receptor antagonist AM251, were applied to NMDA-lesioned OHSC. Propidium iodide (PI) labeling was used as a marker of degenerating neurons and isolectin B(4) (IB(4)) as a marker of microglial cells. Application of both, abn-CBD or 2-AG to lesioned OHSC significantly decreased the number of IB(4)(+) microglial cells and PI(+) neurons in the dentate gyrus. In contrast to AM251, application of O-1918 or CBD antagonized these effects. When microglial cells were depleted by preincubation of OHSC with the bisphosphonate clodronate (100 microg/mL) for 5 days before excitotoxic lesion, 2-AG and abn-CBD lost their neuroprotective effects. We therefore propose that the endocannabinoid 2-AG exerts its neuroprotective effects via activation of abn-CBD-sensitive receptors on microglial cells.