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.

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Endocannabinoids like 2-arachidonoylglycerol strongly modulate the complex machinery of secondary neuronal damage and are shown to improve neuronal survival after excitotoxic lesion. Palmitoylethanolamide (PEA), the naturally occurring fatty acid amide of ethanolamine and palmitic acid, is an endogenous lipid known to mimic several effects of endocannabinoids even without binding to cannabinoid receptors. Here we show that PEA (0.001-1 μM) and the synthetic peroxisome proliferator-activated receptor (PPAR)-alpha agonist 4-chloro-6-(2,3-xylidino)-2-pyrimidinylthio acetic acid (Wy-14,643; 0.1-1 μM) reduced the number of microglial cells and protected dentate gyrus granule cells in excitotoxically lesioned organotypic hippocampal slice cultures (OHSCs). Treatment with the PPAR-alpha antagonist N-((2S)-2-(((1Z)-1-Methyl-3-oxo-3-(4-(trifluoromethyl)phenyl)prop-1-enyl)amino)-3-(4-(2-(5-methyl-2-phenyl-1,3-oxazol-4-yl)ethoxy)phenyl)propyl)propanamide (GW6471; 0.05-5 μM) blocked PEA-mediated neuroprotection and reduction of microglial cell numbers whereas the PPAR-gamma antagonist 2-chloro-5-nitro-N-phenyl-benzamide (GW9662; 0.01-1 μM) showed no effects. Immunocytochemistry and Western blot analyses revealed a strong PPAR-alpha immunoreaction in BV-2 microglial cells and in HT22 hippocampal cells. Intensity and location of PPAR-alpha immunoreaction remained constant during stimulation with PEA (0.01 μM; 1-36 h). In conclusion our data provide evidence that (1) PEA counteracted excitotoxically induced secondary neuronal damage of dentate gyrus granule cells, (2) PPAR-alpha but not PPAR-gamma is the endogenous binding site for PEA-mediated neuroprotection, and (3) PEA may activate PPAR-alpha in microglial cells and hippocampal neurons to exert its neuroprotective effects. In addition to classical endocannabinoids, PEA-mediated PPAR-alpha activation represents a possible target for therapeutic interventions to mitigate symptoms of secondary neuronal damage.

Cannabinoids regulate numerous physiological and pathological events like inflammation or neurodegeneration via CB(1) and CB(2) receptors. The mechanisms behind cannabinoid effects show a high variability and may also involve transient receptor potential channels (TRP) and N-type voltage-gated Ca(2+) channels (Ca(v) 2.2). In the present study we investigated the neuroprotective effects of the synthetic cannabinoid WIN 55,212-2 (WIN) on dentate gyrus (DG) granule cells and elucidated the involvement of TRP and Ca(v) 2.2 that are shown to participate in inflammatory processes. Organotypic hippocampal slice cultures were excitotoxically lesioned using NMDA and subsequently incubated with different WIN concentrations (0.001-10 μM). WIN showed neuroprotective properties in an inverse concentration-dependent manner, most effectively at 0.01 μM. The CB(1) receptor antagonist AM251 blocked neuroprotection mediated by WIN whereas the CB(2) receptor antagonist AM630 showed no effects. Application of the TRPA1 blocker HC-030031 enhanced the neuroprotective efficacy of high (10 μM) WIN concentrations and the number of degenerating neurons became equal to that seen after application of the most effective WIN dose (0.01 μM). In contrast, the application of TRPA1 agonist icilin or allyl isothiocyanate (AITC) led to a stronger neurodegeneration. The use of TRPV1 blocker 6-iodo-nordihydrocapsaicin did not affect WIN-mediated neuroprotection. The selective Ca(v) 2.2 blocker ω-conotoxin (GVIA) completely blocked neuroprotection shown by 10 μM WIN. GVIA and HC-030031 exerted no effects at WIN concentrations lower than 10 μM. Our data show that WIN protects dentate gyrus granule cells in a concentration dependent manner by acting upon CB(1) receptors. At high (10 μM) concentrations WIN additionally activates TRPA1 and Ca(v) 2.2 within the hippocampal formation that both interfere with CB(1) receptor-mediated neuroprotection. This leads to the conclusion that physiological and pharmacological effects of cannabinoids strongly depend on their concentration and the neuroprotective efficacy of cannabinoids may be determined by interaction of activated CB(1) receptor, TRPA1, and Ca(v) 2.2.