The endocannabinoid (eCB) 2-arachidonoylglycerol (2-AG) produced by diacylglycerol lipase α (DGLα) is one of the best-characterized retrograde messengers at central synapses. It has been thought that 2-AG is produced ‘on demand’ upon activation of postsynaptic neurons. However, recent studies propose that 2-AG is pre-synthesised by DGLα and stored in neurons, and that 2-AG is released from such ‘pre-formed pools’ without participation of DGLα. To address whether the 2-AG source for retrograde signalling is the on-demand biosynthesis by DGLα or the mobilization from preformed pools, we examined the effects of acute pharmacological inhibition of DGL by a novel potent DGL inhibitor, OMDM-188, on retrograde eCB signalling triggered by Ca²⁺ elevation, G_q/11-coupled receptor activation or synergy of these two stimuli in postsynaptic neurons. We found that pretreatment for 1 h with OMDM-188 effectively blocked depolarisation-induced suppression of inhibition (DSI), a purely Ca²⁺-dependent form of eCB signalling, in slices from the hippocampus, striatum and cerebellum. We also found that at parallel fibre-Purkinje cell synapses in the cerebellum OMDM-188 abolished synaptically-induced retrograde eCB signalling, which is known to be caused by the synergy of postsynaptic Ca²⁺ elevation and group I metabotropic glutamate receptor (I-mGluR) activation. Moreover, brief OMDM-188 treatments for several minutes were sufficient to suppress both DSI and the I-mGluR-induced retrograde eCB signalling in cultured hippocampal neurons. These results are consistent with the hypothesis that 2-AG for synaptic retrograde signalling is supplied by on demand biosynthesis by DGLα rather than mobilization from presumptive pre-formed pools.
PMID: 23858009 [PubMed – as supplied by publisher]