Postsynaptic diacylglycerol lipase α mediates retrograde endocannabinoid suppression of inhibition in mouse prefrontal cortex

Abstract

Non-technical summary In multiple brain regions, endogenous cannabinoids suppress inhibitory synaptic transmission; however, the biochemical/molecular pathways for endocannabinoid synthesis are poorly understood. Endocannabinoid signalling may be crucial for microcircuit function in the prefrontal cortex (PFC), a cortical region involved in complex behaviours. However, endocannabinoid signalling remains largely unexplored in the PFC. Using enzymatic inhibitors, we show that modulation of inhibitory synaptic transmission in PFC neurons is mediated by the endocannabinoid 2-arachidonoylglycerol synthesized postsynaptically. Interestingly, diacylglycerol lipase (DAGL), the 2-arachidonoylglycerol synthesis enzyme, has two isoforms: DAGLα and DAGLβ. Studying PFC neurons from DAGLα −/− , DAGLβ −/− and wild-type mice, we show that only DAGLα is involved in the suppression of inhibitory transmission in the PFC.Abstract Depolarization-induced suppression of inhibition (DSI) is a prevailing form of endocannabinoid signalling. However, several discrepancies have arisen regarding the roles played by the two major brain endocannabinoids, 2-arachidonoylglycerol (2-AG) and anandamide, in mediating DSI. Here we studied endocannabinoid signalling in the prefrontal cortex (PFC), where several components of the endocannabinoid system have been identified, but endocannabinoid signalling remains largely unexplored. In voltage clamp recordings from mouse PFC pyramidal neurons, depolarizing steps significantly suppressed IPSCs induced by application of the cholinergic agonist carbachol. DSI in PFC neurons was abolished by extra-or intracellular application of tetrahydrolipstatin (THL), an inhibitor of the 2-AG synthesis enzyme diacylglycerol lipase (DAGL). Moreover, DSI was enhanced by inhibiting 2-AG degradation, but was unaffected by inhibiting anandamide degradation. THL, however, may affect other enzymes of lipid metabolism and does not selectively target the α (DAGLα) or β (DAGLβ) isoforms of DAGL. Therefore, we studied DSI in the PFC of DAGLα −/− and DAGLβ −/− mice generated via insertional mutagenesis by gene-trapping with retroviral vectors. Gene trapping strongly reduced DAGLα or DAGLβ mRNA levels in a locus-specific manner. In DAGLα −/− mice cortical levels of 2-AG were significantly decreased and DSI was completely abolished, whereas DAGLβ deficiency did not alter cortical 2-AG levels or DSI. Importantly, cortical levels of anandamide were not significantly affected in DAGLα −/− or DAGLβ −/− mice. The chronic decrease of 2-AG levels in DAGLα −/− mice did not globally alter inhibitory transmission or the response of cannabinoid-sensitive synapses to cannabinoid receptor stimulation, although it altered some intrinsic membrane properties. synaptic inhibition in a DAGLα-dependent manner. These results show that DSI is a prominent form of endocannabinoid signalling in PFC circuits. Moreover, the close agreement between our pharmacological and genetic studies indicates that 2-AG synthesized by postsynaptic DAGLα mediates DSI in P...

Stimulatory and Inhibitory Roles of Brain 2-Arachidonoylglycerol in Bombesin-Induced Central Activation of Adrenomedullary Outflow in Rats

“…In diacylglycerol lipase α-knockout mice, endocannabinoid-mediated retrograde synaptic signaling is totally absent in the cerebellum, hippocampus, striatum, and prefrontal cortex, while the signaling is intact in diacylglycerol lipase β-knockout mouse brain (51 -53). Additionally, 2-AG content is markedly reduced in these brain regions of diacylglycerol lipase α-knockout mice, but not in diacylglycerol lipase β-knockout mice (52,53). These findings suggest that brain diacylglycerol lipase α is mainly responsible for the endocannabinoid 2-AG-mediated retrograde signaling.…”

Endocannabinoid-mediated retrograde modulation of synaptic transmission

“…This model was developed to reconcile the apparent discrepancy in the experimental results between genetic and pharmacological blockade of DGL. DGLα knockout mice were generated independently by three groups, and they all exhibit complete loss of eCB-STD [13][14][15]. In contrast, the reported effects of acute pharmacological blockade of DGL were highly controversial [19].…”

Functional diversity on synaptic plasticity mediated by endocannabinoids

“…By manipulating the synthetic and hydrolytic pathways for each, identification of the eCB involved in DSI/DSE was possible. Deletion of the a isoform of diacylglycerol lipase (DGL), the main enzyme involved in the synthesis of 2-AG, resulted in an inability to trigger eCB-mediated plasticity in cortex, striatum, cerebellum and hippocampus [69][70][71], while inhibition of the anandamide-hydrolysing enzyme fatty acid amide hydrolase (FAAH) did not affect DSI [72], suggesting that anandamide was not involved in this phenomenon.…”