Monoacylglycerol Lipase Limits the Duration of Endocannabinoid-Mediated Depolarization-Induced Suppression of Excitation in Autaptic Hippocampal Neurons

Abstract

Depolarization-induced suppression of excitation (DSE) is a major form of cannabinoid-mediated short-term retrograde neuronal plasticity and is found in numerous brain regions. Autaptically cultured murine hippocampal neurons are an architecturally simple model for the study of cannabinoid signaling, including DSE. The transient nature of DSE-tens of seconds-is probably determined by the regulated hydrolysis of the endocannabinoid 2-arachidonoyl glycerol (2-AG). No less than five candidate enzymes have been considered to serve this role: fatty acid amide hydrolase (FAAH), cyclooxygenase-2 (COX-2), monoacylglycerol lipase (MGL), and ␣/␤-hydrolase domain (ABHD) 6 and 12. We previously found that FAAH and COX-2 do not have a role in determining the duration of autaptic DSE. In the current study, we found that two structurally distinct inhibitors of MGL [N-arachidonoyl maleimide and 4-nitrophenyl 4-(dibenzo [d][1,3]dioxol-5-yl(hydroxy)methyl)piperidine-1-carboxylate (JZL184)] prolong DSE in autaptic hippocampal neurons, whereas inhibition of ABHD6 by N-methyl-N-[[3-(4-pyridinyl) phenyl]methyl]-4Ј-(aminocarbonyl)[1,1Јbiphenyl]-4-yl ester, carbamic acid (WWL70) had no effect. In addition, we developed antibodies against MGL and ABHD6 and determined their expression in autaptic cultures. MGL is chiefly expressed at presynaptic terminals, optimally positioned to break down 2-AG that has engaged presynaptic CB 1 receptors. ABHD6 is expressed in two distinct locations on autaptic islands, including a prominent localization in some dendrites. In summary, we provide strong pharmacological and anatomical evidence that MGL regulates DSE in autaptic hippocampal neurons and, taken together with other studies, emphasizes that endocannabinoid signaling is terminated in temporally diverse ways.

Mutation of Putative GRK Phosphorylation Sites in the Cannabinoid Receptor 1 (CB1R) Confers Resistance to Cannabinoid Tolerance and Hypersensitivity to Cannabinoids in Mice

“…For desensitization experiments, neurons were treated overnight with 100 nM WIN55,212-2 (WIN) in vehicle (0.001% DMSO). We have previously found that overnight treatment with vehicle alone does not affect subsequent responses to cannabinoid agonists (Straiker et al, 2012). WIN washes out with a half-life of 5.5 min (Straiker and Mackie, 2005).…”

“…Tolerance and dependence emerge during heavy cannabis use in humans (D'Souza et al, 2008) and could contribute to cannabis abuse. In preclinical rodent models, repeated administration of ⌬ 9 -THC causes rapid tolerance to ⌬ 9 -THCmediated "tetrad" effects (i.e., antinociception, hypothermia, hypoactivity, and catalepsy; Nguyen et al, 2012) as well as dependence (Tsou et al, 1995).…”

Synapse type-independent degradation of the endocannabinoid 2-arachidonoylglycerol after retrograde synaptic suppression

“…The amount of 2-AG in the brain is markedly increased in global MGL knockout (MGL-KO) mice (26,27). Depolarization-induced suppression of inhibition/excitation (DSI/DSE) as a result of Ca 2+ -ER and mGluR1-mediated synaptic suppression caused by basal RER are significantly prolonged in the hippocampus and cerebellum after treatment of MGL inhibitors (28)(29)(30) or in global MGL-KO mice (31,32). These lines of evidence indicate that MGL significantly contributes to the regulation of 2-AG signaling.…”

“…These lines of evidence indicate that MGL significantly contributes to the regulation of 2-AG signaling. Because MGL is found in the cytoplasm of presynaptic terminals (30,33,34), it is thought that 2-AG is degraded in a "synapse-specific" manner within the cytoplasm of nerve terminals at which 2-AG suppresses transmitter release by activating CB 1 receptors. However, because not all CB 1 + nerve terminals express MGL (35), it is unclear whether MGL regulates 2-AGmediated suppression only at synaptic terminals that express MGL.…”

Implication of the anti-inflammatory bioactive lipid prostaglandin D2-glycerol ester in the control of macrophage activation and inflammation by ABHD6

“…To assess the implication of ABHD6 in 2-AG metabolism by macrophages, J774 macrophages incubated with increasing doses of the selective ABHD6 inhibitor WWL70 (17,23,24) were stimulated with LPS. WWL70 induced a dose-dependent increase in 2-AG levels (Fig.…”