title: The Epac-Phospholipase Cε Pathway Regulates Endocannabinoid Signaling and Cocaine-Induced Disinhibition of Ventral Tegmental Area Dopamine Neurons.
abstract: Exchange protein directly activated by cAMP (Epac) is a direct effector for the ubiquitous second messenger cAMP. Epac activates the phospholipase Cε (PLCε) pathway. PLCβ has been linked to the synthesis of the endocannabinoid 2-arachidonoylglycerol (2-AG). Here, we report that Epac facilitates endocannabinoid-mediated retrograde synaptic depression through activation of PLCε. Intracellular loading of a selective Epac agonist 8-CPT-2Me-cAMP into ventral tegmental area (VTA) dopamine neurons enabled previously ineffective stimuli to induce depolarization-induced suppression of inhibition (DSI) and long-term depression of IPSCs (I-LTD) in the VTA. DSI and I-LTD are mediated by 2-AG since they were blocked by a diacylglycerol lipase inhibitor. The effects of 8-CPT-2Me-cAMP on DSI and I-LTD were absent in Epac2 and PLCε knock-out mice, but remained intact in Epac1 knock-out mice. These results identify a novel mechanism for on-demand synthesis of retrograde signaling 2-AG by the Epac2-PLCε pathway. We investigated the functional significance of Epac2-PLCε-2-AG signaling in regulating inhibitory synaptic plasticity in VTA dopamine neurons induced byin vivococaine exposure. We showed that cocaine place conditioning led to a decrease in the frequency and amplitude of spontaneous IPSCs and an increase in action potential firing in wild-type mice, but not in Epac2 or PLCε knock-out mice. Together, these results indicate that the Epac2-PLCε-2-AG signaling cascade contributes to cocaine-induced disinhibition of VTA dopamine neurons.SIGNIFICANCE STATEMENT2-arachidonoylglycerol (2-AG) is an endogenous cannabinoid that depresses synaptic transmission through stimulation of CB1receptors. Among the six isoforms of phospholipase C (PLC; PLCβ, PLCγ, PLCδ, PLCε, PLCζ, PLCη), only PLCβ has been linked to 2-AG synthesis. Here we demonstrate that 8-CPT-2Me-cAMP, a selective agonist of the cAMP sensor protein Epac, enhances 2-AG-mediated synaptic depression in ventral tegmental area (VTA) dopamine neurons via activation of PLCε. These results identify a novel mechanism for 2-AG synthesis via activation of the Epac-PLCε pathway. Furthermore, we show that cocaine-induced conditioned place preference and disinhibition of VTA dopamine neurons were impaired in mice lacking Epac or PLCε. Thus, the Epac-PLCε signaling pathway contributes to cocaine-induced disinhibition of VTA dopamine neurons and formation of drug-associated memories.
authors.affiliation: Department of Pharmacology and Toxicology, Medical College of Wisconsin, Milwaukee, Wisconsin 53226.
authors.forename: Jiaqing
authors.identifier: 0000-0002-2587-0251
authors.initials: J
authors.lastname: Tong
authors.affiliation: Department of Pharmacology and Toxicology, Medical College of Wisconsin, Milwaukee, Wisconsin 53226.
authors.forename: Xiaojie
authors.identifier: 0000-0001-9843-1674
authors.initials: X
authors.lastname: Liu
authors.affiliation: Department of Pharmacology and Toxicology, Medical College of Wisconsin, Milwaukee, Wisconsin 53226.
authors.forename: Casey
authors.identifier: 0000-0003-0536-1211
authors.initials: C
authors.lastname: Vickstrom
authors.affiliation: Department of Pharmacology and Toxicology, Medical College of Wisconsin, Milwaukee, Wisconsin 53226.
authors.forename: Yan
authors.initials: Y
authors.lastname: Li
authors.affiliation: Department of Pharmacology and Toxicology, Medical College of Wisconsin, Milwaukee, Wisconsin 53226.
authors.forename: Laikang
authors.initials: L
authors.lastname: Yu
authors.affiliation: Neuroscience Center of Excellence and Department of Neurology, Louisiana State University School of Medicine, New Orleans, Louisiana 70112.|Department of Physiology, Tongji Medical College and Institute for Brain Research, Huazhong University of Science and Technology, Wuhan 430030, China, and.
authors.forename: Youming
authors.initials: Y
authors.lastname: Lu
authors.affiliation: Department of Pharmacology, University of Michigan Medical School, Ann Arbor, Michigan 48109.
authors.forename: Alan V
authors.initials: AV
authors.lastname: Smrcka
authors.affiliation: Department of Pharmacology and Toxicology, Medical College of Wisconsin, Milwaukee, Wisconsin 53226, qsliu@mcw.edu.
authors.forename: Qing-Song
authors.identifier: 0000-0003-1858-1504
authors.initials: QS
authors.lastname: Liu
mesh_terms: D000818:Animals; D003042:Cocaine / Q000008:administration & dosage*; D059290:Dopaminergic Neurons / Q000187:drug effects / Q000502:physiology*; D063388:Endocannabinoids / Q000378:metabolism*; D005260:Female; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009433:Neural Inhibition / Q000187:drug effects / Q000502:physiology*; D054801:Phosphoinositide Phospholipase C / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D017557:Ventral Tegmental Area / Q000166:cytology / Q000187:drug effects / Q000502:physiology*
chemical_list: D063388:Endocannabinoids; D054801:Phosphoinositide Phospholipase C; C423320:phospholipase C epsilon; D003042:Cocaine
keywords: Epac; GABAergic disinhibition; endocannabinoid 2-arachidonoylglycerol; phospholipase Cε; retrograde synaptic depression