Activation of Astrocytic μ-opioid Receptor Elicits Fast Glutamate Release Through TREK-1-Containing K2P Channel in Hippocampal Astrocytes

Abstract

Recently, μ-opioid receptor (MOR), one of the well-known Gi-protein coupled receptors (Gi-GPCR), was reported to be highly expressed in the hippocampal astrocytes. However, the role of astrocytic MOR has not been investigated. Here we report that activation of astrocytic MOR by [D-Ala2,N-MePhe4,Gly-ol]-enkephalin (DAMGO), a selective MOR agonist, causes a fast glutamate release using sniffer patch technique. We also found that the DAMGO-induced glutamate release was not observed in the astrocytes from MOR-deficient mice and MOR-short hairpin RNA (shRNA)-expressed astrocytes. In addition, the glutamate release was significantly reduced by gene silencing of the TREK-1-containing two-pore potassium (K2P) channel, which mediates passive conductance in astrocytes. Our findings were consistent with the previous study demonstrating that activation of Gi-GPCR such as cannabinoid receptor CB1 and adenosine receptor A1 causes a glutamate release through TREK-1-containing K2P channel from hippocampal astrocytes. We also demonstrated that MOR and TREK-1 are significantly co-localized in the hippocampal astrocytes. Furthermore, we found that both MOR and TREK-1-containing K2P channels are localized in the same subcellular compartments, soma and processes, of astrocytes. Our study raises a novel possibility that astrocytic MOR may participate in several physiological and pathological actions of opioids, including analgesia and addiction, through astrocytically released glutamate and its signaling pathway.

Activation of Astrocytic μ-Opioid Receptor Causes Conditioned Place Preference

“…We found that DAMGO treatment did not cause any increase of Ca 2+ signals, whereas dramatic Ca 2+ signals were elicited by treatment of TFLLR, a selective agonist for protease-activated receptor-1, which is coupled to both Gq and Gi/o proteins (Figures 3E and 3F). Consistent with our previous report (Woo et al, 2018), these results suggest that activation of astrocytic MOR elicits a glutamate release in a Ca 2+ -independent manner. Further investigation is needed to clarify the role of Ca 2+ in MOR-induced glutamate release in astrocytes.…”

“…This MORdependent enhancement of eEPSC was completely absent in TREK-1-TWIK-1 double-KO mice (Figures 4B, 4C, and S6A-S6D). In the context of a previous finding that activation of astrocytic MOR induces glutamate release by TREK-1-and TWIK-1-containing K2P channels (Woo et al, 2018), these results suggest that astrocytic glutamate is critical for DAMGO-induced enhancement of eEPSC. Consistent with our finding that mGluR1-KO mice did not exhibit DAMGO-induced LTP, the DAMGO-induced enhancement of eEPSC was completely blocked by LY367385, an mGluR1 antagonist, and in mGluR1-KO mice ( Figures 4B and 4C).…”

“…We also have previously reported that the astrocytic glutamate release upon activation of astrocytic Gi-GPCR including MOR could be Ca 2+ independent (Woo et al, 2018;Woo et al, 2012). On the other hand, several reports have demonstrated that activation of astrocytic Gi-GPCR, such as GABA B and hM4Di, evoked Ca 2+ signals (Chai et al, 2017;Durkee et al, 2019).…”

“…According to Hebbian theory, LTP induction at SC-CA1 requires enhanced glutamatergic synaptic transmission. Because we have previously reported that activation of astrocytic Gi-GPCR, such as MOR, elicits a glutamate release in vitro (Woo et al, 2018;Woo et al, 2012), glutamate released from astrocytes upon activation of astrocytic MOR could be the possible transmitter that is involved in this enhancement of glutamatergic synaptic transmission. Before testing this hypothesis, we first tested if activation of astrocytic MOR indeed causes a glutamate release in an ex vivo condition with a recently developed glutamate sensor, iGluSnFr (Marvin et al, 2013).…”

“…On the other hand, it is difficult to imagine that the activation of neuronal MOR could induce NMDAR-dependent LTP because neuronal MOR, an exemplar Gi-protein coupled receptor (Gi-GPCR), usually exerts a powerful inhibitory effect (Johnson and North, 1992). Alternatively, we have recently demonstrated that astrocytes in the CA1 hippocampus exhibit high expressions of MOR , whose activation induces a significant release of glutamate from astrocytes through TREK-1-containing two-pore potassium (K2P) channels (Woo et al, 2018;Woo et al, 2012). Such astrocytic glutamate has a potential to exert a noncanonical excitatory effect of Gi-GPCR onto neighboring neurons to induce LTP.…”