Currents for GPCRs

Science Signaling  18 Jan 2011:
Vol. 4, Issue 156, pp. ec20
DOI: 10.1126/scisignal.4156ec20

Neurotransmitter release at synapses is a rapid calcium-dependent, depolarization-induced exocytosis event. However, neurotransmitter release is also influenced by G protein–coupled receptors, such as the type 2 muscarinic acetylcholine receptor (M2R). M2R associates with the release machinery, and various previous studies suggested that the ligand-bound receptor exerts a tonic brake on neurotransmitter release and that the depolarization triggers a voltage-dependent change in the GPCR to alleviate this tonic inhibition. With this model in mind, Kupchik et al. used sophisticated electrophysiological techniques to monitor gating currents, currents associated with charge movement, in the M2R expressed in Xenopus oocytes and then correlated those findings with experiments in the neuromuscular junction (NMJ) of the mouse diaphragm. Flash activation of the M2R with a caged agonist (CNB-carbachol) allowed microsecond control of receptor activation and revealed that activation of the receptor immediately before depolarization inhibited excitatory postsynaptic currents (EPSCs) in wild-type mice, but not M2R-knockout mice, without affecting calcium currents. To confirm that G protein signaling was not involved, similar experiments were performed with the ligand gallamine, which is an allosteric M2R ligand that does not activate G protein signaling. Gallamine failed to stimulate a G protein effector, dose-dependently inhibited the M2R gating current in oocytes, and dose-dependently decreased EPSCs in the NMJ of wild-type mice. Flash-released CNB-carbachol only reduced EPSCs if the ligand was present before the depolarization that triggered neurotransmitter release. Using complex paradigms of prepulse stimulations that activated the gating current but did not activate calcium influx; timed release of caged CNB-carbachol; and test stimulations that caused neurotransmitter release, the authors provided support for a model in which at resting potentials the release machinery is inhibited by the M2R and depolarization induces a gating current in the M2R that alleviates this block. In wild-type mice, but not M2R-knockout mice, neurotransmitter release also occurred faster in the presence of a prepulse that stimulated the gating currents before the stimulation that triggered the EPSC. The authors propose that these gating currents allow the GPCR to rapidly contribute to neurotransmitter release without requiring signaling through G proteins.

Y. M. Kupchik, O. Barchad-Avitzur, J. Wess, Y. Ben-Chaim, I. Parnas, H. Parnas, A novel fast mechanism for GPCR-mediated signal transduction—Control of neurotransmitter release. J. Cell Biol. 192, 137–151 (2011). [Abstract] [Full Text]