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Differential modulation of T-type voltage gated calcium channels by G-protein coupled receptors. Hildebrand, Michael Earl

Abstract

T-type voltage-gated calcium (Ca2+) channels play critical roles in controlling neuronal excitability, firing patterns, and synaptic plasticity, although the mechanisms and extent to which T-type Ca2+ channels are modulated by G-protein coupled receptors (GPCRs) remains largely unexplored. Investigations into T-type modulation within native neuronal systems have been complicated by the presence of multiple GPCR subtypes and a lack of pharmacological tools to separate currents generated by the three T-type isoforms; Cav3.1, Cav3.2, and Cav3.3. We hypothesize that specific Cav3 subtypes play unique roles in neuronal physiology due to their differential functional coupling to specific GPCRs. Co-expression of T-type channel subtypes and GPCRs in a heterologous system allowed us to identify the specific interactions between muscarinic acetylcholine (mAChR) or metabotropic glutamate (mGluR) GPCRs and individual Cav3 isoforms. Perforated patch recordings demonstrated that activation of Galpha-coupled GPCRs had a strong inhibitory effect on Cav3.3 T-type Ca2+ currents but either no effect or a stimulating effect on Cav3.1 and Cav3.2 peak current amplitudes. Further study of the inhibition of Cav3.3 channels by a specific Galpha-coupled mAChR (M1) revealed that this reversible inhibition was associated with a concomitant increase in inactivation kinetics. Pharmacological and genetic experiments indicated that the M1 receptor-mediated inhibition of Cav3.3 occurs specifically through a Galpha signaling pathway that interacts with two distinct regions of the Cav3.3 channel. As hypothesized, the potentiation of Cav3.1 channels by a Galpha-coupled mGluR (mGluR1) initially characterized in the heterologous system was also observed in a native neuronal system: the cerebellar Purkinje cell (PC). In recordings on PCs within acute cerebellar slices, we demonstrated that the potentiation of Cav3.1 currents by mGluR1 activation is strongest near the threshold of T-type currents, enhancing the excitability of PCs. Ultrafast two-photon Ca2+ imaging demonstrated that the functional coupling between mGluR1 and T-type transients occurs within dendritic spines, where synaptic integration and plasticity occurs. A subset of these experiments utilized physiological synaptic activation and specific mGluR1 antagonists in wild-type and Cav3.1 knock-out mice to show that the mGluR1-mediated potentiation of Cav3.1 T-type currents may promote synapse-specific Ca2+ signaling in response to bursts of excitatory inputs.

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