A Voltage-Independent Calcium Current Inhibitory Pathway Activated by Muscarinic Agonists in Rat Sympathetic Neurons Requires Both Gaq/11 and Gbg

Calcium current modulation by the muscarinic cholinergic agonist oxotremorine methiodide (oxo-M) was examined in sympathetic neurons from the superior cervical ganglion of the rat. Oxo-M strongly inhibited calcium currents via voltage-dependent (VD) and voltage-independent (VI) pathways. These pathways could be separated with the use of the specific M1 acetylcholine receptor antagonist M1-toxin and with pertussis toxin (PTX) treatment. Expression by nuclear cDNA injection of the regulator of G-protein signaling (RGS2) or a phospholipase Cb1 C-terminal construct (PLCb-ct) selectively reduced VI oxo-M modulation in PTX-treated and untreated cells. Expression of the Gbg buffers transducin (Gatr ) and a G-protein-coupled-receptor kinase (GRK3) construct (MAS-GRK3) eliminated oxo-M modulation. Activation of the heterologously expressed neurokinin type 1 receptor, a Gaq/11-coupled receptor, resulted in VI calcium current modulation. This modulation was eliminated with coexpression of Gatr or MAS-GRK3. Cells expressing Gb1g2 were tonically inhibited via the VD pathway. Application of oxo-M to these cells produced VI modulation and reduced the amount of current inhibited via the VD pathway. Together, these results confirm the requirement for Gbg in VD modulation and implicate Gaq-GTP and Gbg as components in the potentially novel VI pathway.