Inositol triphosphate-mediated Ca²⁺ signals direct purinergic P2Y receptor regulation of neuronal ion channels

Purinergic P2Y receptors are one of four types of G_q/11-coupled receptors in rat superior cervical ganglia (SCG) sympathetic neurons. In cultured SCG neurons, purinergic and bradykinin suppression of I_M were similar in magnitude and somewhat less than that by muscarinic agonists. The effects of the P2Y receptor agonist UTP on neuronal excitability and discharge properties were studied. Under current clamp, UTP increased action potential (AP) firing in response to depolarizing current steps, depolarized the resting potential, decreased the threshold current required to fire an AP, and decreased spike-frequency adaptation. These effects were very similar to those resulting from bradykinin stimulation and not as profound as from muscarinic stimulation or full M-current blockade.Wethen examined the P2Y mechanism of action. Like bradykinin, but unlike muscarinic, purinergic stimulation induced rises in intracellular [Ca²⁺]_i. Tests using expression of IP₃"sponge" or IP₃ phosphatase constructs implicated IP₃ accumulation as necessary for purinergic suppression of I_M. Overexpression of wild-type or dominant-negative calmodulin (CaM) implicated Ca²⁺/CaM in the purinergic action. Both sets of results were similar to bradykinin, and opposite to muscarinic, suppression. We also examined modulation of Ca²⁺ channels. As for bradykinin, purinergic stimulation did not suppress I_Ca, unless neuronal calcium sensor-1 (NCS-1) activity was blocked by a dominant-negative NCS-1 construct. Our results indicate that P2Y receptors modulate M-type channels in SCG cells via IP₃-mediated [Ca²⁺]_i signals in concert with CaM and not by depletion of phosphatidylinositol-4, 5-biphosphate. We group purinergic P2Y and bradykinin B₂ receptors together as having a common mode of action.