Gi- and Gs-coupled receptors up-regulate the cAMP cascade to modulate HCN2, but not HCN1 pacemaker channels.

A hallmark of native pacemaker channels is their regulation by neurotransmitters and hormones acting through the second messenger cAMP. In this study, we investigated the modulation of two cloned pacemaker channels, HCN1 and HCN2, by activation of coexpressed inhibitory G protein (Gi)-coupled (p-opioid) or stimulatory G protein (Gs)-coupled [serotonin 5-HT4(a)] receptors in Xenopus oocytes. Both receptors enhanced HCN2, but not HCN1 currents. Receptor activation increased HCN2 current amplitude, increased the activation rate sixfold and decreased the deactivation rate two-fold. In addition, the fully-activated current for HCN2 increased due to a receptor-induced increase of the maximal conductance. These effects were inhibited by 9-(tetrahydro-2'-furyl)adenine (SQ22536), were independent of protein kinases A and C and could be explained by a cAMP-induced shift of the voltage dependence of activation by 15 mV to more positive potentials. The pathway through which these effects occurred involved Gbetagamma-activation of adenylyl cyclase and, in the case of the p-opioid receptor, required co-expression of Galphas. The effect of the 5-HT4(a)-receptor, in part caused by its constitutive activity, occurred directly through Galphas-activation. This suggests that 5-HT4(a) receptors may contribute to functional heterogeneity of pacemaker currents (Ih) in those neurons in which 5-HT4(a)R and HCN2 coexist.