G(i)- and G(s)-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 neurotransmi tters 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 (G(i))-coupled (mu -opioid) or stimulatory G protein (G(s))-coupled [serotonin 5-HT4((a))] rec eptors in Xenopus oocytes. Both receptors enhanced HCN2, but not HCN1 curre nts. Receptor activation increased HCN2 current amplitude, increased the ac tivation rate sixfold and decreased the deactivation rate twofold. In addit ion. the fully-activated current for HCN2 increased due to a receptor-induc ed increase of the maximal conductance. These effects were inhibited by 9-( tetrahydro-2 ' -furyl)adenine (SQ22536), were independent of protein kinase s A and C and could be explained by a cAMP-induced shift of the voltage dep endence of activation by 15 mV to more positive potentials. The pathway thr ough which these effects occurred involved G(beta gamma)-activation of aden ylyl cyclase and, in the case of the mu -opioid receptor, required co-expre ssion of G alpha (s). The effect of the 5-HT4((a))-receptor, in part caused by its constitutive activity, occurred directly through G alpha (s)-activa tion. This suggests that 5-HT4((a)) receptors may contribute to functional heterogeneity of pacemaker currents (I-h) in those neurons in which 5-HT4(( a))R and HCN2 coexist.