Co-activation of GPCRs facilitate GIRK-dependent current

The activity of dopamine neurons is dependent on both intrinsic properties and afferent projections. One potent form of inhibition is mediated by the activation of two inhibitory G protein-coupled receptors, D2 and GABA(B) receptors. Each of these receptors activates G protein-coupled inwardly rectifying potassium (GIRK) channels. Recordings in brain slices have shown that co-activation using saturating concentrations of agonists results in occlusion of the GIRK current. The present study examined the interaction between D2 and GABA(B) receptors using transient applications of sub-saturating concentrations of agonists where the co-application of one agonist resulted in both facilitation and inhibition (desensitization) of the other. The heterologous facilitation was modelled based on the known cooperative interaction between the G protein beta gamma subunits and GIRK channels. The results indicate that a low tonic level of G beta gamma results in facilitation of GIRK current and a high level of G beta gamma results in occlusion. The kinetics of the current induced by transient receptor activation is prolonged in each case. The results suggest that the cooperative interaction between G beta gamma subunits and GIRK channels determines both the amplitude and kinetics of GPCR-dependent current.