Abstract

In the complex signal transduction networks involving G protein-coupled receptors there are numerous examples where G_i -linked receptors augment G_q-dependent signals. The mechanistic basis of such occurrences is thought to entail signal convergence at phospholipase Cβ (PLCβ) via the G protein βγ-dimers. Herein, we explored the possibility that augmentation by βγ-dimers requires preactivation of PLCβ. COS-7 cells were transiently cotransfected with cDNAs encoding various combinations of receptors and G protein subunits. The G_i-coupled δ- and κ-opioid receptors could not stimulate PLCβ unless they were coexpressed with Gα₁₆. The opioid-induced response was dose-dependent and partially inhibited by pertussis toxin or coexpression with transducin, indicating the involvement of βγ-subunits released from the G_iproteins. When PLCβ was preactivated by constitutively active mutants of Gα₁₆, Gα_q, or Gα₁₄, opioids enhanced the activity by 80 to 300% and such responses were mostly pertussis toxin-sensitive. The opioid-induced enhancement was dose-dependent and could not be blocked by staurosporin, a protein kinase C inhibitor. Other G_i-coupled receptors that were ineffective on their own also acquired the ability to stimulate PLCβ in the presence of a constitutively active mutant of Gα_q. Coactivation of endogenous or exogenous G_q-coupled receptors with the δ-opioid receptor produced strong stimulations of PLCβ and such responses could be partially blocked by pertussis toxin. These results show that enhancement of G_q-dependent signals by G_i-coupled receptors requires activated PLCβ and is mediated via the βγ-dimer.