Abstract

A nonisotopic, immunoelectrophoretic technique was used to analyze the characteristics of opioid-evoked activation of G_i2/G_x/z transducer proteins of mouse periaqueductal gray matter membranes. In the presence of picomolar concentrations of guanosine 5′-O-(3-thiotriphosphate), the opioid agonists promoted concentration-dependent increases of immunoreactivity associated with free G_i2α and G_x/zα subunits. [d-Ala²,N-MePhe⁴,Gly-ol⁵]enkephalin and morphine (preferential agonists at mu opioid receptors) and β-endorphin-(1–31) (an agonist atmu/delta opioid receptors) activated G_x/zproteins. In contrast, the agonists of delta opioid receptors, [d-Ala²]deltorphin II and [d-Pen^2,5]enkephalin, displayed little or no activity on this pertussis toxin resistant regulatory protein. Although exhibiting diverse efficacy, all the opioids studied activated G_i2 transducer proteins. [d-Ala²,N-MePhe⁴,Gly-ol⁵]enkephalin and [d-Ala²]deltorphin II were more potent at G_i2α subunits than at G_x/zα subunits. The opioid antagonist naloxone displayed a competitive profile in reducing the activation of G proteins promoted by morphine. Moreover, [d-Pen^2,5]enkephalin antagonized the releasing effect exerted by [d-Ala²]deltorphin II on G_i2α and G_x/zα subunits.N,N-diallyl-Tyr-Aib-Aib-Phe-Leu (ICI-174864) reduced the Gα-related immunosignals promoted by agonists of delta opioid receptors. Therefore, it is suggested that opioids exhibit marked differences in efficacy and/or potency in the activation of G_i2 and G_x/ztransducer proteins in mouse periaqueductal gray matter.