Gx/z is regulated by mu but not delta opioid receptors in the stimulation of the low K-m GTPase activity in mouse periaqueductal grey matter

High affinity low K-m GTPase activity was measured in membrane preparations of adult mouse mesencephalic periaqueductal grey matter (PAG). Opioids displaying selectivity towards mu- or delta-opioid receptors (OR) activated the enzyme in a concentration-dependent manner. Antibodies to mu-OR greatly impaired the potential of mu-agonists, [D-Ala(2),N-MePhe(4),Gly-ol(5)]-enkephalin (DAMGO) and morphine, to increase hydrolysis of GTP. The same antibodies had little effect an [D-Pen(2,5)]enkephalin (DPDPE) and [D-Ala(2)]deltorphin II, both agonists at delta-OR, Stimulation of GTPase by DPDPE and [D-Ala(2)]deltorphin II - but not by morphine or DAMGO - was diminished by antibodies to delta-OR. The blockade of G(i2)alpha subunits by specific antibodies impaired the activation of G alpha-related GTPase by all opioids. Antibodies in vitro, and oligodeoxynucleotides in vivo, prepared against Gx/z alpha subunits, reduced the release of Pi promoted by DAMGO and morphine. The impairment of Gx/z proteins also slightly reduced the effect of the delta 2 agonist [D-Ala(2)]deltorphin II. At delta 1 receptors, DPDPE fully expressed its activation of GTPase. These results indicate that in the FAG, mu-OR and delta-OR couple with Gi2 transducer proteins. Notably, mu-OR also regulates the pertussis toxin-insensitive G-protein Gx/z, an effect poorly exhibited by delta-OR in this tissue.