PLC activity was stimulated either by 1-100 μM of GTP or by 100-3,000 μM Ca2+ in lysed synaptosomal membranes of the guinea pig cerebellum. The κ-opioid receptor agonist selectively inhibited the PLC activity stimulated by 100 μM GTP, but not by 100-3,000 μM Ca2+. Pretreatment of membranes with PTX abolished such a κ-agonist-induced inhibition of PLC activity. The reconstitution of Gi1, but not of Go purified from porcine brains with PTX-treated membranes showed a complete recovery of the κ-agonist-inhibition of PLC activity. These findings suggest that a novel subtype κ-receptor mediates inhibition of PLC through inhibiting the intrinsic activity of PTX-substrate G-proteins.
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