Characterization of high affinity opioid binding sites in rat periaqueductal gray P2 membrane

The periaqueductal gray (PAG) region of the midbrain has been implicated in both stimulation produced and opioid induced analgesia. In the present study the opioid binding characteristics of the PAG were examined with an in vitro radioligand binding technique. [3H]Ethylketocyclazocine (EKC), 2 nM, was used as a tracer ligand to nonselectively label mu, delta, and kappa binding sites in PAG enriched P2 membrane. The mu selective ligand [D-Ala2,N-methylPhe4,Glyol5]enkephalin (DAGO) competed with [3H]EKC for more than one population of binding sites with both high and low affinity. In contrast the delta selective ligand [D-Pen2,D-Pen5]enkephalin (DPDPE) and the kappa selective ligand trans-3,4-dichloro-N-methyl-N-[2-(1- pyrrolidinyl)cyclohexyl]benzeneacetamide, methane sulfonate, hydrate (U50,488H) each competed with [3H]EKC for a single population of binding sites with low affinity. DPDPE and U50,488H also competed with 2 nM [3H]DAGO for a single population of binding sites with similar low affinity. DAGO and not DPDPE competed with 2 nM [3H][D-Ala2,D-Leu5]enkephalin (DADLE) with high affinity. 2 nM [3H]DPDPE did not substantially label PAG enriched P2 membrane, and 1 nM DAGO competed with all specific [3H]DPDPE binding which was observed. These binding data are consistent with the presence of a single population of mu selective high affinity binding sites in PAG enriched P2 membrane to which delta ligands and kappa ligands have low affinity.