Effects of repeated administration of a κ-opioid agonist on phorbol ester binding to membrane-bound protein kinase C in rat brain

The dissociation constant (KD) of [3H] phorbol 12, 13-dibutyrate (PDBu) binding to protein kinase C (PKC) in membranes of rat cortex and midbrain was significantly decreased with no change in the receptor density (Bmax) following 7-d treatment with a κ-opioid agonist, U-50, 488 (20mg/kg/d, i.p.). Neither the Bmax nor KD values in pons/medulla were altered by repreated U-50, 488 treatment. These results suggest that repeated administration of a κ-opioid agonist increases the affinity for PDBu binding to the membrane-bound PKC in rat cortex and midbrain, but not in pons/medulla.