Binding characteristics of mu and kappa agonists in rat brain subcellular fractions

Scatchard analysis of 3H-dihydromorphine (3H-DHM) binding to rat brain subcellular fractions gave a curvilinear plot in the microsomal (P3) and a linear plot in the mitochondrial (P2) fraction, respectively. In contrast, plots of 3H-ethylketocyclazocine (3H-EK) binding were linear in the P3 and curvilinear in the P2 fraction. No differences were seen in the potency of morphine and ketazocine in displacing either 3H-DHM or 3H-EK in the P2 fraction. In the P3 fraction, however, morphine was more potent than ketazocine in displacing 3H-DHM, but the reverse was true when 3H-EK was used as the ligand. Isolation of the P3 fraction may facilitate the demonstration of a kappa receptor in the rat brain.