[Characterization of opioid receptor from bullfrog brain]

Among the tested five species--carp, bullfrog chicken, rat, and mouse--the solubilized fraction derived from the bullfrog brain gave the highest binding activity similar to that of membranes. The difference in their binding activities was demonstrated to depend on the Bmax value, but not on Kd. High-affinity binding of [3H]naloxone and [3H]ethylketocyclazocine (EKC) was proved in supernatant of detergent-treated membranes. The displacement curves of various [3H]ligands binding to the membrane and solubilized fraction by a series of agonists suggested that kappa-receptors are rich in the bullfrog brain. Guanyl-5'yl-imidodiphosphate (GppNHp) and N-ethylmaleimide (NEM) reduced the proportion of opioid receptors with a high affinity for mu-agonist, because the high-affinity states of opioid receptors were converted to the low-affinity states by GppNHp and NEM. The degree in shift of these displacement curves of [3H]naloxone binding by above agonist in bullfrog brain membranes, was slight when compared with that of the rat brain membranes. GppNHp slightly affected the displacement curves of [3H]naloxone binding caused by levorphanol or delta-receptor peptide, but the displacement curve for bremazocine was not affected by GppNHp, suggesting that participation of GTP binding protein is minor in kappa-receptor of the bullfrog.