Fig. 1. Competitive inhibition by buprenorphine, norbuprenorphine, buprenorphine-3-glucuronide (B3G), and norbuprenorphine-3-glucuronide (N3G) of ³H-diprenorphine to human μ (A ), κ (B ), and δ (C ) opioid receptors, and ³H nociceptin binding to the nociceptin receptor (D ). Binding to the opioid receptors and the nociceptin receptor was carried out with 0.4 nM ³H diprenorphine and 0.1 nM ³H-nociceptin, respectively, in the presence or absence of varying concentrations of buprenorphine (red square ), norbuprenorphine (blue circle ), B3G (green triangle ), and N3G (purple pentagon ). Data were normalized to percentage of specific binding. Each data point represents the mean ± SD (n = 9). Lines are predicted results based on the Ki values obtained by nonlinear regression analysis of the observed data, using the equation shown in table 1. Apparent Ki values are shown in table 1.