Down- and up-regulation of opioid receptors in subcellular fractions from brain of buprenorphine-treated adults. SPMs and microsomes (MI) from adult rats treated with saline or buprenorphine (1 mg/kg, 20 hr) were prepared as described in Materials and Methods. Bmax values were estimated with 1 nM [3H]DAMGE (μ), [3H]DSLET (δ), and [3H] U69593 (κ) in homologous competition assays. For microsomes and SPMs, control Kd values were 1.8 ± 0.1 and 1.4 ± 0.07 nM (μ), 1.3 ± 0.2 and 3.3 ± 0.6 nM (δ), and 1.9 ± 0.4 and 1.2 ± 0.1 nM (κ), whereas those for buprenorphine-treated membranes were 5.5 ± 2.5 and 1.9 ± 0.3 nM (μ), 3.7 ± 0.5 and 3.9 ± 0.5 nM (δ), and 3.5 ± 0.1 and 3.3 ± 0.4 nM (κ), respectively. *, Significantly different from saline-treated rats, p < 0.05. Results are from three to five separate experiments.