Buprenorphine Is a Weak Partial Agonist That Inhibits Opioid Receptor Desensitization

Buprenorphine limits ME-induced desensitization. Voltage-clamp recordings made with whole-cell electrodes. A, A control experiment using an untreated slice. ME (30 μm) caused a large outward current that declined during the 10 min application period. B, An experiment taken from a slice that was preincubated with buprenorphine (Bup; 5 nm, 1 h). ME (30 μm) caused a small outward current that did not desensitize during the 10 min application. C, An experiment using a slice that was preincubated with β-CNA (20 nm, 1 h). ME (30 μm) caused a small outward current that desensitized during the 10 min application period. D, Summary of results, plotting the peak amplitude of the current induced by ME (30 μm) against the amount of desensitization (the change in current from the peak to the end of the 10 min application divided by the peak current). The open boxes (vertical line at 35%) indicate experiments done in control slices (A). The amount of desensitization was independent of the initial amplitude of current induced by ME. The gray circles (positive sloping line) are experiments done after buprenorphine (B). In this case, when the current induced by ME was larger, the amount of desensitization was greater. When the ME currents were smaller than 150 pA, the desensitization was eliminated. Filled triangles (negative sloping line) are experiments done after β-CNA (C). In this case, as the current induced by ME decreased, the amount of desensitization was increased.

Buprenorphine blocked desensitization induced by etorphine. A, A control experiment showing the desensitization induced by etorphine (1 μm), the reversal of the current induced by naloxone (Nal; 1 μm), and the current induced by UK14304 (UK; 3 μm). B, An experiment done in a slice that was incubated in buprenorphine (5 nm, 1 h). The current induced by etorphine (1 μm) is significantly smaller and did not desensitize during the 10 min application period. Yoh, Yohimbine.

Incubation of slices with etorphine, oxymorphone, methadone, or oxycodone did not block ME-induced desensitization. A, Sample experiment from a slice that was incubated with etorphine (2 nm, 1 h) before recording the outward current induced by ME (30 μm, 10 min). The ME current peaked (ME1) and declined (ME2) during the 10 min application period. Superfusion with naloxone (Nal; 1 μm) caused an inward current, indicating the presence of etorphine in the slice. B, The same experiment done with slices that were incubated with methadone (1 μm) C, The same experiment done with oxymorphone (1 μm) for 1 h. D, Summarized results plotting the ratio of ME2/ME1 in experiments using slices incubated in buprenorphine (BUP; 5 nm), etorphine (ET; 2 nm), oxymorphone (OM; 1 μm), oxycodone (OC; 1 μm), or methadone (MD; 1 μm) for 1 h before the experiment. There was a marked decrease in the ME current in all experiments except in slices incubated in buprenorphine. Yoh, Yohimbine.

Buprenorphine does not cause receptor internalization and blocked the internalization induced by ME. A, Images of cells in three experiments. Left, Images taken at the beginning of the experiment after incubating slices with M1 anti-Flag antibody without drugs (top 2 images) or β-CNA (10 nm, 1 h; bottom image). The top three images are a control experiment demonstrating the internalization of receptor evoked by ME (30 μm, 15 min). The fluorescence image on the right is only internalized receptors after treatment of the slice with a calcium-free solution to strip antibody bound to extracellular surface. Middle images show that buprenorphine (15 μm) did not induce any internalization. The bottom images show that treatment of slices with a low concentration of β-CNA reduced but did not abolish the ME-induced receptor internalization. Scale bar, 10 μm. B, Summarized results from several experiments showing the amount of internalization (fluorescence as a percentage of the control) induced by ME in control, after treatment of slices with β-CNA (10 nm, 1 h) or buprenorphine (Bup; 5 nm, 1 h). The open bar is the background fluorescence measured after treatment of the slices with calcium-free solution without application of any drug. *p < 0.05.