Stereoselective μ- and δ-opioid receptor-related antinociception and binding with (+)-thebaine

Abstract

In vivo and in vitro binding studies with natural thebaine and its enantiomer, (+)-thebaine were conducted to elucidate further their interactions with the opioid system. (−)-Thebaine a key intermediate in the biosynthesis of morphine in the poppy plant ( Papaver somniferum) and in mammalian tissue, was poorly effective antinociceptively in mice at doses to 30 mg/kg. Its principal behavioral manifestation was lethal convulsions. Naltrindole, at doses of 1 and 10 mg/kg did not block either the convulsions or lethal effects, suggesting that the δ-opioid receptor system was not involved in this action. Surprisingly, the dextrorotatory isomer exhibited significant antinociceptive activity in the tail-flick [ED 50=8.9 (3.4–22.1) mg/kg], hot-plate [ED 50=22.9 (10.9–48.1) mg/kg] and phenylquinone [ED 50=1.9 (1.6–9.5) mg/kg] assays. Studies with opioid receptor-subtype antagonists, β-funaltrexamine, nor-binaltorphimine and naltrindole, indicated that antinociception was associated with μ- and δ-opioid receptors. Results of displacement experiments supported the in vivo data. Significant competition for [ 3 H ]diprenorphine binding with both isomers for cloned μ- and δ-opioid receptors was observed. However, (−)-thebaine was more effective at the δ-opioid receptor ( K i=1.02±0.01 μM) whereas (+)-thebaine was more effective at the μ-opioid receptor ( K i=2.75±0.01 μM). Opioid-induced antinociception associated with unnatural thebaine raises the possibility of additional μ- and δ-opioid receptor sites.