The mixed-action
opioid picenadol (LY150720) is a racemic mixture whose resolution results in a stereoselective separation of agonist and antagonist activity. The effects of
picenadol, its dextrorotatory isomer (LY136596) and
morphine were studied alone and in combination with
naloxone in squirrel monkeys responding under a schedule of electric
shock titration.
Shock intensity was scheduled to increase at 15-sec intervals in 30 steps from 0 to 5.5 mA. Completion of a fixed ratio 5-response requirement terminated the
shock for a 15-sec time-out period after which
shock resumed at the next lower intensity.
Picenadol (0.1-17.5 mg/kg), the d-isomer (0.3-3.0 mg/kg) and
morphine (0.3-5.6 mg/kg) produced dose-related increases in the intensity at which monkeys maintained the
shock without decreasing responding in the presence of
shock.
Shock intensity increases produced by
picenadol occurred over a broader dose range than with either the d-isomer or
morphine. Increases in
shock intensity produced by
picenadol, the d-isomer and
morphine were blocked by
naloxone (0.001-1.0 mg/kg), although the effects of
picenadol were less susceptible to antagonism. The effects of the levorotatory isomer of
picenadol (LY136595) were also examined alone and in combination with
morphine. The l-isomer (0.1-10.0 mg/kg) did not alter
shock intensity when administered alone; however, in combination with
morphine it produced a dose-dependent antagonism of the effects of
morphine. The l-isomer was less potent than
naloxone in this respect. These data support previous suggestions that the antinociceptive properties of
picenadol arise from mu agonist actions of the dextrorotatory isomer and that the levorotatory isomer acts to limit the efficacy of the racemate.