The effects of N-allylnormetazocine on electric shock titration in squirrel monkeys.

The effects of the stereoisomers of the prototypic sigma agonist N-allylnormetazocine were evaluated in squirrel monkeys trained to respond on an electric shock titration schedule. The monkeys responded on a fixed-ratio schedule to decrease the level of shock delivered to their tails. The effects of the isomers were compared to the effects of phencyclidine and morphine administered alone and also in combination with a rate-suppressing dose of morphine. Morphine increased the level at which shock was maintained without decreasing rates of responding in the presence of shock. Both the (+)-isomer and PCP produced increases in the levels at which the monkeys maintained the shock, but only in some animals. The (-)-isomer of N-allylnormetazocine resulted in either decreases or no effect on the level at which the monkeys maintained the shock. The isomers were equipotent in decreasing response rates. When tested in combination with morphine, only the (-)-isomer antagonized the shock-level increasing effects of morphine. Thus, the isomers had similar effects on response rates but the (+)-isomer, like PCP, was more effective in increasing the level at which the monkeys maintained electric shock, while only the (-)-isomer was effective as a morphine antagonist. These results suggest analgesic-like effects for the (+)-isomer of N-allylnormetazocine and substantiate the opiate-antagonist properties of the (-)-isomer.