Effects of morphine, pentazocine and cyclazocine alone and in combination with naloxone on electric shock titration in the squirrel monkey.

The effects of morphine, pentazocine and cyclazocine were examined on a discrete-trial shock titration procedure. Monkeys received continuous shock during 15-sec shock periods. If the monkey did not respond during the shock period, shock remained on for 15 sec and then increased by one increment on the next 15-sec shock period. If five responses were made during the shock period, the shock was immediately terminated for 15 sec during which the chamber was dark (time out). After the 15-sec time out period, the shock resumed at the next lower intensity. Morphine, pentazocine and cyclazocine altered patterns and rates of responding similarly. At a low dose, pentazocine, cyclazocine and morphine decreased median shock levels and increased rates of responding in the presence and in the absence of shock (i.e., during time out). These effects were observed over a wider dose range with pentazocine and cyclazocine than with morphine; moreover, decreases in median shock level and increases in rates of responding in the absence of shock were of greater magnitude after pentazocine and cyclazocine than after morphine. At higher doses, morphine, pentazocine and cyclazocine increased median shock levels and decreased rates of responding in the absence of shock. Rates of responding in the presence of shock were either unchanged or decreased very slightly, suggesting that increases in the intensity at which shock is maintained under shock titration procedures is not necessarily due to decreases in rates of responding. In contrast, naloxone had very little behavioral effect when administered alone. The effects of morphine were antagonized by a dose of naloxone as low as 0.01 mg/kg, whereas higher doses of naloxone were required to antagonize the effects of pentazocine and cyclazocine.