Effects of pentazocine and other opiates on shock detection in the rat: involvement of opiate and dopamine receptors.

The hypothesis that the antinociceptive effects of pentazocine, a mixed agonist-antagonist opiate of the benzomorphan class, are mediated by a dual opiate-dopaminergic mechanism was tested using a two-choice procedure in which rats were required to discriminate the presence or absence of shock. The results showed that pentazocine decreased shock sensitivity and speed of responding, effects that were qualitatively similar to those of morphine. However, while the antinociceptive effect of both pentazocine and morphine could be antagonized by opiate receptor blockage, that of pentazocine, but not of morphine, could also be antagonized by dopamine receptor blockade. Observations with levorphanol and phenazocine suggested further that dopamine, as well as opiate receptor agonism may be characteristic of the benzomorphans.