Effects of morphine and chlorpromazine on the detection of shock.

A discrete-trial, two-choice, 'yes-no' procedure was used to determine the extent to which the perceptual effects of compounds such as morphine and chlorpromazine (CPZ) can be attributed to drug-induced changes in ability to detect shock stimuli (sensitivity). Both morphine (4.0, 5.0, and 6.0 mg/kg) and CPZ (0.25, 0.50, and 1.0 mg/kg) significantly reduced accuracy and increased the times (i.e., lowered the speeds) to initiate trials and to make choice responses. The effects of morphine appeared to be somewhat greater than those of CPZ, particularly at the lowest shock intensity (0.05 mA). When compared to appropriate saline control days, morphine, but not CPZ, significantly reduced accuracy of discrimination on trials when shocks were presented, whereas CPZ, but not morphine, reduced accuracy on no-shock trials. The effects of morphine, but not of CPZ, on accuracy (both overall and on shock trials) decreased as shock intensity increased. The effects of shock intensity were generally inversely related to the effects of morphine and directly related to the effects of CPZ.