The relationship between shock severity and the form of the antinociception observed in pentobarbital-anesthetized rats

Three experiments are reported that explore the opioid/nonopioid nature of environmentally induced antinociception in pentobarbital-anesthetized rats. In Experiment 1, the minimum shock intensity and duration that elicits antinociception was established and two alternative descriptions of the tradeoff between shock intensity and duration were evaluated: the coulometric relationship (duration X intensity) and an extension of Stevens's power law (duration X intensity(3.5)). The results support the power law. In Experiments 2 and 3, the relationship between shock severity and the form of the antinociception elicited was examined. It was found that the least severe shock schedules produced a naltrexone-insensitive ''nonopioid'' antinociception. Increasing shock severity elicited an ''opioid'' antinociception that was attenuated by naltrexone. When shock severity was increased further, the antinociception again became naltrexone insensitive. Implications of the results are discussed.