Opposing effects of apomorphine on pain in rats. Evaluation of the dose-response curve

The effect of apomorphine on a supraspinally mediated response to pain was studied after subcutaneous administration of 10 different doses (25 μg/kg up to 10 mg/kg). Depending on the dose given, apomorphine was found to induce opposing effects on pain, so that low doses, 25–100 μ/kg, dose-dependently increased the sensitivity to pain. This effect then gradually declined in potency with increasing doses and high doses induced antinociception. The data therefore suggest that the net effect recorded involves the sum of responses from at least two functional systems. Using the Hill equation and the digital computer program NONLIN, we have dissociated the observed effect into two components, each having its particular dose-response characteristics: low doses having an ED 50 value of 36 μg/kg produced hyperreactivity to pain, and high doses having an ED 50 of 465 μg/kg (in the absence of hyperalgesia) induced antinociception.