Promethazine both facilitates and inhibits nociception in rats: effect of the testing procedure.

The present study demonstrates that low doses of promethazine (1.25-5 mg/kg SC) dose-dependently facilitate nociception in the vocalization test in rats. However, this effect disappeared gradually with increasing dose, and in contrast, high doses (20-40 mg/kg SC) induced an antinociceptive effect. This indicates that promethazine, depending upon the biophase concentration, has the potential to interact with separate antagonizing or opposing functional systems, producing contrasting effects on nociception. The sigmoid Emax model was fitted to the observed composite effect, and dose-response characteristics for two opposite effects were described. In addition, when suprathreshold stimulation was used to evoke nociception, the stimulus amplified the hyperalgesic efficacy of promethazine but left the potency of this effect unaltered. In this experimental situation only negligible antinociception was observed. Our data thus show that for promethazine, the net effect on nociception in rats is not absolute but is balanced both by the biophase concentration and by the effectiveness of the stimulation used to evoke nociception.