Time course characteristics of acute tolerance development to continuously infused alfentanil in rats

The time course of the development of acute tolerance to alfentanil was characterized in rat experiments using various algorithms of continuous infusion of the drug. To maintain analgesia at a constant level (tail compression test), alfentanil use was steadily increased: Time to 50% increase in alfentanil use was 255 +/- 98 min with analgesia maintained at a level of 50% increase in the pressure threshold, and 203 +/- 116 min with analgesia maintained at a level of 200% increase. With the use of the alfentanil infusion regimen designed to rapidly achieve and maintain the constant alfentanil plasma concentration (120 ng/mL), the rate of development of acute tolerance, measured in terms of time to 50% recovery during continuing infusion, was 81 +/- 26 min with the compression test and 88 +/- 38 min with the hot-plate test, but 374 +/- 46 min with the rotarod test (P < 0.0001). Thus we demonstrated rapid development of acute tolerance to continuously infused alfentanil with different methods of tolerance assessment. The results also indicate that the time course of tolerance development does not depend on the type of noxious stimulation (mechanical versus thermal) if the response is organized predominantly supraspinally, and that acute tolerance to the direct effect of alfentanil on motor functions does not mask the rapid development of tolerance to its analgesic effect.