Behavior maintained by alfentanil or nalbuphine in rhesus monkeys: Fixed-ratio and time-out changes to establish demand curves and relative reinforcing effectiveness

A fixed-ratio, time-out schedule of intravenous alfentanil or nalbuphine delivery was used to maintain responding in rhesus monkeys (Macaca mulatta) during twice-daily 2-hr sessions of unrestricted access. Four doses of each drug were tested under 10 response fixed-ratio and 10-s time-out baseline conditions. Either the fixed ratio or the time-out was periodically increased during single sessions. Alfentanil maintained higher response rates than nalbuphine under conditions in which response rates were limited by the size of the fixed ratio rather than by unconditioned effects. This indicates that alfentanil is a more effective reinforcer than nalbuphine, which is predicted on the basis of the greater intrinsic efficacy of alfentanil relative to nalbuphine at the mu opioid receptor. Unit price analysis of these data demonstrated that a single demand function could be drawn for each drug, indicating that for these opioids in this situation, increasing the dose per injection was equivalent to decreasing the fixed ratio.