Clocinnamox distinguishes opioid agonists according to relative efficacy in normal and morphine-treated rats trained to discriminate morphine

High doses of insurmountable antagonists or frequent administration of high doses of agonists are required to alter the potency of opioid agonists to produce discriminative stimuli. In the present study, insurmountable antagonism and repeated agonist treatment were combined to remove or disable a large enough proportion of μ-opioid receptors to alter the potency or maximal effect for four agonists in male Sprague-Dawley rats trained to discriminate 3.2 mg/kg morphine from saline under a fixed-ratio 15 schedule of food reinforcement. All agonists produced 88 to 100% morphine responding and were differentially sensitive to clocinnamox antagonism (fentanyl < morphine ≤ buprenorphine = nalbuphine). Repeated treatment with 20 mg/kg per day morphine for 6 days decreased by 2- to 3-fold the potency of fentanyl, morphine, and buprenorphine to produce morphine responding. After morphine treatment, 3.2 mg/kg clocinnamox produced a 7-fold further decrease in morphine potency. Clocinnamox (10 mg/kg) produced a 7- and 12-fold further decrease in morphine and fentanyl potency, respectively, a reduction in the slope of the morphine dose response curve, and a suppression of the maximal morphine responding for buprenorphine. Repeated treatment with 10 mg/kg per day morphine for 6 days failed to alter the potency of nalbuphine to produce morphine responding. In these morphine-treated rats, doses of 3.2 or 10 mg/kg clocinnamox suppressed the maximal morphine responding. Taken together, these data indicate that combined insurmountable antagonist and repeated agonist treatment produce additive effects at μ-opioid receptors to diminish discriminative stimulus effects in a manner predicted by the relative efficacy of opioid agonists.