levorphanol and dezocine

Recent studies indicate that morphine is more potent as an antinociceptive agent in male than female rodents and monkeys.. To evaluate the influence of sex, nociceptive stimulus intensity and an opioid's relative efficacy on opioid-induced antinociception in rat strains (F344 and Lewis) that display differential sensitivity to morphine antinociception.. Antinociceptive testing was conducted using a rat warm-water (50-56 degrees C) tail-withdrawal procedure. Dose-response and time-course determinations were performed with various opioids.. Across the nociceptive stimulus intensities tested, the high-efficacy mu opioids morphine, etorphine, and levorphanol were equally effective in males and females, but on average 2.5-fold more potent in males. At moderate stimulus intensities, the low-efficacy mu opioid buprenorphine was approximately 0.4-fold more potent in males, and at higher stimulus intensities more potent and effective (greater maximal effect) in males. At low stimulus intensities, the low-efficacy mu opioid dezocine and the mu/kappa opioid butorphanol were greater than 8.9-fold more potent in males, and at moderate stimulus intensities were more potent and effective in males. At a low stimulus intensity, the mu/kappa opioid nalbuphine was more potent and effective in males. At stimulus intensities in which buprenorphine, dezocine, butorphanol, and nalbuphine produced maximal effects in males but not females, these opioids antagonized the effects of morphine in females. Genotype-related differences were noted as opioids were generally more potent in F344 than Lewis males, whereas no consistent differences were observed between F344 and Lewis females.. That sex differences in the potency and effectiveness of opioids increased with decreases in the opioid's relative efficacy and with increases in the nociceptive stimulus intensity suggests that the relative efficacy of mu opioids as antinociceptive agents is greater in male than female rats.

Topics: Analgesics, Opioid; Animals; Bridged Bicyclo Compounds, Heterocyclic; Buprenorphine; Butorphanol; Cycloparaffins; Dose-Response Relationship, Drug; Etorphine; Female; Genotype; Levorphanol; Male; Morphine; Nalbuphine; Rats; Rats, Inbred F344; Rats, Inbred Lew; Reaction Time; Receptors, Opioid, mu; Sex Factors; Species Specificity; Tetrahydronaphthalenes

We examined the effects of several opioids that vary in intrinsic efficacy at the mu-opioid receptor alone and in combination with morphine in a rat warm water tail withdrawal procedure using 50 degrees C and 52 degrees C water (i.e., low- and high-stimulus intensities). Morphine, levorphanol, dezocine, and buprenorphine produced dose-dependent increases in antinociception using both stimulus intensities. Butorphanol produced maximal levels of antinociception at the low, but not at the high, stimulus intensity, whereas nalbuphine failed to produce antinociception at either stimulus intensity. For cases in which butorphanol and nalbuphine failed to produce antinociception alone, these opioids dose-dependently antagonized the effects of morphine. When levorphanol, dezocine, and buprenorphine were combined with morphine, there was a dose-dependent enhancement of morphine's effects. Similar effects were obtained at the low-stimulus intensity when butorphanol was administered with morphine. In most cases, the effects of these combinations could be predicted by summating the effects of the drugs when administered alone. These results indicate that the level of antinociception produced by an opioid is dependent on the intrinsic efficacy of the drug and the stimulus intensity. Furthermore, the level of antinociception produced by the opioid, not necessarily the opioids' intrinsic efficacy, determines the type of interaction among opioids.. Compared with high-efficacy opioids, lower efficacy opioids produce lower levels of pain relief, especially in situations of moderate to severe pain. When opioids are given in combination, the effects can only be predicted on the basis of the antinociception obtained when the drugs are administered alone.

Topics: Analgesics, Opioid; Animals; Bridged Bicyclo Compounds, Heterocyclic; Buprenorphine; Butorphanol; Cycloparaffins; Dose-Response Relationship, Drug; Drug Combinations; Levorphanol; Morphine; Nalbuphine; Narcotics; Nociceptors; Pain; Physical Stimulation; Rats; Rats, Long-Evans; Receptors, Opioid, mu; Tetrahydronaphthalenes