Predicted structures for kappa opioid G-protein coupled receptor bound to selective agonists.

Human kappa opioid receptor (κ-OR), a G protein-coupled receptor (GPCR), has been identified as a drug target for treatment of such human disorders as pain perception, neuroendocrine physiology, affective behavior, and cognition. In order to find more selective and active agonists, one would like to do structure based drug design. Indeed, there is an X-ray structure for an antagonist bound to κ-OR, but structures for activated GPCRs are quite different from those for the inactive GPCRs. Here we predict the ensemble of 24 low-energy structures of human kappa opioid receptor (κ-OR), obtained by application of the GEnSeMBLE (GPCR Ensemble of Structures in Membrane Bilayer Environment) complete sampling method, which evaluates 13 trillion combinations of tilt and rotation angles for κ-OR to select the best 24. To validate these structures, we used the DarwinDock complete sampling method to predict the binding sites for five known agonists (ethylketocyclazocine, bremazocine, pentazocine, nalorphine, and morphine) bound to all 24 κ-OR conformations. We find that some agonists bind selectively to receptor conformations that lack the salt bridge between transmembrane domains 3 and 6 as expected for active conformations. These 3D structures for κ-OR provide a structural basis for understanding ligand binding and activation of κ-OR, which should be useful for guiding subtype specific drug design.

Discriminative stimulus effects of acute morphine followed by naltrexone in the squirrel monkey: a further characterization.

The discriminative stimulus effects of acute morphine followed by naltrexone have been described previously in nonhuman primates. The purposes of this study were to 1) extend the pharmacological characterization of the discrimination by testing mu-opioid agonists other than morphine and opioid-like compounds other than naltrexone and 2) to examine further the relationship between agonist pretreatment time and manifestation of the cue produced by morphine followed by naltrexone. Subjects were trained to discriminate 1.7 mg/kg morphine --> 0.1 mg/kg naltrexone (MOR --> NTX) versus saline followed by 0.1 mg/kg naltrexone. When combined with 0.1 mg/kg naltrexone, all agonists tested, save buprenorphine, meperidine, and nalbuphine, produced dose-dependent increases in MOR --> NTX-appropriate responding, culminating in criterion levels of responding. Comparing agonist ED50 values revealed a rank order of potency of etorphine >> fentanyl >> levorphanol > heroin > or = methadone > or = nalbuphine > or = morphine. ED50 values for buprenorphine and meperidine could not be calculated. MOR --> NTX-appropriate responding after doses of agonist that produced criterion or near criterion levels of responding was also a function of naltrexone dose. After morphine pretreatment, diprenorphine and nalorphine, but not buprenorphine, dose-dependently substituted for naltrexone. The MOR --> NTX discrimination also depended upon the interval between morphine and NTX administration. Finally, 1-h pretreatment with morphine and etorphine, but not buprenorphine, followed by naltrexone generalized to 4 h MOR --> NTX. These results suggest a minimum efficacy requirement of acutely administered agonists together with the naltrexone training dose for stimulus control of behavior. However, in some cases this requirement can be overcome with higher doses of naltrexone.

[Effects of naloxone, nalorphine and dalargin on the course of shock due to the combination of mechanical trauma and acute blood loss]. / Vliianie naloksona, nalorfina i dalargina na techenie shoka ot sochetaniia mekhanicheskoi travmy i ostroi krovopoteri.

Acute experiments on 75 adult rabbits were made to study action of intravenous injections of naloxone (0.1 mg/kg), nalorphine (0.5 and 2.0 mg/kg) and dalargin (0.1 mg/kg) in early and late periods of irreversible shock caused by combination of non-shockogenic mechanical trauma of the hip and safe blood loss from the femoral artery. In early shock antagonists of opiate receptors did not influence the outcomes while injection of dalargin prolonged survival of the animals noticeably. In late shock injection of the drugs had no significant influence on the course of the pathological process while reinfusion did not help the animals to survive. In the late shock injection of the above drugs after blood reinfusion increased the rabbits' survival.

Social and environmental enrichment enhances sensitivity to the effects of kappa opioids: studies on antinociception, diuresis and conditioned place preference.

Previous studies have reported that social and environmental enrichment can have a marked impact on the functional maturation of the central nervous system and may influence an organism's sensitivity to psychotropic drugs. The purpose of the present study was to examine the effects of social and environmental enrichment on sensitivity to drugs possessing activity at the kappa opioid receptor. Rats were obtained at weaning and randomly assigned to one of two housing conditions: isolated rats were housed individually with no visual or tactile contact with other rats; enriched rats were housed in groups of four in large cages and given various novel objects on a regular basis. After 7 weeks under these conditions, the effects of spiradoline, U69,593 and nalorphine were examined in the warm water, tail-withdrawal procedure. The effects of spiradoline were also examined on urine output and in the conditioned place preference procedure. Enriched rats were more sensitive to the antinociceptive effects of all the opioids examined in the tail-withdrawal procedure, and were more sensitive to the effects of spiradoline on urine output and in the conditioned place preference procedure. Following the conclusion of these tests, housing conditions were reassigned, such that isolated rats were transferred to enrichment cages, and enriched rats were transferred to isolation cages. After 7 weeks under these new conditions, the two groups were equally sensitive to the antinociceptive effects of spiradoline, indicating that the effects of the initial housing conditions were, in part, reversible. Collectively, these data suggest that enriched rats are more sensitive than isolated rats to the effects of kappa opioids, and that the kappa opioid receptor system is sensitive to social and environmental manipulations after weaning.

Sex and rat strain determine sensitivity to kappa opioid-induced antinociception.

RATIONALE: Recent studies indicate that sex and rodent strain are determinants of sensitivity to opioid-induced antinociception. OBJECTIVES: The present study examined the influence of sex and rat strain on kappa opioid-induced antinociception using a series of kappa opioids that vary in their relative effectiveness. METHODS: In a warm-water (50, 52 and 55C) tail-withdrawal procedure, the antinociceptive effects of kappa opioids were determined in male and female rats of the F344, Lewis and Sprague-Dawley (SD) strains. RESULTS: In both males and females of each strain, spiradoline produced high levels of antinociception across all nociceptive stimulus intensities, whereas U50,488 produced high levels only at the low and moderate nociceptive stimulus intensities. Sex differences in the potency and effectiveness of these kappa opioids were relatively small and not consistently obtained. Enadoline, bremazocine and nalorphine were less effective than spiradoline in producing antinociception, and at low and moderate nociceptive stimulus intensities these opioids were both more potent and effective in F344 and SD males than their female counterparts. In contrast, in Lewis rats, only bremazocine was more potent and effective in males. In combination tests, bremazocine shifted the spiradoline dose-effect curve leftward and/or upward in males and rightward in females (i.e., antagonized spiradoline). In contrast, in both males and females enadoline shifted the spiradoline dose-effect curve leftward and/or upward. CONCLUSIONS: These data indicate that kappa opioids were generally more potent and effective as antinociceptive agents in males than females. Similar to data obtained with micro opioids, the magnitude of these sex differences was generally larger with the less effective kappa opioids and determined, in part, by rat strain and nociceptive stimulus intensity.

Discriminative stimulus effects of two doses of fentanyl in rats: pharmacological selectivity and effect of training dose on agonist and antagonist effects of mu opioids.

RATIONALE: Discriminative stimulus effects of mu opioids vary systematically as a function of training dose. Differences among training doses may arise from multiple mechanisms. OBJECTIVES: In vivo apparent pA(2) analyses were used to examine the contributions of opioid mechanisms to stimulus control by low and high training doses of the mu opioid fentanyl. METHODS: Saline and one of two doses of fentanyl, administered s.c., were established as discriminative stimuli in two groups of rats (low training dose group: 0.01 mg/kg; high training dose group: 0.04 mg/kg). Generalization tests and in vivo apparent pA(2) analyses were used to evaluate receptor mechanisms of stimulus control. RESULTS: Fentanyl, etonitazene, methadone, and morphine evoked full fentanyl generalization in both groups but were more potent in the low-dose group. Spiradoline and d-amphetamine did not evoke generalization in either group. Naltrexone antagonized stimulus and rate-altering effects of fentanyl in both groups, with apparent pA(2) values of 7. 6 in the low-dose group and 7.5 in the high-dose group. Nalbuphine and nalorphine evoked full generalization in the low-dose group but less than 40% generalization in the high-dose group. In the high-dose group, nalbuphine and nalorphine antagonized the stimulus and rate-altering effects of fentanyl with apparent pA(2) values of 5.3 and 6.1, respectively, demonstrating lower efficacy mu actions. CONCLUSIONS: Changes in fentanyl training dose preserved the mu opioid selectivity of stimulus control but altered the intensity of the transduced mu opioid stimulus required for generalization. These differences in intensity of the fentanyl stimulus determined whether low efficacy mu opioids would evoke or antagonize fentanyl generalization.

Differential antagonism of the rate-decreasing effects of kappa-opioid receptor agonists by naltrexone and norbinaltorphimine.

Eight kappa-opioid receptor agonists were examined for their effects in squirrel monkeys responding under a fixed interval 3-min schedule of stimulus termination. Six of these kappa-opioid receptor agonists decreased dose-dependently the total number of responses and with an order of potency consistent with kappa-opioid receptor interaction. Three of these kappa-opioid receptor agonists, bremazocine, U69,593 [[(5a,7a,8b)-(+)-N-[7-(1-pyrrolidinyl)-1-oxaspiro(4,5)dec-8-yl)] benzeneacetamide] and enadoline, were evaluated following pretreatment with 1.0 mg/kg of naltrexone or 3.0 mg/kg of norbinaltorphimine. The effects of the three agonists were antagonized significantly by naltrexone, but only those of bremazocine and U69,593 were antagonized significantly by norbinaltorphimine. Statistical analysis of the data averaged over six monkeys revealed that naltrexone was significantly more potent than norbinaltorphimine at antagonizing enadoline and U69,593, but naltrexone and norbinaltorphimine were equipotent at antagonizing bremazocine. Moreover, naltrexone was 8-fold more potent at antagonizing U69,593 and enadoline than at antagonizing bremazocine. These results suggest that under these conditions the effects of U69,593 and enadoline may be mediated, in part, by a different receptor population, perhaps a subtype of kappa-opioid receptors, from the one that mediates the effects of bremazocine.

Nalorphine's ability to substitute for morphine in a drug discrimination procedure is a function of training dose.

Rats trained to discriminate the mu agonists fentanyl or morphine from their respective vehicles generalize to the partial mu agonist nalorphine incompletely and inconsistently. Any number of factors may influence the generalization patterns obtained, one of which being the specific dose of the full opioid agonist used during training, a factor reported to influence generalization with other partial opioid agonists. To assess if training dose influences stimulus generalization to nalorphine and to support its role in the aforementioned variability across studies, in the present experiments rats were trained to discriminate either a low (5.6 mg/kg) or a high (10 mg/kg) dose of morphine from distilled water within the taste aversion baseline of drug discrimination learning. Subjects were then given a range of doses of morphine, nalorphine, methadone, or naloxone to assess the degree of substitution (if any) of these compounds for the training dose of morphine. For all subjects, morphine fully substituted for itself, and the opioid antagonist naloxone failed to substitute for the morphine cue. Rats generalized the morphine cue to nalorphine in subjects trained at the lower dose but not in subjects trained at the higher dose. Rats generalized the morphine cue to methadone in the latter group (the high dose group), indicating that the failure to generalize to nalorphine in this group was not a general inability of an opioid agonist to substitute for morphine. Naloxone blocked morphine stimulus control in all subjects and nalorphine control in the low-dose group for which nalorphine substituted for morphine, suggesting that morphine control (and the nalorphine substitution) was based on opioid activity. These results indicate that the substitution patterns of nalorphine in morphine-trained subjects are a function in part of the dose of morphine used in training and support the position that nalorphine is a partial opioid agonist with intermediate efficacy.

Methodological aspects of the use of a calibrator in in vivo microdialysis-further development of the retrodialysis method.

PURPOSE: To investigate the performance of two alternative retrodialysis recovery methods and to describe the influence of different recoveries on the reliability in estimating unbound extracellular concentrations of morphine. METHODS: Unbound concentrations of morphine in striatum and in blood were determined by microdialysis after a 10 min i.v. infusion in freely moving rats. In vivo recovery of morphine was determined by morphine itself, retrodialysis by drug, and by the calibrator nalorphine, retrodialysis by calibrator. RESULTS: The low calibrator recovery in striatum (8.6%) resulted in large variability in the estimation of unbound extracellular concentrations when retrodialysis by calibrator was used. In blood, where the recovery was higher (36%), the variability was smaller. Also, when retrodialysis by drug was used, the variability remained low. This difference is caused by the propagation of errors in the way retrodialysis recovery is determined. Therefore, calibrator recovery values > or =20% are preferable in concentration estimations using retrodialysis by calibrator. CONCLUSIONS: When no time-dependent change in recovery is observed, retrodialysis by drug determined before the systemic administration is the best method. The calibrator is valuable as a quality control during the experiment.

The mu opioid irreversible antagonist beta-funaltrexamine differentiates the discriminative stimulus effects of opioids with high and low efficacy at the mu opioid receptor.

The purpose of the present study was to determine the relative intrinsic efficacy of various opioids using the irreversible mu opioid antagonist beta-funaltrexamine (betaFNA). To this end, pigeons were trained to discriminate 3.0 (n=6) or 1.8 (n=1) mg/kg morphine from distilled water in a two-key, food-reinforced, drug discrimination procedure. The mu opioids fentanyl, l-methadone, buprenorphine, butorphanol, nalorphine, nalbuphine and levallorphan, as well as the delta opioid BW373U86, substituted completely for the morphine stimulus. The stimulus effects of morphine were antagonized (i.e., produced a significant increase in the ED50 value) by a 10 mg/kg but not a 5 mg/kg dose of betaFNA. Antagonist effects of betaFNA were observed following a 2-h pretreatment, but not following 26-, 50-, 74-, 98- or 146-h pretreatments. The stimulus effects produced by fentanyl, l-methadone and buprenorphine were not antagonized by doses of betaFNA as high as 20, 10 and 10 mg/kg, respectively. The lowest dose of betaFNA required to antagonize the stimulus effects of butorphanol was 10 mg/kg, whereas the effects of nalorphine, nalbuphine and levallorphan were antagonized by a dose of betaFNA as low as 5 mg/kg. The delta BW373U86 substituted for the morphine stimulus, and this effect was not antagonized by 10 mg/kg betaFNA. The pkB values for naloxone (1.0 mg/kg) against the stimulus effects of fentanyl (6.70) and morphine (6.52) were considerably higher than that for BW373U86 (4.60), indicating further that the morphine-like stimulus effects produced by BW373U86 were not mediated by activity at the mu opioid receptor. These findings indicate that the strategy of irreversible antagonism can be used effectively to differentiate opioids with varying degrees of intrinsic efficacy at the mu opioid receptor in a pigeon drug discrimination procedure. In particular, the ranking of these drugs by relative intrinsic efficacy at the mu opioid receptor is: l-methadone=fentanyl> or =buprenorphine> or =morphine> or =butorphanol>nalorphine=nalbuphine=levallorphan. Additionally, the short-acting effect of betaFNA in the pigeon suggests that the recovery of mu opioid receptor function varies across species.

[The mechanism of the action of opiate receptor antagonists in acute shock-induced blood loss]. / K mekhanizmu deistviia antagonistov opiatnykh retseptorov pri ostroi shokogennoi krovopotere.

Experiments on 81 rabbits showed that intravenous infusion of naloxone and nalorphine in different periods of hemorrhagic shock promoted the death of animals. It is concluded that in hypoxia only the antagonistic activity of the drugs is manifested as total inactivation of the endogenous opioid system depriving it of its protective functions. Nalorphine infused after blood reinfusion exhibits agonistic activity and additional activation of the endogenous opioid system promotes the use of the biological reserves by the organism, which saves some of the animals from death or essentially prolongs the survival of the others. Infusion of naloxone after blood transfusion has no effect on the outcome.

Antinociceptive and immunosuppressive effects of opiate drugs: a structure-related activity study.

1. Although it is well known that morphine induces significant immunosuppression, the potential immunosuppressive activity of morphine derived drugs commonly used in the treatment of pain (codeine, hydromorphone, oxycodone) has never been evaluated. 2. We evaluated in the mouse the effect of the natural opiates (morphine and codeine) and synthetic derivatives (hydromorphone, oxycodone, nalorphine, naloxone and naltrexone) on antinociceptive thresholds and immune parameters (splenocyte proliferation, Natural Killer (NK) cell activity and interleukin-2 (IL-2) production). 3. Morphine displayed a potent immunosuppressive effect that was not dose-related to the antinociceptive effect, codeine possessed a weak antinociceptive effect and limited immunosuppressive activity; nalorphine, a mu-antagonist and kappa-agonist, exerted a potent immunosuppressive effect, but had very weak antinociceptive activity. The pure kappa-antagonist nor-BNI antagonized the antinociceptive, but not the immunosuppressive effect of nalorphine. 4. Hydromorphone and oxycodone, potent antinociceptive drugs, were devoid of immunosuppressive effects. 5. The pure antagonists naloxone and naltrexone potentiated immune responses. 6. Our data indicate that the C6 carbonyl substitution, together with the presence of a C7-8 single bond potentiates the antinociceptive effect, but abolishes immunosuppression (hydromorphone and oxycodone). 7. The single substitution of an allyl on the piperidinic ring resulted in a molecule that antagonized the antinociceptive effect but maintained the immunosuppressive effect. 8. Molecules that carry modifications of C6, the C7-8 bond and C14, together with an allyl or caboxymethyl group on the piperidinic ring antagonized both the antinociceptive and the immunosuppressive effect of opiates and were themselves immunostimulants.

Discriminative stimulus and subjective effects of opioids with mu and kappa activity: data from laboratory animals and human subjects.

Although a large and rich body of data is available regarding the discriminative stimulus effects of opioids in laboratory animals and human subjects, it has been difficult to reconcile the data obtained from these two different sources. Therefore, the purpose of this review is to bring together data from both animal and human laboratories and systematically to compare the discriminative stimulus effects of opioids, in particular those with activity at both mu and kappa opioid receptor types (i.e., the mixed action opioids). The data that can be collected from laboratory animals differ from the data that can be collected in human subjects. In general, the advantage of studies in laboratory animals is that they can investigate very broad dose ranges of opioids as well as some very selective opioids that are not available for investigation in human subjects. Although investigations in human subjects are limited by the compounds and doses available for examination, the advantage of these studies is that they can examine the subjective as well as the discriminative stimulus effects of opioids. Taken together, studies conducted in laboratory animals and human subjects indicate that the mixed action opioids are best classified as intermediate efficacy mu agonists with additional activity through other non-mu, possibly kappa opioid systems.

N-cubylmethyl substituted morphinoids as novel narcotic antagonists.

N-Cubylmethylnormorphine (1) and N-cubylmethylnoroxymorphone (2) have been synthesized and found to be more potent ligands at the mu and kappa opioid receptors than morphine and oxymorphone respectively. In the guinea-pig ileum preparation, compounds 1 and 2 were characterized as opioid mu antagonists (Ke = 68 and 16 nM, respectively). Compound 2 also showed effective kappa-antagonism (Ke = 22 nM). The narcotic antagonism activity of 1 has been confirmed by in vivo assays.

Agonist regulation of the expression of the delta opioid receptor in NG108-15 cells.

Exposure of neuronal cells to the chronic presence of opiates leads to a complex series of biochemical events which reflect the changes that result in tolerance and dependence in animals. To achieve a better understanding of the molecular mechanisms underlying these processes, we have examined the effect of agonist efficacy on the regulation of the delta-opioid receptor mRNA in NG108-15 cells. Incubation with various opiates decreased receptor numbers in the order of their efficacy. Northern blot analysis showed that there are 4 size classes of mRNA coding for the delta-opioid receptor in NG108-15 cells even though only one known protein species is found. Moreover, the amount of each transcript is coordinately decreased by long-term etorphine treatment, but not necessarily to the same extent. The etorphine-induced decrease in receptor mRNA was found to be slow in onset, whereas a much more rapid loss of receptor number was observed. This disparity suggests that the down-regulation induced by etorphine can occur both at the levels of receptor protein modification and receptor gene expression, and that the mechanisms of the two processes may be different.

Peptide and nonpeptide ligands for opioid receptors.

Morphine and other natural alkaloid opiates have been used in medicine for centuries. Synthesis of analogs of opiate alkaloids and primary structure activity studies have almost a hundred year history. The endogenous opioid peptides, their genetic expression and enzymatic metabolism, have been described. A number of non-peptide and peptide analogs have led to characterization of opioid receptor types (mu, delta, and kappa) and propose their subtypes. Very recently, all types of opioid receptors of different species have been characterized at the molecular level. The progressive studies of the opioid system have allowed introduction of various new types of drugs. In addition, the opioid system, as one of the best characterized, is often used as a model for studies in neurobiology as well as in bioorganic chemistry. Therefore, opioid system is a good example of the tremendous progress in medicinal chemistry, but also an illustrations of the limitations of scientific tools currently used.

Differential cross-tolerance between analgesia produced by alpha 2-adrenoceptor agonists and receptor subtype selective opioid treatments.

Analgesic cross-tolerance between alpha 2-adrenoceptor and opioid receptor agonists was studied using the mouse tail-flick assay. Mice tolerant to clonidine (0.3 mg/kg s.c.) or xylazine (7 mg/kg s.c.) were cross-tolerant to morphine (5 mg/kg s.c.), nalorphine (70 mg/kg s.c.) and supraspinal [D-Ala2,MePhe4,Gly(ol)5]enkephalin (DAMGO; 4 ng i.c.v.), but not trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)- cyclohexyl] benzeneacetamide methanesulfonate (U50,488; 5 mg/kg s.c.), spinal DAMGO (10 ng i.t.), supraspinal [D-Pen2,D-Pen5]enkephalin (DPDPE; 9 micrograms i.c.v.) or spinal DPDPE (700 ng i.t.). In the complimentary studies, mice tolerant to morphine and nalorphine were cross-tolerant to both of the alpha 2-adrenoceptor agonists, but U50,488 tolerant mice were not. The results suggest differential interactions between alpha 2-adrenoceptor and mu 1-, mu 2-, delta-, kappa 1- and kappa 3-opioid analgesic circuitry.

Nalorphine as a stimulus in drug discrimination learning: assessment of the role of mu- and kappa-receptor subtypes.

Using the conditioned taste aversion baseline of drug discrimination learning, animals were trained to discriminate nalorphine from distilled water. In subsequent generalization tests, the mu-opiate agonist morphine substituted for the nalorphine stimulus in a dose-dependent manner, while the kappa-opiate agonist U50,488H and the mu-opiate antagonists naloxone and naltrexone failed to do so. That the mu-agonist morphine substituted for the nalorphine stimulus while a kappa-agonist and mu-antagonists failed to substitute indicate that the discriminative control that was established with nalorphine in the present study was mu-agonist receptor-mediated. The basis for this selective control by the mu-receptor subtype may be related to the relative salience of receptor activity in opiate-naive animals. The present results suggest that discriminative control by compounds with activity at multiple receptor sites is not uniformly mediated by specific activity at all of those sites. The specific site mediating discriminative control appears to be a function of the specific training drug.

Neural mediation of the cardiovascular responses to intrathecal administration of substance P in the rat: slowing of the cardioacceleration by an adrenal opioid factor.

Substance P, given intrathecally at the second (T2) or ninth (T9) thoracic level in the anesthetized rat, increased heart rate, arterial pressure and circulating catecholamines. At T9 in adrenalectomized animals and at T2 in intact animals, the cardioacceleration was more abrupt than in intact animals injected at T9 suggesting that the adrenals are not necessary for the cardiovascular responses and that the adrenals may have released a factor which slows the neurally mediated cardioacceleration. As opioids are co-released with catecholamines from the adrenals, naloxone (10 mg/kg i.v.) or nalorphine HCl (which does not cross the blood-brain barrier; 10 mg/kg s.c.) was given 5 min before administration of substance P at T9 in intact rats. In both groups the cardioacceleration was similar to that elicited in adrenalectomized animals, indicating that the adrenal factor was opioid and that its action was peripheral rather than central. When propranolol (10 mg/kg i.v.) was given 3 or 15 min before, substance P increased arterial pressure but heart rate was unchanged, indicating that the opioid factor was not slowing the cardioacceleration by a direct effect on the heart. The results indicate that intrathecal administration of substance P produces a neurally mediated increase in arterial pressure and heart rate and induces the release of an adrenal opioid factor which slows the neurally-mediated cardioacceleration by an action in the periphery. This indicates a functional interaction between humoral and neural sympathetic mechanisms regulating the cardiovascular system.

Discriminative-stimulus effects of the low efficacy mu agonist nalbuphine.

The discriminative stimulus effects of nalbuphine were studied in 15 male Sprague-Dawley rats trained to discriminate 3.2 mg/kg of nalbuphine from saline under a fixed-ratio 15 schedule of food delivery. Cumulative doses of nalbuphine produced nalbuphine lever responding at doses of 1.0 to 10 mg/kg and rate-suppressing effects at doses of 3.2 to 32 mg/kg. Experiments to evaluate the contribution of opioid receptor activity suggested that the stimulus effects of nalbuphine were mediated through mu systems, inasmuch as mu agonists (etorphine, fentanyl, morphine, buprenorphine, GPA 1657 and nalorphine) produced nalbuphine lever responding, whereas kappa agonists [EKC and U-50,488H (trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl] benzeneacetamide methanesulfonate hydrate)] and nonopioids (d-pentazocine, d-amphetamine and ketamine) produced saline lever responding. dl-Pentazocine produced nalbuphine lever responding in one-half the rats tested. Both high and low efficacy agonists produced nalbuphine lever responding, but the antagonist naltrexone produced predominantly saline lever responding. Increasing the training dose of nalbuphine by a 0.50 log unit failed to alter the potency of nalbuphine or any other compound to produce nalbuphine lever responding, suggesting that these training doses produce a maximum amount of stimulation at the mu receptor. Naltrexone antagonized the discriminative stimulus but not the rate-decreasing effects of nalbuphine, suggesting that only the discriminative stimulus effects of nalbuphine are mediated by a mu opioid mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)