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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.

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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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-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.

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.

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.

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.

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.

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.

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)

The protein fractions precipitated by ammonium sulfate from the bovine, human and Greenland's seal blood sera enhanced the pain sensitivity of mice, rats and rabbits. The proteins fraction of the seal blood serum was divided in six subfractions by ion-exchange chromatography. One of these subfractions clearly showed hyperalgesic properties, while the others had an opposite effect. The collagenase hydrolysate of the same protein fraction had an analgetic activity. The results of this and previous studies suggest the occurrence of one more nociception-regulating protein-peptide system in mammals.

Using the conditioned taste aversion baseline of drug discrimination learning, animals were trained to discriminate diprenorphine from distilled water. In subsequent generalization tests, the opiate antagonists naltrexone and naloxone and the mixed opiate agonist/antagonist nalorphine substituted for the diprenorphine stimulus in a dose-dependent manner, while the opiate agonist morphine and the nonopiate pentobarbital failed to substitute even at the highest doses tested. That a range of opiate antagonists substituted for the diprenorphine stimulus (and an opiate agonist and a nonopiate failed to substitute) suggest that diprenorphine's antagonist properties may mediate the discrimination, presumably by blocking endogenous opiate activity. The ability of these drugs to substitute for the diprenorphine stimulus may also be a function of this receptor activity. The differences in the specific generalization patterns reported in the present assessment and those of earlier reports were discussed.

In experiments on white outbred male rats the specificity was studied of the influence and mechanisms of action of acute alcoholization (30%-solution of ethanol, intraperitoneally, 0.7 g/kg) on the activity of functionally different neurones of the ventromedial hypothalamus. Experimental results showed that the neurones, the activity of which lowered after saturation (I-st type), increased the discharges frequency at administration of ethanol. Nerve cells, the activity of which increased (II-nd type) and did not change (III-nd type) after saturation, had inhibitory character of reaction in response to alcoholization. The increase of serotonin content in the brain elicited by intraperitoneal administration of 5-OTPh (50 mg/kg) blockaded the action of ethanol on the nerve cells of the I-st type and did not change the effect of the alcohol on the neurones of the II-nd and III-nd types. Preliminary lowering of the noradrenaline level in the brain (disulphiram, intraperitoneally, 100 mg/kg) and blockade of opiate receptors (nalorphine, 5 mg/kg) fully eliminated ethanol influence on the activity of all types of neurones.

1. The influence of opioids in modulating gastric acid secretory responses has been investigated in the continuously perfused stomach of the anaesthetized rat. 2. Intravenous administration of morphine (0.75-3 mg kg-1) or the peripherally acting enkephalin analogue, BW443C (0.75-3 mg kg-1), substantially augmented acid secretion in basal conditions. These effects were significantly inhibited by the opioid antagonists naloxone (1 mg kg-1) and the peripherally acting N-methylnalorphine (2 mg kg-1). When administered alone, neither opioid antagonist influenced basal acid output. 3. Acid secretory responses to different levels of gastric distension (5-20 cmH2O) were significantly and dose-dependently reduced in rats pretreated with morphine (3 mg kg-1) or BW443C (1.5 mg kg-1). Previous administration of either naloxone or N-methyl nalorphine reversed the inhibitory effects of opioids on gastric acid secretion stimulated by distension. Likewise, blockade of opioid receptors with naloxone or N-methylnalorphine significantly increased acid output induced by distension. 4. Levels of serum gastrin in control animals were not increased after intragastric distension (20 cmH2O). Pretreatment with BW443C (1.5 mg kg-1) did not modify the levels of gastrin present during basal or distension stimulated conditions. 5. Pretreatment with morphine or BW443C did not influence the acid responses to i.v. injection of pentagastrin (100 micrograms kg-1), histamine (5 mg kg-1) or carbachol (4 micrograms kg-1). Acid secretion induced by i.v. administration of 2-deoxy-D-glucose (150 mg kg-1) was reduced in rats pretreated with morphine but not with BW443C. Gastric secretory responses to insulin (0.3 i.u. kg-1) were not modified by i.v. morphine.6. These observations support a role for peripherally acting opioids in the regulation of gastric acid secretion during basal and distension-stimulated conditions.

On the basis of their in vivo activity and binding affinity, nalorphine and (-)SKF 10,047 were classified as mixed agonist/antagonist compounds. However, in isolated tissue preparations without a selective antagonist to block their agonist effect, the characterization of these compounds and the determination of their antagonist activity were very difficult. Nor-binaltorphimine, a selective kappa-opioid receptor antagonist, was used in the longitudinal muscle preparations of the guinea pig ileum to block the kappa-agonist activity of nalorphine and (-)SKF 10,047. In the absence of their kappa-agonist activity, we were able to determine the mu-antagonist activity using the mu-selective agonist DAMGO ([D-Ala2,N-Me-Phe4,Gly-ol5]enkephalin). The pA2 values for nalorphine and (-)SKF 10,047 were 7.50 and 7.69, respectively.

The involvement of opioid peptides in the mechanism of action of vouacapan, a new experimental compound extracted from seeds of Pterodon poligalaeflorus Benth, was investigated both in mice utilizing acetic acid writhing response and in rats utilizing inflammatory hyperalgesia induced by carrageenan and modified Randall-Selitto method. Vouacapan, in both models, caused a dose-dependent analgesia when injected p.o., s.c. and i.p. The analgesic effect was partially blocked by naloxone, nalorphine and n-methyl-nalorphine. Significant tolerance to analgesic effect was observed following repeated administration of vouacapan or morphine. On the last day of treatment, cross administration revealed symmetrical and asymmetrical cross-tolerance between vouacapan and morphine, in rats and mice, respectively. We conclude that a release of endorphins could be involved in the analgesic mechanism of vouacapan in both models tudied.

The effects of morphine, naltrexone, and nalorphine were studied in rats trained to lever-press for intravenous heroin and then tested under conditions of non-reinforcement. Animals were reinforced for lever-pressing on a continuous reinforcement schedule (100 micrograms/kg per infusion) for 2-3 h each day following which reinforcement was terminated and animals were studied under extinction conditions for the remainder of the session. Each day following the termination of responding under extinction conditions, animals were given a single injection of saline, morphine, nalorphine, or naltrexone; lever-pressing under the extinction conditions was then observed for several hours. When animals adapted to this regimen, very low levels of responding were seen following saline injections; morphine (2 or 10 mg/kg) reinstated vigorous responding that lasted 1-4 h. Naltrexone (2 mg/kg) suppressed responding below the levels seen after saline, and nalorphine (10 mg/kg) had the same effect as saline. These observations support the view that opioid-seeking behavior is primed by the proponent or opioid-like actions of opioids and not by the opponent or drug-opposite effects associated with opioid withdrawal.

Nonopiate dependent animals were trained to discriminate the opiate antagonist naloxone (1 mg/kg) from distilled water within the conditioned taste aversion baseline of drug discrimination learning. Specifically, rats injected with naloxone prior to a saccharin-LiCl pairing, and with its vehicle prior to saccharin alone, rapidly acquired the drug discrimination, avoiding saccharin following the administration of naloxone and consuming saccharin following its vehicle after only three conditioning trials. Once the discrimination was acquired, generalization tests revealed that the opiate antagonists diprenorphine and naltrexone and the mixed opiate agonist/antagonist nalorphine completely generalized to the naloxone cue at doses of 1.8, 5.6 and 18 mg/kg, respectively. That discriminative control was established with a low dose of naloxone (i.e., 1 mg/kg) and other compounds with opiate antagonist activity generalized to the naloxone cue suggest that the stimulus effects of naloxone were likely mediated through the opiate receptor. Because each of these compounds are reported to bind to the mu receptor (with varying affinities and varying degrees of selectivity), the stimulus properties of naloxone are likely mediated at this specific receptor subtype.

The effects of several opioid compounds were examined in rats and squirrel monkeys responding under a fixed-ratio 30 schedule of food presentation. In rats, dose-effect curves were determined before and after acute pretreatment with 5.6 mg/kg of morphine 5 to 6 hr before the session, as well as during and after the termination of a chronic regimen in which rats received 30.0 mg/kg/day of morphine. In monkeys, dose-effect curves were determined before, during and after the termination of a chronic regimen in which monkeys received 6.0 mg/kg/day of morphine. In morphine-pretreated rats, dose-effect curves for the opioid antagonists naloxone, naltrexone and diprenorphine shifted to the left of those determined when rats were not morphine-pretreated; whereas those for the mu-opioid agonists morphine and I-methadone and mixed-action opioids nalorphine, nalbuphine, butorphanol, pentazocine and bremazocine were unaltered. During chronic morphine administration in rats, dose-effect curves for morphine and butorphanol shifted to the right; whereas the dose-effect curves for naloxone and nalorphine shifted to the left of those determined when rats were not treated with morphine. The effects of pentazocine, nalbuphine, bremazocine and the kappa-opioid agonist U50,488 were unaltered in morphine-maintained rats. In morphine-maintained monkeys, the dose-effect curves for morphine and I-methadone shifted to the right; whereas those for naloxone, nalorphine and nalbuphine shifted to the left of the prechronic dose-effect curves. Dose-effect curves for butorphanol, pentazocine and U50,488 were unaltered. Overall, these results suggest that the chronic morphine administration procedure can be used to distinguish opioid compounds based upon their relative mu agonist and antagonist activity.

Nalorphine is an unusual opiate. Whereas low doses of nalorphine antagonize morphine analgesia, higher nalorphine doses are analgesic, with ED50 values (95% CL) of 13.4 (11.5, 15.8) mg/kg in the writhing and 39.5 (26.6, 60.1) mg/kg in the tail-flick assay. Although nalorphine analgesia is sensitive to naloxone, implying an opioid mechanism, neither beta-funaltrexamine, naltrindole nor nor-binaltorphomine antagonized nalorphine analgesia in the tail-flick assay at doses which reversed equianalgesic doses of their respective selective agonists. Nalorphine and the kappa 3 opiate naloxone benzoylhydrazone demonstrated analgesic cross-tolerance regardless of whether the mice were treated chronically with either nalorphine or naloxone benzoylhydrazone. Animals tolerant to nalorphine were not tolerant to either morphine or U50,488H (trans-3,4-dichloro-N-methyl-N-[2-(pyrrolindinyl)-cyclohexyl]- benzeneacetamide). Furthermore, nalorphine retained its analgesic potency in animals tolerant to U50,488H. Nalorphine exerts its analgesia predominantly through supraspinal mechanisms. Against systemically administered nalorphine, the opiate antagonist WIN44,441 ([2,6,11S-(-)-1-cyclopentyl-5-(1,2,3,4,5,6-hexahydro-8-hydroxy-3,6, 11-trimethyl-2,6-methano-e-benazocine-11-yl)-3-pentanone methylsulfonate) reversed nalorphine analgesia 1500-fold more potently when administered i.c.v. (ID50, 0.1 ng) than when given intrathecally (ID50,159 ng). Together these results indicate that nalorphine analgesia in the tail-flick assay does not involve mu, delta or the U50,488H-sensitive kappa 1 receptor and strongly suggest a role for supraspinal kappa 3 receptors.

This study represents the initial step in assessing the discriminative effects of spiradoline (U62,066), a potent congener of the selective kappa-opioid agonist, U50,488. Separate groups of rats were trained to discriminate between SC injections of saline and either 3.0 mg/kg spiradoline or 3.0 mg/kg morphine in a discrete-trial shock-avoidance/escape procedure. Spiradoline-trained rats generalized completely to U50,488 (ED50S for spiradoline and U50,488 were 0.66 and 8.71 mg/kg, respectively), but selected the choice lever appropriate for saline in generalization tests with graded doses of morphine, phencyclidine, and agonist-antagonist opioids with varying degrees of kappa activity, ethylketocyclazocine, nalorphine, and butorphanol. Morphine-trained rats did not generalize to spiradoline. Naltrexone (0.01 or 0.1 mg/kg) blocked surmountably the discriminative effects of both spiradoline and morphine, but was approximately 10-fold less potent against spiradoline. These results indicate that the discriminative effects of spiradoline are mediated by kappa-opioid receptors; meaningful mu-opioid and PCP/sigma components of action were not in evidence. The potency and apparent pharmacological selectivity of spiradoline suggest the potential value of this drug for studying kappa-opioid-mediated stimulus control of behavior.

The purpose of the present experiment was to determine whether the effects of opioids with varying degrees of efficacy at the mu receptor are differentially altered in morphine-tolerant pigeons. To this end, dose-effect curves were determined for high, intermediate, and low efficacy mu agonists in pigeons responding under a schedule of food presentation prior to, during, and after exposure to a regimen of chronic morphine administration. In pigeons treated with 56 mg/kg/daily morphine, the dose-effect curves for the rate-decreasing effects of the high-efficacy mu agonists morphine and fentanyl were shifted to the right of their prechronic positions (i.e., tolerance). A small degree of tolerance was also conferred to the intermediate-efficacy mu agonists (-)-pentazocine and (-)-metazocine, but not to nalbuphine or butorphanol. In contrast to the effects obtained with these mu agonists, the chronic morphine regimen shifted the dose-effects curves of the mu antagonist naloxone and the low-efficacy mu agonists nalorphine and levallorphan to the left of their prechronic positions (i.e., enhanced sensitivity). These findings demonstrate that morphine tolerance confers cross-tolerance to other high efficacy mu agonists, enhanced sensitivity to mu antagonists and low efficacy mu agonists, and little or no cross-tolerance to intermediate efficacy mu agonists. Disadvantages of using schedule-controlled responding to examine the effects of intermediate efficacy mu agonists are discussed.

In Wistar rats of both sexes stress(restraint)- and drug (indomethacin)-induced gastric, as well, as cysteamine-induced duodenal ulceration was treated by morphine and nalorphine, given either alone or in combination. Neither morphine nor nalorphine had a significant anti-ulcerogenic effect. In contrast, the combined morphine + nalorphine treatment showed a highly significant anti-ulcerogenic action in the case of gastric ulceration but it was ineffective on duodenal ulcer formation. It seems that the chi and sigma endogenous opiate receptors play a significant role in the anti-ulcerogenic effect, while the mu receptors are involved to a considerably lesser degree. Moreover, the experimental results strengthen the view that gastric and duodenal ulceration are two different pathological entities and that ulcer formation is only a common consequence.

The effects of subcutaneous administration of morphine, buprenorphine, pentazocine and nalorphine were studied at two dose levels in rats (low dose x 10 and high dose x 20 of equivalent human dose) on the performance of active avoidance responses using a shuttle box. Pretraining injections of both doses of pentazocine and low dose nalorphine impaired acquisition on day 1 and day 2. Morphine and buprenorphine (at both dose levels) and high dose nalorphine did not affect the acquisition process. Post-training administration of morphine (high dose) and buprenorphine (both doses) delayed extinction of active avoidance responses. Low dose of morphine, high dose of pentazocine and both doses of nalorphine did not appreciably affect the extinction process. Mu opioid receptor agonists probably act as reinforcers to facilitate memory.

The effects of several mixed-action opioid agonist/antagonists were examined alone and in combination with the mu-opioid agonist l-methadone and the kappa-opioid agonist, trans-3,4-dichloro-N-methyl-N[2-(1-pyrrolidinyl)cyclohexyl]benzeneace tam ide (U50,488). Effects were examined in squirrel monkeys responding under a schedule of shock titration. Under this procedure shock was scheduled to increase once every 15 sec from 0.01 to 3.7 mA in 30 steps. Five responses on a lever during the 15-sec shock period terminated the shock for 15 sec, after which the shock resumed at the next lower intensity. The intensity below which the monkeys maintained the shock 50% of the time (median shock level) was determined. l-Methadone and U50,488 produced dose-dependent increases in median shock level. Nalbuphine, butorphanol and levallorphan also increased median shock level, but these increases were much smaller than those observed with l-methadone or U50,488. Nalorphine and naltrexone did not increase median shock level. Butorphanol, levallorphan, nalbupine, nalorphine and naltrexone all produced parallel, rightward shifts in both the l-methadone and U50,488 dose-effect curves. The apparent pA2 values obtained for naltrexone in combination with l-methadone (7.7) were at least one log unit larger than those obtained for naltrexone in combination with U50,488 (6.5). Similar differences were revealed for nalbuphine in combination with l-methadone (6.1) and U50,488 (5.2) as well as for nalorphine in combination with l-methadone (6.0) and U50,488 (5.5).(ABSTRACT TRUNCATED AT 250 WORDS)

In fasting dogs between 29 and 53 per cent (median 42 per cent) of newly produced bile was stored in the gall bladder, with the remainder being released directly into the duodenum. Pretreatment with methadone and atropine resulted in the complete closure of the sphincter of Oddi for three hours, and the complete retention of bile in the gall bladder. This effect could be abolished with nalorphine. The minimum dose of methadone to prevent bile from entering the intestines was 0.25 mg/kg, in combination with 0.1 mg/kg atropine. A meal of canned dog food resulted in the release of between 5 and 65 per cent of gall bladder bile (median 31 per cent) into the duodenum. Infusion with cholecystokinin octapeptide emptied the gall bladder more effectively and predictably and a median of 80 per cent (range 62 to 93 per cent) of the bile had been emptied into the duodenum after one hour. It was concluded that the induction of bile storage in the gall bladder with methadone and the subsequent stimulation of its release into the duodenum by the infusion of cholecystokinin produced an endogenous bile acid load that was three to four times larger than that induced by a meal after a period of fasting.

Effects of opioids and opioid antagonists on citric acid-induced cough and reflex bronchoconstriction have been examined in conscious guinea pigs. Airway reflexes produced by inhaled citric acid are mediated by capsaicin sensitive sensory neurons, and we examined particularly the possibility that inhibitory effects of opioids can be exerted locally in the airway. As expected, systemically administered codeine (1-10 mg/kg), meperidine (3-30 mg/kg) and morphine (1-10 mg/kg) dose-dependently inhibited cough and bronchoconstriction. However, inhalations of nebulized codeine (10-100 mg/ml) and morphine (10-30 mg/ml) also produced these effects. The potency and rapid onset of action of inhaled codeine suggest that it exerted its effects without first being metabolized to morphine. The opioid receptor antagonist naloxone completely (10-100 micrograms/kg), and nalorphine (a mixed agonist/antagonist) (1-3 mg/kg) partly, inhibited codeine's antitussive and antibronchoconstrictor effects. Nalorphine alone (3-30 mg/kg) inhibited citric acid induced reflexes, whereas naloxone was without effect. Prior inhalation of a quaternary opioid receptor antagonist, levallorphan methyl iodide (10 mg/ml), abolished the inhibitory effects of inhaled codeine (30 mg/ml). The present data suggest that inhibition of cough and reflex bronchoconstriction can be produced by opioids, acting on mu and kappa receptors located in the guinea pig tracheobronchial tree. The possible existence in the airways of a unique opioid receptor mediating inhibition of cough (as described in the central nervous system) cannot be excluded.

The effects of opioids on intestinal transit of a charcoal meal were assessed in mice. Morphine (s.c. or i.c.v.) produced a fulldose-response curve and DADLE (s.c. or i.c.v.) produced a dose-response curve with a reduced maximum effect. The highly selective, non-equilibrium mu antagonist, beta-funaltrexamine (beta-FNA), antagonized these effects. Nalorphine (s.c.) induced a maximal 42% inhibition of charcoal meal transit that was not antagonized by beta-FNA. Ethylketazocine (s.c.) produced a full dose-response curve, the higher doses of which were antagonized by beta-FNA. Nalorphine and ethylketazocine had no effect upon charcoal meal transit when given i.c.v.. Tolerance to morphine (s.c.) and to higher doses of ethylketazocine (s.c.) was seen in animals pretreated with a morphine pellet. These data suggest that inhibition of charcoal meal transit in mice can be mediated by a central and a peripheral mu-mediated mechanism and that a peripheral k component may also produce a limited inhibition of intestinal motility. Moreover, since a s.c. dose of beta-FNA produced a 10-and 2-fold shift in the ED50 for morphine given s.c. and i.c.v., respectively, yet an i.c.v. dose of beta-FNA produced a 6- and 200-fold shift in the ED50 for morphine given s.c. and i.c.v., respectively, the effects of a s.c. dose of morphine appear to be mediated predominantly by a peripheral, rather than a central, opioid action.

Brain and adrenal catecholamine turnover in adult female rats treated with morphine was investigated. A different time course response of brain and adrenal catecholamines to alpha-methyl-p-tyrosine methyl-ester (AMT) administration in normal rats was observed; the catecholamine turnover rate in adrenal glands appeared to be much slower than in the brain. Acute morphine increased the turnover of brain dopamine and noradrenaline as well as of adrenal catecholamines, whereas chronic morphine treatment induced a decrease in the turnover of brain noradrenaline. Withdrawal induced by nalorphine produced an increase in the utilization of brain noradrenaline and adrenal catecholamines; this effect could be related to the withdrawl stress situation induced by the opiate antagonist. Although the mechanism of morphine action may implicate other neurotransmitters besides catecholamines, our results contribute to evidence that brain and adrenal catecholamines could be involved in the mechanism of morphine tolerance and/or dependence.

24 hartebeests (Alcelaphus buselaphus major), one waterbuck (Kobus defassa) and 16 kobs (Kobus kob) were immobilized during field work in Upper Volta. The use of the newly developed piperidine derivative R 33799 at weight treated dosage levels can be strongly recommended for the immobilisation of hartebeest. In this species the drug produces a sufficient deep analgesia within a reasonable short period for all handling purposes. It is safe, has a wide therapeutic index, can be used in syringes of not more than 1 ml capacity and is quickly reversed by the antidote (nalorphine hydrobromide). No fatalities occurred. The limited number of kobs immobilized did not allow final conclusions to be drawn on the compatibility of the drug for this game species unless the central nervous side effects as described can be explained better and overcome. Further investigation should be undertaken to study the reaction of kob to lower dosages of R 33799 and other combinations of this analgesic with other neuroleptics. Nalorphine hydrobromide proved to be a useful antidote in hartebeest, but less effective in kob.

Drug self-administration is controlled, in part, by the subject's history of drug exposure. Although a history of drug administration is not necessary for many drugs to function as reinforcers, prior exposure can increase the likelihood that certain drugs, such as ethanol, will maintain behavior. While it has been demonstrated that physiological dependence is not necessary for a drug to function as a reinforcer, the conditions under which such dependence is maintained can control the later self-administration of the drug. Once drug-maintained behaviors are established, the particular drug that maintains behavior can influence the initial pattern of intake of a new drug and thus the dose of that drug that will maintain behavior. Additionally, under certain conditions, similarity between the discriminative stimulus effects of the drug that previously maintained behavior and those of a new drug can increase the likelihood that the new drug will function as a reinforcer. Finally, stimuli that have been paired with drug administration can powerfully control later drug-maintained behavior, the direction of such control being determined by the conditions under which such pairing occurred. In summary, both the type of drug with which a subject has experience as well as the contingencies governing that experience contribute to subsequent drug self-administration.

The effects of morphine, nalorphine and morphine withdrawal on intracerebroventricular kainic acid (0.1 and 0.2 micrometers) lethal toxicity were examined. Morphine (40 mg/kg) potentiated the lethal effect of a low (0.1 micrometer) but not that of a higher dose (0.2 micrometer) of kainic acid. Nalorphine (20 mg/kg) did not affect the toxic action of kainic acid. The mice during morphine withdrawal were very sensitive to the lethal effects of both doses of the amino acid. Kainic acid toxicity seems to be closely related to the functional state of the opiate system. Moreover, it seems that in tolerant animals a supersensitivity of glutamatergic receptors is likely to occur.

Miosis produced by codeine is not antagonized by nalorphine until large oral doses are administered for several days. The present experiment was conducted in order to further study this characteristic of the codeine effect. Eight healthy male volunteers, who were former drug users, were divided into two groups. Subjects in the first group were given a continuous infusion of codeine, 30 mg/hr for 11-16 hr. No subjective effects were reported by the volunteers. In three of the individuals definite miosis antagonized by nalorphine was observed at 9.5 hr. The dose of codeine for the second group was 60 mg/hr for 11 hr. Mild but definite subjective effects were experienced by each of the participants in this group. Miosis appeared between 2 and 6 hr. Challenges at 4 and 6 hr were positive in two subjects and negative or equivocal in the other two. Codeine was excreted in the urine as free and conjugated codeine, morphine, and norcodeine. Maximum rates of excretion were similar for both groups, suggesting that the maximum amount of codeine that can be metabolized is equal or less than 30 mg/hr. Also codeine clearance, being greater than creatinine clearance, suggests that codeine might be excreted by glomerular filtration and tubular secretion. Blood levels of codeine in the 60 mg/hr group were about 10 times those reported as therapeutic. However, morphine or norcodeine were not detectable by the methods used.

The effects of some opiates on the horizontal and vertical components of locomotor activity in mice were measured in actometers fitted with photoelectric cells counting horizontal displacements and the striding over a low partition wall. Morphine and fentanyl, used at low doses which did not modify the horizontal locomotor activity, decreased the striding over of the partition wall. Such a dissociation was not observed with either kappa or sigma agonists which affected in the same way the two components of the locomotor activity: both components were inhibited by kappa agonists and slightly stimulated by the sigma agonist SKF 10047. Although the opiate-induced inhibition of the striding over was induced by low doses of morphine, relatively high doses of naloxone were necessary for its antagonism. This apparently difficult antagonism could result in fact from the sedation which appeared when morphine was associated with naloxone.

The acquisition of shock avoidance behavior by rats was studied in an automated Y-maze which incorporates a simultaneous brightness discrimination paradigm. When administered prior to each of 5 consecutive daily sessions, two opiate derivatives with psychotomimetic properties, cyclazocine and N-allylnormetazocine, impaired the acquisition of brightness discrimination at doses which also increased movements between trials. These effects were similar to those produced by phencyclidine, ketamine, and a high dose of d-amphetamine. Pretreatment with morphine, pentazocine and scopolamine at motor stimulant doses before each training session did not affect acquisition of brightness discrimination. Nalorphine, naltrexone, and chlorpromazine also had no effect on brightness discrimination, even at motor depressant doses. Whereas the motor stimulation produced by morphine or pentazocine was blocked by naltrexone, the motor stimulation and discrimination disruption produced by cyclazocine or N-allylnormetazocine were only incompletely antagonized by naltrexone. The results demonstrate similarities between psychotomimetic opiates and phencyclidine-like compounds and may reflect the sensory or cognitive disturbance produced by these drugs in man.