Differential effects of opioid and nonopioid analgesics on conditional discriminations in pigeons

Under the fixed-consecutive-number schedule (FCN), pigeons were reinforced for responding eight or more times on one response key (work key), and then responding once on a second response key. In one component of this schedule, an external stimulus signalled the completion of the response requirement on the work key (FCN 8-SD), whereas no stimulus change was programmed under the other (FCN 8). Across a range of doses, the mu opioid agonist morphine, the kappa opioid agonist U50,488 and the opioid antagonist naloxone had no consistent effect on accuracy under either FCN schedule. Naloxone and U50,488 produced a general flattening of the conditional probability functions by decreasing the conditional probability of response runs exceeding the minimum response requirement of eight consecutive responses on the work key. The sigma agonists phencyclidine and (+)N-allylnormetazocine and the nonopioid analgesics clonidine and l-nantradol produced large decreases in accuracy under the FCN 8 and small decreases under the FCN 8-SD. With the exception of (+)N-allylnormetazocine, these drugs consistently increased the conditional probability of responses runs shorter than the minimum response requirement on the work key. These findings indicate that the accuracy-altering effects of some opioid and nonopioid analgesics depend in part on the type of discrimination task.     

Evaluation of the effects of opioid agonists and antagonists under a fixed-consecutive-number schedule in rats

The effects of several opioid agonists and the opioid antagonist naloxone were examined in rats responding under a fixed-consecutive-number (FCN) schedule. Under this schedule, a reinforced response run consisted of responding eight or more times on one response lever, and then responding once on a second response lever. In one component of this schedule, an external discriminative stimulus signalled the completion of the response requirement on the first lever, whereas no stimulus change was programmed in the other. Morphine, l-methadone, U50488, ketocyclazocine, phencyclidine, and (+/-)N-allylnormetazocine decreased the percent of reinforced response runs (accuracy) under the FCN schedule without the external discriminative stimulus, but had no effect under the FCN schedule with the external discriminative stimulus. Naloxone and bremazocine, in contrast, had no effect on the accuracy of the discrimination under either FCN schedule. With the exception of bremazocine and U50488, which increased rates of responding at low doses, all drugs produced comparable decreases in rates of responding under both FCN schedules. During tests of antagonism, a 0.1 mg/kg dose of naloxone reversed completely the accuracy-decreasing effects produced by U50488 and morphine. The rate-decreasing effects of morphine and U50488 were reversed completely by a 0.01 and 1.0 mg/kg dose of naloxone, respectively. These results suggest that the addition of an external discriminative stimulus can modulate the disruptive effects of opioids, and that mu, sigma and some kappa agonists produce similar effects when evaluated under the FCN schedules.     

Effects of the delta opioid agonist BW373U86 in pigeons trained to discriminate fentanyl,bremazocine and water in a three-choice drug discrimination procedure

The delta opioid agonist BW373U86 was examined alone and in combination with mu agonists in pigeons trained to discriminate the mu agonist fentanyl (0.056 mg/kg), the kappa agonist bremazocine (0.017 mg/kg), and distilled water in a three-choice drug discrimination procedure. BW373U86 (0.01–10 mg/kg) produced a dose-dependent increase in fentanyl-appropriate responding and complete generalization to fentanyl in four of five subjects. BW373U86 did not elicit bremazocine-appropriate responding in any of the subjects. Fentanyl-appropriate responding elicited by BW373U86 was antagonized by the delta selective antagonist naltrindole (0.1–10 mg/kg) but not by the mu selective antagonist naloxone (0.1–30.0 mg/kg). When BW373U86 was administered in combination with the mu agonists fentanyl, morphine and nalbuphine, a low dose of BW373U86 (0.01 mg/kg) that elicited primarily water-appropriate responding when administered alone did not produce a significant change in the ED₅₀ values for fentanyl, morphine or nalbuphine. Higher doses of BW373U86 (0.1–1.0 mg/kg) increased levels of fentanyl-appropriate responding elicited by low doses of fentanyl, morphine and nalbuphine to levels similar to those produced by BW373U86 alone. These results indicate that BW373U86 shares discriminative stimulus properties with the mu agonist fentanyl in pigeons, possibly by acting at delta opioid receptors. However, BW373U86 does not potentiate the discriminative stimulus effects of mu agonists or share discriminative stimulus effects with the kappa agonist bremazocine.     

Three-choice discrimination in pigeons is based on relative efficacy differences among opioids

RATIONALE: Drug discrimination assays can provide important information on receptor selectivity and relative efficacy to guide the classification and characterization of opioid agonists. OBJECTIVES: A three-choice discrimination was established among high efficacy opioid agonist morphine, low efficacy opioid agonist nalbuphine, and saline to examine the conditions under which differences in relative efficacy might serve as a basis for stimulus control. METHODS: Seven White Carneau pigeons were trained to discriminate among 5.6 mg/kg nalbuphine, 3.2 mg/kg morphine, and saline under fixed ratio 30 (FR30) schedules of food reinforcement. Substitution and antagonism experiments were then conducted with mu, kappa, and delta opioids and naltrexone, respectively and the percent responding appropriate to the training stimuli was determined. RESULTS: Low, intermediate, and high doses of morphine produced > or = 80% saline-, > or = 60% nalbuphine-, and > or = 96% morphine-appropriate responding, respectively. Low and high doses of nalbuphine produced > or = 80% saline- and nalbuphine-appropriate responding, respectively. In substitution tests, low doses of fentanyl and etorphine produced partial nalbuphine-appropriate responding (20-60%) and high doses produced > or = 60-80% morphine-appropriate responding. Intermediate doses of buprenorphine and dezocine produced > or = 60-80% nalbuphine-appropriate responding and high doses produced > or = 80% morphine-appropriate responding. The lower efficacy agonists butorphanol, nalorphine, and levallorphan produced > or = 40-80% nalbuphine-appropriate responding. The kappa agonists spiradoline and U50,488 produced approximately > or = 50% nalbuphine-appropriate responding whereas d-amphetamine, saline, and delta agonists BW373U86 and SNC 80 produced > or = 80% saline-appropriate responding. Naltrexone produced > or = 80% saline-appropriate responding and reversed the stimulus effects of morphine and nalbuphine. CONCLUSIONS: The discrimination between morphine and nalbuphine in pigeons is predominantly based on the relative efficacy differences between morphine, a higher-efficacy mu agonist and nalbuphine, a lower-efficacy mu agonist.     

Differential cross-tolerance to mu and kappa opioid agonists in morphine-tolerant rats responding under a schedule of food presentation

If different populations of opioid receptors mediate the actions of mu and kappa opioid agonists, then tolerance induced by the chronic administration of a mu agonist should confer cross-tolerance to other mu agonists but not necessarily to those compounds whose effects are mediated by the kappa receptor. This hypothesis was evaluated in the present investigation by examining the effects of the mu agonists morphine,l-methadone and fentanyl, the kappa agonists U50,488 and bremazocine, and the mixed kappa/mu agonist ethylketocyclazocine in rats responding under a fixed-ratio 30 schedule of food presentation before, during and after exposure to a regimen of chronic morphine administration. For comparison, naloxone was evaluated as a representative mu antagonist and the phenothiazine chlorpromazine as a control drug. During all phases of the experiment, each of these compounds produced dose-related decreases in rate of responding. During the daily administration of 40 mg/kg morphine, tolerance developed to the rate-decreasing effects of morphine,l-methadone and fentanyl, and an enhanced sensitivity to the effects of naloxone. In contrast to the effects obtained with these mu opioids, there was no evidence that chronic morphine administration produced tolerance or enhanced sensitivity to the rate-decreasing effects of U50,488, bremazocine, ethylketocyclazocine and chlorpromazine. The present findings demonstrate that the chronic administration of morphine results in the selective development of tolerance to other mu agonists. In addition, the lack of cross-tolerance between morphine and the kappa agonists examined demonstrate that this behavioral preparation is a useful tool for differentiating the effects of compounds acting at different opioid receptor types.     

Interactions between mu and kappa opioid agonists in the rat drug discrimination procedure

The present study was designed to explore the nature of the interaction between mu and kappa opioid agonists in the rat drug discrimination procedure. In rats trained to discriminate the kappa agonist U50,488 (5.6 mg/kg) from water, the other kappa agonist bremazocine substituted completely for the U50,488 training stimulus, and the additional kappa agonist tifluadom substituted in three of five of rats tested. In contrast, the mu agonists morphine, fentanyl, and buprenorphine produced primarily vehicle-appropriate responding. When morphine, fentanyl, and buprenorphine were combined with the training dose of U50,488, all three mu agonists reduced U50,488-appropriate responding. In rats trained to discriminate the mu agonist morphine (10.0 mg/kg) from saline, the other mu agonists morphine and buprenorphine all substituted in a dose-dependent manner for the morphine training stimulus, whereas U50,488, bremazocine, and tifluadom produced primarily vehicle-appropriate responding. When combined with the training dose of morphine, bremazocine antagonized morphine's discriminative stimulus effects, whereas U50,488 and tifluadom had no effect. The barbiturate pentobarbital neither substituted for, nor antagonized, the discriminative stimulus effects of either U50,488 or morphine. These results suggest that mu agonists and kappa agonists produce interacting effects in the drug discrimination procedure in rats.     

Differential effects of neuroleptic drugs on the delayed matching-to-sample performance of pigeons

The effects of clozapine, thiothixene, sulpiride, chlorpromazine and loxapine were examined in pigeons responding under a delayed matching-to-sample (DMTS) procedure using 0-, 2- and 8-sec delay intervals. Chlorpromazine (3-100 mg/kg), thiothixene (0.03-1.7 mg/kg), clozapine (0.1-5.6 mg/kg) and loxapine (0.1-10 mg/kg) produced dose-related decreases in the percent of correct responses (accuracy). With the exception of chlorpromazine, the relative magnitude of the accuracy-decreasing effects were unrelated to the length of the delay interval and the nondrug levels of accuracy. In contrast to these accuracy-decreasing effects, sulpiride (3-300 mg/kg) failed to decrease accuracy across the range of doses evaluated. Chlorpromazine, loxapine and clozapine increased response rates at low doses and then decreased response rates as the dose was increased. Thiothixene and sulpiride only decreased response rates in a dose-dependent fashion. The order of potency for the rate-suppressing effects of these drugs was thiothixene greater than clozapine = loxapine greater than chlorpromazine greater than sulpiride. The results of the present investigation suggest that, despite similar dopamine antagonist properties, neuroleptics produce qualitatively different effects in pigeons responding under DMTS procedures.     

Differential neuropharmacological effects of mu,kappa and sigma opioid agonists on cortical EEG power spectra in the rat: Stereospecificity and naloxone antagonism

The study was designed to determine and compare the acute effects of the enantiomers of mu, kappa and sigma opioid agonists on the cortical EEG with the spectral analysis technique. The relative ability of naloxone to antagonize such effects was also assessed. Adult female Sprague-Dawley rats were implanted with chronic cortical EEG and temporalis muscle EMG recording electrodes, and with permanent indwelling external jugular cannulae. (?)-Methadone (mu agonist) produced increases in spectral power over the zero to 10 Hz range, while (?)-ketocyclazocine (kappa agonist) produced increases in the 5–8 Hz band as a predominant peak. The (+)enantiomers of methadone and ketocyclazocine were inactive. The drug (+)-SKF-10,047 (sigma agonist), produced a predominant spectral peak in the 7–9 Hz band that was associated with behavior that suggested psychotomimetic effects. The effects of morphine (mu agonist) on EEG and EEG power spectra were more sensitive to antagonism by naloxone than those produced by ketocyclazocine. The effects of(±)-SKF-10,047 and (+)-SKF-10,047 were not antagonized by 10 mg/kg of naloxone, while the effects of (?)-SKF-10,047 were partially antagonized by lOmg/kg of naloxone. These findings further delineate the specificity of the differential effects of mu, kappa and sigma opioid agonists on the EEG and EEG power spectra in the rat.     

Motivational properties of kappa and mu opioid receptor agonists studied with place and taste preference conditioning

The reinforcing properties of various opioid agonists acting preferentially on the kappa and mu opioid receptors were assessed using taste and place preference conditioning procedures.Kappa receptor agonists produced conditioned aversions. Taste aversions were produced by all of the drugs used, including racemic mixtures of ethylketazocine, tifluadom, and U50-488, and active isomers (+)-tifluadom, (-)-bremazocine, and Mr 2034; corresponding inactive isomers either produced no effect of were less potent. Place aversions were produced by U50-488 and (-)-bremazocine, but not (+)-bremazocine or any of the other kappa receptor agonists tested with the taste procedure. The mu agonists produced predominantly conditioned preferences. Place preferences were produced by morphine, fentanyl and sufentanil. Taste preferences were produced by low doses of these substances; at higher doses the taste preferences were absent or replaced by aversions. Finally, with naloxone and lithium chloride it was shown that the taste procedure was more sensitive to punishing effects than the place procedure.It is concluded that kappa and mu opioid receptor agonists are effective unconditioned stimuli. From the lower portions of the dose response curves it is further concluded that activation of kappa opioid receptors has aversive properties and activation of mu receptors appetitive reinforcing properties. The findings are also discussed with regard to the prevailing notions of taste conditioning with opiates, and the reinforcing properties of activity of the endogenous opioid peptide systems.     

Micro/kappa opioid interactions in rhesus monkeys: implications for analgesia and abuse liability

Micro opioid receptor agonists are clinically valuable as analgesics; however, their use is limited by high abuse liability. Kappa opioid agonists also produce antinociception, but they do not produce micro agonist-like abuse-related effects, suggesting that they may enhance the antinociceptive effects and/or attenuate the abuse-related effects of micro agonists. To evaluate this hypothesis, the present study examined interactions between the micro agonist fentanyl and the kappa agonist U69,593 in three behavioral assays in rhesus monkeys. In an assay of schedule-controlled responding, monkeys responded under a fixed-ratio 30 (FR 30) schedule of food presentation. Fentanyl and U69,593 each produced rate-decreasing effects when administered alone, and mixtures of 0.22:1, 0.65:1, and 1.96:1 U69,593/fentanyl usually produced subadditive effects. In an assay of thermal nociception, tail withdrawal latencies were measured from water heated to 50 degrees C. Fentanyl and U69,593 each produced dose-dependent antinociception, and effects were additive for all mixtures. In an assay of drug self-administration, rhesus monkeys responded for intravenous drug injection, and both dose and FR values were manipulated. Fentanyl maintained self-administration, whereas U69,593 did not. Addition of U69,593 to fentanyl produced a proportion-dependent decrease in rates of fentanyl self-administration. Moreover, addition of U69,593 increased the sensitivity of fentanyl self-administration to increases in the FR value. Taken together, these results suggest that simultaneous activation of mu and kappa receptors, either with a mixture of selective drugs or with a single drug that targets both receptors, may reduce abuse liability without reducing analgesic effects relative to selective micro agonists administered alone.     

Delta opioid-like discriminative stimulus effects of mu opioids in pigeons discriminating the delta opioid BW373U86 from saline

The purpose of this experiment was to examine the substitution patterns produced by opioids with activity at the mu receptor in pigeons trained to discriminate the delta opioid BW373U86 from saline. A low dose of naltrindole (0.1 mg/kg) produced at least a 16-fold rightward shift in the dose-effect curve for the stimulus effects of BW373U86 (yielding a pK(B) = 7.9), whereas a relatively high dose of naloxone (1.0 mg/kg) produced only a 2-fold rightward shift (yielding a pK(B) = 5.6). The delta opioid SNC80 and the mixed mu/kappa opioids ethylketocyclazocine and ketocyclazocine substituted completely for the BW373U86 stimulus. Various opioids with activity at the mu receptor (levallorphan, [-]-cyclazocine, [-]-n-allylnormetazocine, morphine, butorphanol, nalbuphine, [+]-propoxyphene, etorphine, fentanyl) substituted partially for the BW373U86 stimulus. There was no relationship between the substitution patterns produced by these opioids and their relative intrinsic efficacy at the mu receptor, their relative selectivity for the mu receptor or their relative affinity for the delta receptor. Naloxone (1.0 mg/kg) was considerably more effective than naltrindole (0.1 mg/kg) in antagonizing the substitution patterns produced by etorphine, ethylketocyclazocine, ketocyclazocine and butorphanol, suggesting that these effects were not mediated by activity at the delta receptor. There was no evidence that these opioids antagonized the BW373U86 stimulus, suggesting that they were not functioning as low efficacy agonists at the delta receptor. The kappa opioids bremazocine and U50,488, as well as the non-opioids cocaine and pentobarbital, failed to produce appreciable levels of BW373U86 responding. The present findings indicate that in pigeons mu opioids most likely produce delta-like discriminative stimulus effects by activation of mu rather than delta or kappa receptors.     

Changes in sensitivity to the rate-decreasing effects of opioids in pigeons treated acutely or chronically with nalbuphine

This study examined whether tolerance or dependence develops to the effects of the low-efficacy μ opioid agonist nalbuphine on schedule-controlled responding. In untreated pigeons responding under a fixed ratio (FR)-20 schedule of food presentation, nalbuphine, naltrexone, morphine, fentanyl, etonitazene and enadoline decreased response rates. Naltrexone (0.1-10.0mg/kg) did not antagonize the rate-decreasing effects of nalbuphine. In a separate group of pigeons, chronic nalbuphine treatment (1.0-56.0mg/kg/day) did not alter the sensitivity of pigeons to the rate-decreasing effects of nalbuphine or naltrexone. In pigeons treated with 56.0mg/kg/day of nalbuphine, dose-effect curves for μ and kappa agonists were shifted 3- to 10-fold to the right of dose-effect curves determined prior to chronic treatment. The rate-decreasing effects of nalbuphine did not appear to be mediated by opioid receptors, as evidenced by the inability of naltrexone to antagonize nalbuphine and the lack of tolerance development. Although chronic nalbuphine altered the sensitivity to the rate-decreasing effects of μ and kappa agonists, there was no change in sensitivity to naltrexone. To the extent that increased sensitivity to antagonists is indicative of dependence, these data suggest that opioid dependence does not develop to nalbuphine.     

Cross-tolerance and mu agonist efficacy in pigeons treated with LAAM or buprenorphine

The mechanism responsible for decreased opioid use during opioid substitution therapy is not fully understood. To examine whether l-alpha-acetylmethadol (LAAM) or buprenorphine attenuate behavioral effects of opioids through cross-tolerance, discriminative stimulus effects of high and low efficacy mu agonists were examined following 3- or 7-day treatment with LAAM or buprenorphine in pigeons discriminating between saline and heroin or between saline and buprenorphine, respectively. Heroin, buprenorphine and nalbuphine occasioned high levels of drug-appropriate responding in both groups; kappa opioids and non-opioids occasioned predominantly saline-appropriate responding. Administration of LAAM (3.2 mg/kg) or buprenorphine (3.2 mg/kg) occasioned predominantly heroin- or buprenorphine-appropriate responding, respectively. After discontinuation of LAAM treatment, the potency in occasioning heroin-key responding was markedly decreased for nalbuphine, slightly decreased for buprenorphine, and unchanged for heroin. Following discontinuation of buprenorphine treatment, the potency in occasioning buprenorphine-key responding was decreased for nalbuphine and unchanged for buprenorphine and heroin. Thus, greater cross-tolerance developed from LAAM and buprenorphine to low efficacy mu agonists as compared to a higher efficacy agonist. Failure of LAAM and buprenorphine treatment to modify the effects of heroin, under conditions that attenuate the effects of lower efficacy mu opioids, provides a possible rationale for why heroin abuse persists in some patients receiving large doses of agonists in substitution therapy.     

Effects of a novel fentanyl derivative on drug discrimination and learning in rhesus monkeys.

Three monkeys discriminated 1.78 mg/kg of mirfentanil while responding under a fixed-ratio 5 schedule of stimulus-shock termination. Two mirfentanil derivatives, OHM3295 and OHM10579, substituted for mirfentanil in all subjects. However, other drugs produced variable effects among monkeys; for example, mu and kappa opioid agonists and clonidine substituted for mirfentanil on some occasions in two monkeys. Cocaine, amphetamine, and ketamine did not substitute in any subject. Opioid antagonists did not attenuate the effects of mirfentanil. In monkeys responding under a repeated acquisition and performance procedure, errors increased only during the acquisition phase at doses of mirfentanil that decreased response rates. Thus, unlike fentanyl, the discriminative stimulus effects of mirfentanil do not appear to be mediated exclusively through opioid receptors. Finally, mirfentanil does not appear to disrupt complex behavioral processes.     

Differential cross-tolerance to opioid agonists in morphine-tolerant squirrel monkeys responding under a schedule of food presentation

The effects of various mu and kappa opioid agonists were evaluated in three squirrel monkeys responding under a fixed-ratio 30 schedule of food presentation before, during and after a regimen of chronic morphine administration. Initially, dose-effect curves for the mu opioid agonists morphine and l-methadone, the kappa opioid agonists U50,488 and tifluadom, the mixed mu/kappa opioid agonist ethylketocyclazocine, and the non-opioid compound pentobarbital were determined in non-tolerant squirrel monkeys. Subsequently, monkeys were administered up to 3.0 mg/kg of morphine twice daily for 8-9 weeks, which resulted in a 1/2 to 3/4 log unit shift to the right of the morphine dose-effect curve relative to its prechronic position. During the chronic morphine regimen, the l-methadone dose-effect curve shifted to the right approximately 3/4 log unit, while the U50,488 and pentobarbital dose-effect curves did not change. In contrast, the ethylketocyclazocine and tifluadom dose-effect curves shifted to the left approximately 1/4 and 3/4 log unit, respectively. The lack of cross-tolerance between mu and kappa agonists in morphine-tolerant squirrel monkeys observed in the present study provides further support for the differentiation of mu and kappa agonists. The occurrence of leftward shifts in the dose-effect curves of some opioid compounds with kappa agonist activity during the regimen of chronic morphine administration suggests that morphine tolerance modulates their     

Centrally mediated opioid induced depression of hepatic glutathione: effects of intracerebroventricular administration of mu, kappa, sigma and delta agonists

It has recently been demonstrated that morphine produces a loss of hepatocellular glutathione in mice by virtue of its action within the central nervous system. The ability of opioid receptor antagonists to abolish morphine's effect on hepatic glutathione suggests that this action is opioid-receptor mediated. The involvement of opioid receptors in this phenomenon is confirmed in the present study in mice by the ability of naltrexone, 100 micrograms administered intracerebroventricularly (i.c.v.), to completely block the decrease in hepatic glutathione induced by an i.c.v. injection of 100 micrograms of morphine. Intracerebroventricular administration of the selective mu (mu) opioid receptor agonist, (D-Ala2,N-MePhe4,Gly-ol5)enkephalin (DAGO; 25-50 micrograms), or the selective delta (delta) opioid agonist, [D-Pen2,D-Pen5]enkephalin (DPDPE; 3-50 micrograms), like morphine, produced significant decreases in hepatic glutathione 3 h after administration. The selective kappa (kappa) opioid receptor agonists, ethylketocyclazocine (1-30 micrograms) and trans-(+/-)3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl] benzeneacetamide-methane sulfonate (U50 488; 10-300 micrograms), as well as the selective sigma (sigma) opioid agonists, phencyclidine (PCP; 50-300 micrograms) and N-allylnormetazocine (SKF 10,047; 1-30 micrograms), had no effect on the concentrations of glutathione in the liver. It appears from these data that stimulation of mu- or delta-, but not kappa- or sigma-opioid receptors within the central nervous system results in a loss of hepatocellular glutathione.     

Discriminative stimulus properties of (+)- and (-)-n allylnormetazocine: the role of training dose in the stimulus substitution patterns produced by PCP/sigma and mixed action opioid compounds

The discriminative stimulus effects of the stereoisomers of N-allylnormetazocine (NANM) were evaluated in separate groups of pigeons trained to discriminate 3.0 (low) then 7.5 (high) mg/kg (-)-NANM from saline or 3.0 (high) then 1.0 (low) mg/kg (+)-NANM from saline. The stimulus effects of the high and low training dose of (-)-NANM were shared by the PCP/sigma compounds (+)-NANM, phencyclidine, dextrorphan, (+)-cyclazocine and (+)-metazocine but not the putative sigma ligand (+)-pentazocine. Doses of naloxone 3-10 times larger than those which antagonize the effects of mu and kappa opioids failed to alter these substitution patterns. In contrast to the lack of substitution obtained with the mu opioid morphine, kappa opioid bremazocine and opioid antagonist naloxone, the mixed action opioids (-)-pentazocine, butorphanol, nalbuphine and nalorphine substituted completely for the low training dose and partially for the high training dose of (-)-NANM. At the low training dose, the substitution patterns produced by these mixed action opioids were naloxone-reversible. Complete substitution for both training doses of (-)-NANM was also obtained with the mixed action opioids levallorphan and (-)-cyclazocine, although these effects were not naloxone-reversible. In pigeons trained to discriminate the high and low training dose of (+)-NANM from saline, (-)-NANM and the PCP/sigma compounds substituted completely and in a non-naloxone reversible manner. The relative potencies of these PCP/sigma compounds in producing (+)-NANM-like stimulus effects were highly correlated with their relative affinity for the sites labeled by PCP but not (+)-NANM. In these pigeons, naloxone revealed the (+)-NANM-like stimulus effects produced by (-)-pentazocine and (-)-cyclazocine, thus suggesting that these mixed action opioids have a prominent PCP/sigma-like component. When administered alone or in combination with naloxone, the other mixed action opioids evaluated failed to substitute for either the high or low training dose of (+)-NANM. The present results indicate that the stimulus effects of both training doses of (+)-NANM and the high training dose of (-)-NANM are comprised of a PCP/sigma component, whereas that of the low training dose of (-)-NANM has both a PCP/sigma and a naloxone-reversible, opioid component.     

Effects of phenobarbital,clonazepam, valproic acid,ethosuximide, and phenytoin on the delayed matching-to-sample performance of pigeons

The effects of phenobarbital, clonazepam, valproic acid, ethosuximide, and phenytoin were examined in pigeons performing under a delayed matching-to-sample procedure. Clonazepam, valproic acid, ethosuximide, and phenytoin typically reduced the rate of responding to the sample stimulus, whereas phenobarbital usually increased response rates at high doses. Phenobarbital, clonazepam, and valproic acid produced generally dose-dependent decreases in accuracy; ethosuximide and phenytoin failed to do so. These results suggest that there are qualitative as well as quantitative differences in the effects of anticonvulsant drugs under the delayed matching-to-sample procedure.     

Effects of ketazocine, ethylketazocine and phenazocine on schedule-controlled behavior: antagonism by naloxone

Dose-effect curves were determined for phenazocine (0.64-2.5 mg/kg), ketazocine (1.25-80 mg/kg) and ethylketazocine (1.25-80 mg/kg) in pigeons responding under a multiple fixed-ratio 30-response, fixed-interval 5-min schedule of grain presentation. All three opioid agonists decreased responding with the larger doses. The effects of phenazocine were completely antagonized by small doses of naloxone (0.01-1 mg/kg), whereas the effects of ethylketazocine required larger doses of naloxone (1-10 mg/kg) to be completely antagonized. The behavioral effects of ketazocine were partially attenuated by naloxone, but were not antagonized completely even by a 10 mg/kg dose of naloxone. These data from the pigeon are consistent with previous interpretations that the effects of phenazocine are mediated by actions at a mu opioid receptor, whereas the effects of ketazocine and ethylketazocine are mediated by actions at a kappa opioid receptor.     

Effect of mu and kappa opioid receptor agonists on rat plasma corticosterone levels

The effect of several mu and kappa opioid receptor agonists on rat plasma corticosterone levels, measured using radioimmunoassay, was investigated. The mu agonists, morphine and fentanyl, and the kappa agonists, U-50,488, tifluadom and bremazocine, all produced dose-related increases in rat plasma corticosterone levels. The effects of both fentanyl and U-50,488 were reversed by naloxone, indicating an action at opioid receptors. Pretreatment of the rats with the irreversible, mu-selective antagonist, beta-funaltrexamine, reduced the effect of fentanyl, but not that of U-50,488, indicating that both mu and kappa opioid receptors are involved in mediating this effect.