Narcotic cue and narcotic state

Oxycodone-like discriminative stimulus effects of fentanyl-related emerging drugs of abuse in mice

BACKGROUND

Fentanyl and its structurally related compounds have emerged as the most significant contributors to opioid overdose fatalities in recent years. While there is abundant information about the pharmacological effects of fentanyl, far less is known of its more recently abused analogs. The objective of this study was to determine whether fentanyl and several fentanyl-related substances would engender oxycodone-like responding in a mouse model of oxycodone discrimination. Oxycodone was selected as the training drug due to its high selectivity for mu opioid receptors. Compounds that elicited oxycodone-like responding in this procedure would likely evoke overlapping subjective experiences.

METHODS

Adult male C57BL/6 mice were trained to discriminate 1.3 mg/kg oxycodone from vehicle in a food-reinforced, two-lever choice procedure. Generalization tests were conducted with fentanyl and the following fentanyl-related compounds: ocfentanil, 3-furanyl fentanyl, crotonylfentanyl, and valerylfentanyl.

RESULTS

Fentanyl and each of its analogs completely generalized to the 1.3 mg/kg oxycodone discriminative stimulus and naltrexone pretreatment significantly decreased oxycodone-like responding for each compound. Rank order potency for engendering oxycodone-appropriate responding was ocfentanil > fentanyl > 3-furanyl fentanyl ≈ crotonylfentanyl > oxycodone > valerylfentanyl. Drug doses that evoked full substitution also significantly suppressed response rates compared to vehicle.

CONCLUSIONS

These results indicate that the discriminative stimulus, and by extension, the interoceptive and subjective effects of the tested fentanyl analogs, overlap with those of oxycodone. These observations consequentially support the prediction that they would also engender the likelihood for abuse similar to oxycodone. This article is part of the Special Issue entitled 'Opioid Neuropharmacology: Advances in treating pain and opioid addiction'.

Analgesic doses of the enkephalin degrading enzyme inhibitor RB 120 do not have discriminative stimulus properties

The systemically active mixed inhibitor of enkephalin metabolism, N-((S)-2-benzyl-3[(S) 2-amino-4-methylthio)butyldithio-]-1-oxopropyl)-L-alanine benzylester (RB 120), alone or in combination with 4-¿[2-[[3-(1H-indol-3-yl))-2-methyl-1-oxo-2-[[(tricyclo[3.3.1.1. ]dec-2-yloxy) carbonyl]amino¿propyl]amino]-1-phenylethyl]amino¿-4-oxo-[R-(R*, R*)]-butanoate N-methyl-D-glucamine (CI 988; CCK(1) receptor antagonist) was investigated for discriminative and morphine generalisation effects using an operant drug discrimination paradigm in rats. Animals dosed with RB 120 (10 mg/kg) failed to develop a discriminative response. Combined CI 988 (0.3 mg/kg) and RB 120 (10 mg/kg) also failed to elicit a discriminative response. Morphine-trained animals (3.0 mg/kg) did not generalise to RB 120 (10 and 20 mg/kg). Similarly, subsequent retraining of the same animals with 1.5 mg/kg of morphine did not elicit generalisation to RB 120 (10 or 20 mg/kg). Combined RB 120 (10 or 20 mg/kg) and CI 988 (0.3 or 3.0 mg/kg) treatment produced no notable drug lever selection in rats able to discriminate morphine (1.5 mg/kg) from saline. These results suggest that RB 120 may have low abuse potential at analgesic doses.

Drug discrimination in neurobiology

Areas of neurobiological interest are identified towards which drug discrimination (DD) studies have made important contributions. DD allows ligand actions to be analyzed at the whole organism level, with a neurobiological specificity that is exquisite and often unrivalled. DD analyses have thus been made of a vast array of CNS agents acting on receptors, enzymes, or ion channels, including most drugs of abuse. DD uniquely offers access to the study of subjective drug effects in animals, using a methodology that also is transposable to humans and has generated unprecedented models of pathology (e.g., chronic pain, opiate addiction). Parametric studies of such independent variables as training dose and reinforcement provide refined insights into the dynamic psychophysiological mechanisms of both drug effects and behavior. Three different mechanisms have been identified by which discriminative, and perhaps other behaviors, can come about. DD also is superbly sensitive to small, partial activation of molecular substrates; this has enabled DD analyses to pioneer the unravelling of molecular mechanisms of drug action (attributing, f.ex., LSD's particular subjective effects to an unusual, partial activation of 5-HT, and perhaps other receptors). DD has both oriented and served as a tool to conduct drug discovery research (e.g., pirenperone-risperidone, loperamide). The DD response arguably constitutes a quantal, rather than graded, variable, and as such allows a comprehension of molecular, pharmacological, and behavioral mechanisms that would have been otherwise inaccessible. Perhaps most important are the following further contributions. One is the notion that particular, different levels of receptor activation are associated with qualities of neurobiological actions that also differ and are unique, this notion arguably constituting the most significant addition to affinity and intrinsic activity since the earliest theoretical conceptions of molecular pharmacology. Another contribution consists of studies that render redundant the notion of tolerance and identify fundamental mechanisms of signal transduction; these mechanisms account for apparent tolerance, dependence, addiction, and sensitization, and appear to operate ubiquitously in a bewildering array of biological systems.

Empirical evidence that the state dependence and drug discrimination paradigms can generate different outcomes

The study compared the outcomes generated by the State Dependence and Drug Discrimination paradigms with ethanol in the rat. Food-deprived rats learned to complete a fixed-ratio 10 schedule of bar presses for food within 120 s while treated with 320- to 1250-mg/kg doses of ethanol. Subsequent tests of recall of this response with saline failed to generate any evidence that transfer was hampered following the drug-to-saline state change. In contrast, each of 14 rats learned to discriminate 1250 mg/kg ethanol from saline in a Drug Discrimination procedure that also required the animals to press one of two levers for food according to a fixed- ratio 10 schedule. The results offer the first empirical evidence to demonstrate directly that the State Dependence and Drug Discrimination paradigms can generate different outcomes in otherwise identical experimental conditions.

The CCKA receptor antagonist devazepide does not modify opioid self-administration or drug discrimination: comparison with the dopamine antagonist haloperidol

We previously reported that the selective cholecystokininA (CCKA) receptor antagonist, devazepide, blocked the acquisition of a morphine conditioned place preference (ref 28). An interpretation of this finding is that devazepide may either affect an opioid discriminative stimulus and/or modify the rewarding properties of opioids. The present study was designed to investigate these issues by determining the effect of equivalent doses of devazepide in a morphine drug discrimination paradigm and a model of heroin self-administration. In each case, devazepide (0.001-1 mg/kg) was ineffective, i.e there was no antagonism of a morphine discriminative cue, and in a separate group of rats trained to self-administer heroin (0.03 mg/kg/infusion, FR5 schedule, 1h per day), devazepide did not alter the pattern of heroin responding. Because of evidence implicating an interaction between accumbens CCK and dopamine (DA) systems and evidence suggesting an apparent differential involvement of DA in opioid place conditioning, self-administration and drug discrimination behaviour, the effect of the DA antagonist haloperidol was examined in the latter two paradigms. In each test, haloperidol produced an effect inconsistent with a direct DAergic involvement. In a final study the CCKB antagonist L365-260 was also found not to affect an opioid discriminative cue. The present results therefore cast doubt on the potential utility of selective CCKA antagonists as treatments for opioid abuse, and further suggest that CCKB antagonists may not potentiate the subjective effects of opioids, an important finding considering that such drugs have been proposed as adjuncts to opioid therapy for the treatment of pain relief.

Effect of 5-HT3 receptor antagonists on the discriminative stimulus properties of morphine in rats

5-HT3 receptor antagonists, e.g. MDL72222, ondansetron and ICS205-930, have been previously reported to block a morphine (1.5 mg/kg)-induced conditioned place preference in rats. This finding suggests that these drugs may modify the morphine discriminative stimulus which underlies place conditioning. To study this further we have examined the effects of MDL72222, ondansetron and ICS205-930 against a morphine discriminative stimulus using a two-choice, food reinforced, operant paradigm. In an attempt to provide consistency with previous place conditioning studies, a morphine training dose of 1.5 mg/kg was used in addition to a higher 3 mg/kg dose which was studied in separate animals. Stimulus control of behaviour was attained at both morphine training doses, the characteristics of each being consistent with an effect at the mu opioid receptor. Ondansetron (0.001-1 mg/kg), MDL72222 (0.1-3 mg/kg), and ICS205-930 (0.001-1 mg/kg) all failed to consistently antagonise the morphine cue at both training doses, although a mild attenuation was seen in the 1.5 mg/kg group following pretreatment with an intermediate dose of ondansetron and ICS205-930 (both 0.01 mg/kg). The present results therefore suggest hat 5-HT3 antagonists do not block a morphine discriminative state, at least in rats.

Drug abuse potential evaluation in animals

Animal laboratory studies can provide useful information concerning the potential of drugs for abuse. Over the past 50 years, methods have been developed for use with animal subjects which allow the evaluation of pharmacological properties of drugs which are particularly relevant to their abuse. The methods for preclinical drug abuse liability testing are reviewed under six heading: (1) establishment of the degree of pharmacological equivalence to known drugs of abuse, (2) drug discrimination studies, (3) tests of tolerance and cross-tolerance, (4) tests of physical dependence capacity, (5) drug self-administration tests of reinforcing effects, and (6) evaluation of toxicity and performance impairment at self-administered doses. Preclinical studies can be helpful early in drug development to select lead compounds with low abuse potential for further study. In the case of new or already marketed medications, animal testing can often compliment and extend abuse liability evaluation in human subjects. The results of abuse potential evaluation studies can be useful in making decisions about the possible need for regulation under national and international drug laws, and thus play an important role in drug abuse prevention.

Behavioral and 5-HT antagonist effects of ritanserin: a pure and selective antagonist of LSD discrimination in rat

The newly synthesized compound and putative 5-HT2 antagonist ritanserin, but not the structurally related compound R 56413, resembles pirenperone in that it acts as a pure antagonist in an LSD-saline drug discrimination assay in the rat. Ritanserin exceeded pirenperone in terms of behavioral specificity; the lowest effective dose of ritanserin in antagonizing LSD was one order of magnitude higher than that of pirenperone, but the compound depressed rate of operant responding only at doses that were about 1000-fold higher than those at which pirenperone was effective. Ritanserin exerted effects in an open field test which were reminiscent of anxiolytic drug activity in the rat; its effects were greater than those of pirenperone, R 56413 and the benzodiazepines chlordiazepoxide and diazepam. The results of experiments on antagonism of 5-HT-induced hypothermia and of the 5-HTP-induced head-twitch response fail to support the possibility that the putative anxiolytic effects of ritanserin in the rat can be ascribed simply to a pharmacologically defined action at 5-HT receptors.

Discriminative stimulus properties of levo-alpha-acetylmethadol and its metabolites

The discriminative stimulus properties of l-alpha-acetylmethadol (LAAM), its metabolities 1-alpha-acetyl-N-normethadol (NorLaam) and l-alpha-acetyl-N,N-dinormethadol (DiNorLAAM), and methadone were evaluated in rats trained to discriminate between saline and morphine in a two-choice discrete-trial avoidance paradigm. All four compounds produced dose-related increases in the number of trials completed on the morphine-appropriate choice lever after either SC or oral administration indicating that the discriminative stimulus properties of the four compounds and morphine are qualitatively similar. LAAM and DiNorLAAM had a slow onset and long duration of action, and an oral: parenteral potency ratio of 1:3. NorLaam had a more rapid onset and shorter duration of action and was more potent following SC administration than either LAAM or DiNorLAAM; its oral: parenteral potency ratio was 1:10. These results are consistent with evidence from other studies that the pharmacologic activity of LAAM is dependent upon the conversion of LAAM to an active metabolite, probably NorLAAM. The similarities between DiNorLAAM and LAAM suggest that the discriminative stimulus effects of the former compound are also attributable to a metabolite.

Evidence supporting lack of discriminative stimulus properties of a combination of naltrexone and morphine

The aim of the present experiment was to study the potentially discriminable effects of combinations of morphine and naltrexone during long-term treatment. Three groups of gerbils had to discriminate the effects of morphine (12 mg/kg) and those of either saline (4 ml/kg), naltrexone (2 mg/kg), or a combination of this dose of morphine plus naltrexone injected IP 60 min prior to the start of the discriminative training in a T-shaped maze. Rapid development of drug discriminative control of choice behavior (left or right turn in the maze) was evident in these 3 groups which is in marked contrast to the performance of gerbils trained with morphine-naltrexone combination vs. saline or gerbils trained with naltrexone only vs. saline. Neither of these latter groups reached the criterion of performing 8 correct first-trial choices in 10 consecutive training sessions during the 60 training sessions allowed, while the 3 other groups began their criterion performance after only 7--8 training sessions. Thus the discriminative properties of certain combinations of morphine and naltrexone are weak and therfore are not easily discriminable from the effects induced by saline.

Disruption of taste aversion learning by pretreatment with diazepam and morphine

Laboratory rats were pretreated with either morphine (9 mg/kg IP), diazepam (4 mg/kg 1P) or Ringer's solution 2, 3 1/2, and 2 hr, respectively, prior to ingestion of a novel tasting saccharin solution followed immediately by a single injection of one of these agents. Animals pretreated with Ringer's solution followed by an injection of either morphine or diazepam showed a conditioned taste aversion (CTA) as determined by a significant reduction in the mean saccharin intake on a subsequent test trial. Although the drug pretreatments alone produced no conditioned avoidance behavior, the diazepam pretreatment completely blocked the development of both diazepam and morphine-evoked CTAs while the morphine pretreatment prevented a CTA induced by itself but not by diazepam. The results were discussed in terms of the attenuating effects of the pretreatments on the relative saliency of the subsequent conditioning drug injection.

Possible effects of normetabolites on the subjective and reinforcing characteristics of opioids in animals and man

When an opioid capable of forming active metabolites is administered, the total pharmacology is the result of interactions of the opioid and such metabolites, especially normetabolites. Normetabolites may affect the morphine-like characteristics of certain opioids and thus influence their reinforcement in animals and man. Most opioids, when administered in single doses, are positively reinforcing in addicts. Oral administration, as compared with parenteral, facilitates the formation of normetabolites. When chronically administered, many opioids, including acetylmethadol, meperidine, morphine, codeine, propoxyphene, and levorphanol, show evidence of a longer half-life for their normetabolites. Normetabolites may have aversive characteristics and thus impair positive reinforcement of the parent drug in animals and man. For example, addicts do not like chronic oral morphine or chronic oral codeine. Conversely, methadone, the normetabolites of which are inactive, is well accepted during chronic oral administration. Drugs which inhibit N-demethylation will increase the agonist potency of opioids having inactive normetabolites (e.g., methadone) but will decrease the agonist potency of opioids having more potent normetabolites than the parent (e.g., acetylmethadol). The divergent responses of addicts to single doses of opiates as compared with chronic doses indicate that chronic addiction tests in man are needed befored relative abuse liability can be predicted.

Brief communication. Generalization of [DAla2]-enkephalinamide but not of substance P to the morphine cue

Rats were trained to discriminate morphine (7.5 mg/kg, IP) from saline in a two bar positively reinforced lever pressing paradigm on a FR4 schedule. Morphine (IP) showed a naloxone reversible dose-related generalization to the training dose. [DAla2]-Methionine enkephalinamide (DAE) at 1 mg/kg and Substance P (SP) at 0.1 and 0.25 mg/kg showed vehicle appropriate responding after IP injection. DAE (5 mg/kg) disrupted responding completely; SP (0.5 and 0.1 mg/kg) disrupted responding in 50% of the rats. The disruption caused by IP injection of DAE was not naloxone reversible. Intraventricular injection of morphine (5 microgram/rat) and DAE (5 microgram/rat) produced generalization to the opiate cue. The effect of DAE was reversed by naloxone (1 mg/kg, SC). SP (500 and 750 ng/rat, IVT) produced saline-like responding; 1 microgram/rat disrupted responding completely. These data demonstrate that morphine and enkephalin, but not Substance P, share similar discriminative properties.

The effect of morphine on fear extinction in rats

Rats were trained on an appetitive discretetrial discriminated-punishment task in which they learned to suppress responding when an intense flashing light predicting punishment was present and to respond rapidly on trials when the flashing light was absent. Once animals were performing discriminatively, 0.75, 3.0, or 6.0 mg/kg of morphine (base) was administered and a fear extinction session consisting of 60 nonshocked presentations of the flashing light was given. Two saline control groups, one that received fear extinction and one that did not, were also included in the experiment. On the day following fear extinction, all rats were tested in the undrugged state on the discriminated punishment problem, but without shock. The rats receiving 3.0 and 6.0 mg/kg of morphine before the fear extinction session were suppressed by the flashing light more than the saline extinction group or the 0.75 mg/kg morphine treatment group. Moreover, the two higher dose morphine groups were suppressed as readily as the saline group that received no fear extinction. These results are attributed to the antiemotionality effects of morphine.

Drug-cue conditioning to external stimulus conditions

Rats were trained to discriminate 0.04 mg/kg fentanyl from saline in a 2-lever procedure, and to discriminate 10 mg/kg pentobarbital from saline in a maze procedure. Stimulus generalization experiments in rats trained in this double discrimination indicate that the internal cues produced by these training drugs can be conditioned to the external stimulus conditions associated with the different discrimination procedures.

Narcotic cuing and analgesic activity of narcotic analgesics: associative and dissociative characteristics

By using a discrete-trial, two-lever, food-reinforced discrimination learning paradigm, rats were trained to discriminate the narcotic analgesic fentanyl (0.05 mg/kg) from saline. Stimulus generalization experiments with lower fentanyl doses (0.0025 to 0.02 mg/kg) were carried out to generate individual threshold doses. The latter were compared with the sensitivity of the same rats to the analgesic effect of fentanyl, and it was found that there is no correlation between these two sets of data. In a time-effect experiment, the duration of fentanyl's cuing effect was compared with that of its analgesic effect, and it was found that the time-effect characteristics of the narcotic cue are similar to those of analgesia. Again, however, there was no correlation between the duration of both effects within the same group of animals. The results further deliniate the associative and dissociative characteristics of the narcotic cue and narcotic analgesia.

Narcotic cueing properties of intraventricularly administered sufentanil, fentanyl, morphine and met-enkephalin

The narcotic cueing activity of sufentanil, fentanyl, morphine and met-enkephalin was studied upon their injection into the lateral brain ventricle of the rat. Comparative studies on the analgesic activity of the three narcotics support a close correlation between the narcotic cueing and the analgesic activity of narcotic drugs.

Tolerance to morphine-produced discriminative stimuli and analgesia

Male hooded rats were trained in Skinner boxes to press one lever after a morphine injection (10 mg/kg) and another lever after a saline injection (1 ml/kg) on an FR 10 schedule of food reinforcement. After the drug discrimination was well established, the rats were tested for stimulus generalization at different doses of morphine, followed by assessment of tail withdrawal latency as a measure of analgesia. Subjects were then administered increasing doses of morphine sulphate to induce an increased level of tolerance. New dose-response curves indicated that tolerance developed to the morphine-induced discriminative stimulus, and to the analgesic action of morphine, but doses of morphine that failed to cause detectable analgesia still produced a pronounced discriminative stimulus.

Differential haloperidol effect on two indices of fentanyl-saline discrimination

Using a discrete-trial, two-lever, food-reward discrimination learning paradigm, we trained rats (n = 6) to discriminate 0.04 mg/kg fentanyl (s.c. t-30') from saline. Stimulus generalization experiments with an adequate dose range (0.01-0.04 mg/kg) of fentanyl revealed that the ED50 value for drug lever selection is 0.02 mg/kg, irrespective of whether the animals were pretreated (s.c., t-60') with either saline or 0.08 mg/kg haloperidol. With increasing doses of the haloperidol-fentanyl combination, the percentage of total responding on the selected lever progressively decreased, and reached the 50% level at the highest drug combination. It is concluded that this percentage is heavily contaminated by factors unrelated to the discrimination condition being studied; these factors seem to invalidate this percentage as a discrimination index under experimental conditions (e.g., behaviorally toxic doses of drugs) where they are likely to operate. The use of response selection as a discrimination index in drug discrimination research is further argued.