The discriminative stimulus properties of legal, over-the-counter stimulants administered singly and in binary and ternary combinations

Prospective Cross-Sectional Study of the Uses of Non-prescription Analgesics among Hail Residents

<p>The aim of this paper was to describe the general status of the use of non-prescription analgesics (NPA) among the residents of Hail city to ensure the safe use of non-prescription analgesics.  One hundred participants with different diseases were studied consecutively from January to March 2020. Data were collected with an online, semi-structured questionnaire; the obtained data were converted into statistical data using Microsoft Excel 2013. Most participants were taking a single, non-prescription analgesic (87.5%), while combination analgesic products were used by only 12.5% of participants. Using over-the-counter analgesics is common among residents of Hail, and most of those studied use these products in moderation and do not exceed the recommended dose. Acetaminophen is the most widely used product, but people also use NSAIDs and amphetamine-like products. The use of over-the-counter analgesics is common among residents of Hail, and most of the participants in this study use these products in moderation and do not exceed the recommended dose; therefore, exposure of patients to an increased risk of adverse effects of these products is out of the question, and this indicates that a large group of people in this region knows the risk of analgesics overuse. New studies are needed to help improve the approaches to the use of NPA drugs</p>

Current FDA regulatory guidance on the conduct of drug discrimination studies for NDA review: Does the scientific literature support recent recommendations?

BACKGROUND

The Controlled Substances Staff of the Center for Drug Evaluation and Research at the US Food and Drug Administration and the Pharmaceutical Research Manufacturers Association (PhRMA) conducted a series of open forum dialog sessions between 2006 and 2016. A Cross Company Abuse Liability Council (CCALC) was formed during the process of this unique collaborative effort between Industry and Federal Regulators whose goals were to establish the development of standards for the preclinical screening of new molecular entities for schedule control actions required as part of every New Drug Application process. The draft guidance document was published and disseminated in 2010, which allowed for alternative approaches to each study protocol requirement needed for NDA review, if the approach satisfied the requirements of the applicable statutes and regulations (i.e., the controlled substance act). In a series of recent pre-study protocol reviews requested by confidential Pharmaceutical Sponsors of MPI Research, the CSS staff appeared to change its policy and set forth to require all drug discrimination study data to be generated under "extinction" test sessions. MPI Research is a Contract Research Organization acting on behalf of pharmaceutical companies and bound under separate confidentiality agreements.

PURPOSE

The purpose of this review is to highlight the data appearing in peer-reviewed scientific journals that do not support the regulatory administrative constraints on one specific testing methodology (extinction) to the exclusion of another (reinforced test sessions).

CONCLUSION

This mind shift represents a restrictive administrative policy by the FDA that is not supported by the published data.

Reinforcing effect of pseudoephedrine isomers and the mechanism of action

It has been proposed that ephedrine and its isomers may have abuse potential. When made available to rhesus monkeys (n = 4) for self-administration, +-pseudoephedrine functioned as a positive reinforcer in all monkeys, as did (-)-pseudoephedrine in two of three monkeys. Pseudoephedrine isomers were 10- to 33-fold less potent than cocaine. In in vitro binding in monkey brain tissue, both isomers had low affinity for dopamine and serotonin transporters by at least 200-fold relative to cocaine, but comparable affinity for norepinephrine transporters. +-Pseudoephedrine also blocked dopamine uptake in 293 hDAT cells with low potency relative to cocaine. When given in vivo +-pseudoephedrine significantly displaced radioligand binding to dopamine transporters with a potency comparable to that in self-administration. Therefore, pseudoephedrine isomers can function as reinforcers and the mechanism at dopamine transporters may underlie this effect. However, pseudoephedrine appears to be a weak reinforcer and may have relatively low abuse potential.

Discriminative stimulus effects of (-)-ephedrine in rats: analysis with catecholamine transporter and receptor ligands

A drug discrimination procedure was used to examine the neuropharmacology of (-)-ephedrine (5 mg/kg), a sympathomimetic amine found in a variety of dietary supplements. (-)-Ephedrine has caused concern because of its use as a precursor in the manufacture of street drugs (e.g. methamphetamine) and its potential for abuse and toxicity. In the present study, the catecholamine reuptake inhibitors mazindol and nomifensine, the norepinephrine (NE) reuptake inhibitor desipramine, and the dopamine D(2)-like (e.g. D(2), D(3) and D(4)) agonist quinpirole substituted for (-)-ephedrine (>/=80% (-)-ephedrine-lever responding). The NE reuptake inhibitor nisoxetine, the D(1)-like (e.g. D(1) and D(5)) agonists (+/-)-SKF 38393 and SKF 82958, and the mixed D(1)-/D(2)-like agonist apomorphine occasioned intermediate levels of responding (50-79% (-)-ephedrine-lever responding). The (-)-ephedrine cue was antagonized by the D(1)-like antagonist SCH 23390 and the alpha(1)-adrenoceptor antagonist prazosin as well as the D(2)-like antagonists (-)-eticlopride and haloperidol, although only at doses that disrupted responding in some rats. The discriminative stimulus effects of a small dose of (-)-ephedrine (1.25 mg/kg) were enhanced by the alpha(2)-adrenoceptor antagonist idazoxan and to a lesser extent by the beta-adrenoceptor antagonist (-)-propranolol. However, the alpha(2)-adrenoceptor agonist clonidine (0.04 mg/kg) did not attenuate the (-)-ephedrine stimulus. These results suggest that D(1)-, D(2)-like, and alpha(1)-adrenergic receptors mediate the discriminative stimulus effects of (-)-ephedrine. Substitution of desipramine for (-)-ephedrine and not for some other stimulants suggests that NE transmission is a prominent feature of the (-)-ephedrine discriminative stimulus, and that NE underlies therapeutic and abuse-related effects of (-)-ephedrine that diverge from those of other stimulants.

Reinforcing and discriminative-stimulus effects of ephedrine isomers in rhesus monkeys

Ephedrine is a sympathomimetic drug that is currently found in many over-the-counter preparations. This compound exists as four isomers which, in addition to a racemic mixture, were evaluated for their positive reinforcing effects and for their similarity to (+)-amphetamine as a discriminative stimulus. Rhesus monkeys (N=3) with intravenous cocaine (0.1 mg/kg/inj) or saline as a consequence for lever pressing were shown to self-administer all of the ephedrine compounds (range tested: 0.03-3.0 mg/kg/inj), with the exception of (-)-pseudoephedrine, when each drug/dose was substituted for cocaine or saline during test sessions. However, the (-)-pseudoephedrine isomer was evaluated within a limited dose range due to solubility limitations. Systematically increasing the number of responses required for an injection indicated that these isomers were not as effective as reinforcers as was cocaine. Rhesus monkeys (N=3) trained to discriminate intragastric 1.0 mg/kg (+)-amphetamine from saline were given substitution tests with the ephedrine isomers and the racemic mixture. When given intragastrically, at least one dose of all the ephedrine isomers substituted for the (+)-amphetamine discriminative stimulus in at least one of the subjects tested. However, (+)-amphetamine-like effects were not systematically related to dose. When the discriminative-stimulus effects of (-)-ephedrine were also compared with those of (+)-amphetamine across three different routes of administration, full, dose-related, (+)-amphetamine-like responding was observed with both the intramuscular and intravenous routes. Taken together, these results suggest that the ephedrines have psychomotor stimulant-like abuse potential, lower than that of cocaine. Parenteral administration may enhance psychomotor-stimulant-like effects.

Stimulus effects of phenylpropanolamine optical isomers in (+)amphetamine-trained rats

There are eight phenylpropanolamine optical isomers related in structure to the central stimulants methamphetamine and amphetamine. Some of these are quite well known, such as (-)ephedrine, whereas others are relatively obscure, such as (-)cathine. Although certain of these phenylpropanolamines, such as (-)ephedrine and (+)cathine, retain central stimulant activity and are about 10- to 25-fold less potent than (+)amphetamine, the eight phenylpropanolamines have been compared only once before in drug discrimination studies. This latter study employed (-)ephedrine as the training drug. Because there are striking similarities between (-)ephedrine and (+)amphetamine as training drugs, it was of interest to determine and compare the effect of all eight phenylpropanolamines in (+)amphetamine trained animals. Using rats trained to discriminate 1 mg/kg of (+)amphetamine from saline vehicle under a variable interval 15-s (VI 15-s) schedule of reinforcement, the (+)amphetamine stimulus generalized only to (-)ephedrine (ED(50) = 4. 5 mg/kg) and (+)cathine (ED(50) = 8.0 mg/kg), and both agents were at least 10 times less potent that (+)amphetamine (ED(50) = 0.37 mg/kg). These results stand in contrast to those obtained with the (-)ephedrine-trained animals where the ephedrine stimulus generalized to all of the phenylpropanolamines except for (-)pseudoephedrine and (-)cathine. It is concluded that although there might be some similarity between the (-)ephedrine and (+)amphetamine stimuli, there are clear differences between them as determined in tests of stimulus generalization under the conditions employed.

(+)Amphetamine-stimulus generalization to an herbal ephedrine product

We have previously demonstrated that a (+)amphetamine stimulus generalizes both to (-)ephedrine and caffeine. Using rats trained to discriminate intraperitoneal (IP) administration of 1.0 mg/kg of (+)amphetamine (ED(50) = 0.4 mg/kg) from saline vehicle in a standard two-lever drug discrimination procedure, the present investigation shows that the (+)amphetamine stimulus generalizes to (+)amphetamine (ED(50) = 1.0 mg/kg) when administered via the intragastric (IG) route, and that (+)amphetamine appears about 2. 5-fold less potent when administered via the IG route compared to the IP route. Likewise, (-)ephedrine (ED(50) = 10.8 mg/kg) and caffeine (ED(50) = 32.9 mg/kg) are also 2.5-fold less potent when administered via the IG route compared to their potency when administered via the IP route. The (+)amphetamine stimulus also generalizes to an IG-administered herbal preparation (i.e., Herbal XTC; the herbal preparation possesses an approximate potency roughly comparable to what might have been expected on the basis of its reported ephedrine and/or caffeine content. These results demonstrate, for the first time, that an ephedrine-containing herbal preparation can produce a (+)amphetamine-like effect in animals.

Anesthesia is a risk factor for drug and alcohol craving and relapse in ex-abusers

Relapse to cocaine, heroin, and alcohol is a common occurrence in ex-abusers of these substances. Although there are many potential causes for relapse, we postulate that one cause in some people may be re-exposure in an anesthesia setting to a drug similar to the formerly abused drug. We hypothesize, for example, that opioids given during and after surgery may reinstate craving for, and initiate subsequent seeking of, heroin in former abusers. There are a substantial number of animal studies and some human studies documenting a reinstatement phenomenon in which an experimenter-administered psychoactive drug can precipitate drug-seeking behavior in 'abstinent' animals and humans. There is concern amongst health professionals and patients alike on this issue, and we discuss possible avenues of research, both preclinical and clinical, to explore the validity of our hypothesis.

An examination of isomeric phenylpropanolamines in (-)ephedrine-trained rats

A total of eight isomeric phenylpropanolamines are possible when the terminal amine is either an N-monomethylamine or a primary amine: (-)ephedrine, (+)ephedrine, (+)pseudoephedrine, (-)pseudoephedrine, (-)norephedrine, (+)norephedrine, (+)cathine, and (-)cathine. Few previous studies have examined the individual optical isomers of these phenylpropanolamines and, with the exception of one report on locomotor effects, no comparative behavioral data have been published on this series of agents. Using rats trained to discriminate 4 mg/kg of (-)ephedrine (i.p.) from saline vehicle using standard operant conditioning with a VI 15-s schedule of reinforcement, all eight agents were examined in tests of stimulus generalization. The (-)ephedrine stimulus (ED50 = 0.90 mg/kg) generalized to (+)ephedrine (ED50 = 2.64 mg/kg), (+)pseudoephedrine (ED50 = 6.58 mg/kg), (-)norephedrine (ED50 = 1.86 mg/kg), (+)norephedrine (ED50 = 5.75 mg/kg), and (+)cathine (ED50 = 4.87 mg/kg). The (-)ephedrine stimulus failed to generalize to either (-)pseudoephedrine or (-)cathine; the latter agents produced a maximum of 29 and 31% (-)ephedrine-appropriate responding, respectively. Thus, (a) six of the eight phenylpropanolamines produced ephedrine-like stimulus effects, (b) (-)ephedrine was the most potent of the examined agents, and (c) where stimulus generalization occurred, ED50 values spanned less than a tenfold range.

Arylalkylamine drugs of abuse: an overview of drug discrimination studies

Abused arylalkylamines fall into two major categories: the indolealkylamines, and the phenylalkylamines: These agents can be further subclassified on the basis of chemical structure. Examples of these agents possess hallucinogenic, stimulant, and other actions. Drug-discrimination techniques have been used to classify and investigate this large family of agents. Such studies have allowed the formulation of structure-activity relationships and investigations of mechanisms of action. Arylalkylamine designer drugs also possess the same or a combination of actions, and are being investigated by the same methods.

Influence of training paradigm on specificity of drug mixture discriminations

Generalization to different drugs and drug mixtures has been examined in rats trained to discriminate a mixture of amphetamine (0.4 mg/kg) plus pentobarbitone (10 mg/kg) from saline (AND discrimination, n = 8) or to discriminate the same mixture from its component drugs alone (AND-OR discrimination, n = 9). The studies used two-lever operant procedures with a tandem variable interval 1-min fixed-ratio 10 schedule of food reinforcement. There was partial generalization to nicotine and midazolam and no generalization to cocaine, caffeine, or ethanol under AND-discrimination conditions and no generalization to any of these drugs in the AND-OR discrimination. Nicotine or midazolam coadministered with the training doses of pentobarbitone and amphetamine, respectively, produced full generalization in the AND discrimination and partial generalization under AND-OR conditions. Cocaine coadministered with pentobarbitone generalized fully under both procedures, but at larger doses in the AND-OR than in the AND discrimination. Mixtures of either nicotine plus midazolam or caffeine plus ethanol produced very marked generalization under AND-discrimination conditions, but were without significant effect in the AND-OR procedure. The results consistently supported the hypothesis that the AND-OR discrimination procedure increases the specificity of discriminations based on drug mixtures.

Drug mixtures and ethanol as compound internal stimuli

Drug discrimination methods that entail training with mixtures of drugs may shed light on polydrug abuse and on the actions of single drugs that interact with more than one receptor. In AND-discrimination procedures (drug A + drug B vs. vehicle), mixtures are discriminated primarily on the basis of their component drugs: these discriminations may be useful for testing interactions between component drugs in mixtures. The role of training dose, overshadowing and associative blocking in AND-discriminations have been investigated. For example, after prior training with midazolam, it was possible to demonstrate associative blocking of the nicotine element of the mixture stimulus, and vice versa. Using the AND-OR discriminations (drug A + drug B vs. drug A or drug B) increased pharmacological specificity considerably, and these procedures may be valuable for determining whether the effects of a novel mixture are similar to the combined effects of the training drugs. Ethanol is an example of a single drug that may produce a compound cue; rats trained to discriminate ethanol from water generalize (asymmetrically) to GABA(A) enhancers such as chlordiazepoxide (CDP) or pentobarbitone, to NMDA antagonists such as dizocilpine (MK-801), and to some serotonin agonists, such as trifluoromethylphenylpiperazine (5-HT(1B/2C)). In addition, rats trained to discriminate mixtures of either CDP or pentobarbitone plus MK-801 generalize to ethanol. A previous history of training with MK-801 or CDP (prior to ethanol discrimination training) enhanced the MK-801-like and CDP-like effects of ethanol respectively, but associative blocking of proposed elements in the ethanol stimulus was not seen. These studies provide some support for the multielement concept of ethanol discrimination but also suggest that rules governing three-component stimuli (such as those putatively produced by ethanol) may differ from those for the two-component mixtures of drugs studied previously.

Over-the-counter stimulants: abuse and addiction

A widely held belief is that amphetamine-like stimulants have abuse potential, but addiction to these drugs is seldom mentioned. We describe seven patients who fulfilled criteria for amphetamine-like abuse or dependence, based on the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders. This case series lends clinical support to the research findings that amphetamine-like drugs are reinforcing and therefore can be addictive. The use of standardized criteria to classify excessive over-the-counter stimulant use as abuse or dependence distinguishes these case reports from others in the medical literature. Public concern about the misuse of this class of drugs is discussed. We propose that clinicians use standardized criteria to diagnose accurately abuse of or dependence on over-the-counter stimulants for a better understanding of the extent of this problem.

(-)Ephedrine and caffeine mutually potentiate one another's amphetamine-like stimulus effects

Using rats trained to discriminate 1 mg/kg of (+)amphetamine (ED50 = 0.4 mg/kg) from saline vehicle in a two-lever drug discrimination procedure, it was shown that (-)ephedrine (ED50 = 4.5 mg/kg), but not (+)ephedrine, substitutes for the (+)AMPH stimulus. It was also shown that caffeine (ED50 = 12.9 mg/kg) can substitute for (+)amphetamine in a dose-related fashion. Doses of (-)ephedrine and caffeine, which produced < or = 1% drug-appropriate responding when administered alone, were able to enhance each other's stimulus effects when administered in combination such that there was a twofold leftward shift in their respective dose-response curves. Furthermore, stimulus generalization occurred when a dose of caffeine that produced saline-appropriate responding when administered alone was administered in combination with (+)ephedrine. It would appear that low doses of (-)ephedrine and caffeine may mutually potentiate one another's stimulus effects in (+)AMPH-trained rats, and that a combination of caffeine and (+)ephedrine result in altered stimulus character when compared to comparable doses of either agent administered alone.

The stimulus properties of two common over-the-counter drug mixtures: dextromethorphan + ephedrine and dextromethorphan + diphenhydramine

Two groups of rats were trained in a two-choice drug discrimination procedure under a fixed-ratio 10 schedule of food reinforcement. One group of rats (n=12) was trained to discriminate the presence and absence of a drug mixture containing 10 mg/kg dextromethorphan + 10 mg/kg diphenhydramine. The other group of rats (n=12) was trained to discriminate the presence and absence of another drug mixture containing 10 mg/kg dextromethorphan + 10 mg/kg ephedrine. Cross-generalization tests conducted with each of the stimulus elements demonstrated that (1) the drug mixtures were not perceived as new entities distinct from their component elements and (2) the stimulus element saliency may be a factor determining the nature of discriminative control by drug mixtures. Cross-generalization tests conducted with the psychomotor stimulants, cocaine and amphetamine, engendered complete generalization to the training cues in both groups, whereas, pentobarbital engendered predominantly saline- or default-lever responding. These data suggest a potential abuse liability for both of these common over-the-counter drug mixtures and cautions against the use of such combinations in pediatric patients.

(-)-Norpseudoephedrine, a metabolite of cathinone with amphetamine-like stimulus properties, enhances the analgesic and rate decreasing effects of morphine, but inhibits its discriminative properties

Like psychomotor stimulants, a weak amphetamine-like agent, such as phenylpropanolamine, enhances the analgesic effects of morphine (MOR). Thus, it is possible that full psychomotor stimulant potency is not required to increase the analgesic action of opiates. The validity of this assumption is here tested by studying the ability of (-)-norpseudoephedrine (NPE), an enantiomer of phenylpropanolamine and a metabolite of cathinone, to influence both the analgesic effects of MOR and its discriminative stimulus properties. In mice NPE (5.6-10.0-17.0 mg/kg i.p.) did not prolong the latency to lick or to remove paws from a plate warmed at 54 degrees C. However, it significantly potentiated the analgesic effect of 3.2 mg/kg of MOR. These results were replicated in rats by use of the formalin test, which measures the numbers of hind paw flinches produced by injecting 50 microl of formalin into the dorsal surface of the paw. The higher dose of NPE (17 mg/kg) increased the effect of sub-analgesic doses of MOR (0.56 and 1.0 mg/kg). In rats trained to discriminate between 0.5 mg/kg of amphetamine and solvent in a two-lever operant behavior reinforced by water access. NPE induced a dose-dependent increment of drug lever responding from 0% at 1.0 mg/kg to 100% at 32.0 mg/kg. In contrast, NPE did not generalize for the MOR cue up to the dose of 56.0 mg/kg, which produced a substantial reduction of the response rate. However, when given in combination, NPE attenuated the discriminative effects of MOR and potentiated its inhibitory action on the response rate. These results exclude a direct action of NPE on the mu opiate system. In conclusion, NPE preserves amphetamine-like properties and these properties are probably responsible for the interaction of the drug with the analgesic and discriminative effects of MOR. Therefore, this study contradicts the assumption that the analgesic effects of MOR can be enhanced by a sympathomimetic drug that lacks significant psychostimulant actions.

Phentermine+fenfluramine produce cocaine-like discriminative cues

Drug discrimination studies were conducted in six male Sprague-Dawley rats trained to discriminate the interoceptive cues produced by 10 mg/kg cocaine in an effort to investigate if there is stimulus generalization to phentermine or phentermine + fenfluramine. Once having reached criterion performance, these rats were tested with lower doses of cocaine and generated a typical dose-response curve allowing for calculation of an ED50 value: 2.798 mg/kg. Testing of phentermine in doses of 1.25-5.0 mg/kg indicated generalization with the highest dose producing 80% cocaine-appropriate responding and allowing for an ED50 value of 2.356 mg/kg. When the phentermine doses were tested in combination 2.0 mg/kg fenfluramine, however, there was an increase in the discriminability of the highest phentermine dose and a slight decrease in the ED50 value of the combination. Thus, administration of phentermine + fenfluramine, having both dopamine-releasing and serotonin-releasing properties, respectively, may mimic the neurochemical activity by which cocaine acts in the central nervous system and may possibly allow for cocaine-like effects as these two drugs see increased use in obesity control.

Evaluation of the cocaine-like discriminative stimulus effects and reinforcing effects of modafinil

Modafinil [(diphenyl-methyl)sulphinyl-2-acetamide] is a novel psychostimulant drug which is effective in the treatment of narcolepsy and idiopathic hypersomnia. It also has neuroprotective effects in animal models of striatal neuropathology. Although the cellular mechanisms of action of modafinil are poorly understood, it has been shown to have a profile of pharmacological effects that differs considerably from that of amphetamine-like stimulants. There is some evidence that modafinil has central alpha 1-adrenergic agonist effects. In the present study modafinil was evaluated for cocaine-like discriminative stimulus effects in rats and for reinforcing effects in rhesus monkeys maintained on intravenous cocaine self-administration. Modafinil, l-ephedrine and d-amphetamine all produced dose dependent increases in cocaine-lever responding, with maximal levels of 67%, 82% and 100%, respectively. Modafinil produced full substitution in four out of the six rats tested while the highest levels of substitution were associated with substantial response rate decreasing effects. Little evidence was obtained that the discriminative stimulus effects of modafinil were produced by alpha 1-adrenergic activation, based upon results of tests performed in combination with prazosin. In the self-administration procedure, modafinil and l-ephedrine functioned as reinforcers in rhesus monkeys. The reinforcing and discriminative stimulus effects of modafinil-required very high doses: modafinil was over 200 times less potent than d-amphetamine and was also less potent than l-ephedrine. These results show that modafinil has some cocaine-like discriminative stimulus effects and, like other abused stimulants, can serve as a reinforcer at high doses.

Cross-generalization between a cocaine cue and two antihistamines

Rats were trained to discriminate between 10 mg/kg cocaine and saline injections under a fixed ratio 10 schedule of food-motivated lever press responding. Once stimulus control was achieved, reinforced test sessions were conducted to assess the degree of generalization of a wide range of cocaine doses and the cross-generalization between the cocaine training stimulus and two over-the-counter antihistaminic drugs, diphenhydramine and doxylamine, when administered with saline or in drug combinations. Cocaine produced a dose-dependent generalization to the 10 mg/kg training stimulus. Cocaine also produced mild rate-increasing effects at low test doses and response rate suppression at higher doses. Both diphenhydramine and doxylamine produced a partial generalization to the 10 mg/kg cocaine training stimulus. Drug mixtures produced complete cross-generalization with the training cue.

A three-choice haloperidol-saline-cocaine drug discrimination task in rats

This study was conducted to test whether rats could be trained and successfully maintain a three-choice drug discrimination task using 0.1 mg/kg haloperidol (SC, 2-h pretreatment), saline (IP or SC, 2 h and 15 min pretreatment), and 10 mg/kg cocaine (IP, 15-min pretreatment) as training stimuli. Six male Sprague-Dawley rats achieved criterion performance for stimulus control by these training stimuli under a fixed-ratio-5 schedule of food reinforced lever-press responding in an average of 164 training sessions. Dose-response functions for cocaine and haloperidol demonstrated both quantitative and qualitative specificity of the training stimuli. The data also are presented along a single pharmacological continuum (agonist-antagonist) that we hypothesize to represent a parallel subjective or interoceptive stimulus continuum associated with the drug injections. Based on the previous multidimensional model of drug stimuli dimensionality (3), this specific stimulus dimension is characterized as an unidimensional bipolar continuum represented by the hypothetical states of hedonia or euphoria on one end (cocaine) and anhedonia or depression on the opponent end (haloperidol), with a neutral (saline) centroid region. We propose that this specific three-choice drug discrimination task in rats may function as an animal analog of the subjective states associated with cocaine abuse and the subsequent withdrawal or, crash, in humans (7,8,21).

Functional relationships, previous history and the discrimination of a drug mixture in rats

These experiments aim to probe the role of different functional relationships between drugs, responses and reinforcers in studies on the discrimination of drug mixtures. Two-lever discriminations based on mixtures of (+)-amphetamine (0.4 mg/kg) plus pentobarbitone (10 mg/kg) have been compared in three groups of rats (n = 8) trained to discriminate: (i) the mixture from saline, called the AND-discrimination; (ii) either the mixture, amphetamine or pentobarbitone from saline, the OR-discrimination; (iii) the mixture from either amphetamine or pentobarbitone, the AND-OR-discrimination. The rats were trained in a two-bar operant conditioning procedure with a tandem schedule of food reinforcement. The rats trained on the AND procedure acquired the discrimination more rapidly than the rats trained on the other two procedures, but after 60 training sessions, all discriminations were performed with similar (90-94%) accuracy. In rats trained under the AND and the OR procedures, there was full generalization from the mixture to the largest doses used of either amphetamine or pentobarbitone. In contrast, in rats trained under the AND-OR procedure, there was no generalization from the mixture to any dose of either drug separately. The training procedures for the OR and the AND-OR groups were changed to AND-discrimination training; the effects of the different previous histories were found to persist for a prolonged period, despite the fact that training conditions were now the same in all groups. The effects of the drugs on rates of responding were also influenced by the previous training history. Thus, using different training procedures to alter functional relationships can substantially and persistently influence the major characteristics of a discrimination based on a mixture of drugs.

Factors influencing the reinforcing and subjective effects of ephedrine in humans

There has been little study of the abuse liability of ephedrine, a naturally occurring drug used in medicine for thousands of years and currently sold as a "legal" stimulant. The present study measured the reinforcing and subjective effects of ephedrine in a group of 27 adults (18 females and 9 males) with no history of drug dependence. A discrete-trial choice procedure was used to assess the reinforcing effects of a single oral dose of ephedrine selected to produce a moderate subjective response in each subject (range: 37.5-75 mg). A number of variables (gender, current and past drug use, personality, and baseline mood and arousal) were examined in an attempt to identify sources of variability in response to ephedrine. Of the 27 subjects, 5 chose ephedrine on either 2 or 3 out of a possible 3 occasions; overall, ephedrine was chosen on 17% of occasions. In the group as a whole, ephedrine had no effect on ratings of drug liking, but did increase ratings of "high" and scores on the MBG ("euphoria") scale of the Addiction Research Center Inventory. Ephedrine also increased scores on a number of mood scales reflecting CNS stimulation and anxiety. Ephedrine choice was positively associated with current use of marijuana and lower levels of baseline anxiety and hunger, as well as with lower scores on two scales measuring dimensions of the personality trait of harm avoidance. Males and females differed in their response to ephedrine--males chose ephedrine more frequently than females and showed a more positive mood response to the drug.(ABSTRACT TRUNCATED AT 250 WORDS)