Discriminative stimulus properties of d-amphetamine and related compounds in rats

1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane (DOI): From an Obscure to Pivotal Member of the DOX Family of Serotonergic Psychedelic Agents - A Review

1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane (DOI, or DOX where X = -I) was first synthesized in 1973 in a structure-activity study to explore the effect of various aryl substituents on the then newly identified, and subsequently controlled, hallucinogenic agent 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM, or DOX where X = -CH3). Over time, DOI was found to be a serotonin (5-HT) receptor agonist using various peripheral 5-HT receptor tissue assays and later, following the identification of multiple families of central 5-HT receptors, an agonist at 5-HT2 serotonin receptors in rat and, then, human brain. Today, classical hallucinogens, currently referred to as serotonergic psychedelic agents, are receiving considerable attention for their potential therapeutic application in various neuropsychiatric disorders including treatment-resistant depression. Here, we review, for the first time, the historical and current developments that led to DOI becoming a unique, perhaps a landmark, agent in 5-HT2 receptor research.

Is Methylphenidate Beneficial and Safe in Pharmacological Cognitive Enhancement?

Nootropics are drugs used to either treat or benefit cognition deficits. Among this class, methylphenidate is a popular agent, which acts through indirect dopaminergic and noradrenergic agonism and, therefore, is proposed to enhance performance in catecholamine-dependent cognitive domains such as attention, memory and prefrontal cortex-dependent executive functions. However, investigation into the efficacy of methylphenidate as a cognitive enhancer has yielded variable results across all domains, leading to debate within the scientific community surrounding its off-label use in healthy individuals seeking scholaristic benefit or increased productivity. Through analysis of experimental data and methodological evaluation, it is apparent that there are dose-, task- and domain-dependent considerations surrounding the use of methylphenidate in healthy individuals, whereby tailored dose administration is likely to provide benefit on an individual basis dependent on the domain of cognition in which benefit is required. Additionally, it is apparent that there are subjective effects of methylphenidate, which may increase user productivity irrespective of cognitive benefit. Whilst there is not extensive study in healthy older adults, it is plausible that there are dose-dependent benefits to methylphenidate in older adults in selective cognitive domains that might improve quality of life and reduce fall risk. Methylphenidate appears to produce dose-dependent benefits to individuals with attention-deficit/hyperactivity disorder, but the evidence for benefit in Parkinson's disease and schizophrenia is inconclusive. As with any off-label use of pharmacological agents, and especially regarding drugs with neuromodulatory effects, there are inherent safety concerns; epidemiological and experimental evidence suggests there are sympathomimetic, cardiovascular and addictive considerations, which might further restrict their use within certain demographics.

Relationship between drug discrimination and ratings of subjective effects: implications for assessing and understanding the abuse potential of D-amphetamine in humans

The discriminative and subjective effects of drugs in humans are related, but the full extent of this relationship remains to be determined. To further explore this relationship, a retrospective analysis was conducted on data from six studies completed in our laboratory that used identical procedures. The relationship between the discriminative and subjective effects of a range of doses of D-amphetamine (i.e. 2.5-15 mg) was examined using correlational analyses. Significant correlations with discrimination performance were observed on 15 of 20 items from the Drug-Effect Questionnaire across a range of qualities [e.g. Pay For (a positive effect indicative of abuse potential) and Active (a stimulant-like effect)], but the magnitude of these relationships was modest (r<0.52). The current findings demonstrate that diverse subjective effects contribute to the discriminative effects of D-amphetamine and indicate that the former are a more practical means to assess the abuse potential of drugs. Although these procedures are fundamentally related in that they rely on the presence of an interoceptive drug state, they differ in the dimension(s) of the interoceptive effects that participants must quantify. The simultaneous use of drug discrimination and subjective effects may, therefore, reveal complimentary aspects of drug effects that underlie their potential for abuse.

Discriminative stimulus and subject-rated effects of methamphetamine, d-amphetamine, methylphenidate, and triazolam in methamphetamine-trained humans

Methamphetamine abuse is a significant public health concern. Although widely studied in laboratory animals, little is known about the abuse-related behavioral effects of methamphetamine relative to other abused stimulants in controlled laboratory settings in humans. The aim of this study was to examine the discriminative stimulus, subject-rated, performance, and cardiovascular effects of methamphetamine in humans. In the present study, subjects first learned to discriminate 10 mg of oral methamphetamine from placebo. After acquiring the discrimination (> or = 80% drug-appropriate responding on four consecutive sessions), a range of oral doses of methamphetamine (2.5-15 mg), d-amphetamine (2.5-15 mg), methylphenidate (5-30 mg), and triazolam (0.0625-0.375 mg) was tested. Methamphetamine functioned as a discriminative stimulus and produced prototypical stimulant-like subject-rated effects. d-Amphetamine and methylphenidate produced dose-related increases in methamphetamine-appropriate responding, whereas triazolam did not. d-Amphetamine and methylphenidate produced stimulant-like behavioral effects, whereas triazolam produced sedative-like effects. Methamphetamine, but no other drug, increased heart rate, systolic pressure, and diastolic pressure significantly above placebo levels. Performance in the Digit-Symbol Substitution Test was not affected by any of the drugs tested. Overall, these results demonstrate that the acute behavioral effects of methamphetamine, d-amphetamine, and methylphenidate overlap extensively in humans, which is concordant with findings from preclinical studies. Future studies should assess whether the similarity in the behavioral effects of methamphetamine and related stimulants can be extended to other behavioral assays, such as measures of reinforcement, in humans.

Cocaine serves as a peripheral interoceptive conditioned stimulus for central glutamate and dopamine release

Intravenous injections of cocaine HCl are habit-forming because, among their many actions, they elevate extracellular dopamine levels in the terminal fields of the mesocorticolimbic dopamine system. This action, thought to be very important for cocaine's strong addiction liability, is believed to have very short latency and is assumed to reflect rapid brain entry and pharmacokinetics of the drug. However, while intravenous cocaine HCl has almost immediate effects on behavior and extracellular dopamine levels, recent evidence suggests that its central pharmacological effects are not evident until 10 or more seconds after IV injection. Thus the immediate effects of a given intravenous cocaine injection on extracellular dopamine concentration and behavior appear to occur before there is sufficient time for cocaine to act centrally as a dopamine uptake inhibitor. To explore the contribution of peripheral effects of cocaine to the early activation of the dopamine system, we used brain microdialysis to measure the effects of cocaine methiodide (MI)--a cocaine analogue that does not cross the blood brain barrier--on glutamate (excitatory) input to the dopamine cells. IP injections of cocaine MI were ineffective in cocaine-naïve animals but stimulated ventral tegmental glutamate release in rats previously trained to lever-press for cocaine HCl. This peripherally triggered glutamate input was sufficient to reinstate cocaine-seeking in previously trained animals that had undergone extinction of the habit. These findings offer an explanation for short-latency behavioral responses and immediate dopamine elevations seen following cocaine injections in cocaine-experienced but not cocaine-naïve animals.

N-Methyl-1-(4-methoxyphenyl)-2-aminopropane (PMMA) and N-Methyl-1-(3,4-methylenedioxyphenyl)-2-aminopropane (MDMA) produce non-identical discriminative stimuli in rats

N-Methyl-1-(3,4-methylenedioxyphenyl)-2-aminopropane (methylenedioxymethamphetamine, MDMA, Ecstasy) and its structurally abbreviated congener N-methyl-1-(4-methoxyphenyl)-2-aminopropane (para-methoxymethamphetamine, PMMA) are chemically related designer drugs, and PMMA is sometimes sold on the clandestine market as a substitute for MDMA. Prior drug discrimination studies have found that MDMA and PMMA substitute for one another suggesting that they produce similar discriminative stimulus effects in rats. However, there also are some indications that the two agents produce distinct stimulus effects. In this study, further comparisons were made between the stimulus effects of these two agents. Sprague-Dawley rats were trained to discriminate either 1.25 mg/kg of PMMA or 1.5 mg/kg of MDMA from saline vehicle in a two-lever operant paradigm. A structure-activity comparison revealed that MDMA and PMMA behave similarly upon homologation of their terminal amine substituents. In contrast, the PMMA stimulus, unlike an MDMA stimulus, failed to generalize completely to the psychostimulant cocaine, 8-hydroxy-2-(N,N-di-n-propylamino)tetralin (8-OH DPAT), and R(-)-1-(3-methoxyphenyl)-2-aminopropane [R(-)MMA]. In an additional group of animals, a (+)amphetamine stimulus partially generalized to R(-)MMA. Taken together, the results argue and re-emphasize the conclusion that the stimulus effects produced by MDMA and PMMA are similar, but non-identical, and that PMMA is the less "stimulant-like" of the two.

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.

Reinforcing, subject-rated, performance and physiological effects of methylphenidate and d-amphetamine in stimulant abusing humans

Methylphenidate has potential for abuse because it produces behavioural effects similar to those observed with other abused stimulants, such as d-amphetamine and cocaine. The aim of this study was to further characterize the abuse potential of oral methylphenidate relative to oral d-amphetamine. Ten drug-abusing volunteers were recruited to participate in this study, which consisted of seven dose conditions: methylphenidate (16, 32 and 48 mg), d-amphetamine (8, 16 and 24 mg) and placebo. The reinforcing effects of these drugs were assessed during a self-administration session (preceded by a sampling session for each condition) with a modified progressive-ratio procedure. Subject-rated, performance and physiological effects were assessed concurrently during both the sampling and self-administration sessions. The intermediate dose of methylphenidate and d-amphetamine increased responding significantly above placebo levels. Both methylphenidate and d-amphetamine produced dose-dependent increases in stimulant-like subject ratings (e.g. Active, Alert, or Energetic and High), but the effects of these drugs were not isomorphic. These findings are consistent with epidemiological data and previous findings from laboratory studies that suggest methylphenidate has at least some abuse potential.

Effect of PMA optical isomers and 4-MTA in PMMA-trained rats

1-(4-Methoxyphenyl)-2-aminopropane (PMA) and its sulfur analog, 1-(4-methylthiophenyl)-2-aminopropane (4-MTA), have been misrepresented as the controlled substance analog, N-methyl-1-(3,4-methylenedioxyphenyl)-2-aminopropane (MDMA; "Ecstasy"). Because MDMA has been shown to produce both amphetamine-like and N-methyl-1-(4-methoxyphenyl)-2-aminopropane (PMMA)-like stimulus effects in rats, we examined S(+)PMA, R(-)PMA and 4-MTA in rats trained to discriminate either PMMA (1.25 mg/kg) or (+)amphetamine (1.0 mg/kg) from saline vehicle. The sulfur analog of PMMA (i.e., 4-MTMA) was also examined. The PMMA stimulus generalized to R(-)PMA (ED50=0.4 mg/kg), whereas S(+)PMA produced a maximum of 72% PMMA-appropriate responding. 4-MTA (ED50=0.3 mg/kg) also substituted for PMMA, but 4-MTMA produced a maximum of only 36% PMMA-appropriate responding. None of the four agents substituted for (+)amphetamine. Hence, like MDMA, R(-)PMA and 4-MTA are capable of producing PMMA stimulus effects in rats, but unlike MDMA, neither agent substituted for (+)amphetamine.

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.

Effects of training dose on amphetamine drug discrimination: dose-response functions and generalization to cocaine

Rats were trained to discriminate one of three doses of amphetamine (AM), 0.5, 1, or 2 mg/kg, from vehicle (VEH) in a two-lever, food-reinforced, drug-discrimination task. The purpose of the study was to investigate the nature of the shift of the dose-response curve and generalization to cocaine (COC) as a function of training dose. In order to preclude potential differences among the groups in stimulus control, the three training-dose groups were required to perform the discrimination at high and equivalent levels of accuracy. The shift of the dose-response functions to the right as a function of increasing training dose was not parallel. The slope decreased as training dose increased. There was a dose-dependent increase in AM lever responding to test doses of COC that tended to be affected by training dose. The results suggest that proper evaluation of training-dose effects requires that groups be trained to equivalent levels of stimulus control.

Assessing the abuse potential of methylphenidate in nonhuman and human subjects: a review

Methylphenidate (MPH) is widely used for the treatment of Attention Deficit Hyperactivity Disorder (ADHD) in children, adolescents, and adults. Methylphenidate is clearly effective for the treatment of ADHD, but there is controversy as to whether it has significant abuse potential like other psychostimulants (e.g., D-amphetamine and cocaine). In general, the drug is believed to be abused at rates much lower than those for other stimulants. The present review examines studies that investigated the behavioral pharmacological profile of methylphenidate and discusses how results from these studies address its abuse liability. Using MEDLINE search terms methylphenidate, drug discrimination, reinforcement, self-administration, subjective effects, subject-rated effects, abuse potential, and abuse liability, along with a review of the references from identified articles, 60 studies were located in which the reinforcing, discriminative-stimulus, or subjective effects of methylphenidate were directly assessed in nonhumans or humans. Forty-eight (80.0%) of the studies reviewed indicate that methylphenidate either functions in a manner similar to D-amphetamine or cocaine (e.g., functions as a reinforcer, substitutes fully in drug discrimination experiments), or produces a pattern of subjective effects suggestive of abuse potential. The results are discussed as they pertain to factors that may account for the apparent discrepancy in abuse rates between methylphenidate and other stimulants, including characterization of actual abuse rates, defining abuse and misuse, pharmacokinetic factors, and validity of abuse liability assays.

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.

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.

(-)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.

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.

Methylphenidate treatment following ablation-induced hemiplegia in rat: experience during drug action alters effects on recovery of function

Two experiments examined the effects of single or multiple administrations of methylphenidate (MPH; Ritalin) and differing amounts of beam-walking trials (symptom relevant experience) during the period of drug action on recovery from hemiplegia following unilateral sensorimotor cortex ablation in rat. The first study tested multiple doses of MPH (10 mg/kg) or sterile saline given once daily, followed by four beam-walk (BW) trials at 1, 2, 3, and 6 h on 3 consecutive days. A significant and enduring enhancement of recovery was only observed 24 h after the third administration of MPH, compared to saline controls. In the second study, a single dose of MPH (10 mg/kg) or saline was administered 24 h after ablation, followed by 12 BW trials beginning 1 h and continuing at 15-min intervals until 3 h after MPH or saline administration. A significant and enduring facilitation of BW ability was produced by this single MPH treatment regimen. These data further support the importance of an interaction between symptom-relevant experience and drugs that increase norepinephrine transmission to enhance functional recovery after brain damage.

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)

D-amphetamine-like stimulus properties are produced by morphine injections into the ventral tegmental area but not into the nucleus accumbens

We investigated whether injections of morphine into the ventral tegmental area (VTA) or the nucleus accumbens (NAS) could produce amphetamine-like stimulus properties and locomotor stimulant effects. Rats were trained to discriminate 1.0 mg/kg D-amphetamine from saline using both VI-30 and FR-16 reinforcement schedules and they were then tested following bilateral injections of morphine sulfate (2.5, 5.0 and 10.0 micrograms/side) either into the VTA or the NAS. Intra-VTA injections of morphine produced significant increases in amphetamine-lever responding that were comparable to increases observed following intra-NAS D-amphetamine (2.5, 5.0 and 10.0 micrograms/side). Such increases were not observed, however, following intra-NAS injections of morphine. Tests for the effects of intracerebral morphine and D-amphetamine on locomotor activity produced a similar pattern of results. Locomotor activity was increased following intra-VTA morphine and intra-NAS D-amphetamine injections, but not after intra-NAS morphine injections. Together, these findings indicate that activation of opioid receptors within the VTA, but not the NAS, can produce a behavioral state which mimics to some degree the state produced by systemic and intra-NAS injections of D-amphetamine.

Repeated testing within drug discrimination learning: time course studies with cocaine, amphetamine, and 3-PPP

Pigeons were trained to discriminate between 3 mg/kg cocaine and saline. Tests with cocaine and amphetamine were conducted at different intervals after administration to compare the time course of the discriminative stimulus (DS) effects. Tests were of two kinds: a) separate, that is, only one dose and interval were examined on each separate test day; and b) repeated, that is, all three intervals were assessed after a single administration of the drug dose during 1 test day. Separate and repeated determinations of the time course yielded similar estimates. The duration of the DS effects of amphetamine were longer than those of cocaine. No apparent difference, either with regard to duration of effect or potency, existed between (+)- and (-)-amphetamine. The potency of cocaine was similar to that of the amphetamine isomers. The dopamine autoreceptor blockers (+)- and (-)-3-(hydroxyphenyl)-N-n-propylpiperidine (3-PPP) (1-10 mg/kg) engendered less than 44% cocaine-associated responding for the repeatedly examined intervals (15, 60, and 120 min after administration). The results of this study encourage the use of repeated testing methodology to assess the duration of action of the DS effects of drugs.

Drug discrimination by humans compared to nonhumans: current status and future directions

In drug discrimination (DD) procedures, behavior is differentially reinforced depending on the presence or absence of specific drug stimuli. The DD paradigm has been widely adopted by behavioral pharmacologists because of its specificity of stimulus control, concordance with drug action at cellular levels and its use as a preclinical model of subject-rated effects in humans. With the successful extension of DD to humans, a comparison of human and nonhuman DD will help place each in the context of the other. Twenty-eight studies of DD in humans are reviewed, including studies of amphetamine, opioid, benzodiazepine, caffeine, nicotine, marijuana and ethanol discriminative stimuli. Comparison of procedures between studies in humans and nonhumans reveals a common tradition, except the use of instructions appears to facilitate greatly DD acquisition in humans. Findings were qualitatively similar between humans and nonhumans. Potency relationships were quantitatively similar between humans and most, but not all, other species. Areas of human DD needing additional empirical evaluation include the influence of instructions, the effects of training dose and the effects of antagonists. Additionally, antihistamines, barbiturates, nicotine and marijuana are under-represented in human DD.

Discriminative stimulus properties of substituted amphetamine derivatives

Animals were trained to discriminate amphetamine (1 mg/kg) from saline in a fixed-ratio (FR 10), food-reinforced paradigm. Amphetamine-appropriate responding was engendered by the training dose, and by 3 mg/kg, while at lower doses there was a progressive decrease in the extent of responding on the drug-appropriate lever. The following three novel amphetamine derivatives were tested for their ability to produce amphetamine-appropriate responding: 2,5-dimethoxy-4-ethoxy-amphetamine (DMEA); 2,5-dimethoxy-4-methylthio-amphetamine (DMMTA), and 2,4,5-trimethoxy-amphetamine (TMA). DMEA produced only minimal (< 20%) amphetamine-appropriate responding over a dose range of 0.1-10 mg/kg. Substantial decreases in response rate limited testing of the other amphetamines to a dose maximum of 3 mg/kg, but over the range of 0.1-3.0 mg/kg there was little evidence for generalization. At 3 mg/kg of either DMMTA or TMA, only 2 of 10 animals completed at least one uninterrupted FR 10 on either lever, and with either compound only 1 of these 2 animals responded more than 50% on the drug-appropriate lever. Of the three compounds tested, DMMTA had the greatest response rate-decreasing effect.

Opioid-dopaminergic mechanisms in the potentiation of d-amphetamine discrimination by interferon-alpha

In rats trained to discriminate 0.8 mg/kg IP d-amphetamine from 1 ml/kg saline, 4 x 10(6) U/kg of recombinant human interferon-alpha (rIFN-alpha) given intramuscularly 1 h prior to tests potentiated responses elicited by 0.4 mg/kg d-amphetamine. Coadministration of the opioid receptor antagonist naloxone (1 mg/kg IP) with rIFN-alpha suppressed the potentiation of d-amphetamine by the cytokine. Opioid-dopaminergic mechanisms are proposed to explain the action of rIFN-alpha.

Effects of an antitussive mixture and its constituents in rats discriminating methamphetamine from saline

The discriminative effects of over-the-counter antitussive syrup containing dihydrocodeine (DHC), methylephedrine (MEP), caffeine (CAF), and chlorpheniramine (CPA) were compared with those of methamphetamine (MA) in a drug discrimination experiment using rats. Rats were trained to discriminate the effects of MA at 0.5 mg/kg SC and saline for food reinforcement under the fixed-ratio 10 schedule in a two-lever operant chamber situation. In substitution testing using a cumulative dose procedure by the subcutaneous route, DHC (4 and 8 mg/kg, expressed hereafter as referred to cumulative dose) or CPA (16-64 mg/kg) individually did not produce MA lever selection. On the other hand, MEP (128 mg/kg) and CAF (64 mg/kg) produced MA lever selection 41.5 and 57.2% of the time, respectively. The complete mixture (16 mg/kg DHC + 32 mg/kg MEP + 33.2 mg/kg CAF + 6.4 mg/kg CPA) produced MA level selection 65.8% of the time. The partial mixture containing only MEP + CAF at the above doses produced MA lever selection 95.6% of the time. Thus, the complete mixture only partially substituted for MA in rats while the partial mixture containing MEP and CAF completely substituted for MA.

The reinforcing and discriminative stimulus properties of para-ethoxy- and para-methoxyamphetamine

The purpose of this study was two-fold: 1) to assess the degree to which para-methoxyamphetamine and para-ethoxyamphetamine maintain self-administration behavior, and 2) to determine the similarity or difference between these drugs and amphetamine in drug discrimination tests. Animals were trained to self-administer 0.3 mg/kg/infusion cocaine on a fixed-ratio 5 (FR5) schedule of reinforcement. Substitution of para-ethoxyamphetamine (PEA), para-methoxyamphetamine (PMA), or saline produced similar results; in all cases responding decreased substantially. A separate group of animals was trained to discriminate amphetamine (1 mg/kg) from saline in a fixed-ratio (FR10), food-reinforced paradigm. PEA and PMA produced only limited responding on the amphetamine-appropriate lever (maximum of approximately 30%). Both PMA and PEA had effects on response rate which were similar to those of amphetamine, although PMA had slightly greater rate-decreasing effects than the other two compounds. These data suggest that neither PMA nor PEA are reinforcing in rats, and do not possess amphetamine-like discriminative properties.

Stimulus properties of ring-methyl amphetamine analogs

There are three possible ring-substituted methyl amphetamines (or tolylaminopropanes; TAPs): oTAP, mTAP and pTAP. These agents are positional isomers of methamphetamine. Although all three isomers have been previously reported to possess amphetamine-like character, few studies have examined all three agents in comparison with (+)amphetamine. Using rats trained to discriminate 1 mg/kg of (+)amphetamine from saline under a variable-interval 15-sec schedule of reinforcement, tests of stimulus generalization were conducted with the three positional isomers. Only oTAP (ED50 dose = 4.1 mg/kg) completely substituted for (+)amphetamine. mTAP and pTAP resulted only in partial (ca. 50% amphetamine-appropriate responding) generalization. It is concluded that oTAP is capable of producing amphetamine-like stimulus effects and that it is approximately one-tenth as potent as (+)amphetamine; however, because the partial generalization produced by mTAP and pTAP was followed by disruption of behavior at slightly higher doses, it cannot be reliably stated that these latter two isomers lack amphetamine-like character.

Amphetamine-like stimulus properties produced by electrical stimulation of reward sites in the ventral tegmental area

The present study examined whether the stimulus properties of D-amphetamine could be mimicked by electrical stimulation of the ventral tegmental area (VTA). Rats trained to discriminate 1.0 mg/kg D-amphetamine from saline were given generalization tests with a range of D-amphetamine doses administered either alone or in combination with VTA stimulation. The results suggested that the VTA stimulation could enhance the cueing effects of D-amphetamine, as levels of responding on the D-amphetamine-appropriate lever during stimulation trials were increased relative to tests without stimulation. Individual differences were observed in the amount of drug-lever responding elicited by the VTA stimulation during drug-free substitution tests, and the different levels of drug-lever responding correlated positively with the response rates obtained from these rats during subsequent intracranial self-stimulation tests. These finding suggest that VTA stimulation can have D-amphetamine-like stimulus properties and such stimulus properties may be related to the rewarding effects of the brain stimulation.

Self-administration in baboons and the discriminative stimulus effects in rats of bupropion, nomifensine, diclofensine and imipramine

The behavioral effects of the antidepressants nomifensine, diclofensine, bupropion, and imipramine were examined using a cocaine substitution drug self-administration procedure in baboons and a cocaine drug discrimination procedure in rats. Intravenous self-administration of the antidepressants was examined in baboons under conditions in which baseline responding was maintained by intravenous injections of cocaine HCl (0.32 mg/kg/injection). Drug was available under a fixed-ratio 80-response or 160-response schedule of intravenous injection. Each drug injection was followed by a 3-h time-out allowing a maximum of eight injections per day. The antidepressants or their vehicles were substituted for cocaine for a period of 15 days, followed by a return to the cocaine baseline. Nomifensine, diclofensine and bupropion all maintained self-administration behavior at levels above those maintained by their respective vehicles. Some doses of nomifensine, diclofensine, and bupropion maintained levels of behavior similar to those maintained under baseline cocaine conditions. High doses of imipramine maintained levels of behavior above those maintained by its vehicle, but the amount of behavior maintained under these conditions was extremely small. In a second experiment rats were trained to discriminate 32 mumol/kg cocaine (IP 10 min presession) from no drug in a two-lever food reinforced drug discrimination procedure in which responding on one lever was reinforced following ten consecutive responses when the session was preceded by cocaine administration, while responding on the other lever was similarly reinforced in the absence of cocaine pretreatment. Cocaine, nomifensine, diclofensine, and bupropion all dose-dependently occasioned cocaine-appropriate responding. Imipramine did not occasion cocaine-appropriate responding over a range of behaviorally active doses.

Temporal parameters of cathinone, amphetamine and cocaine

Rats were trained to discriminate intraperitoneally administered 0.8 mg/kg 1-cathinone from its vehicle in a two-lever operant procedure. The normal injection-to-session interval was fifteen minutes. When tested in session at 2-180 min postadministration, cathinone discrimination was seen to have a rapid onset (5 minutes) and offset (60 minutes). When the same rats were tested with either 0.8 mg/kg d-amphetamine or 10.0 mg/kg cocaine at the same postinjection time periods, the peak discriminative generalization to each of these other psychostimulants was observed to be later, i.e., an onset of action at 15-30 minutes with a slightly longer duration of action. The results indicate that cathinone exerts discriminative response control within five minutes of intraperitoneal injection and that it has a shorter duration than amphetamine and cocaine.

Discriminative stimulus properties of local anesthetics in d-amphetamine- and pentobarbital-trained pigeons

Pigeons were trained to discriminate either d-amphetamine (1.7 mg/kg, IM) or pentobarbital (10 mg/kg, IM) from saline in a two-key, food-reinforced drug discrimination paradigm. Cocaine, procaine, and lidocaine were administered before test sessions to determine if these local anesthetics shared discriminative stimulus (DS) properties with either training drug. Cocaine (0.1-3.0 mg/kg) substituted for d-amphetamine in all 4 birds from the d-amphetamine-trained group. Procaine (3.0-56 mg/kg) substituted in 3 of the 4 birds from this group, and lidocaine (3.0-30 mg/kg) did not substitute in any bird. In contrast, cocaine, procaine, and lidocaine did not substitute for pentobarbital in any bird in the pentobarbital-trained group. These results suggest that the DS properties of some local anesthetics may be similar to those of psychomotor stimulants. Further, although some local anesthetics may have sedative-like actions, apparently these are not the basis of their DS effects.

An examination of the intravenous self-administration of phenylpropanolamine using a cocaine substitution procedure in the baboon

Intravenous self-administration of phenylpropanolamine HCl (0.10 to 10.0 mg/kg/injection) was examined in baboons under conditions in which baseline responding was maintained by intravenous injections of cocaine HCl (0.32 mg/kg/injection). Drug was available under a FR 160-response schedule of intravenous injection. Each drug injection was followed by a 3-hr time-out allowing a maximum of eight injections per day. Phenylpropanolamine or phenylpropanolamine vehicle (saline) was substituted for cocaine for a period of 15 days followed by a return to the cocaine baseline. Response rates after phenylpropanolamine substitution were similar to those maintained by saline substitution, and lower than those maintained under cocaine baseline conditions. At the two highest doses of phenylpropanolamine tested (3.2 and 10.0 mg/kg/injection) concurrent food maintained behavior was suppressed.

Interactions between stimulants: effects on DRL performance and lethality in rats

The Controlled Substances Act of 1970 drastically reduced the supply of amphetamines available to the public. It also inadvertently prompted the emergence of a new drug industry, namely the marketing of caffeine/phenylethylamine combinations packaged to look like many of the previously available amphetamine preparations. The findings of one recent experimental study corroborate anecdotal evidence that the interoceptive "high" produced by these look-alike stimulants mimics that produced by amphetamine. The present study was designed to further characterize the behavioral effects of caffeine/phenylethylamine combinations. The present findings suggest that adding ephedrine and phenylpropanolamine to caffeine markedly enhances the disruption of DRL performance, as well as the lethality of the drug. In addition, different patterns of interactions were obtained between amphetamine and caffeine versus the caffeine/ephedrine/phenylpropanolamine combination.

Discriminative stimulus effects of a low dose of apomorphine in the rat

The discriminative stimulus (DS) effect of apomorphine was investigated in rats trained in a two-lever, food-reinforcement procedure. Rats were given subcutaneous injections of saline or 0.1 mg/kg apomorphine HCl, 15 min before training sessions. The training dose of apomorphine was chosen to activate dopamine autoreceptors selectively. Stimulus generalization studies demonstrated that the DS effects generalized completely to other direct-acting dopaminergic agonists such as N-n-propylnorapomorphine (NPNA), pergolide, lergotrile, and bromocriptine. The indirect-acting dopamine agonists, (+)amphetamine, cocaine, and methylphenidate produced predominantly saline-appropriate lever responses. The DS effect of apomorphine at the training dose was incompletely antagonized by haloperidol or metoclopramide. The dopaminergic antagonists tested, however, also partially generalized to apomorphine. Both enantiomers of 3-(3-hydroxyphenyl)-N-n-propylpiperidine (3-PPP) produced apomorphine-appropriate lever choice with the (-) enantiomer being slightly more potent. The discriminative property of this (0.1 mg/kg) dose of apomorphine has characteristics consistent with selective dopamine autoreceptor activation.

Cannabimimetic activity (delta 1-THC cue) of cannabidiol monomethyl ether and two stereoisomeric hexahydrocannabinols in rats and pigeons

Animals (rats and pigeons) trained to discriminate between the presence and absence of the effects of delta 1-tetrahydrocannabinol (delta 1-THC; 3 and 0.56 mg/kg, respectively) were tested for generalization with graded doses of delta 1-THC as well as with two 7-hydroxyhexahydrocannabinol epimers which differ in the stereochemistry at the C-1 position only, and a cannabidiol (CBD)-like compound, cannabidiol monomethyl ether (CBDM). delta 1-THC produced dose/time related effects in both rats and pigeons. Both 7-hydroxyhexahydrocannabinols generalized with delta 1-THC in both species. Greater cannabimimetic activity was observed when the substituent at the C-1 position was equatorial (as in compound NL-105) than when the substituent was axial (compound NL-106) (for chemical structures see Fig. 1, below). Thus in the absence of other substituents the planarity at the C-1 position determines cannabimimetic activity. CBDM induced only vehicle appropriate responding at the doses of 3 and 10 mg/kg in both species; 30% delta 1-THC appropriate responding occurred with 17.5 mg/kg (only tested in pigeons), a dose which also appeared to excert rate depressant effects. Thus, like CBD, CBDM has a low degree of cannabimimetic activity.

Structure-activity studies on methoxy-substituted phenylisopropylamines using drug discrimination methodology

Eighteen rats were trained to discriminate 1.0 mg/kg of (+)-amphetamine sulfate from saline in a two-lever operant procedure. Once responding was stable, these animals were administered various doses of sixteen different methoxy-substituted phenylisopropylamines in tests of stimulus generalization. Of three possible mono-methoxyphenylisopropylamines, all three produced amphetamine-appropriate responding, but none was as potent as racemic amphetamine. The amphetamine-stimulus did not completely generalize to any of the di- or tri-methoxyphenylisopropylamines.

Caffeine-phenylethylamine combinations mimic the amphetamine discriminative cue

Although caffeine-phenylethylamine combinations are widely available as over-the-counter medications or as "legal" stimulants, little information is available concerning their behavioral pharmacology or abuse potential. In the present study, rats were trained in a food-reward, two-lever operant drug discrimination paradigm to differentially respond after saline or 0.5 mg/kg amphetamine injections. Tests for generalization to the amphetamine cue indicated only modest amphetamine-lever responding at various doses of caffeine alone or at various doses of ephedrine/phenylpropanolamine (PPA) combinations, but complete generalization to the training cue was found with higher doses of the triple combination (caffeine, ephedrine, and PPA) or with caffeine-ephedrine or caffeine-PPA combinations. All drugs produced response rate decreases at higher doses. These data clearly indicate that certain "legal" stimulants mimic the amphetamine cue and suggest that caffeine may interact additively with phenylethylamines to produce the cue.

Structure-activity studies on amphetamine analogs using drug discrimination methodology

Animals (rats) trained to discriminate 1.0 mg/kg of S(+)-amphetamine sulfate from saline, using a standard operant training procedure, were administered doses of various amphetamine analogs in tests of stimulus generalization in order to study structure-activity relationships (SAR). The types of structural variation of the amphetamine molecule that were investigated included (a) benz-fusion of the aromatic nucleus, (b) alpha-demethylation of the alkyl side chain, (c) conversion of the benzylic methylene to a carbonyl group, and (d) conformational restriction of the side chain. Benz-fusion and alpha-demethylation appear to have a detrimental effect on activity in that none of these analogs produced amphetamine-appropriate responding. However, the carbonylated analog, i.e., cathinone, was found to be equipotent with amphetamine. Furthermore, as with amphetamine, the S-isomer of cathinone was found to be more active than its enantiomer. With respect to the conformationally-restricted analogs, the most potent compound was 2-aminotetralin which was about half as active as racemic amphetamine.

Nisoxetine and amphetamine share discriminative stimulus properties in mice

The interaction of amphetamine with noradrenergic neurons could mediate a portion of the drug's discriminative stimulus properties. To test this hypothesis, mice were trained to discriminate 1.0 or 3.2 mg/kg amphetamine, 32 mg/kg of the selective norepinephrine uptake inhibitor, nisoxetine, or 32 mg/kg nisoxetine + 1.0 mg/kg amphetamine from saline. Differential drug- or saline-appropriate responding was determined using a two photocell-beam procedure with beam interruption as the operant. Reinforcement (5-sec access to evaporated milk) was presented on a fixed-ratio 20 (FR-20) schedule. Mice trained to discriminate 1.0 mg/kg amphetamine from saline generalized to nisoxetine (32 mg/kg) alone and to doses of 0.56 mg/kg amphetamine and above but not to lower doses unless pretreated with nisoxetine (20 or 32 mg/kg). Mice trained to discriminate nisoxetine (32 mg/kg) from saline generalized to 0.56, 1.0 and 3.2 mg/kg amphetamine and generalized to all amphetamine doses when pretreated with nisoxetine (32 mg/kg). Mice trained to discriminate the drug combination from saline generalized to nisoxetine (32 mg/kg) alone, and to 3.2 mg/kg amphetamine tested alone, to 0.56 mg/kg of amphetamine or above when the lower dose of nisoxetine (20 mg/kg) was used, and to all test doses of amphetamine with nisoxetine (32 mg/kg) pretreatment. Mice trained to discriminate 3.2 mg/kg amphetamine from saline generalized to no test dose of amphetamine following either saline or nisoxetine (32 mg/kg) pretreatment. Testing with several doses of pentobarbital (1.0, 3.0, 10.0 and 18.0 mg/kg) resulted in saline-appropriate responding regardless of training group.(ABSTRACT TRUNCATED AT 250 WORDS)

Discriminative stimulus properties of d-amphetamine in pigeons

Two out of four pigeons were successfully trained in an operant procedure to discriminate between the presence and absence of the effects induced by d-amphetamine (final dose: 1.6 mg/kg). The solvent (saline) or d-amphetamine was administered intramuscularly (IM) 30 min prior to training. Tests with other drugs and dosages indicated that l-amphetamine (ED50 = 0.55 mg/kg) and cocaine (ED50 = 1.05 mg/kg) fully generalized to d-amphetamine (ED50 = 0.35 mg/kg), whereas drugs such as p-hydroxy-amphetamine (1.6 and 3.2 mg/kg), morphine (1.5, 3.0 and, 6.0 mg/kg), and delta 9-THC (0.125, 0.25, and 0.50 mg/kg) failed to do so at the doses tested. Apomorphine (0.25 and 0.50 mg/kg) and LSD-25 (0.04 and 0.08 mg/kg) produced intermediate results. Pretreatment with haloperidol (dose range: 0.04 to 1.28 mg/kg), but not propranolol (10 and 20 mg/kg), attenuated significantly the d-amphetamine (1.6 mg/kg) stimulus effects. The two pigeons emitted predominantly d-amphetamine appropriate responses when the training dose (1.6 mg/kg) of d-amphetamine was tested on different occasions 15, 60, and 120 min after the administrations. One bird emitted mostly vehicle appropriate responses when tested 240 min after the d-amphetamine injection whereas the other bird performed d-amphetamine appropriate responses. Selection of the non-drug associated key occurred in the two birds when testing was carried out 480 min (8 hrs) after the administration of d-amphetamine.

Role of training conditions in discrimination of central nervous system stimulants by rats

Amphetamine and some relate compounds were compared in rats trained to discriminate (+)-amphetamine (0.4, 1.0 or 1.6 mg/kg) or cocaine (10.0 mg/kg) from the non drug condition in a standard, two-bar procedure with food reinforcement (n=5-6). Amphetamine and cocaine were generalized completely with each other, in most cases at dose levels which did not greatly reduce the overall numbers of responses. The ED50 values for amphetamine and cocaine varied with the drug and dose used for training, and it was concluded that the stimuli produced by the two drugs were similar but may not be identical. There was an excellent correlation between ED50 values derived from indices of bar selection and percentage-responding on the drug-appropriate bar. Apomorphine was generalized with amphetamine only in the rats trained with the higher doses of amphetamine, and only when administered in doses which greatly reduced the overall number of responses. Para-hydroxyamphetamine increased responding on the drug-appropriate bar only when administered in high doses to the rats trained with the lowest dose of amphetamine (0.4 mg/kg). The results strengthen the evidence that the particular drug and dose level used for training can significantly affect the outcome of generalization tests, and challenge the notion that the discriminability of drugs is an immutable property that is amenable to absolute measurement.

Amphetamine discrimination: onset of the stimulus

Rats were trained to discriminate 1.0 mg/kg (+)-amphetamine sulfate from saline in a two-lever operant procedure. The normal injection-to-session interval was fifteen minutes. When tested with amphetamine immediately after intraperitoneal injection, rats initially responded on the lever paired with saline in training, but quickly shifted to the lever paired with amphetamine in training. When tested with saline immediately after immediately after injection, animals responded appropriately for the saline treatment throughout the extinction test. The results show that (+)-amphetamine exerts discriminative response control within five minutes of intraperitioneal injection.

Effect of neuroleptics and tricyclic antidepressants upon d-amphetamine discrimination

After rats were trained to differentiate between the effects of intraperitqoneal administration of 0.6 mg/kg d-amphetamine and saline, pretreatment with various neuroleptic drugs was observed to significantly inhibit d-amphetamine discrimination. Thus, trifluoperazine, haloperidol, fluphenazine, chlorpromazine and thioridazine, but not clozapine, decreased d-amphetamine-induced control of discriminative performance. The ED50s of the effective neuroleptics for this inhibition were similar to those reported for antagonism of amphetamine-induced stereotypic behavior in the rat and the slopes of the dose-response curves were parallel indicating a common site and mechanism of action, presumably blockade of postsynaptic dopaminergic receptors. In contrast, pretreatment with the tricyclic antidepressant agents, imipramine, nortryptiline and desipramine had no significant effect on the discrimination of a dose of d-amphetamine which produced a low degree of discriminative control. The results are viewed in relation to the "dopamine hypothesis" of schizophrenia and affective disorders and the use of this animal behavioral method for determining brain dopamine interactions is discussed.

MDA and DOM: substituted amphetamines that do not produce amphetamine-like discriminative stimuli in the rat

3,4-Methylenedioxyamphetamine (MDA) and 2,5-dimethoxy-4-methylamphetamine (DOM) are amphetamine congeners that produce both amphetamine-like and LSD-like effects. To evaluate whether MDA and DOM should be classed with amphetamine, their capacity to produce amphetamine-like discriminative stimuli was assessed. Rats were trained to discriminate between saline and 1.0 mg/kg d-amphetamine in a two choice, discrete trial shock avoidance paradigm. Neither MDA nor DOM produced any amphetamine-appropriate responding when tested over a 30-fold dose range. The specificity of the procedure to detect amphetamine-like effects was demonstrated by the failure of LSD to produce any amphetamine-appropriate responding. These results suggest that neither MDA nor DOM should be classed as amphetamine-like agents.

The discriminative stimulus properties of 2,5-dimethoxy-4-methylamphetamine (DOM): differentiation from amphetamine

Rats were trained in a two-lever operant procedure to discriminate either 1.0 mg/kg (+)amphetamine or 1.5 mg/kg DOM from saline. Rats trained to discriminate DOM from saline showed generalization with the DOM training condition when tested with mescaline or 2,5-dimethoxy-4-ethylamphetamine (DOET), but not when tested with (+)amphetamine or methylphenidate. Both isomers of DOM generalized with racemic training compound, the (-)isomer being more potent. The DOM stimulus was completely blocked by the serotonin (5-HT) antagonists cinanserin and methysergide, but not by the peripheral 5-HT antagonist xylamidine nor the dopamine antagonist haloperidol. Rats trained to discriminate (+)amphetamine from saline generalized with the amphetamine training condition when tested with methylphenidate but not when tested with mescaline, DOET, racemic DOM, or either isomer of DOM. The amphetamine stimulus was blocked by pretreatment with haloperidol but not by cinanserin, methysergide, or xylamidine. The results show that, despite their structural similarity, amphetamine and DOM induce pharmacologically distinct stimuli.

Discriminative stimulus properties of psychomotor stimulants in the cat

Cats were trained to choose between two levels or an operant chamber using interoceptive cues provided by d-amphetamine or saline as the discriminative stimuli. Following training, stimulus generalization was observed to additional doses of d-amphetamine and cocaine. but not to morphine. Clozapine blocked the generalization of the drug discrimination response to d-amphetamine, but had no effect on generalization to cocaine. These data indicate that discriminative stimulus properties of psychomotor stimulants, previously described in rats, are similar in cats.