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.     

Drug discrimination studies in rats with caffeine and phenylpropanolamine administered separately and as mixtures

The discriminative stimulus effects of mixtures of caffeine and phenylpropanolamine (PPA) have been investigated because these drugs have been abused together. Rats were trained to discriminate caffeine (20 mg/kg), PPA (20 mg/kg), or a mixture of both drugs, from saline in a two-bar operant conditioning procedure with food reinforcers presented on a tandem VI-FR schedule. Discriminations of mixture, caffeine alone and PPA alone were 90% accurate after 40 sessions. Generalisation to both PPA and caffeine was weak (25–47%) at the doses used in the training mixture, although there was almost complete generalisation to larger doses of PPA. Under these conditions, there was a possible synergistic interaction between caffeine and PPA because the discriminative effect of the mixture could not be fully explained by the combined effects of its component drugs. However, in rats trained on caffeine, PPA had no effect on the dose-response relationship for caffeine; similarly, in rats trained on PPA, caffeine had no effect on the dose-response relationship for PPA (no synergism or antagonism). Generalisation to (+)-amphetamine and cocaine was weakest in rats trained on caffeine, was partial in rats trained on the mixture, and was complete in rats trained on PPA; thus, the mixture of caffeine and PPA was not more like cocaine or amphetamine than PPA alone. The results are in agreement with reports that caffeine and PPA may interact in a complex manner, but do not support the view that the interaction enhances their resemblance to highly abused stimulants such as amphetamine and cocaine.     

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 the nicotinic agonist cytisine

Cytisine binds with high affinity and specificity to neuronal nicotinic receptors but its physiological and behavioural effects are complex and differ from those of nicotine. The present study explores the behavioural aspects further by comparing the discriminative stimulus effects of cytisine with those of nicotine. Two groups of rats were trained to discriminate cytisine (2 mg/kg SC) or nicotine (0.2 mg/kg SC) from saline in a two-lever operant conditioning procedure with food reinforcers presented on a tandem VI FR schedule. A third group of rats was trained to discriminate cytisine (3 mg/kg SC). Rats acquired these discriminations within 50 training sessions. The stimulus effects of both cytisine and nicotine appeared within 4 min of SC injection. In generalization tests, rats trained with either cytisine or nicotine showed steep dose-response curves (generalization gradients) for their respective training drug. However, rats trained with cytisine showed full dose-related, generalization to nicotine (93%), whereas rats trained with nicotine exhibited only partial generalization to cytisine (54%). Rats trained with either cytisine or nicotine exhibited similar, partial generalization (76–77%) to (+)-amphetamine. The nicotine antagonist mecamylamine blocked the discriminative stimulus effects of both cytisine and nicotine; it was confirmed that the block of nicotine (0.2 mg/kg) was complete, whereas the block of cytisine (2 and 3 mg/kg) was incomplete in two separate experiments. Overall, the results showed that cytisine, like nicotine, can serve as a robust discriminative stimulus but, in contrast to its relatively high affinity in binding experiments, cytisine was much less potent than nicotine in the behavioural studies. Although the stimulus effects of the two drugs were very similar, there were some subtle differences such as the asymmetrical cross-generalizations between them and possible small differences in susceptibility to antagonism by mecamyl-amine. These effects were interpreted either in terms of a putative partial agonist effect of cytisine, or by assuming that nicotine produces a compound stimulus. Such a stimulus would be mediated through two or more subtypes of nicotinic receptor, and cytisine would act at some, but not all, of these receptor subtypes. Received: 17 June 1996/Final version: 6 September 1996     

Interaction between chronic amphetamine and neuroleptic treatments on oral behavior in rats

The interaction of concurrently administered amphetamine (AMPH) and haloperidol (HAL) on behavior was examined. Rats were divided into four groups: drug naive controls; HAL-treated for 6 months; AMPH-treated for 1 month; and rats administered both continuous HAL for 6 months and concurrent AMPH treatment during the 2nd month of HAL administration. AMPH alone increased locomotor activity, and this effect was blocked by concurrent haloperidol administration; however, the AMPH-induced reduction of body weight was unaltered by concurrent haloperidol treatment. Oral behavior, monitored both by a human observer and a computerized system, was not significantly altered by HAL alone, or by AMPH alone, but increases in tremorous oral behavior appeared in the concurrent administration group 4 months after AMPH treatment was discontinued. These results could have implications for tardive dyskinesia.     

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

Ninety-six male Sprague-Dawley rats were trained in one of seven drug versus saline (SAL) discrimination (DD) tasks under a variable-ratio 5–15 schedule of food-motivated lever press responding. Three groups of rats (n=12/group) were trained to discriminate between one of the legal over-the-counter (OTC) stimulants — caffeine (CAF), ephedrine (EPHED), phenylpropanolamine (PPA), and SAL. Three other groups (n=2/group) were trained to discriminate between one of three binary stimulant combinations — CAF + EPHED, CAF + PPA, EPHED + PPA, and SAL. The seventh group of rats (n=24) was trained to discriminate between SAL and a ternary combination of the OCT stimulants, CAF + EPHED + PPA. Generalization tests were conducted with each of the OTC stimulants and the controlled stimulants — amphetamine (AMPHET) and cocaine (COC). The data suggest: 1) there is cross-generalization between some OTC combinations and controlled stimulants; 2) full generalization between the OTC and controlled stimulants were demonstrated in rats trained to discriminate two of the binary stimulant combinations from SAL; 3) drug mixtures are not perceived as new entities distinct from their component elements; 4) training dose-ratio may influence the characteristics of mixture discriminations; 5) stimulus overshadowing may be a factor determining drug mixture cues, and 6) the DD properties of aggregate drug compounds may function within a euclidean metric space. We propose that some binary OTC stimulant combinations may effectively function as a methadone-like replacement therapy in cocaine dependence.     

Evidence for perceptual masking of the discriminative morphine stimulus

Morphine-amphetamine and morphine-naltrexone interactions were examined in three groups of White Carneaux pigeons (n=3), which were trained in a twochoice drug discrimination procedure under a FR-30 schedule of food reinforcement using 3.2 mg/kg morphine and saline as discriminative stimuli. Once stimulus control was acquired by these initial training stimuli, the training doses of morphine were gradually changed to 1.0 mg/kg for group A and to 10 mg/kg for group C. The three groups differed in the minimum dose required for stimulus control and the drugs to which the training stimulus generalized. Stimulus generalization to amphetamine was inversely related to training dose. Amphetamine potentiated the discriminative stimulus properties of morphine. Naltrexone blocked the discriminative stimulus properties of morphine to varying degrees, which appeared to be limited by the training dose and the rate-suppressing effects of naltrexone when administered alone. Challenging the morphine stimulus with amphetamine resulted in a qualitatively similar blockade. This blockade was a direct function of the morphine training dose. It is argued that MS-AMP interactions result in perceptual masking of the MS stimulus, which can be differentiated from pharmacological antagonism by NTX. Two other challenge drugs, ketamine, and sodium pentobarbital, did not alter stimulus control by morphine.     

Amphetamine-haloperidol discrimination: effects of chronic drug treatment

Rats responding for food reinforcement were trained in a 2-lever drug discrimination task. Groups of rats were trained to discriminate one of four doses of amphetamine (0.0, 0.1, 0.3, or 0.5 mg/kg) from haloperidol (0.02 mg/kg). Both the rate of acquisition and level of discrimination at asymptote were a function of amphetamine training dose. Following acquisition of this discrimination, choice behavior was assessed in the absence of drug during two test sessions. Twenty-four hours following the second drug-free test session, chronic drug treatment commenced. Half of the animals received 10 mg/kg amphetamine for 10 consecutive days while the other half received 1 mg/kg haloperidol during the same period. Choice behavior was assessed during three 2.5-minute unreinforced drug-free test sessions 24, 48, and 72 hours following the chronic drug regimen. Following chronic haloperidol, animals responded as though a small dose of amphetamine had been administered, while following chronic amphetamine, they responded as though a small dose of haloperidol had been administered. Collectively, these results suggest that animals trained to discriminate amphetamine from haloperidol respond on the basis of a continuum of dopaminergic function. Further, this continuum can be used to elucidate the net effect of pharmacologically-induced alterations in dopaminergic function, as well as the effect of nonpharmacological manipulations that may result in dopaminergic changes.     

Substitution and cross-tolerance profiles of anorectic drugs in rats trained to detect the discriminative stimulus properties of cocaine

Rats were trained to discriminate cocaine, 10.0 mg/kg, using a two-lever operant procedure. Dose-effect data were determined for the substitution of cocaine, diethylpropion, methylphenidate, phenmetrazine, phentermine, and fenfluramine for the cocaine stimulus. All of these drugs, except fenfluramine, substituted fully for the cocaine stimulus. Subsequently, training was halted and cocaine, 20 mg/kg/8 h, was administered for 9 days, and dose-effect data were redetermined for all of these drugs on days 7–9 of chronic administration. Chronic administration of cocaine produced tolerance to the stimulus properties of cocaine, and cross-tolerance to the stimulus properties of methylphenidate, phenmetrazine, and phentermine, such that doses approximately two-fold higher than those used acutely were necessary to reproduce the original effect; the potency for the substitution of diethylpropion for the cocaine stimulus was decreased greater than four-fold; and fenfluramine still failed to substitute for the cocaine stimulus. These data suggest that 1) a common mechanism may mediate tolerance to the discriminative stimulus properties of cocaine, methylphenidate, phenmetrazine, and phentermine, and 2) tolerance in the drug discrimination procedure may have potential for establishing a comprehensive evaluation of dependence liability of CNS stimulants.     

Serotonin receptor mechanisms mediate the discriminative stimulus properties of the atypical antipsychotic clozapine in C57BL/6 mice

Rationale The atypical antipsychotic drug (APD) clozapine (CLZ) has been shown to have a robust discriminative cue in rats, pigeons, and monkeys in two-choice drug discrimination procedures.Objectives The present study determined whether a two-choice drug discrimination procedure with CLZ could be established in C57BL/6 mice and whether this procedure could distinguish between atypical and typical APDs.Methods C57BL/6 male mice were trained to discriminate 2.5 mg/kg CLZ from vehicle in a two-lever drug discrimination procedure.Results Generalization testing with CLZ produced full substitution at the 2.5- and 5.0-mg/kg doses with an ED₅₀ of 1.14 mg/kg. The atypical APDs olanzapine (ED₅₀=0.24 mg/kg), risperidone (ED₅₀=0.072 mg/kg), and ziprasidone (ED₅₀=0.33 mg/kg) fully substituted for CLZs discriminative cue, while the typical APD haloperidol failed to substitute for CLZ. Generalization testing with selective ligands showed that the serotonin (5-HT)_2A/2B/2C antagonist ritanserin fully substituted for CLZ (ED₅₀=2.08 mg/kg) and that the 5-HT receptor agonist quipazine significantly attenuated CLZs discriminative cue without disrupting response rates. The muscarinic receptor antagonist scopolamine, the dopamine agonist amphetamine, and the 5-HT agonist quipazine failed to substitute for CLZ.Conclusions These results demonstrated that antagonism of 5-HT receptors plays an important role in mediating the discriminative stimulus properties of the atypical APD CLZ in C57BL/6 mice. The atypical APDs olanzapine, risperidone, and ziprasidone fully substituted for CLZ, while the typical APD haloperidol did not. These results suggest that CLZ drug discrimination in C57BL/6 mice may be an effective preclinical behavioral assay for screening atypical from typical antipsychotic drugs.     

Lack of generalization of nisoxetine with amphetamine in the rat

Rats were trained to discriminate between the stimulus properties of intraperitoneally administered d-amphetamine (0.8 mg/kg) and its vehicle in a two-lever, food-motivated operant task. Once trained, doses of the norepinephrine reuptake inhibiting agent nisoxetine, ranging from 10 to 20 mg/kg, were administered to investigate if the amphetamine-trained rats would generalize to this agent. This did not, however, occur. Thus, it would seem that noradrenergic mechanisms have a negligible role in the production of the amphetamine-induced discriminative stimulus cue in the rat. Previous evidence that indicated a noradrenergic mediation of amphetamine discrimination in the mouse contrasted with the present results in rats and this discrepancy should warrant caution in comparing results of discriminative studies in these two species.     

Comparative characterisation of the discriminative stimulus properties of clozapine and other antipsychotics in rats

 The discriminative stimulus properties of the prototypical atypical neuroleptic clozapine (5 mg/kg, IP) were characterised in rats using a fixed ratio assay. Clozapine induced full dose-related generalization in the absence of response suppression. Amphetamine and pentylenetetrazol failed to generalise at doses known to be discriminable, showing a degree of specificity for the clozapine cue. The typical neuroleptics haloperidol and loxapine induced minimal (20%) generalization at doses with marked behavioural effects; thus clozapine discrimination dissociates clozapine from typical neuroleptics. Atypical neuroleptics which are not clozapine congeners produced weak partial generalization when tested up to the highest doses that could be studied. The maximal levels of generalization induced by these agents were: amisulpiride 28%, risperidone 40% and sertindole 50%. Clozapine congeners typically caused more generalization, the novel pyridobenzoxapine JL13 inducing 70% maximal generalization. Most generalization (83%) was seen with the clozapine congener seroquel, although in contrast to clozapine, it only generalised at doses with marked effects on responding, so that no drug mimicked clozapine fully. Surprisingly, the clozapine congener olanzapine only induced a maximal level of 38% generalization. This apparently anomalous finding is attributed to an inability to test high doses of the drug due to its rate-suppressant actions. The clozapine cue can be used to rank atypical neuroleptics in terms of their similarity to clozapine in vivo. The clozapine cue is probably a compound cue, since only agents showing “polyvalent” receptor pharmacology induced substantial generalization. Received: 19 May 1997 / Final version:25 July 1997     

In a drug discrimination procedure isolation-reared rats generalize to lower doses of cocaine and amphetamine than rats reared in an enriched environment

Rats with different behavioral histories, defined by rearing and housing in either an enriched condition (EC) or an isolation condition (IC), were trained in a two-lever operant procedure to discriminate 5.0 mg/kg cocaine from saline. In cocaine dose-generalization tests, the IC rats exhibited an ED₅₀ (1.01 mg/kg) significantly lower than the EC rats (ED₅₀:1.55 mg/kg). The cocaine-appropriate responding was emitted when the rats were treated withd-amphetamine, and for thed-amphetamine test doses the ED₅₀ (0.19 mg/kg) was again significantly lower for the IC rats compared to the ECs (ED₅₀:0.33 mg/kg). These data suggest that IC rats are more sensitive to the stimulus properties of indirect dopaminergic agonists than EC rats and highlight the importance of environmental variables in governing an organism's response to the stimulus properties of abused drugs.Supported by DA 05310.     

Tolerance,withdrawal, and supersensitivity to dopamine mediated cues in a drug-drug discrimination

Rats were trained to discriminate between 0.25 mg/kg amphetamine (AMPH) and 0.03 mg/kg haloperidol (HAL) in a two-lever drug discrimination task. In order to test for a drug-induced withdrawal state, animals were assigned to one of three chronic treatment groups and given injections of AMPH, HAL, or distilled water (DW) for 10 consecutive days. Subjects from each treatment condition were then tested at 24, 48, or 72 h after the final injection. At the 24 h retest interval, subjects injected with AMPH responded as though administered an acute dose of HAL (0.028 mg/kg) and subjects injected with chronic HAL responded as though administered an acute dose of AMPH (0.15 mg/kg). By 72 h choice behavior had returned to pretreatment values. To determine whether the rebound observed after 10 days of drug treatment was present after a single injection, independent groups of subjects were injected with single doses of either 10 mg/kg AMPH or 1.0 mg/kg HAL and then retested from 4 h to 48 h later. Single doses of both AMPH and HAL produced significant rebounds that peaked between 20 h (AMPH) and 24 h (HAL) following administration. In a third experiment, animals were tested with or without acute doses of drug following pretreatment with either HAL or AMPH. Receptor supersensitivity accounts for the tolerance observed to HAL 24 h after treatment with 1.0 mg/kg HAL, whereas receptor subsensitivity accounts for the tolerance observed 20 h after treatment with 10 mg/kg AMPH.Some of the data presented here was presented at the meeting of the Society for Neurosciences, New Orleans, 1991.     

Discriminative stimulus properties of the optical isomers of nicotine

Rats were trained to discriminate 200 or 400 g/kg (-)nicotine from saline in a two-bar operant paradigm. Dose-response relationships for optically pure (-)- and (+)nicotine as well as antagonistic effects were examined in both groups of rats. The natural isomer (-)nicotine was approximately nine-times more potent than (+)nicotine. Mecamylamine produced equal blockade of the (-)- and (+)nicotine cues. Hexamethonium and atropine were without effect. These data demonstrate the possible stereospecificity of the central nicotinic receptor that mediates the stimulus effect produced by nicotine.     

Ascorbate antagonizes the behavioral effects of amphetamine by a central mechanism

The behavioral response to amphetamine was monitored in rats that received simultaneous intraventricular infusions of saline or ascorbate. Both groups of animals displayed comparable responses, although ascorbate significantly delayed the onset of amphetamine-induced locomotion and rearing. In rats pretreated with a threshold dose of haloperidol (0.025 mg/kg), virtually all aspects of the amphetamine response were attenuated, and this effect was enhanced by ascorbate. In haloperidol-pretreated rats, ascorbate significantly lowered sniffing and forepaw shuffling throughout the amphetamine response. These results suggest that ascorbate antagonizes dopaminergic transmission by a central mechanism. Offprint requests to: G.V. Rebec     

The role of fentanyl training dose and of the alternative stimulus condition in drug generalization

Different groups of rats were trained to discriminate fentanyl (F) (0.03, 0.02, or 0.01 mg/kg) from saline or to discriminate 0.03 mg/kg fentanyl (F) from alternative stimulus conditions (saline, 0.15 mg/kg nicotine, or 0.01 mg/kg F). When percentage of responses on the drug lever and percentage of time spent responding on the drug lever were used as dependent variables, it was found that training dose and alternative stimulus condition both affected the ED₅₀ and the slope of the F generalization gradient. ED₅₀ and slope values based on group data were not significantly different from values based on individual data. Differences between the results of the first and second 2.5-min period of the extinction test were not significant. ED₅₀ and slope values were unaffected by the preceding training session, except in the group trained to discriminate 0.03 from 0.01 mg/kg F. A lever selection measure showed a significant effect of alternative stimulus condition on ED₅₀ values only.Training dose and alternative stimulus condition also affected the generalization to morphine. Under none of the conditions explored in this study did generalization occur to amphetamine or nicotine. The results are discussed in terms of the relative nature of drug generalization.     

Apomorphine as a discriminative stimulus, and its antagonism by haloperidol.

At a non-stereotypogenic dose (0.16 mg/kg, s.c.), apomorphine was shown to produce a discriminative stimulus in rats. The apomorphine-produced stimulus was found to be effectively antagonized by haloperidol. It is suggested that apomorphine's ability to stimulate central dopamine receptors is directly related to its cueing action.     

An analysis of visual object reversal learning in the marmoset after amphetamine and haloperidol

The effect of amphetamine and haloperidol pretreatment on visual object reversal learning was assessed in the marmoset. Amphetamine induced perseverative responding demonstrated by high reversal learning scores and worse than chance performance in the early stages of reversal. This perseverative responding was prevented by pretreatment with haloperidol. Haloperidol, either alone or in conjunction with amphetamine caused a mild, non-perseverative impairment on reversal learning only.     

Discriminative stimulus effects of nicotine in rats trained under different schedules of reinforcement

There have been few comparisons between different schedules of reinforcement for establishing drugs as discriminative stimuli. Fixed-ratio (FR) 10 and tandem variable-interval 1-min FR-10 schedules have been compared directly in a conventional, nicotine-saline discrimination paradigm with food reinforcement in rats. The discrimination was acquired rapidly under both schedules, with stimulus control by nicotine (0.1 mg/kg SC) being very slightly superior under the FR schedule. In 5-min extinction tests with nicotine, rats maintained under the FR schedule yielded a clear dose-response curve with a bar-selection (quantal) index; in these rats, discrimination of nicotine appeared generally poor, and dose-response curves were shallow, when the percentage of drug-appropriate responding (quantitative index) was calculated. In contrast, rats under the tandem schedule yielded clear dose-response data with both indices. In tests with (+)-amphetamine full generalization was obtained with both schedules, and with both quantitative and quantal indices. Tests of generalization to morphine were negative regardless of the training schedule or index employed. In rats under the FR-10 schedule, overall response rates declined both within and across extinction tests; the relatively high rates of responding maintained by the tandem schedule were more sensitive to the response rate-decreasing effects of morphine and amphetamine. The results confirm that orderly data may be obtained with either a FR or a tandem schedule provided that an appropriate index of discriminative response is employed. The results generally support the validity of current practices, and there will probably be no marked differences between conclusions depending on which schedule is used.