Role of dopamine in d-amphetamine-induced discriminative responding

The co-use of nicotine and prescription psychostimulants: A review of their behavioral and neuropharmacological interactions

BACKGROUND

Nicotine is commonly co-used with other psychostimulants. These high co-use rates have prompted much research on interactions between nicotine and psychostimulant drugs. These studies range from examination of illicitly used psychostimulants such as cocaine and methamphetamine to prescription psychostimulants used to treat attention deficit hyperactivity disorder (ADHD) such as methylphenidate (Ritalin™) and d-amphetamine (active ingredient of Adderall™). However, previous reviews largely focus on nicotine interactions with illicitly used psychostimulants with sparse mention of prescription psychostimulants. The currently available epidemiological and laboratory research, however, suggests high co-use between nicotine and prescription psychostimulants, and that these drugs interact to modulate use liability of either drug. The present review synthesizes epidemiological and experimental human and pre-clinical research assessing the behavioral and neuropharmacological interactions between nicotine and prescription psychostimulants that may contribute to high nicotine-prescription psychostimulant co-use.

METHODS

We searched databases for literature investigating acute and chronic nicotine and prescription psychostimulant interactions. Inclusion criteria were that participants/subjects had to experience nicotine and a prescription psychostimulant compound at least once in the study, in addition to assessment of their interaction.

RESULTS AND CONCLUSIONS

Nicotine clearly interacts with d-amphetamine and methylphenidate in a variety of behavioral tasks and neurochemical assays assessing co-use liability across preclinical, clinical, and epidemiological research. The currently available research suggests research gaps examining these interactions in women/female rodents, in consideration of ADHD symptoms, and how prescription psychostimulant exposure influences later nicotine-related outcomes. Nicotine has been less widely studied with alternative ADHD pharmacotherapy bupropion, but we also discuss this research.

Differential substitution for the discriminative stimulus effects of 3,4-methylenedioxymethamphetamine and methylphenidate in rats

Previous studies have demonstrated that methylphenidate, MDMA (3,4-methylenedioxymethamphetamine), and other psychostimulants exert stimulant-like subjective effects in humans. Furthermore, MDMA and methylphenidate substitute for the discriminative stimulus effects of psychostimulants, such as amphetamine and cocaine, in animals, which suggests that MDMA and methylphenidate may produce similar discriminative stimulus effects in rats. However, there is no evidence regarding the similarities between the discriminative stimulus effects of MDMA and methylphenidate. To explore this issue, cross-substitution, substitution, and combination tests were conducted in rats that had been trained to discriminate between MDMA (2.5 mg/kg) or methylphenidate (5.0 mg/kg) and saline. In the cross-substitution tests, MDMA and methylphenidate did not cross-substitute for each other. In the substitution test, methamphetamine substituted for the discriminative stimulus effects of methylphenidate, but not for those of MDMA. Furthermore, ephedrine and bupropion, which activate dopaminergic and noradrenergic systems, substituted for the discriminative stimulus effects of methylphenidate. On the other hand, serotonin (5-HT) receptor agonists 5-HT1A and 5-HT2 fully substituted for the discriminative stimulus effects of MDMA. These results suggest that activation of the noradrenergic and dopaminergic systems is important for the discriminative stimulus effects of methylphenidate, whereas activation of the serotonergic system is crucial for the discriminative stimulus effects of MDMA. Even though MDMA, like psychostimulants, exerts stimulant-like effects, our findings clearly indicate that the discriminative stimulus effects of MDMA are distinctly different from those of other psychostimulants in rats.

The role of dopaminergic transmission through D1-like and D2-like receptors in amphetamine-induced rat ultrasonic vocalizations

RATIONALE

Systemic amphetamine (AMPH) administration increases the rate of 50-kHz ultrasonic vocalizations (USVs) in adult rats and preferentially enhances the 'trill' subtype; these effects of AMPH critically depend on noradrenergic transmission, but the possible contributions of dopamine are unclear.

OBJECTIVE

To assess the role of dopamine in 50-kHz USVs emitted drug-free and following systemic AMPH administration.

METHODS

Adult male Long-Evans rats pre-selected for high AMPH-induced calling rates were tested with AMPH (1 mg/kg, intraperitoneal (IP)) and saline following pretreatment with the following dopamine receptor antagonists: SCH 23390 (0.005-0.02 mg/kg, subcutaneous (SC)), SCH 39166 (0.03-0.3 mg/kg, SC), haloperidol (0.1, 0.2 mg/kg, IP), sulpiride (20-80 mg/kg, SC), raclopride (0.1-0.5 mg/kg, SC), clozapine (4 mg/kg, SC), risperidone (0.5 mg/kg, SC), and pimozide (1 mg/kg, IP). The dopamine and noradrenaline reuptake inhibitors (GBR 12909 and nisoxetine, respectively) were also tested, alone and in combination.

RESULTS

SCH 23390, SCH 39166, haloperidol, and raclopride dose-dependently inhibited vocalizations under AMPH and suppressed the proportion of trill calls. Sulpiride, however, had no discernable effect on call rate or profile, even at a high dose that reduced locomotor activity. Single doses of clozapine, risperidone, and pimozide all markedly decreased calling under saline and AMPH. Finally, GBR 12909 and nisoxetine failed to promote 50-kHz USVs detectably or alter the subtype profile, when tested alone or in combination.

CONCLUSIONS

The rate of 50-kHz USVs and the call subtype profile following systemic AMPH administration depends on dopaminergic neurotransmission through D1-like and D2-like receptors. However, inhibiting dopamine and/or noradrenaline reuptake appears insufficient to induce calling.

Caffeine induces differential cross tolerance to the amphetamine-like discriminative stimulus effects of dopaminergic agonists

The purpose of this study was to determine if caffeine induces cross tolerance to the amphetamine-like discriminative stimulus effects of dopaminergic drugs that act through distinct mechanisms (e.g., release, uptake inhibition, direct activation of dopamine D(1)- or D(2)-family receptors). Rats were trained to discriminate 1.0 mg/kg d-amphetamine from saline in a two-choice discrete-trial procedure. Stimulus-generalization curves were generated by cumulative dosing for d-amphetamine (0.1-1.0 mg/kg), methylphenidate (0.3-5.6 mg/kg), SKF 81297 (0.3-3.0 mg/kg), and R-(-)-propylnorapomorphine (NPA; 0.001-1.78 mg/kg), as well as for caffeine (3.0-56 mg/kg); curves were re-determined after twice daily injections of caffeine (30 mg/kg) for 3.5 days. The rats generalized dose dependently to the four dopaminergic drugs, but only to a limited extent to caffeine. Twice daily injections of caffeine induced significant cross tolerance (i.e., increased ED(50)) to the amphetamine-like discriminative effects of methylphenidate and SKF 81297, attenuated non-significantly the effects of NPA, and did not alter the effects of amphetamine. Thus, caffeine produces differential cross tolerance to the amphetamine-like discriminative effects of dopaminergic drugs, a phenomenon in which the dopamine D(1) receptor appears to have an important role.

Retention of sequential drug discriminations under fixed-interval schedules for long time periods without training

The experiments showed that sequential drug discriminations can be learned and retained under a fixed-interval (FI) schedule for more than 18 months without additional training under a complex three-choice procedure. Pigeons were trained to discriminate among 5 mg/kg pentobarbital, 2 mg/kg D-amphetamine, and saline. After responding stabilized, dose-response curves were determined for other drugs. Subsequently, pentobarbital was replaced with 5 mg/kg morphine as a training drug, and D-amphetamine was replaced with 30 mg/kg caffeine. After the pigeons learned these new discriminations, dose-response curves were redetermined. Initially, chlordiazepoxide substituted for pentobarbital, cocaine substituted for D-amphetamine, and nicotine partially substituted for D-amphetamine. Morphine, Delta9-tetrahydrocannabinol, and caffeine did not substitute for either drug. After retraining with morphine and caffeine, responding occurred on the pentobarbital/morphine key after pentobarbital, chlordiazepoxide and morphine and on the D-amphetamine/caffeine key after D-amphetamine, cocaine and caffeine. After nicotine and Delta9-tetrahyrdocannabinol, responding occurred on the saline key. These data show that drug discriminations learned under fixed-interval schedules are retained for long time periods, even when discrimination training with other drugs occurs during the retention period.

Serotonergic-dopaminergic mediation of MDMA's discriminative stimulus effects in a three-choice discrimination

(+/-)3,4-Methylenedioxymethamphetamine (MDMA; "Ecstasy") is a common drug of abuse that is often described as both a psychostimulant and a hallucinogen. Two-choice drug discriminations (i.e. drug vs. nondrug) in nonhumans comparing the discriminative stimulus properties of MDMA to psychostimulants or hallucinogens have produced somewhat inconsistent findings. The relative contribution of serotonergic versus dopaminergic actions to MDMA's discriminative stimulus effects may depend on the training stimulus conditions employed. We have previously demonstrated that rats can learn to discriminate the effects of MDMA and D-amphetamine in a three-choice drug discrimination procedure, and that LSD produced nearly complete substitution for MDMA under these conditions, and fenfluramine fully substituted for MDMA. In the present study, 12 rats were trained to discriminate LSD (0.08 mg/kg) and MDMA (1.5 mg/kg) from saline in a three-choice drug discrimination procedure under a fixed-ratio (FR) 10 schedule of food reinforcement. D-Amphetamine produced only partial substitution for MDMA while fenfluramine produced complete stimulus generalization. Low doses of D-amphetamine and fenfluramine produced greater stimulus generalization when administered in combination than when given alone. The serontonin(2) antagonist MDL-100,907 only partially blocked the MDMA cue, but completely antagonized LSD discrimination. The dopamine antagonist haloperidol also failed to block MDMA discrimination. These results indicate that 5-HT release is a salient feature to MDMA's discriminative stimulus effects but that MDMA produces a compound discriminative stimulus.

Attenuation of the amphetamine discriminative cue in rats with the atypical antipsychotic olanzapine

Sixteen male Sprague-Dawley rats were trained to discriminate between saline and amphetamine injections (1.0 mg/kg ip) using a standard two-lever (FR10) drug discrimination paradigm. A baseline dose-effect curve was generated for amphetamine administration alone, using doses both above and below the training dose (0.0-2.2 mg/kg ip). Once completed, a single dose of olanzapine (OLZ; 1.5 mg/kg sc) was tested for its ability to attenuate the amphetamine cue. OLZ pretreatment (60 min) successfully interfered with an animal's ability to discriminate amphetamine injections across various doses. The percentage of correct responding on the amphetamine lever and rate of responding were both significantly decreased across some but not all of the amphetamine doses. Therefore, we believe that this preliminary investigation has successfully shown that an OLZ dose of 1.5 mg/kg sc at 60 min can interfere with an animal's ability to detect some subjective cue(s) associated with amphetamine administration.

Conditioned locomotor activity but not conditioned place preference following intra-accumbens infusions of cocaine

In the first experiment, the conditioned place preference (CPP) paradigm was used to examine the rewarding properties of bilateral microinfusions of cocaine HCl into the nucleus accumbens (0, 12.5, 25, 50, or 100 micrograms). No dose of intra-accumbens cocaine induced a significant CPP. However, bilateral intra-accumbens infusions of d-amphetamine sulfate (10 micrograms) or intraperitoneal administration of cocaine HCl (5 or 10 mg/kg) both produced a significant preference for the drug-paired compartment. In the second experiment, the ability of bilateral intra-accumbens infusions of cocaine HCl (50 micrograms) to elicit conditioned locomotor activity (CLA) was examined. During the conditioning trials, intra-accumbens cocaine significantly increased locomotor activity. On the test day, when no drug was administered, the group that had previously received cocaine in the activity chamber showed significantly greater locomotor activity than the vehicle control group. This demonstration of CLA indicates that rats are able to associate the effects of intra-accumbens infusions of cocaine with environmental stimuli; however, these infusions are not rewarding as measured by the CPP paradigm. In addition, these results may indicate important differences between the neural substrates for cocaine and amphetamine reward and reveal a dissociation between CPP and CLA.

Psychiatric side effects of interferon therapy: prevalence, proposed mechanisms, and future directions

The increasing use of interferon (IFN) in treating a variety of disorders including, malignant melanoma and hepatitis C, has resulted in the identification and increasing concern about the psychiatric side effects that can result from treatment. These effects can occur either shortly after beginning IFN therapy or later as a result of continued treatment. Studies have reported the incidence of later side effects, which include symptoms of depression, anxiety, and occasional suicidal ideation, to be from 0% to 70%. Case studies have demonstrated that pharmacologic interventions are beneficial in reducing iatrogenic psychiatric symptoms while allowing patients to maintain IFN therapy. The present article provides an overview of the psychiatric effects of IFN therapy, the proposed mechanisms of these side effects, and case studies that provide mechanistic support. In addition, limitations of the current literature are provided with suggestions for treating physicians and a discussion of possible future research directions.

The role of the nitric oxide (NO) pathway in the discriminative stimuli of amphetamine and cocaine

To examine the role of the nitric oxide (NO) pathway in the stimulus effects induced by some psychostimulants, separate groups of rats were trained to discriminate between amphetamine (AMPH; 0.5 mg/kg) and saline, or cocaine (COC; 5 mg/kg) and saline using a standard two-lever operant procedure. Substitution studies showed that AMPH and COC generalized for the training drugs in a dose-dependent manner, their ED50, values being 0.1 mg/kg and 1.2 mg/kg, respectively. The dose-response function of both those psychostimulants did not change in the course of the experiment. Moreover, AMPH and COC induced cross-substitution effects towards each other. Successive combination tests demonstrated that injection of a fixed dose of the NO synthase (NOS) inhibitor 7-nitro indazole (7-NI; 25 mg/kg) plus different doses of AMPH or COC resulted in a leftward shift in the dose-response curves of those psychostimulants and a decrease in their ED50 values. On the other hand, pretreatment with the NO donor molsidomine (MOL), injected in a fixed dose of 100 mg/kg before AMPH and COC, shifted the dose-response curves of the psychostimulants to the right and increased their ED50 values. Our results indicate that NO plays an inhibitory role in the dopamine (DA)-evoked discrimination effects of AMPH and COC in rats.

(-)-Nornicotine partially substitutes for (+)-amphetamine in a drug discrimination paradigm in rats

Rats were trained in a two-lever food-reinforced operant task to discriminate (+)-amphetamine (1 mg/kg) from saline. After discrimination training stabilized, test doses of (+)-amphetamine (0.0625-2.0 mg/kg), (-)-nicotine (0.1-1.0 mg/kg), or (-)-nornicotine (1-10 mg/kg) were assessed for their ability to substitute for the (+)-amphetamine training dose during brief test sessions in which food reinforcement was withheld. As expected, as the test dose of (+)-amphetamine increased, there was a dose-related increase in drug-appropriate responding, with both 1 and 2 mg/kg test doses substituting fully for the (+)-amphetamine training dose. Both (-)-nicotine and (-)-nornicotine showed partial substitution (approximately 50% drug-appropriate responding) for the (+)-amphetamine training dose, with (-)-nicotine being more potent than (-)-nornicotine. Rate suppressant effects prevented the assessment of higher doses of (-)-nicotine or (-)-nornicotine. Thus, while (-)-nicotine and (-)-nornicotine share similar discriminative stimulus properties, the mechanism that mediates this effect appears to differ, at least in part, from that activated by (+)-amphetamine.

5-HT2A/2C receptor agonists potentiate the discriminative cue of (+)-amphetamine in the rat

The possible effect of 5-HT2A/2C receptor agonists on an amphetamine-induced behavioral response was examined using the two-lever drug discrimination paradigm. The experiments were designed to investigate an interaction of the hallucinogenic 5-HT2A/2C agonists lysergic acid diethylamide (LSD) and 2,5-dimethoxy-4-iodoamphetamine (DOI), with the discriminative stimulus elicited by a relatively low dose of (+)-amphetamine (1.35 micromol/kg, 0.25 mg/kg, which produced approximately 50% selection of the drug lever). DOI and LSD did not produce amphetamine-like responding at any dose tested or time of administration. However, LSD alone was able to induce a drug-appropriate response in two of nine amphetamine-trained rats. Simultaneous administration of DOI or LSD with amphetamine was not significantly different from the response produced by amphetamine alone. Pre-administration of DOI (3 hr) or of LSD (2 hr) before amphetamine, however, evoked significant enhancement of the amphetamine cue. The results suggest that the enhanced behavioral response to amphetamine may be due either to an increased sensitivity of dopaminergic neurons in the mesolimbic area, or to an enhanced release of dopamine by amphetamine.

Female rats that rapidly acquire a d-amphetamine discrimination generalize more to d-amphetamine

Female Long-Evans rats were trained to discriminate d-amphetamine (0.8 mg/kg) vs. saline in a food-reinforced two-lever operant task. Fifteen rats (fast group) acquired the discrimination rapidly, achieving criterion (eight correct choices within ten sessions) during the first 10 sessions (mean sessions to criterion = 10.0). The remaining eight rats (slow group) made at least three errors during the first 10 sessions and required additional drug discrimination training to achieve criterion (mean sessions to criterion = 15.9). When a rat had completed a minimum of 30 two-lever discrimination training sessions and, in addition, provided 10 correct choices within 10 sessions, generalization testing with lower doses of d-amphetamine was initiated. The fast group made more d-amphetamine-appropriate choices during the generalization test and generalized more to the 0.2 mg/kg d-amphetamine test dose than did the slow group, though the number of training sessions prior to generalization testing was comparable across groups. Results suggest that when the training drug is easily discriminated, fast learners generalize more, even when groups receive comparable amounts of training prior to generalization testing, and this effect is observed in female rats.

Subjective responses to d-amphetamine alone and after pimozide pretreatment in normal, healthy volunteers

The role of dopamine in d-amphetamine-induced euphoria has not been systematically examined in normal volunteers. Therefore, we examined the effects of the dopamine antagonist, pimozide, on responses to d-amphetamine in healthy volunteers, using a within-subjects, double-blind design. Ten subjects received single oral doses of d-amphetamine (0, 10, 20 mg) 2 hours following pretreatment with pimozide (0, 1, 2 mg). Subjective, behavioral, and physiological effects were assessed predrug and for 3 hours after d-amphetamine administration. d-Amphetamine alone produced prototypic effects on a variety of measures, including euphoria and drug liking. Pimozide did not produce any effects when administered alone and produced inconsistent effects on responses to d-amphetamine. Although higher doses of pimozide may be needed to antagonize the euphorigenic effects of d-amphetamine, these results raise the possibility that the role of dopamine in the subjective effects of stimulants may be more complex than initially appreciated.

Psychostimulant-like effects of p-fluoroamphetamine in the rat

The present study was undertaken to compare the pharmacological properties of p-fluoroamphetamine with those of amphetamine and of other halogenated amphetamines, using several in vivo and in vitro tests. These included substitution testing in (+)-amphetamine (1 mg/kg, 5.4 mu mol/kg, i.p.)-, (+)-N-methyl-1-(1,3-benzodioxol-5-yl)-2-butanamine [(+)-MBDB] (1.75 mg/kg, 7.8 mu mol/kg, i.p.)-, and 5-methoxy-6-methyl-2-aminoindan (MMAI) (1.71 mg/kg, 8 mu mol/kg, i.p.)-trained rats, [3H]5-HT and [3H]dopamine uptake inhibition in whole brain synaptosomes, and changes in striatal extracellular levels of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) as measured by in vivo microdialysis in freely moving rats. In drug discrimination substitution tests, p-fluoroamphetamine fully mimicked (+)-amphetamine (ED50 0.43 mg/kg, 2.11 mu mol/kg), whereas 'no substitution' was observed in rats trained to discriminate the serotonin (5-hydroxytryptamine, 5-HT)-releasing agents (+)-MBDB or MMAI from saline. p-Chloroamphetamine did not substitute for amphetamine but fully substituted for the (+)-MBDB and MMAI cues (ED50 0.17 mg/kg, 0.82 mu mol/kg, and 0.14 mg/kg, 0.69 mu mol/kg, respectively). p-Fluoroamphetamine, in comparison with p-chloroamphetamine and p-iodoamphetamine, showed much stronger inhibition of [3H]dopamine than [3H]5-HT uptake into rat brain synaptosomes but was less selective than amphetamine. p-Fluoroamphetamine (7.0 mg/kg, i.p.), 1 h after administration, strongly elevated (849% of baseline) extracellular dopamine in rat striatum measured using in vivo microdialysis. Amphetamine (2 mg/kg, i.p.) increased extracellular dopamine in rat striatum with a maximum at the same time as did p-fluoroamphetamine, but the latter gave a smaller increase. The data presented suggest that p-fluoroamphetamine resembles amphetamie more than it does the 5-HT-releasing type amphetamines.

Neurochemical basis of interleukin 2-modified discrimination behaviour

We trained one group of rats to discriminate 0.8 mg/kg intraperitoneal (i.p.) d-amphetamine from 1 ml/kg saline and the other to discriminate 0.3 mg/kg i.p. (+/-)-ethylketocyclazocine (EKC) from saline. Recombinant human interleukin 2 (rIL-2), 2 x 10(6) U/kg (or 8.2 nmol/kg) given i.p. 1 h prior to tests, potentiated responses elicited by 0.4 mg/kg d-amphetamine. This potentiation of d-amphetamine responses was suppressed by the opioid receptor antagonist naloxone (1 mg/kg) when administered i.p. together with IL-2. IL-2 (4 x 10(6) U/kg) alone produced EKC-like responses in the EKC-trained animals. The cytokine also potentiated 0.1 mg/kg EKC responses at 2 x 10(6) U/kg, an action that was suppressed by 1 mg/kg naloxone. Data from the present study show that IL-2 exerts the same neurochemical action as that previously observed with IFN-alpha for both d-amphetamine and EKC discrimination in rats.

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.

Stimulus effects of d-amphetamine 1: DA mechanisms

As part of a continuing effort to assess the role of monoaminergic neuronal systems in the subjective effects of CNS stimulants, 10 rats trained to discriminate 1.0 mg/kg d-amphetamine from saline were treated with compounds that act through different dopaminergic mechanisms. In substitution (generalization) tests, 20 mg/kg of the dopamine (DA) uptake inhibitor GBR 12909 mimicked the training drug completely; at a dose of 15 mg/kg, GBR 12909 substituted for d-amphetamine incompletely. Neither the D1 agonist SK&F 38393 (1, 10 mg/kg) nor the D2 agonist quinpirole (LY 171555; 0.05-0.2 mg/kg) had amphetamine-like effects. When given in combination with the training drug, the D1 antagonist SCH 23390 blocked the amphetamine cue completely at a dose of 0.05 mg/kg but did not have significant effects at higher or lower doses; the D2 antagonist metoclopramide did not block d-amphetamine at any dose tested (1-5 mg/kg). These data indicate that: a) The discriminable effects of d-amphetamine are due, at least in part, to inhibition of DA uptake; b) direct stimulation of either D1 or D2 receptor sites is not sufficient to evoke d-amphetamine-like responding; and c) blockade of D1 receptors attenuates the subjective effects of d-amphetamine to a greater extent than blockade of D2 receptors.

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.

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.

Studies on the discriminative stimulus properties of apomorphine in rhesus monkeys

Four rhesus monkeys were trained to discriminate the effect of apomorphine (0.1 mg/kg IM) from that of saline injections. The discriminative stimulus (DS) effect of apomorphine generalized to the dopamine D2 receptor agonist quinpirole. The D1 dopamine receptor agonist SKF 38393 elicited responses only on the saline-appropriate lever. Stimulus generalization of the dopamine autoreceptor agonist 3-PPP exhibited stereospecificity favoring the (+) over the (-) isomer. d-Amphetamine, phencyclidine, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), and clonidine did not share the DS effect of apomorphine. The D2-selective antagonists sulpiride and metoclopramide reversed both the DS effect and the response rate reduction produced by the training dose of apomorphine. Chlorpromazine and the D1 antagonist Sch 23390 also antagonized the DS effect, but the antagonism was accompanied by a further rate reduction. Haloperidol and clozapine antagonized the DS effect incompletely. The DS effect produced by apomorphine in this study appears to be mediated predominantly by post-synaptic D2 receptor activation, with contribution also from the D1 receptor.

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.

Behavioural effects of the alpha 2-adrenoceptor antagonists idazoxan and yohimbine in rats: comparisons with amphetamine

Although yohimbine has long been known to increase arousal, reactivity and anxiety in animals and humans, little is known about the behavioural effects of more selective alpha 2-adrenoceptor antagonists such as idazoxan. In a recent experiment, however, it was found that in rats both yohimbine and idazoxan increased low rates of lever pressing, an effect also produced by amphetamine. The purpose of the present study was to investigate further the effects of yohimbine and idazoxan in comparison with those of d-amphetamine on the operant behaviour of rats. In rats trained to press a lever on a FI 60s schedule to obtain food both yohimbine and idazoxan increased response rates, although the effect of yohimbine was considerably greater than that of idazoxan. Lower doses of d-amphetamine had no consistent effect on overall rates of responding whereas a higher dose suppressed responding. Characteristically, d-amphetamine increased responding during early portions of the intervals and decreased responding during the final portions. Idazoxan and yohimbine tended to increase responding throughout the intervals except immediately after reinforcement. When idazoxan was administered in combination with prazosin FI response rates were markedly decreased. Administration of DSP4 did not alter the response rate-increasing effects of either yohimbine or idazoxan. In rats trained to discriminate d-amphetamine from saline both idazoxan and yohimbine gave rise to responding on the saline associated lever. Combination of idazoxan with d-amphetamine did not antagonise the amphetamine cue but produced substantial reductions in response rates, probably due to toxicity.(ABSTRACT TRUNCATED AT 250 WORDS)

Central nervous system stimulants: neuropharmacological mechanisms

The mechanisms underlying CNS-stimulant drug discrimination are discussed. Although different doses of CNS stimulants may produce qualitatively different cues, it appears that a relatively low dose of d-amphetamine (e.g., 1 mg/kg) elicits a "general" CNS-stimulant cue. Presynaptically, this cue may primarily depend on release of endogenous dopamine whereas inhibition of dopamine reuptake, per se, is insufficient to elicit the cue. Postsynaptically, the involvement of both dopamine D-1 and D-2 receptors is implicated. Furthermore, in the drug discrimination situation, D-1/D-2 receptors may be coupled differently than in dopamine-dependent locomotor activation. Anatomically, CNS-stimulant drug discrimination may depend primarily on mesolimbic dopamine systems.

Dissociation of dopaminergic and non-dopaminergic substrates for cues produced by electrical stimulation of the ventral tegmental area

The present study provides evidence for the existence of multiple substrates for cues produced by electrical stimulation of the ventral tegmental area in rats. Two different procedures were employed to assess the effects of amphetamine and haloperidol on the discrimination of high and low intensity cues produced by electrical brain stimulation (EBS). When the procedure involved frequent presentation of brief trials, amphetamine and haloperidol had no effect on the discrimination of EBS. In contrast, when the trials were less frequent and extended in duration, amphetamine enhanced the perceived intensity of the cues whereas haloperidol had the opposite effect. These results indicate that the use of different discrimination procedures may result in the measurement of separate dopaminergic and non-dopaminergic substrates for cue properties of EBS in the ventral tegmental area.

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.

Interaction of nicotine with dopaminergic mechanisms assessed through drug discrimination and rotational behaviour in rats

Biochemical and electrophysiological studies have suggested that nicotine may interact with dopaminergic systems so as to enhance the release and utilization of dopamine. The functional significance of these effects has been assessed using drug discrimination and rotational behaviour in rats. The dopamine antagonists haloperidol and Sch 23390 attenuated the discriminative stimulus effect of nicotine and reduced overall rates of responding. In contrast, droperidol and pimozide were without significant effect on discrimination of nicotine at doses that reduced response rates. There was partial generalization from nicotine to the dopamine D-1 agonist SKF 38393. In rats with unilateral, 6-hydroxydopamine lesions of the nigrostriatal dopamine pathway, nicotine produced rotation towards the side of the lesion, a characteristic effect of indirectly acting dopamine agonists such as amphetamine. The nico tinic-cholinergic antagonist mecamylamine, and haloperidol, blocked rotation produced by nicotine. A dose of nicotine that was too small to produce amphetamine-like effects itself enhanced both the discriminative stimulus and the rotational behaviour produced by amphetamine. However, mecamylamine did not weaken the discriminative effect of amphetamine. The results suggest that there may be a minor component of the nicotine discriminative stimulus that is mediated, indirectly, through a dopaminergic mechanism. There also appears to be a facilitatory, nicotinic influence on behavioural functions linked to the dopamine system.

Further evidence for an amphetamine-like mechanism of action of the alkaloid cathinone

The alkaloid (-)cathinone is a potent stimulant with pharmacological properties closely resembling those of (+)amphetamine. Since (-)cathinone is capable of inducing release at physiological catecholamine storage sites, it has been suggested that (-)cathinone and (+)amphetamine have the same mechanism of action. In the present study, the potency of (-)cathinone in inducing the release of radioactivity from 3H-dopamine prelabelled tissue of the rat caudate nucleus was compared to that of several structural analogs, i.e. to that of four other aminophenones. (-)Cathinone was found to be the most potent of the compounds under investigation, and among these only demethylcathinone had an effect that was within the same order of magnitude as that of (-)cathinone. Furthermore, (-)cathinone and two of its analogs were evaluated in behavioral experiments with regard to their ability to substitute for (+)amphetamine in rats trained to discriminate between (+)amphetamine and saline. It was found that, unlike the other aminophenones, (-)cathinone is capable of producing (+)amphetamine-like stimulus effects, and these can be antagonized by haloperidol in a dose-related manner. Taken together, these findings support the hypothesis that (+)amphetamine and (-)cathinone produce their central stimulant effect via the same dopaminergic mechanism.

Dopaminergic mediation of a behavioral effect of l-cathinone

Ten male rats were trained to discriminate between the stimulus properties of 0.6 mg/kg l-cathinone and saline in a two-lever food-motivated operant task. Once trained, rats showed a dose-dependent increase in discrimination over a dosage range of 0.15-1.2 mg/kg l-cathinone. Analysis of this dose-response relationship indicated an ED50 of 0.27 mg/kg. Pretreatment with 0.2 mg/kg of the specific dopamine blocking drug haloperidol increased this ED50 to 0.47 mg/kg and significantly decreased discriminative performance when co-administered with either 0.15, 0.3, or 0.6 mg/kg l-cathinone. Since the dose-effect curves for cathinone with and without haloperidol pre-treatment were parallel, it is suggested that l-cathinone, the active constituent in khat, produces its discriminative properties, in part, by mediation of dopaminergic neuronal systems.

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.

Prazosin: effect on psychomotor-stimulant cues and locomotor activity in mice

Mice trained to discriminate 1 mg/kg d-amphetamine from saline, or the selective norepinephrine uptake inhibitor, nisoxetine, from saline, cross-generalized to the alternate drug. They also generalized to 5.6 mg/kg cocaine. The cues produced by amphetamine were antagonized by the selective alpha 1-adrenoceptor antagonist, prazosin, and slightly potentiated by the selective alpha 2-adrenoceptor antagonist, yohimbine. The nisoxetine cue was also antagonized by prazosin. In contrast, the peripherally acting sympathomimetics, p-hydroxyamphetamine and phenylpropranolamine, failed to substitute for, and pimozide and propranolol failed to block, either drug cue. In addition, prazosin, at a dose that did not affect either saline-associated locomotor behavior in mice or the locomotor-activity increase produced by the dopamine uptake inhibitor, bupropion, also antagonized the locomotor stimulation produced by amphetamine and cocaine. Thus, in mice, both the cues and locomotor stimulation produced by amphetamine and cocaine appear to be at least partially mediated by central alpha 1-adrenoceptor activation secondary to increased central synaptic concentrations of norepinephrine.

One-way generalization of clonidine to the discriminative stimulus produced by cocaine

Rats were trained to discriminate the stimulus properties of either cocaine or clonidine using a food reinforced two-lever choice paradigm. After training, cocaine was generalized to the cocaine lever in a dose-dependent manner, and clonidine was generalized to the clonidine lever in a dose-dependent manner. Yohimbine, an alpha-2 antagonist, blocked the clonidine stimulus but not the cocaine stimulus. Cocaine was not generalized to the clonidine stimulus; however, clonidine was generalized to the cocaine stimulus, and this generalization was blocked by yohimbine. The one-way generalization of clonidine to cocaine suggests that clonidine has at least two discrete stimulus components: a major component that is not cocaine-like, and a minor component that can be detected by cocaine-trained subjects. In addition, the yohimbine blockade data suggest that both components of the clonidine stimulus are mediated via alpha-2 receptors.

Antagonism of the amphetamine cue by both classical and atypical antipsychotic drugs

Rats were trained to discriminate the stimulus properties of 1 mg/kg of d-amphetamine sulphate (AMPH) from saline in a two-lever task in which correct responding was reinforced with water under a fixed ratio (FR 32) schedule. Classical antipsychotic drugs from different chemical classes were all able to block the AMPH cue. Doses (mg/kg) inhibiting the cueing effect to 50% (ID50) were 0.035 (haloperidol), 0.04 (spiroperidol), 0.09 (cis(Z)-flupenthixol), 0.12 (trifluperazine), 0.15 (perphenazine), 0.92 (chlorpromazine) and 1.40 (pimozide). The AMPH cue was also antagonized by antipsychotic drugs that are considered atypical due to their relative lack of activity in conventional animal models or inability to produce extrapyramidal symptoms in the clinic. The following ID50 values were obtained: 0.88 (molindone), 1.22 (clozapine), 5.48 (metoclopramide), 15.4 (thioridazine) and 52.8 [-)-sulpiride). In addition, the AMPH cue was blocked by the D-1 selective dopamine (DA) antagonist, SCH 23390 (ID50 = 0.014 mg/kg). The abilities of these drugs to block the AMPH cue were unrelated to the drugs' effect upon the rate of responding. For example, some drugs (e.g. haloperidol, spiroperidol and SCH 23390) blocked the AMPH cue completely without any effect on the response rate. Furthermore, the non-antipsychotic phenothiazine, promethazine (2.5-12.5 mg/kg) failed to affect the AMPH cue although the drug strongly suppressed the response rate. However, the potent DA agonists, apomorphine (0.05-0.33 mg/kg) and lisuride (0.02-0.08 mg/kg), and the DA and norepinephrine agonist, DPI (0.4 and 0.8 mg/kg), did not mimic the AMPH cue or did so only partially. These results suggest that the 1 mg/kg AMPH cue depends on (DA) systems other than those involved in the stereotyped motor behavior commonly produced by high doses of AMPH or DA agonists. Low-dose AMPH discrimination may thus serve as a new model for studying antipsychotic drug action.

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.

Discriminative stimulus properties of fenfluramine: evidence for serotonergic involvement

Rats were trained to discriminate 3 mg/kg fenfluramine (FEN) from saline using a milk-reinforced (FR 10 schedule) two-lever operant task. To assess the involvement of the serotonin (5-HT) system in elicitation of the FEN cue, 5-HT compounds were tested for their ability to substitute for or to antagonize the the discriminative stimulus produced by FEN. Following acquisition, the FEN cue was dose-dependent, had a rapid onset (10 min) and a long duration (12 h), and was stereospecific. The putative 5-HT receptor antagonists methysergide and cinanserin antagonized the FEN discriminative stimulus, while chlordiazepoxide, an indirect inhibitor of 5-HT turnover, did not. The FEN cue was also antagonized by the selective 5-HT reuptake inhibitor fluoxetine. Norfenfluramine, p-fluoro-amphetamine, and p-chloroamphetamine, compounds structurally and pharmacologically similar to FEN, substituted for FEN, whereas fluoxetine, cinanserin, methysergide, and chlordiazepoxide did not. The 5-HT precursor 5-hydroxytryptophan partially generalized to the FEN cue. It was further shown that the discriminative stimulus properties of FEN are not based on its anorectic action. These results suggest that the cue properties of FEN might be partially mediated through an interaction with the 5-HT system.

Dopamine and depression: a review of recent evidence. III. The effects of antidepressant treatments

This paper reviews the effects of antidepressants on brain dopamine (DA) function in people and animals. Acute antidepressant treatments have no consistent effect on DA. Chronic treatment does not appear to change DA synthesis or turnover, or to affect post-synaptic DA receptors. However, there is considerable evidence of increased DA function following chronic antidepressant treatments; the site of these effects is at present unclear, but appears to be beyond the DA receptors. There is also evidence for decreased sensitivity of pre-synaptic DA autoreceptors, though this effect is less well established.

Neuropharmacological specificity of the diazepam stimulus complex: effects of agonists and antagonists

A food-reinforced two-lever operant method was used to train rats (n = 10) to discriminate diazepam (3.0 mg/kg i.p.) from saline. Thereafter drugs with supposed agonist and antagonist activity were submitted for test. Pentobarbital was the only agonist able to produce the diazepam discriminative stimulus complex (DSC), while pentylenetetrazol was the only antagonist able to abolish the diazepam DSC. It is concluded that the diazepam DSC is highly specific in this test and is suitable for the investigation of the neuropharmacological action of the benzodiazepines.

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 nicotine: further evidence for mediation at a cholinergic receptor

Rats were trained to discriminate nicotine (0.4 mg/kg SC) from saline in a standard two-bar operant conditioning procedure with food reinforcement. The response to nicotine was dose-related and at the ED50 of 0.14 mg/kg, plasma nicotine concentrations were similar to those reported previously for cigarette smokers who inhale. The nicotine analogues anabasine and cytisine increased nicotine-appropriate responding in a dose-related manner. Animals predominantly responded on the saline-associated lever when administered drugs from a range of pharmacological classes, even at doses that were sufficiently large to reduce the overall numbers of responses. The results confirm that the nicotine discriminative stimulus is highly specific. Previous work has shown anabasine and cytisine to be active at nicotinic-cholinergic binding sites in rat brain. The finding that there is some correlation between the behavioural effects of these compounds and their actions at the nicotine binding site may indicate that the nicotine cue is mediated through a cholinergic receptor.

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.

Drug discrimination learning in lead-exposed rats

Lead acetate (0.02 or 0.5 percent) was administered to dams throughout the lactation period with half of the litters continuing on lead after weaning. Drug thresholds for d-amphetamine were determined by using the drug-discrimination learning paradigm. All the offspring that had been exposed to lead were less sensitive to the stimulus properties of d-amphetamine irrespective of whether or not they had continued on lead after weaning.

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.

A neuropharmacological analysis of the discriminative stimulus properties of fenfluramine

Rats were trained to discriminate fenfluramine (1.0 mg/kg) from saline in a two-lever drug discrimination task. The dose-response curve for this discrimination was orderly with an ED50 of about one-half of the training dose (0.52 mg/kg). In substitution tests, indirect (p-chloroamphetamine) and direct (quipazine, MK-212, lisuride) serotonin (5-HT) agonists substituted for fenfluramine. Since none of these compounds have been reported to be hallucinogenic and the potent hallucinogen LSD did not substitute completely, it was suggested that the discriminative stimulus properties of fenfluramine are not related to its ability to produce hallucinations in humans. The fenfluramine cue, like the quipazine cue, was antagonized by the 5-HT antagonists cyproheptadine and methiothepin. Unlike quipazine, fenfluramine was also partially antagonized by the 5-HT uptake inhibitor, fluoxetine, and the 5-HT synthesis inhibitor, p-chlorophenylalanine. Thus, the fenfluramine cue differs from that of quipazine in that it is mediated via indirect actions on 5-HT receptors. Since the indirect dopamine (DA) agonist d-amphetamine failed to substitute and the DA antagonist haloperidol failed to block the fenfluramine cue, a mediating role for DA was not indicated. Another indirect DA agonist, cocaine, substituted partially for fenfluramine, a result which paralleled that seen with fluoxetine. Both of these partial substitutions were reduced by cyproheptadine; therefore, it was concluded that these effects may be due to the common ability of cocaine, fluoxetine, and fenfluramine to inhibit 5-HT uptake.

A comparison of some behavioural effects of amphetamine and electrical brain stimulation of the mesolimbic dopamine system in rats

Separate groups of rats were trained to press a lever on either a fixed-ratio 40 or fixed-interval 1-min schedule of food reinforcement. Amphetamine increased low rates and decreased high rates of responding. In contrast, electrical stimulation of rewarding sites in the ventral tegmentum was effective only in decreasing high response rates. Stimulation of non-rewarding sites had little effect upon either low-or high-rate responding. Another group of rats was trained to discriminate between amphetamine (1.0 mg/kg) and saline in a standard two-lever procedure with food reinforcement. The internal stimulus produced by the stimulation of rewarding sites did not substitute readily for the discriminative stimulus produced by amphetamine. The results suggest that the operant response-rate decreasing property of amphetamine may be partially mediated through the mesolimbic dopamine system. However, this system may not play a prominent role in mediating either the operant response-rate increasing or discriminable properties of this drug. The idea that the abuse liability of amphetamine may be related to the ability of the drug to interact with one of the central reward systems is discussed.

Different dopaminergic mechanisms for amfonelic acid, amphetamine and apomorphine

Rats were trained to discriminate between the stimulus properties of intraperitoneal 0.16 mg/kg apomorphine and saline in a two-lever, food-motivated operant task. Employing the selected lever and the extended schedule performance measurements to indicate the generalization effect and perseverance of that effect, respectively, neither d-amphetamine nor amfonelic acid produced apomorphine-like discriminative properties. In contrast, administration of 0.1 or 0.2 mg/kg n-propylnoraporphine was observed to produce responses and perseverance on the apomorphine-appropriate lever that was similar to that seen after 0.16 mg/kg administration. The results of this behavioral experimentation are consistent with the notion that d-amphetamine, amfonelic acid and apomorphine may produce their dopaminergic effects by different mechanisms of action and the possibility of two sites of action for n-propylnoraporphine is discussed.