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

Characterization of the discriminative stimulus properties induced by 5-HT1 and 5-HT2 agonists in rats

The effect of different serotonin (5-HT) agonists and antagonists on the discriminative stimulus properties (cue) induced by 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OHDPAT), 1-(m-trifluoromethylphenyl)piperazine (TFMPP) and d-LSD (d-lysergic acid diethylamide) has been investigated. The 8-OHDPAT cue was mimicked by the 5-HT1A agonists ipsapirone, buspirone, gepirone and partially by 5-methoxy-N,N-dimethyltryptamine and d-LSD. 5-HT1B (TFMPP and RU 24969) and 5-HT2 agonists (DOM, DOI and quipazine) were ineffective and induced disruption of responding. The 8-OHDPAT cue was antagonized by spiroxatrine and partially by (-)-alprenolol, whereas selective antagonists of 5-HT2 (ketanserin and ritanserin), 5-HT3 (ICS 205-930), alpha 1-adrenergic (prazosin) and beta-adrenergic receptors (ICI 118.551) were ineffective. The TFMPP cue was mimicked by RU 24969 and partially by quipazine. Other compounds were ineffective. Only (-)-alprenolol antagonized the effect of TFMPP. The d-LSD cue was mimicked by DOM, DOI, quipazine, 5-methoxy-N,N-dimethyltryptamine and partially by ipsapirone, TFMPP and RU 24969. The 3 latter compounds and 5-HT1A agonists induced disruption of responding. The d-LSD cue was antagonized by ketanserin and ritanserin, but not by the other antagonists mentioned above. The specific inhibitor of 5-HT uptake citalopram was not able to substitute for any of the 3 agonists. It is concluded that the drug discrimination technique can be used to identify selective agonists and antagonists of 5-HT receptor subtypes. Compounds with mixed effects on 5-HT receptor subtypes can also be identified. These show additional effects on reaction time and often disrupt responding at higher dosages.

Amphetamine discrimination: effects of dopamine receptor agonists

Dopamine (DA) D-1 and D-2 receptor agonists and antagonists were characterized in receptor binding and adenylate cyclase assays with respect to affinity, selectivity and efficacy. The ability of the ligands to interact with the discriminative stimulus effects of d-amphetamine (AMPH) was then assessed. The D-2 agonists, quinpirole, pergolide and CH 29-717, substituted completely for AMPH while neither partial (SKF 38393 and SKF 75670) nor full D-1 receptor agonists (SKF 89626 and SKF 81297) substituted. On the other hand, the selective D-1 and D-2 antagonists all blocked AMPH. The substitution for AMPH by pergolide was blocked by raclopride but not by SCH 23390, indicating D-2 mediation. In contrast, the motor effects of pergolide were blocked by both raclopride and SCH 23390, indicating mixed D-1/D-2 receptor involvement. These results suggest that D-1 and D-2 are equally involved in the expression of functional effects in the DAergic motor systems. Conversely, D-2 receptors may play a primary role in the DA systems involved in the AMPH cue; furthermore, the D-1 and D-2 receptors in the systems are relatively uncoupled.

Role of specific dopamine receptor subtypes in amphetamine discrimination

Biochemical, electrophysiological, and behavioral experiments suggest that the dopamine D-1 and D-2 receptor subtypes functionally interact. In rats trained to discriminate 1.0 mg/kg d-amphetamine, substitution with the D-2 agonist quinpirole (0.1-2.0 mg/kg) produces amphetamine-lever responding, whereas the D-1 agonist SKF 38393 (0.3-10.0 mg/kg) elicits only saline-appropriate responding. Combining either quinpirole (0.05-0.5 mg/kg) or SKF 38393 (0.5-10.0 mg/kg) with 0.3 mg/kg d-amphetamine results in dose-dependent increases in amphetamine-lever responding. Conversely, the D-1 antagonist SCH 23390 (0.02-0.1 mg/kg) antagonizes the discrimination produced by 0.7 mg/kg d-amphetamine. Additional combination studies examined the effect of DA receptor drugs on discrimination when quinpirole is substituted in d-amphetamine trained rats. SKF 38393 (0.5-7.0 mg/kg) fails to increase the amphetamine-appropriate lever response produced by either 0.05 or 0.2 mg/kg quinpirole. Similarly, SCH 23390 (0.01-0.1 mg/kg) fails to antagonize the amphetamine-lever responding produced by either 0.2 or 0.5 mg/kg quinpirole. Haloperidol (0.02-0.2 mg/kg) does antagonize the amphetamine-appropriate response produced by quinpirole substitution. The d-amphetamine discrimination studies indicate that stimulating D-2 receptors alone or D-1 receptors in the presence of d-amphetamine yields d-amphetamine-lever responding, and suggests that D-1/D-2 receptors can functionally interact to alter discrimination behavior. Quinpirole substitution, on the other hand, shows an insensitivity to D-1 receptor manipulations.

The involvement of dopamine D1 and D2 receptors in the locomotor stimulation produced by (+)-amphetamine in naive and dopamine-depleted mice

The interaction between (+)-amphetamine and dopamine (DA) D1 and D2 receptors was investigated. In naïve mice, i.e., mice with intact stores of DA, both the selective D1 antagonist SCH23390 and the selective D2 antagonist spiperone blocked the locomoter stimulation produced by (+)-amphetamine. The selective D1 agonist SKF38393 (6 mg/kg intraperitoneally) did not produce a consistent dose-dependent effect on the response to (+)-amphetamine in naïve mice. In mice depleted of DA with reserpine 24 hr before a challenge with (+)-amphetamine, neither SCH23390 nor spiperone were completely effective in blocking (+)-amphetamine. A combination of spiperone plus SCH23390 was, however, more effective than either drug alone, although significant activity remained even after the combination. In mice pretreated with reserpine and various doses of alpha methyl-p-tyrosine (alpha MPT, intraperitoneally), the degree of stimulation produced by (+)-amphetamine was dependent on the amount and frequency of alpha MPT dosage - the higher and more frequent the dose, the more effective the blockade. In these animals, both SKF38393 and the selective D2 agonist quinpirole potentiated the stimulation induced by (+)-amphetamine when the dose of alpha MPT was not maximal. However, in those animals pretreated with reserpine plus two doses each of 400 mg/kg alpha MPT, neither SKF38393 nor quinpirole were effective in potentiating (+)-amphetamine. Nevertheless, when SKF38393 and quinpirole were administered simultaneously to these mice, marked locomotor stimulation occurred implying that the pretreatment itself had not rendered the mice incapable of locomotion.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Differential tolerance to cataleptic effects of SCH 23390 and haloperidol after repeated administration

The development of tolerance to the cataleptic effect of the selective D-1 antagonist SCH 23390 (0.5 mg/kg/day SC or 0.1 mg/kg/day SC) and haloperidol (1 mg/kg/day SC) during repeated administration was investigated. Catalepsy in rats was measured using the horizontal bar method. SCH 23390 induced a dose-related cataleptic effect of short duration, whereas the cataleptic effect of haloperidol appeared more slowly and lasted longer. Marked tolerance to the cataleptic effect of haloperidol developed already 6 days from the beginning of the treatment. The cataleptic effect of the higher dose regimen of SCH 23390 was also significantly reduced after 6 days' treatment. However, unlike haloperidol, this subacute tolerance was gradually reversed and was no longer significant after 12 and 18 days. The cataleptic response to the lower dose of SCH 23390 (0.1 mg/kg/day) was not significantly altered during the treatment and no initial catalepsy tolerance was observed with this dose regimen. These results suggest that different mechanisms are involved in the expression of cataleptic behaviour during chronic treatment with SCH 23390 and classical antipsychotics, such as haloperidol.

Effects of stimulation and blockade of dopamine receptor subtypes on the discriminative stimulus properties of cocaine

The involvement of dopamine (DA) receptor subtypes in the behavioral effects of CNS stimulants was studied in rats trained to discriminate cocaine from saline. In substitution tests, the stimulus effects of 10 mg/kg of this substance generalized to d-amphetamine (0.25-1.0 mg/kg) and the selective D2 against LY-171555 (0.05-0.25 mg/kg) but not to the D1 agonist SKF-38393 (5.0-15.0 mg/kg); in combination tests, the D1 antagonist Sch-23390 (0.0625-0.5 mg/kg) significantly blocked, and the D2 antagonist spiperone (0.25-0.5 mg/kg) partially blocked the cocaine cue. These data suggest that the involvement of DA systems in the behavioral effects of cocaine is more complex than either D1 or D2 receptor activation; for example, the stimulus properties of this substance might involve both D1 and D2 receptor activation.

Comparison of the pharmacological characteristics of [3H]raclopride and [3H]SCH 23390 binding to dopamine receptors in vivo in mouse brain

In vivo binding of the benzamide derivative [3H]raclopride was studied in mouse brain. The binding was saturable, reversible and stereospecific. Non-specific binding was 5-15% of the total binding. Pharmacological characterization of the binding indicated labelling of dopamine D2 receptors since the binding was potently inhibited by compounds with high affinity for this receptor in vitro. On the other hand, compounds with low affinity in vitro i.e., dopamine D1-selective compounds were weak or inactive as inhibitors of [3H]raclopride binding. A comparison of the pharmacological characteristics of [3H]raclopride and [3H]SCH 23390 binding in vivo indicated that compounds with selectivity in vitro retained this selectivity in vivo. Thus, spiroperidol, haloperidol, 1-sulpiride, clebopride, LY 171555 and (-)-NPA ((-)-N-propyl-norapomorphine) were D2 selective while SCH 23390, SKF 38393 and SKF 75670 were D1 selective. Clozapine, tilozepine, cis-flupentixol, chlorpromazine and butaclamol were non-selective both in vitro and in vivo. However, a few compounds changed profile in vivo compared to in vitro. Thus, fluperlapine and fluphenazine had a dual D1-D2 receptor profile in vitro but were D1- or D2-selective in vivo, respectively. Pergolide and molindone which were D2-selective in vitro both had a dual D1-D2 receptor profile in vivo. In conclusion, [3H]raclopride, in vivo, selectively labels the dopamine D2 receptor. Comparison of the pharmacological characteristics of [3H]raclopride and [3H]SCH 23390 binding in vivo supported the that the dopamine D1 receptor is an important target for a variety of neuroleptics, especially of the clozapine type. This may indicate that blockade of the dopamine D1 receptor conveys antipsychotic action.

The discriminative stimulus properties of the D-1 agonist SK&F 38393 and the D-2 agonist (-)-NPA are mediated by separate mechanisms

The dopamine D-1 agonist SK&F 38393 (10 mg/kg) the D-2 agonist (-)-NPA (0.04 mg/kg) and d-amphetamine (1.0 mg/kg) were established as discriminative stimuli versus saline in rats. The stimulus induced by SK&F 38393 was stereoselective, since the R-(+)-, but not the S-(-)-enantiomer was effective. It was mimicked by two partial D-1 agonists with central effects, SK&F 75670 and Lu 24-040, but not by the peripheral agonist fenoldopam. D-2 agonists and d-amphetamine were ineffective. The effect of SK&F 38393 was antagonized by SCH 23390, but not by its inactive enantiomer SCH 23388 or by the D-2 antagonist YM 09151-2. The (-)-NPA stimulus was dependent on postsynaptic D-2 receptors: It was mimicked by quinpirole and pergolide in stimulant dosages, whereas the partial D-2 agonist (-)-3-PPP inhibited the effect of (-)-NPA. The dopamine synthesis inhibitor alpha-methyl-p-tyrosine did not antagonize the effect of (-)-NPA. Likewise, the above-mentioned D-1 agonists produced saline responding. D-amphetamine produced partial substitution to (-)-NPA. The (-)-NPA stimulus was blocked by YM 09151-2, but not by SCH 23390. In d-amphetamine-trained rats, quinpirole, (-)-NPA and pergolide produced generalization, whereas SK&F 38393 was ineffective. Both SCH 23390 and YM 09151-2 antagonized the effect of d-amphetamine. It is concluded that the cues induced by SK&F 38393 and (-)-NPA are mediated by separate D-1 and D-2 sites, whereas both sites contribute to the effect of d-amphetamine.

Effects of nimodipine on the discriminative stimulus properties of d-amphetamine in rats

The discriminative stimulus (DS) properties of d-amphetamine (AMP) are thought to be mediated by enhanced release of catecholamines, which may involve neuronal calcium influx through voltage sensitive channels. The present study examined the influence of nimodipine, a calcium channel blocker, on the DS properties of AMP. Rats (N = 8) were trained to discriminate AMP (0.5 mg/kg, IP) from saline in a two-lever, food-reinforced, drug discrimination paradigm. Nimodipine alone (2.0-5.6 mg/kg, IP) did not substitute for AMP. When given in combination with AMP, 2.0 mg/kg nimodipine increased by less than 2-fold the AMP dose necessary to induce AMP-appropriate responses. Higher doses of nimodipine combined with AMP did not increase the magnitude of this effect. Nimodipine enhanced the effects of AMP on response rate. Haloperidol (0.125 mg/kg) increased by approximately 4-fold, whereas diazepam (0.5 or 1.0 mg/kg) and morphine (5.0 mg/kg) increased by approximately 2-fold the AMP dose necessary to induce AMP-appropriate responses. The interaction with AMP was associated with enhanced reduction of response rate in the tests with diazepam and morphine but not haloperidol. These results suggest that nimodipine attenuates the DS properties of AMP, probably in a non-specific way, due to the ability of nimodipine itself to induce a discriminable internal state.

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.

Stimulus properties of dopaminergic drugs: comparisons involving selective agonists and antagonists

Rats were trained to discriminate the putatively selective dopamine (DA) receptor agonists SKF 38393 (10 mg/kg) or Ly 171555 (0.025 mg/kg) from saline in a two-lever situation involving fixed-ratio (FR 20), extinction schedules of water reinforcement. During substitution tests, no dose of any compound [apomorphine, Ly 171555, lisuride, LSD, amphetamine, cocaine, (-) 3-PPP, or SKF 82526] mimicked SKF 38393, the effects of which were blocked by the D1 antagonist Sch 23390 but not by haloperidol. Postsynaptic and DA "autoreceptor" agonists [apomorphine, (-) 3-PPP], as well as dopaminergic ergot derivatives (bromocriptine, lergotrile, lisuride) and Sch 23390, substituted for Ly 171555, a partial ergoline which has behavioral effects that are blocked by haloperidol and molindone, but not by either Sch 23390 or serotonin (5-HT) antagonists (ketanserin, pizotifen). Amphetamine and cocaine did not substitute for either SKF 38393 or Ly 171555. These results suggest that the stimulus properties of a variety of neuropharmacologically important and clinically useful compounds are transduced at (pre- or postsynaptic) D2 receptors. However, this conclusion must be tempered by evidence that actions at D1 and D2 receptors may not be entirely independent. The behavioral effects of abused psychomotor stimulants probably involve mechanisms other than "direct" agonist activity at either D1 or D2 receptors.

Cholinergic mediation of the discriminative stimulus properties of clozapine

Rats were trained to discriminate clozapine (CLZ; 5.76 mg/kg, IP t-30 min) in a two-lever operant task in which responding on the correct lever was reinforced with water under a fixed ratio 32 schedule. The ED50 of CLZ was 1.1 mg/kg. The CLZ cue was generalised to atropine (ED50 = 8.7 mg/kg), scopolamine (ED50 = 0.37 mg/kg) and fluperlapine (ED50 = 4.0 mg/kg), but not to non-cholinergic compounds, i.e. buspirone, diazepam, ketanserin, prazosin or SCH 23390. The peripherally-acting muscarinic antagonist methylscopolamine did not substitute for CLZ. Furthermore, the CLZ cue was marginally attenuated by d-amphetamine; a high dose of oxotremorine (1 mg/kg) appeared to block the CLZ cue (to 22%). However, this effect could not be evaluated statistically due to an insufficient number of animals responding. These results may indicate that the discriminative stimulus effects of CLZ primarily involve antagonism of central muscarinic acetylcholine receptors.

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.

Effects of heavy metal cations and other sulfhydryl reagents on brain dopamine D1 receptors: evidence for involvement of a thiol group in the conformation of the active site

To investigate aspects of the biochemical nature of membrane-bound dopamine D1 receptors, rat striatal homogenates were pretreated with heavy metal cations and some other chemical agents, and their effects on D1 receptors were subsequently determined using a standard [3H](R)-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1-N-3- benzazepine([3H]SCH 23390) binding assay. Incubation of striatal membranes with as little as 1 microM Hg2+, 10 microM Cu2+, and 10 microM Cd2+ completely prevented specific [3H]SCH 23390 binding. The effect of Cu2+, 1.5 microM, was noncompetitive in nature, whereas 3-5 microM Cu2+ afforded mixed-type inhibition. The inhibitory effect of Cu2+ was fully reversed by dithiothreitol (0.1-1 mM). Cu2+ (2 microM) did not affect the affinity of cis-flupenthixol or clozapine for remaining [3H]SCH 23390 sites. A second series of cations, Co2+ (30 microM), Ni2+ (30 microM), Mn2+ (1 mM), Ca2+ (25 mM), and Ba2+ (20 mM), inhibited specific [3H]SCH 23390 binding by 50% at the concentrations indicated. The thiol alkylating reagent N-ethylmaleimide (NEM) (0.2 mM) reduced specific binding by 70%. The effect of NEM was completely prevented by coincubation with a D1 receptor saturating concentration of SCH 23390 (20 nM) or dopamine (10 microM). The results indicated that the dopamine D1 receptor is a thiol protein and that a thiol group is essential for the ligand binding.

Low doses of cis-flupentixol attenuate motor performance

We evaluated the effects of cis-flupentixol on reinforced responding. The experimental subjects were rats and the reinforced response was a lever press. The procedure was a five-component multiple schedule that provided five different reinforcement rates. Cis-flupentixol produced dose-dependent decreases in reinforced responding. An equation, the matching law, was fitted to the results. One parameter of this equation represents the estimated response rate asymptote. Cis-flupentixol produced dose-dependent decreases in the asymptotes. A second parameter of the equation represents the rate of reinforcement that maintains a one-half asymptotic response rate. Cis-flupentixol did not appear to affect this measure. There is evidence that the response rate asymptote measures motor components of response rate and that the reinforcement parameter measures the efficacy of the reinforcement maintaining the response. According to these results, cis-flupentixol systematically affected the motor-component of reinforced responding-it slowed down lever pressing-without affecting the subject's sensitivity to the reinforcer maintaining the response. In contrast, other neuroleptics have decreased the subjects' sensitivity to reinforcement, according to the matching law measures.

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.

D1 dopamine receptor--the search for a function: a critical evaluation of the D1/D2 dopamine receptor classification and its functional implications

The present review focuses on the hypothesized D1/D2 dopamine (DA) receptor classification, originally based on the form of receptor coupling to adenylate cyclase activity. The pharmacological effects of compounds exhibiting putative selective agonist or antagonist profiles at those DA receptors positively coupled to adenylate cyclase activity (D1 DA receptors) are extensively reviewed. Comparisons are made with the effects of putative selective D2 DA receptor agonists and antagonists, and on the basis of this work, the DA receptor classification is critically evaluated. A variety of biochemical, behavioral, and electrophysiological evidence is presented which supports the view that D1 and D2 DA receptors can interact in both an opposing and synergistic fashion. Particular attention is focused on the possibility that D1 receptor stimulation is required to enable the expression of certain D2 receptor-mediated effects, and the functional consequences of this form of interaction are considered. A hypothetical model is presented which considers how both the opposing and enabling forms of interaction between D1 and D2 DA receptors can control behavioral expression. Finally, the clinical relevance of this work is discussed and the potential use of selective D1 receptor agonists and antagonists in the treatment of psychotic states and Parkinson's disease is considered.

Dihydroergotoxine potentiation of thioridazine-induced hypomotility

Dihydroergotoxine (DHET), a dopamine (DA) receptor agonist, induced hypomotility in rats at doses of 0.125-0.5 mg/kg s.c., (-)sulpiride a D-2 receptor antagonist, a dose which did not affect spontaneous activity (10 mg/kg s.c.) counteracted the hypomotility induced by DHET. Thioridazine (THR), a widely used neuroleptic, reduced motility (2.5-10 mg/kg i.p.) when given alone; this effect was potentiated by DHET when the two drugs administered in doses of 1 mg/kg THR/0.05 mg/kg DHET and 2.5 mg/kg THR/0.125 mg/kg DHET; higher doses of THR (5 and 10 mg/kg i.p.) which by themselves induced marked inhibition of motility, were not potentiated by doses of DHET of 0.25 and 0.5 mg/kg s.c. THR and DHET influenced in an opposite manner the levels of DA-metabolites dihydroxyphenylacetic acid and homovanillic acid in the caudate; while THR (1-10 mg/kg i.p.) increased, DHET (0.25-0.5 mg/kg s.c.), decreased DA-metabolite levels. The association of THD and DHET resulted in a reciprocal antagonism of their effects on DA-metabolite levels.

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.

Cueing effects of amphetamine and LSD: elicitation by direct microinjection of the drugs into the nucleus accumbens

Two groups of rats were trained to discriminate either d-amphetamine sulphate (AMPH; 1 mg/kg) or d-lysergic acid diethylamide bitartrate (LSD; 0.16 mg/kg). Microinjection of AMPH into the nucleus accumbens elicited a cueing effect which was similar to that of systemically administered AMPH in AMPH-trained animals (ED50 was 0.24 micrograms). Co-injection of (-)-sulpiride (50 or 100 ng) into the accumbens antagonized the effect of a fixed dose of AMPH (1 microgram) which, alone, produced 76% of the systemic AMPH cue effect. Microinjections of AMPH (1-5 micrograms) into either the anterior dorsomedial or the anterior ventrolateral striatum failed to elicit the cueing effect of AMPH. In LSD-trained animals a dose of 1 microgram LSD injected into the accumbens produced 84% of the systemic cueing effect of LSD. These results suggest that dopamine (DA) receptors in the nucleus accumbens are involved in AMPH discrimination. Furthermore, since both classical and atypical antipsychotic drugs block the AMPH cue, the results provide indirect evidence for involvement of mesolimbic DA in antipsychotic drug action.