Repeated administration of MK-801 produces sensitization to its own locomotor stimulant effects but blocks sensitization to amphetamine

Repeated amphetamine administration produced behavioral sensitization to subsequent amphetamine challenge. The development of sensitization was blocked by coadministration of the N-methyl-D-aspartate (NMDA) antagonist MK-801. Conditioned locomotion, as revealed by saline challenge, was also blocked by MK-801, suggesting that behavioral sensitization and conditioned locomotion may share a requirement for NMDA receptor stimulation. Repeated MK-801 administration produced behavioral sensitization to MK-801 but not amphetamine challenge, suggesting that MK-801 itself produces sensitization through a different mechanism than amphetamine.

[Effect of the calcium channel blockers on rotation induced by apomorphine and amphetamine in rats lesioned with 6-hydroxydopamine]

We have studied the effects of calcium channel blockers cinarizine, flunarizine, nicardipine, nifedipine, verapamil and diltiazem on the motor anomalies which can be found in rats with unilateral nigrostriated lesion due to 6-OH-DA. All of them reduce rotation induced by apomorphine according to this order: nifedipine greater than nicardipine greater than diltiazem greater than flunarizine greater than verapamil greater than cinarizine. No effects on rotation induced by amphetamine have been observed. Our data suggest that calcium channel blockers cause dopamine receptor blockade but they do not modify the calcium-non-depending releasing of dopamine induced by amphetamine.

Chlordiazepoxide nonspecifically enhances consumption of saccharin solution

Following the establishment of a saccharin-amphetamine, saccharin-lithium or saccharin-saline association, rats were given a two-bottle test of preference for saccharin and water. Thirty min prior to the test, half of the rats were pretreated with chlordiazepoxide (9 mg/kg) and half of the rats were pretreated with saline. The results revealed that pretreatment with chlordiazepoxide (CDP) nonselectively enhanced saccharin consumption regardless of whether the flavored solution had been paired with amphetamine, lithium or saline. These results provide evidence that CDP enhances the palatability of flavored solutions.

Facilitating effect of amphetamine on the response of rabbit aortic strips to adrenaline, dopamine and serotonin

Amphetamine increased the response of rabbit aortic strips to adrenaline, dopamine and serotonin at consistently lower doses than those exerting a direct contracting effect. The amphetamine-facilitated contraction had the same shape as that produced by biogenic amines alone, whereas the contraction produced by amphetamine alone was more delayed and flatter. Serotonin and dopamine facilitated each other, but less markedly and with a narrower interval between facilitating and contracting doses than amphetamine. Pargyline exerted no facilitating effect on biogenic amines. Phentolamine and prazosin inhibited the direct response to adrenaline, dopamine and amphetamine, and the amphetamine-facilitated response to adrenaline and dopamine; they were inactive against serotonin alone and combined with a facilitating dose of amphetamine or dopamine. Cyproheptadine inhibited the direct response to serotonin and amphetamine, and the amphetamine-facilitated response to serotonin; it was inactive against dopamine and adrenaline both alone and combined with a facilitating dose of amphetamine or serotonin.

Effects of selective drugs for dopaminergic D1 and D2 receptors on conditioned locomotion in rats

Classically conditioned locomotor activity has been demonstrated by pairing injections of dopamine agonists or antagonists with specific environmental stimuli. The present studies investigated conditioning using drugs with varying selectivity for the dopamine D1 or D2 receptor. Experiment 1 assessed conditioning in groups of rats using the indirect acting agonist (+)-amphetamine (2.0 mg/kg), and the D1 agonist SKF 38393 (10.0 mg/kg), the D2 agonist quinpirole (2.5 mg/kg), the D1 and D2 antagonists, SCH 23390 (0.05 mg/kg) and metoclopramide (25.0 mg/kg), respectively. Paired groups received nine 2-h drug-environment (automated activity monitoring chambers) pairings whereas Unpaired groups received the stimuli explicitly unpaired. Test revealed conditioned hyperactivity with each agonist and metoclopramide whereas conditioned hypoactivity was seen with SCH 23390. Experiment 2 assessed the interaction of these agonists and antagonists on the establishment of conditioned activity. Paired groups received an agonist and antagonist during conditioning sessions. SCH 23390 blocked conditioning based on (+)-amphetamine and SKF 38393 but not quinpirole. Metoclopramide (10.0 mg/kg) blocked conditioning based on quinpirole but not SKF 38393. Metoclopramide (25.0 mg/kg) also did not block (+)-amphetamine-induced conditioning. These studies suggested that drug-induced alterations at either D1 or D2 receptors may be involved in conditioned locomotion.

Calcium channel blockers and behavioral sensitization

Behavioral sensitization to amphetamine-induced stereotypy was previously shown to consist of two separable phenomena, induction and expression, both of which involve the excitatory amino acids (EAA). In the present experiments, the calcium channel blockers (CCB), nifedipine, diltiazem and verapamil, were shown to block both phenomena; these results are similar to those reported earlier for DNQX, an antagonist of the non-N-methyl-D-aspartate receptors for the EAA. The CCB, like DNQX, affect only that percentage of the stereotypic response which results from the sensitization reaction, without affecting the quantitative portion of the response attributable to the acute effect of amphetamine. The results support previous conclusions that the sensitization response consists of two quantitative components, only one of which involves the EAA. The antagonism exhibited by the CCB suggests that behavioral sensitization involves Ca++ and L-type calcium channels.

Naloxone blockade of amphetamine place preference conditioning

Amphetamine and naloxone were examined in place conditioning, in order to study possible interactions between endogenous opioids and catecholamines in reinforcement. After initial preferences were determined, animals were conditioned with amphetamine alone (1.0 mg/kg SC), naloxone alone (0.02, 0.2 or 2.0 mg/kg SC) or combinations of amphetamine plus naloxone. A reliable, long-lasting preference for the compartment associated with amphetamine was observed, reflecting the reinforcing properties of this drug. No preference or aversion was observed in animals that received saline in both compartments. Naloxone (0.02, 0.2 and 2.0 mg/kg) produced a dose-dependent place aversion; while the lowest dose had effects similar to saline, the higher doses produced significant place aversions. Naloxone, at all three doses examined, prevented the ability of amphetamine to produce a place preference. Thus, the lowest dose of naloxone, having no effects alone in place conditioning was still able to block the reinforcing effects of amphetamine. These results suggest that the reinforcing effects of amphetamine are dependent on activation of opiate receptors, and provide further evidence that interactions between endogenous opioids and catecholamines may be important in reinforcement.

Studies on the participation of the dopaminergic system in the central effects of chelidonine

Chelidonine administered to rats in doses of 25, 50, 100 and 200 mg/kg ip exerted an inhibitory effect on the dopaminergic structures in the rat. It was shown that chelidonine decreased amphetamine- and apomorphine-induced hyperactivity and inhibited amphetamine and apomorphine sterotype. Besides, chelidonine significantly inhibited the yawning and penile erection produced by apomorphine. However, chelidonine potentiated also catalepsy caused by haloperidol. In doses of 100 and 200 mg/kg ip chelidonine depressed the whole brain dopamine (DA) concentrations and enhanced DA utilization.

Antagonism of cocaine, amphetamine, and methamphetamine toxicity

The effect of diazepam, haloperidol, MK-801, and propranolol in antagonizing behavioral symptoms induced by lethal doses of cocaine, amphetamine, and methamphetamine were studied in a rat model. Animals were first pretreated IP with potential antagonists, diazepam (2, 5, and 10 mg/kg), haloperidol (5, 10, and 20 mg/kg), propranolol (5, 10, and 20 mg/kg), MK-801 (0.5, 1.0, and 2.5 mg/kg), and then were challenged IP with cocaine (70 mg/kg) (LD85), d-amphetamine (75 mg/kg) (LD100), and methamphetamine (100 mg/kg) (LD90). Diazepam, at all doses, provided significant protection against cocaine- (p less than or equal to 0.01) and methamphetamine- (p less than or equal to 0.05) induced seizures and produced a dose-dependent effect against amphetamine-induced seizures. MK-801, at all doses, reduced seizures in all groups (p less than or equal to 0.01). Propranolol altered the incidence of methamphetamine-induced seizures. Significant protection against cocaine-induced death was afforded by diazepam (p less than or equal to 0.01) and propranolol (p less than or equal to 0.05). Significant protection against amphetamine-induced death was provided by haloperidol (all doses, p less than or equal to 0.1), MK-801 (all doses, p less than or equal to 0.1), and propranolol (10 and 20 mg/kg, p less than or equal to 0.1). No agent reduced the incidence of methamphetamine- (50 or 100 mg/kg) induced death. The failure of d-amphetamine antagonists to protect against methamphetamine-induced toxicity and death suggest that different mechanisms of toxicity may exist between these drugs.

Effects of ICI 169,369, a selective serotonin2 antagonist, in electrophysiological tests predictive of antipsychotic activity

Extracellular single unit recording techniques were used to compare the effects of ICI 169,369, a selective serotonin2 receptor antagonist, with the reference antipsychotic (AP) agents clozapine and haloperidol, in electrophysiological tests that may predict AP activity. ICI 169,369 was found to reverse the inhibitory actions of amphetamine on A9 and A10 dopamine (DA) neurons, a common property shared by other AP drugs, and was comparable in potency to clozapine. In cell population studies, acute treatment with ICI 169,369 (at a low dose only) and clozapine selectively increased the number of spontaneously active A10 DA cells, which was found to correlate with the ability of both these drugs to cause depolarization inactivation (DI) of A10 DA cells after chronic administration. Interestingly, chronic treatment with ICI 169,369 also caused a significant increase in the number of actively discharging A9 DA cells, an effect not predicted on the basis of the acute data. A similar effect was noted for clozapine, although the magnitude did not reach statistical significance. This profile of activity was unlike that of haloperidol, which acutely caused a nonselective increase in the number of active A9 and A10 DA cells, associated with the ability of this agent to cause DI of both A9 and A10 DA cells after chronic treatment. Inasmuch as DI of A10 DA cells may be correlated with AP efficacy whereas DI of A9 DA cells may predict the ability of an AP to cause extrapyramidal side effects, ICI 169,369, like clozapine, may be a potential AP with a reduced likelihood for producing extrapyramidal side effects.

[D-TRP11]-neurotensin, unlike alpha-flupenthixol, may not block amphetamine-induced hyperactivity in rats

Recent reports have suggested that neurotensin (NT) and some of its analogues resemble neuroleptics, for example alpha-flupenthixol (FLU), in their ability to suppress locomotor activity. The results obtained support this conclusion, but only if total photocell counts (the "traditional" index) are considered. An improved method of measuring activity--where subjects have to interrupt photocell beams in sequence before a ("conditional") count is registered--suggested, in direct contrast to the total counts index, that the stable analogue of neurotensin [D-Trp11]-neurotensin (DTNT) increased activity slightly (Sahgal and Keith, 1986). The present report is on the effects of DTNT (0-8 microgram/rat, i.c.v.) and FLU (0-1 mg/kg, i.p.) on hyperactivity induced by D-amphetamine (1.5 mg/kg, i.p.). The usual total counts index of activity suggested that FLU and DTNT blocked the increase. On the other hand, conditional count data suggested that only FLU was effective. Both measures indicated that FLU and amphetamine, administered separately, suppressed and enhanced activity, respectively. In contrast, DTNT at these doses had no significant effect on the conditional counts but markedly suppressed total counts. Subsequent observation of DTNT-treated rats placed in small open fields, suggested that the peptide induced marked circling (ambulatory) behaviour, at the cost of other behavioural categories, especially rearing and grooming. It is argued that (a) DTNT may not resemble neuroleptics in its effects on motor behaviour and (b) conditional activity counts, and also measures relating to brief interruptions of photocell beams, can provide useful additional information concerning motor activity.

5HT3 receptor antagonists block morphine- and nicotine- but not amphetamine-induced reward

The effect of two potent and specific antagonists of 5HT3 receptors, ICS 205-930 and MDL 72222, on the reinforcing properties of amphetamine, morphine and nicotine was studied in rats. Drug-induced reinforcement was assessed by measuring drug-conditioned place preference. ICS 205-930 and MDL 72222 dose-dependently reduced the place preference induced by morphine (1.0 mg/kg SC). At doses of 0.030 mg/kg SC the two antagonists completely blocked morphine-induced place preference while doses of 0.015 mg/kg SC significantly reduced it. ICS 205-930 and MDL 72222 at doses of 0.030 mg/kg SC also prevented the place preference induced by nicotine (0.6 mg/kg SC). In contrast, ICS 205-930 and MDL 72222 up to doses of 0.030 mg/kg SC failed to modify the place preference elicited by amphetamine (1.0 mg/kg SC). The results indicate that 5HT3 receptors are specifically involved in the reinforcing properties of morphine and nicotine.

Similarities between the stimulus properties of phenylpropanolamine and amphetamine

Rats at 80% body weight were trained to discriminate 1.0 mg/kg d-amphetamine versus saline in a two-lever, discrete trial drug discrimination task to obtain food pellets. After reliable discriminative control of lever choice was established, various doses of d,l-phenylpropanolamine (PPA, i.e., d,l-norephedrine) were substituted for the amphetamine training dose in non-reinforced test trials. Test doses of 10, 20, and 40 mg/kg PPA resulted in over 90% responses on the amphetamine-appropriate lever. Lower doses (1.25, 2.5, and 5.0 mg/kg) resulted in predominantly saline-appropriate responses. The generalization seen after the 20 mg/kg dose of phenylpropanolamine was blocked by pretreatment with 0.5 mg/kg haloperidol, suggesting that the generalization from amphetamine to PPA was mediated by a dopaminergic mechanism.

Block of conditioned avoidance responding in the rat by substituted phenylpiperazines

Ortho-methoxyphenylpiperazine (OMPP) and meta-substituted chlorophenylpiperazine (MCPP) blocked conditioned avoidance responding (CAR) in the rat (ED50 values = 5.6 (4.6, 7.3) and 2.4 (1.9, 2.9) mg/kg i.p. (95% confidence limits), respectively) without markedly altering escape responding. Since this test predicts antipsychotic efficacy, the piperazines were examined in radioligand binding assays and found to have no affinity for dopamine (DA) binding sites, but were active at serotonin binding sites. OMPP displaced ligands for the 5-HT1A binding site with high affinity (Ki = 9.5 (5.4, 17.9) nM) but was inactive at 5-HT2 sites (Ki greater than 1000 nM). MCPP, on the other hand, displaced ligands for 5-HT1, 5-HT1A and 5-HT2 binding sites with similar potencies (Ki values = 25 (3, 67), 23 (14, 40) and 40 (33, 48) nM, respectively). Pretreatment with metergoline (1.0 mg/kg i.p. -30 min) reduced MCPP- but not OMPP-induced block of CAR. OMPP, on the other hand, acted as a DA receptor antagonist in vivo blocking amphetamine-induced stereotyped behavior, whereas MCPP did not. Neither produced catalepsy even given in doses 8-10 times those required to block CAR. Insofar as these compounds lack antidopaminergic activity in vivo, yet are active in a test (CAR) predictive of antipsychotic activity in which DA receptor antagonists are active, they may be novel antipsychotic agents, or, perhaps, false positives in the CAR paradigm.

Naltrexone blocks amphetamine-induced hyperactivity, but not disruption of social and agonistic behavior in mice and squirrel monkeys

Significant anatomical overlap of opioid and dopamine receptors as well as reciprocity of control over synthesis, metabolism, and release of opioid peptides and dopamine in brain suggests functional interactions between the two systems. In the first of two studies, the behavioral effects of amphetamine and naltrexone alone, and in combination were studied in established groups of socially interacting squirrel monkeys. Naltrexone (0.1-10.0 mg/kg, IM) increased locomotion and marking behavior in subordinate monkeys. The frequency of social initiatives directed at treated subordinate monkeys by untreated members of the group was also increased. The behavior of dominant monkeys was relatively unaffected, except at the highest dose when autonomic distress was also evident. The frequency of walking bouts by both dominant and subordinate monkeys was increased by amphetamine (0.1-0.6 mg/kg, IM), and the social behavior of dominant monkeys was disrupted by drug treatment. Naltrexone (0.1 mg/kg, IM) significantly antagonized amphetamine's effects on motor behavior, and enhanced or did not affect amphetamine's effects on social behavior. In a second study, the interaction of amphetamine (0.63-10.0 mg/kg, IP) and naltrexone (0.1-10.0 mg/kg, IP) on the behavior of resident male mice during confrontations with a male intruder was studied. Naltrexone selectively reduced the frequency of attack at the highest dose tested. Amphetamine increased locomotor activity and decreased attack and threat behavior in resident mice. A low dose of naltrexone (1.0 mg/kg, IP) blocked amphetamine's effects on locomotion and enhanced the disruption of aggressive behavior. The amphetamine-naltrexone interaction on locomotor activity in mice and monkeys is consistent with opioid receptor modulation of dopamine mediated functions.(ABSTRACT TRUNCATED AT 250 WORDS)

Metaphit prevents locomotor activation induced by various psychostimulants and interferes with the dopaminergic system in mice

Metaphit, an isothiocyanate analog of phencyclidine and a proposed phencyclidine receptor acylator, inactivated the carrier involved in the neuronal uptake of dopamine in in vitro experiments with preparations of the striatum in the mouse. In ex vivo experiments 2 and 24 hr after the intravenous administration of metaphit, no changes were observed either in the binding of [3H]cocaine to striatal membranes or in the uptake of [3H]dopamine into synaptosomes or slices. In in vivo experiments 24 hr after pretreatment with metaphit, selective labelling of uptake sites for dopamine in the striatum of the mouse with [3H]GBR 12935 was unaffected. In these in vivo experiments, however, metaphit antagonized the locomotor stimulation induced by blockers of the uptake of dopamine (methylphenidate, mazindol, cocaine, GBR 12909) but not that induced by drugs that affect locomotion by other mechanisms (amphetamine, phencyclidine). Twenty-four hours after treatment with metaphit there was an increase in homovanillic acid in all regions of the brain studied (striatum, olfactory tubercle, cerebral cortex). There was no effect of metaphit on the disappearance rate of 3,4-dihydroxyphenylacetic acid and homovanillic acid from the striatum during the inhibition of monoamine oxidase with pargyline. If the increase in homovanillic acid reflected a greater rate of dopamine catabolism in metaphit-treated mice, it could explain the lack of locomotor stimulation of blockers uptake of the dopamine in these animals, resulting from a rapid breakdown of extracellularly accumulated dopamine.

The selective dopamine antagonist properties of BRL 34778: a novel substituted benzamide

BRL 34778, exo-4-amino-5-chloro-2-methoxy-N-[9-(4-fluorophenylmethyl)-9-azabi cyclo(3.3.1) non-3-yl]-benzamide, is a very potent and specific dopamine D2-receptor antagonist. It exhibits a Ki value of 2.14 nM for dopamine D2-receptors but much lower affinity for D1-(Ki 5700 nM) and for many other receptor types. Its action is potent and of long duration in models for antipsychotic activity (4 h ED50s PO are 0.017 mg/kg for antagonism of apomorphine-induced climbing in mice and 0.028 mg/kg for antagonism of amphetamine-induced locomotion in rats). In contrast to its high potency in models for antipsychotic activity, BRL 34778 is much weaker in models for extrapyramidal effects and sedation (4 h ED50s PO are 2.4 mg/kg for inducing catalepsy in rats and 1.14 mg/kg for inhibition of rearing behaviour in rats). These data indicate potent activity of BRL 34778 in models for antipsychotic activity but low activity in models for extrapyramidal effects and sedation.

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.

[Pyrido[2,3-b][1,4]diazepinones with CNS activity]

In the interests of developing our research on compounds with a pyrazinone nucleus, cyclohomologues, characterised by the presence of one diazepinone nucleus, were prepared. The 5-[(dialkylamino)alkyl]-3,5-dihydro-2-methyl/phenyl-4H-pyrido[2,3- b][1,4]diazepin-4-ones obtained by means of condensation of the 2-(dialkylamino)alkylamino-3-aminopyridines with ethyl acetyl- or benzoyl- acetate, were subjected to pharmacological experimentation in order to evaluate their effect upon mice with regard to exploratory activity, motor coordination, and spontaneous activity. In addition their analgesic activity was evaluated and also their anti-strychnine, anti-cardiazole, anti-amphetamine and anti-reserpine activities.

Antagonism of AP-5- and amphetamine-induced behaviour by timelotem as compared with clozapine and haloperidol

Bilateral intrastriatal injection of DL-2-amino-5-phosphonovaleric acid (AP-5), that blocks glutamatergic transmission at the N-methyl-d-aspartate preferring receptor, induces sniffing and body turns and reduces grooming in rats. Timelotem, a representative of the newly developed chemical class of anellated benzodiazepines antagonized specifically AP-5-induced sniffing and body turns. Classical (haloperidol) as well as atypical (clozapine) neuroleptics had recently been shown to antagonize AP-5-induced sniffing; clozapine, like timelotem, but not haloperidol, additionally antagonized AP-5-induced body turns. Further, timelotem antagonized amphetamine-induced stereotyped behaviour in rats, but was found less active than haloperidol in this test. Comparing the activity of drugs in both paradigms revealed that haloperidol inhibited AP-5-induced sniffing and amphetamine-induced stereotypies within the same dose range, but timelotem and clozapine were found more potent in the AP-5 test than in the amphetamine test. Thus, detailed drug profiles discriminate timelotem and clozapine from haloperidol, linking timelotem again to atypical antipsychotic compounds.

Electrophysiological evaluation of a partial agonist of dopamine receptors

The ergot derivative trans-dihydrolisuride (TDHL) was tested for its effects on firing rates of dopaminergic (DA) neurons located in the substantia nigra pars compacta of chloral hydrate-anesthetized rats. Using extracellular single-barreled microelectrodes, DA neurons were identified by their long duration, positive-negative action potentials and their slow bursting pattern of spontaneous firing, as well as by the location of recording sites through histological recoveries of dye deposits. Like haloperidol and clozapine, which are full DA receptor antagonists, TDHL antagonized the depression in DA neuron firing induced by systemic amphetamine. However, where full antagonists completely reversed the amphetamine effect, TDHL could do so only partially, the maximal effect being around half. Like DA agonists, but unlike DA antagonists, TDHL also depressed the spontaneous firing rates of DA neurons. But whereas the full agonist apomorphine completely inhibited firing of DA neurons, TDHL only depressed firing rates by about half, even at high doses. These data support the contention that TDHL is a partial DA agonist.

Blockade of D-1 receptors by SCH 23390 antagonizes morphine- and amphetamine-induced place preference conditioning

Morphine (0.5 mg/kg s.c.) and amphetamine (1 mg/kg s.c.) used in a two-compartment place preference test induced strong place preference when paired to the non-preferred environments. They did not modify preference under basal conditions but completely reversed morphine- and amphetamine-induced place preference. Pairing of haloperidol (0.2 mg/kg s.c.) to both environments also abolished the morphine-induced place preference.

Prevention of amphetamine-induced behavioral hypersensitivity by concomitant treatment with microgram doses of apomorphine

Behavioral hypersensitivity induced by subchronic administration of amphetamine in mice was inhibited by concomitant administration of apomorphine in microgram doses. Acutely, amphetamine followed in 15 min by apomorphine, produces a normal rise in locomotor activity subsequent to the action of amphetamine and an inhibition of that activity by the modulatory action of apomorphine. We suggest that apomorphine acts at a presynaptic neuronal site to interrupt the process(es) responsible for the development of stimulant-induced sensitization.

Effect of cysteine on the persistent depletion of brain monoamines by amphetamine, p-chloroamphetamine and MPTP

The administration of L-cysteine (500 mg/kg i.p.) 30 min before and 5 h after the administration of (+)-amphetamine sulfate markedly attenuated the persistent decreases in striatal dopamine (DA), dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in rats one week after the administration of a single dose of amphetamine (9.2 mg/kg i.p.) to iprindole-treated animals and in mice one week after the last of four daily injections of amphetamine (30 mg/kg i.p.). Cysteine prevented the persistent decreases in striatal serotonin (5HT) and 5-hydroxyindoleacetic acid (5HIAA) one week after the administration of p-chloroamphetamine to rats, but failed to alter the persistent decreases in striatal DA, DOPAC and HVA in mice one week after the last of four daily doses of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, 30 mg/kg s.c.). The results suggest that the mechanisms by which amphetamine and p-chloroamphetamine, but not MPTP, produce persistent depletions of striatal monoamines involve the generation of neurotoxic electrophilic intermediates which can be inactivated by the administration of cysteine.