Evidence that alpha-methyl-p-tyramine is implicated in behavioural augmentation to amphetamine

The role of CYP2D in rat brain in methamphetamine-induced striatal dopamine and serotonin release and behavioral sensitization

RATIONALE

Cytochrome P450 2D (CYP2D) enzymes metabolize many addictive drugs, including methamphetamine. Variable CYP2D metabolism in the brain may alter CNS drug/metabolite concentrations, consequently affecting addiction liability and neuropsychiatric outcomes; components of these can be modeled by behavioral sensitization in rats.

METHODS

To investigate the role of CYP2D in the brain in methamphetamine-induced behavioral sensitization, rats were pretreated centrally with a CYP2D irreversible inhibitor (or vehicle) 20 h prior to each of 7 daily methamphetamine (0.5 mg/kg subcutaneous) injections. In vivo brain microdialysis was used to assess brain drug and metabolite concentrations, and neurotransmitter release.

RESULTS

CYP2D inhibitor (versus vehicle) pretreatment enhanced methamphetamine-induced stereotypy response sensitization. CYP2D inhibitor pretreatment increased brain methamphetamine concentrations and decreased the brain p-hydroxylation metabolic ratio. With microdialysis conducted on days 1 and 7, CYP2D inhibitor pretreatment exacerbated stereotypy sensitization and enhanced dopamine and serotonin release in the dorsal striatum. Day 1 brain methamphetamine and amphetamine concentrations correlated with dopamine and serotonin release, which in turn correlated with the stereotypy response slope across sessions (i.e., day 1 through day 7), used as a measure of sensitization.

CONCLUSIONS

CYP2D-mediated methamphetamine metabolism in the brain is sufficient to alter behavioral sensitization, brain drug concentrations, and striatal dopamine and serotonin release. Moreover, day 1 methamphetamine-induced neurotransmitter release may be an important predictor of subsequent behavioral sensitization. This suggests the novel contribution of CYP2D in the brain to methamphetamine-induced behavioral sensitization and suggests that the wide variation in human brain CYP2D6 may contribute to differential methamphetamine responses and chronic effects.

p-Hydroxyamphetamine causes prepulse inhibition disruption in mice: contribution of serotonin neurotransmission

p-Hydroxyamphetamine (p-OHA) has been shown to have a number of pharmacological actions, including causing abnormal behaviors such as increased locomotor activity and head-twitch response in rodents. We have recently reported that intracerebroventricular (i.c.v.) administration of p-OHA dose-dependently induces prepulse inhibition (PPI) disruption in mice, which is attenuated by pretreatment with haloperidol, clozapine or several dopaminergic agents. Haloperidol and clozapine have affinities for serotonergic (especially 5-HT(2A)) receptors. To investigate the involvement of the central serotonergic systems in p-OHA-induced PPI disruption, herein we tested several serotonergic agents to determine their effects on p-OHA-induced PPI disruption. p-OHA-induced PPI disruption was attenuated by pretreatment with 5,7-dihydroxytryptamine (5,7-DHT, a neurotoxin which targets serotonin-containing neurons) and p-chlorophenylalanine (PCPA, a serotonin synthesis inhibitor). p-OHA-induced PPI disruption was also attenuated by pretreatment with ketanserin (a 5-HT(2A/2C) receptor antagonist) and MDL100,907 (a selective 5-HT(2A) receptor antagonist). These data suggest that p-OHA-induced PPI disruption may involve increased serotonin release into the synaptic cleft, which then interacts with the post-synaptic 5-HT(2A) receptor.

p-Hydroxyamphetamine causes prepulse inhibition disruptions in mice: contribution of dopamine neurotransmission

It is well known that amphetamine induces disrupted prepulse inhibition (PPI) in humans and rodents. We have previously reported that intracerebroventricular (i.c.v.) administration of p-hydroxyamphetamine (p-OHA) induces multiple behavioral responses, such as increased locomotor activity and head-twitch response in rodents. To reveal the characteristics of p-OHA on sensorimotor function in rodents, herein we tested the effects of p-OHA on PPI in mice. i.c.v. administration of p-OHA dose-dependently induced PPI disruptions for all prepulse intervals tested. This effect of p-OHA on PPI was attenuated by pretreatment with haloperidol or clozapine. p-OHA-induced PPI disruptions were also attenuated by pretreatment with L-741,626 (a selective D(2) receptor antagonist), L-745,870 (a selective D(4) receptor antagonist) or 6-hydroxydopamine (a neurotoxin which targets DA-containing neurons), but not by SCH 23390 (a selective D(1) receptor antagonist), eticlopride (a D(2)/D(3) receptor antagonist) or GBR 12909 (a DA-reuptake inhibitor). These results indicate that selective blockade of either the D(2) or D(4) receptor subtype may prevent disruption of PPI induced by p-OHA via presynaptic DA release.

Central administration of p-hydroxyamphetamine produces a behavioral stimulant effect in rodents: evidence for the involvement of dopaminergic systems

RATIONALE AND OBJECTIVES

It is well-known that amphetamine induces increased locomotor activity in rodents. We previously found that intracerebroventricular (i.c.v.) administration of p-hydroxyamphetamine (p-OHA), an amphetamine metabolite, increases synaptic dopamine (DA) levels in the striatum. In the present study, we investigated the effect of p-OHA on locomotor activity in rodents.

RESULTS

In mice, i.c.v. administration of p-OHA significantly increased locomotor activity in a dose-dependent manner. p-Hydroxynorephedrine, another amphetamine metabolite, did not increase locomotor activity. This effect of p-OHA was inhibited by pretreatment with nomifensine, a dopamine-uptake inhibitor, but not by fluoxetine, a serotonin-uptake inhibitor, or diethyldithiocarbamate, a dopamine-beta-hydroxylase inhibitor. Furthermore, we tested the effects of microinjections of p-OHA into the rat nucleus accumbens (NAc) on locomotor activity. Local infusion of p-OHA into the NAc significantly increased locomotor activity. As in mice, the increased locomotor activity induced by p-OHA microinjection into the NAc in rats was inhibited by nomifensine.

CONCLUSIONS

These data suggest that dopaminergic systems in the NAc may play important roles in p-OHA-induced locomotor activity in rodents.

Reduced CYP2D6 activity is a negative risk factor for methamphetamine dependence

Because methamphetamine (METH) is metabolized by CYP2D6 at the first step of hydroxylation and demethylation, it is possible that functional variants of CYP2D6 alter susceptibility to methamphetamine-induced dependence. We genotyped CYP2D6*1, *4, *5, *10, and *14 for 202 patients with METH dependence and 337 controls in a Japanese population and found a significant association of the CYP2D6 gene with METH dependence (p=0.0299). The patients had fewer *10 and *14 alleles, which are hypofunction alleles, than the controls. CYP2D6 genotypes were divided into three phenotypes: extensive metabolizers, intermediate metabolizers, and poor metabolizers. There was no poor metabolizer among our Japanese subjects, and intermediate metabolizers of CYP2D6 were significantly fewer in methamphetamine-dependent subjects than in controls (p=0.0212), with an odds ratio of 0.62 (95% confidence interval: 0.51-0.76). The present study demonstrated that reduced CYP2D6 activity was a negative risk factor for methamphetamine dependence.

Pharmacologic and toxicologic effects of di(beta-phenylisopropyl)amine (DPIA) in rats and mice

1. Di(beta-phenylisopropyl)amine (DPIA) given i.p. to mice and rats in sublethal doses caused increased motility, mild stereotypic behavior and suppression of food intake. Repeated daily doses led to enhanced motor stimulation, and in one group, 40% lethality, indicating development of "reverse tolerance". Brain monoamine modifiers prevented DPIA-induced motor activity. 2. Treatment with toxic i.p. doses of DPIA enabled determination of the LD50, which was 106.8 mg/kg for isolated mice and 89.7 mg/kg for mice kept in aggregation after dosing. Possible antidotal agents given before a high DPIA dose (LD50) protected significantly against lethality. 3. Combinations of DPIA with (+)-amphetamine in mice at lethal doses showed a subadditive synergism. 4. Effects of DPIA on the cardiovascular system, both i.v. in anesthetized rats and in isolated atrial preparations, were mainly opposite to those of (+)-amphetamine, namely decreases in blood pressure, force of contraction and heart rate.

Modification of behavioral responses to methamphetamine evoked by the stimulant's metabolite p-hydroxynorephedrine in rats

The central effect of p-hydroxynorephedrine (OH-NE), one of the p-hydroxylated metabolites of methamphetamine (MAP) and amphetamine (AMP), was investigated in rats. Locomotion and stereotypy were examined after SC injections of 0.5-5 mg/kg of MAP or 0.02-0.5 mg/kg of apomorphine (APO) in animals treated with either saline or 5-50 mg/kg of OH-NE IP 20 hr before behavioral assessment. The locomotor stimulating effect of both 0.5-2 mg/kg of MAP and 0.2 mg/kg of APO was enhanced by 5 mg/kg of OH-NE. On the other hand, 30 mg/kg of OH-NE severely suppressed the stimulating effect of MAP but had no influence on that induced by 0.2 mg/kg of APO. The stereotypy induced by 5 mg/kg of MAP or 0.5 mg/kg of APO was enhanced and prolonged in the OH-NE-treated rats. Subsequently, examinations were performed to determine whether OH-NE had any effect on the dopaminergic mechanism. Hypomotility induced by 0.02 mg/kg of APO was alleviated by 5 mg/kg of OH-NE, but was aggravated by 30 mg/kg. These results suggest that OH-NE administered prior to SC injections of MAP or APO influences their behavioral effects via the dopaminergic mechanism. The possibility that other neural mechanisms may be involved in this OH-NE-induced behavioral modification is also discussed.