Subchronic methamphetamine treatment enhances methamphetamine- or cocaine-induced dopamine efflux in vivo

Involvement of the dopaminergic system in the reward-related behavior of pregabalin

There has been an increase in cases of drug addiction and prescription drug abuse worldwide. Recently, pregabalin abuse has been a focus for many healthcare agencies, as highlighted by epidemiological studies. We previously evaluated the possibility of pregabalin abuse using the conditioned place preference (CPP) paradigm. We observed that a 60 mg/kg dose could induce CPP in mice and that pregabalin-rewarding properties were mediated through glutamate neurotransmission. Notably, the dopaminergic reward circuitry is also known to play a crucial role in medication-seeking behavior. Therefore, this study aimed to explore the possible involvement of dopaminergic receptor-1 in pregabalin-induced CPP. Mice were randomly allocated to receive saline or the dopamine-1 receptor antagonist SKF-83566 (0.03 mg/kg, intraperitoneal). After 30 min, the mice received either saline or pregabalin (60 mg/kg) during the conditioning phase. Among the control groups that received saline or SKF-83566, the time spent in the two conditioning chambers was not significantly altered. However, among the pregabalin-treated group, there was a marked increase in the time spent in the drug-paired chamber compared to the time spent in the vehicle-paired chamber. Notably, blocking dopamine-1 receptors with SKF-83566 completely prevented pregabalin-induced place preference, thus demonstrating the engagement of the dopaminergic system in pregabalin-induced reward-related behavior.

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.

Sub-chronic taurine administration induces behavioral sensitization but does not influence ethanol-induced dopamine release in the nucleus accumbens

Preclinical studies have shown that the amino acid taurine is of importance for the dopamine elevating properties of ethanol. Taurine intake has escalated over the last decade due to increased consumption of taurine-containing energy drinks and dietary supplements. Whether long-term intake of large amounts of taurine induces adaptations affecting ethanol-induced dopamine elevation is not clear. Thus the aim of the present studies was to explore the impact of repeated administration of large amounts of taurine on ethanol-induced behavior and dopamine neurotransmission. Repeated daily systemic administration of taurine increased taurine-induced locomotor activity and rearing. Acute administration of taurine and ethanol in naïve animals produced an additive effect on extracellular taurine but no alteration of the ethanol-induced dopamine elevation, as measured by in vivo microdialysis. Sub-chronic administration of taurine did not modify the taurine- or dopamine-elevating properties of ethanol. Daily taurine treatment also failed to change the mRNA expression of the taurine transporter and GABA_A- and glycine-receptor subunits, as measured by qPCR in nucleus accumbens tissue. We conclude that systemic administration of taurine may have long lasting central effects, here displayed as behavioral sensitization. However, repeated daily exposure to taurine does not appear to influence the dopamine elevating properties of ethanol.

A Case Study of Acute Stimulant-induced Psychosis

Psychosis resulting from stimulant overuse is commonly observed in clinical practices today. This is in large part due to the significant increase in attention-deficit/hyperactive disorder (ADHD) diagnoses in recent years, the increase in methamphetamine trafficking, and the prevalence of stimulant use in the day-to-day activities by many of those living in today’s fast-paced society. The current inability to predict those users who will experience stimulant-induced psychosis from those users who will not needs further investigation.

In this case study, we examine why one particular stimulant user experiences stimulant-induced psychosis. We give an account of a patient with an acute psychotic episode admitted to the emergency room at a local hospital. During initial evaluation, it was discovered that he had been diagnosed with ADHD one month ago and that the onset of his symptoms was likely related to an overuse of amphetamines. The patient had no personal history or family history of any psychiatric or mood disorders. He had no history of recreational drug use prior to this incident, and had no history of seizures or head trauma. After appropriate pharmacological intervention with antipsychotics and psychological intervention, the patient showed gradual improvement over the next five days of his hospitalization. After the five days, he was allowed to be discharged.

We conclude with the cautionary advice that a patient’s susceptibility of experiencing stimulant-induced psychosis should be assessed before prescribing stimulants. In cases where stimulant use is not supervised by a physician, appropriate understanding of management of stimulant-induced psychosis is of utmost importance in order to provide the very best patient education and care.

Significance of protein kinase C in the neuropsychotoxicity induced by methamphetamine-like psychostimulants

The abuse of methamphetamine (MA), an amphetamine (AMPH)-type stimulant, has been demonstrated to be associated with various neuropsychotoxicity, including memory impairment, psychiatric morbidity, and dopaminergic toxicity. Compelling evidence from preclinical studies has indicated that protein kinase C (PKC), a large family of serine/threonine protein kinases, plays an important role in MA-induced neuropsychotoxicity. PKC-mediated N-terminal phosphorylation of dopamine transporter has been identified as one of the prerequisites for MA-induced synaptic dopamine release. Consistently, it has been shown that PKC is involved in MA (or AMPH)-induced memory impairment and mania-like behaviors as well as MA drug dependence. Direct or indirect regulation of factors related to neuronal plasticity seemed to be critical for these actions of PKC. In addition, PKC-mediated mitochondrial dysfunction, oxidative stress or impaired antioxidant defense system has been suggested to play a role in psychiatric and cognitive disturbance induced by MA (or AMPH). In MA-induced dopaminergic toxicity, particularly PKCδ has been shown to trigger oxidative stress, mitochondrial dysfunction, pro-apoptotic changes, and neuroinflammation. Importantly, PKCδ may be a key mediator in the positive feedback loop composed of these detrimental events to potentiate MA-induced dopaminergic toxicity. This review outlines the role of PKC and its individual isozymes in MA-induced neuropsychotoxicity. Better understanding on the molecular mechanism of PKCs might provide a great insight for the development of potential therapeutic or preventive candidates for MA (or AMPH)-associated neuropsychotoxicity.

Differential roles of dopamine D1 and D2 receptor-containing neurons of the nucleus accumbens shell in behavioral sensitization

The nucleus accumbens (Nac) mediates the reinforcing and motor stimulating properties of psychostimulants. It receives dopaminergic afferents from the ventral midbrain and is divided into two distinct subregions: shell and core. Each of these contains two subtypes of medium spiny neurons, which express either dopamine D1 (D1R) or D2 (D2R) receptors. However, functional dissociation between the two subtypes in psychostimulant response remains to be elucidated. We performed selective ablation of each subtype in the Nac shell in mice, using immunotoxin-mediated cell targeting, and examined the behavioral sensitization evoked by repeated administration of methamphetamine. The D1R cell-ablated mice exhibited delayed induction of sensitized locomotion compared to control mice, whereas the D2R cell-ablated mice showed a mildly enhanced rate of induction of sensitization. In vivo microdialysis revealed a marked blockade of the increase in extracellular dopamine in the Nac of the D1R cell-ablated animals in response to methamphetamine, indicating that the observed delay in behavioral sensitization in these mice involves an impairment in accumbal dopamine release. Our results reveal differential roles of D1R- and D2R-containing accumbal shell neurons in the development of behavioral sensitization to psychostimulants. Behavioral sensitization, enhanced motility by repetitive psychostimulant administration, is a model of drug addiction. Here, we show that the nucleus accumbens (Nac) shell neurons containing dopamine D1 receptor (D1R) or D2 receptor (D2R) play distinct roles in behavioral sensitization triggered by methamphetamine, and that D1R-containing neurons enhance the induction of behavioral sensitization at the early phase, whereas D2R-containing neurons act to suppress the rate of development of the behavior.

Methamphetamine downregulates striatal glutamate receptors via diverse epigenetic mechanisms

BACKGROUND

Chronic methamphetamine (METH) exposure causes neuroadaptations at glutamatergic synapses.

METHODS

To identify the METH-induced epigenetic underpinnings of these neuroadaptations, we injected increasing METH doses to rats for 2 weeks and measured striatal glutamate receptor expression. We then quantified the effects of METH exposure on histone acetylation. We also measured METH-induced changes in DNA methylation and DNA hydroxymethylation.

RESULTS

Chronic METH decreased transcript and protein expression of GluA1 and GluA2 alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) and GluN1 N-methyl-D-aspartate receptor subunits. These changes were associated with altered electrophysiological glutamatergic responses in striatal neurons. Chromatin immunoprecipitation-polymerase chain reaction revealed that METH decreased enrichment of acetylated histone H4 on GluA1, GluA2, and GluN1 promoters. Methamphetamine exposure also increased repressor element-1 silencing transcription factor (REST) corepressor 1, methylated CpG binding protein 2, and histone deacetylase 2 enrichment, but not of sirtuin 1 or sirtuin 2, onto GluA1 and GluA2 gene sequences. Moreover, METH caused interactions of REST corepressor 1 and methylated CpG binding protein 2 with histone deacetylase 2 and of REST with histone deacetylase 1. Surprisingly, methylated DNA immunoprecipitation and hydroxymethylated DNA immunoprecipitation-polymerase chain reaction revealed METH-induced decreased enrichment of 5-methylcytosine and 5-hydroxymethylcytosine at GluA1 and GluA2 promoter sequences. Importantly, the histone deacetylase inhibitor, valproic acid, blocked METH-induced decreased expression of AMPAR and N-methyl-D-aspartate receptor subunits. Finally, valproic acid also attenuated METH-induced decrease H4K16Ac recruitment on AMPAR gene sequences.

CONCLUSIONS

These observations suggest that histone H4 hypoacetylation may be the main determinant of METH-induced decreased striatal glutamate receptor expression.

Effects of amphetamine on subcellular distribution of dopamine and DOPAC

Amphetamine effects on distribution of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), and amphetamine in vesicular, cytosolic, and extracellular compartments associated with a striatal varicosity were estimated through use of a computer simulation model. In addition, contribution to overall effects of amphetamine by each of five actions--transport by dopamine transporter (DAT), transport by vesicular monoamine transporter, stimulation of reverse transport, inhibition of monoamine oxidase (MAO), and slowing of dopamine cell firing rate--were evaluated. Amphetamine enters a varicosity almost entirely by DAT and accumulates to very high levels within the varicosity. Both reverse transport by DAT and passive diffusion contribute to continual amphetamine egress across the plasma membrane. Amphetamine enters storage vesicles by both transport and diffusion. The transport portion competes with dopamine storage, resulting in redistribution of approximately half of dopamine from vesicles to cytosol. The high concentration of amphetamine in the cytosol inhibits MAO, protecting cytosolic dopamine. A very small fraction of cytosolic dopamine is moved to extracellular compartment via reverse transport by DAT. The amount of dopamine moved by reverse transport is limited because of competition by very high cytosolic levels of amphetamine. In the presence of amphetamine, rate of dopamine transfer to extracellular compartment is less than control; however, high levels of extracellular dopamine are maintained because amphetamine occupies the DAT, thus limiting dopamine reuptake. Simulation output from a model using exchange-diffusion mechanism of reverse transport does not match all published data that were simulated, suggesting that inward transport of a substrate is not required to initiate reverse transport.

Distinct neurochemical adaptations within the nucleus accumbens produced by a history of self-administered vs non-contingently administered intravenous methamphetamine

Methamphetamine is a highly addictive psychomotor stimulant yet the neurobiological consequences of methamphetamine self-administration remain under-characterized. Thus, we employed microdialysis in rats trained to self-administer intravenous (IV) infusions of methamphetamine (METH-SA) or saline (SAL) and a group of rats receiving non-contingent IV infusions of methamphetamine (METH-NC) at 1 or 21 days withdrawal to determine the dopamine and glutamate responses in the nucleus accumbens (NAC) to a 2 mg/kg methamphetamine intraperitoneal challenge. Furthermore, basal NAC extracellular glutamate content was assessed employing no net-flux procedures in these three groups at both time points. At both 1- and 21-day withdrawal points, methamphetamine elicited a rise in extracellular dopamine in SAL animals and this effect was sensitized in METH-NC rats. However, METH-SA animals showed a much greater sensitized dopamine response to the drug challenge compared with the other groups. Additionally, acute methamphetamine decreased extracellular glutamate in both SAL and METH-NC animals at both time-points. In contrast, METH-SA rats exhibited a modest and delayed rise in glutamate at 1-day withdrawal and this rise was sensitized at 21 days withdrawal. Finally, no net-flux microdialysis revealed elevated basal glutamate and increased extraction fraction at both withdrawal time-points in METH-SA rats. Although METH-NC rats exhibited no change in the glutamate extraction fraction, they exhibited a time-dependent elevation in basal glutamate levels. These data illustrate for the first time that a history of methamphetamine self-administration produces enduring changes in NAC neurotransmission and that non-pharmacological factors have a critical role in the expression of these methamphetamine-induced neurochemical adaptations.

Association between the Regulator of G-protein Signaling 9 Gene and Patients with Methamphetamine Use Disorder and Schizophrenia

The regulator of G-protein signaling (RGS) modulates the functioning of heterotrimeric G protein. RGS9-2 is highly expressed in the striatum and plays a role in modulating dopaminergic receptor-mediated signaling cascades. Previous studies suggested that the RGS9 gene might contribute to the susceptibility to psychotic diseases. Therefore, we investigated the association between the RGS9 gene and two related dopamine psychoses, schizophrenia and methamphetamine use disorders. The subjects comprised 487 patients of schizophrenia and 464 age- and sex-matched healthy controls and 220 patients of methamphetamine use disorder and 289 controls. We genotyped two nonsynonymous polymorphisms, rs12452285 (Leu225Ser) and rs34797451 (His498Arg), of the RGS9 gene. Rs34797451 showed monomorphism in the present Japanese population, but rs12452285 showed polymorphism. There were no significant differences in genotypic or allelic distributions of rs12452285 between patients with schizophrenia and the corresponding control or between patients with methamphetamine use disorder and the corresponding control. We also analyzed the clinical features of methamphetamine use disorder. We found a significant association in allelic distribution with the phenotypes of age at first consumption (p=0.047). The present study suggested that the RGS9 gene is unlikely to play a major role in schizophrenia and methamphetamine dependence liability and/or the development of methamphetamine induced psychosis, at least in a Japanese population.

Methamphetamine-associated psychosis

Methamphetamine (METH) is a frequent drug of abuse in U.S. populations and commonly associated with psychosis. This may be a factor in frequent criminal justice referrals and lengthy treatment required by METH users. Persecutory delusions and auditory hallucinations are the most consistent symptoms of METH-associated psychosis (MAP). MAP has largely been studied in Asian populations and risk factors have varied across studies. Duration, frequency and amount of use as well as sexual abuse, family history, other substance use, and co-occurring personality and mood disorders are risk factors for MAP. MAP may be unique with its long duration of psychosis and recurrence without relapse to METH. Seven candidate genes have been identified that may be associated with MAP. Six of these genes are also associated with susceptibility, symptoms, or treatment of schizophrenia and most are linked to glutamatergic neurotransmission. Animal studies of pre-pulse inhibition, attenuation of social interaction, and stereotypy and alterations in locomotion are used to study MAP in rodents. Employing various models, rodent studies have identified neuroanatomical and neurochemical changes associated with METH use. Throughout this review, we identify key gaps in our understanding of MAP and suggest potential directions for future research.

Selective breeding for magnitude of methamphetamine-induced sensitization alters methamphetamine consumption

RATIONALE

Genetically determined differences in susceptibility to drug-induced sensitization could be related to risk for drug consumption.

OBJECTIVES

Studies were performed to determine whether selective breeding could be used to create lines of mice with different magnitudes of locomotor sensitization to methamphetamine (MA). MA sensitization (MASENS) lines were also examined for genetically correlated responses to MA.

METHODS

Beginning with the F2 cross of C57BL/6J and DBA/2J strains, mice were tested for locomotor sensitization to repeated injections of 1 mg/kg MA and bred based on magnitude of sensitization. Five selected offspring generations were tested. All generations were also tested for MA consumption, and some were tested for dose-dependent locomotor-stimulant responses to MA, consumption of saccharin, quinine, and potassium chloride as a measure of taste sensitivity, and MA clearance after acute and repeated MA.

RESULTS

Selective breeding resulted in creation of two lines [MA high sensitization (MAHSENS) and MA low sensitization (MALSENS)] that differed in magnitude of MA-induced sensitization. Initially, greater MA consumption in MAHSENS mice reversed over the course of selection so that MALSENS mice consumed more MA. MAHSENS mice exhibited greater sensitivity to the acute stimulant effects of MA, but there were no significant differences between the lines in MA clearance from blood.

CONCLUSIONS

Genetic factors influence magnitude of MA-induced locomotor sensitization and some of the genes involved in magnitude of this response also influence MA sensitivity and consumption. Genetic factors leading to greater MA-induced sensitization may serve a protective role against high levels of MA consumption.

One-trial cocaine-induced behavioral sensitization in preweanling rats: role of contextual stimuli

Using a one-trial procedure, preweanling rats exhibit robust sensitization regardless of whether drug pretreatment and testing occur in the same or different environments. The purpose of the present study was to determine whether one-trial context-specific and context-independent sensitization of preweanling rats could be dissociated by varying the pretreatment dose of cocaine, by varying the pretreatment drug, or by minimizing interoceptive cues. In Experiments 1a and 1b, rats were pretreated with a broad dose range of cocaine (0-40 mg/kg) before placement in a novel activity chamber or the home cage. In Experiment 2, rats were pretreated with a locomotor-enhancing drug (e.g., methylphenidate, U50,488, or MK-801) before placement in a novel activity or anesthesia chamber. In Experiment 3, rats were anesthetized with isoflurane before cocaine administration to minimize the effects of interoceptive and injection cues. In all experiments, rats were challenged with cocaine on the test day (24 hr later), with locomotion being measured in activity chambers. Results showed that (a) the pretreatment dose of cocaine (10-40 mg/kg) did not differentially affect context-specific and context-independent sensitization; (b) cross-sensitization between methylphenidate and cocaine was observed in the context-specific condition, but not when using a context-independent procedure; and (c) sensitization was evident if injection and interoceptive cues were minimized. One possibility is that associative processes do not modulate the one-trial sensitization of preweanling rats. Alternatively, "unitization" may cause preweanling rats to treat the different environments as equivalent, thus permitting robust sensitization even when drug pretreatment and testing occur in different environments.

A single high dose of methamphetamine increases cocaine self-administration by depletion of striatal dopamine in rats

Psychostimulant addicts often take high doses of drugs, and high doses of psychostimulants such as methamphetamine (METH) are neurotoxic to striatal dopamine (DA) terminals. Yet, the effects of high doses of METH on drug-seeking and drug-taking behavior have not been examined. In the present study, we found that single high doses of METH in rats (10-20 mg/kg) dose-dependently increased cocaine self-administration under fixed-ratio 2 (FR2) reinforcement conditions, while higher doses (40 mg/kgx1 or 10 mg/kg/2 hx4) caused high mortality among rats maintained on daily cocaine self-administration. The increased cocaine self-administration appeared to be a compensatory response to reduced cocaine reward after METH, because the same doses of METH caused a dose-dependent reduction both in "break-point" levels for cocaine self-administration under progressive-ratio reinforcement and in nucleus accumbens DA response to acute cocaine. Further, METH (10-20 mg/kg) produced large DA release (4000%-6000% over baseline), followed by a significant reduction in striatal DA and 3,4-dihydroxyphenylacetic acid (DOPAC) contents, but without significant changes in striatal DA transporter levels. These findings suggest that the present high doses of METH caused striatal DA depletion or hypofunction without severe damage in DA terminals, which may contribute to the increased cocaine-taking behavior observed in the present study. Provided that the present doses of METH may mimic METH overdose incidents in humans, the present findings suggest that METH-induced DA depletion or neurotoxicity may lead to an increase in subsequent drug-taking and drug-seeking behavior.

Intermittent Dopaminergic stimulation causes behavioral sensitization in the addicted brain and parkinsonism

The gold standard therapy for Parkinson's disease (PD) consists in chronic administration of pulses of the dopamine (DA) precursor l-dihydroxyphenylalanine (l-DOPA). Although the main brain area which is DA-deficient is the dorsal striatum (more the putamen than the caudate nucleus), other DA-innervated brain regions (i.e., the ventral striatum and other limbic areas) are affected by systemic administration of l-DOPA. While such a therapy produces an increase in synaptic and nonsynaptic DA, which replace the neurotransmitter deficiency, peaks of extracellular DA in the course of disease progression produce abnormal involuntary movements related to behavioral sensitization. Methamphetamine (METH), a widely abused drug, is known to produce behavioral sensitization, related to DA release (more in the ventral than dorsal striatum as well as other limbic regions). The present review discusses the overlapping between these treatments, based on pulses of DA stimulation with an emphasis on the class of DA receptors; signal transduction pathways; rearranged expression of neurotransmitters, cotransmitters, and their receptors coupled with ultrastructural changes. In fact, all these levels of synaptic plasticity show a surprising homology following these treatments, posing the mechanisms of behavioral sensitization during DA-replacement therapy in PD very close to the neurobiological mechanisms operating during METH abuse. In line with this view is the growing evidence of addictive behaviors in PD patients during the course of DA-replacement therapy.

The Frizzled 3 gene is associated with methamphetamine psychosis in the Japanese population

Background

Frizzled 3 (Fzd3) is a receptor required for the Wnt-signaling pathway, which has been implicated in the development of the central nervous system, including synaptogenesis and structural plasticity. We previously found a significant association between the FZD3 gene and susceptibility to schizophrenia, but subsequent studies showed inconsistent findings. To understand the roles of the FZD3 gene in psychotic disorders further, it should be useful to examine FZD3 in patients with methamphetamine psychosis because the clinical features of methamphetamine psychosis are similar to those of schizophrenia.

Methods

Six SNPs of FZD3, rs3757888 in the 3' flanking region, rs960914 in the intron 3, rs2241802, a synonymous SNP in the exon5, rs2323019 and rs352203 in the intron 5, and rs880481 in the intron 7, were selected based on the previous schizophrenic studies and analyzed in 188 patients with methamphetamine psychosis and 240 age- and gender-matched controls.

Results

A case-control association analyses revealed that two kinds of FZD3 haplotypes showed strong associations with methamphetamine psychosis (p < 0.00001). Having the G-A-T-G or A-G-C-A haplotype of rs2241802-rs2323019-rs352203-rs880481 was a potent negative risk factor (odds ratios were 0.13 and 0.086, respectively) for methamphetamine psychosis.

Conclusion

Our present and previous findings indicate that genetic variants of the FZD3 gene affect susceptibility to two analogous but distinct dopamine-related psychoses, endogenous and substance-induced psychosis.

Alpha2-containing GABA(A) receptors are involved in mediating stimulant effects of cocaine

alpha2 subunit-containing GABA(A) receptors are involved in incentive learning associated with cocaine, and in cocaine addiction. Deletion of alpha2-containing receptors abolishes cocaine-induced behavioural sensitisation (BS), while selective activation of alpha2 receptors, achieved using Ro 15-4513's agonist properties in alpha2(H101R) mice, induced BS. Here, we investigate further the mechanisms underlying Ro 15-4513-induced behavioural sensitisation in alpha2(H101R) mice. alpha2(H101R) mice sensitised to Ro 15-4513 (10 mg/kg) showed an enhanced stimulant response to cocaine (10 mg/kg). In contrast, cocaine (10 mg/kg)-sensitised alpha2(H101R) mice did not show enhanced sensitivity to the stimulant effects of Ro 15-4513 (1, 3 and 10 mg/kg), suggesting that the neural adaptations underlying Ro 15-4513 induced BS are related to, but not identical with those associated with cocaine-induced plasticity. Secondly, we investigated whether alpha2-containing receptors are involved in mediating the ability of BZs to facilitate cocaine-induced activity. The non-selective (i.e., alpha1, alpha2, alpha3 and alpha5 subtype) benzodiazepine GABA(A) receptor agonist midazolam (10 and 30 mg/kg) potentiated cocaine (10 mg/kg) hyperactivity in wildtype mice, but not in alpha2(H101R) mice, in which alpha2-containing receptors are insensitive to benzodiazepines. To determine where alpha2 receptors are localised we compared BZ-insensitive sites between wildtype (alpha4 and alpha6) and alpha2(H101R) (alpha2, alpha4 and alpha6) mice, using quantitative autoradiography to estimate [(3)H]Ro 15-4513 binding in the presence of 10 muM diazepam. alpha2 receptors were found in projection areas of the mesolimbic dopamine pathway including accumbens, central amygdala, and basolateral amygdala as well as CA1 and CA3 areas of the hippocampus. The involvement of the alpha2-containing receptor in mediating BZ's potentiating effect on cocaine hyperactivity suggests that the locomotor stimulant effects of BZs and psychostimulants may be mediated by a common neural system, but the lack of cross sensitisation to Ro 15-4513 in cocaine-sensitised alpha2(H101R) mice, suggests that this form of BS may occur downstream of plastic events underlying cocaine sensitisation.

Methamphetamine-elicited alterations of dopamine- and serotonin-metabolite levels within mu-opioid receptor knockout mice: a microdialysis study

mu-Opioid receptors (mu-ORs) modulate methamphetamine (MA)-induced behavioral responses, increased locomotor activity and stereotyped behavior in the mouse model. We investigated the changes in dopamine (DA) and serotonin (5-HT) metabolism in the striatum following either acute or repeated MA treatment using in vivo microdialysis. We also studied the role of mu-ORs in the modulation of MA-induced DA and 5-HT metabolism within mu-OR knockout mice. Subsequent to either acute or repeated intraperitoneal administration of MA, wild-type mice revealed decreases in extracellular concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) in a dose-dependent manner. Moreover, wild-type mice had reductions in basal concentrations of DOPAC and HVA following repeated MA treatment with a higher dose. The effects of acute, repeated or challenge MA administration upon extracellular levels of DOPAC and HVA within mu-OR knockout mice significantly differed from the wild-type controls. The duration of recovery to the basal levels of extracellular DA and 5-HT metabolites induced by MA were much longer in wild-type mice than for mu-OR knockout mice. These findings suggest that mu-ORs play a modulatory role in MA-induced DA and 5-HT metabolism in the mouse striatum. This possible mechanism of MA-induced behavioral change as modulated by mu-OR merits further study.

Oxidative stress in methamphetamine-induced self-injurious behavior in mice

Previous studies have shown that N-methyl-D-aspartate, the formation of free radicals and poly(ADP-ribose) polymerase are related to methamphetamine-induced neurotoxicity. This study was designed to investigate the involvement of oxidative stress in methamphetamine-induced self-injurious behavior in mice. In this study, methamphetamine (20 mg/kg) induced continuous self-injurious behavior in six of seven mice. N-methyl-D-aspartate-receptor antagonists (MK801 and 3-((R)-2-carboxypiperazin-4-yl) propyl-1-phosphonic acid) significantly attenuated this methamphetamine-induced self-injurious behavior. These results suggest that the activation of N-methyl-D-aspartate receptors is involved in methamphetamine-induced self-injurious behavior. Furthermore, we found that the nonselective nitric oxide synthase inhibitor l-N-nitro-L-arginine methyl ester hydrochloride and the neuronal nitric oxide synthase inhibitor 7-nitroindazole, but not the inducible nitric oxide synthase inhibitor aminoguanidine, the free-radical inhibitors fullerene and 3-methyl-1-phenyl-2-pyrazolin-5-one-186, or the poly(ADP-ribose) polymerase inhibitor benzamide, significantly attenuated methamphetamine-induced self-injurious behavior. The present results show that oxidative stress, which is mediated by the activation of neuronal nitric oxide synthase, is associated with methamphetamine-induced self-injurious behavior. These findings may help us to better understand the clinical phenomenon of self-injurious behavior.

Chronic benzylpiperazine (BZP) exposure produces behavioral sensitization and cross-sensitization to methamphetamine (MA)

BACKGROUND

Like other psychostimulant drugs, acute exposure to benzylpiperazine (BZP) increases dopaminergic neurotransmission, producing hyperactivity and stereotypy. The consequences of repeated BZP exposure have not however been investigated. The effects of acute and repeated BZP and methamphetamine (MA) exposure on locomotor activity and stereotypy were measured in order to determine whether there was sensitization and cross-sensitization between these two psychostimulant drugs.

METHODS

The effects of acute treatment with MA (0.0, 0.5, 1.0 and 2.0 mg/kg, intraperitoneal (IP)) or BZP (0.0, 5.0, 10.0, 20.0 and 40.0 mg/kg, IP) on locomotor activity and stereotypy were determined. Effects of repeated exposure were determined in other groups that received five daily injections of 2.0 mg/kg MA, 20.0 mg/kg BZP or vehicle. Following a 2-day withdrawal period, rats from each treatment group received either a low dose MA (0.5 mg/kg) or BZP (10.0 mg/kg).

RESULTS

MA and BZP produced dose-dependent hyperactivity and stereotypy. Repeated MA and BZP resulted in a potentiated locomotor but not stereotypy response. Following the withdrawal period, MA pretreated rats exhibited a sensitized locomotor and stereotypy response to the low dose MA and a conditioned response to saline. BZP pretreated rats also demonstrated a sensitized locomotor response to the low dose of BZP and MA.

CONCLUSIONS

The present findings indicate that repeated exposure to BZP results in sensitization and cross-sensitization to MA.

Association between gene polymorphisms of SLC22A3 and methamphetamine use disorder

BACKGROUND

Methamphetamine (MAP) is one of the most frequently used illegal substances in Japan, and family and twin studies have suggested that genetic factors contribute to psychostimulant dependence, including MAP dependence. Organic cation transporter 3 (OCT3) has been reported to be involved in the disposition of MAP as well as MAP-induced behavioral changes in animals. Moreover, SLC22A3 (which encodes OCT3) is a candidate gene for MAP dependence because it is located within a chromosomal region associated with substance dependence.

METHODS

Using 96 healthy control subjects, linkage disequilibrium (LD) within the SLC22A3 was investigated, and 5 single-nucleotide polymorphisms (SNPs) were selected as haplotype tag SNPs to search for an association with MAP dependence. Single-marker analyses and haplotype analyses of these SNPs were performed in 213 subjects with MAP dependence and 443 healthy controls.

RESULTS

SLC22A3 polymorphisms were not significantly associated with MAP dependence in any of the single-marker and haplotype analyses. When subjects with MAP dependence were divided into polysubstance and single-MAP users, genotype and allele frequency of SNP2 (p=0.024, p=0.011, respectively), allele frequency of SNP3 (p=0.037), and haplotypic frequencies for these 2 SNPs (p=0.0438) differed significantly between groups.

CONCLUSIONS

These results suggest that polymorphisms of SLC22A3 are related to the development of polysubstance use in Japanese patients with MAP dependence.

Acute and chronic continuous methamphetamine have different long-term behavioral and neurochemical consequences

We compared two different methamphetamine dosing regimens and found distinct long-term behavioral and neurochemical changes. Adult rats were treated with 1-day methamphetamine injection (3x5 mg/kg s.c., 3 h apart) or 7-day methamphetamine minipump (20 mg/kg/day s.c.). The minipump regimen models the sustained methamphetamine plasma levels in some human bingers whereas the 1-day regimen models a naive user overdose. On withdrawal days 7 and 28, rats were acutely challenged with cocaine to test for behavioral sensitization and subsequently sacrificed for caudate and accumbens dopamine tissue content. Other rats were analyzed on withdrawal days 3, 7 or 28 using voltammetry in caudate slices. On withdrawal days 7 and 28, the methamphetamine injection but not the minipump rats showed behavioral cross-sensitization to cocaine. There was no change in baseline dopamine release, reuptake or sensitivity to quinpirole in any treatment group on either withdrawal day. However, consistent with the behavioral sensitization, cocaine had a greater effect in potentiating dopamine release and in blocking dopamine reuptake in methamphetamine injection versus saline irrespective of withdrawal day. The minipump group showed tolerance to the dopamine releasing effect of cocaine on withdrawal day 28 and had lower dopamine tissue content in the caudate versus the methamphetamine injection group. Dopamine turnover as measured by the DOPAC/dopamine ratio tended to be higher in the minipump-treated rats. These data suggest that the behavioral cross-sensitization seen in the methamphetamine injection rats could be in part due to the increased potency of cocaine in blocking dopamine reuptake and in increasing dopamine release. The decreased potency of cocaine in the caudate slices from the minipump-treated group may be related to decreased dopamine tissue content.

Behavioral sensitization to the discriminative stimulus effects of methamphetamine in rats

Sensitization to the discriminative stimulus effects of psychostimulants is not fully understood. Therefore, the present study was designed to investigate the development of sensitization to the discriminative stimulus of methamphetamine in rats. A dose-response curve for methamphetamine and a generalization test for cocaine were recorded in rats trained to discriminate between 1.0 mg/kg methamphetamine and saline. A significant leftward shift of the dose-response curve for methamphetamine and of the dose-generalization curve for cocaine was observed following repeated administration of methamphetamine (2.0 mg/kg) instead of saline. These findings suggest that repeated administration of methamphetamine can produce behavioral sensitization to the discriminative stimulus effects of methamphetamine in rats.

Enhancement of central dopaminergic activity in the kainate model of temporal lobe epilepsy: implication for the mechanism of epileptic psychosis

There is an increased incidence of schizophrenia-like psychosis in temporal lobe epilepsy (TLE), and several risk factors have been implicated, including the duration of epilepsy and temporal lobe neuropathology. To investigate the biological mechanism of epileptic psychosis, we examined alterations of central dopaminergic systems in the kainate model of TLE. In adult rats, kainate was microinjected into the left amygdala to induce status epilepticus. An indirect dopamine agonist methamphetamine (MAP, 2 mg/kg, i.p.) was administered before and 1 month after the kainate treatment. MAP-induced locomotor activity was significantly enhanced in the kainate group compared with the baseline (pre-kainate) level, which was antagonized by pretreatment with haloperidol. The enhancement of locomotor activity in the kainate group was significantly correlated with the density of hippocampal CA1 neurons. Although the basal extracellular dopamine concentration was significantly lower in the striatum in the kainate group than in the control group (5.5 vs 39.2 fmol/20-min sample), the maximal concentration following MAP administration did not differ between the two groups. These results clearly demonstrate that hypersensitivity of the dopamine systems develops in the chronic phase of the kainate-induced TLE model, which may be responsible for the mechanism of epileptic psychosis.

The location of persistent amphetamine-induced changes in the density of dendritic spines on medium spiny neurons in the nucleus accumbens and caudate-putamen

Repeated intermittent treatment with amphetamine increases the density of dendritic spines on medium spiny neurons (MSNs) in the nucleus accumbens (NAcc). The purpose of this study was two-fold: (1) to determine whether amphetamine has similar effects on MSNs in the caudate-putamen (CPu) and (2) to determine if this effect is localized to distal dendrites, the site of convergence of dopamine (DA) and glutamate synapses in the striatum. An amphetamine treatment regimen that produced behavioral sensitization increased the density of dendritic spines on MSNs in both the NAcc and dorsolateral CPu. This effect was long lasting, because it was evident 3.5 months after the discontinuation of drug treatment. The increase in spine density was confined to distal dendrites of MSNs, and was not apparent on dendrites close to the cell body. It is concluded that amphetamine may preferentially reorganize synapses at the site of DA-glutamate interaction in the striatum, which may alter DA-glutamate signaling and thereby contribute to some of the persistent behavioral and psychological consequences of repeated exposure to psychostimulant drugs.

Changes in basal and cocaine-evoked extracellular dopamine uptake and release in the rat nucleus accumbens during early abstinence from cocaine: quantitative determination under transient conditions

Despite an abundance of studies on mechanisms of behavioral sensitization, considerable uncertainty exists as to whether alterations in dopamine neurotransmission underlie the exacerbated behavioral effects of cocaine observed during the early stages of abstinence. One of the factors contributing to the uncertainty and controversy may be the limitations in utilized measurement techniques (mostly conventional microdialysis). The techniques of quantitative microdialysis under transient conditions and rotating disk electrode voltammetry were used to characterize basal dopamine dynamics as well as time-related changes in extracellular dopamine concentrations and dopamine uptake that occur in response to an acute drug challenge in control animals and animals with previous history of cocaine. Basal extracellular dopamine concentrations were unaltered on abstinence day 3 from repeated cocaine administration (5 days, 20 mg/kg, i.p.). The extraction fraction of dopamine, an indirect measure of dopamine uptake, was significantly lower in cocaine-sensitized animals relative to controls. These two facts, taken together, suggest that basal dopamine release is depressed in cocaine-sensitized animals on abstinence day 3. At the same time, a cocaine challenge decreased the extraction fraction and increased the extracellular dopamine concentration in both experimental groups. The magnitude of the increase in extracellular dopamine concentration was greater in cocaine-sensitized animals, while the ability of cocaine to decrease the extraction fraction was unaltered, suggesting that the increase in extracellular dopamine concentration reflects an increase in drug-evoked dopamine release. Moreover, cocaine-pretreated rats demonstrated greater depolarization-induced dopamine release and the ability of dopamine D(2) receptor agonist, quinpirole, to inhibit release was decreased in these animals. These data demonstrate that a cocaine treatment regimen resulting in behavioral sensitization is associated with a reduction in basal dopamine release, an enhancement in both cocaine and K(+)-evoked dopamine release, and a subsensitivity of dopamine D(2) autoreceptors that regulate dopamine release in the nucleus accumbens.

Stimulant-induced psychosis and schizophrenia: the role of sensitization

Three different conditions, psychostimulant-induced behavioral sensitization in rodents, psychostimulant-induced psychoses in human, and chronic schizophrenia show similar longitudinal alternations, progressively enhanced susceptibility to abnormal behaviors, psychotic state, and relapse. Sensitization phenomena to the drugs or endogenous dopamine should be involved in the mechanisms underlying the development of such susceptibility. Recently, an enhanced dopamine release in vivo by amphetamine administration in the striatum has been shown in schizophrenics, which is a replication of that previously proven in the behavioral sensitization in rats. Accordingly, common molecular mechanisms of sensitization phenomena must develop in these three conditions, and are overviewed in this review

Chronic intermittent amphetamine pretreatment enhances future appetitive behavior for drug- and natural-reward: interaction with environmental variables

Appetitive behavior for drug and sexual reward is enhanced in animals with a history of amphetamine-experience. The present experiment investigated whether prior exposure to a sensitizing regimen of amphetamine treatment would 'globally' enhance future appetitive behaviors of adult male Sprague-Dawley rats, and whether the drug preexposure-environment or intermittency of administration would affect this development. Reward appetite was compared in drug-experienced versus drug-naive rats using amphetamine place-preference conditioning (CPP) and a natural-incentive sensitization task, which measured appetitive approach for food and sexual reward. Experiment I found that 10 daily exposures to 1 mg/kg amphetamine did not alter future psychostimulant CPP, regardless of abstinence schedule. Although daily exposure to a higher amphetamine dose also did not alter appetitive behavior when measured after 2-weeks drug abstinence in Experiment II, alternate-day amphetamine experience (5.0 mg/kg, twice-a-day) in an initially unfamiliar environment persistently enhanced future amphetamine CPP and appetitive behavior for natural reward. Identical treatment administered in the homecage did not. Furthermore, sensitized reward-seeking behaviors were not globally evident. Animals that showed sensitized amphetamine CPP did not show sensitized food-seeking behavior and vice versa. Thus, the environment surrounding chronic psychostimulant drug experience can greatly affect subsequent reward appetite, but the sensitized expression may be individually determined.

Ethanol administration potentiates cocaine-induced dopamine levels in the rat nucleus accumbens

Ethanol and cocaine are frequently co-abused, and the drug combination has been reported to produce an increased and prolonged subjective euphoria as compared to when either drug is administered alone. Acute administration of ethanol or cocaine increases the extracellular dopamine (DA) concentration in the nucleus accumbens (NAcc), a terminal region of the mesolimbic dopaminergic pathway. In the present study, the effects of separate and concurrent administration of cocaine and ethanol on DA concentrations in the NAcc were studied in rats pretreated with ethanol. Four groups of rats received either ethanol (2 g/kg, i.p.) or saline twice daily for 6 consecutive days. Thereafter, rats were given injections of saline or cocaine for another 2 days (i.e. treatment days 7 and 8) using a 'binge' administration pattern (three i.p. injections of 15 mg/kg each with 1-h interval starting 40 min after the first of the two daily doses of ethanol/saline). Stereotypic behavior was scored after each 'binge' of cocaine or saline on days 7 and 8. The DA and DA metabolite concentrations were measured using microdialysis on day 8. Ethanol enhanced the effect of cocaine on DA concentration in the NAcc as compared to a single administration of cocaine. The DA levels increased and reached their maximum values within 20-40 min after the cocaine administration, then gradually declined until the next injection 1-h later. Cocaine-induced stereotypic behavior was significantly increased in both saline and ethanol pretreated groups, though there was no significant difference between the two groups. The results of this study suggest that the enhanced DA transmission may be related to the experience produced by concurrent abuse of ethanol and cocaine in humans.

[Positron emission tomography (PET) study of the alterations in brain pharmacokinetics of methamphetamine in methamphetamine sensitized animals]

I investigated the differences in brain pharmacokinetics of [11C]methamphetamine ([11C]MAP) in normal and MAP sensitized animals using positron emission tomography (PET). [11C]MAP was synthesized by an automated on-line [11C]methylation system. I newly produced MAP sensitized dog and monkey by repeated MAP treatment. The maximal level of accumulation of [11C]MAP in the sensitized dog brain was 1.4 times higher than that in the control. This result suggests the changes in the pharmacokinetic profile of MAP in the brain affect the development or expression of MAP-induced behavioral sensitization. However, the overaccumulation of [11C]MAP in the sensitized monkey brain was not observed due to the influence of anesthesia.

D3 dopamine receptor, behavioral sensitization, and psychosis

Behavioral sensitization is a progressive, enduring enhancement of behaviors that develops following repeated stimulant administration. It is mediated in part by dopaminergic pathways that also modulate a number of psychiatric conditions including the development of psychosis. We propose that down-regulation of D3 dopamine receptor function in critical brain regions contributes to sensitization. Rodent locomotion, a sensitizable behavior, is regulated by the opposing influence of dopamine receptor subtypes, with D3 stimulation opposing concurrent D1 and D2 receptor activation. The D3 dopamine receptor has a 70-fold greater affinity for dopamine than D1 or D2 dopamine receptors. This imbalance in ligand affinity dictates greater occupancy for D3 than D1 or D2 receptors at typical dopamine concentrations following stimulant drug administration, resulting in differences in the relative tolerance at D3 vs D1 and D2 receptors. Sensitization may therefore result in part from accommodation of the inhibitory D3 receptor 'brake' on D1/D2 mediated behaviors, leading to a progressive locomotion increase following repeated stimulant exposure. The requirement for differential tolerance at D3 vs D1 and D2 receptors may explain the observed development of sensitization following application of cocaine, but not amphetamine, directly into nucleus accumbens. If correct, the 'D3 Dopamine Receptor Hypothesis' suggests D3 antagonists could prevent sensitization, and may interrupt the development of psychosis when administered during the prodromal phase of psychotic illness. Additional study is needed to clarify the role of the D3 dopamine receptor in sensitization and psychosis.

Kappa-opioid receptor activation prevents alterations in mesocortical dopamine neurotransmission that occur during abstinence from cocaine

In vivo microdialysis was used to characterize basal dopamine dynamics and cocaine-evoked dopamine levels in the medial prefrontal cortex of male Sprague-Dawley rats that had previously received once daily injections of cocaine (days 1-5; 20mg/kg, i.p.) in combination with the selective kappa-opioid receptor agonist U-69593 (days 3-5; 0.32mg/kg, s.c.) or its vehicle. The influence of these treatments on [3H]dopamine uptake in medial prefrontal cortex synaptosomes was also determined. Three days following the cessation of drug treatment, animals with prior history of cocaine administration exhibited enhanced psychomotor stimulation in response to a subsequent cocaine challenge. This effect was not apparent in animals that had previously received the cocaine treatment regimen in combination with the kappa-opioid receptor agonist U-69593. Cocaine challenge increased prefrontal dopamine levels in all pretreatment groups, but cocaine-pre-exposed animals had lower cocaine-evoked dopamine levels and higher basal in vivo extraction fraction, indicative of an increase in basal dopamine uptake relative to controls. Pretreatment with U-69593 prevented these effects of cocaine. Measurement of [3H]dopamine uptake in synaptosomes revealed a significant increase in uptake three days after the cessation of cocaine treatment. No increase in uptake was observed in animals that had received the cocaine treatment regimen in combination with U-69593. These results demonstrate that the early phase of abstinence from cocaine is associated with marked alterations in medial prefrontal cortex dopamine neurotransmission and that these neuroadaptations are prevented by the activation of kappa-opioid receptors. Furthermore, they raise the possibility that mesocortical dopamine neurons may be an important neural substrate upon which kappa-opioid agonists act to prevent the development of cocaine-induced behavioral sensitization.

Pharmacology and behavioral pharmacology of the mesocortical dopamine system

The prefrontal cortex (PFC) has long been known to be involved in the mediation of complex behavioral responses. Considerable research efforts are directed towards refining the knowledge about the function of this brain area and the role it plays in cognitive performance and behavioral output. In the first part, this review provides, from a pharmacological perspective, an overview of anatomical, electrophysiological and neurochemical aspects of the function of the PFC, with an emphasis on the mesocortical dopamine system. Anatomy of the mesocortical system, basic physiological and pharmacological properties of neurotransmission within the PFC, and interactions between dopamine and glutamate as well as other transmitters within the mesocorticolimbic circuit are included. The coverage of these data is largely restricted to what is relevant for the second part of the review which focuses on behavioral studies that have examined the role of the PFC in a variety of phenomena, behaviors and paradigms. These include reward and addiction, locomotor activity and sensitization, learning, cognition, and schizophrenia. Although the focus of this review is on the mesocortical dopamine system, given the intricate interactions of dopamine with other transmitter systems within the PFC and the importance of the PFC as a source of glutamate in subcortical areas, these aspects are also covered in some detail where appropriate. Naturally, a topic as complex as this cannot be covered comprehensively in its entirety. Therefore this review is largely limited to data derived from studies using rats, and it is also specifically restricted to data concerning the medial PFC (mPFC). Since in several fields of research the findings concerning the function or role of the mPFC are relatively inconsistent, the question is addressed whether these inconsistencies might, at least in part, be related to the anatomical and functional heterogeneity of this brain area.

Studies of amphetamine or methamphetamine psychosis in Japan: relation of methamphetamine psychosis to schizophrenia

There exist clinical characteristics of methamphetamine (MAP) psychosis in the Japanese population. MAP psychosis involves paranoid-hallucinatory states indistinguishable from paranoid schizophrenia, with residual volitional disturbances (e.g., loss of spontaneity and idleness). Paranoid-hallucinatory states persist after the pharmacological effects of MAP have worn off and readily reappear upon a reinjection of MAP. Individuals with a history of MAP psychosis further undergo spontaneous recurrence of their paranoid-hallucinatory states in response to stress. The development of MAP psychosis might therefore be related to persisting brain damage or changes in brain metabolism induced by repeated MAP use, and thus studies of the clinical course and neurological basis of MAP psychosis could provide insights into the pathophysiology of schizophrenia. Accordingly, psychiatrists have studied the clinical characteristics of MAP psychosis and examined the neurobiological basis of MAP-induced behavioral sensitization, using animals. MAP-induced behavioral sensitization might well be related to dopamine supersensitivity; however, the contribution of presynaptic autoreceptors remains controversial, and other hypotheses should be considered. Recently, the process that triggers spontaneous recurrence of MAP psychosis (flashbacks) and corresponding peripheral neurotransmitter functions has been studied. Stress sensitization associated with noradrenergic hyperactivity, involving increased dopamine release, appears to be crucial in the development of flashbacks. Overall, MAP-induced susceptibility to paranoid-hallucinatory states and to abnormal behavior (e.g., stereotyped behavior) in animals is examined as a model for predicting relapses of paranoid schizophrenia. Further extensive studies on the neurobiological and molecular mechanisms of this susceptibility are required.

Context determines the type of sensitized behaviour: a brief review and a hypothesis on the role of environment in behavioural sensitization

Behavioural sensitization to psychostimulants may develop context-dependency in certain circumstances. Animals given a stimulant repeatedly in a test cage but not in other environments may show enhanced drug-induced behaviour in the test cage. Conditioning mechanisms have been claimed to be responsible for these phenomena. However, several recent findings are not properly accounted for by conditioning. In addition, growing evidence supports the hypothesis that behavioural sensitization reflects neural changes induced by repeated exposure to psychostimulants (the pharmacological hypothesis). However, the pharmacological hypothesis itself fails to account for environmental influences. In this paper, we propose a hypothesis on the role of environment that is complementary to the pharmacological hypothesis. According to our hypothesis, environment does not have a causal role in the development of sensitization, but it modifies the mode of expression of the sensitized behaviour. Sensitization primarily reflects a neuroadaptive change induced by repeated exposure of the neural system to psychostimulants. However, psychostimulants are known to induce different behaviours in different environments. Therefore, repeated administration of a psychostimulant in different environments would result in augmentation of different behaviours. Our hypothesis potentially accommodates various previous observations. We briefly review the literature and present our hypothesis.

The role of endogenous sensitization in the pathophysiology of schizophrenia: implications from recent brain imaging studies

Long-term sensitization is a process whereby exposure to a given stimulus such as a drug or a stressor results in an enhanced response at subsequent exposures. Sensitization of mesolimbic dopamine systems has been postulated by several authors to underlie the development of dopaminergic abnormalities associated with schizophrenia. In this review, core features of stimulant-induced sensitization of dopamine systems in rodents are briefly reviewed, as well as the behavioral and clinical evidence suggesting the relevance of this process to drug-induced psychosis and schizophrenia. Results of recent brain imaging studies relevant to the question of sensitization in schizophrenia are then discussed. These studies indicate that schizophrenia is associated with increased amphetamine-induced dopamine release. This exaggerated response was detected in patients experiencing an episode of clinical deterioration but not in clinically stable patients. Since increased stimulant-induced dopamine release is a hallmark of sensitization, these results support the view that schizophrenia is associated with a process of endogenous sensitization. Based on the preclinical evidence that dopamine projection to the prefrontal cortex acts as a buffer that oppose the development of sensitization in subcortical dopamine projections, we propose that, in schizophrenia, neurodevelopmental abnormalities of prefrontal dopaminergic systems might result in a state of enhanced vulnerability to sensitization during late adolescence and early adulthood. It is also proposed that D(2) receptor blockade, if sustained, might allow for an extinction of this sensitization process, with possible re-emergence upon treatment discontinuation. A better understanding of the neurocircuitry associated with endogenous sensitization and its consequence in schizophrenia might be important for the development of better treatment and relapse prevention strategies.

Dopamine as the wind of the psychotic fire: new evidence from brain imaging studies

Abnormalities of dopamine function in schizophrenia are suggested by the common antidopaminergic properties of antipsychotic medications. However, direct evidence of a hyperdopaminergic state in schizophrenia has been difficult to demonstrate, given the difficulty of measuring dopamine transmission in the living human brain. This situation is rapidly changing. Recent developments in positron emission tomography and single-photon emission tomographic techniques enabled measurement of acute fluctuation of synaptic dopamine in the vicinity of D2 receptors. Using this technique, we, and others, measured the increase in dopamine transmission following acute amphetamine challenge in untreated patients with schizophrenia and matched healthy subjects. Following a brief overview of these new brain imaging techniques, the main results derived with this method in patients with schizophrenia are described: (1) amphetamine-induced dopamine release is elevated in patients with schizophrenia, supporting the idea that schizophrenia is associated with dysregulation of dopamine transmission; (2) following amphetamine, hyperactivity of dopamine transmission is associated with activation of psychotic symptomatology; (3) this dysregulation of dopamine release is not a long-term consequence of previous neuroleptic treatment, and is detected in never-medicated patients experiencing a first episode of the illness; and (4) in contrast, this exaggerated response of the dopamine system to amphetamine exposure is not detected in patients studied during a period of illness stabilization, suggesting that the hyperdopaminergic state associated with schizophrenia fluctuates over time. In conclusion, a hyperdopaminergic state might be present in schizophrenia during the initial episode and subsequent relapses, but not during periods of remission. This finding has important consequences for the development of new treatment strategies for the remission phase.

Increased dopamine transmission in schizophrenia: relationship to illness phases

BACKGROUND

Abnormalities of dopamine function in schizophrenia are suggested by the common antidopaminergic properties of antipsychotic medications. However, direct evidence of a hyperdopaminergic state in schizophrenia has been difficult to demonstrate, given the difficulty to measure dopamine transmission in the living human brain. Such evidence has recently emerged. Three studies reported an increase in dopamine transmission following acute amphetamine challenge in patients with schizophrenia compared to matched healthy control subjects, thus demonstrating a dysregulation of dopamine in schizophrenia. In all studies, a large variance was observed within the schizophrenic group in the magnitude of this finding, and clinical predictors of this effect could not be identified.

METHODS

In this paper, we combined previously published and newly acquired data to obtain sufficient power to address this question.

RESULTS

The most important findings derived from this extended data set are: 1) dysregulation of dopamine function revealed by the amphetamine challenge is present at onset of illness and in patients never previously exposed to neuroleptic medications; 2) this dysregulation was observed in patients experiencing an episode of illness exacerbation, but not in patients studied during a remission phase.

CONCLUSIONS

A hyperdopaminergic state is present in schizophrenia during the initial episode and subsequent relapses, but not in periods of remission. This finding has important consequences for the development of new treatment strategies for the remission phase.

Stability of [123I]IBZM SPECT measurement of amphetamine-induced striatal dopamine release in humans

Binding competition between endogenous dopamine (DA) and the D2 receptor radiotracer [123I]IBZM allows measurement of the change in synaptic DA following amphetamine challenge with SPECT in the living human brain. Previous investigations using this technique in healthy subjects have shown that the magnitude of amphetamine effect on [123I]IBZM binding potential (BP) is small (range between 5 to 15% decrease), and that a large between-subject variability in this effect is observed. Therefore, it was unclear how much of the apparent between-subject variability was due to a low signal-to-noise ratio in the measurement, vs. true between-subject differences in the magnitude of the response. The goals of this investigation were to test the within-subject reproducibility and reliability of amphetamine-induced decrease in [123I]IBZM BP with a test/retest paradigm, and to establish the presence or absence of tolerance or sensitization to single administration ofi.v. amphetamine. Six healthy male subjects, never previously exposed to psychostimulants, twice underwent measurement of striatal amphetamine-induced DA release (between-measurement interval 16 +/- 10 days) using SPECT and the [123I]IBZM constant infusion technique. Results demonstrated an excellent within-subject reproducibility of amphetamine-induced DA release: amphetamine-induced decreases in [123I]IBZM BP were significant on each day, and had an intraclass correlation coefficient (ICC) of 0.89. Moreover, values from the second experiment were not significantly different from first experiment, suggesting the absence of either sensitization or tolerance to the effect of amphetamine on DA release in these experimental conditions. The subjective activation, as rated by the subjects on analog scales, was also highly reproducible. In conclusion, this scanning technique provides a reliable measurement of amphetamine-induced reduction of [123I]IBZM BP and enables detection of between-subject differences that appear stable over time.

Importance of initial environments in the development of ambulatory sensitization to methamphetamine and cocaine in mice

Repeated administration of CNS stimulants such as amphetamines and cocaine induces behavioural sensitization which can be influenced by the animal's environment. This study has evaluated the effect of restraint on the development and maintenance of ambulatory sensitization to methamphetamine and cocaine in mice. Subcutaneous administration of the CNS stimulants methamphetamine (2 mg kg(-1)) and cocaine (20 mg kg(-1)) seven times at three-day intervals resulted in ambulatory sensitization when the mice were placed in 20-cm diameter activity cages after each dose of the drug. However, if methamphetamine or cocaine was administered when the mice were in small jars (6-cm diameter) in which expression of ambulation, but not of circling and rearing, was completely restricted, the development of ambulatory sensitization was retarded or inhibited, with circling behaviour concurrently increased, when subsequent repeated doses of the drug were administered in the activity cage. Subsequent repeated treatment of ambulatory-sensitized mice with the drug or saline when the mice were in the jars did not change the levels of the ambulatory sensitization or the circling behaviour. These results suggest that the mice are sensitized to the behavioural effect of CNS stimulants which can be expressed in the environment in which the drug is administered. It is also considered that the established sensitization is strongly retained and is responsible for retardation or suppression of the development of sensitization to other behavioural stimulant effects.

Neurochemical studies of schizophrenia in Japan

Neurobiological research in schizophrenia has been hampered by several confounding factors such as the heterogeneity of the illness and the paucity of biological markers. Recent progress in research methods, however, has enabled the improvement in our understanding its pathophysiology. This paper reviews recent neurochemical investigations of schizophrenia and its animal models which were conducted in Japan in the last decade. The research areas reviewed are (i) monoamine and their metabolites in body fluids, (ii) phospholipids and prostaglandins, (iii) neurochemistry in autopsy brains, (iv) immunological measures, (v) magnetic resonance spectroscopy, (vi) regional cerebral blood flows (rCBF), (vii) molecular genetics, and (viii) animal models. It is worth noting that there exist abnormalities of amino acidergic (glutamatergic and GABAergic) neurotransmission as well as monoaminergic (dopaminergic and serotonergic) one in postmortem schizophrenic brains. These abnormalities and also the findings of altered rCBF indicate the existence of disturbed neuronal circuits that contribute to the diverse symptoms of schizophrenia. Also, dysfunction of membrane phospholipids derived from studies on magnetic resonance spectroscopy may underlie negative symptoms in schizophrenia. Given that schizophrenia is considered to comprise a group of disorders with a diverse heterogeneity of etiologies, research in the next decade is expected to identify putative genes that are involved in vulnerability to schizophrenic phenotype.

Expression of dopamine transporter and vesicular monoamine transporter 2 mRNAs in rat midbrain after repeated amphetamine administration

The dopamine transporter (DAT) in pre-synaptic membranes and the vesicular monoamine transporter 2 (VMAT2) in membranes of synaptic vesicles are involved in mediating the acute effects of amphetamine on dopamine transmission. Therefore, using a quantitative method of in situ hybridization and computerized image analysis, the expression of DAT and VMAT2 mRNAs was examined in rats treated for 5 days with amphetamine and killed 3 or 14 days after the last injection. We examined ventral tegmental area (VTA), substantia nigra (SN) and the transitional zone between VTA and SN. Each of these regions was further subdivided into rostral, intermediate and caudal portions. In control rats, autoradiographs revealed a gradient of both DAT and VMAT2 mRNA levels, decreasing gradually from rostral to caudal rat midbrain. After 3 days of withdrawal, a significant increase in DAT mRNA levels was found in rostral portions of VTA (117.9 + 5.8% of control group), SN (116.5 + 4.5%) and the transitional zone (119.6 + 5.6%) and in the intermediate portion of SN (113.5 + 4.3%). VMAT2 mRNA was significantly increased only in rostral and intermediate portions of the transitional zone (120.9 + 4.8 and 113.6 + 4.1%). After 14 days of withdrawal, there was a trend towards increased DAT mRNA levels in intermediate-caudal portions of midbrain, but a statistically significant increase was observed only in the intermediate portion of VTA (120.2 + 7.9%). No changes in VMAT2 mRNA levels were found. Thus, repeated amphetamine administration exerts modest and regionally selective effects on DAT and VMAT2 mRNA expression in subpopulations of midbrain dopamine neurons.

Effects of acute and repeated administration of N-methyl-D-aspartate (NMDA) into the ventral tegmental area: locomotor activating effects of NMDA and cocaine

Repeated, intermittent administration of psychostimulants produces an enhancement of the subsequent behavioral effects of these drugs. This behavioral sensitization has been implicated in maintenance of and relapse to drug-taking. As a result, there has been great interest in elucidating the mechanisms underlying both the development and expression of sensitization. An accumulation of data from studies of stimulant-induced locomotor activity has implicated excitatory amino acids in the development of behavioral sensitization. In the present study, N-methyl-D-aspartate (NMDA) (0.6, 1.25 or 2.5 microg) infused bilaterally into the ventral tegmental area (VTA) produced dose-dependent locomotor activation. The locomotor activating effect of NMDA was increased following repeated NMDA administration (two exposures to intra-VTA NMDA), suggesting sensitization. However, repeated intra-VTA NMDA failed to sensitize rats to the locomotor activating effects of systemically administered cocaine (5.0, 10.0 or 20.0 mg/kg). These findings are consistent with the notion that repeated activation of NMDA receptors is sufficient for the development of behavioral sensitization to NMDA. Other neuroadaptations produced by repeated psychostimulant administration are required in order for the development of sensitization to the behavioral effects of those drugs.

Tolerance to the neurotoxic effect of methamphetamine in rats behaviorally sensitized to methamphetamine or amphetamine

A series of experiments was conducted to examine whether rats behaviorally sensitized to methamphetamine (MA) would show supersensitivity or tolerance to the MA-induced neurotoxic effects on dopaminergic and serotonergic nerve terminals in the striatum (ST), nucleus accumbens (NA) and medial frontal cortex (MFC). Moderate to high doses of MA (3, 4 and 5 mg HCl salt/kg, s.c., at 2 h intervals, four injections) dose-relatedly decreased the contents of dopamine (DA), dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in ST, and the content of 5-HIAA in NA and that of 5-HT in MFC. These neurotoxic effects in ST were significantly attenuated in rats behaviorally sensitized to MA (4 mg HCl salt/kg, s.c., for 10 days). To examine the possibility that the attenuation in the toxic effects in sensitized rats was due to an accelerated metabolism from MA to amphetamine (AMPH), a high dose of MA (5 mg HCl salt/kg, s.c., at 2 h intervals, four injections) was administered to rats behaviorally sensitized to AMPH (4 mg HCl salt/kg, s.c., for 10 days). It was revealed that the MA-induced decrease in the striatal contents of DOPAC, homovanillic acid (HVA), 5-HT and 5-HIAA were attenuated in rats behaviorally sensitized to AMPH. The MA-induced decrease in the striatal DA content tended to be attenuated in AMPH-sensitized rats. These data suggest that rats behaviorally sensitized to MA or AMPH develop tolerance to MA-induced striatal dopaminergic and serotonergic neurotoxicity. It is speculated that the mechanism of tolerance might be mediated by an altered central response rather than peripheral metabolism.

Changes of behavior and monoamine metabolites in the rat brain after repeated methamphetamine administration: effects of duration of repeated administration

1. The authors studied the mechanism of the reverse-tolerance phenomenon caused by long-term administration of central stimulant drugs. Methamphetamine(MAP) was chronically administered to rats, and the reverse-tolerance phenomenon was studied in terms of behavioral changes and changes in monoamine metabolites, the latter being examined by in vivo microdialysis of the extracellular compartment of the corpus-striatum. The authors also studied [3H]SCH23390 and [3H]spiperone binding to striatal membranes after chronic MAP administration. 2. MAP(4 mg/kg) or saline was administered intraperitoneally once daily to male rats. In Groups 1 and 2, 10 and 30 injections of MAP were given, respectively. In Groups 3 and 4, animals received 10 and 30 injections of saline as controls. One week after the final injection, all rats were challenged with 4 mg/kg MAP. 3. Groups 1 and 2 displayed more intense stereotypy than Groups 3 and 4, indicating that behavioral sensitization had been achieved in the former. Dopamine(DA) levels increased rapidly in response to MAP challenge in all groups, with the increases in Groups 1 and 2 being more marked than that in Groups 3 and 4. Group 1 showed greater persistence and a higher rate of DA increase than Group 2. 4. The number of D1 and D2 dopamine receptors did not change after the repeated MAP administration. 5. The rate of increase in DA release induced by MAP was dependent on the duration of repeated administration, and there was no correlation between the intensity of stereotypy and the rate of increase in DA release induced by MAP. These findings suggest that enhancement in DA release is unlikely to be the sole cause of behavioral sensitization.

Positron emission tomography study of the alterations in brain distribution of [11C]methamphetamine in methamphetamine-sensitized dog

We newly prepared a MAP-sensitized dog by repeated MAP treatment and studied the brain distribution of [11C]MAP in the normal and the MAP-sensitized dog using PET. The maximal level of accumulation of [11C]MAP in the sensitized dog brain was 1.4 times higher than that in the control. No difference was found in the metabolism of MAP between the two conditions. The significant increase of [11C]MAP in the MAP-sensitized brain indicates that subchronic MAP administration causes some functional change in the uptake site of MAP. The pharmacokinetic change may, in part, account for behavioral sensitization.

Prevention of methamphetamine-induced behavioral sensitization in rats by a cyclic AMP phosphodiesterase inhibitor, rolipram

Effects of an interaction between rolipram, a cyclic adenosine 3', 5'-monophosphate (cyclic AMP) phosphodiesterase inhibitor, and methamphetamine on the development of behavioral sensitization were observed in rats. In vivo microdialysis showed that a single dose of 4 mg/kg methamphetamine (i.p.) significantly increased striatal dopamine levels while coadministration with 4 mg/kg rolipram (i.p.) did not affect these levels. Also, methamphetamine alone did not alter striatal cyclic AMP levels but coadministration with rolipram and rolipram alone significantly increased these levels. The administration of 4 mg/kg methamphetamine (i.p.) once a day for 5 days significantly enhanced hyperlocomotion and rearing induced by a 2-mg/kg methamphetamine challenge (i.p.) after a 1-week withdrawal period, compared with controls or coadministration with 4 mg/kg rolipram (i.p.). Striatal dopamine levels, detected by in vivo microdialysis, were increased following the challenge but were comparable between the groups. These findings suggest that rolipram prevents methamphetamine-induced behavioral sensitization by increasing cyclic AMP levels while not affecting dopamine-releasing processes.