Extended-access, but not limited-access, methamphetamine self-administration induces behavioral and nucleus accumbens dopamine response changes in rats.

To better understand the neurobiology of methamphetamine (METH) dependence and the cognitive impairments induced by METH use, we compared the effects of extended (12 h) and limited (1 h) access to METH self-administration on locomotor activity and object place recognition, and on extracellular dopamine levels in the nucleus accumbens and caudate-putamen. Rats were trained to self-administer intravenous METH (0.05 mg/kg). One group had progressively extended access up to 12-h sessions. The other group had limited-access 1-h sessions. Microdialysis experiments were conducted during a 12-h and 1-h session, in which the effects of a single METH injection (self-administered, 0.05 mg/kg, i.v.) on extracellular dopamine levels were assessed in the nucleus accumbens and caudate-putamen compared with a drug-naive group. The day after the last 12-h session and the following day experimental groups were assessed for their locomotor activities and in a place recognition procedure, respectively. The microdialysis results revealed tolerance to the METH-induced increases in extracellular dopamine only in the nucleus accumbens, but not in the caudate-putamen in the extended-access group compared with the control and limited-access groups. These effects may be associated with the increased lever-pressing and drug-seeking observed during the first hour of drug exposure in the extended-access group. This increase in drug-seeking leads to higher METH intake and may result in more severe consequences in other structures responsible for the behavioral deficits (memory and locomotor activity) observed in the extended-access group, but not in the limited-access group.

Alterations in the striatal dopamine system during intravenous methamphetamine exposure: effects of contingent and noncontingent administration.

The continuing spread of methamphetamine (METH) abuse has stimulated research aimed at understanding consequences of its prolonged exposure. Alterations in nigrostriatal dopamine (DA) system parameters have been characterized in experimental studies after discontinuation of long-term METH but fewer studies have included similar assessments during METH exposure. Here, we report METH plasma pharmacokinetics and striatal DA system alterations in rat after noncontingent and contingent METH administration for 7.5 weeks. Escalating METH exposure was delivered by dynamic infusion (DI) that incorporated a "humanized" plasma METH half life or by intravenous self-administration (IVSA) that included binge intakes. Kinetic modeling of DI and IVSA for 24 h periods during the final week of METH exposure showed that plasma METH levels remained between 0.7 and 1.5 µM. Animals were sacrificed during their last METH administration for autoradiography assessment using [³H]ligands and D2 agonist-induced [³5S]GTPγS binding. DA transporter binding was decreased (DI, 34%; IVSA, 15%) while vesicular monoamine transporter binding and substantia nigra DA cell numbers were unchanged. Decreases were measured for D2 receptor (DI and IVSA, 15-20%) and [³5S]GTPγS binding (DI, 35%; IVSA, 18%). These similar patterns of DI and IVSA associated decreases in striatal DA markers reflect consequences of cumulative METH exposure and not the drug delivery method. For METH IVSA, individual differences were observed, yet each animal's total intake was similar within and across three 24-h binges. IVSA rodent models may be useful for identifying molecular mechanisms that are associated with METH binges in humans.

Development of stereotyped behaviors during prolonged escalation of methamphetamine self-administration in rats.

RATIONALE: Experimental animal studies have shown that repeated administration of psychostimulants, such as methamphetamine (METH), results in an altered behavioral response profile, which includes the sensitization of both locomotor and stereotyped behaviors. Although sensitization of these behaviors has been characterized in detail during bolus, investigator-administered drug administration, little is known about the development or expression of stereotypies during psychostimulant self-administration. OBJECTIVE/METHODS: The present study investigated in rats the expression of focused stereotyped behaviors during an extended access, escalation procedure of METH self-administration. Over several weeks during stepwise-extended daily access to METH (3, 6, and 12 h) followed by exposure to 24-h "binges," rats gradually increased daily drug intake. RESULTS: During the escalation procedure, the rats' behavioral response evolved from locomotor activation to progressively more focused stereotypies, culminating in continuous oral behaviors (licking, gnawing, and chewing), interrupted only by episodic lever presses. Sensitization of stereotyped behaviors was evident, particularly with regard to oral behaviors that exhibited a more rapid onset and intensification in the apparent absence of greater drug intake. CONCLUSIONS: Our data demonstrate that stepwise-extended daily access to METH (3, 6, 12, and 24 h) self-administration in rats closely approximates motivational, pharmacokinetic, as well as behavioral patterns of human METH abuse. The accompanied appearance of sensitization of intense focused stereotyped behaviors, which is probably a consequence of escalation of drug intake, resembles stereotypies associated with investigator-initiated METH administration and may parallel the development of stimulant-induced psychosis seen in human abusers.

Repeated phencyclidine administration alters glutamate release and decreases GABA markers in the prefrontal cortex of rats.

Repeated phencyclidine (PCP) administration induces cognitive disruptions resembling those seen in schizophrenia. Alterations in glutamate transmission and γ-aminobutyric acid (GABA) function in the prefrontal cortex (PFC) have been implicated in these PCP-induced deficits, as well as in cognitive symptoms of schizophrenia. PCP-induced cognitive deficits are reversed by chronic treatment with the atypical antipsychotic clozapine in rats. We investigated the effects of a single injection vs. repeated administration of PCP on glutamate levels in the PFC using in vivo microdialysis. Furthermore, we examined how these PCP regimens affect GABA neuronal markers in the PFC. Finally, we investigated the effects of clozapine on disruptions in glutamate levels and GABA neuronal markers induced by repeated PCP administration. Acute PCP administration (2 mg/kg) increased extracellular PFC glutamate; this increase appeared blunted, but was not eliminated, after repeated PCP pretreatment. PCP administration also strongly decreased levels of parvalbumin and glutamic acid decarboxylase-67 (two markers of GABA function) in the PFC, an effect that was maintained after a 10 day drug-free washout period and unaltered by the resumption of repeated PCP injections. All of the observed PCP effects were attenuated by chronic treatment with clozapine, an atypical antipsychotic that has partial effectiveness on cognitive impairment in schizophrenia. These findings suggest that abnormal cortical glutamate transmission, possibly driven by pathological changes in GABA function in parvalbumin-positive fast-spiking interneurons, may underlie some of the cognitive deficits in schizophrenia. A better understanding of glutamate and GABA dysregulation in schizophrenia may uncover new treatment targets for schizophrenia-related cognitive dysfunction.

The metabotropic glutamate 2/3 receptor agonist LY379268 blocked nicotine-induced increases in nucleus accumbens shell dopamine only in the presence of a nicotine-associated context in rats.

The metabotropic glutamate 2/3 (mGlu2/3) receptor agonist LY379268 ([-]-2-oxa-4-aminobicyclo [3.1.0] hexane-4,6-dicarboxylate) attenuates both nicotine self-administration and cue-induced nicotine seeking in rats. In this study, the effects of LY379268 (1 mg/kg) or saline pretreatment on nicotine-induced increases in nucleus accumbens (NAcc) shell dopamine were evaluated using in vivo microdialysis under different experimental conditions: (i) nicotine (0.4 mg/kg, base) was experimenter-administered subcutaneously to nicotine-naïve rats; (ii) nicotine was experimenter-administered either subcutaneously (0.4 mg/kg) or by a single experimenter-administered infusion (0.06 mg/kg, base) in rats with a history of nicotine self-administration (nicotine experienced) in the absence of a nicotine-associated context (ie, context and cues associated with nicotine self-administration); (iii) nicotine (0.06 mg/kg) was self-administered or experimenter-administered in nicotine-experienced rats in the presence of a nicotine-associated context. In saline-pretreated nicotine-naïve and nicotine-experienced rats, nicotine increased NAcc shell dopamine regardless of the context used for testing. Interestingly, LY379268 pretreatment blocked nicotine-induced increases in NAcc shell dopamine in nicotine-experienced rats only when tested in the presence of a nicotine-associated context. LY379268 did not block nicotine-induced increases in NAcc shell dopamine in nicotine-naïve or -experienced rats tested in the absence of a nicotine-associated context. These intriguing findings suggest that activation of mGlu2/3 receptors negatively modulates the combined effects of nicotine and nicotine-associated contexts/cues on NAcc dopamine. Thus, these data highlight a critical role for mGlu2/3 receptors in context/cue-induced drug-seeking behavior and suggest a neurochemical mechanism by which mGlu2/3 receptor agonists may promote smoking cessation by preventing relapse induced by the combination of nicotine and nicotine-associated contexts and cues.

Extended access to methamphetamine self-administration affects sensorimotor gating in rats.

Disturbed information processing observed in neuropsychiatric disorders is reflected by deficient sensorimotor gating, measured as prepulse inhibition (PPI) of the acoustic startle response (ASR). Long-term, higher dose methamphetamine (METH) abuse patterns are associated with cognitive impairments, mania and/or schizophrenia-like psychosis. The present study investigated in rats METH-induced impairment of sensorimotor gating using an intravenous self-administration (IVSA) escalating dose procedure. In this procedure, rats escalated drug intake during weekly extended access periods to METH IVSA (1, 3, and 6h), where PPI was assessed after each access period and thus at various times of drug exposure. Despite increased drug intake over the course of extended access to METH, disruption of sensorimotor gating was only seen after the access period of 6h. The data suggest that METH-induced impairment of sensorimotor gating in IVSA-tasks is rather attributed to continuous and higher dose exposure than to actual amounts of drug present at the time of testing. IVSA procedures, comprising stepwise stimulant escalation may serve as a useful translational model in rats that approximate important aspects of human abuse pattern in the context of stimulant-induced cognitive and behavioral deficits.

Evidence for genetic association of RORB with bipolar disorder.

BACKGROUND: Bipolar disorder, particularly in children, is characterized by rapid cycling and switching, making circadian clock genes plausible molecular underpinnings for bipolar disorder. We previously reported work establishing mice lacking the clock gene D-box binding protein (DBP) as a stress-reactive genetic animal model of bipolar disorder. Microarray studies revealed that expression of two closely related clock genes, RAR-related orphan receptors alpha (RORA) and beta (RORB), was altered in these mice. These retinoid-related receptors are involved in a number of pathways including neurogenesis, stress response, and modulation of circadian rhythms. Here we report association studies between bipolar disorder and single-nucleotide polymorphisms (SNPs) in RORA and RORB. METHODS: We genotyped 355 RORA and RORB SNPs in a pediatric cohort consisting of a family-based sample of 153 trios and an independent, non-overlapping case-control sample of 152 cases and 140 controls. Bipolar disorder in children and adolescents is characterized by increased stress reactivity and frequent episodes of shorter duration; thus our cohort provides a potentially enriched sample for identifying genes involved in cycling and switching. RESULTS: We report that four intronic RORB SNPs showed positive associations with the pediatric bipolar phenotype that survived Bonferroni correction for multiple comparisons in the case-control sample. Three RORB haplotype blocks implicating an additional 11 SNPs were also associated with the disease in the case-control sample. However, these significant associations were not replicated in the sample of trios. There was no evidence for association between pediatric bipolar disorder and any RORA SNPs or haplotype blocks after multiple-test correction. In addition, we found no strong evidence for association between the age-at-onset of bipolar disorder with any RORA or RORB SNPs. CONCLUSION: Our findings suggest that clock genes in general and RORB in particular may be important candidates for further investigation in the search for the molecular basis of bipolar disorder.

Human methamphetamine pharmacokinetics simulated in the rat: behavioral and neurochemical effects of a 72-h binge.

Bingeing is one pattern of high-dose methamphetamine (METH) abuse, which involves continuous drug taking over several days and can result in psychotic behaviors for which the brain pathology remains poorly defined. A corresponding animal model of this type of METH exposure may provide novel insights into the neurochemical and behavioral sequelae associated with this condition. Accordingly, to simulate the pharmacokinetic profile of a human METH binge exposure in rats, we used a computer-controlled, intravenous METH procedure (dynamic infusion, DI) to overcome species differences in METH pharmacokinetics and to replicate the human 12-h plasma METH half-life. Animals were treated over 13 weeks with escalating METH doses, using DI, and then exposed to a binge in which drug was administered every 3 h for 72 h. Throughout the binge, behavioral effects included unabated intense oral stereotypies in the absence of locomotion and in the absence of sleep. Decrements in regional brain dopamine, norepinephrine, and serotonin levels, measured at 1 and 10 h after the last injection of the binge, had, with the exception of caudate-putamen dopamine and frontal cortex serotonin, recovered by 48 h. At 10 h after the last injection of the binge, [(3)H]ligand binding to dopamine and vesicular monoamine transporters in caudate-putamen were reduced by 35 and 13%, respectively. In a separate METH binge-treated cohort, post-binge behavioral alterations were apparent in an attenuated locomotor response to a METH challenge infusion at 24 h after the last injection of the binge. Collectively, the changes we characterized during and after a METH binge suggest that for human beings under similar exposure conditions, multiple time-dependent neurochemical deficits contribute to their behavioral profiles.

Escalating dose-multiple binge methamphetamine exposure results in degeneration of the neocortex and limbic system in the rat.

Abuse of stimulant drugs such as methamphetamine (METH) and cocaine has been associated with long-lasting persistent behavioral alterations. Although METH-induced changes in the striatal dopaminergic system might play a role in these effects, the potential underlying neuroanatomical substrate for the chronic cognitive dysfunction in METH users is unclear. To investigate the involvement of non-dopaminergic systems in the neurotoxic effects of METH, we treated rats with an escalating dose-multiple binge regimen, which we have suggested may more closely simulate human METH exposure profiles. Combined neuropathological and stereological analyses showed that 30 days after the last binge, there was shrinkage and degeneration in the pyramidal cell layers of the frontal cortex and in the hippocampal CA3 region. Further immunocytochemical analysis showed that METH exposure resulted in loss of calbindin interneurons in the neocortex and selective damage to pyramidal neurons in the CA3 region of the hippocampus and granular cells in the dentate gyrus that was accompanied by microglial activation. Taken together, these studies suggest that selective degeneration of pyramidal neurons and interneurons in the neocortex and limbic system might be involved in the cognitive alterations in METH users.

Escalating dose, multiple binge methamphetamine regimen does not impair recognition memory in rats.

Rats exposed to methamphetamine (METH) in an acute high dose "binge" pattern have been reported to exhibit a persistent deficit in a novel object recognition (NOR) task, which may suggest a potential risk for human METH abusers. However, most high dose METH abusers initially use lower doses before progressively increasing the dose, only eventually engaging in multiple daily administrations. To simulate this pattern of METH exposure, we administered progressively increasing doses of METH to rats over a 14 day interval, then treated them with daily METH binges for 11 days. This treatment resulted in a persistent deficit in striatal dopamine (DA) levels of approximately 20%. We then tested them in a NOR task under a variety of conditions. We could not detect a deficit in their performance in the NOR task under any of the testing conditions. These results suggest that mechanisms other than or additional to the decrement in striatal DA associated with an acute METH binge are responsible for the deficit in the NOR task, and that neuroadaptations consequential to prolonged escalating dose METH pretreatment mitigate against these mechanisms.

Escalating dose pretreatment induces pharmacodynamic and not pharmacokinetic tolerance to a subsequent high-dose methamphetamine binge.

A major feature of human methamphetamine (METH) abuse is the gradual dose escalation that precedes high-dose exposure. The period of escalating doses (EDs) is likely associated with development of tolerance to aspects of METH's pharmacologic and toxic effects but the relative contributions of pharmacokinetic and pharmacodynamic factors have not been well defined. In our prior studies in rats, we showed that pretreatment with an ED-METH regimen (0.1-4.0 mg/kg over 14 days) attenuated the toxicity of a subsequently administered high-dose METH binge (4 x 6 mg/kg at 2 h interval) that itself produced behavioral stereotypy, increases in core temperature, and decreases in DA system phenotypic markers in caudate-putamen (CP). Using those ED-METH and binge protocols in the present studies, pharmacokinetic and pharmacodynamic parameters that may have contributed to the apparent neuroprotection afforded by ED-METH were assessed. The ED-METH regimen itself reduced [(3)H]WIN35,428 (WIN) binding to the dopamine transporter (DAT) by 15% in CP, but did not affect DA content. During the METH binge, ED-METH pretreated animals showed attenuated increases in core temperature while concurrent microdialysis studies in CP showed a reduced DA response despite unaltered extracellular levels of METH. At 1 h after the binge, concentrations of METH and its metabolite amphetamine in brain and plasma were unaffected by the ED-METH. The results show that ED-METH pretreatment produces reductions in DAT binding and the DA response during a subsequent METH binge by altering pharmacodynamic and not pharmacokinetic parameters.

Differential regulation of immediate-early gene expression in the prefrontal cortex of rats with a high vs low behavioral response to methamphetamine.

Methamphetamine (METH) administration mimics many of the symptoms of mania and can produce psychosis after chronic use. Both rodents and man display interindividual variation in response to METH. The molecular mechanisms underlying these differences might be relevant to both stimulant addiction and endogenous psychosis. We treated 50 Sprague-Dawley rats acutely with METH (4.0 mg/kg) and 10 control rats with saline, and measured their behavior for 3 h after drug administration. Animals were divided into high responders (HR) (top 20%) and low responders (LR) (lowest 20%) based on their stereotypy response. They were killed 24 h after injection. Total RNA was extracted from the prefrontal cortex (PFC) and the expression of approximately 30 000 transcripts were analyzed using Affymetrix 230 2.0 GeneChips. Real-time reverse transcription-polymerase chain reaction was used to validate the expression of a select group of genes. Forty-three genes exhibited significant differences in expression in HR vs LR 24 h after METH treatment including a group of immediate-early genes (IEGs) (eg, c-fos, junB, NGFI-B, serum-regulated glucocorticoid kinase). These IEG expression differences were accompanied by the significant downregulation of many of these genes compared to saline in the HR but not LR, suggesting a differential responsiveness of signal transduction pathways in these two groups of rats. In addition, the expression of other transcription factors in the PFC was significantly different in HR compared to LR. These gene expression changes may contribute to individual differences in responsiveness to stimulants and the development of mania and psychosis.

Forebrain D1 function and sensorimotor gating in rats: effects of D1 blockade, frontal lesions and dopamine denervation.

Prefrontal D1 hypoactivity is implicated in the pathophysiology of schizophrenia, and might contribute to sensorimotor gating deficits in schizophrenia patients, based on evidence that D1 blockade in the medial prefrontal cortex (MPFC) reduces prepulse inhibition of startle (PPI) in animal models. PPI is disrupted by systemic and intra-MPFC infusion of the D1 antagonist, SCH23390. We investigated the role of the MPFC in the PPI-disruptive effects of systemic SCH23390 administration, and more generally, in the dopaminergic regulation of PPI. PPI was measured in rats after forebrain manipulations, including systemic administration of SCH23390, ibotenic acid lesions of the MPFC, and 6OHDA-induced dopamine (DA) depletion from MPFC or nucleus accumbens. Systemic SCH23390 disrupted PPI; these effects were not opposed by ibotenic acid lesions of the MPFC. PPI remained intact after MPFC DA depletion, but--as predicted by Bubser and Koch [M. Bubser, M. Koch, Prepulse inhibition of the acoustic startle response of rats is reduced by 6 hydroxydopamine lesions of the medial prefrontal cortex, Psychopharmacology 113 (1994) 487-492]--a reduction in PPI from pre- to post-surgery correlated significantly with MPFC DA loss. The effects of systemic SCH23390 were not opposed by NAC DA depletion. D1 receptors regulate PPI in rats, but this effect does not appear to be mediated either by the MPFC or by increased mesolimbic DA activity.

Human methamphetamine pharmacokinetics simulated in the rat: single daily intravenous administration reveals elements of sensitization and tolerance.

We developed a computer-controlled intravenous methamphetamine (METH) administration procedure (dynamic infusion), which enables us to compensate for an important pharmacokinetic difference between rats and humans by imposing a 12-h half-life for the drug in rats. Dynamic infusion of 0.5 mg/kg METH produced a pharmacokinetic profile that closely simulates the METH exposure pattern in humans, including an apparent half-life of 11.6+/-1.3 h, and an area under the concentration vs time curve of 9.4 microM h, about 20-fold larger than results obtained with typical rat pharmacokinetics. Using this procedure, METH produced a prolonged behavioral stimulation and elevation in caudate extracellular dopamine (DA). Both the behavioral and the DA effects exhibited tolerance to the sustained plasma METH exposure. Single daily dynamic infusion of 0.5 mg/kg METH for 15 days resulted in a progressive enhancement of the behavioral response until about Day 10. On subsequent days, in addition to continued evidence of sensitization, tolerance in the form of a marked decrease in the duration of the behavioral activation became a prominent feature of the response. Qualitative changes in the behavior also emerged. Resumption of METH treatment following 4 days of withdrawal revealed that sensitization was apparent during the first dynamic infusion, and that tolerance re-emerged within two additional days of drug administration. These results showed that a human-like METH exposure pattern produced behavioral and striatal DA response profiles that are both quantitatively and qualitatively different from the effects typically observed with single daily METH injections in rats. Thus, simulation of human METH exposure patterns may be a critical prerequisite to identifying mechanisms relevant to the chronic use of this drug in humans.

Prolonged exposure of rats to intravenous methamphetamine: behavioral and neurochemical characterization.

The translational value of preclinical models of methamphetamine abuse depends in large part on the degree to which the drug regimens used in animals produce methamphetamine exposure patterns similar to those experienced by human methamphetamine abusers. To approximate one common form of methamphetamine abuse, we studied the effects of a schedule of intravenous methamphetamine administration in rats which included 2 weeks of progressively more frequent drug injections (0.125 mg/kg/injection) followed by 40 maintenance days during which animals received 40 daily injections (at 15-min intervals), with the dose gradually increasing (0.125-0.25 mg/kg per injection) every 5-10 days. This treatment produced an emerging behavioral profile characterized by gradually more continuous periods of activation consisting of progressively more intense, focused stereotypy interrupted by episodic bursts of locomotion. We also assessed markers of dopamine neurotransmission (dopamine transporter, vesicular monoamine transporter, and dopamine D1 and D2 receptors) at 15 min and (including dopamine levels) at 6 and 30 days following cessation of methamphetamine treatment. All dopamine components measured in caudate-putamen were significantly reduced at 15 min and 6 days after the final methamphetamine injection. Dopamine D1 and D2 receptors fully recovered after 30 days of drug abstinence, whereas dopamine and the dopamine transporter exhibited significant but incomplete recovery by this time point. In contrast, only the vesicular monoamine transporter exhibited no evidence of recovery over the 30-day withdrawal period. These data are discussed in terms of damage to dopamine terminals and compensatory adjustments in mechanisms maintaining functional dopaminergic transmission.

Stimulant actions in rodents: implications for attention-deficit/hyperactivity disorder treatment and potential substance abuse.

Most evidence supports the continued use of stimulants as the best available pharmacotherapy for the treatment of children with attention-deficit/hyperactivity disorder (ADHD), but little is known about possible enduring behavioral and neuroadaptational consequences of long-term stimulant exposure. Although a variety of preclinical studies, particularly those using methylphenidate (MP), have attempted to address these issues, most of these studies have used procedures that might not adequately simulate clinical treatment conditions, and results have not been entirely consistent. In particular, the rationale for selection of MP doses that simulate clinical exposure has not been well defined. We suggest that the use of more appropriate treatment conditions, including doses that result in plasma drug levels comparable to therapeutic levels, will provide a more accurate model for adequately assessing the therapeutic mechanisms and potential long-term consequences of stimulant psychotherapy in the treatment of ADHD.

Neurochemical analysis of rat strain differences in the startle gating-disruptive effects of dopamine agonists.

The disruption of prepulse inhibition (PPI) in rats by dopamine (DA) agonists is used to study the neural basis of strain differences in dopaminergic function. We reported that, compared to Long-Evans (LEH) rats, Sprague-Dawley (SDH) rats are more sensitive to the PPI-disruptive effects of the direct D1/D2 agonist apomorphine (APO) and the indirect DA agonist d-amphetamine (AMPH). This strain difference is heritable, with PPI drug sensitivity following a generational pattern (SDH>N2>F1>LEH) suggestive of additive effects of multiple genes. Here, we assessed the neurochemical bases for these heritable strain differences by measuring tissue levels of dopamine, serotonin (5HT) and their respective metabolites in several forebrain regions after vehicle, APO or AMPH administration. SDH rats were more sensitive than LEH rats to the PPI-disruptive effects of both APO (0.5 mg/kg) and AMPH (4.5 mg/kg). Several significant SDH vs. LEH strain differences in regional neurochemical levels were detected, as were drug effects on these chemicals. However, SDH, LEH and F1 rats did not exhibit differential drug sensitivity in any neurochemical indices measures. These findings suggest that inherited differences in the dopaminergic regulation of sensorimotor gating do not likely reflect differences in presynaptic forebrain dopaminergic or serotonergic processes.

Altered extracellular dopamine concentration in the brains of cholecystokinin-A receptor deficient rats.

The gut-brain peptide cholecystokinin (CCK) has been implicated in the regulation of dopamine (DA) transmission in the brain. CCK agonists have been shown to modify baseline and stimulant-induced DA release in the brain via CCK-A mediated mechanisms. However, the role of endogenous CCK in regulating brain DA via CCK-A receptors has not been fully elucidated. Recently, a strain of rats (Otsuka Long Evans Tokushima Fatty (OLETF)), lacking the CCK-A receptor due to a genetic mutation, was discovered, providing a potentially useful tool to study the DA regulatory role of CCK-A receptors. In order to further clarify the role of CCK-A receptors in the regulation of central DA transmission, extracellular DA levels in the nucleus accumbens (NAC) and the caudate-putamen (CP) of OLETF rats, and their non-mutant counterparts, Long Evans Tokushimo Otsuka rats, was assessed by microdialysis at baseline and in response to cocaine (15 mg/kg) and amphetamine (0.5 mg/kg) administration. Baseline levels of extracellular DA were significantly elevated in the CP but not in the NAC of OLETF rats. In contrast, the NAC exhibited a greater DA response to cocaine (15 mg/kg) and amphetamine (0.5 mg/kg) in OLETF rats. This is the first direct evidence, of which we are aware, supporting altered DA regulation in OLETF rats. These findings suggest that CCK-A receptors play an important role in the regulation of central DA transmission, and support the notion that the OLETF rat is a useful model to study this regulation.

Escalating dose methamphetamine pretreatment alters the behavioral and neurochemical profiles associated with exposure to a high-dose methamphetamine binge.

The neurotoxic effects of methamphetamine (METH) have been characterized primarily from the study of high-dose binge regimens in rodents. However, this drug administration paradigm does not include a potentially important feature of stimulant abuse in humans, that is, the gradual escalation of stimulant doses that frequently occurs prior to high-dose exposure. We have argued that pretreatment with escalating doses (EDs) might significantly alter the neurotoxic profile produced by a single high-dose binge. In the present study, we tested this hypothesis by pretreating rats with saline or gradually increasing doses of METH (0.1-4.0 mg/kg over 14 days), prior to an acute METH binge (4 x 6 mg/kg at 2 h intervals). These animals, whose behavior was continuously monitored throughout drug treatment, were then killed 3 days later for determination of caudate-putamen dopamine (DA) content, levels of [(3)H]WIN 35,428 binding to the DA transporter, and levels of [(3)H]dihydrotetrabenazine ([(3)H]DTBZ) binding to the vesicular monoamine transporter. ED pretreatment markedly attenuated the stereotypy response, as well as the hyperthermia and indices of sympathetic activation associated with the acute binge. In addition, ED pretreatment prevented the decline in [(3)H]WIN 35,428 binding, and significantly diminished the decrease in DA levels, but did not affect the decrease in [(3)H]DTBZ binding associated with the acute binge. We suggest that further study of the effects produced by a regimen which includes a gradual escalation of doses prior to high-dose METH binge exposure could more accurately identify the neurochemical and behavioral changes relevant to those that occur as a consequence of high-dose METH abuse in humans.

Oxidative mechanisms and tardive dyskinesia.

Tardive dyskinesia has been and continues to be a significant problem associated with long-term antipsychotic use, but its pathophysiology remains unclear. In the last 10 years, preclinical studies of the administration of antipsychotics to animals, as well as clinical studies of oxidative processes in patients given antipsychotic medications, with and without tardive dyskinesia, have continued to support the possibility that neurotoxic free radical production may be an important consequence of antipsychotic treatment, and that such production may relate to the development of dyskinetic phenomena. In line with this hypothesis, evidence has accumulated for the efficacy of antioxidants, primarily vitamin E (alpha-tocopherol), in the treatment and prevention of tardive dyskinesia. Early studies suggested a modest effect of vitamin E treatment on existing tardive dyskinesia, but later studies did not demonstrate a significant effect. Because evidence has continued to accumulate for increased oxidative damage from antipsychotic medications, but less so for the effectiveness of vitamin E, especially in cases of long-standing tardive dyskinesia, alternative antioxidant approaches to the condition may be warranted. These approaches may include the use of antioxidants as a preventive measure for tardive dyskinesia or the use of other antioxidants or neuroprotective drugs, such as melatonin, for established tardive dyskinesia.