Serum neurofilament light levels in natalizumab-treated patients with multiple sclerosis who switch to extended interval dosing from every-4-week dosing in real-world clinical practice.

BACKGROUND: Serum levels of neurofilament light chain (sNfL) are a potentially useful biomarker for assessing the efficacy of multiple sclerosis (MS) treatments. OBJECTIVE: To compare levels of sNfL in patients with MS who switched from natalizumab every 4 weeks (Q4W) to extended interval dosing (EID) and patients who remained on Q4W dosing in real-world clinical practice. METHODS: This was a retrospective analysis of samples from patients treated with natalizumab from 2010 to 2015 at a single center in the United States. Levels of sNfL were compared in patients who stayed on Q4W dosing or who switched to EID (parallel-arm analyses) and during Q4W and EID periods in patients who switched to EID (pre- and post-switch analyses). RESULTS: The analysis included 139 patients (Q4W: n = 79; EID: n = 60). After adjustment, levels of sNfL did not significantly differ between patients who remained on Q4W dosing and those who switched to EID in parallel-arm analyses (adjusted Q4W-EID difference = 0.51 pg/mL; p = 0.60) or pre- and post-switch analyses (adjusted difference = 0.96 pg/mL; p = 0.10). CONCLUSION: These sNfL biomarker results suggest that the effectiveness of natalizumab is maintained in patients who switch from Q4W dosing to EID.

Progressive Multifocal Leukoencephalopathy Risk Perception in Patients Considering Natalizumab for Multiple Sclerosis.

Background: Progressive multifocal leukoencephalopathy (PML) remains a concern when considering natalizumab for multiple sclerosis (MS) treatment. Extensive research has identified factors that increase PML risk, and it is important that providers and patients accurately understand risk to make appropriate benefit-risk decisions. Methods: One hundred adult US patient-candidates for natalizumab therapy were questioned about their PML risk perception, the maximum PML risk they deemed acceptable, and sources of information used to understand risk. Differences in group distributions were compared. Results: Patients estimated their potential PML risk from 0.1% to 87% (mean, 31.5%). Maximum PML risk deemed acceptable ranged from 0.1% to 45% (mean, 14.5%). Actual risk (mean, 0.01%), based on published risk estimates, was calculated as a function of time receiving therapy, anti-John Cunningham virus antibody index, and previous use of immunosuppressants. The sexes perceived their risks similarly and had similar risk acceptance. Patient perception of PML risk increased with age, whereas willingness to accept risk remained similar among all ages. Higher levels of education correlated with more accurate risk perception and lower risk tolerance. Neither risk perception nor tolerance was correlated with disability level. Sixty-three percent of patients indicated that their primary/referring physician's concern level regarding potential risk of PML during the benefit-risk discussion was their main source of information about risk. Conclusions: Patients with MS substantially overestimated their PML risk, often by three orders of magnitude. Patients with MS could benefit from accurate risk education, and providers could play an essential role in presenting PML risk information in a manner understandable to each individual patient.

Risk of natalizumab-associated PML in patients with MS is reduced with extended interval dosing.

OBJECTIVE: To use the large dataset from the Tysabri Outreach: Unified Commitment to Health (TOUCH) program to compare progressive multifocal leukoencephalopathy (PML) risk with natalizumab extended interval dosing (EID) vs standard interval dosing (SID) in patients with multiple sclerosis (MS). METHODS: This retrospective cohort study included anti-JC virus antibody-positive patients (n = 35,521) in the TOUCH database as of June 1, 2017. The effect of EID on PML risk was evaluated with 3 planned analyses using Kaplan-Meier methods stratified by prior immunosuppressant use. Risk of PML was analyzed by Cox regression adjusted for age, sex, prior immunosuppressants, time since natalizumab initiation, and cumulative number of infusions. RESULTS: This study included 35,521 patients (primary analysis: 1,988 EID, 13,132 SID; secondary analysis: 3,331 EID, 15,424 SID; tertiary analysis: 815 EID, 23,168 SID). Mean average dosing intervals were 35.0 to 43.0 and 29.8 to 30.5 days for the EID and SID cohorts, respectively. Hazard ratios (95% confidence intervals) of PML risk for EID vs SID were 0.06 (0.01-0.22, p < 0.001) and 0.12 (0.05-0.29, p < 0.001) for the primary and secondary analyses, respectively. Relative risk reductions were 94% and 88% in favor of EID for the primary and secondary analyses, respectively. The tertiary analysis included no cases of PML with EID. CONCLUSION: Natalizumab EID is associated with clinically and statistically significantly lower PML risk than SID. CLASSIFICATION OF EVIDENCE: This study provides Class III evidence that for patients with MS, natalizumab EID is associated with a lower PML risk than SID.

Methylphenidate administration alters vesicular monoamine transporter-2 function in cytoplasmic and membrane-associated vesicles.

In vivo methylphenidate (MPD) administration increases vesicular monoamine transporter-2 (VMAT-2) immunoreactivity, VMAT-2-mediated dopamine (DA) transport, and DA content in a nonmembrane-associated (referred to herein as cytoplasmic) vesicular subcellular fraction purified from rat striatum: a phenomenon attributed to a redistribution of VMAT-2-associated vesicles within nerve terminals. In contrast, the present study elucidated the nature of, and the impact of MPD on, VMAT-2-associated vesicles that cofractionate with synaptosomal membranes after osmotic lysis (referred to herein as membrane-associated vesicles). Results revealed that, in striking contrast to the cytoplasmic vesicles, DA transport velocity versus substrate concentration curves in the membrane-associated vesicles were sigmoidal, suggesting positive cooperativity with respect to DA transport. Additionally, DA transport into membrane-associated vesicles was greater in total capacity in the presence of high DA concentrations than transport into cytoplasmic vesicles. Of potential therapeutic relevance, MPD increased DA transport into the membrane-associated vesicles despite rapidly decreasing (presumably by redistributing) VMAT-2 immunoreactivity in this fraction. Functional relevance was suggested by findings that MPD treatment increased both the DA content of the membrane-associated vesicle fraction and K(+)-stimulated DA release from striatal suspensions. In summary, the present data demonstrate the existence of a previously uncharacterized pool of membrane-associated VMAT-2-containing vesicles that displays novel transport kinetics, has a large sequestration capacity, and responds to in vivo pharmacological manipulation. These findings provide insight into both the regulation of vesicular DA sequestration and the mechanism of action of MPD, and they may have implications regarding treatment of disorders involving abnormal DA disposition, including Parkinson's disease and substance abuse.

Therapeutic doses of amphetamine and methylphenidate selectively redistribute the vesicular monoamine transporter-2.

High-dose administration of psychostimulants traffics the vesicular monoamine transporter-2 (VMAT-2), as assessed by subcellular fractionation of rat striatal tissue. This study demonstrates that administration of low doses of amphetamine or methylphenidate differentially traffic VMAT-2 within nerve terminals, with effects similar to those observed after high-dose administration. Trafficking of vesicular glutamate, acetylcholine, or GABA transporters was not altered by high-or low-dose amphetamine or methylphenidate treatment. These data represent the first report that amphetamine redistributes VMAT-2 protein. In addition, these data demonstrate that the trafficking of VMAT-2 after amphetamine or methylphenidate is selective for monoaminergic neurons.

New insights into the mechanism of action of amphetamines.

Amphetamine is a psychostimulant commonly used to treat several disorders, including attention deficit, narcolepsy, and obesity. Plasmalemmal and vesicular monoamine transporters, such as the neuronal dopamine transporter and the vesicular monoamine transporter-2, are two of its principal targets. This review focuses on new insights, obtained from both in vivo and in vitro studies, into the molecular mechanisms whereby amphetamine, and the closely related compounds methamphetamine and methylenedioxymethamphetamine, cause monoamine, and particularly dopamine, release. These mechanisms include amphetamine-induced exchange diffusion, reverse transport, and channel-like transport phenomena as well as the weak base properties of amphetamine. Additionally, amphetamine analogs may affect monoamine transporters through phosphorylation, transporter trafficking, and the production of reactive oxygen and nitrogen species. All of these mechanisms have potential implications for both amphetamine- and methamphetamine-induced neurotoxicity, as well as dopaminergic neurodegenerative diseases.

Mechanisms of methamphetamine-induced dopaminergic neurotoxicity.

Methamphetamine (METH) is a powerful stimulant of abuse with potent addictive and neurotoxic properties. More than 2.5 decades ago, METH-induced damage to dopaminergic neurons was described. Since then, numerous advancements have been made in the search for the underlying mechanisms whereby METH causes these persistent dopaminergic deficits. Although our understanding of these mechanisms remains incomplete, combinations of various complex processes have been described around a central theme involving reactive species, such as reactive oxygen and/or nitrogen species (ROS and RNS, respectively). For example, METH-induced hyperthermia, aberrant dopamine(DA), or glutamate transmission; or mitochondrial disruption leads to the generation of reactive species with neurotoxic consequences. This review will describe the current understanding of how high-dose METH administration leads to the production of these toxic reactive species and consequent permanent dopaminergic deficits.

Polymorphisms and haplotypes of the regulator of G protein signaling-2 gene in normotensives and hypertensives.

Regulator of G protein signaling (RGS) proteins stimulate the GTPase activity of Galpha subunits of heterotrimeric G proteins, thereby negatively regulating G protein-coupled receptor signaling. RGS2, which preferentially alters Galphaq-mediated signaling, may be important for cardiovascular health, because knockout of RGS2 in mice is associated with altered smooth muscle relaxation and hypertension. In this study, we determined genetic variation in the human RGS2 gene by sequencing DNA in normotensive and hypertensive populations of whites (n=128) and blacks (n=122). We identified 14 single nucleotide polymorphisms and 2 two-base insertion/deletions (in/del; 1891 to 1892 TC and 2138 to 2139 AA). Although most of the genetic variants were found at low allelic frequency, in particular in coding regions, the 1891 to 1892 TC and 2138 to 2139 AA intronic in/del were in linkage disequilibrium and were associated with hypertension in blacks (P<0.05). We defined several haplotypes for the RGS2 gene, certain of which showed striking differences between whites and blacks. Additionally, 2 haplotypes had significantly different frequencies between hypertensive and normotensive black groups (P<0.05). We conclude that RGS2 is genetically conserved within coding regions but that the intronic in/del define ethnicity-specific haplotypes. Moreover, certain RGS2 variants that occur at greater frequency in hypertensive blacks may serve as ethnicity-specific genetic variants for this disease.

Multi-tasking RGS proteins in the heart: the next therapeutic target?

Regulator of G-protein-signaling (RGS) proteins play a key role in the regulation of G-protein-coupled receptor (GPCR) signaling. The characteristic hallmark of RGS proteins is a conserved approximately 120-aa RGS region that confers on these proteins the ability to serve as GTPase-activating proteins (GAPs) for G(alpha) proteins. Most RGS proteins can serve as GAPs for multiple isoforms of G(alpha) and therefore have the potential to influence many cellular signaling pathways. However, RGS proteins can be highly regulated and can demonstrate extreme specificity for a particular signaling pathway. RGS proteins can be regulated by altering their GAP activity or subcellular localization; such regulation is achieved by phosphorylation, palmitoylation, and interaction with protein and lipid-binding partners. Many RGS proteins have GAP-independent functions that influence GPCR and non-GPCR-mediated signaling, such as effector regulation or action as an effector. Hence, RGS proteins should be considered multifunctional signaling regulators. GPCR-mediated signaling is critical for normal function in the cardiovascular system and is currently the primary target for the pharmacological treatment of disease. Alterations in RGS protein levels, in particular RGS2 and RGS4, produce cardiovascular phenotypes. Thus, because of the importance of GPCR-signaling pathways and the profound influence of RGS proteins on these pathways, RGS proteins are regulators of cardiovascular physiology and potentially novel drug targets as well.

Role of monoamine transporters in mediating psychostimulant effects.

Monoamine transporters such as the dopamine (DA) transporter (DAT) and the vesicular monoamine transporter-2 (VMAT-2) are critical regulators of DA disposition within the brain. Alterations in DA disposition can lead to conditions such as drug addiction, Parkinson's disease, and schizophrenia, a fact that underscores the importance of understanding DAergic signaling. Psychostimulants alter DAergic signaling by influencing both DAT and VMAT-2, and although the effects of these drugs result in increased levels of synaptic DA, the mechanisms by which this occurs and the effects that these drugs exert on DAT and VMAT-2 vary. Many psychostimulants can be classified as releasers (ie, amphetamine analogs) or uptake blockers (ie, cocaine-like drugs) based on the mechanism of their acute effects on neurotransmitter flux through the DAT. Releasers and uptake blockers differentially modulate the activity and subcellular distribution of monoamine transporters, a phenomenon likely related to the neurotoxic potential of these drugs to DAergic neurons. This article will review some of the recent findings whereby releasers and uptake blockers alter DAT and VMAT-2 activity and how these alterations may be involved in neurotoxicity, thus providing insight on the neurodegeneration observed in Parkinson's disease.

Psychostimulants and vesicle trafficking: a novel mechanism and therapeutic implications.

The monoamine vesicular transporter 2 (VMAT-2) has been associated with dopamine (DA) sequestration and protection against neurodegeneration caused by the intracellular oxidation of this monoamine. The data presented herein suggest that methylphenidate treatment enhances the amount of VMAT-2 protein and possibly its activity in the presynaptic cytosol, where it is able to increase the sequestration of DA and likely protect against its instability. In contrast, methamphetamine (METH) has an opposite effect on cytosolic VMAT-2 resulting in degradation of DA terminals. The fact that posttreatment of methylphenidate after a neurotoxic regimen of METH protects against resulting loss of DA parameters suggests that treatment with methylphenidate, or other DA transporter blockers, may be protective against degenerative disorders of DA pathways, such as Parkinson's disease.

Methamphetamine increases dopamine transporter higher molecular weight complex formation via a dopamine- and hyperthermia-associated mechanism.

Multiple high-dose administrations of methamphetamine (METH) both rapidly (within hours) decrease plasmalemmal dopamine (DA) uptake and cause long-term deficits in DA transporter (DAT) levels and other dopaminergic parameters persisting weeks to months in rat striatum. In contrast, either a single administration of METH or multiple administrations of methylenedioxymethamphetamine (MDMA) cause less of an acute reduction in DA uptake and little or no persistent dopaminergic deficits. The long-term dopaminergic deficits caused by METH have been suggested, in part, to involve the DAT. Hence, this study assessed the impact of METH and MDMA administration on the DAT protein per se. Results revealed that multiple administrations of METH promoted formation of higher molecular weight (>170 kDa) DAT-associated protein complexes 24-48 hr after treatment. This increase was attenuated by either preventing hyperthermia or pretreatment with the tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine; notably, each of these manipulations has also been demonstrated previously to prevent the persistent deficits in dopaminergic function caused by METH treatment. In contrast, either a single injection of METH or multiple injections of MDMA caused little or no formation of these DAT complexes. The addition of the reducing agent beta-mercaptoethanol to samples prepared from METH-treated rats diminished the intensity of these complexes. Taken together, these data are the first to demonstrate higher molecular weight DAT complex formation in vivo and that such formation can be altered by both pharmacological and physiological manipulations. The implications of this phenomenon with regard to the neurotoxic potential of these stimulants are discussed.

Methamphetamine rapidly decreases mouse vesicular dopamine uptake: role of hyperthermia and dopamine D2 receptors.

Multiple high-dose administrations of the dopamine-releasing agent, methamphetamine, rapidly and persistently decrease vesicular dopamine uptake in purified vesicles prepared from striata of treated rats. Because important differences in the neurotoxic effects of stimulants have been documented in rats and mice, the purpose of this study was to determine if methamphetamine-induced effects in rats occur in mice and to elucidate mechanisms underlying these effects. Results reveal methamphetamine treatment rapidly decreased mouse striatal vesicular dopamine uptake; a phenomenon associated with a subcellular redistribution of vesicular monoamine transporter-2 (VMAT-2) immunoreactivity. Both methamphetamine-induced hyperthermia and dopamine D2 receptor activation contributed to the stimulant-induced deficits in vesicular dopamine uptake. Unlike methamphetamine, the dopamine reuptake inhibitors, methylphenidate and cocaine, rapidly increased vesicular dopamine uptake. The implications of these phenomena are discussed.

Methylphenidate alters vesicular monoamine transport and prevents methamphetamine-induced dopaminergic deficits.

It has been hypothesized that high-dose methamphetamine treatment rapidly redistributes cytoplasmic dopamine within nerve terminals, leading to intraneuronal reactive oxygen species formation and well characterized persistent dopamine deficits. We and others have reported that in addition to this persistent damage, methamphetamine treatment rapidly decreases vesicular dopamine uptake, as assessed in purified vesicles prepared from treated rats; a phenomenon that may contribute to aberrant intraneuronal dopamine redistribution proposedly caused by the stimulant. Interestingly, post-treatment with dopamine transporter inhibitors protect against the persistent dopamine deficits caused by methamphetamine; however, mechanisms underlying this phenomenon have not been elucidated. Also of interest are findings that dopamine transporter inhibitors, including methylphenidate, rapidly increase 1) vesicular dopamine uptake, 2) vesicular monoamine transporter-2 (VMAT-2) ligand binding, and 3) VMAT-2 immunoreactivity in a vesicular subcellular fraction prepared from treated rats. Therefore, we hypothesized that methylphenidate post-treatment might protect against the persistent striatal dopamine deficits caused by methamphetamine by rapidly affecting VMAT-2 and vesicular dopamine content. Results reveal that methylphenidate post-treatment both prevents the persistent dopamine deficits and reverses the acute decreases in vesicular dopamine uptake and VMAT-2 ligand binding caused by methamphetamine treatment. In addition, methylphenidate post-treatment reverses the acute decreases in vesicular dopamine content caused by methamphetamine treatment. Taken together, these findings suggest that methylphenidate prevents persistent methamphetamine-induced dopamine deficits by redistributing vesicles and the associated VMAT-2 protein and presumably affecting dopamine sequestration. These findings not only provide insight into the neurotoxic effects of methamphetamine but also mechanisms underlying dopamine neurodegenerative disorders, including Parkinson's disease.

Ceramide-induced alterations in dopamine transporter function.

The purpose of this study was to determine the effects of ceramide on dopamine and serotonin (5-HT, 5-hydroxytryptamine) transporters. Exposure of rat striatal synaptosomes to C2-ceramide caused a reversible, concentration-dependent decrease in plasmalemmal dopamine uptake. In contrast, ceramide exposure increased striatal 5-HT synaptosomal uptake. This increase did not appear to be due to an increased uptake by the 5-HT transporter. Rather, the increase appeared to result from an increase in 5-HT transport through the dopamine transporter, an assertion evidenced by findings that this increase: (1) does not occur in hippocampal synaptosomes (i.e., a preparation largely devoid of dopamine transporters), (2) occurs in striatal synaptosomes prepared from para-chloroamphetamine-treated rats (i.e., a preparation lacking 5-HT transporters), (3) is attenuated by pretreatment with methylphenidate (i.e., a relatively selective dopamine reuptake inhibitor) and (4) is inhibited by exposure to exogenous dopamine (i.e., which presumably competes for uptake with 5-HT). Taken together, these results reveal that ceramide is a novel modulator of monoamine transporter function, and may alter the affinity of dopamine transporters for its primary substrate.

Methylphenidate redistributes vesicular monoamine transporter-2: role of dopamine receptors.

It is well accepted that methylphenidate (MPD) inhibits dopamine (DA) transporter function. In addition to this effect, this study demonstrates that MPD increases vesicular [3H]DA uptake and binding of the vesicular monoamine transporter-2 (VMAT-2) ligand dihydrotetrabenazine (DHTBZ) in a dose- and time-dependent manner in purified striatal vesicles prepared from treated rats. This change did not result from residual MPD introduced by the original in vivo treatment, because application of MPD in vitro (< or =1 miccrom) was without effect, and higher concentrations decreased vesicular [3H]DA uptake. In addition, MPD treatment increased and decreased VMAT-2 immunoreactivity in striatal vesicle subcellular and plasmalemmal membrane fractions, respectively. The MPD-induced increase in both VMAT-2 immunoreactivity and DHTBZ binding was attenuated by pretreatment in vivo with either the DA D(1) receptor antagonist SCH23390 or the DA D2 receptor antagonist eticlopride. Coadministration of these antagonists in vivo inhibited completely the MPD-induced increase in DHTBZ binding in the purified vesicular preparation. These observations suggest a role for DA in the MPD-induced redistribution of VMAT-2. The implications of this phenomenon will be discussed.

Differential trafficking of the vesicular monoamine transporter-2 by methamphetamine and cocaine.

High-dose administration of cocaine or methamphetamine to rats acutely (< or = 24 h) alters vesicular dopamine transport. This study elucidates the nature of these changes. Results reveal a differential redistribution of the vesicular monoamine transporter-2 (VMAT-2) within striatal synaptic terminals after drug treatment. In particular, cocaine shifts VMAT-2 protein from a synaptosomal membrane fraction to a vesicle-enriched fraction, as assessed ex vivo in fractions prepared from treated rats. In contrast, methamphetamine treatment redistributes VMAT-2 from a vesicle-enriched fraction to a location that is not retained in a synaptosomal preparation. These data suggest that psychostimulants acutely and differentially affect VMAT-2 subcellular localization.

A single methamphetamine administration rapidly decreases vesicular dopamine uptake.

Recent studies demonstrated that vesicular dopamine (DA) uptake can be rapidly altered in synaptic vesicles purified from the striata of stimulant-treated rats. Specifically, a single administration of the plasmalemmal DA transporter inhibitor, cocaine, or the DA D(2) agonist, quinpirole, increases vesicular DA uptake in vesicles purified from the striata of treated rats. These effects of cocaine are prevented by pretreatment with a D(2), but not D(1), DA receptor antagonist. The purpose of the present study was to characterize the effect of a mechanistically different psychostimulant, methamphetamine (METH), on vesicular DA uptake. Results demonstrated that a single administration of this DA-releasing agent rapidly and reversibly decreased vesicular DA uptake. The METH-related decrease in vesicular DA uptake was attenuated by pretreatment with the D(2) antagonist, eticlopride, but not the D(1) antagonist, SCH23390 (R-[+]-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine). Core body temperature did not contribute to the effects of METH on vesicular DA uptake. Neither quinpirole nor cocaine increased vesicular DA uptake when rats were concurrently treated with METH. These studies provide further evidence that psychostimulants rapidly and differentially modify vesicular DA uptake. In addition, these studies demonstrate a complex role for D(2) DA receptors in altering vesicular DA transport.

Methylenedioxymethamphetamine decreases plasmalemmal and vesicular dopamine transport: mechanisms and implications for neurotoxicity.

Administration of a high-dose regimen of methamphetamine (METH) rapidly and profoundly decreases plasmalemmal and vesicular dopamine (DA) transport in the striatum, as assessed in synaptosomes and purified vesicles, respectively. To determine whether these responses were common to other amphetamines of abuse, effects of methylenedioxymethamphetamine (MDMA) on the plasmalemmal DA transporter (DAT) and vesicular monoamine transporter-2 (VMAT-2) were assessed. Similar to effects of METH reported previously, multiple high-dose MDMA administrations rapidly (within 1 h) decreased plasmalemmal DA uptake, as assessed ex vivo in synaptosomes prepared from treated rats. Unlike effects of multiple METH injections, this deficit was reversed completely 24 h after drug treatment. Also in contrast to effects of multiple METH injections, 1) MDMA caused little or no decrease in binding of the DAT ligand WIN35428, and 2) neither prevention of hyperthermia nor prior depletion of DA prevented the MDMA-induced reduction in plasmalemmal DA transport. However, a role for phosphorylation was suggested because pretreatment with protein kinase C inhibitors attenuated the deficit caused by MDMA in an in vitro model system. In addition to affecting DAT function, MDMA rapidly decreased vesicular DA transport as assessed in striatal vesicles prepared from treated rats. Unlike effects of multiple METH injections reported previously, this decrease partially recovered by 24 h after drug treatment. Taken together, these results reveal several differences between effects of MDMA and previously reported METH on DAT and VMAT-2; differences that may underlie the dissimilar neurotoxic profile of these agents.

Tolerance to the neurotoxic effects of methamphetamine in young rats.

The present study examined whether exposure to methamphetamine during adolescence (as determined in post-natal day 40 rats) might alter its effects when used in young adulthood (as assessed in post-natal day 90 rats). Results confirm that high-dose methamphetamine administration (4x10 mg/kg/injection, s.c., 2-h intervals) decreases striatal dopamine uptake and transporter ligand binding in post-natal day 90 rats; effects that were blocked if animals received six biweekly methamphetamine pretreatments (15 mg/kg; s.c.) beginning at post-natal day 40. This neuroprotection was not likely due to pharmacokinetic tolerance, since brain methamphetamine concentrations did not differ 1 h after the high-dose methamphetamine regimen among treated rats regardless of pretreatment. The methamphetamine biweekly pretreatment attenuated the hyperthermia caused by the neurotoxic methamphetamine regimen; a phenomenon that may have contributed to the neuroprotection.