Mechanisms for tolerance to methamphetamine effects

The Influence of Prenatal Exposure to Methamphetamine on the Development of Dopaminergic Neurons in the Ventral Midbrain

Methamphetamine, a highly addictive central nervous system (CNS) stimulant, is used worldwide as an anorexiant and attention enhancer. Methamphetamine use during pregnancy, even at therapeutic doses, may harm fetal development. Here, we examined whether exposure to methamphetamine affects the morphogenesis and diversity of ventral midbrain dopaminergic neurons (VMDNs). The effects of methamphetamine on morphogenesis, viability, the release of mediator chemicals (such as ATP), and the expression of genes involved in neurogenesis were evaluated using VMDNs isolated from the embryos of timed-mated mice on embryonic day 12.5. We demonstrated that methamphetamine (10 µM; equivalent to its therapeutic dose) did not affect the viability and morphogenesis of VMDNs, but it reduced the ATP release negligibly. It significantly downregulated Lmx1a, En1, Pitx3, Th, Chl1, Dat, and Drd1 but did not affect Nurr1 or Bdnf expression. Our results illustrate that methamphetamine could impair VMDN differentiation by altering the expression of important neurogenesis-related genes. Overall, this study suggests that methamphetamine use may impair VMDNs in the fetus if taken during pregnancy. Therefore, it is essential to exercise strict caution for its use in expectant mothers.

A Zebrafish Model of Neurotoxicity by Binge-Like Methamphetamine Exposure

Hyperthermia is a common confounding factor for assessing the neurotoxic effects of methamphetamine (METH) in mammalian models. The development of new models of methamphetamine neurotoxicity using vertebrate poikilothermic animals should allow to overcome this problem. The aim of the present study was to develop a zebrafish model of neurotoxicity by binge-like methamphetamine exposure. After an initial testing at 20 and 40 mg/L for 48 h, the later METH concentration was selected for developing the model and the effects on the brain monoaminergic profile, locomotor, anxiety-like and social behaviors as well as on the expression of key genes of the catecholaminergic system were determined. A concentration- and time-dependent decrease in the brain levels of dopamine (DA), norepinephrine (NE) and serotonin (5-HT) was found in METH-exposed fish. A significant hyperactivity was found during the first hour of exposure, followed 3 h after by a positive geotaxis and negative scototaxis in the novel tank and in the light/dark paradigm, respectively. Moreover, the behavioral phenotype in the treated fish was consistent with social isolation. At transcriptional level, th1 and slc18a2 (vmat2) exhibited a significant increase after 3 h of exposure, whereas the expression of gfap, a marker of astroglial response to neuronal injury, was strongly increased after 48 h exposure. However, no evidences of oxidative stress were found in the brain of the treated fish. Altogether, this study demonstrates the suitability of the adult zebrafish as a model of METH-induced neurotoxicity and provides more information about the biochemical and behavioral consequences of METH abuse.

Differential effects of synthetic psychoactive cathinones and amphetamine stimulants on the gut microbiome in mice

The list of pharmacological agents that can modify the gut microbiome or be modified by it continues to grow at a high rate. The greatest amount of attention on drug-gut microbiome interactions has been directed primarily at pharmaceuticals used to treat infection, diabetes, cardiovascular conditions and cancer. By comparison, drugs of abuse and addiction, which can powerfully and chronically worsen human health, have received relatively little attention in this regard. Therefore, the main objective of this study was to characterize how selected synthetic psychoactive cathinones (aka “Bath Salts”) and amphetamine stimulants modify the gut microbiome. Mice were treated with mephedrone (40 mg/kg), methcathinone (80 mg/kg), methamphetamine (5 mg/kg) or 4-methyl-methamphetamine (40 mg/kg), following a binge regimen consisting of 4 injections at 2h intervals. These drugs were selected for study because they are structural analogs that contain a β-keto substituent (methcathinone), a 4-methyl group (4-methyl-methamphetamine), both substituents (mephedrone) or neither (methamphetamine). Mice were sacrificed 1, 2 or 7 days after treatment and DNA from caecum contents was subjected to 16S rRNA sequencing. We found that all drugs caused significant time- and structure-dependent alterations in the diversity and taxonomic structure of the gut microbiome. The two phyla most changed by drug treatments were Firmicutes (methcathinone, 4-methyl-methamphetamine) and Bacteriodetes (methcathinone, 4-methyl-methamphetamine, methamphetamine, mephedrone). Across time, broad microbiome changes from the phylum to genus levels were characteristic of all drugs. The present results signify that these selected psychoactive drugs, which are thought to exert their primary effects within the CNS, can have profound effects on the gut microbiome. They also suggest new avenues of investigation into the possibility that gut-derived signals could modulate drug abuse and addiction via altered communication along the gut-brain axis.

Does Prenatal Methamphetamine Exposure Induce Sensitization to Drugs in Adulthood?

Behavioral sensitization is defined as augmented psychomotor activity, which can be observed after drug re-administration following withdrawal of repeated drug exposure. It has been shown that abuse of one drug can lead to increased sensitivity to certain other drugs. This effect of developed general drug sensitivity is called cross-sensitization and has been reported between drugs with similar as well as different mechanisms of action. There is growing evidence that exposure to drugs in utero not only causes birth defects and delays in infant development, but also impairs the neural reward pathways, in the brains of developing offspring, in such a way that it can increase the tendency for drug addiction later in life. This review summarizes the results of preclinical studies that focused on testing behavioral cross-sensitization, after prenatal methamphetamine exposure, to drugs administered in adulthood, with both similar and different mechanisms of action. Traditionally, behavioral sensitization has been examined using the Open field or the Laboras Test to record locomotor activity, and the Conditioned Place Preference and Self-administration test to examine drug-seeking behavior. However, it seems that prenatal drug exposure can sensitize animals not only to the locomotor-stimulating and conditioning effects of drugs, but may also be responsible for modified responses to various drug effects.

Does Prenatal Methamphetamine Exposure Induce Cross-sensitization to Cocaine and Morphine in Adult Male Rats?

The aim of the present study was to examine the cross-sensitization induced by prenatal methamphetamine (MA) exposure to challenge dose of cocaine or morphine. Rat mothers received a daily injection of MA (5 mg/kg) or saline throughout the gestation period. Adult male offspring (prenatally MA- or saline-exposed) were divided to groups with challenge doses of saline (1 ml/kg), cocaine (5 mg/kg) or morphine (5 mg/kg). Behavior in unknown environment was examined in Laboras, nociception in Plantar test, and active drug-seeking behavior in conditioned place preference (CPP). Our data demonstrate that cocaine increased the exploratory activity in Laboras test in prenatally saline-exposed, but decreased it in prenatally MA-exposed rats. An analgesic effect of cocaine was demonstrated only by the tail withdrawal and it was independent of the prenatal drug exposure. CPP test showed that prenatal MA exposure induced rather tolerance than sensitization to cocaine. In contrast to cocaine effects, morphine decreased rearing activity in both, prenatally MA-exposed and saline-exposed rats, and locomotion only in prenatally MA-exposed rats in the Laboras. In the Plantar test, the results demonstrated that morphine had an analgesic effect in prenatally saline-exposed rats but this effect was suppressed in prenatally MA-exposed rats. In the CPP test morphine induced drug-seeking behavior, which however was not affected by prenatal drug exposure. Thus, our data demonstrate that there is a cross-effect between prenatal MA exposure and the challenge dose of other drug in adulthood, however drug-seeking behavior is not increased by prenatal MA exposure as we expected.

Do prenatally methamphetamine-exposed adult male rats display general predisposition to drug abuse in the conditioned place preference test?

Drug abuse of pregnant women is a growing problem. The effect of prenatal drug exposure may have devastating effect on development of the offsprings that may be long-term or even permanent. One of the most common drug abused by pregnant women is methamphetamine (MA), which is also the most frequently abused illicit drug in the Czech Republic. Our previous studies demonstrated that prenatal MA exposure alters behavior, cognition, pain and seizures in adult rats in sex-specific manner. Our most recent studies demonstrate that prenatal MA exposure makes adult rats more sensitive to acute injection of the same or related drugs than their controls. The aim of the present study was to examine the effect of prenatal MA exposure on drug-seeking behavior of adult male rats tested in the Conditioned place preference (CPP). Adult male rats were divided to: prenatally MA-exposed (5 mg/kg daily for the entire prenatal period), prenatally saline-exposed (1 ml/kg of physiological saline) and controls (without maternal injections). The following drugs were used in the CPP test in adulthood: MA (5 mg/kg), amphetamine (5 mg/kg), cocaine (5 and 10 mg/kg), morphine (5 mg/kg), MDMA (5 mg/kg) and THC (2 mg/kg). Our data demonstrated that prenatally MA-exposed rats displayed higher amphetamine-seeking behavior than both controls. MA as well as morphine induced drug-seeking behavior of adult male rats, however this effect did not differ based on the prenatal MA exposure. In contrast, prenatal MA exposure induced rather tolerance to cocaine than sensitization after the conditioning in the CPP. MDMA and THC did not induce significant effects. Even though the present data did not fully confirmed our hypotheses, future studies are planned to test the drug-seeking behavior also in self-administration test.

Long-term protective effects of methamphetamine preconditioning against single-day methamphetamine toxic challenges

Methamphetamine (METH) use is associated with neurotoxic effects which include decreased levels of dopamine (DA), serotonin (5-HT) and their metabolites in the brain. We have shown that escalating METH dosing can protect against METH induced neurotoxicity in rats sacrificed within 24 hours after a toxic METH challenge. The purpose of the current study was to investigate if the protective effects of METH persisted for a long period of time. We also tested if a second challenge with a toxic dose of METH would cause further damage to monoaminergic terminals. Saline-pretreated rats showed significant METH-induced decreases in striatal DA and 5-HT levels in rats sacrificed 2 weeks after the challenge. Rats that received two METH challenges showed no further decreases in striatal DA or 5-HT levels in comparison to the single METH challenge. In contrast, METH-pretreated rats showed significant protection against METH-induced striatal DA and 5-HT depletion. In addition, the METH challenge causes substantial decreases in cortical 5-HT levels which were not further potentiated by a second drug challenge. METH preconditioning provided almost complete protection against METH -induced 5-HT depletion. These results are consistent with the idea that METH pretreatment renders the brain refractory to METH-induced degeneration of brain monoaminergic systems.

Methamphetamine treatment during development attenuates the dopaminergic deficits caused by subsequent high-dose methamphetamine administration

Administration of high doses of methamphetamine (METH) causes persistent dopaminergic deficits in both nonhuman preclinical models and METH-dependent persons. Noteworthy, adolescent [i.e., postnatal day (PND) 40] rats are less susceptible to this damage than young adult (PND90) rats. In addition, biweekly treatment with METH, beginning at PND40 and continuing throughout development, prevents the persistent dopaminergic deficits caused by a "challenge" high-dose METH regimen when administered at PND90. Mechanisms underlying this "resistance" were thus investigated. Results revealed that biweekly METH treatment throughout development attenuated both the acute and persistent deficits in VMAT2 function, as well as the acute hyperthermia, caused by a challenge METH treatment. Pharmacokinetic alterations did not appear to contribute to the protection afforded by the biweekly treatment. Maintenance of METH-induced hyperthermia abolished the protection against both the acute and persistent VMAT2-associated deficits suggesting that alterations in thermoregulation were caused by exposure of rats to METH during development. These findings suggest METH during development prevents METH-induced hyperthermia and the consequent METH-related neurotoxicity.

Methamphetamine toxicity and messengers of death

Methamphetamine (METH) is an illicit psychostimulant that is widely abused in the world. Several lines of evidence suggest that chronic METH abuse leads to neurodegenerative changes in the human brain. These include damage to dopamine and serotonin axons, loss of gray matter accompanied by hypertrophy of the white matter and microgliosis in different brain areas. In the present review, we summarize data on the animal models of METH neurotoxicity which include degeneration of monoaminergic terminals and neuronal apoptosis. In addition, we discuss molecular and cellular bases of METH-induced neuropathologies. The accumulated evidence indicates that multiple events, including oxidative stress, excitotoxicity, hyperthermia, neuroinflammatory responses, mitochondrial dysfunction, and endoplasmic reticulum stress converge to mediate METH-induced terminal degeneration and neuronal apoptosis. When taken together, these findings suggest that pharmacological strategies geared towards the prevention and treatment of the deleterious effects of this drug will need to attack the various pathways that form the substrates of METH toxicity.

Methamphetamine preconditioning: differential protective effects on monoaminergic systems in the rat brain

Pretreatment with methamphetamine (METH) can attenuate toxicity due to acute METH challenges. The majority of previous reports have focused mainly on the effects of the drug on the striatal dopaminergic system. In the present study, we used a regimen that involves gradual increases in METH administration to rats in order to mimic progressively larger doses of the drug used by some human METH addicts. We found that this METH preconditioning was associated with complete protection against dopamine depletion caused by a METH challenge (5 mg/kg x 6 injections given 1 h apart) in the striatum and cortex. In contrast, there was no preconditioning-mediated protection against METH-induced serotonin depletion in the striatum and hippocampus, with some protection being observed in the cortex. There was also no protection against METH-induced norepinephrine (NE) depletion in the hippocampus. These results indicate that, in contrast to the present dogmas, there might be differences in the mechanisms involved in METH toxicity on monoaminergic systems in the rodent brain. Thus, chronic injections of METH might activate programs that protect against dopamine toxicity without influencing drug-induced pathological changes in serotoninergic systems. Further studies will need to evaluate the cellular and molecular bases for these differential responses.

The effects of methamphetamine on core body temperature in the rat--part 2: an escalating regimen

RATIONALE

Methamphetamine (METH) induces hyperthermia, which is diminished with chronic treatment in a dose-dependent manner. Our objective was to determine whether the temperature responses produced by a chronic, escalating-dose METH regimen and a chronic, 5.0 mg/kg dose regimen.

METHODS

Rats received pretreatment injections of saline, 5.0 mg/kg METH, 10.0 mg/kg METH (second comparison group), or an escalating-METH regimen (2-9 mg/kg) for 12 days. On day 13, all four groups were challenged with 10.0 mg/kg METH. Temperature measurements were made telemetrically at 24 degrees C ambient temperature.

RESULTS

Escalating pretreatment produced hyperthermia; with successive exposures, the hyperthermic peak shifted to the right. The 5.0-mg/kg-pretreatment group initially showed no change in temperature at 60 min post-treatment but developed hypothermia at 60 min with chronic treatment; at 3 h post-treatment, significant hyperthermia was present and did not diminish with chronic treatment. After the 10.0-mg/kg-METH challenge, the saline-pretreatment group was hyperthermic, and the 10.0-mg/kg-pretreatment group was hypothermic; the 5.0 mg/kg and escalating pretreatment groups were intermediate and were not different from each other. At 3 h post-challenge, no group differences were apparent. Dopamine (DA) and serotonin (5-HT) were not depleted when measured 2 weeks after treatment ended.

CONCLUSIONS

(1) Escalating and 5.0-mg/kg regimens produced different temperature profiles during the 12-day pretreatment period but a similar diminished response to the 10.0-mg/kg-METH challenge on day 13. (2) The diminished temperature responses with chronic treatment occurred in the absence of long-term DA and 5-HT depletions.

The effects of methamphetamine on core body temperature in the rat--part 1: chronic treatment and ambient temperature

RATIONALE

Stimulants such as methamphetamine (METH) alter core temperature in a manner that is dependent on ambient temperature and that shows tolerance after chronic use. Our objectives were to (1) determine whether tolerance to METH-induced hyperthermia was a consequence of neurotoxicity to dopamine or serotonin and (2) determine the relationship between ambient temperature and chronic treatment on the METH-induced temperature response.

MATERIALS AND METHODS

Rats were treated with 1.0, 5.0, or 10.0 mg/kg METH at 24 degrees C (experiment 1) or treated with 5.0 mg/kg METH at 20 degrees C, 24 degrees C, or 28 degrees C (experiment 2). Treatment occurred for 12 days, and temperature measurements were made once per minute telemetrically during 7-h sessions in computer-regulated environments.

RESULTS

Peak increases in core temperature occurred at 60 min post-treatment for the 1.0 and 10.0 mg/kg doses, and at 180 min for the 5.0 mg/kg dose. Tolerance-like effects were seen with chronic 5.0 (mixed results) and 10.0 mg/kg METH in the absence of dopamine or serotonin depletions measured 2 weeks after the completion of treatment. After 5.0 mg/kg METH, variations in ambient temperature resulted in an early flexible change in core temperature (phase 1) (hyperthermia at 28 degrees and hypothermia at 20 degrees ) and a later inflexible hyperthermia (phase 2).

CONCLUSIONS

The results suggest that (1) the peak effect of different doses of METH occurs at different times (24 degrees ), (2) the diminished temperature response with chronic METH treatment was not associated with long-term dopamine and serotonin depletions, and (3) a two-phase temperature response to METH may reflect two independent mechanisms.

Differential neurochemical consequences of an escalating dose-binge regimen followed by single-day multiple-dose methamphetamine challenges

Chronic intake of methamphetamine (METH) causes tolerance to its behavioral and subjective effects. To better mimic human patterns of drug abuse, the present study used a rodent model that took into account various facets of human drug administration and measured METH-induced effects on brain monoamine levels. Adult male Sprague-Dawley rats were injected with METH or saline according to an escalating dose schedule for 2 weeks. This was followed by a challenge regimen of either saline or one of two doses of METH (3 x 10 mg/kg every 2 h or 6 x 5 mg/kg given every hour, both given within a single day). Both challenge doses of METH caused significant degrees of depletion of dopamine in the striatum and norepinephrine and serotonin in the striatum, cortex, and hippocampus. Animals pre-treated with METH showed significant attenuation of METH-induced striatal dopamine depletion but not consistent attenuation of norepinephrine and serotonin depletion. Unexpectedly, METH pre-treated animals that received the 3 x 10 mg/kg challenge showed less increases in tympanic temperatures than saline pre-treated rats whereas METH pre-treated animals that received the 6 x 5 mg/kg METH challenge showed comparable increases in temperatures to saline pre-treated rats. Therefore, pre-treatment-induced partial protection against monoamine depletion is probably not because of attenuated METH-induced hyperthermia in those rats.

Sex differences in (+)-amphetamine- and (+)-methamphetamine-induced behavioral response in male and female Sprague-Dawley rats

(+)-Methamphetamine (METH) and (+)-amphetamine (AMP) are structurally similar drugs that are reported to induce similar pharmacological effects in rats of the same sex. Because pharmacokinetic data suggest female rats should be more affected than males, the current studies sought to test the hypothesis that the behavioral and temporal actions of METH and AMP should be greater in female Sprague-Dawley rats than in males. Using a dosing regimen designed to reduce the possibility of tolerance and sensitization, rats were administered 1.0 and 3.0 mg/kg intravenous drug doses. Distance traveled, rearing events and focal stereotypies (e.g., head weaving, sniffing) were measured. Female rats traveled significantly greater distances and displayed a greater number of rearing events than males after both doses. Analysis of stereotypy ratings after 3.0 mg/kg revealed that focal stereotypies were more pronounced and lasted longer in females. The second study compared the potencies of METH and AMP in inducing locomotor activity and focal stereotypies in each sex. No differences in potency were found when METH and AMP effects were compared within males or females. In summary, these studies showed female rats displayed greater and longer-lasting locomotor activity and more stereotypic behaviors, supporting earlier evidence of significant sexual dimorphism in pharmacokinetics.

Clinical effects and management of methamphetamine abuse

Methamphetamine is a highly potent and addictive drug that is abused in the United States and around the world. The drug is inexpensive and easily manufactured from simple chemicals such as pseudoephedrine. These features, coupled with its long half-life and highly addictive nature, contribute to the increasing problem of illicit methamphetamine use. Abuse of this agent has both acute and chronic serious health consequences. Policy makers and public health officials must continue to develop programs that educate the public and limit the abuse associated with methamphetamine.

Persistence of tolerance to methamphetamine-induced monoamine deficits

Methamphetamine is a highly addictive and potent stimulant, the use of which has increased significantly in recent years. In addition to the severe behavioral and societal consequences associated with methamphetamine abuse, methamphetamine can cause persistent damage to monoaminergic nerve terminals in rats, as measured by either monoamine concentrations or activity of the rate limiting synthetic enzymes, tyrosine hydroxylase and tryptophan hydroxylase. Repeated, sub-neurotoxic doses of methamphetamine, however, can cause rats to become resistant to the neurotoxic effects of multiple high-dose administrations of methamphetamine; a phenomenon known as tolerance. This study investigates the persistence of tolerance evoked by pretreatment with escalating-dose administrations of methamphetamine. Rats were pretreated over several days with low, escalating doses of methamphetamine, followed by high-dose methamphetamine challenge after variable recovery periods. Results revealed that tolerance to monoaminergic deficits persisted for at least one week, but was completely eliminated by 31 days. There were no differences in the distribution of methamphetamine or its major metabolite, amphetamine, between methamphetamine-pretreated animals and saline-pretreated animals 2 h after the final methamphetamine challenge injection, and there were no regional differences in methamphetamine concentrations between the frontal cortex, hippocampus or striatum. We also observed that while methamphetamine pretreatment attenuated the hyperthermia caused by the high-dose methamphetamine challenge, significant reductions in methamphetamine-induced hyperthermia were not required for the development of tolerance with this regimen.

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.

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.

Plasma drug concentrations and physiological measures in 'dance party' participants

The increasing use of (+/-) 3,4-methylenedioxymethamphetamine (MDMA) in the setting of large dance parties ('raves') and clubs has been the source of some concern, because of potential acute adverse events, and because animal studies suggest that MDMA has the potential to damage brain serotonin (5-HT) neurons. However, it is not yet known whether MDMA, as used in the setting of dance parties, leads to plasma levels of MDMA that are associated with toxicity to 5-HT neurons in animals. The present study sought to address this question. Plasma MDMA concentrations, vital signs, and a variety of blood and urine measures were obtained prior to, and hours after, individuals attended a dance party. After the dance party, subjects were without clinical complaints, had measurable amounts of residual MDMA in plasma, and nearly half of the subjects also tested positive for methamphetamine, another amphetamine analog that has been shown to have 5-HT neurotoxic potential in animals. Plasma concentrations of MDMA did not correlate with self-reported use of 'ecstasy' and, in some subjects, overlapped with those that have been associated with 5-HT neurotoxicity in non-human primates. Additional subjects were likely to have had similar concentrations while at the dance party, when one considers the reported time of drug ingestion and the plasma half-life of MDMA in humans. Hematological and biochemical analyses were generally unremarkable. Moderate increases in blood pressure, heart rate and body temperature were observed in the subjects with the highest MDMA plasma concentrations. These findings are consistent with epidemiological findings that most people who use MDMA at dance parties do not develop serious clinical complications, and suggest that some of these individuals may be at risk for developing MDMA-induced toxicity to brain serotonin neurons.

Relationship between temperature, dopaminergic neurotoxicity, and plasma drug concentrations in methamphetamine-treated squirrel monkeys

To examine the relationship between temperature (ambient and core), dopaminergic neurotoxicity, and plasma drug [methamphetamine (METH)] and metabolite [amphetamine (AMPH)] concentrations, two separate groups of squirrel monkeys (n = 4-5 per group) were treated with METH (1.25 mg/kg, given twice, 4 h apart) or vehicle (same schedule) at two different ambient temperatures (26 and 33 degrees C). Core temperatures and plasma drug concentrations were measured during the period of drug exposure; striatal monoaminergic neuronal markers in the same monkeys were determined 1 week later. At the temperature range examined, the higher ambient temperature did not significantly enhance METH-induced hyperthermia or METH-induced dopaminergic neurotoxicity, although there were trends toward increases. Acute METH-induced increases in core temperature correlated highly and directly with subsequent decreases in striatal dopaminergic markers. Squirrel monkeys with the greatest increases in core temperature (and largest dopaminergic deficits) had the highest plasma drug metabolite (AMPH) concentrations. There was substantial interanimal variability, both with regard to elevations in core temperature and plasma drug concentrations. Pharmacokinetic studies in six additional squirrel monkeys revealed comparable individual differences in METH metabolism. These results, which provide the first available data on the within-subject relationship between temperature (ambient and core), plasma concentrations of METH (and AMPH), and subsequent dopaminergic neurotoxic changes, suggest that, as in rodents, core temperature can influence METH neurotoxicity in primates. In addition, they suggest that interanimal differences presently observed in thermal and neurotoxic responses to METH may be related to individual differences in drug metabolism.

Disparity in the temporal appearance of methamphetamine-induced apoptosis and depletion of dopamine terminal markers in the striatum of mice

Methamphetamine (METH) causes damage in the striatum at pre- and post-synaptic sites. Exposure to METH induces long-term depletions of dopamine (DA) terminal markers such as tyrosine hydroxylase (TH) and DA transporters (DAT). METH also induces neuronal apoptosis in some striatal neurons. The purpose of this study is to demonstrate which occurs first, apoptosis of some striatal neurons or DA terminal toxicity in mice. This is important because the death of striatal neurons leaves the terminals in a state of deafferentation. A bolus injection (i.p.) of METH (30 mg/kg) induces apoptosis (TUNEL staining) in approximately 25% of neurons in the striatum at 24 h after METH. However, in contrast to apoptosis, depletion of TH (Western blotting) begins to appear at 24 h after METH in dorsal striatum while the ventral striatum is unaffected. The peak of TH depletion (approximately 80% decrease relative to control) occurs at 48 h after METH. Autoradiographic analysis of DAT sites showed that depletion begins to appear 24 h after METH and peaks at 2 days (approximately 60% depletion relative to control). Histological analysis of the induction of glial fibrillary acidic protein (GFAP) by METH in striatal astrocytes revealed an increase at 48 h after METH that peaked at 3 days. These data demonstrate that striatal apoptosis precedes the depletion (toxicity) of DA terminal markers in the striatum of mice, suggesting that the ensuing state of deafferentation of the DA terminals may contribute to their degeneration.

Monoclonal IgG affinity and treatment time alters antagonism of (+)-methamphetamine effects in rats

The roles of monoclonal antibody affinity and treatment time of (+)-methamphetamine-induced pharmacological effects in rats were studied using two anti-(+)-methamphetamine monoclonal antibodies. These studies tested the preclinical protective effects of monoclonal antibody antagonists in (+)-methamphetamine overdose and pretreatment scenarios. The higher affinity antibody (mAb6H4; KD=11 nM for (+)-methamphetamine) more effectively antagonized (+)-methamphetamine-induced behavioral effects (distance and rearing) than the low affinity antibody (designated mAb6H8; KD=250 nM) and had a longer duration of action. Both antibodies more effectively reduced (+)-methamphetamine effects in the overdose model than in the pretreatment model. (+)-Methamphetamine pharmacokinetic studies showed the mAb6H4 significantly reduced brain concentrations over time in both models. However, while mAb6H4 immediately reduced brain concentrations in the overdose model, it did not prevent the initial distribution of (+)-methamphetamine into the brain in the pretreatment model. Thus, anti-(+)-methamphetamine monoclonal antibody affinity and administration time (relative to (+)-methamphetamine dosing) are critical determinants of therapeutic success.

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.

Attenuated microglial activation mediates tolerance to the neurotoxic effects of methamphetamine

Methamphetamine causes persistent damage to dopamine nerve endings of the striatum. Repeated, intermittent treatment of mice with low doses of methamphetamine leads to the development of tolerance to its neurotoxic effects. The mechanisms underlying tolerance are not understood but clearly involve more than alterations in drug bioavailability or reductions in the hyperthermia caused by methamphetamine. Microglia have been implicated recently as mediators of methamphetamine-induced neurotoxicity. The purpose of the present studies was to determine if a tolerance regimen of methamphetamine would attenuate the microglial response to a neurotoxic challenge. Mice treated with a low-dose methamphetamine tolerance regimen showed minor reductions in striatal dopamine content and low levels of microglial activation. When the tolerance regimen preceded a neurotoxic challenge of methamphetamine, the depletion of dopamine normally seen was significantly attenuated. The microglial activation that occurs after a toxic methamphetamine challenge was blunted likewise. Despite the induction of tolerance against drug-induced toxicity and microglial activation, a neurotoxic challenge with methamphetamine still caused hyperthermia. These results suggest that tolerance to methamphetamine neurotoxicity is associated with attenuated microglial activation and they further dissociate its neurotoxicity from drug-induced hyperthermia.

Alterations in vesicular dopamine uptake contribute to tolerance to the neurotoxic effects of methamphetamine

Previous studies demonstrated that tolerance to the long-term neurotoxic effects of methamphetamine on dopamine neurons could be induced by pretreating with multiple injections of escalating doses of methamphetamine. The mechanism(s) underlying this tolerance phenomenon is unknown. Some recent studies suggested that aberrant vesicular monoamine transporter-2 (VMAT-2) and dopamine transporter function contribute to neurotoxic effects of methamphetamine. Hence, the purpose of this study was to explore the role of the VMAT-2 and dopamine transporter in the induction of tolerance to the longterm persistent dopaminergic deficits caused by methamphetamine. A second purpose was to investigate the potential role of hyperthermia and alterations in brain methamphetamine distribution in this tolerance. Results revealed that the methamphetamine pretreatment regimen attenuated both the acute methamphetamine-induced decrease in VMAT-2 function 2 h after the methamphetamine challenge administration and its resulting persistent dopamine deficits without attenuating the acute methamphetamine-induced decreases in dopamine transporter uptake. Furthermore, pretreatment with methamphetamine prior to a high-dose methamphetamine challenge administration also attenuated the acute methamphetamine-induced redistribution of VMAT-2 immunoreactivity within the nerve terminal. This protection was not due to alterations in concentration of methamphetamine in the brain because both the methamphetamine- and saline-pretreated rats had similar amounts of methamphetamine and amphetamine at 30 min to 2 h after the last methamphetamine challenge injection. In summary, these data are the first to demonstrate an association between the prevention of acute alterations in vesicular dopamine uptake and the development of tolerance to the neurotoxic effects of methamphetamine.

The role of hyperthermia and metabolism as mechanisms of tolerance to methamphetamine neurotoxicity

Pretreatment with multiple methamphetamine injections prior to a high-dose methamphetamine challenge administration can attenuate long-term deficits in striatal and hippocampal serotonin content caused by the stimulant. The present data extend previous findings by demonstrating that rats pretreated with escalating doses methamphetamine did not exhibit dopamine deficits in the striatum, nor serotonin deficits in striatal, frontal cortical, or hippocampal tissues, 7 days after a challenge methamphetamine administration. This protection was not due to attenuation of methamphetamine-induced hyperthermia or altered brain methamphetamine concentrations. These data differ from previous findings thereby highlighting that different mechanisms contribute to the tolerance of the neurotoxic effects.

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.

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.