Differential regional effects of methamphetamine on dopamine transport.

Multiple high-dose methamphetamine administrations cause long-lasting (>1 week) deficits in striatal dopaminergic neuronal function. This stimulant likewise causes rapid (within 1 h) and persistent (at least 48 h) decreases in activities of striatal: 1) dopamine transporters, as assessed in synaptosomes; and 2) vesicular monoamine transporter -2 (VMAT-2), as assessed in a non-membrane-associated (referred to herein as cytoplasmic) vesicular subcellular fraction. Importantly, not all brain areas are vulnerable to methamphetamine-induced long-lasting deficits. Similarly, the present study indicates that methamphetamine exerts differential acute effects on monoaminergic transporters according to brain region. In particular, results revealed that in the nucleus accumbens, methamphetamine rapidly, but reversibly (within 24 h), decreased plasmalemmal dopamine transporter function, without effect on plasmalemmal dopamine transporter immunoreactivity. Methamphetamine also rapidly and reversibly (within 48 h) decreased cytoplasmic VMAT-2 function in this region, with relatively little effect on cytoplasmic VMAT-2 immunoreactivity. In contrast, methamphetamine did not alter either dopamine transporter or VMAT-2 activity in the hypothalamus. Noteworthy, the nucleus accumbens and hypothalamus did not display the persistent long-lasting striatal dopamine depletions caused by the stimulant. Taken together, these data suggest that deficits in plasmalemmal and vesicular monoamine transporter activity lasting greater than 24-48 h may be linked to the long-lasting dopaminergic deficits caused by methamphetamine and appear to be region specific.

Age-dependent effects of methamphetamine on VMAT-2.

Abuse of methamphetamine (METH) among adolescents and young adults is concerning since studies have demonstrated that multiple administrations of high-dose METH induce persistent dopaminergic deficits. METH has also been shown to reduce dopamine (DA) uptake by the vesicular monoamine transporter-2 (VMAT-2) and to reduce the amount of VMAT-2 protein in a purified vesicular fraction. VMAT-2 plays a critical role in the sequestration of DA in dopaminergic nerve terminals. This function is important since DA can oxidize rapidly to form highly reactive species. It is likely that disruption of this normal intracellular processing of DA contributes to oxidative sequences ultimately leading to persistent deficits. Interestingly, METH appears to be less toxic in adolescent rats compared to young adult rats. VMAT-2 is proposed to play an important role in the age-dependent difference. Since the effect of METH on the function and quantity of VMAT-2 has primarily been studied in young adult rats and since developmental changes in the dopaminergic system are reported to occur between adolescence and adulthood, it is important to determine if there is an age-dependent difference in response of VMAT-2 to METH.

Age-dependent methamphetamine-induced alterations in vesicular monoamine transporter-2 function: implications for neurotoxicity.

Tens of thousands of adolescents and young adults have used illicit methamphetamine. This is of concern since its high-dose administration causes persistent dopaminergic deficits in adult animal models. The effects in adolescents are less studied. In adult rodents, toxic effects of methamphetamine may result partly from aberrant cytosolic dopamine accumulation and subsequent reactive oxygen species formation. The vesicular monoamine transporter-2 (VMAT-2) sequesters cytoplasmic dopamine into synaptic vesicles for storage and perhaps protection against dopamine-associated oxidative consequences. Accordingly, aberrant VMAT-2 function may contribute to the methamphetamine-induced persistent dopaminergic deficits. Hence, this study examined effects of methamphetamine on VMAT-2 in adolescent (postnatal day 40) and young adult (postnatal day 90) rats. Results revealed that high-dose methamphetamine treatment caused greater acute (within 1 h) decreases in vesicular dopamine uptake in postnatal day 90 versus 40 rats, as determined in a nonmembrane-associated subcellular fraction. Greater basal levels of VMAT-2 at postnatal day 90 versus 40 in this purified fraction seemed to contribute to the larger effect. Basal tissue dopamine content was also greater in postnatal day 90 versus 40 rats. In addition, postnatal day 90 rats were more susceptible to methamphetamine-induced persistent dopaminergic deficits as assessed by measuring VMAT-2 activity and dopamine content 7 days after treatment, even if drug doses were adjusted for age-related pharmacokinetic differences. Together, these data demonstrate dynamic changes in VMAT-2 susceptibility to methamphetamine as a function of development. Implications with regard to methamphetamine-induced dopaminergic deficits, as well as dopamine-associated neurodegenerative disorders such as Parkinson's disease, are discussed.

Dopamine D2 receptor activation increases vesicular dopamine uptake and redistributes vesicular monoamine transporter-2 protein.

Recent studies demonstrate that multiple dopamine receptor subtypes contribute to the regulation of vesicular monoamine transporter-2 (VMAT-2) activity. The present studies extend these findings by demonstrating that administration of the nonselective dopamine D2 receptor family agonist, quinpirole, rapidly increased vesicular dopamine uptake in purified rat striatal vesicles. This effect occurred in both postnatal day 40 and 90 rats, and was associated with redistribution of the vesicular monoamine transporter-2 (VMAT-2) within nerve terminals. Neither a full nor a partial dopamine D1 receptor family agonist (SKF81297 nor SKF38393, respectively) affected vesicular dopamine uptake per se, nor the effect of quinpirole. Neither the dopamine D3 nor the D4 receptor antagonists, NGB2904 and clozapine, respectively, altered the quinpirole-mediated increase in uptake. However, the nonselective dopamine D2 receptor family antagonist, eticlopride, prevented the quinpirole-induced increase. Taken together, these data demonstrate that dopamine D2 receptor subtype activation increases vesicular dopamine uptake. Implications of this phenomenon with regard to the treatment of Parkinson's disease will be discussed.

Apomorphine increases vesicular monoamine transporter-2 function: implications for neurodegeneration.

Apomorphine is a nonselective dopamine D1/D2 receptor agonist used in Europe to treat symptoms resulting from the dopaminergic degeneration associated with Parkinson's disease. In addition, neuroprotective effects of this agent in rodent models have been reported. Recent studies indicate that treatments that alter vesicular monoamine transporter-2 (VMAT-2) function may be protective in models of dopaminergic degeneration. Hence, the purpose of the present study was to examine the effect of apomorphine on VMAT-2 function. Results revealed that apomorphine rapidly and reversibly increased vesicular dopamine uptake, as determined in purified striatal vesicles obtained from treated rats. This increase occurred in both postnatal day 40 and postnatal day 90 rats, and was associated with a redistribution of VMAT-2 protein within nerve terminals. This effect of apomorphine on vesicular dopamine uptake was blocked by pretreating with eticlopride, a dopamine D2 receptor antagonist. The implications of these findings relevant to the treatment of neurodegeneration are discussed.

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.

Pramipexole increases vesicular dopamine uptake: implications for treatment of Parkinson's neurodegeneration.

Pramipexole is a dopamine D2/D3 receptor agonist used to treat Parkinson's disease. Both human and animal studies suggest that pramipexole may exhibit neuroprotective properties involving dopamine neurons. However, mechanisms underlying its neuroprotective effects remain uncertain. The present results reveal a novel cellular action of this agent. Specifically, pramipexole rapidly increases vesicular dopamine uptake in synaptic vesicles prepared from striata of treated rats. This effect is: (1) associated with a redistribution of vesicular monoamine transporter-2 (VMAT-2) immunoreactivity within nerve terminals; and, (2) prevented by pretreatment with the dopamine D2 receptor antagonist, eticlopride. The implications of this finding relevant to the treatment of neurodegenerative disorders are discussed.