Dopamine receptor regulation of vesicular monoamine transporter-2: implications for neurotoxicity and Parkinson's neurodegenration.

Alterations in vesicular monoamine transporter-2 (VMAT-2) function could regulate intra- and extra-neuronal dopamine levels, which may modify cytoplasmic dopamine levels. An aberrantly high level of cytoplasmic dopamine has been linked to neuronal toxicity. Recent studies have indirectly demonstrated that dopamine D1 and D2 receptor classes influence vesicular dopamine uptake, however, no studies to date have shown that direct dopamine receptor stimulation would alter VMAT-2 function. Several studies have also suggested that dopamine receptor agonists may be protective in neurodegenerative models such as methamphetamine (METH)-induced toxicity, although the underlying mechanism is unknown. Interestingly, this METH-induced neurotoxicity is observed in older but not younger rodent models. In this dissertation, the hypothesis that METH and dopamine receptor agonists differentially alter vesicular dopamine uptake will be tested. In the first part of this dissertation, the effect of direct dopamine receptor stimulation on VMAT-2 function is examined. Results reveal that dopamine receptor agonists (apomorphine, pramipexole, and quinpirole) increase vesicular dopamine uptake and redistribute VMAT-2 within nerve terminals. This effect occurs in both postnatal day (PND) 40 and 90 rats. Pretreating with a dopamine D3 or D4 receptor antagonist does not prevent the quinpirole-induced increase in vesicular dopamine uptake. Pretreating with the nonselective dopamine D2 receptor antagonist, eticlopride, blocks the apomorphine-, pramipexole-, and quinpirole-induced increases in vesicular dopamine uptake. These data demonstrate that the dopamine D2 receptor subtype underlies the dopamine receptor agonist-induced increase in vesicular dopamine uptake. The present results also demonstrate that, in contrast to dopamine receptor agonists, a neurotoxic regimen of METH significantly decreases vesicular dopamine uptake in purified vesicles of PND 90 but not of PND 40 rats. This greater effect caused by METH that was observed in PND 90 rats was not due to differences in body temperature, as both treated groups were similarly hyperthermic. However, the effect is associated with greater brain METH concentration in PND 90 compared to PND 40 rats. This difference in pharmacokinetics likely did not contribute to the difference in vesicular dopamine uptake, as administering doses of METH that produced greater or equivalent brain METH levels in PND 40 compared to PND 90 rats did not cause a greater decrease in dopamine uptake in PND 40 rats. These findings suggest a novel mechanism whereby dopamine receptor agonists may be protective and signify a possible explanation for METH-induced toxicity observed in PND 90 but not PND 40 rats.