Interaction of Amphetamines and Related Compounds at the Vesicular Monoamine Transporter

Abstract

Amphetamine-type agents interact with the vesicular monoamine transporter type 2 (VMAT₂), promoting the release of intravesicular neurotransmitter and an increase in cytoplasmic neurotransmitter. Some compounds, such as reserpine, “release” neurotransmitter by inhibiting the ability of VMAT₂ to accumulate neurotransmitter in the vesicle, whereas other types of compounds can release neurotransmitter via a carrier-mediated exchange mechanism. The purpose of this study was to determine, for 42 mostly amphetamine-related compounds, their mode of interaction with the VMAT₂. We used a crude vesicular fraction prepared from rat caudate to assay VMAT₂ activity. Test compounds were assessed in several assays, including 1) inhibition of [³H]dihydrotetrabenazine binding, 2) inhibition of vesicular [³H]dopamine uptake, and 3) release of preloaded [³H]dopamine and [³H]tyramine. Several important findings derive from this comprehensive study. First, our work indicates that most agents are VMAT₂ substrates. Second, our data strongly suggest that amphetamine-type agents deplete vesicular neurotransmitter via a carrier-mediated exchange mechanism rather than via a weak base effect, although this conclusion needs to be confirmed via direct measurement of vesicular pH. Third, our data fail to reveal differential VMAT₂ interactions among agents that do and do not produce long-term 5-hydroxytryptamine depletion. Fourth, the data reported revealed the presence of two pools of [³H]amine within the vesicle, one pool that is free and one pool that is tightly associated with the ATP/protein complex that helps store amine. Finally, the VMAT₂ assays we have developed should prove useful for guiding the synthesis and evaluation of novel VMAT₂ agents as possible treatment agents for addictive disorders.