Drug abuse involves the pharmacologic activation of neural pathways involved in reward. Dopamine mediates these pathways and drugs of abuse bypass the normal stimuli for reward by increasing dopamine release more directly. This non-physiological activation then produces changes in synaptic transmission that presumably underlie tolerance, physical dependence and drug craving. Drug abuse therefore provides a model for neural plasticity in the reward pathway that has clear relevance for behavior. Amphetamines induce the release of dopamine stores from synaptic vesicles into the cytoplasm and from the cytoplasm into the synapse. Studies of the mechanism indicate a role for monoamine transporters. Consistent with this, Dr. Sulzer has recently shown that amphetamine reduces quantal size. Interestingly, amphetamine also reduces the frequency of release and FD2 dopamine receptor stimulation alone produces similar changes, suggesting that regulation of transmitter release may have a normal role in the reward pathway and other neural systems. Changes in quantal size and frequency of release may also contribute to drug addiction. Recent developments now enable us to identify the presynaptic mechanisms that regulate transmitter release. Dr. Sulzer will use direct, voltammetric measurement of dopamine in both PC12 cells and primary neuronal; cultures to characterize the effect of D2 receptor activation on dopamine release. To identify the molecular mechanisms responsible for changes in quantal size, Dr. Edwards has isolated cDNA clones encoding the vesicular monoamine transporters and will characterize their potential to reverse (and its role in amphetamine action), their regulation by phosphorylation and their intracellular trafficking. Recent developments in understanding regulated exocytosis, endocytosis and the biogenesis of synaptic vesicles enable Dr. Kelly to determine the role of the synaptic vesicle cycle in regulating transmitter release. Dr. Sulzer will then use voltammetry and primary neuronal cultures to determine the significance of these biochemical mechanisms for dopamine release. In summary, this program will allow three investigators with complementary expertise to explore the role of recently identified molecular mechanisms in synaptic transmission and neural plasticity. The focus on dopamine provides an important new context to address these general issues, one that has central importance for the reward pathway and drug abuse.
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