MECHANISM OF ACTION OF DRUGS OF ABUSE--AMPHETAMINE

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DESCRIPTION: (Adapted From The Applicant's Abstract) This application seeks renewal of research support to continue to study the sites and mechanisms of action of psychomotor stimulants. Its general aim is to study the effects of amphetamine on striatal glutamate and dopamine neurotransmission and the sites of termination of glutamate and dopamine axons relative to the different neostriatal output cells. Specifically, it will address three issues related to the effects of systemic amphetamine: the presynaptic mechanisms contributing to neostriatal changes associated with addiction, and the circuitry that brings about the amphetamine-induced increase in neuronal activity of the direct striatal output pathway. The motor effects of these drugs depend on their ability to increase dopamine (DA) transmission. Several motor responses are enhanced by repeated amphetamine administration; a phenomenon termed behavioral sensitization. Changes in the sensitivity of DA somatodendritic and terminal axon autoreceptors may contribute to behavioral sensitization. Experiments are proposed using terminal excitability, an in vivo electro- physiological measure of presynaptic receptor stimulation, and microdialysis to further study the bases for presynaptic changes in nigrostriatal DA axon terminals in sensitized rats. DA terminal fields in nucleus accumbens and prefrontal cortex will also be examined. Alterations in glutamatergic transmission may also be critical in the development of sensitization. Excitability measurements will assess possible presynaptic changes in the glutamatergic afferents to the neostriatum, nucleus accumbens and ventral tegmental area. Other studies will examine the relation between impulse-induced long-lasting changes in presynaptic corticostriatal excitability and postsynaptic expressions of long- term potentiation or depression. Electron microscopic studies are proposed to further elucidate the neostriatal circuitry involving the DA and glutamate afferent systems. It will be determined whether DA inputs onto spiny dendrites are associated with inputs from specific cortical and thalamic regions and if these patterns of convergence differ for spiny neurons identified as belonging to the direct and indirect output pathways. The thalamus is a major excitatory input to the striatum, yet little is known regarding differences in innervation from specific thalamic regions. Differences in the thalamic input onto cholinergic and spiny neurons participating in the two output pathways will be examined and the morphology and other afferents of these cells determined. The possibility of nonsynaptic release sites on nigrostriatal DA afferents will be assessed by labeling components of the release mechanism and then correlating them with the locations of dopamine receptors.