Nigral tissue prepared as synaptosomes demonstrates both high and low affinity uptake of [3H]dopamine. Recently accumulated [3H]dopamine is releasable by 35 mM K+. Substance P increases both uptake and release of dopamine by nigral synaptosomes; γ-aminobutyric acid (GABA) inhibits release with no effect on uptake at concentrations less than 10-4 M. In the striatum, substance P inhibits both uptake and release of dopamine. The results support the existence of dopamine-containing terminals in substantia nigra tissue. The differences in response to substance P and GABA found between nigra and striatum may reflect structural differences in dopamine-containing processes in these areas, related to their proposed origin as dend-tritic (substantia nigra) and axonal (striatal) terminals.
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