STUDIES ON THE NATURE OF THE RELEASABLE POOL OF DOPAMINE IN SYNAPTOSOMES FROM RAT CORPUS STRIATUM - DEPOLARIZATION-INDUCED RELEASE OF DOPAMINE-H-3 FROM SUPER-FUSED SYNAPTOSOMES LABELED UNDER VARIOUS CONDITIONS

Synaptosomal preparations, obtained from rat corpus striatum, were superfused with Krebs-Ringer medium. During superfusion the synaptosomes were exposed to various concentrations of 3H-dopamine for a few minutes. Depolarization was effected by superfusion with medium containing elevated K+-concentrations. Both the accumulation and the subsequent K+-induced release of 3H-dopamine were quantified in the same preparation. This method was used to detect the presence of 3H-dopamine in different pools in isolated dopaminergic nerve terminals. The data obtained can be summarized as follows: 1. The percentage of 3H-dopamine released and, hence, the distribution of 3H-dopamine into intrasynaptosomal transmitter pools, was independent of the amount of 3H-dopamine accumulated in the synaptosomes. 2. Upon repeated depolarization, 3H-dopamine release was markedly depressed. Supplying the synaptosomes with 3H-dopamine between two stimulations fully restored the release in response to depolarization. Therefore, the depression of 3H-dopamine release upon repeated depolarization is probably due to the selective depletion of a releasable pool and not to an impairment of stimulus-secretion coupling processes. 3. When synaptosomes were subjected twice to K+-stimulation and exposed to 3H-dopamine either before the first stimulation or between both stimulations, the most recently accumulated 3H-dopamine was released (by the second stimulation) in preference to 3H-dopamine accumulated previously. 4. 14C-Dopamine synthesized in the synaptosomes from 14C-tyrosine was not released in preference to 3H-dopamine that entered synaptosomal pools through the high affinity uptake mechanism. 5. Our results strongly support the view that dopamine is stored heterogeneously in the varicosities of nigro-striatal neurones in the rat. The remarkably slow redistribution of stored 3H-dopamine into synaptosomal pools indicates that differences in vesicle localization with respect to the nerve terminal membrane might explain the occurrence of different dopamine pools. © 1979 Springer-Verlag.