Differential Transmitter Release from Nerve Terminals Isolated from Basal Ganglia and Substantia Nigra

The K+‐induced release of amino acids and dopamine from synaptosomes of basal ganglia and substantia nigra of sheep was studied. K+ (56 mM) caused an increase in the release of GABA from caudate, putamen, globus pallidus, and substantia nigra, the increased release being 227, 171, 198, and 366%, respectively, compared with samples incubated without stimulation. The release of glutamate was also increased by 56 mM‐K+ (136–183%) from all regions except the globus pallidus, and a significant release of aspartate was only seen in response to K+ stimulation of synaptosomes from putamen (50%). Veratrine (75 μM) also stimulated a similar pattern of amino acid release from these regions. Regional correlation was shown between the presence of an uptake system for an amino acid and its evoked release. [14C]Dopamine formed from L‐[U‐14C]tyrosine was released only from caudate and putamen synaptosomes by K+ stimulation, the increases being 105% and 74%, respectively. Synthesis of [14C]dopamine from L‐[U‐14C]tyrosine occurred only in synaptosomes prepared from these two regions and was not detected in synaptosomes from substantia nigra or globus pallidus although whole‐tissue homogenates of substantia nigra were able to synthesise dopamine.