Rapid Release of [3H]Dopamine from Median Eminence and Striatal Synaptosomes

Release of preaccumulated, tritium-labeled dopamine (3H]DA) from preparations of isolated nerve terminals (synaptosomes) of rat median eminence (ME) and corpus striatum (CS) was examined over short time intervals (1-20 s). In both preparations, basal efflux of 3H]DA was linear with time. Depolarization with high K+ resulted in an initial rapid release of 3H]DA which stabilized by 20 s, whereas veratridine elicited an increased rate of release over basal levels that was linear over the first 20 s. The calculated rate constants of release for both the initial phase of K+- and the veratridine-stimulated release were approximately threefold greater in CS than in ME synaptosomes. The major component of the high K+-induced release of 3H]DA from both synaptosome preparations increased as a graded function of Ca2+]o. However, a smaller component, independent of external Ca2+, existed in both ME and CS synaptosomes. Increasing the Mg2+] in the external solution resulted in a right shift of both the K+]o and the Ca2+]o dose-response curves, consistent with actions of Mg2+ on screening surface membrane charges and blocking voltage-dependent Ca2+ channels. In all studies, steady-state uptake of the 3H]DA was about twofold greater into CS than into ME synaptosomes. Moreover, the fraction of incorporated 3H]DA released by stimulation from the CS was much greater than that released from ME synaptosomes. These data are consistent with differences between these two types of dopaminergic terminals with respect to packaging and/or distribution of the accumulated neurotransmitter in intraneuronal pools, as well as marked differences in the apparent kinetics of DA release.