ISSN:
1573-6903
Keywords:
Neuronal transmission
;
synaptic plasma membranes
;
GABA release
Source:
Springer Online Journal Archives 1860-2000
Topics:
Medicine
Notes:
Abstract Synaptic plasma membrane (SPM) vesicles were isolated under conditions which preserve most of their biochemical properties. Therefore, they appeared particularly useful to study the cytoplasmic GABA release mechanism through its neuronal transporter without interference of the exocytotic mechanism. In this work, we utilized SPM vesicles isolated from sheep brain cortex to investigate the process of [3H]GABA release induced by ouabain, veratridine and Na+ substitution by other monovalent cations (K+, Rb+, Li+, and choline). We observed that ouabain is unable to release [3H]GABA previously accumulated in the vesicles and, in our experimental conditions, it does not act as a depolarizing agent. In contrast, synaptic plasma membrane vesicles release [3H]GABA when veratridine is present in the external medium, and this process is sensitive to extravesicular Na+ and it is inhibited by extravesicular Ca2+ (1 mM) under conditions which appear to permit its entry. However, veratridine-induced [3H]GABA release does not require membrane depolarization, since this drug does not induce any significant alteration in the membrane potential, which is determined by the magnitude of the ionic gradients artificially imposed to the vesicles. The substitution of Na+ by other monovalent cations promotes [3H]GABA release by altering the Na+ concentration gradient and the membrane potential of SPM vesicles. In the case of choline and Li+, we observed that the fraction of [3H]GABA released relatively to the total amount of neurotransmitter released by K+ or Rb+ is about 28% and 68%, respectively. Since the replacement of Na+ by K+, Rb+, and Li+ causes different levels of membrane depolarization, and the replacement of Na+ by choline causes hyperpolarization of the vesicles, these results suggest that, in parallel to the [3H]GABA release, which is directly proportional to the level of membrane depolarization, this neurotransmitter can be released by decreasing the external Na+, which reflects an elevation of the Na+ concentration gradient (in→out). Like veratridine-induced release, the depolarization-induced release of [3H]GABA by SPM vesicles is inhibited by Ca2+, which suggests that this divalent cation interfers with the cytoplasmic GABA release mechanism.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00970542
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