ON THE SITE OF ORIGIN OF TRANSMITTER AMINO ACIDS RELEASED BY DEPOLARIZATION OF NERVE TERMINALS <i>IN VITRO</i>

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<jats:p><jats:bold>Abstract—</jats:bold> —The site of origin of transmitter amino acids released by depolarizing agents from nerve endings was studied. The model used was the incubated and depolarized synaptosome preparation from which the component soluble, synaptic vesicle, membrane and mitochondrial sub‐fractions were obtained. Synaptosomal amino acids were radioactively labelled from D‐[U‐<jats:sup>14</jats:sup>C]glucose <jats:italic>in vivo</jats:italic> by intraventricular injection and <jats:italic>in vitro</jats:italic> during subsequent incubation. The specific radioactivities of amino acids released in response to K<jats:sup>+</jats:sup> (56 mM) or veratrine (75 μM) were found to closely resemble those of the soluble cytoplasmic fraction, in most cases differing significantly from those of the other fractions. The specific radioactivity of the GABA and aspartate released by K<jats:sup>+</jats:sup> stimulation and the GABA and glutamate released by veratrine were significantly different from that of the vesicles in each case. The specific radioactivities of glutamate released by both agents, and also GABA with K<jats:sup>+</jats:sup> stimulation, were approximately double that of the amino acid released in control conditions. Depletion of the soluble cytoplasmic pools of glutamate, GABA and aspartate occurred following stimulation, corresponding to the induced‐release of these compounds. Turnover of the amino acids in the other subfractions was too low to account for their participation in the release process in addition to the soluble cytoplasmic pool. A cytoplasmic origin of release of neurotransmitter amino acids from nerve endings is proposed.</jats:p>

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