The release of endogenous amino acids from the rat visual cortex.

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Abstract
The release of endogenous taurine, GABA, glycine, aspartate, glutamate, glutamine and alanine from the rat visual cortex was measured using a cortical cup technique. The electrocorticogram (EGoG) was monitored throughout most experiments. Spreading depression, evoked by the dropwise placement of 10% KCl solution on to the brain outside the cup was associated with a significant increase in the release of GABA and glutamine but a marked fall in that of glutamate. The evoked release of GABA and glutamate but not of glutamine was Ca2+-dependent. A solution containing 50 mM-K+ placed within the cup elicited a significant increase in the release of taurine and GABA, whereas 100 mM K+ additionally released aspartate and glutamate. The K+-evoked release of these amino acids with the exceptions of taurine and glutamine was Ca2+-dependent. Three series of experiments were carried out in which the preparations were stimulated electrically. Bipolar stimulation (100 Hz, 1 ms pulse width, 2.5 mA for 5 min) with the electrode within the cup was followed by significant increases in taurine, GABA and glutamate release; using a 5 mA current, there was an additional release of aspartate and alanine. Only the evoked release of GABA and glutamate was Ca2+-dependent. In the 2nd and 3rd series of experiments, the electrode was sited adjacent to the cup or on the contralateral cortex, respectively. Following stimulation (100 Hz, 1 ms pulse width, 2.5 mA for 5 min) there was a significant increase in taurine and GABA release and a significant fall in the release of aspartate and glutamate. With the exception of taurine, these changes in release were Ca2+-dependent. Reducing the stimulus current to 1.5 mA or the period of stimulation to 2.5 min initiated similar but statistically insignificant changes in release. A range (10-100 Hz) of stimulation frequencies was examined: the evoked release of GABA was linearly related to frequency whereas that of taurine was frequency-independent. The fall in aspartate and glutamate release was maximal at a frequency of about 50 Hz. The possible sites of release of the amino acids and the proposed neurotransmitter roles of the physiologically active amino acids were discussed.