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

Abstract

SUMMARY1. 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 (ECoG) was monitored throughout most experiments.2. 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.3. 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.4. Three series of experiments were carried out in which the preparations were stimulated electrically. Bipolar stimulation (100 Hz, 1 msec 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.5. In the second and third series of experiments, the electrode was sited adjacent to the cup or on the contralateral cortex respectively. Following stimulation (100 Hz, 1 msec 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 1P5 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 R. M. CLARK AND G. G. S. COLLINS linearly related to frequency whereas that of taurine was frequencyindependent. The fall in aspartate and glutamate release was maximal at a frequency of about 50 Hz.6. The results are discussed in relation to (a) the possible sites of release of the amino acids and (b) the proposed neurotransmitter roles of the physiologically active amino acids.

Evidence of a neurotransmitter role for aspartate and γ‐aminobutyric acid in the rat olfactory cortex

“…In none of the present experiments was a consistent, significant glutamate release detected: even direct electrical stimulation of the slice enclosed within the cup was unaccompanied by significant glutamate release (see Table 3) unless the stimulus current was increased to 5 mA (not shown). Under such conditions, release of glutamate was accompanied by release of alanine and glycine in addition to aspartate, taurine and GABA (see also Clark & Collins, 1976). Moreover, although in the present bulbectomy experiments there was a slow but perceptible fall in glutamate levels (see Fig.…”

“…The animals were allowed to recover and then killed at various time intervals thereafter. Slices of olfactory cortex were prepared in the usual manner and their amino acid content extracted and estimated using the methods described previously (Clark & Collins, 1976). Release of amino acids from slices of cortex from such bulbectomized rats was also assessed.…”

“…of mean, the number of experiments is shown in parentheses) cut from the olfactory cortex slices which had been incubated (see Pickles & Simmonds, 1976) but not perfused. For method of extraction and assay, see Clark & Collins (1976). t The resting amino acid efflux is from slices either incubated and perfused with normal Ringer solution or Ca2+-free Ringer solution (for composition, see Methods section). Each value is the mean resting efflux + S.E.…”

“…The amount of aspartate, glutamate, GABA, glycine, taurine, glutamine and alanine in the samples was estimated using a double-label modification of the microdansylation procedure of Briel & Neuhoff (1972). For full details of the assay procedure, see Clark & Collins (1976).…”

Possible presynaptic actions of 2‐amino‐4‐phosphonobutyrate in rat olfactory cortex

“…The experimental procedure employed in preparing and perfusing the tissue fragments is described elsewhere (Collins et al, 1981). The amino acids were estimated by a double label microdansylation procedure (Clark & Collins, 1976).…”