Noradrenaline inhibition of acetylcholine release from guinea-pig brain

— A correlation has been attempted between the uptake characteristics of l- and d-homocysteate and the time courses of neuronal excitation by these and other amino acids related to l-glutamate.



The uptake of l- and d-homocysteate and of l-[35S]homocysteate was studied in individual slices of rat cerebral cortex at 37°C. Tissue: medium ratios attained over l0 min for the unlabelled enantiomers at 2.5 mM were 3.7 for l-homocysteate but only 0.8 for the d-isomer. The uptake of l-[35S]homocysteate over the concentration range 0.09 μm-2 mm can be attributed mainly to a low-affinity transport process with Km approx 3 mm and Vmax 1.7 μmol/g/min, but a high-affinity process of low Vmax may make a minor contribution at the lower concentrations within this range. In terms of dependence on energy metabolism and [Na+], and on inhibition by p-chloromercuriphenylsulphonate, ouabain and structural analogues of the amino acid, the main uptake system for L-[35S]homocysteate appears to be similar to that mediating low-affinity uptake of l-glutamate and other acidic amino acids. d-Homocysteate was but a weak inhibitor of this uptake system compared with other structural analogues.



The time courses of excitation by 6 amino acids were determined by microelectrophoretic application to rat spinal neurones. d-Homocysteate induced responses with recovery times considerably longer than those of the other amino acids; this correlates with the absence of rapid uptake systems demonstrated for this amino acid in cortical tissue. d-Glutamate and l-homocysteate, which are only accumulated by low-affinity transport mechanisms, induced responses with recovery periods similar to those of l-glutamate, l-aspartate and d-aspartate, which are accumulated by both high- and low-affinity uptake systems. Although contributions of other factors to the observed time courses, such as rates of association and dissociation of the amino acid-receptor complexes, cannot be excluded, the present results are consistent with the hypothesis that low-affinity uptake systems of high Vmax play an important role in the rapid termination of the effects of amino acid excitants.

ACTIONS OF l‐ AND d‐HOMOCYSTEATE IN RAT CNS: A CORRELATION BETWEEN LOW‐AFFINITY UPTAKE AND THE TIME COURSES OF EXCITATION BY MICROELECTROPHORETICALLY APPLIED L‐GLUTAMATE ANALOGUES

Publisher Summary This chapter discusses ion and energy metabolism of the brain at the cellular level. The comparison between metabolism in the brain in vivo and in brain slices and isolated cells has highlighted that ion and energy metabolism of the dissociated cell types corresponds to that in more complex in vitro preparation and under in vivo conditions. Therefore, observations in such preparations of individual cells are relevant for interpretation of physiological and pathological events at the cellular level of the brain cortex. The chapter also focuses the effects of ions (especially K+) on energy metabolism and of adverse metabolic conditions on ion distribution. From a physiological point of view, it is probably of major importance that the potassium concentration in the interstitial fluid becomes considerably increased during neuronal activity. Owing to the pronounced influence of K+ on membrane polarization, efficient and fast removal of excess extracellular potassium is essential.

Ion and energy metabolism of the brain at the cellular level.

Effects of nicotine on distribution and release of 14C-norepinephrine and 14C-dopamine in rat brain striatum and hypothalamus slices.

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The uptake and release of glutamate and of GABA, as well as the effect of high potassium concentrations (35 or 80 mM) hereupon, were studied by aid of 14C-labelled amino acids in brain cortex slices from rats of different ages between birth and adulthood. Both the extent of the uptake (i.e. the tissue/medium ratio of 14C at, or close to, equilibrium) and the rate of uptake (i.e. the tissue/ medium ratio of 14C after short (5 min) incubation periods) increased with age. Differences were, however, found between glutamate and GABA, and the extent of the GABA uptake had a distinct maximum during the second postnatal week. At all ages, high concentrations of potassium caused a decrease in the rate of GABA uptake but were without effect on the rate with which glutamate was taken up. The release of the two amino acids occurred with approximately the same half-time (50 min) in slices from animals of at least 14 days of age. Before that time the release of glutamate was somewhat faster, whereas that of GABA was much slower, especially during the first postnatal week (half-time 90 min). The ontogenetic alterations in the effect of excess potassium were complex and varied both between the two potassium concentrations used and between the two amino acids. The results are thus compatible with the existence of different transport systems for the two amino acids, They also suggest that glutamate may exert other functions in addition to its role as a putative transmitter.

Postnatal alterations in effects of potassium on uptake and release of glutamate and GABA in rat brain cortex slices.

A new type of tissue holder is described in which the slices are freely floating. Rat brain-cortex slices incubated in this holder show a potassium concentration of 70.4 μmoles/g final wet wt. after incubation and transfer for 1 hr.

L. Lolle

Papers

An improved holder for transfer of brain slices during in vitro incubation