ACTIONS OF l ‐GLUTAMATE AND RELATED AMINO ACIDS ON OXYGEN UPTAKE, LACTATE PRODUCTION AND NADH LEVELS OF RAT BRAIN IN VITRO

A range of acidic amino acids differing in (i) their potency as neuronal excitants, (ii) their transport properties and (iii) their ability to act as substrates for metabolism have been compared with respect to their effects on energy metabolism of rat cerebral cortex in vitro.l‐Glutamate, and d‐ and l‐homocysteate, increased tissue slice NADH levels, and the same three amino acids, together with d‐glutamate and kainate, increased oxygen uptake by the slices. It was concluded that these effects were predominantly due to neuronal depolarization and the ensuing activation of ion pump mechanisms. l‐Glutamate, d‐glutamate and l‐homocysteate increased lactate production by the slices, whereas d‐homocysteate and kainate did not. Since the two latter amino acids are the strongest neuroexcitants but probably the least rapidly transported, it is suggested that stimulation of lactate production in slices by amino acid excitants is a consequence of the energy requirements of active uptake of the amino acids, and probably occurs mainly in glial cells. Although the metabolism of l‐glutamate appeared not to be an essential requirement for the effects observed with this amino acid in the present work, such metabolism may make a proportionately greater contribution under sub‐optimal conditions of slice preparation and incubation, where electrical activity of the tissue may be impaired.