THE UPTAKE OF [³H]GABA BY SLICES OF RAT CEREBRAL CORTEX

Abstract

—A rapid accumulation of [3H]GABA occurs in slices of rat cerebral cortex incubated at 25° or 37° in a medium containing [3H]GABA. Tissue medium ratios of almost 100:1 are attained after a 60 min incubation at 25°. At the same temperature no labelled metabolites of GABA were found in the tissue or the medium. The process responsible for [3H]GABA uptake has many of the properties of an active transport mechanism: it is temperature sensitive, requires the presence of sodium ions in the external medium, is inhibited by dinitrophenol and ouabain, and shows saturation kinetics. The estimated Km value for GABA is 2·2 × 10−5m, and Vmax is 0·115 μmoles/min/g cortex. There is only negligible efflux of the accumulated [3H]GABA when cortical slices are exposed to a GABA‐free medium. [3H]GABA uptake was not affected by the presence of large molar excesses of glycine, l‐glutamic acid, l‐aspartic acid, or β‐aminobutyrate, but was inhibited in the presence of l‐alanine, l‐histidine, β‐hydroxy‐GABA and β‐guanidinopropionate. It is suggested that the GABA uptake system may represent a possible mechanism for the inactivation of GABA or some related substance at inhibitory synapses in the cortex.

Role of Transmitter Uptake Mechanisms in Synaptic Neurotransmission

“…10), leading to tissue to medium ratios as high as 100 to 1 when small slices of cortex are incubated with the radioactive amino-acid in an artificial saline medium. This rate of uptake is even more rapid than that of labelled noradrenaline by the isolated rat heart; it corresponds to a complete clearance of GABA from the extracellular space of the cortical slices in (Iversen & Neal, 1968). less than 2 seconds. Uptake of GABA is similar to that of noradrenaline in that it appears to be mediated by an active transport system which is temperature dependent, saturable, sodium dependent and highly specific.…”

“…In both crustacean nerve-muscle systems and mammalian brain an uptake of exogenous GABA has been observed (Sisken & Roberts, 1964;Blasberg & Lajtha, 1965 ;Weinstein, Varon, Muhleman & Roberts, 1965;Iversen & Kravitz, 1968;Iversen & Neal, 1968;Strasberg & Elliott, 1967). The uptake system which we are most interested in is that present in rat cerebral cortex (Iversen & Neal, 1968). This is capable of a very rapid concentration of extracellular tritium labelled GABA (Fig.…”

Frequency‐dependent depression of inhibition in guinea‐pig neocortex in vitro by GABAB receptor feed‐back on GABA release.

“…Initially, GABA uptake was proposed to remove synaptically liberated GABA and thereby terminate transmitter action (Iversen & Neal, 1968), analogous to the function of acetylcholinesterase at the end-plate (Katz & Miledi, 1973). Enhancement of GABA responses by nipecotic acid, a specific blocker of GABA uptake (Krogsgaard-Larsen & Johnston, 1975) in peripheral ganglion cells (Brown & Galvan, 1977;Gallagher et at.…”

Sodium‐dependent suppression of gamma‐aminobutyric‐acid‐gated chloride currents in internally perfused frog sensory neurones.

“…When the Na+ gradient is reduced by blocking the Na-K pump activity with ouabain, GABA uptake is decreased (Iversen & Neal, 1968). We consider that the importance of the Na+ electrochemical potential gradient in controlling the GABA uptake can best be analysed electrophysiologically, if this ionic factor is well controlled.…”