GABA and glycine transport in frog CNS: high affinity uptake and potassium-evoked release in vitro.

Slices of frog cerebrum, optic tectum, medulla and spinal cord rapidly accumulate 3H]GABA and 3H]glycine from the surrounding medium so that after 10 min tissue:medium ratios as high as 113 for GABA (optic tectum) and 18.5 for glycine (medulla) may be achieved. Kinetic analysis revealed two distinct saturable uptake systems for each amino acid in the 4 CNS areas. The high affinity systems (apparent Km: 9-22 muM for GABA; 5-35 muM for glycine) required sodium ions in the medium and were relatively substrate specific. Significant release of 3H]GABA and 3H]glycine, but not of L-3H]leucine, was evoked by exposure to medium containing potassium ions in a concentration of 40 mM. The process of release was calcium-dependent. The importance of these results with regard to the roles of GABA and glycine as neurotransmitters in both spinal and supraspinal levels of the amphibian neuraxis is discussed.