Sulphydryl groups and GABA uptake in slices of rat cerebral cortex

GABA 'desensitization' of frog primary afferent fibers

GABA (gamma-aminobutyric acid) depolarizes the terminals of primary afferent fibers of the in vitro hemisected frog spinal cord. During sustained or repetitive exposure to GABA or to muscimol, the amplitude of the depolarization is characterized by a rapid and exponential decline to a steady plateau level (desensitization). Desensitization to muscimol was eliminated by removal of Ca2+ and addition of Mn2+ to the superfusate--a finding consistent with the presence of 'receptor' ('true') desensitization (i.e., receptor inactivation). GABA desensitization was significantly reduced by exposure of the cord to either low Na+, low temperature, ouabain, dinitrophenol, (+/-)-nipecotic acid, or cis-1,3-aminocyclohexanecarboxylic acid. These treatments also significantly decreased the high affinity uptake of GABA when the latter process was studied by incubating frog spinal slices in Ringer's solution containing a low concentration of [3H]GABA. These results suggest that cellular transport processes can influence the form of GABA responses and indicate that neuronal removal of GABA is responsible in part for GABA desensitization.

Effect of homo-beta-proline and other heterocyclic GABA analogues on GABA uptake in neurons and astroglial cells and on GABA receptor binding

Two groups of GABA (gamma-aminobutyric acid) analogues, one comprising derivatives of beta-proline and the other compounds structurally related to nipecotic acid, were investigated as potential inhibitors of high-affinity GABA transport in neurons and glial cells, as well as displacers of GABA receptor binding. In addition to cis-4-hydroxynipecotic acid, which is known as a potent inhibitor of GABA uptake, homo-beta-proline was the only compound which proved to be a potent inhibitor of glial as well as neuronal GABA uptake. IC50 values for GABA uptake into glial cells and brain cortex "prisms" were 20 and 75 micro M, respectively, and the IC50 value obtained for GABA uptake into cultured neurons was 10 micro M. A kinetic analysis of the action of homo-beta-proline on GABA uptake into cultured astrocytes and neurons showed that this compound acts as a competitive inhibitor of GABA uptake in both cell types. From the apparent Km values, Ki values for homo-beta-proline of 16 and 6 micro M could be calculated for glial and neuronal uptake, respectively. This mechanism of action strongly suggests that homo-beta-proline interacts with the GABA carriers. Furthermore, homo-beta-proline also displaced GABA from its receptor with an IC50 value of 0.3 micro M. The cis-4-hydroxynipecotic acid analogues, cis- and trans-4-mercaptonipecotic acid, had no inhibitory effect on glial or neuronal GABA uptake. Other SH reagents, PCMB, NEM and DTNB, were shown to be relatively weak inhibitors of GABA uptake into cultured astrocytes, suggesting that SH groups are not directly involved in the interaction between GABA and its transport carrier.

Hypotaurine transport in brain slices: comparison with taurine and GABA

Hypotaurine uptake was compared to taurine and GABA uptakes in brain slices under identical experimental conditions. The slices effectively concentrated both hypotaurine and GABA from the medium, whereas taurine was taken up more slowly. The uptakes of these three structurally related amino acids were all saturable, consisting of one low- and one high-affinity transport component. The kinetic parameters of hypotaurine uptake were of the same order of magnitude as those of GABA uptake. All uptake systems were sensitive to temperature, metabolic poisons, and sodium omission. Hypotaurine uptake was inhibited by GABA, L-2,4-diaminobutyric acid (L-DABA), cysteic acid, and beta-alanine, but not by taurine. Taurine uptake was strongly reduced by hypotaurine, beta-alanine, and L-DABA, as well as by GABA, whereas GABA uptake was affected only by cystamine, L-DABA, and nipecotic acid. The uptake processes of hypotaurine, taurine, and GABA were thus fairly similar and showed properties characteristic for neurotransmitter uptake. Hypotaurine uptake resembled more GABA than taurine uptake. The present inhibition studies suggest that there may exist only one common two-component transport system for these three amino acids.