Effect of Homo-?-Proline and Other Heterocyclic GAB A Analogues on GABA Uptake in Neurons and Astroglial Cells and on GABA Receptor Binding

Abstract

Two groups of GABA analogs, 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 mouse and rat neurons and glial cells and displacers of GABA receptor binding. In addition to cis-4-hydroxynipecotic acid, which is a potent inhibitor of GABA uptake, homo-.beta.-proline was the only compound which was a potent inhibitor of glial and neuronal GABA uptake. IC50 [median inhibitory concentration] values for GABA uptake into glial cells and brain cortex prisms were 20 and 75 .mu.M, respectively, and the IC50 value obtained for GABA uptake into cultured neurons was 10 .mu.M. A kinetic analysis of the action of homo-.beta.-proline on GABA uptake into cultured astrocytes and neurons showned 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 .mu.M could be calculated for glial and neuronal uptake, respectively. This mechanism of action strongly suggests that homo-.beta.-proline interacts with the GABA from its receptor with an IC50 value of 0.3 .mu.M. The cis-4-hydroxynipecotic acid analogs, cis- and trans-4-mercaptonipecotic acid, had no inhibitory effect on glial or neuronal GABA uptake. Other SH reagents, PCMB [p-chloromercuribenzoate], NEM [N-ethylmaleimide] and DTNB [dithio bis(nitrobenzoic acid)], were 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.