Endogenous GABA determines the characteristics of [3H]GABA-binding.

Since the identification of sodium-independent GABA-binding sites in membrane preparations from the mammalian central nervous system numerous publications have dealt with the effects of putative endogenous inhibitors of GABA-binding. However, controversy has arisen over the existence and identity of such inhibitors and the possible artifactural role of residual endogenous GABA in the interpretation of GABA-binding data. Using direct determination of GABA concentrations we have demonstrated that it is extremely difficult to remove endogenous GABA from rat brain membrane preparations and have shown that, even in well-washed preparations, there is a source of GABA production which compounds this problem. When care has been taken to ensure that GABA concentrations are minimal, analysis of binding data reveals the presence of two classes of GABA-binding sites with dissociation constants of the order of 9 nM and 318 nM and maximum binding capacities of 0.63 and 1.65 pmol/mg protein respectively (i.e., 63 and 165 pmol/g original wet tissue). A theoretical analysis of the effects of endogenous GABA demonstrates how failure to take into account even very low concentrations of GABA leads to misinterpretation of the results.