Differential effects of nipecotic acid and 4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol on extracellular gamma-aminobutyrate levels in rat thalamus.

Using the microdialysis technique and HPLC (high-performance liquid chromatography) determination of amino acids, the extracellular concentrations of gamma-aminobutyrate (GABA), glutamate, aspartate and a number of other amino acids were determined in rat thalamus during infusion through the microdialysis tubing of the GABA transport inhibitors 4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol (THPO) and nipecotic acid. Administration of 5.0 mM THPO led to a 200% increase in the extracellular GABA concentration. Simultaneous infusion of THPO and GABA (50 microM) increased the extracellular GABA concentration to 1200% of the basal level whereas GABA alone was found to increase the GABA level to 500%. If nipecotic acid (0.5 mM) was administered together with GABA (50 microM) the extracellular concentration of GABA was not increased further. While administration of GABA alone or GABA together with nipecotic acid had no effect on the extracellular levels of glutamate and aspartate it was found that GABA plus THPO increased the extracellular concentration of these amino acids. GABA administered alone or together with nipecotic acid or THPO led to relatively small but significant increases in the extracellular concentrations of the amino acids glycine, glutamine, serine and threonine. The results demonstrate that THPO, which preferentially inhibits glial GABA uptake and which is not a substrate for the GABA carriers, was more efficient increasing the extracellular concentration of GABA than nipocotic acid which is a substrate and an inhibitor of both neuronal and glial GABA uptake. This indicates that GABA uptake inhibitors that are not substrates for the carrier and which preferentially inhibit glial GABA uptake may constitute a group of drugs by which the efficacy of GABAergic neurotransmission may be enhanced.