IN VIVO RELEASE OF ENDOGENOUS GABA FROM RAT SUBSTANTIA NIGRA MEASURED BY A NOVEL METHOD

A sensitive GABA assay using HPLC coupled with fluorimetric detection with o-phthalaldehyde is described. GABA, lysine and ethanolamine can be measured within approx 12 min. The detection limits for these compounds (signal/noise = 3) is 0.67 pmol, 1.8 pmol and 0.73 pmol respectively. Using this assay the in vivo release of GABA from rat substantia nigra was studied with a push-pull perfusion technique. A pronounced increase in the rate of endogenous GABA release was observed after addition of depolarizing amounts of K⁺ to the perfusion medium, whereas the concentrations of lysine and ethanolamine in the perfusate did not change. This enhanced release of GABA was not diminished after omission of Ca²⁺ and Mg²⁺ from the medium. Increasing the Mg²⁺ concentration and leaving out Ca²⁺ however, resulted in a marked depression in the K⁺-induced GABA release. Electrical stimulation of the striatum also produced an increase in release of GABA from the substantia nigra. Inhibition of glutamic acid decarboxylase (with 3-mercaptopropionic acid) caused an immediate decrease in GABA release. Inhibition of GABA transminase (with aminooxyacetic acid) leads to an increased release of GABA after approx 15 min. These findings suggest that the technique is suitable for measuring neuronal release of endogenous GABA in vivo