Recordings of extracellular glutamate and gamma-aminobutyric acid in the striatum of non-anesthetized rats. K(+)-stimulation, its Ca(2+)-dependence and lack of effects of drugs acting on dopamine receptors.

The extracellular concentrations of glutamate (GLU) or gamma-aminobutyric acid (GABA) in the striatum were determined in non-anesthetized, non-restrained rats using the microdialysis method, and the effects of some dopaminergic drugs were studied. Since a great part of these amino acids in the extracellular space is probably of origin not related to neurotransmission, the percentage of amino acids of neuronal origin was increased by stimulation with K+ added to the perfusion medium, and the Ca(2+)-dependence of K+ stimulated release was checked. This stimulation was performed twice in most of the experiments, under standard condition and after specific manipulations. K+ (100 mmol/l) produced pronounced increases in the overflow of GLU and even more of GABA. Omission of Ca2+ and addition of 20 mmol/l of Mg2+ in the medium significantly decreased the K(+)-stimulated overflow of GLU, but not of GABA. However, when in the whole perfusion procedure K+ was elevated and, initially, the medium was kept Ca(2+)-free, but contained 20 mmol/l of Mg2+, and subsequently was replaced by a medium containing Ca2+, but no Mg2+, then the shift to the second medium led to considerable increases in overflow both of GLU and GABA indicating a partial Ca(2+)-dependence of K(+)-stimulated overflow and suggesting at least a partial neuronal origin of the extracellular amino acids. In subsequent experiments, dopaminergic drugs were administered systemically before the second stimulation with K+. Apomorphine (2 mg/kg s.c.) had no effect on the K(+)-stimulated GABA overflow.(ABSTRACT TRUNCATED AT 250 WORDS)