Release of endogenous GABA from the substantia nigra is not controlled by GABA autoreceptors

The characteristics of the release of GABA from slices of the rat substantia nigra, elicited by electrical stimulation at frequencies of 0.5-48 Hz and by elevated K+ concentrations ranging from 15-35 mmol/l, was studied. Comparisons were made with cortical slices where the data were not available from previous studies. No GABA release could be evoked from rat nigral slices by electrical stimulation between 0.5 and 4 Hz, in contrast to cortical slices, in which this pool is sensitive towards inhibition by (-)-baclofen. Also, comparatively less GABA release could be evoked from nigral than from cortical slices by K+ concentrations between 15 and 25 mmol/l. While (-)-baclofen at 10 mumol/l inhibited release caused by 15 mumol/l K+ in cortical, it did not in nigral slices. GABA release caused by higher frequencies (8-48 Hz) or 30 mmol/l K+ concentrations was Ca2+-dependent and in the former case also tetrodotoxin-sensitive. It had similar characteristics as in cortical slices and was insensitive towards (-)-baclofen, muscimol and bicuculline. Even more markedly than in the cortex, 30 mmol/l K+ released greater amounts of GABA than electrical stimulation at 24 Hz of a similar duration, suggesting the existence of one or several additional pool(s) of lesser excitability.(ABSTRACT TRUNCATED AT 250 WORDS)