Abstract
The release of 3H-GABA evoked by exposure to 30 mM potassium during 1 min was found to be calcium independent in the rat occipital cortex and calcium dependent in the substantia nigra. Exposure to either muscimol 1 μM or GABA 1 μM inhibited the potassium-evoked release release of 3H-GABA from the substantia nigra but not from the occipital cortex. The inhibitory effect of muscimol 1 μM on the pitassium-evoked release of 3H-GABA from the substantia nigra was significantly antagonized by picrotoxin 10 μM. Exposure to either 10 or 100 μM pricotoxin alone did not affect the potassium-evoked release of 3H-GABA. These results are compatible with the presence of a negative feedback mechanism in gabaergic nerve terminals of the rat substantia nigra which is mediated by presynaptic GABA autoreceptors. In addition, these results emphasize the importance of the calcium-dependent dependent nature of the release of the neurotransmitter for demonstrating the modulation of transmitter release through presynaptic receptors.
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