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Neuronal G protein-gated inwardly rectifying potassium (GIRK) channels mediate the slow inhibitory effects of many neurotransmitters post-synaptically. However, no evidence exists that supports that GIRK channels play any role in the inhibition of glutamate release by GABAB receptors. In this study, we show for the first time that GABAB receptors operate through two mechanisms in nerve terminals from the cerebral cortex. As shown previously, GABAB receptors reduces glutamate release and the Ca2+ influx mediated by N-type Ca2+ channels in a mode insensitive to the GIRK channel blocker tertiapin-Q and consistent with direct inhibition of this voltage-gated Ca2+ channel. However, by means of weak stimulation protocols, we reveal that GABAB receptors also reduce glutamate release mediated by P/Q-type Ca2+ channels, and that these responses are reversed by the GIRK channel blocker tertiapin-Q. Consistent with the functional interaction between GABAB receptors and GIRK channels at nerve terminals we demonstrate by immunogold electron immunohistochemistry that pre-synaptic boutons of asymmetric synapses co-express GABAB receptors and GIRK channels, thus suggesting that the functional interaction of these two proteins, found at the post-synaptic level, also occurs at glutamatergic nerve terminals.