GTP-binding protein βγ subunits mediate presynaptic calcium current inhibition by GABAB receptor

A variety of GTP-binding protein (G protein)-coupled receptors are expressed at the nerve terminals of central synapses and play modulatory roles in transmitter release. At the calyx of Held, a rat auditory brainstem synapse, activation of presynaptic gamma -aminobutyric acid type B receptors (GABA(B) receptors) or metabotropic glutamate receptors inhibits presynaptic P/Q-type Ca2+ channel currents via activation of C proteins, thereby attenuating transmitter release. To identify the heterotrimeric G protein subunits involved in this presynaptic inhibition, we loaded G protein beta gamma subunits (G beta gamma) directly into the calyceal nerve terminal through whole-cell patch pipettes. G beta gamma slowed the activation of presynaptic Ca2+ currents (I-pCa) and attenuated its amplitude in a manner similar to the externally applied baclofen, a GABA(B) receptor agonist. The effects of both G beta gamma and baclofen were relieved after strong depolarization of the nerve terminal. In addition, G beta gamma partially occluded the inhibitory effect of baclofen on I-pCa. In contrast, guanosine 5 ' -O-(3-thiotriphosphate)-bound G(o)alpha loaded into the calyx had no effect. Immunocytochemical examination revealed that the subtype of G proteins G(o), but not the G(i), subtype, is expressed in the calyceal nerve terminal. These results suggest that presynaptic inhibition mediated by C protein-coupled receptors occurs primarily by means of the direct interaction of G(o) beta gamma subunits with presynaptic Ca2+ channels.