Presynaptic GABA B autoreceptors suppress neurotransmitter release during repetitive stimulation via the Gβγ-SNARE pathway

ABSTRACT GABAergic signaling provides the brain’s primary inhibitory mechanism with defects linked to epilepsy, anxiety, depression, insomnia, schizophrenia and neurodegeneration. A key regulatory mechanism is autoinhibition of GABA release during repetitive activity via presynaptic G i/o -coupled GABA B receptors, supporting synaptic tuning and memory formation, and limiting neurotransmitter spillover. Exogenous GABA B receptor agonists reduce presynaptic Ca 2+ entry by inhibiting calcium channels. However, using transgenic mice expressing a mutant SNAP25 with diminished ability to bind Gβγ (SNAP25Δ3), we show that suppression by GABA B autoreceptors requires intact Gβγ-SNARE interactions. Imaging of presynaptic Ca 2+ transients in GABAergic axons showed no GABA-mediated autoreceptor suppression of Ca 2+ entry during stimulus trains. In contrast, application of the exogenous GABA B receptor agonist baclofen profoundly inhibited Ca 2+ entry, which could be partially reversed by exogenously elevating cAMP, indicating a complementary role of inhibition of adenylyl cyclase. Baclofen reduced spontaneous IPSC frequency and amplitude and both effects were diminished in SNAP25Δ3 mice, consistent with inhibition at Ca 2+ channels and SNARE complexes. Physiological GABA-mediated and exogenous GABA B receptor activation thus produce distinct outcomes on GABAergic neurotransmission, indicating that synthetic drug application to neurons does not faithfully recapitulate endogenous signaling pathways. We conclude that endogenous rapid GABA B autoreceptor signaling inhibits neurotransmitter release primarily by Gβγ-mediated inhibitions of SNARE mechanisms, whereas prolonged agonist application additionally suppresses Ca 2+ influx via cAMP signaling.