Syntaxin 1A occludes GABAB receptor-induced inhibition of exocytosis downstream of Ca2+ entry in mouse hippocampal neurons

The mechanisms underlying gamma-amino butyric acid (GABAB) receptor-mediated inhibition of exocytosis have been characterized in a variety of synapses. Using patch-clamp recording methods, we attempted to clarify the intracellular mechanisms underlying presynaptic inhibition in autaptic synapses of isolated mouse hippocampal neurons. Baclofen, a selective GABAB receptor agonist, decreased the frequency of glutamatergic miniature excitatory postsynaptic currents (mEPSCs) without changing their amplitude in Ca2+-free extracellular solution, suggesting that baclofen inhibits exocytosis downstream of Ca2+ entry. Syntaxin 1A is known to modulate exocytosis and suppress neuronal sprouting. Antisense oligonucleotide-mediated knockdown of syntaxin 1A increased the frequency of mEPSCs under Ca2+-free condition. Estimation of the number of functional release sites by staining with FM1-43 indicated that the increased frequency of mEPSCs was induced by facilitation of exocytosis at each site, rather than by an increased number of release sites due to neuronal sprouting. Baclofen reduced mEPSC frequency in syntaxin 1A-knockdown neurons to the same level as that in nonsense oligonucleotide transfected neurons under Ca2+-free condition. These results suggest that the GABAB receptor- and syntaxin 1A-induced inhibitions of exocytosis occlude one another and that the GABAB receptor shares a common intracellular pathway with syntaxin 1A in inhibiting transmitter release downstream of Ca2+ entry.