The hippocampal mossy fiber bouton to CA3 pyramidal neuron (MFB-PN) synapse has an important role in information processing and storage, whose efficacy is under profound modulation. Activation of presynaptic GABAB receptors (GABABRs) reduces extracellularly evoked EPSC amplitudes and mediates heterosynaptic depression at MFB-PN synapses.1,2 To decipher the mechanisms underlying GABABR-mediated modulation at these synapses, we performed paired pre-postsynaptic patch-clamp recordings and ultrastructural analysis using cryo-fixation and 2D electron microscopy (EM) in acute slices of mouse hippocampus. 50µM of the GABABR agonist baclofen suppressed the evoked EPSC amplitudes by 88.3±2.2%. Coefficient-of-variation analysis indicated that baclofen had a presynaptic site of action. The presynaptic Ca2+ current was reduced by 27.7%, only partially accounting for the reduction in EPSC. Cumulative EPSC analysis using train stimulation revealed that presynaptic GABABR activation significantly reduced the size of readily releasable pool (RRP) by 72.1±6.9%. Moreover, the RRP refilling rate was reduced by 33.0±6.8%, while the release probability of a single vesicle dropped by 54.6±4.4%. EM analysis of baclofen-incubated (50µM, 5min) and cryo-fixated acute slices revealed a median of 0.27 docked vesicles per 100nm profile length, suggesting that baclofen might decrease the number of docked vesicles at the active zone. Preliminary experiments with pre-postsynaptic recording combined with optogenetic stimulation of hippocampal interneurons suggested that synaptically released GABA might also inhibit MFB-PN EPSCs, implying a physiological role of GABABR-mediated regulation. Collectively, our results suggest that presynaptic GABABRs, activated by endogenous GABA, shape synaptic efficacy via regulation of vesicular release mechanisms at hippocampal MFB-PN synapses.References:1. Vogt KE, Nicoll RA. Glutamate and gamma-aminobutyric acid mediate a heterosynaptic depression at mossy fiber synapses in the hippocampus. Proc Natl Acad Sci U S A. 1999;96(3):1118-1122. doi:10.1073/pnas.96.3.11182. Guetg N, Seddik R, Vigot R, et al. The GABA B1a isoform mediates heterosynaptic depression at hippocampal mossy fiber synapses. Journal of Neuroscience. 2009;29(5):1414-1423. doi:10.1523/JNEUROSCI.3697-08.2009