Release-independent short-term facilitation at GABAergic synapses in the olfactory bulb

Neurones of the olfactory bulb are innervated by GABA-releasing axons and dendrites of diverse origin. Here, I studied GABAergic neurotransmission in juxtaglomerular cells using whole-cell voltage-clamp recordings in acute olfactory bulb slices. Spontaneous IPSCs were fully blocked by the GABA(A) receptor antagonist SR95531 (40 microM) and the sodium channel blocker tetrodotoxin (1 microM). The IPSCs had mean amplitudes of 125+/-86 pA and relatively slow biexponential decay times (tau(1)=4.3+/-1.0 ms (67+/-12%), tau(2)=16.9+/-2.7 ms) at physiological temperatures. Short-term plasticity of evoked IPSCs showed two distinct patterns: depressing (n=4 cells) and facilitating-depressing (n=9). In two cells, postsynaptic responses were mediated by single functional release sites. During a train of stimuli (4 stimuli at 20 Hz), the release probability increased by two-fold, whereas the potency (postsynaptic responses excluding failures) decreased by ~15%. The increase in release probability for the second stimulus in the train also occurred when the first action potential failed to release transmitter. However, the decrease in the potency was only observed if the preceding action potential released transmitter. These results reveal a heterogeneity in the short-term plasticity of evoked IPSCs in juxtaglomerular cells and demonstrate that the short-term facilitation at some GABAergic synapses is independent of release.