At immature mossy fibers-CA3 connections, activation of presynaptic GABAB receptors by endogenously released GABA contributes to synapses silencing

Abstract

Early in postnatal life correlated GABAergic activity in the hippocampus is thought to play a crucial role in synaptogenesis and in the development of adult neuronal networks. Unlike adulthood, at this developmental stage, mossy fibers (MF) which are the axons of granule cells, release GABA into CA3 principal cells and interneurons. Here, we tested the hypothesis that at MF-CA3 connections, tonic activation of GABAB autoreceptors by GABA is responsible for the low probability of release and synapse silencing. Blocking GABAB receptors with CGP55845 enhanced the probability of GABA release and switched on silent synapses while the opposite was observed with baclofen. Both these effects were presynaptic and were associated with changes in paired-pulse ratio and coefficient of variation. In addition, enhancing the extracellular GABA concentration by repetitive stimulation of MF or by blocking the GABA transporter GAT-1, switched off active synapses, an effect that was prevented by CGP55845. In the presence of CGP55845, stimulation of MF-induced synaptic potentiation. The shift of EGABA from the depolarizing to the hyperpolarizing direction with bumetanide, a blocker of the cation-chloride co-transporter NKCC1, prevented synaptic potentiation and caused synaptic depression, suggesting that the depolarizing action of GABA observed in the presence of CGP55845 is responsible for the potentiating effect. It is proposed that, activation of GABAB receptors by spillover of GABA from MF terminals reduces the probability of release and contributes to synapses silencing. This would act as a filter to prevent excessive activation of the auto-associative CA3 network and the emergence of seizures.

In the Developing Rat Hippocampus, Endogenous Activation of Presynaptic Kainate Receptors Reduces GABA Release from Mossy Fiber Terminals

“…Depolarizing the cell to positive potentials (ϩ40 mV) produced bicuculline-sensitive outward currents with latency, onset, and deactivation kinetics similar to those obtained at Ϫ70 mV, further indicating that they were mediated by GABA A receptors (data not shown) (Safiulina et al, 2006). Consistent with previous observations (Safiulina et al, 2006;Safiulina and Cherubini, 2009), bath application of GYKI 52466 (30 M), which at this concentration selectively blocks AMPA receptors, did not modify the amplitude, the shape, the latency, and rise time of individual responses, indicating that AMPA receptors do not contribute to synaptic currents ( Fig. 1 A-D).…”

“…In agreement with previous studies (Gutiérrez et al, 2003;Kasyanov et al, 2004;Safiulina et al, 2006;Safiulina and Cherubini, 2009;Sivakumaran et al, 2009), during the first week of postnatal life, minimal stimulation of granule cells in the dentate gyrus elicited lowprobability GPSCs, which were completely blocked by bicuculline (10 M) or picrotoxin (100 M). Depolarizing the cell to positive potentials (ϩ40 mV) produced bicuculline-sensitive outward currents with latency, onset, and deactivation kinetics similar to those obtained at Ϫ70 mV, further indicating that they were mediated by GABA A receptors (data not shown) (Safiulina et al, 2006).…”

“…In six cases, stimulation of granule cells in the dentate gyrus failed to produce any synaptic response to the first stimulus (over at least 30 consecutive trials). However, occasional responses to the second stimulus suggested that these synapses were "presynaptically" silent (Gasparini et al, 2000;Kasyanov et al, 2004;Safiulina and Cherubini, 2009;Sivakumaran et al, 2009). Application of UBP 302 to "presynaptically" silent cells induced the appearance of synaptic responses to the first stimulus (supplemental Fig.…”

“…To exclude the possibility of an indirect effect via kainateinduced modulation of other receptors known to depress transmitter release such as GABA B receptors (Safiulina and Cherubini, 2009), nicotine and muscarinic acetylcholine receptors (Maggi et al, 2004), purinergic P2Y receptors (Zhang et al, 2003;Safiulina et al, 2005), and mGluRs (Scanziani et al, 1997), in a set of experiments (n ϭ 18) DNQX was applied in the presence of CGP 52432 (1 M), DH␤E (50 M), atropine (1 M), PPADS (50 M), and LY341495 (100 M), selective antagonists for these receptors. Also in this case, DNQX induced a significant increase in GPSCs amplitude similar to that obtained in the absence of the blockers, suggesting a direct effect of DNQX on kainate receptors localized on MF terminals (supplemental Fig.…”

Spike-Timing-Dependent Plasticity Induces Presynaptic Changes at Immature Hippocampal Mossy Fiber Synapses

“…Moreover, in a recent paper the authors showed that the "whispering" state of mossy fiber synapses at this age seems to be related to presynaptic tonic GABA B receptor activation inhibiting synaptic release (Safiulina and Cherubini, 2009). In future work, it would be interesting to investigate whether "fully speaking" synapse induced by the pairing protocol operates by modulating tonic GABA B receptor activation.…”