Bidirectional Hebbian Plasticity at Hippocampal Mossy Fiber Synapses on CA3 Interneurons

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Abstract
Hippocampal area CA3 is critically involved in the formation of nonoverlapping neuronal subpopulations (“pattern separation”) to store memory representations as distinct events. Efficient pattern separation relies on the strong and sparse excitatory input from the mossy fibers (MFs) to pyramidal cells and feedforward inhibitory interneurons. However, MF synapses on CA3 pyramidal cells undergo long-term potentiation (LTP), which, if unopposed, will degrade pattern separation because MF activation will now recruit additional CA3 pyramidal cells. Here, we demonstrate MF LTP in stratum lacunosum-moleculare (L-M) interneurons induced by the same stimulation protocol that induces MF LTP in pyramidal cells. This LTP was NMDA receptor (NMDAR) independent and occurred at MF Ca2+-impermeable AMPA receptor synapses. LTP was prevented by with voltage clamping the postsynaptic cell soma during high-frequency stimulation (HFS), intracellular injections of the Ca2+chelator BAPTA (20 mm), or bath applications of the L-type Ca2+channel blocker nimodipine (10 μm). We propose that MF LTP in L-M interneurons preserves the sparsity of pyramidal cell activation, thus allowing CA3 to maintain its role in pattern separation. In the presence of the mGluR1α antagonist LY367385 [(S)-(+)-a-amino-4-carboxy-2-methylbenzeneacetic acid] (100 μm), the same HFS that induces MF LTP in naive slices triggered NMDAR-independent MF LTD. This LTD, like LTP, required activation of the L-type Ca2+channel and also was induced after blockade of IP3receptors with heparin (4 mg/ml) or the selective depletion of receptor-gated Ca2+stores with ryanodine (10 or 100 μm). We conclude that L-M interneurons are endowed with Ca2+signaling cascades suitable for controlling the polarity of MF long-term plasticity induced by joint presynaptic and postsynaptic activities.