^{Hippocampal Combinatorial Plasticity Induced by Conditioning of Converging Schaffer-CA1 Pathways in Rat in vivo}

Hippocampal plasticity in two conditioning pathways, i.e. hippocampal combinatorial plasticity, can be induced by conditioning of converging Schaffer-CA1 pathways in Sprague-Dawley rats under pentobarbital sodium anesthesia. During conditioning, paired pulses were delivered at 5 Hz（600 pairs）and the inter-peak interval of the field excitatory postsynaptic potentials (fEPSP) of the paired-pulse stimulation was 10 ms. Our results showed that long-term potentiation (LTP) and long-term depression (LTD) might be induced simultaneously by conditioning of two pathways whether two conditioning pathways were independent or not. Thus hippocampal combinatorial plasticity, i.e. LTP/LTD, can be induced successfully. Our results demonstrated that conditioning paradigm could enable the induction of bidirectional hippocampal synaptic plasticity and the direction of synaptic plasticity was determined by the state of synapses. Our results indicate that hippocampal combinatorial plasticity induced by cellular conditioning paradigm may encode hippocampus dependent learning and memory much better compared with LTP and LTD induced by low frequency stimulation or high frequency stimulation independently and may reflect the flexibility and stability of hippocampal synaptic plasticity.