Abstract
CA3–CA3 recurrent excitatory synapses are thought to play a key role in memory storage and pattern completion. Whether the plasticity properties of these synapses are consistent with their proposed network functions remains unclear. Here, we examine the properties of spike timing-dependent plasticity (STDP) at CA3–CA3 synapses. Low-frequency pairing of excitatory postsynaptic potentials (EPSPs) and action potentials (APs) induces long-term potentiation (LTP), independent of temporal order. The STDP curve is symmetric and broad (half-width ∼150 ms). Consistent with these STDP induction properties, AP–EPSP sequences lead to supralinear summation of spine [Ca2+] transients. Furthermore, afterdepolarizations (ADPs) following APs efficiently propagate into dendrites of CA3 pyramidal neurons, and EPSPs summate with dendritic ADPs. In autoassociative network models, storage and recall are more robust with symmetric than with asymmetric STDP rules. Thus, a specialized STDP induction rule allows reliable storage and recall of information in the hippocampal CA3 network.
Highlights
The amount of potentiation was significantly smaller than that induced by the corresponding pairing protocol lacking the hyperpolarizing current pulse (9 cells, P 1⁄4 0.03; Fig. 3j versus Fig. 1e; Table 1). These results indicate that the ADP effectively propagated into the dendrites of CA3 pyramidal neurons, leading to nearly symmetric voltage changes during pre–postsynaptic and post–presynaptic sequences
Anti-Hebbian STDP induction rules were previously reported[25,31,32,33,34,35,36], none of the prior studies demonstrated a broad and symmetric potentiation curve. Both blockers of NMDA receptors and L-type Ca2 þ channels blocked STDP, showing that two independent Ca2 þ sources were required for plasticity induction at these synapses[15,37]
Spine [Ca2 þ ] transients mediated by NMDA receptors and L-type Ca2 þ channels showed temporally symmetric summation
Summary
The specific dendritic properties of CA3 pyramidal neurons[21] may play a role in the generation of a broad, temporally symmetric STDP induction rule. In our model, the symmetric STDP induction rule facilitated the storage and recall of information by incomplete input, conveying the ability of pattern completion.
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