The glutamatergic synapse in hippocampal neurons is the best described interface between electrical activity and memory encoding in the central nervous system. We have found that ClC-3 chloride channels localize to the post-synaptic plasma membranes of hippocampal neurons where they are both spatially and functionally linked to NMDA receptors.  NMDA dependent-Ca²⁺ entry, activation of CaMKII, and subsequent phosphorylation/gating of ClC-3 by CaMKII links the two channels via a Ca²⁺ -mediated positive feedback loop.  ClC-3 channels increase neuronal excitability post-synaptically in immature neurons by increasing post-synaptic potentials (EPSPs) at a time when [Cl^-]_i is high.  In stark contrast, ClC-3 channel activation is inhibitory in mature neurons when [Cl^-]_I is low.  Now the opening of ClC-3 channels linked to the Ca²⁺ influx through NMDA channels serves as a charge shunt pathway giving rise to membrane hyperpolarization, reduction in EPSP amplitude, and promotion of the block of NMDRs by Mg²⁺.   Given the pivotal new role for ClC-3 chloride channels as regulators of synaptic efficacy, experiments in the laboratory currently explore the influence of ClC-3 chloride channels on the induction of synaptic plasticity at hippocampal synapses as well as their role in shaping Ca²⁺ transients at synaptic sites.  It is our working hypothesis that ClC-3 channels in immature neurons facilitate the induction of long-term potentiation (LTP) at glutamatergic synapses through an enhancement of the NMDAR-dependent Ca²⁺ signals.  Activation of ClC-3 channels in mature neurons reduces the LTP of synaptic transmission and shifts the frequency threshold at which LTP is first observed and at which the transition from LTD to LTP occurs, to higher stimulation frequencies.

       Synaptic development and plasticity are integral to our understanding of neuronal function and disease, so understanding the events that underlie these processes, including the role of ClC-3 channels, is of fundamental significance. Chloride channels are a physiologically important, yet under-studied, class of channels in the brain. A role for depolarizing GABA-mediated Cl^- driven excitation is now widely accepted as a model for neuronal maturation in cortical circuits; however, a role for ClC channels in the phenomenon has not previously been explored. ClC-3 is unique among its family members in that it is gated by Ca²⁺ dependent phosphorylation.  Our recent published studies ¹, demonstrate that ClC-3 channels colocalize with NMDA receptors at hippocampal synapses.  Calcium flowing through NMDA receptors activates ClC-3 channels enhancing the excitatory postsynaptic potential (EPSP) in immature neurons and reducing the EPSP in mature neurons.  Long-term potentiation (LTP) and its dependence upon NMDA-receptor mediated calcium entry has been widely studied as a mechanism underlying learning and memory.  Based upon our data, expression of ClC-3 channels would enhance NMDA-receptor dependent calcium signaling and thereby facilitate the induction of long-term potentiation at early times in development and conversely depress both calcium signaling and induction of LTP in mature neurons.  Thus, Ca²⁺-dependent ClC-3 channels and their unique relationship with NMDA receptor provide a new and important level of regulation in the modulation of synaptic plasticity.  In that ClC-3 channels are ubiquitously expressed in the brain ², the relationship between ClC-3 channels and NMDA receptors is not likely to be restricted to the  hippocampus, however, this remains an open question. Our studies are targeted at an integrated understanding of the role postsynaptically expressed ClC-3 plays in the fundamental aspects of hippocampal neuronal excitability as well as synaptic plasticity.

REFERENCES

1.         Wang, X.Q. et al. CLC-3 Channels Modulate Excitatory Synaptic Transmission in Hippocampal Neurons. Neuron 52, 321-333 (2006).

2.         Kawasaki, M. et al. Cloning and expression of a protein kinase C-regulated chloride channel abundantly expressed in rat brain neuronal cells. Neuron 12, 597-604 (1994).