In contrast to physiological conditions with 10 mM [Cl−]in (ECl near the resting potential), whole-cell recording with 130 mM [Cl−]in revealed that, while the membrane voltage and input resistance remained unchanged, the spike duration is longer and the threshold for spike generation is lowered (Table 1; Figure S2). R428 order While the former effect is attributable to CaCC, the latter may also involve Cl− channels that are constitutively active in a resting neuron. Whereas under physiological conditions both K+ efflux through K+ channels and Cl− influx through Cl− channels that are constitutively active counter sodium

channel activation during depolarization in setting the threshold for spike generation, elevating internal Cl− leads to Cl− efflux through these Cl− channels Selleck Ku 0059436 thereby enhancing rather than dampening the excitability. During normal development, a neuron switches from higher internal Cl− to lower internal Cl−, shifting ECl and converting Cl− channel activity from excitatory to inhibitory (Ben-Ari, 2002). Whereas most mature neurons normally have low internal Cl− (5–10 mM) to allow Cl− channels to provide inhibition, extended periods of high neuronal activity or pathological conditions such as seizures and brain traumas can lead to accumulation

of internal Cl− and revert Cl− channel activity back to an excitatory conductance as that during development (Blaesse et al., 2009, De Koninck, 2007 and Payne and et al., 2003). The susceptibility of cation-chloride cotransporters to modulation renders the Cl− gradient a dynamic readout of neuronal activity. For example, in mature hippocampal neurons, spike firing can alter the Cl− gradient via activity-dependent phosphorylation of KCC2, a K+-Cl− co-transporter that normally extrudes Cl−, resulting in Cl− accumulation and a positive shift in ECl (Fiumelli et al., 2005 and Woodin et al., 2003). This activity-dependent shift in the Cl− gradient will cause CaCCs to progressively lose their grip over the action potential

duration and threshold for spike initiation by synaptic potentials, as well as EPSP amplitude and summation. Epilepsy patients exhibit upregulation of NKCC1 (Cl− accumulator) and downregulation of KCC2 (Cl− extruder) in the temporal lobe, resulting in a positive shift in ECl (Palma et al., 2006). In hippocampal slices, KCC2 undergoes downregulation after sustained interictal-like activity in zero-Mg2+ conditions (Rivera et al., 2004). Similar positive shift in ECl due to altered Cl− gradient also takes place with brain trauma (Bonislawski et al., 2007) and axonal injury (Nabekura et al., 2002). Thus, impairments of Cl− homeostasis would turn CaCC modulation into positive feedback to further exacerbate the excitotoxicity.