Disrupted Dentate Granule Cell Chloride Regulation Enhances Synaptic Excitability during Development of Temporal Lobe Epilepsy

Abstract

GABA A receptor-mediated inhibition depends on the maintenance of intracellular ClϪ concentration ([Cl Ϫ ] in ) at low levels. In neurons in the developing CNS, [Cl Ϫ ] in is elevated, E GABA is depolarizing, and GABA consequently is excitatory. Depolarizing GABAergic synaptic responses may be recapitulated in various neuropathological conditions, including epilepsy. In the present study, rat hippocampal dentate granule cells were recorded using gramicidin perforated patch techniques at varying times (1-60 d) after an epileptogenic injury, pilocarpine-induced status epilepticus (STEP). In normal, non-epileptic animals, these strongly inhibited dentate granule cells act as a gate, regulating hippocampal excitation, controlling seizure initiation and/or propagation. For 2 weeks after STEP, we found that E GABA was positively shifted in granule cells. This shift in E GABA altered synaptic integration, increased granule cell excitability, and resulted in compromised "gate" function of the dentate gyrus. E GABA recovered to control values at longer latencies post-STEP (2-8 weeks), when animals had developed epilepsy. During this period of shifted E GABA , expression of the Cl Ϫ extruding K ϩ /Cl Ϫ cotransporter, KCC2 was decreased. Application of the KCC2 blocker, furosemide, to control neurons mimicked E GABA shifts evident in granule cells post-STEP. Furthermore, post-STEP and furosemide effects interacted occlusively, both on E GABA in granule cells, and on gatekeeper function of the dentate gyrus. This suggests a shared mechanism, reduced KCC2 function. These findings demonstrate that decreased expression of KCC2 persists for weeks after an epileptogenic injury, reducing inhibitory efficacy and enhancing dentate granule cell excitability. This pathophysiological process may constitute a significant mechanism linking injury to the subsequent development of epilepsy.

KCC2 activity is critical in limiting the onset and severity of status epilepticus

“…KCC2 dysfunction is thought to contribute to traumatic brain injury, neuropathic pain, and acute stress (10). Consistent with this, deficits in the activity and expression levels of KCC2 develop rapidly in animal models of SE and persist after its termination (11)(12)(13).…”

“…Consistent with their role in mediating GABAergic inhibition, deficits in the cell surface levels of GABA A Rs and the amplitudes of miniature inhibitory postsynaptic currents are evident in animal models of SE (22)(23)(24). KCC2 deficits are also clear in patients with intractable epilepsy and animal models of seizures (11,13,25). Here we have examined the mechanisms that underlie KCC2 inactivation during neuronal hyperexcitability and whether they contribute to the pathophysiology of SE.…”

“…1A). These results suggested that deficits in KCC2 surface activity could contribute to elevated levels of intracellular Cl -that develop rapidly in animal models of SE and persist after its termination (11)(12)(13).…”

Perirhinal cortex hyperexcitability in pilocarpine‐treated epileptic rats

“…Indeed, we have found that KCC2 immunoreactivity is consistently lower in epileptic rats, in the PC as well as in other parahippocampal regions (de Guzman et al, 2006). A transient decrease in KCC2 expression associated with increased excitability was also described in granule cells of the dentate gyrus, during the first 2 weeks after pilocarpine treatment (Pathak et al, 2007). Upregulation of KCC2 in cultures of CA3 neuronal cells prevents the development of hyperexcitability, whereas the downregulation of KCC2 provokes seizure susceptibility (Zhu et al, 2008).…”

Disease-Modifying Effects of Phenobarbital and the NKCC1 Inhibitor Bumetanide in the Pilocarpine Model of Temporal Lobe Epilepsy

“…Because GABA is typically inhibitory in adults but excitatory at an early developmental stage, the shift to depolarizing action of GABA after brain insults indicates a replay of development programs (Payne et al, 2003;Ben-Ari and Holmes, 2005). In the pilocarpine model of TLE, this shift in GABA equilibrium potential (E GABA ) is temporally restricted to the period of epileptogenesis and may constitute a significant mechanism linking injury to the subsequent development of epilepsy (Pathak et al, 2007).…”