The role of KCC2 in hyperexcitability of the neonatal brain

The hyperpolarizing activity of γ-aminobutyric acid A (GABA_A) receptors depends on the intracellular chloride gradient that is developmentally regulated by the activity of the chloride extruder potassium (K) chloride (Cl) cotransporter 2 (KCC2). In humans and rodents, KCC2 expression can be detected at birth. In rodents, KCC2 expression progressively increases and reaches adult-like levels by the second postnatal week of life. Several studies report changes in KCC2 expression levels in response to early-life injuries. However, the functional contribution of KCC2 in maintaining the excitation-inhibition balance in the neonatal brain is not clear. In the current study, we examined the effect of KCC2 antagonism on the neonatal brain activity under hyperexcitable conditions ex vivo and in vivo.

Ex vivo electrophysiology experiments were performed on hippocampal slices prepared from 7 to 9 days-old (P7-P9) male rats. Excitability of CA1 pyramidal neurons bathed in zero-Mg²⁺ buffer was measured using single-unit extracellular (loose) or cell-attach protocol before and after application of VU0463271, a specific antagonist of KCC2. To examine the functional role of KCC2 in vivo, the effect of VU0463271 on hypoxia-ischemia (HI)-induced ictal (seizures and brief runs of epileptiform discharges - BREDs), and inter-ictal spike and sharp-wave activity was measured in P7 male rats. A highly sensitive LC-MS/MS method was used to determine the distribution and the concentration of VU0463271 in the brain.

Ex vivo blockade of KCC2 by VU0463271 significantly increased the frequency of zero-Mg²⁺-triggered spiking in CA1 pyramidal neurons. Similarly, in vivo administration of VU0463271 significantly increased the number of ictal events, BREDs duration, and spike and sharp-wave activity in HI rats. LC-MS/MS data revealed that following systemic administration, VU0463271 rapidly reached brain tissues and distributed well among different brain regions.

The results suggest that KCC2 plays a critical functional role in maintaining the balance of excitation-inhibition in the neonatal brain, and thus it can be used as a therapeutic target to ameliorate injury associated with hyperexcitability in newborns.