Mariya George

Background: Traumatic brain injury (TBI) is a disruption in normal brain function caused by external mechanical force and remains a leading cause of acquired epilepsy worldwide.² Post-traumatic epilepsy (PTE) develops in approximately 10–20% of patients after moderate to severe TBI and often occurs after a latent period of months to years, suggesting a progressive biological process rather than immediate neuronal damage.² Normal cortical stability depends on a balance between excitatory glutamatergic pyramidal neurons and inhibitory GABAergic interneurons, particularly parvalbumin-positive (PV) interneurons, which provide fast perisomatic inhibition critical for network synchronization and prevention of runaway excitation.³ Emerging evidence suggests that synaptic disinhibition, microglial activation, and chloride dysregulation contribute to chronic cortical hyperexcitability following TBI.¹˒⁴ Current therapies suppress seizures but do not correct the underlying synaptic imbalance associated with epileptogenesis.²

Objective: This narrative literature review examined how microglia-mediated interneuron dysfunction and chloride dysregulation contribute to cortical hyperexcitability and post-traumatic epileptogenesis following traumatic brain injury. This review also evaluated whether inhibition of NKCC1 with bumetanide may preserve inhibitory signaling and stabilize cortical network activity after injury.

Search Methods: Literature searches were conducted using PubMed and related biomedical databases for studies published from 2018–2023. Search terms and MeSH keywords included “traumatic brain injury,” “post-traumatic epilepsy,” “microglia,” “parvalbumin interneurons,” “chloride dysregulation,” “NKCC1,” “bumetanide,” “synaptic disinhibition,” “cortical hyperexcitability,” and “epileptogenesis.” Primary experimental and clinical studies evaluating inflammatory signaling, inhibitory circuit dysfunction, and chloride transporter regulation following TBI were reviewed.¹⁻⁵

Results: Experimental studies demonstrated that TBI induces persistent microglial activation, inflammatory signaling, and progressive disruption of inhibitory circuitry.¹˒² Activated microglia release inflammatory mediators that contribute to synaptic remodeling, neuronal death, and interneuron vulnerability, leading to reduced inhibitory signaling.²˒⁵ Imaging and electrophysiologic studies demonstrated decreased PV interneuron density, reduced inhibitory synaptic currents, impaired interneuron firing properties, increased pyramidal neuron firing, and enhanced cortical synchronization following injury.¹˒³ Reduced inhibitory control from PV interneuron loss promotes synaptic disinhibition and lowers seizure threshold.³ Altered chloride transporter expression, particularly increased NKCC1 activity, disrupted intracellular chloride homeostasis, and converted GABA signaling from inhibitory to depolarizing, further increasing neuronal excitability.⁴ Tessier et al. demonstrated that bumetanide-mediated NKCC1 inhibition preserved PV interneuron survival, restored microglia–interneuron structural interactions, reduced epileptiform activity, and improved oscillatory stability after controlled cortical impact injury.¹ Functional recovery reflected restoration of inhibitory balance rather than increased neuronal excitability.¹ Clinical studies additionally demonstrated that elevated inflammatory cytokines, including IL-1β, were associated with increased risk of developing PTE following TBI.⁵

Conclusion: Current evidence suggests that TBI disrupts inhibitory signaling through both microglia-mediated interneuron dysfunction and chloride dysregulation, producing cortical hyperexcitability and epileptogenesis. Experimental findings demonstrated that bumetanide-mediated NKCC1 inhibition restored inhibitory balance, preserved inhibitory interneuron survival, and reduced network hyperexcitability after injury.¹˒⁴ These findings support chloride homeostasis and microglia–interneuron signaling as potential therapeutic targets for preventing post-traumatic epileptogenesis rather than solely suppressing seizures after they occur.

Works Cited

1. Tessier J, et al. Bumetanide induces post-traumatic microglia–interneuron interactions and prevents interneuron loss after traumatic brain injury. Brain. 2023;146(2):650-667.
2. Agoston DV, Shultz SR. Post-traumatic epilepsy: pathophysiology and treatment. Curr Opin Neurol. 2019;32(2):200-207.
3. Vascak M, Jin X, Jacobs KM. Mild traumatic brain injury induces structural and functional changes in parvalbumin interneurons. J Neurotrauma. 2018;35(13):1578-1591.
4. He Q, Nomura T, Xu J, Contractor A. The developmental switch in GABA polarity is delayed and contributes to circuit hyperexcitability. Neuron. 2019;101(3):1-15.
5. Diamond ML, Ritter AC, Failla MD, et al. IL-1β associations with post-traumatic epilepsy development after traumatic brain injury. Neurology. 2018;91(9):e799-e808.