Ceftriaxone Treatment Preserves Cortical Inhibitory Interneuron Function via Transient Salvage of GLT-1 in a Rat Traumatic Brain Injury Model

Abstract

Traumatic brain injury (TBI) results in a decrease in glutamate transporter-1 (GLT-1) expression, the major mechanism for glutamate removal from synapses. Coupled with an increase in glutamate release from dead and dying neurons, this causes an increase in extracellular glutamate. The ensuing glutamate excitotoxicity disproportionately damages vulnerable GABAergic parvalbumin-positive inhibitory interneurons, resulting in a progressively worsening cortical excitatory:inhibitory imbalance due to a loss of GABAergic inhibitory tone, as evidenced by chronic post-traumatic symptoms such as epilepsy, and supported by neuropathologic findings. This loss of intracortical inhibition can be measured and followed noninvasively using long-interval paired-pulse transcranial magnetic stimulation with mechanomyography (LI-ppTMS-MMG). Ceftriaxone, a β-lactam antibiotic, is a potent stimulator of the expression of rodent GLT-1 and would presumably decrease excitotoxic damage to GABAergic interneurons. It may thus be a viable antiepileptogenic intervention. Using a rat fluid percussion injury TBI model, we utilized LI-ppTMS-MMG, quantitative PCR, and immunohistochemistry to test whether ceftriaxone treatment preserves intracortical inhibition and cortical parvalbumin-positive inhibitory interneuron function after TBI in rat motor cortex. We show that neocortical GLT-1 gene and protein expression are significantly reduced 1 week after TBI, and this transient loss is mitigated by ceftriaxone. Importantly, whereas intracortical inhibition declines progressively after TBI, 1 week of post-TBI ceftriaxone treatment attenuates the loss of inhibition compared to saline-treated controls. This finding is accompanied by significantly higher parvalbumin gene and protein expression in ceftriaxone-treated injured rats. Our results highlight prospects for ceftriaxone as an intervention after TBI to prevent cortical inhibitory interneuron dysfunction, partly by preserving GLT-1 expression.

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“…Glutamate transport thus constitutes a powerful antiexcitotoxic mechanism in the brain. Rat TBI models show a large, transient posttraumatic decline in GLT‐1 mRNA and protein levels and transporter function that starts within 24 hours of injury and normalizes by 6 weeks after injury . Possible explanations for these changes include increased endocytosis due to glutamate‐induced clustering, caspase‐3–mediated proteolysis of existing protein, and diminished de novo gene transcription …”

“…Progressively worsening oxidative stress, coupled with the breakdown of the protective perineuronal net surrounding these cells, leads to their gradual loss, whereas excitatory neurons remain largely unaffected . Preferential loss of these inhibitory interneurons results in a progressive loss of GABAergic intracortical inhibition and may contribute to epileptogenesis …”

“…In vivo ceftriaxone results in a threefold increase in GLT‐1 protein expression in healthy rodent brains 5‐7 days after start of treatment . The GLT‐1 normalization after TBI may result from increase by ceftriaxone of the glutamate transporter gene SLC1A2 expression, which is reduced after TBI …”

“…Although GLT‐1 upregulation does not seem to last long after cessation of treatment, ceftriaxone's stabilization of GLT‐1 levels in the acute and subacute posttraumatic period partially protects against delayed excitotoxic damage to the vulnerable GABAergic inhibitory interneuron system and preserves both intracortical inhibition (as measured by transcranial magnetic stimulation) and cortical Parvalbumin expression up to 6 weeks after TBI …”

“…Rat TBI models show a large, transient posttraumatic decline in GLT-1 mRNA and protein levels and transporter function that starts within 24 hours of injury and normalizes by 6 weeks after injury. [163][164][165][166] Possible explanations for these changes include increased endocytosis due to glutamate-induced clustering, 167 caspase-3-mediated proteolysis of existing protein, 168 and diminished de novo gene transcription. 169,170 Excessive glutamate leakage from injured neurons together with transiently deranged synaptic glutamate clearance results in increased extracellular glutamate, which persists for days after trauma in humans as well as rodents.…”