Abstracts

Rationale: Excess of extracellular glutamate after traumatic brain injury (TBI) contributes to excitotoxic cell death and posttraumatic epilepsy. The only known mechanism of extracellular glutamate clearance is glutamate transport, which is mediated by glutamate transporters. GLT-1, the analog of human EAAT2, is the major glutamate transporter in the rat brain and expressed in astrocytes, axon terminals and oligodendrocytes. We previously identified that GLT-1 expression is suppressed focally after TBI, and is is restored after treatment with ceftriaxone, a well-tolerated β-lactam antibiotic previously shown to enhance GLT-1 expression. Here, we test whether ceftriaxone treatment reduces the extent of gliosis and mitigates posttraumatic epilepsy. Methods: Under isoflurane-inhaled anesthesia, rats were exposed to moderate lateral fluid percussion injury (LFPI, 2.26 ± 0.12 atm) via a 4 mm diameter craniectomy over the left parietal region. Ceftriaxone was injected acutely after TBI (200 mg/kg, ip), and then daily for 7 days. 7 days after TBI, left lesioned and right nonlesioned cortex tissue was dissected separately for analysis in each rat. GLT-1 and GFAP expression was quantified by immunoblot; β-actin was used as a loading control. Rats were implanted with wireless EEG transmitters to record seizure activity 12 weeks after TBI. Seizures were detected by an automatic seizure detection algorithm and validated by visual inspection. Results: Seven days after TBI, relative to saline control, daily ceftriaxone treatments restored lesioned cortex (n = 7) expression of GLT-1 to near control levels and increased its expression in the nonlesioned cortex by 29% (p = 0.048). The increase in GFAP after TBI is reduced with ceftriaxone. There is a 43% (p = 0.031) decrease in GFAP expression in the lesioned cortex of ceftriaxone treated rats compared to the saline control group. In addition, seizure frequency per 24 hours of recording, with seizures defined as runs of spikes 10 sec or longer in duration, was significantly reduced to 46.8 ± 15.4 (mean ± SEM) seizures in the ceftriaxone group (n = 6) relative to 151.3 ± 43.6 seizures (p = 0.038) in the saline control group (n = 5). Conclusions: We conclude that one week of ceftriaxone administration can restore GLT-1 expression to near normal levels in lesioned cortex and cause an over expression in the nonlesioned cortex. Our data also imply that improved glutamate clearance after ceftriaxone treatment may contribute to reduced regional gliosis and reduced frequency of posttraumatic seizures.