Abstracts

Rationale: Traumatic brain injury (TBI) results in excessive glutamate (glu) release from dead and dying neurons, which contributes to excitotoxic cell injury and posttraumatic epilepsy (PTE). Glu transport is the only known mechanism for its clearance, largely via the GLT1 astrocyte glutamate transporter whose expression can be upregulated by beta lactam antibiotics. Previously, we found that GLT1 expression is depressed in lesional tissue after TBI, and that ceftriaxone (Cef) treatment after TBI attenuates this loss, decreases regional gliosis, and suppresses posttraumatic seizures (Goodrich et. al., 2013). We also reported a progressive loss of intracortical inhibition (ICI) as measured by paired pulse transcranial magnetic stimulation (ppTMS), along with a gradual loss of cortical parvalbumin (PV) interneurons following LFPI (Lee et al., SFN 2013). We now investigate whether Cef treatment preserves cortical inhibition and inhibitory interneuron function after motor cortex lateral fluid percussion injury (LFPI), the most common PTE model in rats. Methods: Adult male rats (n=33) received moderate lateral fluid percussion injury (LFPI; 2.3 ± 0.1 atm) via a craniotomy over the motor cortex, and were divided into 2 groups: to receive Cef (200mg/kg/day IP, n=16 Cef-TBI) or saline (n=17), daily, for one week after TBI. A sham group (n=5) received all surgical procedures except LFPI, and did not receive IP injections. ICI was assessed using a mechanomyographic (MMG) response to ppTMS weekly for 8 consecutive weeks. In ppTMS the cortex is stimulated by electrical currents induced by an extracranial magnetic field. In our setup, the size of the resultant hind limb movement was measured by accelerometry to generate an MMG (Hsieh et. al., 2012). ppTMS was applied as paired stimuli with a 200 msec interpulse interval at 100% motor threshold to estimate the ratio of two consecutive evoked MMG responses as a measure of cortical inhibition. GAD1 and PV expression in lesional tissue was measured using real time PCR at two and four weeks after injury. Results: Repeated measures ANOVA of the ppTMS cortical inhibition metric indicated significant treatment (p<0.05) and time effect (<0.001), and interaction between the factors (p<0.01). Post hoc tests showed significantly greater loss of inhibition in the Sal-TBI group (n=7) compared to Cef-TBI (n=6) from three weeks and onwards after TBI (p<0.05). GAD1 expression is transiently depressed in the Sal-TBI group two weeks after LFPI, while there is no GAD1 depression in the Cef-TBI group (n=5 each, p<0.05). In contrast to GAD1, PV is depressed in the Sal-TBI group relative to Cef-TBI only four weeks after LFPI (n=5 each, p<0.05). Conclusions: These data suggest that Cef treatment preserves intracortical inhibition and interneuron function at the level of calcium binding protein gene expression, and are a step toward understanding the Ceftriaxone anti-epileptogenic effect that we reported previously (Goodrich et al., 2013).