Gabapentin decreases epileptiform discharges in a chronic model of neocortical trauma.

Severe traumatic brain injury is a significant risk factor for development of epilepsy that often becomes clinically apparent after a latent period of months to years (Salazar et al, 1985; Annegers et al, 1998). Unfortunately, a large percentage of patients with symptomatic epilepsies due to trauma or other etiologies are not seizure free on appropriate pharmacological antiseizure treatment (Kwan and Brodie, 2000). The high incidence of epileptogenesis following severe injury in war time (Salazar et al., 1985) and the expected marked increase due to the conflicts in Iraq and Afghanistan, emphasize the importance of developing prophylactic strategies that might be applied during the latent period between injury and the onset of seizures (Garga and Lowenstein, 2006). To date, multiple clinical trials aimed at prevention of epilepsy after traumatic brain injury (TBI) with older antiseizure agents (Glotzner et al, 1983; Young et al, 1983a,b; Temkin et al. 1990, 1999), glucocorticoids (Watson et al, 2004) and magnesium (Temkin et al. 2007) have been unsuccessful (see Temkin 2001, 2009 for reviews). In previous experiments, we used the partial cortical isolation (“undercut”or “UC” ) model of chronic posttraumatic epileptogenesis (Echlin, 1959; Sharpless and Halpern, 1962) to assess cellular mechanisms that might underlie injury-induced focal cortical hyperexcitability (Tseng and Prince, 1993; Salin et al, 1995; Hoffman et al, 1994; Li and Prince, 2002; Jin et al 2006; Faria and Prince, 2010; Jin et al, 2011; reviewed in Graber and Prince, 2006; Prince et al, 2009; Li et al, 2010). Rodent in vitro slices from the undercut (UC) cortex generate spontaneous and evoked epileptiform discharges after a latent period of ~7–14 days (Tseng and Prince, 2003; Hoffman et al, 1994; Salin et al, 1995). Focal suppression of cortical activity with tetrodotoxin (TTX) during a critical period of 3 days after injury reduces subsequent epileptogenesis in this model, providing a proof in principle that prophylaxis of posttraumatic epileptogenesis is possible (Graber and Prince 1999, 2004). The mechanisms for this effect have not been fully explored; however, recent data indicate that TTX treatment reduces immunocytochemical evidence for axonal and terminal sprouting (Prince et al, 2009). Enhanced excitatory connectivity, is a prominent pathophysiological alteration in injured epileptogenic neocortex (Salin et al, 1995; Li and Prince 2002; Jin et al., 2006) and hippocampus (Tauck and Nadler 1985, McKinney et al 1997, Yang et al 1995), as well as in human epileptogenic temporal lobes (Babb et al, 1991, 1992; Masukawa et al, 1992; Isokawa et al 1993; Marco and DeFelipe, 1997). Layer V excitatory pyramidal (Pyr) neurons in UC neocortex develop elaborate sprouting of axonal arbors with increased branching and a higher density of boutons on their axons (Salin et al. 1995), and whole cell recordings from these cells show an increase in the frequency of spontaneous (s) and miniature (m) excitatory postsynaptic currents (EPSCs) (Li and Prince, 2002). Experiments in which laser scanning photostimulation of caged glutamate has been used to map excitatory circuitry have also revealed a more extensive functional excitatory connectivity in the UC cortex (Jin et al. 2006). These findings have led to the hypothesis that an intervention to decrease new excitatory synapse formation after injury might be one approach to antiepileptogenesis in the undercut model of posttraumatic epileptogenesis. Formation of new synaptic connections is an important event during development in cortical structures (Sutor and Luhmann, 1995, Sur and Leamey, 2001) that appears to be recapitulated after injury (Salin et al, 1995; Carmichael and Chesselet, 1992). A potential approach to limiting excessive excitatory synapse formation, and possibly epileptogenesis following injury, was suggested by recent results that have shown a prominent role for thrombospondins (TSPs), and perhaps other astrocyte-secreted proteins, in excitatory synapse formation during development (Christopherson et al 2005; Eroglu et al, 2009). The receptor for TSPs is the calcium channel α2δ1 subunit (Gee et al., 1996; Field et al., 2006), which is significantly upregulated by nerve and brain injury (Luo et al 2001, 2002 ) and isalso the receptor for the antiepileptic/antiallodynic drugs gabapentin(GBP) (Eroglu et al, 2009; Gee et al., 1996) and pregabalin. GBP also has potentialneuroprotective properties (Rothstein and Kuncl, 1995). High affinity binding of GBP inhibits TSP-induced excitatory synapse formation during development both in vitro and in vivo, providing it is administered within a ~5 days of formation of a new synapse; already existing synapses are unaffected (Eroglu et al., 2009). These developmental results lead to the hypothesis that GBP treatment might also block or limit excitatory synaptogenesis following injury, and reduce posttraumatic hyperexcitability in this model. We therefore treated animals with GBP via ip injections or sc via infusion pumps following injury and used the expression of epileptiform activity in in vitro slices, recordings of sEPSCs and mEPSCs in layer V Pyr neurons, and anatomical measures of excitatory synapses to assess drug effects. Results suggest that GBP administered after cortical injury limits excitatory synapse formation, reduces the frequency of excitatory postsynaptic currents (EPSCs), decreases the incidence of epileptiform discharges in neocortical slices and has neuroprotective effects. Portions of these results were published in an abstract (Li et al., 2009).