Abstracts

Rationale: Focal Cortical dysplasias (FCDs) are frequent causes of medically intractable epilepsy, where the first seizure may occur at any point during the patient s life. In most adults, the initial seizure can be provoked by a 2nd hit brain insult such as a minor head trauma, brain infection, or cerebral ischemia. After a certain latent period following the 2nd hit, seizures become more difficult to control with anticonvulsants. We hypothesize that during this latent period, certain molecular/cellular mechanisms (such as aberrant axonal sprouting) develop in the FCD brain to enable the growth of the epileptic network, thus rendering FCDs highly excitable. GAP-43 is not only responsible for normal neural circuitry formation during development, but also for pathological axonal growth. This study investigates the expression of GAP-43 in an adult rat model of radiation-induced focal cortical dysplasia by administering a single dose injection of Pentylenetetrazol (PTZ) to provoke seizures as a 2nd hit. Methods: Time pregnant rats were irradiated (XRT) on E17 with 145 cGy the standard protocol for inducing focal cortical dysplasias in our lab. Both the adult rats (PND 70) that received in utero irradiation and those that served as controls were injected with a single dose of PTZ (40 mg/kg). All rats subsequently exhibited acute clinical seizures (0.5 hr- 3 hrs after injection). The rats were sacrificed for GAP-43 immunohistochemistry (IHC) and histological staining at 2 days (2d) and 15 days (15d) following the PTZ injection. All XRT rats had acquired FCD, as evidenced by histopathological examinations. The 13 rats were divided into 5 groups: 1) -XRT/-PTZ (n=3); 2) XRT/PTZ-2d (n=3); 3) -XRT/PTZ-2d (n=3); 4) XRT/PTZ-15d (n=2); 5) XRT/PTZ-15d (n=2). Results: Compared to the level of GAP-43 expression in group 1 (see Methods section), GAP-43 IHC stainings increased in the inner molecular layers (IML) of the fascia dentata and in the cortex of rats in groups 2-5. More importantly, there was a differential gradient of increased GAP-43 staining intensity with the highest magnitude in group 4 >group 2> group 3 >group 5.Conclusions: Seizures by PTZ injection induced the expression of GAP-43 in the IMLs and cortices of both control and FCD (XRT) rats with a greater degree of protein expression in the FCD rats. Furthermore, while the presence of GAP-43 was only temporarily induced in the control rats, its expression was sustained and continued to increase in FCD (XRT) rats for up to 15 days post- 2nd hit. If confirmed in a study involving a greater number of rats, this preliminary finding suggests that FCD may have the intrinsic capability to develop an epileptic synaptic network by aberrant axonal sprouting triggered by the 2nd hit brain insult.