Abstracts

Rationale: Developmental cortical malformations including polymicrogyria and tuberous sclerosis are associated with intractable epilepsy. To understand the pathophysiology of epilepsies associated with cortical malformation, we have utilized glutamate imaging in the freeze-lesion (FL) model of polymicrogyria. In this model neurons that form deep cortical layers are lost, resulting in a microgyrus enriched in layer II/III neurons. Our preliminary findings have shown that glutamate levels are increased in concentration and duration in the cortical areas surrounding the FL (microgyral zone, MZ) following afferent fiber stimulation. We hypothesized that glutamate levels are increased due to decreased glutamate reuptake capacity in the MZ or by increased afferent input into the MZ. Methods: Microgyri were created by briefly placing a freezing probe on the skulls of neonatal rats. Neocortical brain slices from sham operated and freeze lesioned rats were prepared 14-128 days later. Brain slices were loaded with glutamate FRET nanosensor and images were collected simultaneously with extracellular field recordings. For more detailed methods please see Dulla, et al. J.Neurosci. Meth. 168(2) p.306 Results: In order to understand the basis of increased glutamate signal in FL animals we first tested the hypothesis that glutamate reuptake capacity is decreased the MZ. We applied 1 and 5 mM glutamate via local perfusion to normal and FL brain slices. Glutamate reuptake capacity was decreased in FL slices especially during the initial exposure to high-levels of glutamate. We next tested the possibility that cortical areas surrounding the FL receive increased ascending input and that projections from other areas tend to converge in this zone. Glutamate release was evoked in normal and FL cortex by stimulating the external capsule while blocking both glutamate transporters, to eliminate changes in glutamate reuptake, and glutamate receptors, to eliminate multi-synaptic glutamate release. Our findings suggest that ascending input into FL cortex is capable of releasing more glutamate and that pattern of glutamate release is skewed toward the FL area, consistent with a convergence of afferent fibers into this region. Conclusions: Consistent with previous reports we find that functional glutamate reuptake is decreased in the epileptogenic cortical areas directly surrounding a cortical microgyrus. We also found that functional ascending input is increased in the MZ and that the region of synaptic glutamate release is skewed toward the microgyrus itself. Both of these alterations could contribute to the generation of epileptiform activity seen in FL brain slices. Reduced glutamate reuptake in the MZ would prolong the duration as well as the amplitude of evoked glutamate transients near the microgyrus. Increased ascending input into the MZ would compound this reuptake deficiency and could lead to significant developmental alterations in the malformed cortical circuitry.