Abstracts

Rationale: Cortical microgyri are a defining feature of many congenital cortical malformations and can be induced experimentally by perinatal freeze lesions. Areas immediately adjacent to cortical microgyri (paramicrogyral area: PMG) are prone to epileptiform activity and are known to have altered neuronal properties as well as both increased excitatory and inhibitory input. Furthermore cortical connectivity is modified in the PMG which may exacerbate susceptibility to epileptiform activity. Using glutamate nanosensor imaging paired with cortical field recording we set out to answer the following questions: Is propagation of electrical activity altered in the PMG? Does activity initiated in layer II/III of nearby cortex activate the PMG and does the PMG and nearby cortex form a recurrent sub-circuit that drives epileptiform activity? Methods: Microgyri were created by briefly placing a freezing probe on the skulls of neonatal rat pups. Neocortical brain slices from sham operated and freeze lesioned rats were prepared 14-28 days later. Brain slices were loaded with glutamate FRET nanosensor and images were collected simultaneously with extracellular field recordings. Dulla, et al. J.Neurosci. Meth.168(2)p.306 Results: Using a FRET-based glutamate nanosensor we examined glutamate release patterns as a measure of network activation in cortical brain slices from control animals as well as animals with induced migrogyri. This technique was paired with extracellular field recordings made in the PMG as well as just outside the PMG. Cortical network activation, as measured by increases in glutamate concentration, in both control and microgyrus-containing slices showed initial activation on-column with the stimulation electrode. Activity initiated in the outer layers of cortex (II/III) and then moved into the deeper layers (V/VI). Little glutamate release was seen in layer IV. In slices with a microgyrus, after the initial activation of layer II/III (first 10-50 ms) an intense glutamate signal was seen throughout the PMG extending to the very edge of the microgyrus which persisted for 100’s of milliseconds. Activation within the PMG persisted longer in deeper cortical layers and activity often oscillated between deep layers of the PMG and deep layers of nearby normally layered cortex. Recurrent activation of the PMG and nearby cortex was common. The cortex directly adjacent to the microgyrus, but on the other side with respect to the simulation electrode was often activated approximately 300 ms after stimulation suggesting that prolonged activity in the PMG led to secondary activation across the microgyrus. Conclusions: Microgyri alter cortical excitability and cause epileptiform activity. Based on FRET-based glutamate imaging, slices containing a microgyrus have increased network activation, specifically the PMG is activated more intensely and for a longer duration. Furthermore, recurrent excitation between the PMG and nearby normally layered cortex occurred which lasted hundreds of milliseconds and may underlie the epileptiform activity commonly associated with microgyria.