Abstracts

Rationale: Dysfunction of synaptic inhibition is a major mechanism underlying epilepsy. This birthed the hypothesis that enhancing inhibition within via cell transplantation of GABAergic interneurons could potentially reduce epileptic activity within neural circuits. GABAergic interneuron progenitor cells isolated from the embryonic medial ganglionic eminence (MGE) migrate, mature, and integrate into the native circuitry of the brains of transplant recipient mice (Alvarez-Dolado et al 2006). Importantly, MGE cells largely differentiate into somatostatin- (Sst) and parvalbumin-positive (PV) interneuron subtypes, both of which provide strong inhibition to cortical and hippocampal pyramidal neurons. Transplantation of MGE cells into the neonatal or adult brains of epileptic mice normalized decreased levels of synaptic inhibition and reduced both seizures and behavioral comorbidities associated with epilepsy (Baraban et al 2009, Hunt et al 2013). The integration of these cells can also induced homeostatic plasticity within the native circuitry in response to altered levels of inhibition (Howard et al 2014). While transplantation of GABAergic interneurons has clear potential for epilepsy treatment, relatively little is known about the basic processes by which non-native neurons integrate into the recipient brain. Methods: To begin to understand the mechanisms by which transplanted interneurons connect with native neural circuitry, we have begun a thorough analysis of the development of excitatory synaptic transmission onto MGE neurons transplanted into the mouse cortex. We isolated MGE progenitor cells from GFP+ mice at embryonic day 12-13 and transplanted these cells bilaterally into the cortices of wild type mice on postnatal day 2-3. We then prepared acute cortical slices for electrophysiology 7, 14, 21 and 35 days after transplant (dat). These time points were chosen to cover a broad range of the "development" of transplanted neurons. Previous studies have determined that transplanted MGE cells may receive synaptic input early on, but take weeks to exhibit fully mature action potential firing properties and form inhibitory synapses onto native pyramidal cells (Alvarez-Dolado et al 2006, Baraban et al 2009). Whole cell recordings were made from GFP+ cells within the slices. Current clamp recordings were made to divide transplant-derived interneurons into classes based on action potential firing patterns: fast spiking or regular spiking/non-pyramidal. Voltage clamp recordings were made of spontaneous miniature postsynaptic potentials (sEPSCs). Results: The frequency, amplitude, and kinetics of these events were analyzed between classes and compared with values from recorded native interneurons. Additionally, various glutamate receptor subtypes (i.e., kainate vs. AMPA) were pharmacologically blocked to determine the molecular makeup of the neurotransmitter receptors at the synapses of transplanted cells at different time points. Conclusions: Supported by NIH/NINDS R01-NS071785.