Abstracts

Rationale: Previously, we demonstrated that transplantation of interneuron precursors into epileptic mice dramatically suppresses spontaneous seizures (Baraban et al. 2009). Precursors are obtained from a transient embryonic structure - the medial ganglionic eminence (MGE) - that produces the majority of cortical interneurons. MGE cells dissected from mouse embryos and transplanted into a postnatal mouse brain exhibit an impressive capacity to migrate and differentiate into various functional interneuron subtypes. Here, we further examined how grafted interneuron precursors anatomically and functionally integrate into the host neocortex. Methods: Interneuron precursors from MGE were isolated from E13.5 GFP+ mouse embryos. MGE cells were injected into the neocortices of postnatal day 3 host mice and graft recipients were sacrificed 30-40 days later. Immunostaining for GAD67, parvalbumin (PV), somatostatin (SOM), neuropeptide Y (NPY), calretinin (CR), DAPI and layer-specific markers was performed. GABA-mediated tonic currents were recorded from host L2/3 pyramidal cells in the presence of GABA uptake blockers followed by bath application of SR95531 (100 μM). GFP+ cell number within a 40x field of view that neighbored the recorded pyramidal neuron was counted in the living slice Results: Grafted interneuron precursor cells preferentially migrated to GAD67-immunoreactive (IR) and cell sparse layers of the host neocortex (L1 and L5), and filled these layers with extensive neuritic processes. GFP+ cell somas and processes formed a distinct band within L1 and L5 such that grafted cells appeared to encounter a barrier repelling them from neighboring layers. Further, we are examining whether grafted interneurons subtypes, namely the CR-, PV- and SOM-IR cells, exhibit the same layer preferences as their endogenous counterparts in the host brain. Given that GFP cell density is not uniform throughout the neocortex, we recorded tonic GABA currents from host pyramidal cells surrounded by varying numbers of grafted derived GFP+ cells. We found that tonic current increases proportionally with the number of neighboring GFP+ cells. Conclusions: Grafted MGE-derived interneuron somas and their processes prefer GAD67 and cell sparse neocortical layers of the host brain. These studies suggest that grafted cells may (i) respond to endogenous GABA within these neocortical layers or (ii) prefer neocortical layers that are less densely packed and can thereby accommodate new neurons and synapses. Further, the spatial variations in interneuron density following transplantation may affect the amount of tonic inhibition in the host brain.