Synaptic inhibition in the vertebrate central nervous system is largely mediated by type A GABA receptors (GABA(_A)R). The clustering of (GABA(_A)R) at discrete and functionally significant domains on the nerve cell surface is an important determinant in the integration of synaptic inputs. To discern the role that specific GABA(_A)R subunits play in determining the receptor's cell surface topography and mobility, recombinant GABA(_A)Rs, comprising different GABA(_A)R subunit combinations, were transiently expressed in COS7, HEK293 and PC12 cells. In addition, a series of domain swapping experiments were performed in order to elucidate which regions of the protein are important in mobility/anchoring of receptors. The cellular localization and lateral mobility of the recombinantly expressed GABA(_A)Rs were determined by immunocytochemistry and Fluorescence Photobleach Recovery (FPR), respectively. The results presented in this thesis show that GABA(_A)R al subunits are recruited by the β3 subunits from an internally sequestered pool and assembled into a population of GABA(_A)Rs that are spatially segregated into clusters and also immobilised on the cell surface. FPR experiments on recombinant GABA(_A)R containing al-a6 subunits expressed in COS? cells showed restricted mobilities consistent with mobility constants determined for native GABA(_A)Rs expressed on cerebellar granule cells. Furthermore, the intracellular loop domain M3/M4 of the a1 subunits was found to be required for anchoring recombinantly expressed GABA(_A)Rs in C0S7 and cerebellar granule cells in culture, but not for GABA(_A)R clustering at the cell surface.

Posttraumatic epilepsy is a common consequence of brain trauma. However, little is known about how long-term changes in local excitatory and inhibitory synaptic networks contribute to epilepsy after closed-head brain injury. This study adapted a widely used model of experimental brain injury as a mouse model of posttraumatic epilepsy. Behavioral seizure activity and alterations in synaptic circuitry in the dentate gyrus were examined in mice after experimental cortical contusion brain injury. Spontaneous behavioral seizures were observed in 20% of mice after moderate injury and 36-40% of mice weeks after severe injury. In the dentate gyrus, most mice displayed regionally localized mossy fiber reorganization ipsilateral to the injury that was absent in control mice or sections contralateral to the injury. Extracellular field and whole-cell patch clamp recordings were performed in acute brain slice preparations of the dentate gyrus. Dentate granule cells displayed spontaneous and evoked activity that was consistent with network synchronization and the formation of recurrent excitatory network only in slices that had posttraumatic mossy fiber sprouting. The excitability of surviving hilar GABAergic interneurons, which provide important feedback inhibition to granule cells, was examined at similar time points. Cell-attached and whole-cell voltage-clamp recordings revealed increased spontaneous and glutamate photostimulation-evoked excitatory input to hilar GABA neurons ipsilateral to the injury, versus control and contralateral slices. Despite increased excitatory synaptic input to interneurons, whole-cell voltage-clamp recordings revealed a reduction in inhibitory synaptic input to granule cells. These findings suggest that there are alterations in excitatory and inhibitory circuits in mice with posttraumatic mossy fiber sprouting and seizures after cortical contusion head injury.