Dr McBain's lab continues to investigate the differential mechanisms underlying synaptic transmission and plasticity onto both principal neurons and inhibitory interneurons within the hippocampal formation of the mammalian cortex. To this end we have established novel roles for both ionotropic and metabotropic glutamte receptors. Furthermore we have explored the role of intrinsic voltage-gated channels in regulating individual neuronal- and network-excitability with the use of high-resolution whole-cell patch clamp recording techniques in brain slices of hippocampus. We have also explored the neurogenesis, migration and development of specific cohorts of local circuit GABAergic interneurons arising from the ganglionic eminences. Cells originating from the medial ganglionic eminence give rise to distinct populations of interneurons that then migrate to and populate the developing hippocampus. For all of these studies we use a combinaiton of high resolution electrophysiological tools, genetic, molecular and biochemical techniques as well as confocal and two-photon imaging. We continue to explore novel forms of long lasting synaptic and cellular plasticity (both long term depression and long lasting potentiation) observed at glutamatergic excitatory synaptic connections between dentate gyrus granule cells and interneurons of the CA3 hippocampus. Previously we have shown that the dentate gyrus mossy fiber-CA3 system engages their interneuron targets via multiple parallel systems that differentially utilize glutamate receptors to endow distinct synaptic properties and computational outcomes for the postsynaptic target neurons. In this cycle we have completed the most detailed analysis to the roles played by inhibitory interneurons within the feedforward and feedback inhibitory circuits across a wide developmental age range. Our data suggest that a fine balance between GABAergic feedforward and feedback inhibitory systems maintains a narrow temporal window for glutamatergic derived excitation for CA3 principal cells. The spatiotemporal origins of hippocampal interneuron diversity Although vastly outnumbered, inhibitory interneurons critically pace and synchronize excitatory principal cell populations to coordinate cortical information processing. Precision in this control relies upon a remarkable diversity of interneurons primarily determined during embryogenesis by genetic restriction of neuronal potential at the progenitor stage. Like their neocortical counterparts, hippocampal interneurons arise from medial and caudal ganglionic eminence (MGE and CGE) precursors. However, while studies of the early specification of neocortical interneurons are rapidly advancing, much to our surprise similar lineage analyses of hippocampal interneurons have lagged. We investigate the spatiotemporal origins of hippocampal interneurons using transgenic mice that specifically reported MGE- and CGE-derived interneurons either constitutively or inducibly. We found that hippocampal interneurons are produced in two neurogenic waves between E9-E12 and E12-E16 from MGE and CGE, respectively. These cells migrate through the marginal zone and subventricular zone to populated the stratum lacunosum moleculare prior to their final destination within the hippocampus proper. Migration from the MGE and CGE into the hippocampus takes varying amounts of time with cells born at later embryonic stages taking less time despite the increased dimensions of the migratory path length. In the mature hippocampus, CGE-derived interneurons primarily localize to superficial layers in strata lacunosum moleculare and deep radiatum, while MGE-derived interneurons readily populate all layers with preference for strata pyramidale and oriens. Disrupted excitatory synapse maturation in GABAergic interneurons may promote neuropsychiatric disorders such as schizophrenia. However, establishing developmental programs for nascent synapses in GABAergic cells is confounded by their sparsity, heterogeneity and late acquisition of subtype-defining characteristics. We investigated synaptic development in mouse interneurons targeting cells by lineage from medial ganglionic eminence (MGE) or caudal ganglionic eminence (CGE) progenitors. MGE-derived interneuron synapses were dominated by GluA2-lacking AMPA-type glutamate receptors (AMPARs), with little contribution from NMDA-type receptors (NMDARs) throughout development. In contrast, CGE-derived cell synapses had large NMDAR components and used GluA2-containing AMPARs. In neonates, both MGE- and CGE-derived interneurons expressed primarily GluN2B subunit-containing NMDARs, which most CGE-derived interneurons retained into adulthood. However, MGE-derived interneuron NMDARs underwent a GluN2B-to-GluN2A switch that could be triggered acutely with repetitive synaptic activity. Our findings establish ganglionic eminence-dependent rules for early synaptic integration programs of distinct interneuron cohorts, including parvalbumin- and cholecystokinin-expressing basket cells. Currently we are exploring the impact of selective elimination of particular glutamate receptor subunits (GluA1/2, GluN1) in cell embryogenesis, migration and maturation of nascent cortical circuits.
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