Ramesh Chittajallu

Hippocampal LTD Expression Involves a Pool of AMPARs Regulated by the NSF–GluR2 Interaction

We investigated whether the interaction between the N-ethyl-maleimide-sensitive fusion protein (NSF) and the AMPA receptor (AMPAR) subunit GluR2 is involved in synaptic plasticity in the CA1 region of the hippocampus. Blockade of the NSF-GluR2 interaction by a specific peptide (pep2m) introduced into neurons prevented homosynaptic, de novo long-term depression (LTD). Moreover, saturation of LTD prevented the pep2m-induced reduction in AMPAR-mediated excitatory postsynaptic currents (EPSCs). Minimal stimulation experiments indicated that both pep2m action and LTD were due to changes in quantal size and quantal content but were not associated with changes in AMPAR single-channel conductance or EPSC kinetics. These results suggest that there is a pool of AMPARs dependent on the NSF-GluR2 interaction and that LTD expression involves the removal of these receptors from synapses.

Kainate receptors: subunits, synaptic localization and function.

Although it is well established that kainate receptors constitute an entirely separate group of proteins from AMPA receptors, their physiological functions remain unclear. The molecular cloning of subunits that form kainate receptors and the ability to study recombinant receptors is leading to an increased understanding of their functional properties. Furthermore, the development of kainate receptor-selective agonists and antagonists over the past few years is now allowing the physiological roles of these receptors and, in some cases, specific subunits to be investigated. As a consequence, the synaptic activation of postsynaptic kainate receptors and the presence of presynaptic kainate receptors that serve to regulate excitatory and inhibitory synaptic transmission have been described, and will be discussed in this article by Ramesh Chittajallu, Steven Braithwaite, Vernon Clarke and Jeremy Henley.

Expression of the green fluorescent protein in the oligodendrocyte lineage: a transgenic mouse for developmental and physiological studies.

We generated a transgenic mouse expressing the enhanced green fluorescent protein (EGFP) under the control of the 2′‐3′‐cyclic nucleotide 3′‐phosphodiesterase (CNP) promoter. EGFP+ cells were visualized in live tissue throughout embryonic and postnatal development. Immunohistochemical analysis in brain tissue and in sciatic nerve demonstrated that EGFP expression was restricted to cells of the oligodendrocyte and Schwann cell lineages. EGFP was also strongly expressed in “adult” oligodendrocyte progenitors (OPs) and in gray matter oligodendrocytes. Fluorescence‐activated cell sorting allowed high‐yield purification of EGFP+ oligodendrocyte‐lineage cells from transgenic brains. Electrophysiological patch clamp recordings of EGFP+ cells in situ demonstrated that OP cells displayed large outward tetraethylammonium (TEA)‐sensitive K+ currents and very small inward currents, whereas mature oligodendrocytes were characterized by expression of large inward currents and small outward K+ currents. The proliferation rate of EGFP+ cells in developing white matter decreased with the age of the animals and was strongly inhibited by TEA. Oligodendrocyte development and physiology can be studied in live tissue of CNP‐EGFP transgenic mice, which represent a source of pure EGFP+ oligodendrocyte‐lineage cells throughout development.

Shaker-type potassium channel subunits differentially control oligodendrocyte progenitor proliferation

Oligodendrocyte precursor (OP) cells are exposed to multiple extrinsic signals that control their proliferation and differentiation. Previous cell proliferation studies and electrophysiological analysis in cultured cells and in brain slices have suggested that outward potassium channels, particularly Kv1 subunits, may have a prominent role in OP cell proliferation. In the present study, we assessed to what extent overexpression of Kv1.3, Kv1.4, Kv1.5, and Kv1.6 can affect OP cell proliferation and differentiation in culture. We observed that overexpression of Kv1.3 or Kv1.4 increased OP cell proliferation in the absence of mitogens, whereas Kv1.6 overexpression inhibited mitogen‐induced OP cell cycle progression. Interestingly, Kv1.3, Kv1.4, Kv1.5, and Kv1.6 overexpression did not interfere with the kinetics of oligodendrocyte differentiation. This study represents the first demonstration that the activity of potassium channels containing distinct Kv1 subunit proteins directly controls oligodendroglial proliferation in the presence of mitogens, as well as in growth factor‐free conditions.

Long-Term, Selective Gene Expression in Developing and Adult Hippocampal Pyramidal Neurons Using Focal In Utero Electroporation

An important step toward understanding brain function is the development of robust methods to molecularly perturb neurons in the developing and adult CNS. A technique of general use needs to allow targeting of specific tissues or cell types and be reasonably cost and time effective. Most important,

Satellite NG2 Progenitor Cells Share Common Glutamatergic Inputs with Associated Interneurons in the Mouse Dentate Gyrus

Several studies have provided evidence that NG2-expressing (NG2+) progenitor cells are anatomically associated to neurons in gray matter areas. By analyzing the spatial distribution of NG2+ cells in the hilus of the mouse dentate gyrus, we demonstrate that NG2+ cells are indeed closely associated to interneurons. To define whether this anatomical proximity reflected a specific physiological interaction, we performed patch-clamp recordings on hilar NG2+ cells and interneurons between 3 and 21 postnatal days. We first observed that hilar NG2+ cells exhibit spontaneous glutamatergic EPSCs (sEPSCs) whose frequency and amplitude increase during the first 3 postnatal weeks. At the same time, the rise time and decay time of sEPSCs significantly decreased, suggesting that glutamatergic synapses in NG2+ cells undergo a maturation process that is reminiscent of what has been reported in neurons during the same time period. We also observed that hilar interneurons and associated NG2+ cells are similarly integrated into the local network, receiving excitatory inputs from both granule cells and CA3 pyramidal neurons. By performing pair recordings, we found that bursts of activity induced by GABAergic antagonists were strongly synchronized between both cell types and that the amplitude of these bursts was positively correlated. Finally, by applying carbachol to increase EPSC activity, we observed that closely apposed cells were more likely to exhibit synchronized EPSCs than cells separated by >200 μm. The finding that NG2+ cells are sensing patterns of activity arising in closely associated neurons suggests that NG2+ cell function is finely regulated by the local network.

Pentraxins Coordinate Excitatory Synapse Maturation and Circuit Integration of Parvalbumin Interneurons.

Circuit computation requires precision in the timing, extent, and synchrony of principal cell (PC) firing that is largely enforced by parvalbumin-expressing, fast-spiking interneurons (PVFSIs). To reliably coordinate network activity, PVFSIs exhibit specialized synaptic and membrane properties that promote efficient afferent recruitment such as expression of high-conductance, rapidly gating, GluA4-containing AMPA receptors (AMPARs). We found that PVFSIs upregulate GluA4 during the second postnatal week coincident with increases in the AMPAR clustering proteins NPTX2 and NPTXR. Moreover, GluA4 is dramatically reduced in NPTX2(-/-)/NPTXR(-/-) mice with consequent reductions in PVFSI AMPAR function. Early postnatal NPTX2(-/-)/NPTXR(-/-) mice exhibit delayed circuit maturation with a prolonged critical period permissive for giant depolarizing potentials. Juvenile NPTX2(-/-)/NPTXR(-/-) mice display reduced feedforward inhibition yielding a circuit deficient in rhythmogenesis and prone to epileptiform discharges. Our findings demonstrate an essential role for NPTXs in controlling network dynamics highlighting potential therapeutic targets for disorders with inhibition/excitation imbalances such as schizophrenia.

Emergence of cortical inhibition by coordinated sensory–driven plasticity at distinct synaptic loci

Feedforward GABAergic inhibition sets the dendritic integration window, thereby controlling timing and output in cortical circuits. However, the manner in which feedforward inhibitory circuits emerge is unclear, despite this being a critical step for neocortical development and function. We found that sensory experience drove plasticity of the feedforward inhibitory circuit in mouse layer 4 somatosensory barrel cortex in the second postnatal week via two distinct mechanisms. First, sensory experience selectively strengthened thalamocortical-to-feedforward interneuron inputs via a presynaptic mechanism but did not regulate other inhibitory circuit components. Second, experience drove a postsynaptic mechanism in which a downregulation of a prominent thalamocortical NMDA excitatory postsynaptic potential in stellate cells regulated the final expression of functional feedforward inhibitory input. Thus, experience is required for specific, coordinated changes at thalamocortical synapses onto both inhibitory and excitatory neurons, producing a circuit plasticity that results in maturation of functional feedforward inhibition in layer 4.

Dual origins of functionally distinct O-LM interneurons revealed by differential 5-HT 3A R expression

Forebrain circuits rely upon a relatively small but remarkably diverse population of GABAergic interneurons to bind and entrain large principal cell assemblies for network synchronization and rhythmogenesis. Despite the high degree of heterogeneity across cortical interneurons, members of a given subtype typically exhibit homogeneous developmental origins, neuromodulatory response profiles, morphological characteristics, neurochemical signatures and electrical features. Here we report a surprising divergence among hippocampal oriens-lacunosum moleculare (O-LM) projecting interneurons that have hitherto been considered a homogeneous cell population. Combined immunocytochemical, anatomical and electrophysiological interrogation of Htr3a-GFP and Nkx2-1-cre:RCE mice revealed that O-LM cells parse into a caudal ganglionic eminence–derived subpopulation expressing 5-HT3A receptors (5-HT3ARs) and a medial ganglionic eminence–derived subpopulation lacking 5-HT3ARs. These two cohorts differentially participate in network oscillations, with 5-HT3AR-containing O-LM cell recruitment dictated by serotonergic tone. Thus, members of a seemingly uniform interneuron population can exhibit unique circuit functions and neuromodulatory properties dictated by disparate developmental origins.

Ca2+ and synaptic plasticity

A major effort in neuroscience is directed towards understanding the roles of Ca2+ signalling in the induction of synaptic plasticity. Here, we summarize the evidence concerning Ca2+ signalling, paying particular attention to CA1 excitatory synapses, and its relationship to the induction of long-term potentiation and long-term depression. We discuss the ways in which synaptic activation can elevate Ca2+ postsynaptically and how dendritic spines may act as a Ca2+ compartment which can both isolate and integrate Ca2+ signals.