Neuronal precursor cell migration in the developing mammalian brain is a complex process requiring the coordinated interaction of numerous proteins. We have recently shown that amyloid precursor protein (APP) plays a role in migration into the cortical plate through its interaction with two cytosolic signaling proteins, disabled 1 (DAB1) and disrupted in schizophrenia 1 (DISC1). In order to identify extracellular factors that may signal through APP to regulate migration, we performed an unbiased mass spectrometry-based screen for factors that bind to the extracellular domain of APP in the rodent brain. Through this screen, we identified an interaction between APP and pancortins, proteins expressed throughout the developing and mature cerebral cortex. Via co-immunoprecipitation, we show that APP interacts with all four of the mammalian pancortin isoforms (AMY, AMZ, BMY, BMZ). We demonstrate that the BMZ and BMY isoforms of pancortin can specifically reduce β-secretase- but not α-secretase-mediated cleavage of endogenous APP in cell culture, suggesting a biochemical consequence of the association between pancortins and APP. Using in utero electroporation to overexpress and knock down specific pancortin isoforms, we reveal a novel role for pancortins in migration into the cortical plate. Interestingly, we observe opposing roles for alternate pancortin isoforms, with AMY overexpression and BMZ knock down both preventing proper migration of neuronal precursor cells. Finally, we show that BMZ can partially rescue a loss of APP expression and that APP can rescue effects of AMY overexpression, suggesting that pancortins act in conjunction with APP to regulate entry into the cortical plate. Taken together, these results suggest a biochemical and functional interaction between APP and pancortins, and reveal a previously unidentified role for pancortins in mammalian cortical development.

We have developed a Drosophila melanogaster model of Alzheimer's disease (AD) by expressing beta-amyloid peptides (Abeta) in the neurons of the fly. The fly develops neurodegeneration, a locomotor defect, and has a shorter lifespan. An unbiased genetic screen of a library of 3000 enhancing genetic element insertions identified the fly ortholog of puromycin-sensitive aminopeptidase (PSA) as a potent suppressor of Abeta toxicity. PSA is an exopeptidase that catalyzes the hydrolysis of amino acid residues from the amino terminus of peptide substrates and is inhibited by puromycin. PSA has been shown to digest polyglutamine sequences in vitro such as those in Huntington's disease as well as cleave tau both in vitro, in cultured SH-SY5Y cells, and in vivo in a mouse model of frontotemporal dementia (FTD). Furthermore, overexpression of PSA upregulates autophagy thereby protecting against the accumulation of aggregation-prone proteins associated with neurodegenerative diseases including: polyQ, ataxin-3 (spinocerebellar ataxia type 3), alpha-synuclein (Parkinson's disease), and SOD1 (amyotrophic lateral sclerosis, ALS). PSA and the catalytically inactive zinc binding domain (ZBD) mutant (E309A) were tagged with GFP and expressed in 7PA2 cells. These are CHO cells that overexpress the amyloid precursor protein (APP) with the V717F mutation and secrete Abeta. The activity of GFP-tagged PSA in cell lysates was assessed by its ability to digest the chromogenic substrate, leucine p-nitroanilide. The localization of GFP-tagged PSA was determined by direct fluorescence and by density gradient ultracentrifugation. The toxicity of Abeta1-42 peptide added to neuronal SH-SY5Y cells transiently transfected with GFP-tagged PSA was quantified using a fluorescent dye-based assay that measured the number of live and dead cells. GFP-tagged PSA, but not the inactive ZBD mutant, was active in cell lysates against the chromogenic substrate and could be inhibited by puromycin. Direct fluorescence of transiently transfected 7PA2 cells revealed that PSA was mainly localized in the cytoplasm. Density gradient ultracentrifugation demonstrated that PSA was not membrane-bound. The toxicity of Abeta1-42 peptide added exogenously to SH-SY5Y cells was rescued by the overexpression of GFP-tagged PSA. We are currently trying to elucidate the cellular mechanism by which PSA rescues Abeta toxicity. This study shows that GFP-tagged PSA is cytoplasmic, active against a chromogenic substrate, and can rescue the toxicity of the Abeta1-42 peptide in cell culture.

Alzheimer's disease (AD) continues to be one of most difficult human diseases to treat. The past 18 months have been a cruel reminder of the challenges of finding new and effective treatments. In 2010, several large Phase III clinical trials were terminated for lack of therapeutic efficacy. Concurrently, an NIH expert review panel was resigned to conclude that there was insufficient scientific evidence to recommend any treatment choices for slowing the progression of AD. Why has this disease proved so daunting? The answer is complex. To begin, it is still not clear whether AD is one disease with a single cause or multiple syndromes with common symptoms and/or a common pathology. Resolving this question is a prerequisite for forecasting whether to expect a 'magic bullet' therapy or only incremental progress in select patient populations. This review will explore some of the details of recent clinical trials and consider some of the lessons learned. As therapies approach clinical trials, it is essential to understand the expectations of regulatory agencies such as the FDA and EMA to obtain approval. Lastly, we will cover some of the essential gaps in our scientific understanding about the disease process and the impact this has on target validation. The hope of finding of a quick cure for AD without a complete understanding of the disease may have been too optimistic. However, a prudent review of the scientific evidence, a clear understanding of the expectations of regulators, and careful attention to patient needs may still lead to good therapies in the foreseeable future.

The beta-amyloid peptide (Abeta) is thought to play a critical role in the pathophysiology of Alzheimer's disease (AD). To study the effects of Abeta on the brain, transgenic mouse models have been developed that express high levels of Abeta. These mice show some features of AD, including amyloid plaques and mild cognitive impairment, but not others such as progressive neurodegeneration. We investigated the age-dependent effects of Abeta on synaptic physiology in Tg2576 mice that express human Abeta. We report that both basal synaptic activity and long-term potentiation (LTP), as measured in the CA1 region of the hippocampus, were compromised by 7 months of age before plaque deposition. Despite a persistent increase in Abeta levels with age, LTP recovered in 14-month-old mice, with no further loss of basal activity compared with activity measured in 7-month-old mice. Previous work has shown that inhibitors of gamma-secretase, an enzyme critical for Abeta synthesis, can significantly reduce Abeta production and plaque formation in Tg2576 mice. Our data demonstrate that 7-month-old Tg2576 mice treated with an orally available gamma-secretase inhibitor showed a significant improvement in synaptic function and plasticity within days, and the effect was correlated with the extent and duration of Abeta reduction. These results indicate that recovery from Abeta-mediated synaptotoxicity can occur rapidly with Abeta-lowering therapies. These findings highlight some of the strengths and limitations of using Abeta-overexpressing mouse models for Alzheimer's drug discovery.

Synaptic degeneration, including impairment of synaptic plasticity and loss of synapses, is an important feature of Alzheimer disease pathogenesis. Increasing evidence suggests that these degenerative synaptic changes are associated with an accumulation of soluble oligomeric assemblies of amyloid beta (A beta) known as ADDLs. In primary hippocampal cultures ADDLs bind to a sub-population of neurons. However the molecular basis of this cell type-selective interaction is not understood. Here, using siRNA screening technology, we identified alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunits and calcineurin as candidate genes potentially involved in ADDL-neuron interactions. Immunocolocalization experiments confirmed that ADDL binding occurs in dendritic spines that express surface AMPA receptors, particularly the calcium-impermeable type II AMPA receptor subunit (GluR2). Pharmacological removal of the surface AMPA receptors or inhibition of AMPA receptors with antagonists reduces ADDL binding. Furthermore, using co-immunoprecipitation and photoreactive amino acid cross-linking, we found that ADDLs interact preferentially with GluR2-containing complexes. We demonstrate that calcineurin mediates an endocytotic process that is responsible for the rapid internalization of bound ADDLs along with surface AMPA receptor subunits, which then both colocalize with cpg2, a molecule localized specifically at the postsynaptic endocytic zone of excitatory synapses that plays an important role in activity-dependent glutamate receptor endocytosis. Both AMPA receptor and calcineurin inhibitors prevent oligomer-induced surface AMPAR and spine loss. These results support a model of disease pathogenesis in which A beta oligomers interact selectively with neurotransmission pathways at excitatory synapses, resulting in synaptic loss via facilitated endocytosis. Validation of this model in human disease would identify therapeutic targets for Alzheimer disease.

Characterized as a peripheral metabolic disorder and a degenerative disease of the central nervous system respectively, it is now widely recognized that type 2 diabetes mellitus (T2DM) and Alzheimer's disease (AD) share several common abnormalities including impaired glucose metabolism, increased oxidative stress, insulin resistance and amyloidogenesis. Several recent studies suggest that this is not an epiphenomenon, but rather these two diseases disrupt common molecular pathways and each disease compounds the progression of the other. For instance, in AD the accumulation of the amyloid-beta peptide (Aβ), which characterizes the disease and is thought to participate in the neurodegenerative process, may also induce neuronal insulin resistance. Conversely, disrupting normal glucose metabolism in transgenic animal models of AD that over-express the human amyloid precursor protein (hAPP) promotes amyloid-peptide aggregation and accelerates the disease progression. Studying these processes at a cellular level suggests that insulin resistance and Aβ aggregation may not only be the consequence of excitotoxicity, aberrant Ca2+ signals, and proinflammatory cytokines such as TNF-α, but may also promote these pathological effectors. At the molecular level, insulin resistance and Aβ disrupt common signal transduction cascades including the insulin receptor family/PI3 kinase/Akt/GSK3 pathway. Thus both disease processes contribute to overlapping pathology, thereby compounding disease symptoms and progression.

Alzheimer's disease is characterized by the pathological accumulation of the beta-amyloid peptide (Aβ) in brain regions that subserve memory and cognition. In animal models, numerous reports have now demonstrated that soluble forms of Aβ can interfere with synaptic plasticity (the continuous experience-dependent remodeling of brain connectivity that is likely to underlie learning and memory). This deficit in synaptic plasticity can be rapidly induced in rodents and is one of the earliest impairments that can be reliably measured. Emerging evidence also indicates that regulation of chromatin remodeling and gene transcription by inhibitors of histone-deacetylases (HDACs) can affect synaptic plasticity. Two independent laboratories have now reported that the cognitive deficits observed in the p25 and APP/PS1 transgenic mouse models of AD can be ameliorated by HDAC inhibition. To replicate and expand on these results, we measured long-term potentiation (LTP) in brain slices prepared from 7–8 month-old APP transgenic mice (prior to plaque formation) or wild-type mice treated with cell-derived Aβ. In both models, we observed a robust impairment in LTP in the CA1 region of the hippocampus. Acute application (40 minutes prior to high-frequency stimulation) of HDAC inhibitors offsets the deficit in synaptic plasticity caused by Aβ. Ongoing work will test subtype selective HDAC inhibitors. In addition, we performed mRNA profiling of the hippocampus to identify transcripts that are altered by HDAC antagonists that may be relevant to the expression of LTP. Our results support a role for HDAC inhibition in moderating the effects of Aβ on this form of synaptic plasticity.

Numerous studies have now shown that the amyloid beta-protein (Abeta), the principal component of cerebral plaques in Alzheimer disease, rapidly and potently inhibits certain forms of synaptic plasticity. The amyloid (or Abeta) hypothesis proposes that the continuous disruption of normal synaptic physiology by Abeta contributes to the development of Alzheimer disease. However, there is little consensus about how Abeta mediates this inhibition at the molecular level. Using mouse primary hippocampal neurons, we observed that a brief treatment with cell-derived, soluble, human Abeta disrupted the activation of three kinases (Erk/MAPK, CaMKII, and the phosphatidylinositol 3-kinase-activated protein Akt/protein kinase B) that are required for long term potentiation, whereas two other kinases (protein kinase A and protein kinase C) were stimulated normally. An antagonist of the insulin receptor family of tyrosine kinases was found to mimic the pattern of Abeta-mediated kinase inhibition. We then found that soluble Abeta binds to the insulin receptor and interferes with its insulin-induced autophosphorylation. Taken together, these data demonstrate that physiologically relevant levels of naturally secreted Abeta interfere with insulin receptor function in hippocampal neurons and prevent the rapid activation of specific kinases required for long term potentiation.

Alzheimer's disease (AD) is characterized by decreased synapse density in hippocampus and neocortex, and synapse loss is the strongest anatomical correlate of the degree of clinical impairment. Although considerable evidence supports a causal role for the amyloid-β protein (Aβ) in AD, a direct link between a specific form of Aβ and synapse loss has not been established. We demonstrate that physiological concentrations of naturally secreted Aβ dimers and trimers, but not monomers, induce progressive loss of hippocampal synapses. Pyramidal neurons in rat organotypic slices had markedly decreased density of dendritic spines and numbers of electrophysiologically active synapses after exposure to picomolar levels of soluble oligomers. Spine loss was reversible and was prevented by Aβ-specific antibodies or a small-molecule modulator of Aβ aggregation. Mechanistically, Aβ-mediated spine loss required activity of NMDA-type glutamate receptors (NMDARs) and occurred through a pathway involving cofilin and calcineurin. Furthermore, NMDAR-mediated calcium influx into active spines was reduced by Aβ oligomers. Partial blockade of NMDARs by pharmacological antagonists was sufficient to trigger spine loss. We conclude that soluble, low-noligomers of human Aβ trigger synapse loss that can be reversed by therapeutic agents. Our approach provides a quantitative cellular model for elucidating the molecular basis of Aβ-induced neuronal dysfunction.

Despite progress in defining a pathogenic role for amyloid β protein (Aβ) in Alzheimer's disease, orally bioavailable compounds that prevent its effects on hippocampal synaptic plasticity and cognitive function have not yet emerged. A particularly attractive therapeutic strategy is to selectively neutralize small, soluble Aβ oligomers that have recently been shown to mediate synaptic dysfunction.

The normal function of APP is incompletely understood. Previous studies have shown that APP may have neurotrophic effects and injection of APPs can enhance memory function in mice. Moreover, the mechanism by which Aβ inhibits certain forms of synaptic plasticity remain unknown. To study the acute effects of soluble APP and Aβ on synaptic function and signal transduction cascades in wild–type mouse hippocampal neurons. We have used biochemical and electrophsyiological techniques on acute hippocampal slices or cultured primary neurons. We now report that APP processing plays an essential role in the normal synaptic function of hippocampal neurons. Application of exogenous APPs to mouse hippocampal slices caused a rapid potentiation of glutamatergic synapses. APPs increased the amplitudes of evoked AMPA receptor currents and miniature EPSPs. This effect was blocked 1) by preclearing the perfusate with anti–APPs antibody; 2) by the G–protein antagonist, NF023; and 3) by secreted Aβ. In view of these data, we tested multiple α, β, and γ–secretase inhibitors for effects on LTP, and found that acute application of several BACE antagonists inhibited LTP. This was rescued with exogenous APPs. Since oligomers of Aβ inhibit the potentiation of synapses by APPs or high–frequency stimulation, we examined how Aβ affects the signal transduction cascades that mediate LTP. While some kinases such as PKA and PKC were activated normally by an LTP–inducing stimulus, Aβ prevented the activation of the Erk/MAPK, CaMKII, and PI3K pathways. Using a panel of antagonists, we examined whether inhibition of any single cell–surface receptor could replicate the signal–transduction pattern seen with Aβ. To date, antagonists to the mGluR and insulin receptor appear to replicate the pattern Aβ–mediated signal transduction inhibition. These data show that proteolytic processing of APP by BACE, is necessary for the normal expression of LTP. Furthermore, our results suggest that small, soluble Aβ oligomers interfere selectively with certain signal transduction pathways necessary for LTP, offering a mechanism by which Aβ could lead to subtle amnestic deficits early in AD.

Converging lines of evidence suggest a central role for amyloid β–protein (Aβ) in the genesis of Alzheimer's disease. Studies we've conducted over the past decade suggest that small, soluble oligomers of Aβ rather than insoluble amyloid fibrils may be early effectors of synaptic dysfunction. We initially observed that stable CHO cell lines expressing wt APP (7W) or FAD mutant APP (7PA2) secrete high–picomolar concentrations of SDS–stable oligomers, providing a ready source for examining the biological activity of natural oligomers of human Aβ in vitro and in vivo. To characterize further the neurobiological properties of soluble oligomers and assess ways to block their synaptotoxicity. 7PA2 and parental CHO– cells were conditioned with or without putative inhibitors of fibrillogenesis. For additional biochemical specificity, media were fractionated by SEC and the activities of oligomer and monomer fractions compared. With our collaborators, bioactivity was assessed in rats and mice in several ways, including measuring a) hippocampal LTP, an electrophysiological correlate of learning and memory; b) performance in remembering a complex learned behavior; c) effects of certain small–molecule modulators of Aβ; and d) effects of exogenous or endogenous anti–Aβ antibodies. Aβ oligomers could be quantitatively separated from monomers by non–denaturing SEC. The cell–derived oligomers did not affect baseline synaptic transmission but consistently blocked hippocampal LTP, with trimers appearing more potent than dimers. Monomers had no effect. Aβ vaccination in wt rats rescued the inhibition of hippocampal LTP caused by ICV microinjection of the oligomers, and the extent of rescue correlated with the levels of anti–oligomer antibodies. We identified several small molecules that decreased oligomer levels and abrogated their block of LTP. ICV injection of oligomers in rats interfered transiently and potently with performance on a complex lever–pressing task. We are now searching for longer–term effects of the oligomers on neuronal and glial biochemistry, including tau alteration. Disease–relevant concentrations of naturally secreted oligomers of human Aβ alter both synaptic plasticity and cognitive behavior in vivo. Moreover, anti–Aβ antibodies and certain small molecules that retard oligomer formation can each rescue oligomer–mediated electrophysiological effects, providing a rational mechanism for therapeutic intervention in AD.

Recent studies support the hypothesis that soluble oligomers of amyloid β-peptide (Aβ) rather than mature amyloid fibrils are the earliest effectors of synaptic compromise in Alzheimer's disease. We took advantage of an amyloid precursor protein-overexpressing cell line that secretes SDS-stable Aβ oligomers to search for inhibitors of the pathobiological effects of natural human Aβ oligomers. Here, we identify small molecules that inhibit formation of soluble Aβ oligomers and thus abrogate their block of long-term potentiation (LTP). Furthermore, we show that cell-derived Aβ oligomers can be separated from monomers by size exclusion chromatography under nondenaturing conditions and that the isolated, soluble oligomers, but not monomers, block LTP. The identification of small molecules that inhibit early Aβ oligomer formation and rescue LTP inhibition offers a rational approach for therapeutic intervention in Alzheimer's disease and highlights the utility of our cell-culture paradigm as a useful secondary screen for compounds designed to inhibit early steps in Aβ oligomerization under biologically relevant conditions.

Here, we present a short interfering RNA (siRNA) application that reduces the expression of NR1 the obligate subunit of the NMDA receptor (NMDAR) and virtually eliminates NMDAR function in a small subset of neurons within otherwise normally developing Xenopus laevis tadpoles. We designed two plasmids each containing a CMV promoter driving a dsRed "reporter" cDNA and DNA coding for one short hairpin RNA (shRNA) under the control of the U6 promoter. The shRNA was cleaved to produce a siRNA against NR1 transcript (iNR1). NR1 transcript and protein, differentiation and survival of NR1 knockdown neurons, were assayed in vivo. iNR1 effects on NMDAR function used Xenopus tectal neuron cultures, Ca2+ imaging and patch-clamp electrophysiology. NR1 transcript, protein and NMDAR function was significantly reduced or eliminated in iNR1-expressing neurons. Protein, transcript and function for the closely related AMPA receptor (AMPAR) subunit GluR1 was unperturbed. In vivo imaging of small groups of iNR1 neurons at two different ages revealed that siRNA knockdown of the NMDAR does not affect differentiation or survival of young neurons. Thus, with siRNA, molecular hypotheses about synaptic mechanisms of circuit formation can be rapidly tested in intact developing vertebrates using normal neurons in the same animals as controls.

We describe a homeostatic mechanism that limits NMDA receptor currents in response to early light activation of a developing visual pathway. During the second postnatal week of rodent retinocollicular development, the Ca2+-activated phosphatase calcineurin (CaN) mediates a rapid, activity-induced shortening in the decay time of NMDA receptor (NMDAR) currents. We show that protein kinase A acts in opposition to CaN to maintain NMDAR currents with long decay times. The CaN-mediated change is coincident with the initial expression of the NMDAR subunit NR2A. Using NR2A knock-out mice and dialyzing neurons with a constitutively active CaN, we demonstrate that NR2A subunits are necessary for the effect of CaN on NMDAR current kinetics. In wild-type mice, Ser900 of NR2A, previously implicated in CaN-mediated glycine-independent desensitization, becomes chronically dephosphorylated by postnatal day 11 as NMDAR current decay times become faster. Pharmacologically disrupting early photoreceptor-driven activity in the retina eliminates the dephosphorylation of NR2A and prevents the shortening in NMDAR current decay time. These data suggest that the developmental onset of retinal activity increases CaN-mediated dephosphorylation of NR2A subunits newly incorporated into synaptic NMDARs of the superior colliculus, thereby providing a mechanism for the early and rapid reduction of NMDAR current decay time in visual neurons.

Background: MicroRNAs are a large new class of tiny regulatory RNAs found in nematodes, plants, insects and mammals. MicroRNAs are thought to act as post-transcriptional modulators of gene expression. In invertebrates microRNAs have been implicated as regulators of developmental timing, neuronal differentiation, cell proliferation, programmed cell death and fat metabolism. Little is known about the roles of microRNAs in mammals.Results: We isolated 18-26 nucleotide RNAs from developing rat and monkey brains. From the sequences of these RNAs and the sequences of the rat and human genomes we determined which of these small RNAs are likely to have derived from stem-loop precursors typical of microRNAs. Next, we developed a microarray technology suitable for detecting microRNAs and printed a microRNA microarray representing 138 mammalian microRNAs corresponding to the sequences of the microRNAs we cloned as well as to other known microRNAs. We used this microarray to determine the profile of microRNAs expressed in the developing mouse brain. We observed a temporal wave of expression of microRNAs, suggesting that microRNAs play important roles in the development of the mammalian brain.Conclusion: We describe a microarray technology that can be used to analyze the expression of microRNAs and of other small RNAs. MicroRNA microarrays offer a new tool that should facilitate studies of the biological roles of microRNAs. We used this method to determine the microRNA expression profile during mouse brain development and observed a temporal wave of gene expression of sequential classes of microRNAs.

The N -methyl- d -aspartate (NMDA) glutamate receptor (NMDAR), long implicated in developmental plasticity, shows decay time kinetics that shorten postnatally as NR2A subunits are added to the receptor. Neither the mechanism nor immediate effect of this change is known. We studied developing NMDAR currents by using visual neurons in slices from NR2A knockout (NR2AKO) and WT mice. Both strains show increased dendritic levels of synaptic density scaffolding protein PSD-95 with age. Dendritic levels of NR2A increased at the same time in WT and immunoprecipitated with PSD-95. PSD-95/NMDAR binding was significantly decreased in the NR2AKO. Moreover, NMDAR miniature currents (minis) were lost and rise times of NMDAR evoked currents increased in mutant mice. Age-matched WT cells showed NR2A-rich receptors predominating in minis, yet slow NR2B mediated currents persisted in evoked currents. Disrupting photoreceptor activation of retinal ganglion cells eliminated increases in PSD-95 and NR2A in superior collicular dendrites of WT mice and slowed the loss of miniature NMDAR currents in NR2AKOs. These data demonstrate that NMDARs that respond to single quantal events mature faster during development by expressing the NR2A subunit earlier than NMDARs that respond to evoked release. We hypothesize that NR2A-rich NMDARs may be localized to the center of developing synapses by an activity-dependent process that involves the targeting of PSD-95 to the postsynaptic density. Neonatal receptors become restricted to perisynpatic or extrasynaptic sites, where they participate primarily in evoked currents.

Slabs of slow-release plastic (Elvax) containing NMDA or solvent were implanted over the rat colliculus beginning on postnatal day 8 (P8). Whole-cell patch clamping in the superficial superior collicular layers (sSCs) from P10 to P21 demonstrated a severe decrease in spontaneous EPSC frequency after chronic NMDA treatment. The decrease was not attributable to an increase in GABA(A) receptor-mediated inhibition and was present only when NMDA receptor (NMDAR) current was blocked by Mg2+. Analysis of miniature EPSCs indicated that many active sites on NMDA-treated neurons lacked functional AMPA and kainate receptor (AMPA/KAR) currents, and AMPA/KAR:NMDAR current ratios of evoked EPSCs were also significantly reduced. In addition, the normal downregulation of NMDAR decay time in sSC neurons at P11 was absent after NMDA treatment. Nevertheless, neither AMPA nor NMDA receptor subunit expression was altered by NMDA treatment, and experiments with the NMDAR antagonist ifenprodil suggested that incorporation of NR2A-containing NMDARs at the sSC synapses was unperturbed. Thus, disrupting but not blocking NMDARs suppresses the development of AMPA/KAR currents. The absence of the P11 NMDAR current downregulation is likely a secondary effect resulting from the reduction of AMPA/KAR function. Chronic agonist application reduces but does not eliminate NMDAR conductances. Therefore these data support an active role for NMDAR currents in synaptic development. Prolonged NMDA treatment in vivo, which couples reduced postsynaptic Ca2+ responses with normally developing afferent activity, produces a long-lasting synaptic depression and stalls glutamatergic synaptogenesis, suggesting that the correlation between robust NMDAR activation and afferent activity is an essential component during normal development.

Whole-cell recording in the superficial layers of the developing superior colliculus (sSC) reveals a large drop in NMDA receptor (NMDAR) current decay time synchronized across all neurons and occurring consistently between P10 and P11. We show that blocking the Ca2+/calmodulin–dependent phosphatase calcineurin (CaN) in the postsynaptic neuron can abolish this drop. The regulation is induced prematurely by 1–2 hr of electrical stimulation in P10 collicular slices only if CaN and NMDAR currents can be activated in the neuron. These data suggest that a long-lasting, CaN-mediated control of NMDAR kinetics is rapidly initiated by heightened activity of the NMDAR itself and demonstrate a novel developmental and tonic function of CaN that can play an important role in modulating the plasticity of the developing CNS.