A common single nucleotide variant in the cytokine receptor-like factor-3 (CRLF3) gene causes neuronal deficits in human and mouse cells.

Single nucleotide variants in the general population are common genomic alterations, where the majority are presumed to be silent polymorphisms without known clinical significance. Using human induced pluripotent stem cell (hiPSC) cerebral organoid modeling of the 1.4 megabase Neurofibromatosis type 1 (NF1) deletion syndrome, we previously discovered that the cytokine receptor-like factor-3 (CRLF3) gene, which is co-deleted with the NF1 gene, functions as a major regulator of neuronal maturation. Moreover, children with NF1 and the CRLF3L389P variant have greater autism burden, suggesting that this gene might be important for neurologic function. To explore the functional consequences of this variant, we generated CRLF3L389P-mutant hiPSC lines and Crlf3L389P-mutant genetically engineered mice. While this variant does not impair protein expression, brain structure, or mouse behavior, CRLF3L389P-mutant human cerebral organoids and mouse brains exhibit impaired neuronal maturation and dendrite formation. In addition, Crlf3L389P-mutant mouse neurons have reduced dendrite lengths and branching, without any axonal deficits. Moreover, Crlf3L389P-mutant mouse hippocampal neurons have decreased firing rates and synaptic current amplitudes relative to wild type controls. Taken together, these findings establish the CRLF3L389P variant as functionally deleterious and suggest that it may be a neurodevelopmental disease modifier.

Periodic and aperiodic changes to cortical EEG in response to pharmacological manipulation.

Cortical electroencephalograms (EEGs) may help understanding of neuropsychiatric illness and new treatment mechanisms. The aperiodic component (1/f) of EEG power spectra is often treated as noise, but recent studies suggest that changes to the aperiodic exponent of power spectra may reflect changes in excitation/inhibition balance, a concept linked to antidepressant effects, epilepsy, autism, and other clinical conditions. One confound of previous studies is behavioral state, because factors associated with behavioral state other than excitation/inhibition ratio may alter EEG parameters. Thus, to test the robustness of the aperiodic exponent as a predictor of excitation/inhibition ratio, we analyzed video-EEG during active exploration in mice of both sexes during various pharmacological manipulations with the fitting oscillations and one over f (FOOOF) algorithm. We found that GABAA receptor (GABAAR)-positive allosteric modulators increased the aperiodic exponent, consistent with the hypothesis that an increased exponent signals enhanced cortical inhibition, but other drugs (ketamine and GABAAR antagonists at subconvulsive doses) did not follow the prediction. To tilt excitation/inhibition ratio more selectively toward excitation, we suppressed the activity of parvalbumin-positive interneurons with Designer Receptors Exclusively Activated by Designer Drugs (DREADDs). Contrary to our expectations, circuit disinhibition with the DREADD increased the aperiodic exponent. We conclude that the aperiodic exponent of EEG power spectra does not yield a universally reliable marker of cortical excitation/inhibition ratio.NEW & NOTEWORTHY Neuropsychiatric illness may be associated with altered excitation/inhibition balance. A single electroencephalogram (EEG) parameter, the aperiodic exponent of power spectra, may predict the ratio between excitation and inhibition. Here, we use cortical EEGs in mice to evaluate this hypothesis, using pharmacological manipulations of known mechanism. We show that the aperiodic exponent of EEG power spectra is not a reliable marker of excitation/inhibition ratio. Thus, alternative markers of this ratio must be sought.

Cognitive deficits and impaired hippocampal long-term potentiation in KATP-induced DEND syndrome.

ATP-sensitive potassium (KATP) gain-of-function (GOF) mutations cause neonatal diabetes, with some individuals exhibiting developmental delay, epilepsy, and neonatal diabetes (DEND) syndrome. Mice expressing KATP-GOF mutations pan-neuronally (nKATP-GOF) demonstrated sensorimotor and cognitive deficits, whereas hippocampus-specific hKATP-GOF mice exhibited mostly learning and memory deficiencies. Both nKATP-GOF and hKATP-GOF mice showed altered neuronal excitability and reduced hippocampal long-term potentiation (LTP). Sulfonylurea therapy, which inhibits KATP, mildly improved sensorimotor but not cognitive deficits in KATP-GOF mice. Mice expressing KATP-GOF mutations in pancreatic ß-cells developed severe diabetes but did not show learning and memory deficits, suggesting neuronal KATP-GOF as promoting these features. These findings suggest a possible origin of cognitive dysfunction in DEND and the need for novel drugs to treat neurological features induced by neuronal KATP-GOF.

The Taylor Family Institute at Washington University: Novel Treatments in Psychiatry.

Efforts to develop more effective treatments for psychiatric illnesses will require innovative approaches that address changes in brain networks underlying cognition, emotion and motivation, cardinal symptoms that cut across all psychiatric disorders. This effort will include new molecular entities that modulate neuronal excitability, synapses, cellular stress and inflammation. Other opportunities will come from repurposing existing treatments. This overview highlights current and future goals in treatment development in the Taylor Family Institute for Innovative Psychiatric Research at Washington University.

A Proinflammatory Stimulus Disrupts Hippocampal Plasticity and Learning via Microglial Activation and 25-Hydroxycholesterol.

Inflammatory cells, including macrophages and microglia, synthesize and release the oxysterol 25-hydroxycholesterol (25HC), which has antiviral and immunomodulatory properties. Here, we examined the effects of lipopolysaccharide (LPS), an activator of innate immunity, on 25HC production in microglia, and the effects of LPS and 25HC on CA1 hippocampal synaptic plasticity and learning. In primary microglia, LPS markedly increases the expression of cholesterol 25-hydroxylase (Ch25h), the key enzyme involved in 25HC synthesis, and increases the levels of secreted 25HC. Wild-type microglia produced higher levels of 25HC than Ch25h knock-out (KO) microglia with or without LPS. LPS treatment also disrupts long-term potentiation (LTP) in hippocampal slices via induction of a form of NMDA receptor-dependent metaplasticity. The inhibitory effects of LPS on LTP were mimicked by exogenous 25HC, and were not observed in slices from Ch25h KO mice. In vivo, LPS treatment also disrupts LTP and inhibits one-trial learning in wild-type mice, but not Ch25h KO mice. These results demonstrate that the oxysterol 25HC is a key modulator of synaptic plasticity and memory under proinflammatory stimuli.SIGNIFICANCE STATEMENT Neuroinflammation is thought to contribute to cognitive impairment in multiple neuropsychiatric illnesses. In this study, we found that a proinflammatory stimulus, LPS, disrupts hippocampal LTP via a metaplastic mechanism. The effects of LPS on LTP are mimicked by the oxysterol 25-hydroxycholesterol (25HC), an immune mediator synthesized in brain microglia. Effects of LPS on both synaptic plasticity and one-trial inhibitory avoidance learning are eliminated in mice deficient in Ch25h (cholesterol 25-hydroxylase), the primary enzyme responsible for endogenous 25HC synthesis. Thus, these results indicate that 25HC is a key mediator of the effects of an inflammatory stimulus on hippocampal function and open new potential avenues to overcome the effects of neuroinflammation on brain function.

Correcting mitochondrial fusion by manipulating mitofusin conformations.

Mitochondria are dynamic organelles that exchange contents and undergo remodelling during cyclic fusion and fission. Genetic mutations in MFN2 (the gene encoding mitofusin 2) interrupt mitochondrial fusion and cause the untreatable neurodegenerative condition Charcot-Marie-Tooth disease type 2A (CMT2A). It has not yet been possible to directly modulate mitochondrial fusion, in part because the structural basis of mitofusin function is not completely understood. Here we show that mitofusins adopt either a fusion-constrained or a fusion-permissive molecular conformation, directed by specific intramolecular binding interactions, and demonstrate that mitofusin-dependent mitochondrial fusion can be regulated in mouse cells by targeting these conformational transitions. On the basis of this model, we engineered a cell-permeant minipeptide to destabilize the fusion-constrained conformation of mitofusin and promote the fusion-permissive conformation, reversing mitochondrial abnormalities in cultured fibroblasts and neurons that harbour CMT2A-associated genetic defects. The relationship between the conformational plasticity of mitofusin 2 and mitochondrial dynamism reveals a central mechanism that regulates mitochondrial fusion, the manipulation of which can correct mitochondrial pathology triggered by defective or imbalanced mitochondrial dynamics.

A complement-microglial axis drives synapse loss during virus-induced memory impairment.

Over 50% of patients who survive neuroinvasive infection with West Nile virus (WNV) exhibit chronic cognitive sequelae. Although thousands of cases of WNV-mediated memory dysfunction accrue annually, the mechanisms responsible for these impairments are unknown. The classical complement cascade, a key component of innate immune pathogen defence, mediates synaptic pruning by microglia during early postnatal development. Here we show that viral infection of adult hippocampal neurons induces complement-mediated elimination of presynaptic terminals in a murine WNV neuroinvasive disease model. Inoculation of WNV-NS5-E218A, a WNV with a mutant NS5(E218A) protein leads to survival rates and cognitive dysfunction that mirror human WNV neuroinvasive disease. WNV-NS5-E218A-recovered mice (recovery defined as survival after acute infection) display impaired spatial learning and persistence of phagocytic microglia without loss of hippocampal neurons or volume. Hippocampi from WNV-NS5-E218A-recovered mice with poor spatial learning show increased expression of genes that drive synaptic remodelling by microglia via complement. C1QA was upregulated and localized to microglia, infected neurons and presynaptic terminals during WNV neuroinvasive disease. Murine and human WNV neuroinvasive disease post-mortem samples exhibit loss of hippocampal CA3 presynaptic terminals, and murine studies revealed microglial engulfment of presynaptic terminals during acute infection and after recovery. Mice with fewer microglia (Il34(-/-) mice with a deficiency in IL-34 production) or deficiency in complement C3 or C3a receptor were protected from WNV-induced synaptic terminal loss. Our study provides a new murine model of WNV-induced spatial memory impairment, and identifies a potential mechanism underlying neurocognitive impairment in patients recovering from WNV neuroinvasive disease.

Oxysterols Modulate the Acute Effects of Ethanol on Hippocampal N-Methyl-d-Aspartate Receptors, Long-Term Potentiation, and Learning.

Ethanol is a noncompetitive inhibitor of N-methyl-d-aspartate receptors (NMDARs) and acutely disrupts hippocampal synaptic plasticity and learning. In the present study, we examined the effects of oxysterol positive allosteric modulators (PAMs) of NMDARs on ethanol-mediated inhibition of NMDARs, block of long-term potentiation (LTP) and long-term depression (LTD) in rat hippocampal slices, and defects in one-trial learning in vivo. We found that 24S-hydroxycholesterol and a synthetic oxysterol analog, SGE-301, overcame effects of ethanol on NMDAR-mediated synaptic responses in the CA1 region but did not alter acute effects of ethanol on LTD; the synthetic oxysterol, however, overcame acute inhibition of LTP. In addition, both oxysterols overcame persistent effects of ethanol on LTP in vitro, and the synthetic analog reversed defects in one-trial inhibitory avoidance learning in vivo. These results indicate that effects of ethanol on both LTP and LTD arise by complex mechanisms beyond NMDAR antagonism and that oxysterol NMDAR PAMS may represent a novel approach for preventing and reversing acute ethanol-mediated changes in cognition. SIGNIFICANCE STATEMENT: Ethanol acutely inhibits hippocampal NMDARs, LTP, and learning. This study found that certain oxysterols that are NMDAR-positive allosteric modulators can overcome the acute effects of ethanol on NMDARs, LTP, and learning. Oxysterols differ in their effects from agents that inhibit integrated cellular stress responses.

δ subunit-containing GABAA IPSCs are driven by both synaptic and diffusional GABA in mouse dentate granule neurons.

KEY POINTS: Current views suggest γ2 subunit-containing GABAA receptors mediate phasic IPSCs while extrasynaptic δ subunits mediate diffusional IPSCs and tonic current. We have re-examined the roles of the two receptor populations using mice with picrotoxin resistance engineered into receptors containing the δ subunit. Using pharmacological separation, we find that in general δ and γ IPSCs are modulated in parallel by manipulations of transmitter output and diffusion, with evidence favouring modestly more diffusional contribution to δ IPSCs. Our findings also reveal that spontaneous δ IPSCs are mainly driven by channel deactivation, rather than by diffusion of GABA. Understanding the functional contributions of the two receptor classes may help us understand the actions of drug therapies with selective effects on one population over the other. ABSTRACT: GABAA receptors mediate transmission throughout the central nervous system and typically contain a δ subunit (δ receptors) or a γ2 subunit (γ2 receptors). δ IPSCs decay slower than γ2 IPSCs, but the reasons are unclear. Transmitter diffusion, rebinding, or slow deactivation kinetics of channels are candidates. We used gene editing to confer picrotoxin resistance on δ receptors in mice, then pharmacologically isolated δ receptors in mouse dentate granule cells to explore IPSCs. γ2 and δ components of IPSCs were modulated similarly by presynaptic manipulations and manipulations of transmitter lifetime, suggesting that GABA release recruits δ receptors proportionally to γ2 receptors. δ IPSCs showed more sensitivity to altered transmitter release and to a rapidly dissociating antagonist, suggesting an additional spillover contribution. Reducing GABA diffusion with 5% dextran increased the peak amplitude and decreased the decay of evoked δ IPSCs but had no effect on δ or dual-component (mainly γ2-driven) spontaneous IPSCs, suggesting that GABA actions can be local for both receptor types. Rapid application of varied [GABA] onto nucleated patches from dentate granule cells demonstrated a deactivation rate of δ receptors similar to that of δ spontaneous IPSCs, consistent with the idea that deactivation and local GABA actions drive δ spontaneous IPSCs. Overall, our results indicate that δ IPSCs are activated by both synaptic and diffusional GABA. Our results are consistent with a functional relationship between δ and γ2 GABAA receptors akin to that of slow NMDA and fast AMPA EPSCs at glutamate synapses.

Chemogenetic Isolation Reveals Synaptic Contribution of δ GABAA Receptors in Mouse Dentate Granule Neurons.

Two major GABAA receptor classes mediate ionotropic GABA signaling, those containing a δ subunit and those with a γ2 subunit. The classical viewpoint equates γ2-containing receptors with IPSCs and δ-containing receptors with tonic inhibition because of differences in receptor localization, but significant questions remain because the populations cannot be pharmacologically separated. We removed this barrier using gene editing to confer a point mutation on the δ subunit in mice, rendering receptors containing the subunit picrotoxin resistant. By pharmacologically isolating δ-containing receptors, our results demonstrate their contribution to IPSCs in dentate granule neurons and weaker contributions to thalamocortical IPSCs. Despite documented extrasynaptic localization, we found that receptor localization does not preclude participation in isolated IPSCs, including mIPSCs. Further, phasic inhibition from δ subunit-containing receptors strongly inhibited summation of EPSPs, whereas tonic activity had little impact. In addition to any role that δ-containing receptors may play in canonical tonic inhibition, our results highlight a previously underestimated contribution of δ-containing receptors to phasic inhibition.SIGNIFICANCE STATEMENT GABAA receptors play key roles in transient and tonic inhibition. The prevailing view suggests that synaptic γ2-containing GABAA receptors drive phasic inhibition, whereas extrasynaptic δ-containing receptors mediate tonic inhibition. To re-evaluate the impact of δ receptors, we took a chemogenetic approach that offers a sensitive method to probe the synaptic contribution of δ-containing receptors. Our results reveal that localization does not strongly limit the contribution of δ receptors to IPSCs and that δ receptors make an unanticipated robust contribution to phasic inhibition.

Positive Allosteric Modulation as a Potential Therapeutic Strategy in Anti-NMDA Receptor Encephalitis.

N-methyl-d-aspartate receptors (NMDARs) are ionotropic glutamate receptors important for synaptic plasticity, memory, and neuropsychiatric health. NMDAR hypofunction contributes to multiple disorders, including anti-NMDAR encephalitis (NMDARE), an autoimmune disease of the CNS associated with GluN1 antibody-mediated NMDAR internalization. Here we characterize the functional/pharmacological consequences of exposure to CSF from female human NMDARE patients on NMDAR function, and we characterize the effects of intervention with recently described positive allosteric modulators (PAMs) of NMDARs. Incubation (48 h) of rat hippocampal neurons of both sexes in confirmed NMDARE patient CSF, but not control CSF, attenuated NMDA-induced current. Residual NMDAR function was characterized by lack of change in channel open probability, indiscriminate loss of synaptic and extrasynaptic NMDARs, and indiscriminate loss of GluN2B-containing and GluN2B-lacking NMDARs. NMDARs tagged with N-terminal pHluorin fluorescence demonstrated loss of surface receptors. Thus, function of residual NMDARs following CSF exposure was indistinguishable from baseline, and deficits appear wholly accounted for by receptor loss. Coapplication of CSF and PAMs of NMDARs (SGE-301 or SGE-550, oxysterol-mimetic) for 24 h restored NMDAR function following 24 h incubation in patient CSF. Curiously, restoration of NMDAR function was observed despite washout of PAMs before electrophysiological recordings. Subsequent experiments suggested that residual allosteric potentiation of NMDAR function explained the persistent rescue. Further studies of the pathogenesis of NMDARE and intervention with PAMs may inform new treatments for NMDARE and other disorders associated with NMDAR hypofunction.SIGNIFICANCE STATEMENT Anti-N-methyl-d-aspartate receptor encephalitis (NMDARE) is increasingly recognized as an important cause of sudden-onset psychosis and other neuropsychiatric symptoms. Current treatment leaves unmet medical need. Here we demonstrate cellular evidence that newly identified positive allosteric modulators of NMDAR function may be a viable therapeutic strategy.

Differential Presynaptic ATP Supply for Basal and High-Demand Transmission.

The relative contributions of glycolysis and oxidative phosphorylation to neuronal presynaptic energy demands are unclear. In rat hippocampal neurons, ATP production by either glycolysis or oxidative phosphorylation alone sustained basal evoked synaptic transmission for up to 20 min. However, combined inhibition of both ATP sources abolished evoked transmission. Neither action potential propagation failure nor depressed Ca2+ influx explained loss of evoked synaptic transmission. Rather, inhibition of ATP synthesis caused massive spontaneous vesicle exocytosis, followed by arrested endocytosis, accounting for the disappearance of evoked postsynaptic currents. In contrast to its weak effects on basal transmission, inhibition of oxidative phosphorylation alone depressed recovery from vesicle depletion. Local astrocytic lactate shuttling was not required. Instead, either ambient monocarboxylates or neuronal glycolysis was sufficient to supply requisite substrate. In summary, basal transmission can be sustained by glycolysis, but strong presynaptic demands are met preferentially by oxidative phosphorylation, which can be maintained by bulk but not local monocarboxylates or by neuronal glycolysis.SIGNIFICANCE STATEMENT Neuronal energy levels are critical for proper CNS function, but the relative roles for the two main sources of ATP production, glycolysis and oxidative phosphorylation, in fueling presynaptic function in unclear. Either glycolysis or oxidative phosphorylation can fuel low-frequency synaptic function and inhibiting both underlies loss of synaptic transmission via massive vesicle release and subsequent failure to endocytose lost vesicles. Oxidative phosphorylation, fueled by either glycolysis or endogenously released monocarboxylates, can fuel more metabolically demanding tasks such as vesicle recovery after depletion. Our work demonstrates the flexible nature of fueling presynaptic function to maintain synaptic function.

Ketamine: NMDA Receptors and Beyond.

Human studies examining the effects of the dissociative anesthetic ketamine as a model for psychosis and as a rapidly acting antidepressant have spurred great interest in understanding ketamine's actions at molecular, cellular, and network levels. Although ketamine has unequivocal uncompetitive inhibitory effects on N-methyl-d-aspartate receptors (NMDARs) and may preferentially alter the function of NMDARs on interneurons, recent work has questioned whether block of NMDARs is critical for its mood enhancing actions. In this viewpoint, we examine the evolving literature on ketamine supporting NMDARs as important triggers for certain psychiatric effects and the possibility that the antidepressant trigger is unrelated to NMDARs. The rapidly evolving story of ketamine offers great hope for untangling and treating the biology of both depressive and psychotic illnesses.

Contributions of space-clamp errors to apparent time-dependent loss of Mg2+ block induced by NMDA.

N-methyl-d-aspartate receptors (NMDARs) govern synaptic plasticity, development, and neuronal response to insult. Prolonged activation of NMDARs such as during an insult may activate secondary currents or modulate Mg2+ sensitivity, but the conditions under which these occur are not fully defined. We reexamined the effect of prolonged NMDAR activation in juvenile mouse hippocampal slices. NMDA (10 µM) elicited current with the expected negative-slope conductance in the presence of 1.2 mM Mg2+ However, several minutes of continued NMDA exposure elicited additional inward current at -70 mV. A higher concentration of NMDA (100 µM) elicited the current more rapidly. The additional current was not dependent on Ca2+, network activity, or metabotropic NMDAR function and did not persist on agonist removal. Voltage ramps revealed no alteration of either reversal potential or NMDA-elicited conductance between -30 mV and +50 mV. The result was a more linear NMDA current-voltage relationship. The current linearization was also induced in interneurons and in mature dentate granule neurons but not immature dentate granule cells, dissociated cultured hippocampal neurons, or nucleated patches excised from CA1 pyramidal neurons. Comparative simulations of NMDA application to a CA1 pyramidal neuron and to a cultured neuron revealed that linearization can be explained by space-clamp errors arising from gradual recruitment of distal dendritic NMDARs. We conclude that persistent secondary currents do not strongly contribute to NMDAR responses in juvenile mouse hippocampus and careful discernment is needed to exclude contributions of clamp artifacts to apparent secondary currents.NEW & NOTEWORTHY We report that upon sustained activation of NMDARs in juvenile mouse hippocampal neurons there is apparent loss of Mg2+ block at negative membrane potentials. However, the phenomenon is explained by loss of dendritic voltage clamp, leading to a linear current-voltage relationship. Our results give a specific example of how spatial voltage errors in voltage-clamp recordings can readily be misinterpreted as biological modulation.

Loss of Local Astrocyte Support Disrupts Action Potential Propagation and Glutamate Release Synchrony from Unmyelinated Hippocampal Axon Terminals In Vitro.

Neuron-astrocyte interactions are critical for proper CNS development and function. Astrocytes secrete factors that are pivotal for synaptic development and function, neuronal metabolism, and neuronal survival. Our understanding of this relationship, however, remains incomplete due to technical hurdles that have prevented the removal of astrocytes from neuronal circuits without changing other important conditions. Here we overcame this obstacle by growing solitary rat hippocampal neurons on microcultures that were comprised of either an astrocyte bed (+astrocyte) or a collagen bed (-astrocyte) within the same culture dish. -Astrocyte autaptic evoked EPSCs, but not IPSCs, displayed an altered temporal profile, which included increased synaptic delay, increased time to peak, and severe glutamate release asynchrony, distinct from previously described quantal asynchrony. Although we observed minimal alteration of the somatically recorded action potential waveform, action potential propagation was altered. We observed a longer latency between somatic initiation and arrival at distal locations, which likely explains asynchronous EPSC peaks, and we observed broadening of the axonal spike, which likely underlies changes to evoked EPSC onset. No apparent changes in axon structure were observed, suggesting altered axonal excitability. In conclusion, we propose that local astrocyte support has an unappreciated role in maintaining glutamate release synchrony by disturbing axonal signal propagation. SIGNIFICANCE STATEMENT: Certain glial cell types (oligodendrocytes, Schwann cells) facilitate the propagation of neuronal electrical signals, but a role for astrocytes has not been identified despite many other functions of astrocytes in supporting and modulating neuronal signaling. Under identical global conditions, we cultured neurons with or without local astrocyte support. Without local astrocytes, glutamate transmission was desynchronized by an alteration of the waveform and arrival time of axonal action potentials to synaptic terminals. GABA transmission was not disrupted. The disruption did not involve detectable morphological changes to axons of glutamate neurons. Our work identifies a developmental role for astrocytes in the temporal precision of excitatory signals.

Comparison of Steroid Modulation of Spontaneous Inhibitory Postsynaptic Currents in Cultured Hippocampal Neurons and Steady-State Single-Channel Currents from Heterologously Expressed α1ß2γ2L GABA(A) Receptors.

Neuroactive steroids are efficacious modulators of γ-aminobutyric acid type A receptor (GABA(A)) receptor function. The effects of steroids on the GABA(A) receptor are typically determined by comparing steady-state single-channel open probability or macroscopic peak responses elicited by GABA in the absence and presence of a steroid. Due to differences in activation conditions (exposure duration, concentration of agonist), it is not obvious whether modulation measured using typical experimental protocols can be used to accurately predict the effect of a modulator on native receptors under physiologic conditions. In the present study, we examined the effects of 14 neuroactive steroids and analogs on the properties of spontaneous inhibitory postsynaptic currents (sIPSCs) in cultured rat hippocampal neurons. The goal was to determine whether the magnitude of modulation of the decay time course of sIPSCs correlates with the extent of modulation and kinetic properties of potentiation as determined in previous single-channel studies. The steroids were selected to cover a wide range of efficacy on heterologously expressed rat α1ß2γ2L GABA(A) receptors, ranging from essentially inert to highly efficacious (strong potentiators of single-channel and macroscopic peak responses). The data indicate a strong correlation between prolongation of the decay time course of sIPSCs and potentiation of single-channel open probability. Furthermore, changes in intracluster closed time distributions were the single best predictor of prolongation of sIPSCs. We infer that the information obtained in steady-state single-channel recordings can be used to forecast modulation of synaptic currents.

Endogenous 24S-hydroxycholesterol modulates NMDAR-mediated function in hippocampal slices.

N-methyl-D-aspartate receptors (NMDARs), a major subtype of glutamate receptors mediating excitatory transmission throughout the central nervous system (CNS), play critical roles in governing brain function and cognition. Because NMDAR dysfunction contributes to the etiology of neurological and psychiatric disorders including stroke and schizophrenia, NMDAR modulators are potential drug candidates. Our group recently demonstrated that the major brain cholesterol metabolite, 24S-hydroxycholesterol (24S-HC), positively modulates NMDARs when exogenously administered. Here, we studied whether endogenous 24S-HC regulates NMDAR activity in hippocampal slices. In CYP46A1(-/-) (knockout; KO) slices where endogenous 24S-HC is greatly reduced, NMDAR tone, measured as NMDAR-to-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) excitatory postsynaptic current (EPSC) ratio, was reduced. This difference translated into more NMDAR-driven spiking in wild-type (WT) slices compared with KO slices. Application of SGE-301, a 24S-HC analog, had comparable potentiating effects on NMDAR EPSCs in both WT and KO slices, suggesting that endogenous 24S-HC does not saturate its NMDAR modulatory site in ex vivo slices. KO slices did not differ from WT slices in either spontaneous neurotransmission or in neuronal intrinsic excitability, and exhibited LTP indistinguishable from WT slices. However, KO slices exhibited higher resistance to persistent NMDAR-dependent depression of synaptic transmission induced by oxygen-glucose deprivation (OGD), an effect restored by SGE-301. Together, our results suggest that loss of positive NMDAR tone does not elicit compensatory changes in excitability or transmission, but it protects transmission against NMDAR-mediated dysfunction. We expect that manipulating this endogenous NMDAR modulator may offer new treatment strategies for neuropsychiatric dysfunction.

Fast phasic release properties of dopamine studied with a channel biosensor.

Few other neurotransmitters are of as intense interest to neuropsychiatry and neurology as dopamine, yet existing techniques to monitor dopamine release leave an important spatiotemporal gap in our understanding. Electrochemistry and fluorescence imaging tools have been developed to fill the gap, but these methods have important limitations. We circumvent these limitations by introducing a dopamine-gated chloride channel into rat dorsal striatal medium spiny neurons, targets of strong dopamine innervation, thereby transforming dopamine from a slow transmitter into a fast transmitter and revealing new opportunities for studying moment-to-moment regulation of dopamine release. We demonstrate pharmacological and biophysical properties of the channel that make it suitable for fast, local dopamine measurements, and we demonstrate for the first time spontaneous and evoked responses to vesicular dopamine release in the dorsal striatum. Evoked dopamine currents were separated into a fast, monosynaptic component and a slower-rising and decaying disynaptic component mediated by nicotinic receptor activation. In summary, LGC-53 represents a dopamine biosensor with properties suitable for temporal separation of distinct dopamine signals in targets of dopamine innervation.

A mechanism regulating G protein-coupled receptor signaling that requires cycles of protein palmitoylation and depalmitoylation.

Reversible attachment and removal of palmitate or other long-chain fatty acids on proteins has been hypothesized, like phosphorylation, to control diverse biological processes. Indeed, palmitate turnover regulates Ras trafficking and signaling. Beyond this example, however, the functions of palmitate turnover on specific proteins remain poorly understood. Here, we show that a mechanism regulating G protein-coupled receptor signaling in neuronal cells requires palmitate turnover. We used hexadecyl fluorophosphonate or palmostatin B to inhibit enzymes in the serine hydrolase family that depalmitoylate proteins, and we studied R7 regulator of G protein signaling (RGS)-binding protein (R7BP), a palmitoylated allosteric modulator of R7 RGS proteins that accelerate deactivation of Gi/o class G proteins. Depalmitoylation inhibition caused R7BP to redistribute from the plasma membrane to endomembrane compartments, dissociated R7BP-bound R7 RGS complexes from Gi/o-gated G protein-regulated inwardly rectifying K(+) (GIRK) channels and delayed GIRK channel closure. In contrast, targeting R7BP to the plasma membrane with a polybasic domain and an irreversibly attached lipid instead of palmitate rendered GIRK channel closure insensitive to depalmitoylation inhibitors. Palmitate turnover therefore is required for localizing R7BP to the plasma membrane and facilitating Gi/o deactivation by R7 RGS proteins on GIRK channels. Our findings broaden the scope of biological processes regulated by palmitate turnover on specific target proteins. Inhibiting R7BP depalmitoylation may provide a means of enhancing GIRK activity in neurological disorders.

Quantification of bursting and synchrony in cultured hippocampal neurons.

It is widely appreciated that neuronal networks exhibit patterns of bursting and synchrony that are not captured by simple measures such as average spike rate. These patterns can encode information or represent pathological behavior such as seizures. However, methods for quantifying bursting and synchrony are not agreed upon and can be confounded with spike rate measures. Previous validation has largely relied on in silico networks and single experimental conditions. How published measures of bursting and synchrony perform when applied to biological networks of varied average spike rate and subjected to varied experimental challenges is unclear. In multielectrode array recordings of network activity, we found that two mechanistically distinct drugs, cyclothiazide and bicuculline, produced equivalent increases in average spike rate but differed in bursting and synchrony. We applied several measures of bursting to the recordings (2 threshold interval methods and a surprise-based method) and found that a measure based on an average critical interval, adjusted for the array-wide spike rate, performed best in quantifying differential drug effects. To quantify synchrony, we compared a coefficient of variation-based measure, the recently proposed spike time tiling coefficient, the SPIKE-distance measure, and a global synchrony index. The spike time tiling coefficient, the SPIKE-distance measure, and the global synchrony index all captured a difference between drugs with the best performance exhibited by the global synchrony index. In summary, our exploration should aid other investigators by highlighting strengths and limitations of current methods.