Tereza Smejkalova

Estradiol Acutely Potentiates Hippocampal Excitatory Synaptic Transmission through a Presynaptic Mechanism

Although recent evidence suggests that the hippocampus is a source of 17β-estradiol (E2), the physiological role of this neurosteroid E2, as distinct from ovarian E2, is unknown. One likely function of neurosteroid E2 is to acutely potentiate excitatory synaptic transmission, but the mechanism of this effect is not well understood. Using whole-cell voltage-clamp recording of synaptically evoked EPSCs in adult rat hippocampal slices, we show that, in contrast to the conclusions of previous studies, E2 potentiates excitatory transmission through a presynaptic mechanism. We find that E2 acutely potentiates EPSCs by increasing the probability of glutamate release specifically at inputs with low initial release probability. This effect is mediated by estrogen receptor β (ERβ) acting as a monomer, whereas ERα is not required. We further show that the E2-induced increase in glutamate release is attributable primarily to increased individual vesicle release probability and is associated with higher average cleft glutamate concentration. These two findings together argue strongly that E2 promotes multivesicular release, which has not been shown before in the adult hippocampus. The rapid time course of acute EPSC potentiation and its concentration dependence suggest that locally synthesized neurosteroid E2 may activate this effect in vivo.

Estrogen Mobilizes a Subset of Estrogen Receptor-α-Immunoreactive Vesicles in Inhibitory Presynaptic Boutons in Hippocampal CA1

Although the classical mechanism of estrogen action involves activation of nuclear transcription factor receptors, estrogen also has acute effects on neuronal signaling that occur too rapidly to involve gene expression. These rapid effects are likely to be mediated by extranuclear estrogen receptors associated with the plasma membrane and/or cytoplasmic organelles. Here we used a combination of serial-section electron microscopic immunocytochemistry, immunofluorescence, and Western blotting to show that estrogen receptor-α is associated with clusters of vesicles in perisomatic inhibitory boutons in hippocampal CA1 and that estrogen treatment mobilizes these vesicle clusters toward synapses. Estrogen receptor-α is present in approximately one-third of perisomatic inhibitory boutons, and specifically in those that express cholecystokinin, not parvalbumin. We also found a high degree of extranuclear estrogen receptor-α colocalization with neuropeptide Y. Our results suggest a novel mode of estrogen action in which a subset of vesicles within a specific population of inhibitory boutons responds directly to estrogen by moving toward synapses. The mobilization of these vesicles may influence acute effects of estrogen mediated by estrogen receptor-α signaling at inhibitory synapses.

Multiple ERβ antisera label in ERβ knockout and null mouse tissues

In the process of characterizing a custom-made affinity-purified antiserum for estrogen receptor beta (ERbeta), ck5912, we used a number of common tests for specificity of ck5912 along with that of 8 commercially available ERbeta antisera: Affinity Bioreagents PA1-310B, Invitrogen D7N, Upstate 06-629, Santa Cruz H150, Y19, L20, 1531, and Abcam 9.88. We tested their recognition of recombinant ERbeta (rERbeta) versus rERalpha, ERbeta versus ERalpha transfected into cell lines, as well as labeling in wildtype (WT) versus estrogen receptor beta knockout (betaERKO) and null (ERbeta(ST)(L-/L-)) mouse ovary, hypothalamus, and hippocampus. To our surprise, we found that while most of these antisera passed some tests, giving the initial impression of specificity, western blot analysis showed that all of them recognized apparently identical protein bands in WT, betaERKO and ERbeta(ST)(L-/L-) tissues. We share these results with the goal of helping other researchers avoid pitfalls in interpretation that could come from use of these ERbeta antisera.

Estradiol Facilitates the Release of Neuropeptide Y to Suppress Hippocampus-Dependent Seizures

About one-third of women with epilepsy have a catamenial seizure pattern, in which seizures fluctuate with the menstrual cycle. Catamenial seizures occur more frequently when the ratio of circulating estradiol to progesterone is high, suggesting that estradiol is proconvulsant. We used adult female rats to test how estradiol-induced suppression of GABAergic inhibition in the hippocampus affects behavioral seizures induced by kainic acid. As expected, estradiol decreased the latency to initiate seizures, indicating increased seizure susceptibility. At the same time, however, estradiol also shortened the duration of late-stage seizures, indicating decreased seizure severity. Additional analyses showed that the decrease in seizure severity was attributable to greater release of the anticonvulsant neuropeptide, neuropeptide Y (NPY). First, blocking hippocampal NPY during seizures eliminated the estradiol-induced decrease in seizure duration. Second, light and electron microscopic studies indicated that estradiol increases the potentially releasable pool of NPY in inhibitory presynaptic boutons and facilitates the release of NPY from inhibitory boutons during seizures. Finally, the presence of estrogen receptor-α on large dense-core vesicles (LDCVs) in the hippocampus suggests that estradiol could facilitate neuropeptide release by acting directly on LDCVs themselves. Understanding how estradiol regulates NPY-containing LDCVs could point to molecular targets for novel anticonvulsant therapies.

Cholesterol modulates open probability and desensitization of NMDA receptors

NMDA receptors (NMDARs) are tetrameric cation channels permeable to calcium; they mediate excitatory synaptic transmission in the CNS and their excessive activation can lead to neurodegeneration. Although these receptors are in direct contact with plasma membrane, lipid–NMDAR interactions are little understood. Using cultured rat cerebellar granule cells, we show that acute and chronic pretreatments resulting in cell cholesterol depletion profoundly diminish NMDAR responses and increase NMDAR desensitization, and also that cholesterol enrichment potentiates NMDAR responses; however, cholesterol manipulation has no effect on the amplitude of AMPA/kainate receptor responses. Diminution of NMDAR responses by cholesterol depletion is the result of a reduction of the ion channel open probability, whereas the increase in receptor desensitization is the result of an increase in the rate constant of entry into the desensitized state. These results demonstrate the physiological role of membrane lipids in the modulation of NMDAR activity.

Distribution and posttranslational modification of synaptic ERα in the adult female rat hippocampus.

Acute 17β-estradiol (E2) signaling in the brain is mediated by extranuclear estrogen receptors. Here we used biochemical methods to investigate the distribution, posttranslational modification, and E2 regulation of estrogen receptor-α (ERα) in synaptosomal fractions isolated by differential centrifugation from the adult female rat hippocampus. We find that ERα is concentrated presynaptically and is highly enriched with synaptic vesicles. Immunoisolation of vesicles using vesicle subtype-specific markers showed that ERα is associated with both glutamate and γ-aminobutyric acid-containing neurotransmitter vesicles as well as with some large dense core vesicles. Experiments using broad spectrum and residue-specific phosphatases indicated that a portion of ERα in synaptosomal fractions is phosphorylated at serine/threonine residues leading to a mobility shift in SDS-PAGE and creating a double band on Western blots. The phosphorylated form of ERα runs in the upper of the two bands and is particularly concentrated with synaptic vesicles. Finally, we used E2 with or without the acyl protein thioesterase 1 inhibitor, Palmostatin B, to show that 20 min of E2 treatment of hippocampal slices depletes ERα from the synaptosomal membrane by depalmitoylation. We found no evidence that E2 regulates phosphorylation of synaptosomal ERα on this time scale. These studies begin to fill the gap between detailed molecular characterization of extranuclear ERα in previous in vitro studies and acute E2 modulation of hippocampal synapses in the adult brain.

Block of NMDA receptor channels by endogenous neurosteroids: implications for the agonist induced conformational states of the channel vestibule.

N-methyl-D-aspartate receptors (NMDARs) mediate synaptic plasticity, and their dysfunction is implicated in multiple brain disorders. NMDARs can be allosterically modulated by numerous compounds, including endogenous neurosteroid pregnanolone sulfate. Here, we identify the molecular basis of the use-dependent and voltage-independent inhibitory effect of neurosteroids on NMDAR responses. The site of action is located at the extracellular vestibule of the receptor’s ion channel pore and is accessible after receptor activation. Mutations in the extracellular vestibule in the SYTANLAAF motif disrupt the inhibitory effect of negatively charged steroids. In contrast, positively charged steroids inhibit mutated NMDAR responses in a voltage-dependent manner. These results, in combination with molecular modeling, characterize structure details of the open configuration of the NMDAR channel. Our results provide a unique opportunity for the development of new therapeutic neurosteroid-based ligands to treat diseases associated with dysfunction of the glutamate system.

Arctic lakes show strong decadal trend in earlier spring ice-out.

The timing of the seasonal freeze-thaw cycle of arctic lakes affects ecological processes and land-atmosphere energy fluxes. We carried out detailed ice-phenology mapping of arctic lakes, based on daily surface-reflectance time series for 2000–2013 from MODIS at 250 m spatial resolution. We used over 13,300 lakes, area >1 km2, in five study areas distributed evenly across the circumpolar Arctic — the first such phenological dataset. All areas showed significant trends towards an earlier break-up, stronger than previously reported. The mean shift in break-up start ranged from −0.10 days/year (Northern Europe) to −1.05 days/year (central Siberia); the shift in break-up end was between −0.14 and −0.72 days/year. Finally, we explored the effect of temperature on break-up timing and compared results among study areas. The 0 °C isotherm shows the strongest relationship (r = 0.56–0.81) in all study areas. If the trend in early break-up continues, rapidly changing ice phenology will likely generate significant, arctic-wide impacts.

Preferential Inhibition of Tonically over Phasically Activated NMDA Receptors by Pregnane Derivatives.

Postsynaptic N-methyl-d-aspartate receptors (NMDARs) phasically activated by presynaptically released glutamate are critical for synaptic transmission and plasticity. However, under pathological conditions, excessive activation of NMDARs by tonically increased ambient glutamate contributes to excitotoxicity associated with various acute and chronic neurological disorders. Here, using heterologously expressed GluN1/GluN2A and GluN1/GluN2B receptors and rat autaptic hippocampal microisland cultures, we show that pregnanolone sulfate inhibits NMDAR currents induced by a prolonged glutamate application with a higher potency than the NMDAR component of EPSCs. For synthetic pregnanolone derivatives substituted with a carboxylic acid moiety at the end of an aliphatic chain of varying length and attached to the steroid skeleton at C3, the difference in potency between tonic and phasic inhibition increased with the length of the residue. The steroid with the longest substituent, pregnanolone hemipimelate, had no effect on phasically activated receptors while inhibiting tonically activated receptors. In behavioral tests, pregnanolone hemipimelate showed neuroprotective activity without psychomimetic symptoms. These results provide insight into the influence of steroids on neuronal function and stress their potential use in the development of novel therapeutics with neuroprotective action. SIGNIFICANCE STATEMENT Synaptic activation of N-methyl-d-aspartate receptors (NMDARs) plays a key role in synaptic plasticity, but excessive tonic NMDAR activation mediates excitotoxicity associated with many neurological disorders. Therefore, there is much interest in pharmacological agents capable of selectively blocking tonically activated NMDARs while leaving synaptically activated NMDARs intact. Here, we show that an endogenous neurosteroid pregnanolone sulfate is more potent at inhibiting tonically than synaptically activated NMDARs. Further, we report that a novel synthetic analog of pregnanolone sulfate, pregnanolone hemipimelate, inhibits tonic NMDAR currents without inhibiting the NMDAR component of the EPSC and shows neuroprotective activity in vivo without inducing psychomimetic side effects. These results suggest steroids may have a clinical advantage over other known classes of NMDAR inhibitors.

7-Methoxyderivative of tacrine is a ‘foot-in-the-door’ open-channel blocker of GluN1/GluN2 and GluN1/GluN3 NMDA receptors with neuroprotective activity in vivo

Abstract N‐methyl‐d‐aspartate receptors (NMDARs) are ionotropic glutamate receptors that mediate excitatory neurotransmission in the mammalian central nervous system (CNS), and their dysregulation results in the aetiology of many CNS syndromes. Several NMDAR modulators have been used successfully in clinical trials (including memantine) and NMDARs remain a promising pharmacological target for the treatment of CNS syndromes. 1,2,3,4‐Tetrahydro‐9‐aminoacridine (tacrine; THA) was the first approved drug for Alzheimers disease (AD) treatment. 7‐methoxyderivative of THA (7‐MEOTA) is less toxic and showed promising results in patients with tardive dyskinesia. We employed electrophysiological recordings in HEK293 cells and rat neurones to examine the mechanism of action of THA and 7‐MEOTA at the NMDAR. We showed that both THA and 7‐MEOTA are “foot‐in‐the‐door” open‐channel blockers of GluN1/GluN2 receptors and that 7‐MEOTA is a more potent but slower blocker than THA. We found that the IC50 values for THA and 7‐MEOTA exhibited the GluN1/GluN2A < GluN1/GluN2B < GluN1/GluN2C = GluN1/GluN2D relationship and that 7‐MEOTA effectively inhibits human GluN1/GluN2A‐M817V receptors that carry a pathogenic mutation. We also showed that 7‐MEOTA is a “foot‐in‐the‐door” open‐channel blocker of GluN1/GluN3 receptors, although these receptors were not inhibited by memantine. In addition, the inhibitory potency of 7‐MEOTA at synaptic and extrasynaptic hippocampal NMDARs was similar, and 7‐MEOTA exhibited better neuroprotective activity when compared with THA and memantine in rats with NMDA‐induced lesions of the hippocampus. Finally, intraperitoneal administration of 7‐MEOTA attenuated MK‐801‐induced hyperlocomotion and pre‐pulse inhibition deficit in rats. We conclude that 7‐MEOTA may be considered for the treatment of diseases associated with the dysfunction of NMDARs. Highlights7‐MEOTA is a potent “foot‐in‐the‐door” open‐channel blocker of NMDA receptors.7‐MEOTA potently inhibits GluN1/GluN2A‐M817V receptors with a pathogenic mutation.7‐MEOTA exhibits neuroprotective activity in rats with NMDA‐induced lesions.7‐MEOTA attenuates MK‐801‐induced pre‐pulse inhibition deficit in rats.