Quantitative and correlative extreme ultraviolet coherent imaging of mouse hippocampal neurons at high resolution

Imaging using coherent extreme ultraviolet radiation from laser sources provides high-resolution images of biological samples.

Ptychographic coherent diffractive imaging with orthogonal probe relaxation

Ptychography is a scanning coherent diffractive imaging (CDI) technique that relies upon a high level of stability of the illumination during the course of an experiment. This is particularly an issue for coherent short wavelength sources, where the beam intensity is usually tightly focused on the sample in order to maximize the photon flux density on the illuminated region of the sample and thus a small change in the beam position results in a significant change in illumination of the sample. We present an improved ptychographic method that allows for limited stability of the illumination wavefront and thus significantly improve the reconstruction quality without additional prior knowledge. We have tested our reconstruction method in a proof of concept experiment, where the beam instability of a visible light source was emulated using a piezo driven mirror, and also in a short wavelength microscopy CDI setup using a high harmonic generation source in the extreme ultraviolet range. Our work shows a natural extension of the ptychography method that paves the way to use ptychographic imaging with any limited pointing stability coherent source such as free electron or soft X-ray lasers and improve reconstruction quality of long duration synchrotron experiments.

Bispyridinium Compounds Inhibit Both Muscle and Neuronal Nicotinic Acetylcholine Receptors in Human Cell Lines

Standard treatment of poisoning by organophosphorus anticholinesterases uses atropine to reduce the muscarinic effects of acetylcholine accumulation and oximes to reactivate acetylcholinesterase (the effectiveness of which depends on the specific anticholinesterase), but does not directly address the nicotinic effects of poisoning. Bispyridinium molecules which act as noncompetitive antagonists at nicotinic acetylcholine receptors have been identified as promising compounds and one has been shown to improve survival following organophosphorus poisoning in guinea-pigs. Here, we have investigated the structural requirements for antagonism and compared inhibitory potency of these compounds at muscle and neuronal nicotinic receptors and acetylcholinesterase. A series of compounds was synthesised, in which the length of the polymethylene linker between the two pyridinium moieties was increased sequentially from one to ten carbon atoms. Their effects on nicotinic receptor-mediated calcium responses were tested in muscle-derived (CN21) and neuronal (SH-SY5Y) cells. Their ability to inhibit acetylcholinesterase activity was tested using human erythrocyte ghosts. In both cell lines, the nicotinic response was inhibited in a dose-dependent manner and the inhibitory potency of the compounds increased with greater linker length between the two pyridinium moieties, as did their inhibitory potency for human acetylcholinesterase activity in vitro. These results demonstrate that bispyridinium compounds inhibit both neuronal and muscle nicotinic receptors and that their potency depends on the length of the hydrocarbon chain linking the two pyridinium moieties. Knowledge of structure-activity relationships will aid the optimisation of molecular structures for therapeutic use against the nicotinic effects of organophosphorus poisoning.

A super-oscillatory lens optical microscope for subwavelength imaging

The past decade has seen an intensive effort to achieve optical imaging resolution beyond the diffraction limit. Apart from the Pendry-Veselago negative index superlens, implementation of which in optics faces challenges of losses and as yet unattainable fabrication finesse, other super-resolution approaches necessitate the lens either to be in the near proximity of the object or manufactured on it, or work only for a narrow class of samples, such as intensely luminescent or sparse objects. Here we report a new super-resolution microscope for optical imaging that beats the diffraction limit of conventional instruments and the recently demonstrated near-field optical superlens and hyperlens. This non-invasive subwavelength imaging paradigm uses a binary amplitude mask for direct focusing of laser light into a subwavelength spot in the post-evanescent field by precisely tailoring the interference of a large number of beams diffracted from a nanostructured mask. The new technology, which--in principle--has no physical limits on resolution, could be universally used for imaging at any wavelength and does not depend on the luminescence of the object, which can be tens of micrometres away from the mask. It has been implemented as a straightforward modification of a conventional microscope showing resolution better than λ/6.

Protection against nerve agent poisoning by a noncompetitive nicotinic antagonist

The acute toxicity of organophosphorus (OP) nerve agents arises from accumulation of acetylcholine (ACh) and overstimulation of ACh receptors. The mainstay of current pharmacotherapy is the competitive muscarinic antagonist, atropine. Nicotinic antagonists have not been used due to the difficulties of administering a dose of a competitive neuromuscular blocker sufficient to antagonise the effects of excessive ACh, but not so much that it paralyses the muscles. An alternative approach would be to use a noncompetitive antagonist whose effects would not be overcome by increasing ACh concentrations. This study demonstrates that the compound 1,1'-(propane-1,3-diyl)bis(4-tert-butylpyridinium), which blocks open nicotinic ion channels noncompetitively, is able to reverse the neuromuscular paralysis after nerve agent poisoning in vitro and to protect guinea pigs against poisoning by nerve agents when used as part of a therapeutic drug combination including a muscarinic antagonist. In contrast to the oxime HI-6, this compound was equally effective in protecting against poisoning by sarin or tabun. Further studies should identify more effective compounds with this action and optimise doses for protection against nerve agent poisoning in vivo.

Kainic acid induces rapid cell death followed by transiently reduced cell proliferation in the immature granule cell layer of rat organotypic hippocampal slice cultures

Brain injury due to seizures results in transiently increased cell proliferation and neurogenesis in the subgranular zone of the adult dentate gyrus. In contrast, the immature postnatal brain appears to be more resistant to cell death after seizure-induced brain injury and paradoxically reacts to seizures by reducing SGZ proliferation. Organotypic hippocampal slice cultures are a useful paradigm for modelling the early postnatal hippocampus. We have investigated the temporal relationship between cell death and cell proliferation after kainate in the granule cell layer of rat organotypic hippocampal slice cultures equivalent to post natal day 11 animals. We found stable numbers and densities of mature thionine stained cells in the granule cell layer over 72 h in control cultures grown in defined medium. We also found a slowly declining cell proliferation rate over the same time period under control conditions. We report evidence of early cell death in the granule cell layer after just 2 h exposure to 5 microM kainate, followed by a significant decrease in cell proliferation in the granule cell layer at 24 h. In contrast to control conditions, cell proliferation rose significantly in the kainate exposed cultures by 72 h back to levels seen at 2 h. There were no significant changes in cell labelling with antibody to activated caspase-3 between kainate treated and control cultures at any time point examined. Our results suggest that kainate-induced injury in the early postnatal hippocampus damages precursor cells contributing to a reduction in granule layer cell proliferation.

Intraischaemic hypothermia reduces free radical production and protects against ischaemic insults in cultured hippocampal slices

Hypothermia has been demonstrated to be an effective neuroprotective strategy in a number of models of ischaemic and excitotoxic neurodegeneration in vitro and in vivo. Reduced glutamate release and free radical production have been postulated as potential mechanisms underlying this effect but no definitive mechanism has yet been reported. In the current study, we have used oxygen-glucose deprivation in organotypic hippocampal slice cultures as an in vitro model of cerebral ischaemia. When assessed by propidium iodide fluorescence, reducing the temperature during oxygen-glucose deprivation to 31-33 degrees C was significantly neuroprotective but this effect was lost if the initiation of hypothermia was delayed until the post-insult recovery period. The neuroprotective effects of hypothermia were associated with a significant decrease in both nitric oxide production, as assessed by 3-amino-4-aminomethyl-2',7'-difluorofluorescein fluorescence, and superoxide formation. Further, hypothermia significantly attenuated NMDA-induced nitric oxide formation in the absence of hypoxia/hypoglycaemia. We conclude that the neuroprotective effects of hypothermia are mediated through a reduction in nitric oxide and superoxide formation and that this effect is likely to be downstream of NMDA receptor activation.

4,5-diaminofluoroscein imaging of nitric oxide synthesis in crayfish terminal ganglia

We have analyzed the synthesis of nitric oxide in the terminal abdominal ganglion of the crayfish using the fluorescent probe 4,5-Diaminofluoroscein diacetate, DAF-2 DA. Following DAF-2 loading, ganglia showed cell-specific patterns of fluorescence in which the occurrence of strongly fluorescent cell bodies was highest in specific anterior, central, and posterior regions. We found that preincubation with the nitric oxide synthase (NOS) inhibitor L-NAME prevented much of the initial development of DAF-2 fluorescence, whereas the inactive isomer D-NAME had no effect. Washout of preincubated L-NAME caused increased cell-specific fluorescence due to endogenous NOS activity. Application of the NOS substrate L-arginine also resulted in an increase of DAF-2 fluorescence in a cell-specific manner. We bath applied the NO donor SNAP to increase exogenous NO levels which resulted in DAF-2 fluorescence increases in most cells. We therefore presume that the cell-specific pattern of DAF-2 fluorescence indicates the distribution of neurones actively synthesizing NO. The similarity between the DAF-2 staining pattern and previously published studies of NOS activity are discussed.

Involvement of ryanodine receptors in EPYLRFamide-mediated reduction of acetylcholine-induced inward currents in helix lucorum identified neurones

The effects of several modulators of ryanodine receptors (RYRs) on the reduction of acetylcholine induced inward current (ACh-current) evoked by EPYLRFamide (5 microM, bath application), the potent N-terminally modified analogue of the endogenous Helix heptapeptide SEPYLRFamide, were investigated. These modulators were applied intracellularly. Inward currents were recorded from identified Helix lucorum LPa2, LPa3, RPa3, RPa2 neurones in ganglia preparations using the two-electrode voltage clamp technique. ACh was applied ionophoretically. BAPTA (0.1 mM), chelator of intracellular Ca(2+), ryanodine (0.1 mM), agonist/antagonist of RYRs and dantrolene (0.1 mM), antagonist of RYRs decrease the effect of EPYLRFamide. Adenosine (1 mM), alpha,beta-methylene ATP (0.1 mM), the nonhydrolisable ATP analogue and cyclic adenosine diphosphate ribose (0.1 mM) (agonists of RYRs) potentiate the modulatory effect of EPYLRFamide. Ruthenium red (1 mM), antagonist of RYRs and caffeine (1 mM), agonist of RYRs do not change the modulatory effect of EPYLRFamide. These data suggest that intracellular Ca(2+) and RYRs are involved in the modulatory effect of EPYLRFamide on ACh-currents. It was concluded that EPYLRFamide decreases ACh-current through elevation of basal intracellular level of a putative endogenous agonist of RYRs which activates RYR-dependent mobilization of Ca(2+) by binding to the adenine nucleotide site of the ryanodine receptor-channel complex and does not bind the site activated by caffeine.

Reduced Mg2+ blockade of synaptically activated N-methyl-D-aspartate receptor-channels in CA1 pyramidal neurons in kainic acid-lesioned rat hippocampus

Unilateral kainic acid lesion in the hippocampus caused a long-term change in the balance between excitatory and inhibitory drive onto CA1 pyramidal cells, making these cells hyperexcitable several weeks post-lesion. In this study, we have shown an enhanced N-methyl-D-aspartate receptor-mediated component in the excitatory synaptic transmission together with a reduced GABA(A) receptor-mediated inhibition in CA1 pyramidal cells one-week post kainic acid lesion. In these cells, pharmacologically isolated N-methyl-D-aspartate receptor-mediated whole-cell excitatory postsynaptic currents were significantly larger at negative holding potentials, and the voltage-dependence of N-methyl-D-aspartate receptor channels was shifted in the hyperpolarizing direction. The plot of relative conductance (g/gMax) shifted significantly (P<0.01) to more negative holding potentials by 19 mV (-28+/-4 mV in control slices and -47+/-4 mV in kainic acid slices) at the half maximal conductance point (g/gMax =0.5). This shift gives a larger N-methyl-D-aspartate receptor-mediated component in the excitatory synaptic transmission at resting membrane potentials (around -60 mV). The shifted voltage dependence is highly sensitive to extracellular Mg2+ ions. Moderate increases in [Mg2+]o from 1 mM to 2.6 mM more than compensated for the negative shift and effectively suppressed the population epileptiform bursting activity. Fitting the voltage dependence to an ionic block model revealed a higher dissociation constant of N-methyl-D-aspartate receptor channels for Mg2+ in kainic acid-lesioned slices (52 mM at 0 mV; 330 microM at -60 mV) than in control slices (7.7 mM at 0 mV; 93 microM at -60 mV). While a simple single site model adequately fitted the control data for [Mg2+]o at 1 mM and 2.6 mM, no consistent model of this form was found for the kainic acid-lesioned slices. These results revealed changed properties of N-methyl-D-aspartate receptor channels in the kainic acid-lesioned model of epilepsy. The reduced Mg2+ blockade of N-methyl-D-aspartate receptor channels contributed significantly to the epileptiform bursting activity.

Role of ERK1/ERK2 and p70S6K pathway in insulin signalling of protein synthesis

The signalling pathways by which insulin triggers protein synthesis were studied using an antisense strategy to deplete ERK1/ERK2 and rapamycin to inhibit the p70S6K pathway. The results indicated that ERK1/ERK2 principally regulated the amount of the protein synthesis machinery available in the cell while the p70S6K pathway contributed to modulating its activation in response to insulin. ERK1/ERK2 also mediated in a small proportion of insulin-stimulated protein synthesis which included the induction of c-fos protein. When c-fos induction was blocked the majority of insulin-stimulated protein synthesis still occurred and thus did not require transcriptional regulation of c-fos or its targets.

The actions of muscle relaxants at nicotinic acetylcholine receptor isoforms

The pharmacological diversity of the different isoforms of the nicotinic acetylcholine receptor arises from the diversity of the subunits that assemble to form the native receptors. The aim of this study was to investigate the actions of the muscle relaxants d-tubocurarine, pancuronium and vecuronium on different isoforms of nicotinic acetylcholine receptors (mouse foetal muscle, mouse adult muscle and a rat neuronal), using the Xenopus oocyte expression system. Oocytes were injected with cRNAs for alpha, beta, gamma, delta subunits (the native foetal muscle subunit combination), or with cRNAs for alpha, beta, epsilon, delta subunits (the native adult muscle subunit combination), or with cRNAs for alpha4beta2 subunits (a putative native neuronal subunit combination). Acetylcholine had a similar potency at all three subunit combinations (EC50 11.6, 17.4 and 19.1 microM, respectively). At all three receptor types, d-tubocurarine and pancuronium blocked the responses elicited by acetylcholine in a reversible manner. Furthermore, the inhibition of the acetylcholine currents for the foetal and adult nicotinic acetylcholine receptor by pancuronium and d-tubocurarine was independent of the holding voltage over the range -100 to -40 mV. In oocytes expressing the foetal muscle nicotinic acetylcholine receptors the inhibition of the current in response to 100 microM acetylcholine by 10 nM d-tubocurarine was 29 +/- 5% (mean +/- S.E.M.; n = 7), and the inhibition by 10 nM pancuronium was 39 +/- 6% (mean +/- S.E.M.; n = 8; P > 0.05 vs. d-tubocurarine). However, in the adult form of the muscle nicotinic acetylcholine receptor, 10 nM d-tubocurarine and 10 nM pancuronium were both more effective at blocking the response to 100 microM acetylcholine compared to the foetal muscle nicotinic acetylcholine receptor, with values of 55 +/- 5% (P < 0.01; n = 12) and 60 +/- 4% (P < 0.001; n = 10), respectively. Thus the developmental switch from the gamma to the epsilon subunit alters the antagonism of the nicotinic acetylcholine receptor for both pancuronium and d-tubocurarine. Vecuronium was more potent than pancuronium. One nM vecuronium reduced the response to 100 microM acetylcholine by 71 +- 6% (n = 10) for foetal and 63 +/- 5% (n = 4) for adult nicotinic acetylcholine receptors. In the alpha4beta2 neuronal nicotinic acetylcholine receptor combination, 10 nM pancuronium was a more effective antagonist of the response to 100 microM acetylcholine (69 +/- 6%, n = 6) than 10 nM d-tubocurarine (30 +/- 5%; n = 6; P < 0.05 compared to pancuronium). This is in contrast to the adult muscle nicotinic acetylcholine receptor, where pancuronium and d-tubocurarine were equieffective. The expression of the beta2 subunit with muscle alpha, epsilon and delta subunits formed a functional receptor which was blocked by pancuronium and d-tubocurarine in a similar manner to the alphabeta1epsilondelta subunit consistent with the hypothesis that the beta subunit is not a major determinant in the action of this drug at the adult muscle nicotinic acetylcholine receptor.

Pro-epileptic changes in synaptic function can be accompanied by pro-epileptic changes in neuronal excitability

Repetitive sensory input, stroboscopic lights or repeated sounds can induce epileptic seizures in susceptible individuals. In order to understand the process we have to consider multiple factors. The output of a set of neurones is determined by the amount of excitatory synaptic input, the degree of positive feedback and their inherent electrical excitability, which can be modified by synaptic inhibition. Recent research has shown that it is possible to separate these phenomena, and that they do not always behave in unison.

Neuroprotection by both NMDA and non-NMDA receptor antagonists in in vitro ischemia

We have investigated the relative contributions of oxygen and glucose deprivation to ischaemic neurodegeneration in organotypic hippocampal slice cultures. Cultures prepared from 10-day-old rats were maintained in vitro for 14 days and then deprived of either oxygen (hypoxia), glucose (hypoglycaemia), or both oxygen and glucose (ischaemia). Hypoxia alone induced degeneration selectively in CA1 pyramidal cells and this was greatly potentiated if glucose was removed from the medium. We have also characterised the effects of both pre- and post-treatment using glutamate receptor antagonists and the sodium channel blocker tetrodotoxin (TTX). Neuronal death following either hypoxia or ischaemia was prevented by pre-incubation with CNQX, MK-801 or tetrodotoxin. MK-801 or CNQX also prevented death induced by either hypoxia or ischaemia if added immediately post-insult, however, post-insult addition of TTX prevented hypoxic but not ischaemic damage. Organotypic hippocampal slice cultures are sensitive to both NMDA and non-NMDA glutamate receptor blockade and thus represent a useful in vitro system for the study of ischaemic neurodegeneration paralleling results reported using in vivo models of ischaemia.

Synaptic release rather than failure in the conditioning pulse results in paired-pulse facilitation during minimal synaptic stimulation in the rat hippocampal CA1 neurones

In this paper we report our observations of the relationship of paired-pulse facilitation (PPF) with synaptic release or release failure at small numbers of synaptic sites. Minimal stimulation protocols were employed to enable the activation of only one or a few axons which synapse onto CA1 pyramidal cells in the hippocampus. Excitatory postsynaptic currents (EPSCs) in response to paired stimulation were measured. On the analysis of the data, we examined the effects of failure and synaptic release on PPF, and found that PPF was observed as a decrease in failures of synaptic release in response to test stimuli which were preceded by conditioning stimuli that evoked synaptic release. The test-pulse failure rate following conditioning pulse failures was indistinguishable from the overall conditioning pulse failure rate. It is postulated that the mechanism of paired-pulse facilitation is presynaptic and associated with successful synaptic release in response to the conditioning stimulus.

Activation of nicotinic acetylcholine receptors expressed in quail fibroblasts: effects on intracellular calcium

1. The aim of these experiments was to determine the ability of the muscle-type nicotinic acetylcholine receptor (AChR) stably expressed in quail fibroblasts (QF18 cells) to elevate intracellular calcium ([Ca2+]i) upon activation. Ratiometric confocal microscopy, with the calcium-sensitive fluorescent dye Indo-1 was used. 2. Application of the nicotine agonist, suberyldicholine (SDC), to the transfected QF18 cells caused an increase in [Ca2+]i. Control [Ca2+]i levels in QF18 cells were found to be 164 +/- 22 nM (mean +/- s.e. mean; n = 40 cells) rising to 600 +/- 81 nM on addition of SDC (10 microM; n = 15 cells), whereas no increase in [Ca2+]i was seen in non-transfected control QT6 fibroblasts (before: 128 +/- 9 nM, n = 40; after; 113 +/- 13 nM, n = 15). 3. The increase in [Ca2+]i caused by application of SDC was dose-dependent, with an EC50 value of 12.7 +/- 5.9 microM (n = 14). 4. The responses to SDC in QF18 cells were blocked by prior application of alpha-bungarotoxin (200 nM), by the addition of Ca2+ (100 microM), by removal of Na+ ions from the extracellular solution, or by the voltage-sensitive calcium channel blockers nifedipine and omega-conotoxin, which act with IC50 values of 100 nM and 100 pM respectively. 5. We conclude that activation of the nicotinic AChRs leads to a Na(+)-dependent depolarization and hence activation of endogenous voltage-sensitive Ca2+ channels in the plasma membrane and an increase in [Ca2+]i. There is no significant entry of Ca2+ through the nicotinic receptor itself.

Stimulation of endocytosis in mouse blastocysts by insulin: a quantitative morphological analysis

The effects of insulin on the endocytic activity of mouse blastocysts in vitro were investigated using confocal laser scanning microscopy, quantitative image analysis and electron microscopy. Confocal studies showed that fluorescein isothiocyanate-labelled markers, dextran (fluid phase) and albumin (combined membrane and fluid phase), were endocytosed by blastocysts and localized within vesicles (about 2.5 microns in diameter) in the outer trophectoderm cells. No labelling was detected in the inner cell mass cells or the blastocoel cavity. Treatment with 170 nmol insulin l-1 stimulated the endocytosis of fluorescently labelled dextran in freshly collected blastocysts, increasing mean vesicle diameter per embryo by 15% (P < 0.05) after incubation with insulin for 2.5 h and mean vesicle number per embryo by 56% (P < 0.01) after 6 h. Both effects were also evident in blastocysts that had been cultured from the late eight-cell stage. Blastocysts incubated for 6 h with insulin displayed increased convolutions in the trophectoderm apical membrane compared with controls, indicating increased membrane activity and suggesting macropinosome formation. Collectively, these results suggest that insulin enhances endocytosis in the trophectoderm by stimulating uptake at the apical membrane into larger and more numerous endocytic vesicles and with some evidence of vesicle fusion. This mechanism may provide a metabolic basis for the stimulation by insulin of biosynthesis, proliferation and morphological development in early embryos.

Increased perinuclear Ca2+ activity evoked by metabotropic glutamate receptor activation in rat hippocampal neurones

1. The effect of metabotropic glutamate receptor activation on intracellular Ca2+ activity (alpha Cai) of rat hippocampal pyramidal neurones in vitro was examined using ratiometric confocal laser scanning microscopy with the Ca(2+)-sensitive fluorescent probe indo-1 AM. 2. Metabotropic receptors were selectively activated with 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD; 100 microM) in the presence of D-2-amino 5-phosphonovaleric acid (D-APV), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and CdCl2. Most pyramidal neurones (77/84) responded with an elevation in Ca2+ activity, maximal after 3-5 min. Fluorescence ratio responses were concentration dependent (EC50 approximately 10 microM) and were blocked by prior application of the antagonist (RS)-4-carboxy-3-hydroxyphenylglycine (RS-CHPG, 300 microM). 3. Responses to 1S,3R-ACPD (100 microM) also caused acidification of the neurones, from estimated control pH 7.2 to pH 6.6 (measured with the pH-sensitive dye SNAFL-calcein). The correction factor for indo-1 determination of Ca2+ was estimated to be x 1.4. 4. Elevations in alpha Cai were greater within the perinuclear region (> 1000 nM), than in the cytoplasm (approximately 200 nM). This region was devoid of staining by the endoplasmic reticulum staining dye 3,3'-dihexyloxacarbocyanine iodide (DiOC6(3)). 5. It is concluded that activation of metabotropic receptors in immature rat hippocampal pyramidal neurones leads to a large increase in perinuclear Ca2+ which would be well positioned to interact with the genome.

Bicuculline enhances the late GABAB receptor-mediated paired-pulse inhibition observed in rat hippocampal slices

The inhibition of CA1 pyramidal neurones in rat hippocampal slices was studied using extracellular recordings of population spike potential responses to paired orthodromic stimulation. Variation of the interpulse interval allowed the separation of an early phase of inhibition (interpulse interval 5-20 ms), blocked by the GABAA receptor antagonist bicuculline (1 microM; n = 11), and a late phase (interpulse interval 200-400 ms) blocked by the GABAB receptor antagonist phaclofen (1 mM; n = 5) but enhanced by bicuculline (n = 11). Similar enhancement was not observed when conditioning response amplitudes were increased by increasing the stimulus strength, rather than bicuculline. Orthodromic stimulation leads to synaptic excitation of both pyramidal neurones and inhibitory interneurones, and may also lead to activation of inhibitory inputs onto interneurones. Bicuculline could prevent inhibition of the interneurones, and hence enhance the late, GABAB receptor-mediated inhibition. Conversely, the therapeutic administration of benzodiazepines would be postulated to enhance the inhibition of inhibitory interneurones, leading to an iatrogenic decrease in GABAB receptor-mediated inhibition.

Enhancement by 5-hydroxytryptamine and analogues of desensitization of neuronal and muscle nicotinic receptors expressed in Xenopus oocytes

1. The action of 5-hydroxytryptamine (5-HT) on neuronal and muscle nicotinic acetylcholine receptors (nicotinic AChR) expressed in Xenopus oocytes was studied. 2. 5-HT enhanced the rate of desensitization of the acetylcholine (ACh) current response in all receptor subtypes investigated (muscle, alpha beta 2 gamma delta and alpha 4 beta 2), acting in a dose-dependent manner. 3. 5-HT also reduced the peak current elicited by ACh in a dose-dependent manner. The IC50 value for the muscle type receptor was 227 +/- 0.44 microM, and 166 +/- 0.47 microM and 283 +/- 0.28 microM for the combinations alpha beta 2 gamma delta and alpha 4 beta 2 respectively. 4. The effect of 5-HT on the responses to ACh (10 microM) was found to be independent of membrane voltage over the range tested (-80 to -10 mV), and to be readily reversed by washout. 5. The action of 5-HT could be mimicked by structurally similar molecules. The homologue tryptamine was less potent than 5-HT in blocking the ACh current, with an IC50 of 1.0 +/- 0.02 mM. Ketanserin, a 5-HT2 receptor antagonist, was more potent than 5-HT, the IC50 being 49.0 +/- 1.4 microM. 6. We postulate that a highly conserved portion of the tertiary structure of nicotinic AChRs, which includes some part of the ACh binding site, has affinity for 5-HT and structural analogues.

2-Hydroxy-saclofen causes a phaclofen-reversible reduction in population spike amplitude in the rat hippocampal slice

2-Hydroxy-saclofen is known to be active at GABAB receptors in the mammalian central nervous system, and we have investigated its effects on synaptic transmission in the rat hippocampal slice preparation. Orthodromic stimuli were applied to the stratum radiatum, and population spike responses from the CA1 pyramidal cell layer were recorded extracellularly. A second, identical stimulus was applied at a variable interpulse interval (IPI) after the initial conditioning stimulus. GABAergic synaptic inhibition was observed as a decrease in the spike amplitude of the second response compared to the first. Both the GABAB receptor antagonist phaclofen (1 mM) and 2-hydroxy-saclofen (200 microM) prevented a slow phase of inhibition for IPIs of 200-400 ms. However, these agents differed markedly in their effects on overall synaptic transmission. Phaclofen had no effect on the amplitude of the initial conditioning spike amplitude, whereas 2-hydroxy-saclofen reduced it significantly, in a manner similar to baclofen (1 microM). The direct actions of 2-hydroxy-saclofen were unexpected for a pure antagonist of GABAB receptors, but could be prevented by the co-administration of phaclofen (1 mM), but not bicuculline (1 microM). Reduction in conditioning spike amplitude due to antagonism of GABAB autoreceptors on inhibitory interneurones and subsequent enhancement of GABAA tonic inhibition would have been blocked by bicuculline. The blockade of the 2-hydroxy-saclofen effect by phaclofen implies a GABAB receptor partial agonist action. The possible sites of this action are discussed.

Membrane clustering and bungarotoxin binding by the nicotinic acetylcholine receptor: role of the beta subunit

Nicotinic acetylcholine receptors (nAChRs) are localised at morphologically distinct regions of the post-synaptic membrane by interactions between the receptor subunits and cytoskeletal proteins, such as the 43-kDa protein. We have used Xenopus oocytes to examine the localisation and pharmacological properties of muscle nAChRs associated with 43-kDa protein and to compare them with hybrid muscle nAChRs containing a beta subunit derived from a neuronal source. Receptors expressed on the oocyte outer membrane were visualised using confocal scanning laser microscopy. Coexpression of mouse muscle subunit alpha 1 beta 1 gamma delta and 43-kDa protein transcripts produced discrete receptor aggregates with a diameter of 1-5 microns whose function was partially blocked by application of neuronal bungarotoxin (NBT) at 100 nM. Substitution of the beta 1 subunit by the neuronal beta 2 protein produced a functioning receptor that did not aggregate in the presence of 43-kDa protein and was substantially blocked by the same concentration of NBT. Hybrid alpha 1 beta 4 gamma delta receptors exhibited a combination of characteristics in that they clustered like normal muscle subunits in the presence of 43-kDa protein, but showed a sensitivity to NBT intermediate between that of muscle receptors and that of hybrids containing beta 2. These results suggest that the beta subunit is an important determinant in receptor localisation and sensitivity to NBT.

Residues 1 to 80 of the N-terminal domain of the beta subunit confer neuronal bungarotoxin sensitivity and agonist selectivity on neuronal nicotinic receptors

Standard two electrode voltage clamp techniques were used to investigate the response of neuronal nicotinic acetylcholine receptors, expressed in Xenopus oocytes, to various agonists and neuronal bungarotoxin (NBT). The beta subunit is an important determinant of the receptor's pharmacological profile. Co-expression of alpha 4 and beta 2 subunits produced a receptor that was relatively insensitive to cytisine and nicotine and inhibited by NBT, whilst the alpha 4 beta 4 combination produced a receptor that was highly sensitive to cytisine and nicotine but resistant to toxin. The first 80 amino acids of the N-terminal domain of the beta subunit are implicated in these characteristics, since the combination of alpha 4 with a hybrid beta subunit comprising amino acids 1-->80 of beta 2 and 81-->416 of beta 4 became relatively insensitive to nicotine and cytisine and resistant to inhibition by neuronal bungarotoxin.

An enzymatic mechanism for calcium current inactivation in dialysed Helix neurones

'Wash-out' and inactivation of the Ca current were examined in dialysed, voltage-clamped neurones of Helix aspersa under conditions that isolate the Ca current virtually free of other currents. EGTA or other internal Ca2+ chelators were routinely omitted from the dialysate. The time-dependent loss, or wash-out, of Ca current was slowed by addition to the dialysing solution of agents, such as dibutyryl adenosine 3'-5'-cyclic monophosphate (dibutyryl cyclic AMP), Mg adenosine 5'-triphosphate (ATP) and the catalytic subunit of cyclic-AMP-dependent protein kinase, that promote protein phosphorylation and by EGTA. However, neither the phosphorylation-promoting agents nor internal EGTA prevented wash-out entirely, nor did they significantly restore previously 'washed-out' current. With phosphorylating agents in the dialysing solution, the irreversible development of wash-out was greatly reduced by introduction of leupeptin, an inhibitor of protease activity. Thus, the irreversible component of wash-out appears to result from a Ca-dependent proteolytic process. In the presence of leupeptin alone, Ca current amplitude continued to decline: however, the current could be largely or fully restored with addition of catalytic subunit, dibutyryl cyclic AMP, and Mg ATP to the dialysing solution. Thus, inhibition of proteolysis revealed a reversible component of wash-out that appears to result from dephosphorylation. During perfusion with leupeptin, Mg ATP, dibutyryl cyclic AMP and catalytic subunit the Ca current remained stable for up to several hours without addition of internal Ca2+ buffer. The rate of inactivation of the current that occurs during a depolarizing step showed only a very gradual decline during this time. Under these conditions, perfusion with calcineurin, a Ca-calmodulin-dependent phosphatase, caused a significant increase in the rate of Ca current inactivation. This inactivation was virtually eliminated by introduction of EGTA or by replacement of external Ca2+ with Ba2+, which is consistent with the ion dependency for calmodulin-dependent activation of calcineurin. When ATP in the dialysate was replaced with ATP-gamma-S (adenosine 5'-O-(thiotriphosphate], an analogue that donates a thiophosphate group resistant to hydrolysis, the rate of inactivation slowed. Since Ca-dependent inactivation during step depolarizations is enhanced by conditions that promote dephosphorylation, and Ca current wash-out is slowed by conditions that promote phosphorylation, inactivation and reversible wash-out appear to be related.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium domains associated with individual channels can account for anomalous voltage relations of CA-dependent responses

Computer-assisted modeling of calcium influx through voltage-activated membrane channels predicted that buffer-limited elevation of cytoplasmic free calcium ion concentration occurs within microscopic hemispherical "domains" centered upon the active Ca channels. With increasing depolarization, the number of activated channels, and hence the number of Ca domains, should increase; the single-channel current should, however, decrease, thereby decreasing Ca2+ accumulation in each domain relative to the macroscopic current. Such voltage dependence of the microscopic distribution of Ca2+ may influence relations between total Ca2+ entry and Ca-dependent processes. Ca-mediated inactivation of Ca channels in Aplysia neurons exhibits behavior consistent with the calcium domain hypothesis.