1,721,463 research outputs found
Noise analysis of drug-induced voltage clamp currents in denervated frog muscle fibres
No full textVoltage clamp currents were recorded during iontophoretic application of steady doses of acetylcholine (ACh), carbachol or suberyldicholine to hyperpersensitive extrasynaptic regions of chronically denervated frog muscle fibers. Autocorrelation functions of drug induced current fluctuations were calculated and estimates of conductance gamma and average open time tau of the extrasynaptic ion channels were derived. 2. The average open time of an extrajunctional channel induced by acetylcholine is tauACh = 11 +/- 1-6 msec (+/- S.E.) at -80 mV and 8 degrees C. Carbachol and suberyldicholine open channels of tauCarb = 3-9 +/- 0-4 msec and tauSubCh = 19 +/- 2-5 msec (+/- S.E.) duration under the same conditions. The average open time of the extrasynaptic channel produced by each drug is three to five times longer than the value found for junctional channels in normal fibres. 3. The average open time of the extrajunctional channel is dependent on temperature and membrane potential. Lowering the temperature or increasing the membrane potential increases the average open time of the channels induced by any one of the drugs. 4. The conductance of a single extrajunctional channel opened by the action of acetylcholine is estimated to be gammaextra = 15 +/- 1-8 pmho (+/- S.E.). This is somewhat lower than the value of gammaep = 23 +/- 2 pmho (+/- S.E.) found for the conductance of a single open channel in the junctional membrane of normal fibres. The extrasynaptic channels opened by the action of carbachol and suberyldicholine have similar conductances to those produced by ACh. 5. The autocorrelation function of drug-induced current fluctuations, recorded at the former end-plate region of chronically denervated fibres often shows both a fast and a slow time constant. They correspond in value to the time constant of the autocorrelation function obtained from end-plate currents in normal fibres and from extrasynaptic currents in denervated fibres respectively. This could indicate that two populations of channels exist at the former end-plate region of denervated muscle fibres
Voltage-dependence of drug-induced conductance in frog neuromuscular junction
No full textMembrane currents from voltage-clamped frog muscle fibers were recorded during iontophoretic application of steady doses of carbachol, acetylcholine, and suberyldicholine to the endplate region. In the presence of these drugs, an exponentially relaxing current was observed after step changes of membrane potential. The time constant of relaxation was found to be voltage-dependent. It was equal to the time constant obtained from the autocorrelation function of drug-induced conductance fluctuations measured under similar conditions. Analysis of instantaneous current at the on- and offsets of voltageclamp pulses showed that there is no shift in equilibrium potential during the pulses
Rat brain serotonin receptors in Xenopus oocytes are coupled by intracellular calcium to endogenous channels
No full textSerotonin activates chloride currents in Xenopus oocytes injected with a subfraction of rat brain poly(A)+ mRNA. Patch-clamp recordings from cell-attached patches showed that serotonin, applied locally outside the patch, caused the opening of channels of approximately equal to 3 pS conductance and an average lifetime of approximately equal to 100 msec. The extrapolated reversal potential indicated that the channels are chloride-selective. Single-channel currents with similar characteristics were observed in inside-out patches from native oocytes in response to elevated calcium concentrations on the cytoplasmic side. Measurements of intracellular calcium concentration ([Ca2+]i) by fura-2 fluorescence showed approximately equal to 10-fold increases in [Ca2+]i in response to serotonin application in both normal and calcium-free Ringer solution in mRNA-injected oocytes. Little or no response to serotonin was observed in native oocytes. These results suggest that serotonin activation of receptors that are inserted into the oocyte membrane following injection of rat brain poly(A)+ mRNA can induce calcium release from intracellular stores. The increase in [Ca2+]i subsequently activates calcium-dependent chloride channels. Because calcium-dependent chloride channels and a receptor-controlled mechanism of internal calcium release have been shown to exist in native oocytes, we conclude that the newly inserted serotonin receptors utilized the endogenous second-messenger-mediated calcium release to activate endogenous calcium-dependent chloride channels
Single acetylcholine-activated channels show burst-kinetics in presence of desensitizing concentrations of agonist
No full textHigh resolution measurements of the current through individual ion channels activated by acetylcholine (AChR-channels) in frog muscle have shown that these currents are discrete pulse-like events with durations of a few milliseconds1,2. Fluctuation and relaxation measurements of end-plate currents have led to the conclusion that the rate of channel opening increases with agonist concentration, and that the channel, once open, closes spontaneously3–6. Katz and Thesleff have shown, however, that in the continued presence of ACh, the initial end-plate current declines to an equilibrium value with a time constant of several seconds7. This reversible phenomenon is referred to as receptor desensitization. We report here that in the presence of ACh concentrations sufficient to cause desensitization, single channel current pulses appear in groups. From the temporal sequence of the pulses, we have derived estimates of the rates of activation and desensitization of the AChR-channel
Transmitter release modulation by intracellular Ca2+ buffers in facilitating and depressing nerve terminals of pyramidal cells in layer 2/3 of the rat neocortex indicates a target cell-specific difference in presynaptic calcium dynamics
Full text linkIn connections formed by nerve terminals of layer 2/3 pyramidal cells onto bitufted interneurones in young (postnatal day (P)14-15) rat somatosensory cortex, the efficacy and reliability of synaptic transmission were low. At these connections release was facilitated by paired-pulse stimulation (at 10 Hz). In connections formed by terminals of layer 2/3 pyramids with multipolar interneurones efficacy and reliability were high and release was depressed by paired-pulse stimulation. In both types of terminal, however, the voltage-dependent Ca2+ channels that controlled transmitter release were predominantly of the P/Q- and N-subtypes. The relationship between unitary EPSP amplitude and extracellular calcium concentration ([Ca2+]o) was steeper for facilitating than for depressing terminals. Fits to a Hill equation with nH = 4 indicated that the apparent KD of the Ca2+ sensor for vesicle release was two- to threefold lower in depressing terminals than in facilitating ones. Intracellular loading of pyramidal neurones with the fast and slowly acting Ca2+ buffers BAPTA and EGTA differentially reduced transmitter release in these two types of terminal. Unitary EPSPs evoked by pyramidal cell stimulation in bitufted cells were reduced by presynaptic BAPTA and EGTA with half-effective concentrations of ~0.1 and ~1 mM, respectively. Unitary EPSPs evoked in multipolar cells were reduced to one-half of control at higher concentrations of presynaptic BAPTA and EGTA (~0.5 and ~7 mM, respectively). Frequency-dependent facilitation of EPSPs in bitufted cells was abolished by EGTA at concentrations of > 0.2 mM, suggesting that accumulation of free Ca2+ is essential for facilitation in the terminals contacting bitufted cells. In contrast, facilitation was unaffected or even slightly increased in the terminals loaded with BAPTA in the concentration range 0.02-0.5 mM. This is attributed to partial saturation of exogenously added BAPTA. However, BAPTA at concentrations > 1 mM also abolished facilitation. Frequency-dependent depression of EPSPs in multipolar cells was not significantly reduced by EGTA. With BAPTA, the depression decreased at concentrations > 0.5 mM, concomitant with a reduction in amplitude of the first EPSP in a train. An analysis is presented that interprets the effects of EGTA and BAPTA on synaptic efficacy and its short-term modification during paired-pulse stimulation in terms of changes in [Ca2+] at the release site ([Ca2+]RS) and that infers the affinity of the Ca2+ sensor from the dependence of unitary EPSPs on [Ca2+]o. The results suggest that the target cell-specific difference in release from the terminals on bitufted or multipolar cells can be explained by a longer diffusional distance between Ca2+ channels and release sites and/or lower Ca2+ channels density in the terminals that contact bitufted cells. This would lead to a lower [Ca2+] at release sites and would also explain the higher apparent KD of the Ca2+ sensor in facilitating terminals
Fractional calcium currents through recombinant GluR channels of the NMDA, AMPA and kainate receptor subtypes
Full text link1. Simultaneous fluorescence and whole-cell current measurements using the calcium indicator dye fura-2 were made in HEK 293 cells expressing recombinant glutamate receptor (GluR) channels, and fractional Ca 2+ currents (the proportion of whole-cell current carried by Ca 2+; Pf) were determined. 2. Cells expressing N-methyl-D-aspartate receptor (NMDAR) channels showed glutamate activated Ca 2+ inflow in the voltage range -60 to 40 mV in normal extracellular solution. Ca 2+ inflow decreased in a voltage-dependent manner at membrane potentials more negative than -30 mV due to Mg 2+ block. Voltage dependence of block at negative potentials was stronger in cells expressing the NR1-NR2A as compared with cells expressing NR1-NR2C subunits. 3. Fractional Ca 2+ currents through NMDARs were independent of extracellular Mg2+ and varied between 8.2% (NR1-NR2C subunits) and 11% (NR1-NR2A subunits) in normal extracellular solution (1.8 mM Ca 2+) at -60 mV membrane potential. Pf values increased with increasing [Ca2+]o in the range of 0.5-10mM [Ca 2+]o in a saturating fashion. 4. In cells expressing alpha- amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor (AMPAR) subunits which were unedited at the Q/R site of the putative transmembrane segment TM2 (Q-form), or in cells coexpressing unedited and edited subunits (R-form), the glutamate-evoked Ca 2+ inflow increased from 20 to -80 mV in an almost linear way. 5. Fractional Ca 2+ currents through AMPAR channels depended on subunit composition. Pf values of Q-form homomeric channels at -60 mV and 1.8 mM [Ca 2+]o were between 3.2 and 3.9%. They were slightly voltage dependent and increased with [Ca 2+] in the range 1.8-10mM. Pf values in cells co- expressing Q- and R-form subunits were almost one order of magnitude smaller (0.54%). 6. Relative concentrations of Q-form and R-form GluR-B subunit-specific cDNAs used for cell transfection determined the expression of functionally different heteromeric AMPARS. Pf decreased with increasing relative concentration of R-form encoding CDNAs from 3.4 to 1.4%, demonstrating that editing of the Q/R site of GluR-B subunits decreases Ca 2+ inflow through heteromeric AMPARs. 7. Cells expressing the GluR-6 subunit of the kainate receptor (KAR) family were characterized by Pf values which depended on the editing in the TM1 and TM2 segments. Pf values were largest for the Q-form (1.55-2.0%) and lowest for R-form channels (< 0.2%), suggesting that Q/R site editing also decreases Ca 2+ inflow through KAR channels. Cells co-expressing both subunit forms showed an intermediate value (0.58%)
Neuroscience history interview with Professor Bert Sakmann, Nobel Laureate in Physiology or Medicine (1991), Max Planck Society, Germany
No full textDr. Bert Sakmann (b. 1942) studied at the Universities of Tuebingen, Freiburg, Berlin, Paris, and Munich, graduating in 1967. Much of his professional life has been spent in various institutes of the Max Planck Society. In 1971, a British Council Fellowship took him to the Department of Biophysics of University College London to work with Bernard Katz (1911-2003). In 1974, he obtained his Ph.D. from the University of Goettingen and, with Erwin Neher (b. 1944) at the Max Planck Institute for Biophysical Chemistry, began work that would transform cellular biology and neuroscience, resulting in the 1991 Nobel Prize for Physiology or Medicine. In 2008, Dr. Sakmann returned to Munich, where he headed the research group "Cortical Columns in Silico" at the Max Planck Institute of Neurobiology in Martinsried. Here, their group discovered the cell-type specific sensory activation patterns in different layers of a column in the vibrissal area of rodents' somatosensory cortices
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
- …
