Linda M. Nowak

Bio: Linda M. Nowak is an academic researcher from Cornell University. The author has contributed to research in topics: Depolarization & AMPA receptor. The author has an hindex of 22, co-authored 35 publications receiving 6743 citations. Previous affiliations of Linda M. Nowak include University of Michigan & École Normale Supérieure.

Magnesium gates glutamate-activated channels in mouse central neurones

Abstract: The responses of vertebrate neurones to glutamate involve at least three receptor types. One of these, the NMDA receptor (so called because of its specific activation by N-methyl-D-aspartate), induces responses presenting a peculiar voltage sensitivity. Above resting potential, the current induced by a given dose of glutamate (or NMDA) increases when the cell is depolarized. This is contrary to what is observed at classical excitatory synapses, and recalls the properties of 'regenerative' systems like the Na+ conductance of the action potential. Indeed, recent studies of L-glutamate, L-aspartate and NMDA-induced currents have indicated that the current-voltage (I-V) relationship can show a region of 'negative conductance' and that the application of these agonists can lead to a regenerative depolarization. Furthermore, the NMDA response is greatly potentiated by reducing the extracellular Mg2+ concentration [( Mg2+]o) below the physiological level (approximately 1 mM). By analysing the responses of mouse central neurones to glutamate using the patch-clamp technique, we have now found a link between voltage sensitivity and Mg2+ sensitivity. In Mg2+-free solutions, L-glutamate, L-aspartate and NMDA open cation channels, the properties of which are voltage independent. In the presence of Mg2+, the single-channel currents measured at resting potential are chopped in bursts and the probability of opening of the channels is reduced. Both effects increase steeply with hyperpolarization, thereby accounting for the negative slope of the I-V relationship of the glutamate response. Thus, the voltage dependence of the NMDA receptor-linked conductance appears to be a consequence of the voltage dependence of the Mg2+ block and its interpretation does not require the implication of an intramembrane voltage-dependent 'gate'.

The role of divalent cations in the N-methyl-D-aspartate responses of mouse central neurones in culture.

Abstract: 1. Single-channel currents activated by N-methyl-D-aspartate (NMDA) agonists were analysed in the presence of various extracellular concentrations of divalent cations in outside-out patches from mouse neurones in primary culture. 2. In nominally Mg2+-free solutions the opening and closing of the channels leads to rectangular current pulses, the mean duration of which varies little with membrane potential. After addition of Mg2+, the single-channel currents recorded at negative potentials appear in bursts of short openings separated by brief closures. 3. The duration of the short openings decreases with increasing Mg2+ concentration, while the duration of the short closures is independent of the Mg2+ concentration. Depolarization increases the duration of the short openings and decreases the duration of the short closures. 4. The dependence of the burst structure on the Mg2+ concentration and on membrane potential is compatible to a first approximation with a model in which Mg2+ ions enter the open channel and block it by binding at a deep site. A better approximation requires, however, additional assumptions such as Mg2+ permeation and/or interactions between Ca2+ and Mg2+. 5. Increasing the extracellular Ca2+ concentration from 1 to 100 mM produces three effects on the currents flowing through NMDA channels. It shifts the reversal potential towards a positive value (+30 mV); it reduces the outward current flowing through the NMDA channels at very positive potentials; it reduces the inward current flowing at negative potentials. 6. The interpretation of the effects of Ca2+ appears to require three hypotheses: that Ca2+ permeates the NMDA channel, that there exists a significant surface potential at the entrance of the NMDA channel in physiological solutions and that both Ca2+ and monovalent cations bind to the channel, binding being stronger in the case of Ca2+ ions. 7. While Co2+ and, to a lesser extent, Mn2+ mimic the effects of Mg2+ on the NMDA channel, Ca2+, Ba2+ and Cd2+ do not. The distinction between Mg2+-like and Ca2+-like divalent cations corresponds to a difference in the speed of exchange of the water molecules surrounding the cations in solutions. Thus, it is possible that permeation occurs for all the divalent cations, but is slower for those which are slowly dehydrated.

N‐methyl‐D‐aspartate‐activated channels of mouse central neurones in magnesium‐free solutions.

Abstract: 1 The whole-cell and outside-out configurations of the patch-clamp method were used to investigate the properties of the channels activated by N-methyl-D-aspartate (NMDA channels) in mouse central neurones in culture Recording was made in Mg2+-free solutions 2 In the whole-cell recording mode the currents induced by both NMDA and L-glutamate were accompanied by a large increase in noise In both cases the noise power spectra were well fitted by single Lorentzian functions and the corresponding mean time constant, tau, was about 6 ms at room temperature The single-channel conductance, gamma n, estimated from the ratio of the noise variance to the total current, varied between 22 and 40 pS 3 Endogenous amino acids known to activate NMDA receptors (L-glutamate, L-aspartate, L-cysteine sulphinate and quinolinate) as well as exogenous NMDA agonists such as ibotenate and trans-2,3-piperidine dicarboxylate (trans-PDA) all produced similar responses In particular, analysis of the current noise yielded tau values between 4 and 8 ms in all cases 4 NMDA responses were antagonized by 2-amino-5-phosphonovalerate (APV) without any effect on gamma n or tau values measured by noise analysis; NMDA responses were also diminished by D-alpha-aminoadipate and cis-2,3-piperidine dicarboxylate 5 In outside-out patches, it was observed that the single-channel current amplitude varies linearly as a function of membrane potential between -80 and +60 mV The reversal potential is near 0 mV NMDA channels are permeable to Na+, K+ and Cs+, but blocked by choline The single-channel conductance, gamma e, varies between 40 and 50 pS at room temperature 6 The NMDA channels open in bursts of short openings interrupted by brief closures At -60 mV, the closures had a mean duration, tc, of 04 +/- 02 ms The mean channel open time, to, was 59 +/- 10 ms for NMDA and 53 +/- 17 ms for L-glutamate The mean burst duration, tb, was 105 +/- 07 ms for NMDA and 85 +/- 20 ms for L-glutamate 7 When the temperature was increased between 14 and 24 degrees C, the NMDA channel conductance increased with a Q10 of 16 while the mean open time decreased with a Q10 close to 2 8 The NMDA channel showed, in addition to the 'main' conductance state (40-50 pS), smaller conductance states of 15 and 35 pS(ABSTRACT TRUNCATED AT 400 WORDS)

Ionic channels in mouse astrocytes in culture.

Abstract: We observed Na, K, and Cl voltage-dependent currents in a patch-clamp study of mouse brain astrocytes. In whole-cell recordings, depolarizations activated inward currents that were identified as Na currents since they were blocked by TTX, although complete block required high concentrations (greater than 1 microM). The corresponding single-channel Na currents were observed in outside-out patches. The channels were opened by a depolarizing pulse applied from a holding potential identical to the resting potential (-70 to -80 mV). Therefore, they may be considered functional Na channels. After addition of veratridine and an alpha-scorpion toxin, the decay of Na currents in whole-cell recordings was slower than observed under control conditions. At the single-channel level, the channels appeared to open in bursts. Depolarization did not increase the duration of the bursts, but inside each burst, increased the time spent in the open state. The K currents observed in the whole-cell recording mode were separated into inactivating and noninactivating currents. The inactivating current resembled the A current in its kinetics, its insensitivity to tetraethylammonium, and its sensitivity to 4- aminopyridine. At the single-channel level, at least 3 classes of K channels were observed at steady depolarized potentials. They resembled the K channels found in chromaffin cells by Marty and Neher (1985). Large conductance channels (385 pS) activated around 0 mV were identified as Cl channels.

A synaptic model of memory: long-term potentiation in the hippocampus

Abstract: Long-term potentiation of synaptic transmission in the hippocampus is the primary experimental model for investigating the synaptic basis of learning and memory in vertebrates. The best understood form of long-term potentiation is induced by the activation of the N-methyl-D-aspartate receptor complex. This subtype of glutamate receptor endows long-term potentiation with Hebbian characteristics, and allows electrical events at the postsynaptic membrane to be transduced into chemical signals which, in turn, are thought to activate both pre- and postsynaptic mechanisms to generate a persistent increase in synaptic strength.

The glutamate receptor ion channels

Abstract: The ionotropic glutamate receptors are ligand-gated ion channels that mediate the vast majority of excitatory neurotransmission in the brain. The cloning of cDNAs encoding glutamate receptor subunits, which occurred mainly between 1989 and 1992 ([Hollmann and Heinemann, 1994][1]), stimulated this

Cloned Glutamate Receptors

Abstract: The application of molecular cloning technology to the study of the glutamate receptor system has led to an explosion of knowledge about the structure, expression, and function of this most important fast excitatory transmitter system in the mammalian brain. The first functional ionotropic glutamate receptor was cloned in 1989 (Hollmann et al 1989) , and the results of this molecular-based approach over the past three years are the focus of this review. We discuss the implications of and the new questions raised by this work-which is probably only a glance at this fascinating and complex signaling system found in brains from the snails to man. Glutamate receptors are found throughout the mammalian brain, where they constitute the major excitatory transmitter system. The longest-known and best-studied glutamate receptors are ligand-gated ion channels, also called ionotropic glutamate receptors , which are permeable to cations. They have traditionally been classified into three broad subtypes based upon pharmaco­ logical and electrophysiological data: a-amino-3-hydroxy-5-methyl-4isoxazole propionate (AMPA) receptors, kainate (KA) receptors , and N-methyl-D-aspartate (NMDA) receptors. Recently, however, a family of G protein-coupled glutamate receptors , which are also called metabotropic glutamate or transl -aminocyclopentanel ,3-dicarboxylate (tACPD) recep­ tors, was identified (Sugiyama et al 1987) . (For reviews of the classification and the pharmacological and electrophysiological properties of glutamate receptors see Mayer & Westbrook 1987, Collingridge & Lester 1989, Honore 1989, Monaghan et al 1989, Wroblewski & Danysz 1 989, Hansen &

Selective impairment of learning and blockade of long-term potentiation by an N -methyl-D-aspartate receptor antagonist, AP5

Abstract: Recent work has shown that the hippocampus contains a class of receptors for the excitatory amino acid glutamate that are activated by N-methyl-D-aspartate (NMDA) and that exhibit a peculiar dependency on membrane voltage in becoming active only on depolarization. Blockade of these sites with the drug aminophosphonovaleric acid (AP5) does not detectably affect synaptic transmission in the hippocampus, but prevents the induction of hippocampal long-term potentiation (LTP) following brief high-frequency stimulation. We now report that chronic intraventricular infusion of D,L-AP5 causes a selective impairment of place learning, which is highly sensitive to hippocampal damage, without affecting visual discrimination learning, which is not. The L-isomer of AP5 did not produce behavioural effects. AP5 treatment also suppressed LTP in vivo. These results suggest that NMDA receptors are involved in spatial learning, and add support to the hypothesis that LTP is involved in some, but not all, forms of learning.