Excitatory amino acids and intracellular pH in motoneurons of the isolated frog spinal cord

Double-barrelled pH-sensitive micro-electrodes were used to measure changes of intracellular and extracellular pH in and around motoneurons of the isolated frog spinal cord during application of excitatory amino acids. It was found that N-methyl- -aspartate, quisqualate and kainate produced a concentration-dependent intracellular acidification. Extracellularly, triphasic pH changes (acid-alkaline-acid going pH transients) were observed during the action of these amino acids. The possible significance of such pH changes for the physiological and pathophysiological effects of excitatory amino acids are discussed

Glucose availability and sensitivity to anoxia of isolated rat peripheral nerve

The contrast between resistance to ischemia and ischemic lesions in peripheral nerves of diabetic patients was explored by in vitro experiments. Isolated and desheathed rat peroneal nerves were incubated in the following solutions with different glucose availability: 1) 25 mM glucose, 2) 2.5 mM glucose, and 3) 2.5 mM glucose plus 10 mM 2-deoxy-D-glucose. Additionally, the buffering power of all of these solutions was modified. Compound nerve action potential (CNAP), extracellular pH, and extracellular potassium activity (aKe) were measured simultaneously before, during, and after a period of 30 min of anoxia. An increase in glucose availability led to a slower decline in CNAP and to a smaller rise in aKe during anoxia. This resistance to anoxia was accompanied by an enhanced extracellular acidosis. Postanoxic recovery of CNAP was always complete in 25 mM HCO3(-)-buffered solutions. In 5 mM HCO3- and in HCO3(-)-free solutions, however, nerves incubated in 25 mM glucose did not recover functionally after anoxia, whereas nerves bathed in solutions 2 or 3 showed a complete restitution of CNAP. We conclude that high glucose availability and low PO2 in the combination with decreased buffering power and/or inhibition of HCO3(-)-dependent pH regulation mechanisms may damage peripheral mammalian nerves due to a pronounced intracellular acidosis

Changes in extracellular pH during electrical stimulation of isolated rat vagus nerve

Double-barrelled pH-sensitive micro-electrodes were used to record changes of extracellular pH during repetitive stimulation of isolated rat vagus nerves. It was found that a small initial alkaline shift was followed by a prolonged acidification. The acidification was correlated in time with the poststimulus undershoot of the extracellular K+ activity and with the recovery phase of the nerve conduction velocity. In the presence of ouabain, the acid component of the pH change was completely abolished (indicating a metabolic origin), whereas the alkaline component remained unaltered. These pH changes were too small to make a significant contribution to the activity-related changes in conduction velocity of the vagal C-fibres

A test to determine the site of abnormal neuromuscular refractoriness

Objective: The relative refractory period (RRP) of motor axons is an important parameter in nerve excitability tests of the recovery cycle (RC). Abnormalities may have a site in the axonal membrane, the neuromuscular junction, or in a dysfunction of the muscle. We aimed in this study to determine the site of abnormality, using a modified protocol of the conventional RC test, whereby an additional supramaximal stimulus is added at the same interstimulus interval as in RC recordings (RCSM). Methods: Twenty-four healthy subjects aged 37.8 ± 2.4 years (mean ± SE) were examined with median nerve excitability testing using RC and RCSM protocols at normal temperature (34.1 ± 0.2 °C). The recordings were repeated in 12 subjects after selective cooling of the thenar muscle (25.2 ± 0.7 °C) and in 12 subjects after cooling the nerve trunk at the wrist (24.9 ± 0.3 °C). Results: After cooling the nerve, RRP measured with RC and RCSM were prolonged similarly (medians by 1.8 ms, and 2.1 ms respectively). In contrast, cooling the muscle prolonged RRP measured with RC (by 1.3 ms), but did not significantly prolong RRP measured with RCSM. RRPs measured by RC and RCSM were significantly different when cooling was at the muscle (P = 5.10-4), but not when cooling was at the nerve (P = 0.57). Conclusions: A difference between RC and RCSM indicates abnormal excitability distal to the axonal membrane under the stimulating electrode. Significance: Combining RCSM with the conventional RC protocol should help to localize the site of abnormal neuromuscular refractoriness

Effects of lithium on electrical activity and potassium ion distribution in the vertebrate central nervous system

Three different regions of the vertebrate central nervous system maintained in vitro (frog spinal cord, guinea pig olfactory cortex and hippocampus) have been used to investigate how Li+ influences membrane potential, membrane resistance, action potentials, synaptic potentials and the transmembrane K+-distribution of neurons and glial cells. In view of the therapeutic action of Li+ in manicdepressive disease, a special effort was made to determine the threshold concentration for the actions of Li+ on the parameters described above. It was observed that Li+ induced a membrane depolarization of both neurons and glial cells, a decrease of action potential amplitudes, a facilitation of monosynaptic excitatory postsynaptic potentials and a depression of polysynaptic reflexes. The membrane resistance of neurons was not altered. Li+ also induced an elevation of the free extracellular potassium concentration and a decrease of the free intracellular potassium concentration. Furthermore, in the presence of Li+ a slowing of the recovery of the membrane potential of neurons and glial cells, and of the extracellular potassium concentration after repetitive synaptic stimulation was observed. The threshold concentrations for the effects of Li+ were below 5 mmol/l in the frog spinal cord and below 2 mmol/l in the guinea pig olfactory cortex and hippocampus. The basic mechanism underlying the action of Li+ may be an interaction with the transport-function of the Na+/K+ pump

NMR Evidence for Charge Inhomogeneity in Stripe Ordered La_{1.8-x}Eu_{0.2}Sr_{x}CuO_4

We report ^{17}O Nuclear Magnetic Resonance (NMR) results in the stripe ordered La_{1.8-x}Eu_{0.2}Sr_{x}CuO_4 system. Below a temperature T_q ~ 80K, the local electric field gradient (EFG) and the absolute intensity of the NMR signal of the planar O site exhibit a dramatic decrease. We interpret these results as microscopic evidence for a spatially inhomogeneous charge distribution, where the NMR signal from O sites in the domain walls of the spin density modulation are wiped out due to large hyperfine fields, and the remaining signal arises from the intervening Mott insulating regions.Comment: 4 pages, to appear in Phys. Rev. Let

Adenosine, ‘pertussis-sensitive’ G-proteins, and K+ conductance in central mammalian neurones under energy deprivation

There is a striking similarity between the effects of adenosine and of hypoxia or glucose depletion on membrane potential and conductance of hippocampal neurones in tissue slices of rat brain. Both induce a membrane hyperpolarization by an increase in potassium conductance. It seemed likely, therefore, that a rise in extracellular adenosine concentration during energy deprivation may link neuronal metabolism with membrane K+ conductance. To test this hypothesis, we have now investigated the effects of hypoxia/glucose deprivation on hippocampal neurones from pertussis toxin-treated rats. In such slices adenosine had no effect on postsynaptic membrane potential and input resistance. Nevertheless, hypoxia or glucose depletion were as effective as in controls. These data provide evidence against adenosine as the main mediator between cell metabolism and potassium conductance

AC susceptibility investigation of vortex dynamics in nearly-optimally doped REFeAsO1−x_{1-x}Fx_{x} superconductors (RE = La, Ce, Sm)

Ac susceptibility and static magnetization measurements were performed in the nearly-optimally doped LaFeAsO$_{0.9}$F$_{0.1}$ and CeFeAsO$_{0.92}$F$_{0.08}$ superconductors, complementing earlier results on SmFeAsO$_{0.8}$F$_{0.2}$ [Phys. Rev. {\bf B 83}, 174514 (2011)]. The magnetic field -- temperature phase diagram of the mixed superconducting state is drawn for the three materials, displaying a sizeable reduction of the liquid phase upon increasing $T_{c}$ in the range of applied fields ($H \leq 5$ T). This result indicates that SmFeAsO$_{0.8}$F$_{0.2}$ is the most interesting compound among the investigated ones in view of possible applications. The field-dependence of the intra-grain depinning energy $U_{0}$ exhibits a common trend for all the samples with a typical crossover field value (2500 Oe $\lesssim H_{cr} \lesssim 5000$ Oe) separating regions where single and collective depinning processes are at work. Analysis of the data in terms of a simple two-fluid picture for slightly anisotropic materials allows to estimate the zero-temperature penetration depth $\lambda_{ab}(0)$ and the anisotropy parameter $\gamma$ for the three materials. Finally, a sizeable suppression of the superfluid density is deduced in a $s^{\pm}$ two-gap scenario