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Papers by Sameh Malek
Brain Research, 2005
Na(+)-K(+)-ATPase pump failure during either anoxia or ouabain perfusion induces rapid axonal dep... more Na(+)-K(+)-ATPase pump failure during either anoxia or ouabain perfusion induces rapid axonal depolarization by dissipating ionic gradients. In this study, we examined the interplay between cation and anion transporting pathways mediating axonal depolarization during anoxia or selective Na(+)-K(+)-ATPase inhibition. Compound resting membrane (V(m)) potential of rat optic nerve was measured in a grease gap at 37 degrees C. Chemical anoxia (2 mM NaCN or NaN(3)) or ouabain (1 mM) caused a loss of resting potential to 42 +/- 11% and 47 +/- 2% of control after 30 min, respectively. Voltage-gated Na(+)-channel blockade was partially effective in abolishing this depolarization. TTX (1 microM) reduced depolarization to 73 +/- 10% (chemical anoxia) and 68 +/- 4% (ouabain) of control. Quaternary amine Na(+) channel blockers QX-314 (1 mM) or prajmaline (100 microM) produced similar results. Residual ionic rundown largely representing co-efflux of K(+) and Cl(-) during chemical anoxia in the pr...
A detailed understanding of injury mechanisms in peripheral nerve fibers will help guide successf... more A detailed understanding of injury mechanisms in peripheral nerve fibers will help guide successful design of therapies for peripheral neuropathies. This study was therefore undertaken to examine the ionic mechanisms of Ca2+ overload in peripheral myelinated fibers subjected to chemical inhibition of energy metabolism. Myelinated axons from rat dorsal roots were co‐loaded with Ca2+‐sensitive (Oregon Green BAPTA‐1) and Ca2+‐insensitive (Alexa Fluor 594) dextran‐conjugated fluorophores and imaged using confocal laser scanning microscopy. Axoplasmic regions were clearly outlined by the Ca2+‐insensitive dye, from which axonal Ca2+‐dependent fluorescence changes (FCa.ax) were measured. Block of Na+–K+ ATPase (ouabain), opening of Na+ channels (veratridine), and inhibiting energy metabolism (iodoacetate + NaN3) caused a rapid rise in FCa.ax to 96% above control after 30 min. Chemical ischemia (iodoacetate + NaN3) caused a more gradual increase in FCa.ax (54%), which was almost completely ...
Journal of Neurophysiology
Receptor-mediated calcium signaling in axons of mouse and rat optic nerves was examined by select... more Receptor-mediated calcium signaling in axons of mouse and rat optic nerves was examined by selectively staining the axonal population with a calcium indicator. Nicotine (1-50 microM) induced an axonal calcium elevation that was eliminated when calcium was removed from the bath, suggesting that nicotine induces calcium influx into axons. The nicotine response was blocked by d-tubocurarine and mecamylamine but not alpha-bungarotoxin, indicating the presence of calcium permeable, non-alpha7 nicotinic acetylcholine receptor (nAChR) subtype. Agonist efficacy order for eliciting the axonal nAChR calcium response was cytisine approximately nicotine > acetylcholine. The nicotine-mediated calcium response was attenuated during the process of normal myelination, decreasing by approximately 10-fold from P1 (premyelinated) to P30 (myelinated). Nicotine also caused a rapid reduction in the compound action potential in neonatal optic nerves, consistent with a shunting of the membrane after ope...
The Journal of neuroscience, 2003
Rundown of ionic gradients is a central feature of white matter anoxic injury; however, little is... more Rundown of ionic gradients is a central feature of white matter anoxic injury; however, little is known about the contribution of anions such as Cl Ϫ. We used the in vitro rat optic nerve to study the role of aberrant Cl Ϫ transport in anoxia/ischemia. After 30 min of anoxia (NaN 3 , 2 mM), axonal membrane potential (V m) decreased to 42 Ϯ 11% of control and to 73 Ϯ 11% in the presence of tetrodotoxin (TTX) (1 M). TTX ϩ 4,4Ј-diisothiocyanatostilbene-2,2Ј disulfonic acid disodium salt (500 M), a broad spectrum anion transport blocker, abolished anoxic depolarization (95 Ϯ 8%). Inhibition of the K-Cl cotransporter (KCC) (furosemide 100 M) together with TTX was also more effective than TTX alone (84 Ϯ 14%). The compound action potential (CAP) area recovered to 26 Ϯ 6% of control after 1 hr anoxia. KCC blockade (10 M furosemide) improved outcome (40 Ϯ 4%), and TTX (100 nM) was even more effective (74 Ϯ 12%). In contrast, the Cl Ϫ channel blocker niflumic acid (50 M) worsened injury (6 Ϯ 1%). Coapplication of TTX (100 nM) ϩ furosemide (10 M) was more effective than either agent alone (91 Ϯ 9%). Furosemide was also very effective at normalizing the shape of the CAPs. The KCC3a isoform was localized to astrocytes. KCC3 and weaker KCC3a was detected in myelin of larger axons. KCC2 was seen in oligodendrocytes and within axon cylinders. Cl Ϫ gradients contribute to resting optic nerve membrane potential, and transporter and channel-mediated Cl Ϫ fluxes during anoxia contribute to injury, possibly because of cellular volume changes and disruption of axo-glial integrity, leading to propagation failure and distortion of fiber conduction velocities.
Brain Research, 2005
Na(+)-K(+)-ATPase pump failure during either anoxia or ouabain perfusion induces rapid axonal dep... more Na(+)-K(+)-ATPase pump failure during either anoxia or ouabain perfusion induces rapid axonal depolarization by dissipating ionic gradients. In this study, we examined the interplay between cation and anion transporting pathways mediating axonal depolarization during anoxia or selective Na(+)-K(+)-ATPase inhibition. Compound resting membrane (V(m)) potential of rat optic nerve was measured in a grease gap at 37 degrees C. Chemical anoxia (2 mM NaCN or NaN(3)) or ouabain (1 mM) caused a loss of resting potential to 42 +/- 11% and 47 +/- 2% of control after 30 min, respectively. Voltage-gated Na(+)-channel blockade was partially effective in abolishing this depolarization. TTX (1 microM) reduced depolarization to 73 +/- 10% (chemical anoxia) and 68 +/- 4% (ouabain) of control. Quaternary amine Na(+) channel blockers QX-314 (1 mM) or prajmaline (100 microM) produced similar results. Residual ionic rundown largely representing co-efflux of K(+) and Cl(-) during chemical anoxia in the pr...
A detailed understanding of injury mechanisms in peripheral nerve fibers will help guide successf... more A detailed understanding of injury mechanisms in peripheral nerve fibers will help guide successful design of therapies for peripheral neuropathies. This study was therefore undertaken to examine the ionic mechanisms of Ca2+ overload in peripheral myelinated fibers subjected to chemical inhibition of energy metabolism. Myelinated axons from rat dorsal roots were co‐loaded with Ca2+‐sensitive (Oregon Green BAPTA‐1) and Ca2+‐insensitive (Alexa Fluor 594) dextran‐conjugated fluorophores and imaged using confocal laser scanning microscopy. Axoplasmic regions were clearly outlined by the Ca2+‐insensitive dye, from which axonal Ca2+‐dependent fluorescence changes (FCa.ax) were measured. Block of Na+–K+ ATPase (ouabain), opening of Na+ channels (veratridine), and inhibiting energy metabolism (iodoacetate + NaN3) caused a rapid rise in FCa.ax to 96% above control after 30 min. Chemical ischemia (iodoacetate + NaN3) caused a more gradual increase in FCa.ax (54%), which was almost completely ...
Journal of Neurophysiology
Receptor-mediated calcium signaling in axons of mouse and rat optic nerves was examined by select... more Receptor-mediated calcium signaling in axons of mouse and rat optic nerves was examined by selectively staining the axonal population with a calcium indicator. Nicotine (1-50 microM) induced an axonal calcium elevation that was eliminated when calcium was removed from the bath, suggesting that nicotine induces calcium influx into axons. The nicotine response was blocked by d-tubocurarine and mecamylamine but not alpha-bungarotoxin, indicating the presence of calcium permeable, non-alpha7 nicotinic acetylcholine receptor (nAChR) subtype. Agonist efficacy order for eliciting the axonal nAChR calcium response was cytisine approximately nicotine > acetylcholine. The nicotine-mediated calcium response was attenuated during the process of normal myelination, decreasing by approximately 10-fold from P1 (premyelinated) to P30 (myelinated). Nicotine also caused a rapid reduction in the compound action potential in neonatal optic nerves, consistent with a shunting of the membrane after ope...
The Journal of neuroscience, 2003
Rundown of ionic gradients is a central feature of white matter anoxic injury; however, little is... more Rundown of ionic gradients is a central feature of white matter anoxic injury; however, little is known about the contribution of anions such as Cl Ϫ. We used the in vitro rat optic nerve to study the role of aberrant Cl Ϫ transport in anoxia/ischemia. After 30 min of anoxia (NaN 3 , 2 mM), axonal membrane potential (V m) decreased to 42 Ϯ 11% of control and to 73 Ϯ 11% in the presence of tetrodotoxin (TTX) (1 M). TTX ϩ 4,4Ј-diisothiocyanatostilbene-2,2Ј disulfonic acid disodium salt (500 M), a broad spectrum anion transport blocker, abolished anoxic depolarization (95 Ϯ 8%). Inhibition of the K-Cl cotransporter (KCC) (furosemide 100 M) together with TTX was also more effective than TTX alone (84 Ϯ 14%). The compound action potential (CAP) area recovered to 26 Ϯ 6% of control after 1 hr anoxia. KCC blockade (10 M furosemide) improved outcome (40 Ϯ 4%), and TTX (100 nM) was even more effective (74 Ϯ 12%). In contrast, the Cl Ϫ channel blocker niflumic acid (50 M) worsened injury (6 Ϯ 1%). Coapplication of TTX (100 nM) ϩ furosemide (10 M) was more effective than either agent alone (91 Ϯ 9%). Furosemide was also very effective at normalizing the shape of the CAPs. The KCC3a isoform was localized to astrocytes. KCC3 and weaker KCC3a was detected in myelin of larger axons. KCC2 was seen in oligodendrocytes and within axon cylinders. Cl Ϫ gradients contribute to resting optic nerve membrane potential, and transporter and channel-mediated Cl Ϫ fluxes during anoxia contribute to injury, possibly because of cellular volume changes and disruption of axo-glial integrity, leading to propagation failure and distortion of fiber conduction velocities.