PubMed Health⌕ Search

Biomedical subjects

J M Ritchie

Publications and source records attributed to J M Ritchie.

At least 55 records · Page 3Linked to original sources
PubMed1986

Events in degenerating cat peripheral nerve: induction of Schwann cell S phase and its relation to nerve fibre degeneration.

Severance of a peripheral nerve leads to a characteristic series of events in the distal stump, including the dissolution of axons and myelin and the proliferation of Schwann cells within their basal lamina. This study examines the relationship between the spatial-temporal pattern of the induction of the Schwann cell S phase, loss of the structural and functional properties of axolemma, and the clearance of myelin debris in the cat tibial nerve. Nerve transection stimulated a monophasic increase in [3H]thymidine incorporation that peaked at 4 days post-transection throughout an 80-mm length of distal stump. Light microscope autoradiography revealed prominent incorporation into Schwann cells of myelinated fibres. Treatment of distal stumps with mitomycin C at the time of nerve transection greatly retarded thymidine incorporation and clearance of myelin debris, but not the time course of axonal degeneration, decline in the synthesis of the major myelin glycoprotein, P0, or the onset of ovoid formation. Nerve transection also greatly reduced the specific uptake of [3H]saxitoxin (STX), a ligand which binds to voltage-sensitive sodium channels. Binding in the distal stump fell precipitously to 20% of the normal at 4 days post-transection, concurrent with the peak of thymidine incorporation. This low level of binding was maintained for periods of up to 70 days, demonstrating that some STX binds to structures other than axons in denervated distal stumps. Prior treatment with mitomycin C delayed the loss of specific STX binding. In conclusion, these studies suggest that: Schwann cell DNA replication and/or mitosis regulates other events during Wallerian degeneration, including myelin degeneration, catabolism of P0 and the clearance of sodium channels from nodal axolemma; the decline in P0 synthesis and/or shift to synthesis of less extensively processed P0 is independent of the induction of Schwann cell S phase; and Schwann cells enveloping myelinated axons enter S phase within a 24-h period throughout the entire 80-mm length of distal stump.

Animals↗
PubMed1985

The role of post-traumatic mitosis in elevation of anaerobic metabolism enzyme (lactic acid dehydrogenase) activity in degenerating central and peripheral nerve.

Glial and Schwann cells undergo marked biochemical and morphological alterations following axonal injury. In the present experiments, the extent of enzyme activity associated with anaerobic (LDH, lactic dehydrogenase) vs aerobic (SDH, succinic dehydrogenase) respiration was assessed distal to the site of nerve fiber injury. Studies were performed in rat central (optic) and peripheral (sciatic) nerves at 2, 7 and 14 days postoperatively (d.p.o.). In sciatic nerves, LDH activity rose 3-fold in traumatized (vs unoperated control) nerve tissue between 2 and 7 d.p.o. and remained elevated at 14 d.p.o. SDH activity in traumatized nerve was equal to that in unoperated nerve at 7 d.p.o., but decreased at 14 d.p.o. LDH activity in optic nerve at 2 d.p.o. was equivalent to that in control nerve, but rose approximately two-fold by 7 d.p.o. However, unlike peripheral nerve, activity in traumatized optic nerve decreased to control levels at 14 d.p.o. SDH activity in traumatized optic nerve remained unchanged at any timepoint examined. Taken in concert, these data are consistent with the hypothesis that there is an overall shift in CNS glial and Schwann cell metabolism from aerobic to anaerobic respiration following nerve injury. Additional studies were performed to determine if this shift requires prior Schwann or glial cell mitosis. Administration of mitotic inhibitor (AraC, cytosine arabinofuranoside) inhibited post-traumatic elevations in LDH activity in optic, but not peripheral nerve. No significant effect of the drug on axonal degeneration (as assessed by saxitoxin binding) was observed.

Animals↗
PubMed1985

The presence of voltage-gated sodium, potassium and chloride channels in rat cultured astrocytes.

Patch-clamp recording from the plasmalemma of rat cultured astrocytes reveals the presence of both voltage-dependent sodium and voltage-dependent potassium conductances. These conductances are similar but not identical to the corresponding conductances in the axolemma. Whereas the h infinity relation of the sodium channels has the same voltage dependence as in the nodal axolemma, the peak current-voltage relation is shifted by about 30 mV along the voltage axis in the depolarizing direction. It is speculated that the glial cells synthesize sodium and potassium channels for later insertion into the axolemma of neighbouring axons. The astrocytes also express a plasmalemmal voltage-dependent anion conductance that is turned on at about -40 mV (that is, near the resting potential of the cultured astrocytes). The channels involved are large enough to be just permeable to glutamate but not to ascorbate. It is suggested that the conductance of this channel for chloride plays a physiological role in the spatial buffering of potassium by glial cells.

Animals↗
PubMed1985

Organization of ion channels in the myelinated nerve fiber.

The functional organization of the mammalian myelinated nerve fiber is complex and elegant. In contrast to nonmyelinated axons, whose membranes have a relatively uniform structure, the mammalian myelinated axon exhibits a high degree of regional specialization that extends to the location of voltage-dependent ion channels within the axon membrane. Sodium and potassium channels are segregated into complementary membrane domains, with a distribution reflecting that of the overlying Schwann or glial cells. This complexity of organization has important implications for physiology and pathophysiology, particularly with respect to the development of myelinated fibers.

Animals↗
PubMed1985

Voltage-dependent sodium and potassium channels in mammalian cultured Schwann cells.

Cultured Schwann cells from sciatic nerves of newborn rabbits and rats have been examined with patch-clamp techniques. In rabbit cells, single sodium and potassium channels have been detected with single channel conductances of 20 pS and 19 pS, respectively. Single sodium channels have a reversal potential within 15 mV of ENa, are blocked by tetrodotoxin, and have rapid and voltage-independent inactivation kinetics. Single potassium channels show current reversal close to EK and are blocked by 4-aminopyridine. From these results, and from comparisons of single-channel and whole-cell data, we show that these Schwann cells contain voltage-dependent sodium and potassium channels that are similar in most respects to the corresponding channels in mammalian axonal membranes. Cultured rat Schwann cells also have sodium channels, but at a density about 1/10th that of rabbit cells, a result in agreement with saxitoxin binding experiments on axon-free sectioned nerves. Saxitoxin binding to cultured cells suggests that there are up to 25,000 sodium channels in a single rabbit Schwann cell. We speculate that in vivo Schwann cells in myelinated axons might act as a local source for sodium channels at the nodal axolemma.

Animals↗
PubMed1984

A calcium-activated cation-selective channel in rat cultured Schwann cells.

Calcium-activated channels, in the plasma membrane of rat cultured Schwann cells were studied in isolated 'inside-out' membrane patches. With identical (150 mM NaCl) solutions on either side of the membrane, a single channel conductance of 32 pS was calculated for inward current; the conductance was somewhat less for outward current. The channel is about equally permeable to sodium and potassium ions, but is not detectably permeable to either chloride or calcium. Under our experimental conditions the channel is activated by high (more than 10(-4) M) concentrations of calcium and is sensitive to voltage, channel activity increasing with membrane depolarization.

Animals↗
PubMed1984

On the binding of labelled saxitoxin to the squid giant axon.

Measurement of the maximum saturable binding capacity for 3H-labelled saxitoxin in well cleaned giant axons from Loligo forbesi gave a value of 290 +/- 65 (s.e.m.) binding sites per square micrometre of membrane. In samples of the fin nerve the total saturable uptake was 12.4 +/- 2.4 fmol mg-1 wet mass, corresponding to 94 sites per square micrometre for fibres whose mean diameter after fixation and embedding was estimated as 16 micron.

Animals↗
PubMed1984

Sodium channels in the axolemma of normal and degenerating rabbit optic nerve.

Section of a rabbit peripheral nerve leads to axonal degeneration and a proliferation of Schwann cells, and it is known to lead to a profound increase in the saxitoxin binding capacity of its distal portion, suggesting that Schwann cells may bind this marker for sodium channels. The present study shows, however, that crush with subsequent axonal degeneration of the central axons of the rabbit optic nerve leads to a slow monotonic fall in the saxitoxin binding capacity, which by 100 days after crush is not significantly different from zero. This suggests that central glial cells (astrocytes and oligodendrocytes) do not bind saxitoxin, and that the saturable binding of saxitoxin to this nerve is entirely to the axolemma. On this basis, the value for total saxitoxin binding capacity of the normal rabbit optic nerve, taken together with the morphometric data of D. I. Vaney & A. Hughes (J. comp. Neurol. 80, 241-252 (1976)), yields a sodium channel density of about 400-700 channels per square micrometre nodal axolemma.

Animals↗
PubMed1984

Saxitoxin binding to central and peripheral nervous tissue of the myelin deficiency (md) mutant rat.

Tritium-labeled saxitoxin binding was assayed in the CNS and PNS of normal and of 'myelin deficiency' (md) mutant rat pups. This mutant in the Wistar rat is characterized by a virtually complete amyelination of the CNS, the PNS being normal. No significant difference was found in the saxitoxin binding capacity of md and normal brain, spinal cord, or sciatic nerves. This suggests that the presence or absence of myelin does not influence the number of voltage-sensitive sodium channels in the rat.

Animals↗
PubMed1984

High conductance anion-selective channels in rat cultured Schwann cells.

Anion-selective channels, with very large single unit conductances, are present in the cell membrane of rat cultured Schwann cells measured with the patch-clamp technique. In inside-out membrane patches, channels with a conductance of about 450 pS (in symmetrical 150 mM NaCl) were observed. These channels did not become active until several minutes after the cytoplasmic surface had been exposed to the bathing medium, suggesting that these channels may normally be kept in an inactive state by some as yet unknown internal factor. The channel opened over a relatively small potential range (-10 mV to +20 mV) and closed rapidly at more positive and more negative potentials with voltage-dependent kinetics. Although the channels showed a slight permeability towards small cations the major permeability was to anions. The order of permeability was: I greater than Br greater than Cl greater than methyl SO4 greater than SO4 greater than acetate = isethionate. Aspartate and glutamate were not detectably permeant. The physiological role of these channels remains unknown.

Animals↗
PubMed1984

On the physiological role of internodal potassium channels and the security of conduction in myelinated nerve fibres.

A theoretical analysis of the passive electrical properties of normal myelinated nerve suggests that the function of the voltage-dependent potassium channels in the internodal axolemma under the myelin sheath is to permit the generation of an internodal resting potential. Calculation shows that if this internodal potential were not present, the nodal potential would be reduced (by electrotonic short-circuiting) thus impairing the security of conduction. This impairment is particularly pronounced with smaller diameter fibres.

Animals↗
PubMed1983

Reactive glial protein synthesis and early disappearance of saxitoxin binding in degenerating rat optic nerve.

The biochemistry of gliotic CNS tissue was assessed by monotoring changes in de novo protein synthesis distal to site of crush in the rat optic nerve between 3 and 20 days post-operatively. Radioactivity profiles on 12% polyacrylamide SDS gels showed reproducible peaks associated with protein(s) with dissociated molecular weights of 57K, 51K, 42K, 40K, 37K and 23K. Differences between crush and control nerves were observed with respect to the latter two peaks. De novo synthesis of 23K protein (comigrant with myelin proteolipid protein) was evident in control but not crushed nerves. Synthesis of 37K protein (identity unknown) was evident at 7, 10 and (to a lesser extent) 20 days post-operatively in crushed nerves, but not in crushed nerves at 3 days post-operatively or in unoperated nerves at any time point. The appearance of the synthesis of the 37K protein coincides with a drop in the level of functional axolemma (assessed by [3H]saxitoxin binding) in crushed nerves from 78% to 36% of control levels between 3 and 7 days post-operatively.

Animals↗
PubMed1983

Extraneuronal saxitoxin binding sites in rabbit myelinated nerve.

The changes in binding of 3H-labeled saxitoxin (STX) to rabbit sciatic nerve during axonal regeneration (after nerve crush) and during axonal degeneration (after nerve section) were measured and compared with the corresponding changes in the sciatic nerves of other mammals (rat, guinea pig, and cat). In the rabbit and rat, regeneration after nerve crush is associated with a 2- to 4-fold increase in STX binding capacity, consistent with the known corresponding increase in the number of nodes of Ranvier in regenerating nerve. Furthermore, consistent with the disappearance of nodes that occurs with Wallerian degeneration, nerve section leads to a disappearance of all, or most, of the STX binding in rat and guinea pig nerve, similar to that previously found for cat nerve. However, in the rabbit, nerve section leads to a large maintained increase in STX binding. Intraneural injection of diphtheria toxin, which is known to damage Schwann cells and which causes an increase in STX binding in intact nerves, abolishes the binding in cut nerves. It is suggested that the increased binding in cut nerves is to nonneuronal sites situated on the surface membrane of the Schwann cells, which have greatly proliferated in number as axonal degeneration has progressed. The reason for the difference between rabbits and other species and the possibility that the binding sites of rabbit Schwann cells represent functional sodium channels remain to be investigated.

Amphibian Proteins↗