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D O Smith

Publications and source records attributed to D O Smith.

At least 55 records · Page 3Linked to original sources
PubMed1987

Acetylcholine metabolism and choline availability at the neuromuscular junction of mature adult and aged rats.

1. The balance between acetylcholine (ACh) synthesis and degradation and the availability of choline were studied in the neuromuscular junction of rats aged 10 (mature adult) and 28 (aged) months. Endogenous and 2H4-labelled variants of ACh and choline were assayed during steady-state stimulated and resting conditions using gas chromatography-mass spectrometry. 2. Endogenous ACh levels were 34% less per nerve terminal in the older rats; in non-innervated tissue, the levels were 12-14% as large as in innervated tissue. Endogenous choline levels in innervated and non-innervated tissue were similar and were 28% higher in the older animals. 3. Accumulation of 2H4-labelled choline was significantly greater in the 28-month animals; the specific activity at equilibrium was 60% higher in the older rats. 4. Incorporation of [2H4]choline into [2H4]ACh was significantly faster in the aged rats; times to half-maximal values were 2.0 and 0.6 min in the 10- and the 28-month animals, respectively. There was no measurable synthesis of ACh in the non-innervated tissue. 5. There were no major age-related differences in the amount of ACh released during stimulation between 1 and 20 Hz. In contrast, choline efflux during stimulation was significantly greater in the aged animals; flux values (+/- S.E. of mean) were 0.48 (+/- 0.014) and 0.66 (+/- 0.038) nmol/min for the 10- and the 28-month rats, respectively. 6. Under resting conditions, endogenous ACh efflux was significantly greater in the older rats; the rates (+/- S.E. of mean) were 2.00 (+/- 0.21) and 3.05 (+/- 0.43) pmol/min for the younger and the older animals, respectively; similarly, choline efflux was greater in the aged tissue. 7. These results indicated that lower intracellular ACh levels can be attributed to enhanced leakage rather than to decreased synthesis. Greater ACh efflux is accompanied by correspondingly greater choline uptake in the 28-month rats; this is associated with greater choline efflux and higher steady-state choline levels in the aged tissue.

Acetylcholine↗
PubMed1987

Extracellular ATP modulates calcium uptake and transmitter release at the neuromuscular junction.

Adenosine, AMP, ADP, and ATP were tested for their ability to modulate evoked quantal transmitter release at excitor-opener nerve terminals in the crayfish walking leg. Only ATP was found to have a significant effect, inhibiting release by 43%. To determine whether the effects of extracellular ATP on transmitter release were related to changes in free Ca2+ levels in the nerve terminal, net 45Ca uptake was measured in regions of the nerve near the terminal; although resting 45Ca uptake was increased by 5 mM exogenous ATP. Thus, extracellular ATP normally inhibits evoked transmitter release; this is associated with reduced stimulation-induced net Ca2+ uptake into the synaptic terminal. To explore the possibility that metabolic factors might be involved, nerve ATP levels were reduced by greater than or equal to 50% by omitting glucose from the bathing solution. Although quantal content and synaptic delay were unaffected by reduced intracellular ATP, under these conditions 5 mM exogenous ATP failed to reduce quantal release. Five millimolars of ATP did increase synaptic facilitation, however, which is consistent with a reduced ability to regulate intracellular free Ca2+. Therefore, under conditions of reduced intracellular ATP, the increased resting Ca2+ uptake produced by 5 mM ATP is not buffered completely. This leads to elevated free Ca2+ levels in the nerve terminal, increasing the amount of transmitter released following an action potential.

Acetylcholine↗
PubMed1986

The use of the signal averaging computer for evaluation of peripheral nerve problems.

Utilization of signal averaging computers to analyze evoked nerve potentials generated by an electrical stimulus is a useful adjunct to clinical evaluation. Currently, somatosensory evoked responses are useful for the evaluation of the peripheral nerve with suspected compression syndromes, neuropathies of unknown etiologies, post-traumatic injuries, neuromas-in-continuity, and brachial plexus injuries. Further refinements in the system will result in increased clinical application of this useful modality. Present data indicate that neuromas with a conduction velocity of less than 10 meters per second or an amplitude of less than one fourth of normal should be considered unfavorable neuromas, and if clinically correlated, resection and repair anticipated. Conduction velocity slowing of greater than 20 meters per second across a potential site of compression neuropathy at the wrist using short-segment study should also be viewed as undesirable and will usually correlate with clinical findings on an abnormal von Frey test. The somatosensory response in compression neuropathy usually correlates better with the clinical finding than with an EMG.

Adult↗
PubMed1986

Increased presynaptic ATP levels coupled to synaptic activity at the crayfish neuromuscular junction.

Levels of ATP and related adenylates were measured in the terminal region of efferent nerves in the crayfish opener muscle using the luciferin-luciferase method. Following 1 min of stimulation at 50 Hz, the average (+/- SE) ATP content rose from 13.4 (+/- 1.5) to 19.0 (+/- 2.1) nmol/mg dry weight. The amounts of ADP, AMP, and the phosphagen phosphoarginine did not change significantly. Thus, the increased ATP was not derived from any of these potential sources. The increase was found to depend on synaptic activation, however, for its magnitude was directly related to the concentration of extracellular Ca2+, and it was blocked when CoCl2, verapamil, ruthenium red, or gamma-methylglutamate and picrotoxin were added to the bath. Addition of ATP to the bath solution also increased nerve ATP levels. Based upon measurements of sucrose distribution, only 50% of this increase was in the extracellular water space. The remainder of the ATP had either entered the nerve, become adsorbed extracellularly, or both. Addition of 2-deoxy-D-glucose and gamma-32P-ATP to the bath resulted in the formation of 32P-2-deoxy-D-glucose-6-P by the nerve. This suggests that a fraction of the extracellular ATP does enter the neuron chemically intact. To determine whether exogenous ATP is the source of the increased ATP measured in the nerve following stimulation, the bath was assayed for ATP. Stimulation did cause ATP levels to increase significantly; however, the maximum concentration was 3 orders of magnitude lower than that required to increase ATP levels in resting nerve.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphate↗
PubMed1985

Electrophysiological measurement of glucose uptake into motor nerve terminals of mature adult and aged rats.

Glucose uptake into motor nerve terminals innervating the extensor digitorum longus (EDL), diaphragm and soleus, was monitored indirectly in mature adult and aged rats by recording changes in quantal transmitter release induced by glucose. Glucose entered the nerve terminals innervating the EDL more rapidly than either the diaphragm or the soleus. There was an age-related decrease in glucose uptake into the nerve terminals innervating all 3 muscles, although this change was not significant statistically.

Age Factors↗
PubMed1985

Differential changes in the end-plate architecture of functionally diverse muscles during aging.

Architectural changes at the neuromuscular junctions of three functionally diverse muscles, the diaphragm, extensor digitorum longus (EDL), and soleus, were studied in rats from adulthood (10 mo) through senescence (31 mo). End-plate structure was examined using a silver-gold impregnation of the axon and its terminal arborization. The mean number of terminal branches per end plate, including those that appeared to exhibit signs of sprouting or degeneration, were examined as a function of age. Nerve terminal branch length, the number of major myelinated axon branches per end plate, and end-plate area were also examined. To ascertain trends in the data, smoothing of mean values for time-series analysis was performed. Both fast-twitch diaphragm and EDL muscles revealed more pronounced fluctuations in nerve terminal branch number during aging than did the slow-twitch soleus. Smoothed trends of the mean number of nerve terminal branches per end plate show that nerve terminal proliferation increased with age at the diaphragm neuromuscular junction but decreased at the hindlimb junctions. Age-related changes in average terminal branch number progressed gradually in all muscles until 25 mo of age, after which we observed a precipitous increase at 28 mo and subsequent decline at 31 mo. Trends of change indicate that although sprouting within the diaphragm was maintained during aging, both EDL and soleus experienced a general decline in sprouting. Degeneration generally decreased at all nerve-muscle junctions with aging, although the decline began at a later age in the diaphragm. Analysis of mean data values indicates that in both the EDL and soleus, sprouting and degeneration attained minimum values at 25 mo of age. This was followed by a significant increase at 28 mo and then a decline by 31 mo of age. In general, sprouting fluctuated more severely than degeneration. We conclude that despite the variability from one age to the next, age-related trends of change are evident in nerve terminal branch number. The direction of these changes appears to be influenced by the degree to which muscle activity is maintained during aging. However, regardless of functional status, all three muscles experienced dramatic changes in end-plate morphology after 25 mo of age.

Aging↗
PubMed1984

Coated vesicle morphology and sub-populations at the neuromuscular junction.

Serial sections of frog cutaneous pectoris neuromuscular junctions were examined to determine if isolated coated vesicles in one section are connected to infoldings of the presynaptic membrane in adjacent sections or are truly pinched off from the plasmalemma. Twenty percent of the coated vesicles examined serially were isolated from plasmalemma. In addition, two populations of coated vesicles were observed: those the size of synaptic vesicles and a smaller population (8% of total) of larger diameter (100 nm).

Animals↗
PubMed1984

Factors governing speed of action potential conduction and neuromuscular transmission in aged rats.

Impaired motor coordination is a widespread attribute of senescence. As this might be due to slower action potential propagation in peripheral nerves or to prolonged delays of neuromuscular transmission, axon conduction velocity, synaptic delays, and associated morphologic features governing speed of conduction were studied in the phrenic-diaphragm preparations of rats aged 10 (mature adult) and 28 (aged) months. Conduction velocity and synaptic delay of the motoneurons were measured with electrophysiologic recording techniques. Speed of conduction did not change with age, but the average synaptic delay increased from 0.28 in the younger rats to 0.40 ms in the 28-month-old animals. Nerve composition was examined using the electron microscope. There were no age-related changes in the number of myelinated or nonmyelinated axons, capillaries, or myelin sheath invaginations. Moreover, fiber diameter and myelin sheath width did not vary with age. The total extracellular space was about 32% larger in the aged rats: the synaptic cleft widths were 42.7 and 50.6 nm in the 10- and the 28-month animals, respectively. We concluded that in the rat age-related changes in the transmission properties of peripheral nerve are minor. Furthermore, temporal motor deficits during aging must be associated primarily with changes in central pathways or at the muscular apparatus.

Action Potentials↗
PubMed1984

Acetylcholine storage, release and leakage at the neuromuscular junction of mature adult and aged rats.

Acetylcholine (ACh) content, release during stimulation and spontaneous leakage were studied at the diaphragm neuromuscular junction in rats aged 10 and 28 months. In addition, quantal release statistics were determined. The results were related to the number of nerve terminals per end-plate and to the age of the animals. Nerve terminal ACh content was lower in the 28-month-old rats; in nonstimulated tissue the average (+/- S.E.) ACh levels were 29.3 (+/- 4.1) and 17.2 (+/- 2.4) fmol per end-plate in the 10- and 28-month-old rats, respectively. During nerve stimulation, greater amounts of ACh were released by the older rats. After 1000 impulses, the average (+/- S.E.) ACh release was 3.2 (+/- 0.2) and 3.9 (+/- 0.2) fmol per end-plate in the 10- and the 28-month-old animals, respectively; however, the amounts released per nerve terminal were not significantly different. The fraction of the resting ACh content released per action potential was greater in the aged rats; this may be the cause of enhanced synaptic depression in the older animals. Leakage of ACh from cytoplasmic sources was assessed by measuring the hyperpolarization following application of d-tubocurarine (dTC). The hyperpolarizing responses were of similar magnitude in both age groups; however, the older rats were less sensitive to ACh, indicating that cytoplasmic leakage was greater in these animals. Miniature end-plate potentials (m.e.p.p.s) occurred twice as frequently in the aged rats; this increase could be attributed to an age-related increase in the number of nerve terminals per end-plate. After correcting for variations in membrane input resistance and resting potential, m.e.p.p. amplitudes were compared and found to be significantly smaller in the aged rats. Quantal release during nerve stimulation was measured and found to obey binomial statistics. Estimated values of m and n were significantly larger in the older rats; there were no age-related differences in p. The increases in m and n could be attributed to the larger number of nerve terminals per end-plate in the aged animals. Acetylcholinesterase (AChE) activity was assayed. In innervated tissue there were no significant differences with age; however, in non-innervated tissue AChE activity was significantly higher in the 28-month-old rats.

Acetylcholine↗
PubMed1983

Variable activation of synaptic release sites at the neuromuscular junction.

To study variable activation of synaptic release sites during repetitive stimulation, quantal release was measured at two synaptic sites innervated by the same presynaptic axon on the opener muscle of the crayfish walking leg using focal extracellular recording techniques. Binomial statistics describing this release were then calculated. Values of m, the average amount of quanta released in response to a presynaptic action potential, measured at one site were sometimes found to exhibit stationarity while those at the other site on the same fiber were not. This nonstationarity was attributed to conduction failures in the terminal arborization. To minimize bias in the estimates of the parameters n and p, the data were screened carefully for nonuniformity of the release probabilities and nonstationarity. Immediately from the onset of stimulation of frequencies from 5 to 30 Hz, 65% of the sites exhibited nonuniformity. After 1 min of repetitive stimulation, this was reduced to 10%. Using uniform and stationary data, the parameters n and p were estimated during facilitation and depression. Facilitation was associated with an increase only in n at lower frequencies (5 to 10 Hz) of repetitive stimulation and in p, and occasionally n, at the higher frequencies (10 to 30 Hz). Depression was associated primarily with decreases in n. The results indicate frequency-dependent variability in the activation of synaptic release sites.

Action Potentials↗
PubMed1983

Extracellular potassium levels and axon excitability during repetitive action potentials in crayfish.

Changes in extracellular K+ levels were measured during repetitive stimulation of the excitor axon of the opener muscle of the crayfish walking leg. Mesaxon channels, through which K+ might diffuse away from the periaxonal ('Frankenhaeuser-Hodgkin') space, were examined in electron micrographs; they were seen every 2-10 micron along the axon, and their average (+/- S.D.) length and width were 3.0 (+/- 1.6) micron and 19.8 (+/- 8.9) nm, respectively. Intracellular recordings revealed a 40 ms after-depolarization following an action potential; this was attributed to elevated levels of extracellular K+. During stimulation at 50 Hz, this resulted in a depolarizing shift of the membrane potential between impulses; the average depolarization was 9.3 mV, which corresponds to a 4.3 mM increase in extracellular K+. Using K+-selective micro-electrodes, changes in extracellular K+ activity, delta aK, were measured at distances ranging from 10 to 50 micron from the axon; during 50 Hz stimulation, delta aK rose within 15 s to a maximum value of 1.1 mM which was maintained at a steady level in most preparations. Conduction failure occurred in several preparations after at least 90 s of stimulation; levels of delta aK were not abnormally high in these cases. Soaking the axon for at least 15 min in saline with extracellular K+ levels at least 18 mM above normal values was necessary to cause blockage in unstimulated nerves. Soaking the preparation for 30 min in 10(-3) M-ouabain resulted in a 48% increase in the maximum values of delta aK during 50 Hz stimulation. It is concluded that K+ accumulates extracellularly during axon stimulation and that the extent of this accumulation is reduced by active uptake mechanisms; however, this accumulation probably cannot directly block action potential conduction, for neither the magnitude nor the kinetics of K+ build-up approach values shown to reduce excitability.

Action Potentials↗
PubMed1983

Axon conduction failure under in vivo conditions in crayfish.

Action potential discharge in an identified axon was recorded in 'intact' crayfish, and conduction block occurred during prolonged high-frequency activity. These in vivo recordings were also used to trigger stimulation of the same axon in vitro; conduction block occurred during high-frequency bursts in small terminal branches. Quantal release was distributed in a bimodal fashion, with numerous failures, due to these occurrences of conduction block. These results indicate that conduction block may occur in intact animals and that it may also cause selective recruitment of specific nerve terminals during normal behaviour.

Action Potentials↗
PubMed1982

Restricted diffusion of extracellular potassium at the neuromuscular junction of aged rats.

Changes in the extracellular K+ activity, aK, were recorded at the neuromuscular junction in aged and young rats. After the onset of stimulation, aK rose to a peak value which was maintained until stimulation was terminated. The peak value was greater in the older rats at any given distance. Also, it took longer for aK to attain one-half its peak value in the older animals. It is concluded that diffusion of K+ away from the end plate is probably slower in aged than in young rats.

Acetylcholine↗
PubMed1982

Effects of hypertonic solutions on quantal transmitter release at the crayfish neuromuscular junction.

1. The effects of a hypertonic bathing medium, containing either NaCl or melezitose, on the average number (m) and the time course of quantal release following an action potential were studied using focal extracellular recording methods at synaptic sites on the opener muscle of the crayfish leg.2. After the application of hypertonic saline, the rate of spontaneous quantal release increased but m decreased to a new level within 1 min; the extent of depression depended on the magnitude of the increase in tonicity until the osmolarity was 50% greater than the normal value of 0.43 osmol/l (Osm), but greater increases in tonicity exerted little further effect.3. The synaptic delays were increased and distributed over a longer range of time in hypertonic solutions; also, the latency between the first and second quantal releases in a multiple response to a single nerve impulse was also increased.4. Hypertonicity had no significant effect on the conduction velocity of the action potential, the independence of successive quantal releases in the same response, or the uniformity of the rise and fall of the probability of quanta release following an action potential, alpha(t).5. The time course of alpha(t) is prolonged in hypertonic solutions; this was manifested as an increase in the time constant of the exponential decline in alpha(t) from its peak value following the nerve impulse.5. When the potentiating agent 5-hydroxytryptamine (5-HT) was added to the hypertonic saline, m was increased, and the time course of alpha(t) was prolonged further than in the hypertonic solution alone; 5-HT produced no change in the time course of alpha(t) when it was added to normal saline, although m was increased.6. It is concluded that in this preparation hypertonicity decreases the rate of release of each quantum from the nerve terminal following a nerve impulse.

Action Potentials↗
PubMed1982

Anion blockers inhibit impulse-evoked quantal transmitter release at the crayfish neuromuscular junction.

1. The effects of the anion-transport blocking reagents Probenecid, 4,4'diisothiocyanostilbene-2,2'-disulphonate (DIDS), and pyridoxal phosphate on the number of quanta released (m) and the time course of the probability of quantal release, alpha(t), following an action potential were studied at single synaptic sites on the opener muscle in the crayfish leg.2. Low concentrations (10-50 muM) of DIDS or Probenecid produced decreases in m to about 30-50% of the initial value; at higher concentrations (100 muM or greater), evoked release failed. Much greater concentrations (500 muM) of pyridoxal phosphate were necessary to produce decreases in m.3. Synaptic depression in 50 muM-Probenecid was reversible by removing the agent; after treatment with 50 muM-DIDS, however, m partially regained the initial value upon removal of the drug.4. The time course of alpha(t) after an action potential was unaffected by any of the reagents.5. It is concluded that the reagents decrease the number of quanta evoked by a nerve impulse by decreasing the magnitude, but not the time course, of the average rate of quantal release following an action potential; it is suggested that nerves and other secretory cells may share a common mechanism during exocytosis that is subject to modification by these agents.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1980

Mechanisms of action potential propagation failure at sites of axon branching in the crayfish.

1. The phenomena leading to action potential conduction block during repetitive stimulation of the excitor axon of the opener muscle in the crayfish walking leg were studied. 2. Action potentials, recorded extracellularly with micro-electrodes, failed to propagate past sites of axonal bifurcation following at least 3000 impulses; reduction of the rate or brief cessation of stimulation resulted in restored conduction. 3. Failure occurred initially at branch points located most peripherally and then more centrally as stimulation continued; this centripetal progression of the site of block resulted in a stepwise reduction of the number of synaptic terminals from which transmitter was released. 4. Prior to conduction failure, the conduction velocity and the sodium inward current of the action potentials decreased. 5. Local application of hyperpolarizing current or of physiological saline with low [K+] in the vicinity of a block can restore propagation; thus depolarization of the membrane most probably causes failure. 6. Soaking the preparation for as long as 2 hr in the metabolic inhibitor 2,4-dinitrophenol had no effect on the number of stimulus impulses before initial conduction block; however, the time required for recovery from the failure was prolonged. 7. The number of impulses prior to block was related directly to the temperature of the preparation; this had a Q10 calculated to be about 1 . 3. 8. It is suggested that during repetitive activity, the K+ gradient across the membrane is reduced, resulting in depolarization and eventually in conduction failure.

Action Potentials↗
PubMed1980

Morphological aspects of the safety factor for action potential propagation at axon branch points in the crayfish.

1. In the crayfish, regions of axonal bifurcation where action potential propagation failed during repetitive activity were visualized using modulation-contrast optics. To determine if a reduced safety factor for conduction at the branch sites has a morphological basis, the geometrical ratio (GR) of axon diameters, where GR = (d(1) (3/2)+d(2) (3/2))/d(p) (3/2), was calculated; d(1), d(2), and d(p) are the diameters of the two daughter branches and the parent axon, respectively.2. Values of GR at sites of conduction block were usually slightly higher than 1, indicating a small drop in safety factor; however the values were well below the level at which failure might occur due to an impedance mismatch.3. Calculated values of GR were generally higher in the most peripheral axon regions where propagation was observed to block most readily.4. Short collaterals containing numerous synaptic vesicles and apparent release sites were observed to have relatively low values of GR (about 0.8); collaterals containing densely populated mitochondria were characterized by relatively high values of GR (about 1.3).5. It is concluded that the morphological characteristics of the axon in the branching region are conducive to an inherent low safety factor for conduction, but that they cannot play a major role in block development.

Action Potentials↗