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Biomedical subjects

S H Young

Publications and source records attributed to S H Young.

At least 37 records · Page 2Linked to original sources
PubMed1994

Quantal and non-quantal ACh release at developing Xenopus neuromuscular junctions in culture.

1. Single acetylcholine receptor (AChR) channel openings, detected by the whole-cell patch clamp technique, were used to monitor quantal and non-quantal ACh release at synapses in 1- and 2-day-old co-cultures of Xenopus embryonic motoneurons and muscle cells. motoneuron growth cones in ways that presumably reflect muscle-nerve inductive influences and the development of neurotransmitter release mechanisms. 2. Miniature endplate currents (MEPCs) occurred at a mean frequency of approximately 0.6 s-1 with a skewed distribution and mean amplitude of about twenty channel openings. In addition, occasional brief episodes of rapid deviations in the baseline were observed in some cells, with mean amplitudes of 4-8 pA and durations of a few hundred milliseconds. However, these episodes did not closely resemble summated openings of AChR channels. Moreover, where tested, these episodes were not blocked by curare; and comparable episodes were seen in an uninnervated myocyte. Thus they appear not to reflect ACh release from the nerve terminal. 3. Single-channel openings that might have been responses to non-quantal release of ACh were observed at rates of 0.9-12.3 min-1 (mean 3.0 min-1), only 1-5 times the rate of spontaneous AChR channel openings in uninnervated myocytes (mean 1.4 min-1). 4. We conclude that there is no significant non-quantal ACh leak from the presynaptic contacts in these immature synapses under these culture conditions. This is in disagreement with other, less direct, experimental reports, but consistent with findings in mature frog motor nerve terminals.

Acetylcholine↗
PubMed1993

Prevention of intraocular pressure rise following intravitreal injection.

The intraocular pressure change produced by an intravitreal injection of ganciclovir, 2 mg in 0.1 ml, was studied in patients with cytomegalovirus retinitis. Using a Tono-pen XL to measure the intraocular pressure (IOP) of four patients (six eyes) we found the mean pressure immediately following injection was 44.5 mm Hg. Measurements taken on separate occasions after a 30 mm Hg decompression of the eye for 15 minutes before the injection showed a mean IOP of only 20.6 mm Hg after the injection. Mercury bag decompression of the eye significantly reduced the rise in IOP following intravitreal injection (difference in the mean IOP rise = 26.4 mm Hg, df = 54, t = 7.67, p < 0.001). Without ocular decompression before injection, all patients complained of temporary loss of vision, and reflux of the injection solution was frequently observed. Use of ocular decompression also reduced the discomfort of the injection. Throughout the full course of treatment by this means there were no adverse effects on the visual acuity. This technique is recommended to those performing intravitreal injections.

Cytomegalovirus Infections↗
PubMed1992

High dose intravitreal ganciclovir in the treatment of cytomegalovirus retinitis.

OBJECTIVE: To assess the role of intravitreal administration of high doses of ganciclovir as a supplement and alternative to intravenous administration in the treatment of cytomegalovirus (CMV) retinitis in patients with acquired immunodeficiency syndrome (AIDS). DESIGN: A retrospective study of visual outcome, relapse and complications of intravenous and high dose intravitreal administration of ganciclovir alone and in combination. METHOD: Twenty-three patients with AIDS and CMV retinitis (37 eyes) were examined by the authors and notes, fundal drawings and photographs reviewed. Initially patients were treated with intravenous ganciclovir alone and given supplementary intravitreal therapy for relapse or vision-threatening retinitis; however, later patients were managed with combination maintenance ganciclovir or maintenance intravitreal treatment alone. RESULTS: Relapse and loss of vision occurred frequently in patients treated with intravenous ganciclovir alone or in combination with intermittent intravitreal therapy. Eyes managed with maintenance high dose intravitreal ganciclovir alone or in combination with intravenous treatment did not relapse or lose vision. The most important complication of intravenous administration of ganciclovir was neutropenia (73% of patients), whereas that of intravitreal therapy was endophthalmitis (three eyes). CONCLUSION: High dose ganciclovir given intravitreally effectively suppressed CMV retinitis and preserved vision without adverse systemic effects or deterioration of quality of life.

AIDS-Related Opportunistic Infections↗
PubMed1990

Diffusional and electrokinetic redistribution at the synapse: a physicochemical basis of synaptic competition.

Coinnervating nerve terminals may compete for "stabilizing factors" confined within the postsynaptic cell. The competition could be achieved through a diffusion-mediated trapping the factor, facilitated by an activity-dependent electrokinetic migration of the factor toward the synaptic site. We have examined the evidence for diffusional and electrokinetic motions of cell surface and cytoplasmic components, the profile and magnitude of the electric field produced by the synaptic current, and the plausibility that these motions underlie the process of synaptic competition.

Animals↗
PubMed1989

Constraints on the interpretation of nonquantal acetylcholine release from frog neuromuscular junctions.

In the frog nerve-muscle preparation, there is evidence for nonquantal release of acetylcholine (ACh) at a level 100 times that attributable to spontaneous quantal events (miniature endplate potentials). It is widely assumed that nonquantal release occurs near the sites of quantal release (active zones) in the nerve terminal, close to the postsynaptic muscle membrane. This high level of nonquantal ACh release has led to the suggestion that it may serve a trophic function at the nerve-muscle junction. However, the precise origin and mechanism of nonquantal release have not been determined. We have used outside-out patches of ACh receptor-rich membrane as a sensitive technique for the direct measurement of ACh release from highly localized regions of enzymatically treated nerve terminals and have found little detectable ACh "leakage" from active-zone regions. If all of the nonquantal ACh release were localized to the under surface of the nerve terminal, we estimate that we would have detected more than 10 times the low level detected with our patch probe. Furthermore, although quantal release was easily measurable, vesicular exocytosis (hypothesized to insert ACh transport proteins into the plasma membrane, thereby producing the leak) did not increase single-channel activity in the patch probe above that attributable to quantal release. We conclude that, at rest, the active-zone region of nerve terminals is not a major source of nonquantal ACh release and that vesicular exocytosis does not noticeably increase the level of nonquantal release from the nerve terminal. Thus, with biochemical measurements that indicate that spontaneous ACh release is relatively unchanged by prior denervation, these results question the assumed source and mechanism of nonquantal release and the suggestion that leakage of ACh from active-zone regions plays a trophic role in nerve-muscle interaction.

Acetylcholine↗
PubMed1989

Direct measurement of ACh release from exposed frog nerve terminals: constraints on interpretation of non-quantal release.

1. Acetylcholine (ACh) release from enzymatically exposed frog motor nerve terminals has been measured directly with closely apposed outside-out clamped patches of Xenopus myocyte membrane, rich in ACh receptor channels. When placed close to the synaptic surface of the terminal, such a membrane patch detects both nerve-evoked patch currents (EPCs) and spontaneous quantal 'miniature' patch currents (MPCs), from a few micrometres length of the terminal, in response to ACh release from the nearest three to five active zones. 2. Chemical measurements of ACh efflux from whole preparations revealed a spontaneous release rate of 4.1 pmol (2 h)-1, and no significant difference in resting efflux between enzyme-treated and control preparations. The ratio of enzyme-treated to contralateral control muscle efflux averaged 1.17, indicating that enzyme treatment did not affect spontaneous ACh release. Vesamicol (1.7 microM), which blocks the ACh transporter in synaptic vesicles, decreased the spontaneous release of ACh to 67% of control. 3. In the absence of nerve stimulation, the frequency of single-channel openings recorded by outside-out patch probes adjacent to nerve terminals was very low (1-2 min-1), and little different at a distance of hundreds of micrometres, suggesting that if ACh was continually leaking from the terminal in a non-quantal fashion, the amount being released near active zone regions on the terminal was below the limit of detection with the patches. 4. Direct measurements of the sensitivity of the patches, coupled with calculated ACh flux rates, lead to the conclusion that the amount of ACh released non-quantally from the synaptic surface of the frog nerve terminal is less than one-tenth the amount expected if all non-quantal release is from this region of the terminal membrane. 5. Following a series of single nerve shocks or a 50 Hz train of nerve stimuli, the frequency of asynchronous single-channel openings increased for several seconds. This transient increase in channel openings was not sensitive to movement of the patch electrode a significant distance (4 microns) away from the active sites, or to manipulations previously reported to block non-quantal transmitter leakage, including addition of 10 mM-Ca2+ or 1.7 microM-vesamicol to the bath. These channel openings appear to be due to an accumulation of ACh which originated from many evoked quanta, and not the effect of locally increased non-quantal ACh release due to nerve stimulation. 6. We conclude that transmitter leakage at adult frog terminals is either localized to a source other than the synaptic surface of the nerve terminal, or released in a widespread and diffuse fashion from many sources, which may include the nerve terminal.

Acetylcholine↗
PubMed1988

Improved reporting of adverse drug reactions.

A program using satellite pharmacists to improve adverse drug reaction (ADR) reporting in an 1100-bed teaching hospital is described. Because relying on physicians to report ADRs had met with little success (only six reports in seven years), the pharmacy department proposed that pharmacists in satellite pharmacies on patient-care units be called upon to identify and report ADRs. To begin this program, an ADR team composed of a physician, pharmacist, and nurse made weekly rounds of the satellite pharmacies to assist pharmacists in identifying potential ADRs. The FDA definition of an ADR was adopted. Also, inservice education programs about ADR reporting were conducted for pharmacists and nurses. Currently, suspected ADRs are reported to the satellite pharmacist, who forwards a completed drug reaction report form to the assistant director for clinical pharmacy services. Reports are discussed quarterly by the ADR subcommittee of the pharmacy and therapeutics committee; the sub-committee members determine whether any follow-up action is needed. In the first three years after implementation of this program, 306 ADRs were reported; 90% of the reports were filed by pharmacists. An ADR reporting system based on reporting by staff pharmacists has been effective in increasing the number of reported reactions and pharmacist involvement in monitoring patients for ADRs.

Drug-Related Side Effects and Adverse Reactions↗
PubMed1987

Quantal release of transmitter is not associated with channel opening on the neuronal membrane.

The traditional view that quantal release of neurotransmitter results from the fusion of transmitter-containing vesicles with the neuronal membrane has been recently challenged. Although various alternative mechanisms have been proposed, a common element among them is the release of cytoplasmic transmitter, which, in one view, could occur through large conductance channels on the presynaptic membrane. Six nerve-muscle cell pairs were examined with a whole-cell patch clamp for the presence of such channels that are associated with the production of miniature end-plate potentials. Examination of the neuronal membrane current during the occurrence of 822 miniature end-plate potentials produced no evidence of large channels. Thus it is unlikely that quantal release is mediated by such channels in the neuromuscular junction.

Animals↗
PubMed1987

Opening of single gap junction channels during formation of electrical coupling between embryonic muscle cells.

Gap junctions, which are low-resistance intercellular pathways, may contribute to normal embryogenesis by allowing cell-to-cell passage of as yet unidentified regulatory or inductive signals. But little is known about the properties of newly formed single junctional channels which are the basis of the communicating junctions. Reported here are the first direct measurements of current passing through single junctional channels as they form. Individual pairs of embryonic Xenopus muscle cells in culture were manipulated into contact, allowing control of the onset time and area of cellular contact, and current was recorded with the patch clamp technique. The opening of single channels which pass current between the two cells at a conductance of about 100 pS was observed within minutes of cell-cell contact. The channels opened one-at-a-time, and once formed, remained open for long periods of time, with infrequent brief closures. This suggests that formation of electrical coupling between two cells proceeds by addition of single conducting junctional channels one channel-at-a-time.

Animals↗
PubMed1986

Spontaneous release of transmitter from the growth cones of Xenopus neurons in vitro: the influence of Ca2+ and Mg2+ ions.

To determine whether spontaneous release of transmitter from the growth cones of neurons exhibits properties similar to the spontaneous release which occurs from the neurons at the neuromuscular junction, release of transmitter from the growth cones of Xenopus neurons in culture was monitored in salines containing varying calcium and magnesium concentrations. Release was monitored by use of an outside-out piece of muscle membrane attached to a patch clamp electrode. Spontaneous release of transmitter from the growth cones in standard saline (2 mM CaCl2, 1 mM MgCl2) produces clusters of single-channel openings in the muscle membrane. Clusters are seen to consist of two types: a series of high-frequency channel openings, called "bursts," and clusters of low-frequency channel openings called "singles." The bursts were identified and examined for their possible relationship to MEPP-producing release, and the singles were identified and examined for their possible relationship to "leak" release of the neuromuscular junction. When the external saline contains high calcium (10 mM CaCl2, 1 mM MgCl2) or high magnesium (2 mM CaCl2, 9 mM MgCl2), the frequencies of both "bursts" and "singles" was greatly reduced. This reduction in release persists if the neurons are grown in the high-calcium or high-magnesium solutions. When the saline is a low-calcium solution (0 mM CaCl2, 3 mM MgCl2) the growth cones release transmitter at rates similar to those from standard saline. These results indicate that although the spontaneous release from the growth cone shares one characteristic with the leak release, neither the burst nor the singles release from the growth cones share exact relationship with either the MEPP producing release or the leak release. This suggests that further development of the mechanisms for spontaneous release of neurotransmitter occurs after nerve-muscle contact.

Acetylcholine↗
PubMed1985

Response of nerve growth cone to focal electric currents.

Monopolar electric current pulses were focally applied through a micropipette to the growth cone of Xenopus embryonic neurons in culture. Application of the current directly in front of the growth cone modulated the rate of growth cone extension: Negative (sink) currents increased the growth rate, while positive (source) currents reduced the growth rate. When the currents were applied in a direction perpendicular to the direction of the neurite growth, both negative and positive currents produced inhibitory effects. Application of a negative focal current at a 45 degree angle with respect to the direction of neurite growth resulted in an oriented growth of the neurite toward the current sink. However, after the growth cone had been attracted to the vicinity of a current sink, further extension of the neurite was inhibited. These current effects occur rapidly after the onset of the current application, and are at least partially reversible within 1 hr after the termination of the current. The magnitude of current density required to induce a growth cone response was found to be in the order of a few pA per micron2. Such current density is close to that which may be generated at the muscle cell surface by the acetylcholine molecules released from the growth cone during the early phase of nerve-muscle contact.

Animals↗
PubMed1985

Three types of transmitter release from embryonic neurons.

Prior to the contact with their target muscle cells in culture, growth cones of many isolated Xenopus embryonic neurons release acetylcholine (ACh) spontaneously. Using patch clamp techniques, this release can be detected by an outside-out patch of muscle membrane placed near the growth cone. Intracellular recording from innervated muscle cells showed spontaneous miniature endplate potentials (MEPPs) of varying amplitudes. Amplitude histograms showed a skewed distribution with multiple peaks, suggesting the existence of subunits in either the quantal packages of ACh released by the nerve terminal or in the postsynaptic muscle response. In addition to the quantal ACh release reflected by MEPPs, nerve terminal also release a large amount of ACh in a non-quantal fashion. This non-quantal ACh release is revealed by the hyperpolarization of the muscle membrane following extracellular application of curare or alpha-bungarotoxin, as well as by denervation of the muscle cell.

Acetylcholine↗
PubMed1984

Ferritin conjugates as specific magnetic labels. Implications for cell separation.

Concanavalin A coupled to the naturally occurring iron storage protein ferritin is used to label rat erythrocytes and increase the cells' magnetic susceptibility. Labeled cells are introduced into a chamber containing spherical iron particles and the chamber is placed in a uniform 5.2 kG (gauss) magnetic field. The trajectory of cells in the inhomogeneous magnetic field around the iron particles and the polar distributions of cells bound to the iron particles compare well with the theoretical predictions for high gradient magnetic systems. On the basis of these findings we suggest that ferritin conjugated ligands can be used for selective magnetic separation of labeled cells.

Animals↗
PubMed1983

Rapid lateral diffusion of extrajunctional acetylcholine receptors in the developing muscle membrane of Xenopus tadpole.

We have studied the lateral diffusion of acetylcholine (ACh) receptors in the extrajunctional region of developing myotomal muscle cell membrane of Xenopus tadpoles by a technique of local inactivation. The myotomal muscle cell surfaces of Xenopus tadpoles were exposed to external solution by gently removing the skin of the tail. The density of ACh receptors was monitored by membrane depolarizations in response to iontophoretically applied pulses of ACh. A pulse of alpha-bungarotoxin was pressure ejected onto the exposed fiber surface, resulting in a rapid local inactivation of the ACh receptors. With time, the functional ACh receptors diffused into the region of inactivation, producing a recovery of ACh response. That the observed recovery of ACh sensitivity is due to diffusion of ACh receptors from the unexposed undersurface of the fiber to the inactivated region was evidenced by the following: (1) no recovery was observed following prolonged toxin application; (2) pretreatment of the muscle cells with concanavalin A, which cross-links and immobilizes ACh receptors, prevented recovery; (3) mapping of ACh response along the muscle cell axis showed that the recovery cannot be accounted for by diffusion along the longitudinal axis of the fiber; and (4) the diffusion coefficients observed after scaling the recovery rate with fiber radius fell within a small range (1.5 to 4.0 X 10(-9) cm2/sec), consistent with diffusion of ACh receptors around the fiber circumference. This finding of rapid lateral diffusion within developing tadpole myotomal muscle membrane supports the notion that the localization of ACh receptors induced by innervation could be achieved by a "diffusion-trap" mechanism where the nerve contact region serves as a trap for rapidly diffusing receptors in the membrane.

Acetylcholine↗