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Biomedical subjects

R Miledi

Publications and source records attributed to R Miledi.

At least 235 records · Page 13Linked to original sources
PubMed1979

End-plate currents and acetylcholine noise at normal and myasthenic human end-plates.

1. The amplitudes and time courses of miniature end-plate currents (m.e.p.c.s) have been compared at normal and myasthenic (MG) human end-plates studied under voltage clamp. The m.e.p.c. amplitude at MG end-plates is reduced to about one third normal; mean m.e.p.c. (normal) = 2.6 +/- 0.2 nA, mean m.e.p.c. (MG) = 1.0 +/- 0.1 nA. The decay time constant of m.e.p.c.s (tau m.e.p.c.) is very similar at normal and MG end-plates; tau m.e.p.c. (normal) = 1.70 +/- 0.1 msec, tau m.e.p.c. (MG) = 1.80 +/- 0.13 msec (Vm = - 80 mV. T = 23 degrees C). 2. The equilibrium potential of the end-plate current (e.p.c.) at normal and myasthenic human end-plates is close to 0 mV. 3. Decay time constants tau e.p.c. and tau m.e.p.c. increase exponentially with membrane hyperpolarization. The voltage sensitivity of the time constants was similar at normal and MG end-plates. 4. Both normal and myasthenic e.p.c.s are greatly prolonged in the presence of neostigmine (10(-6) g/ml.). At the same time the voltage sensitivity of tau e.p.c. is slightly reduced. 5. In response to steady ionophoretically applied ACh the mean membrane currents obtained at MG end-plates were smaller than the normal under similar conditions. 6. Analysis of end-plate current noise obtained during the steady application of acetylcholine (ACh) to voltage clamped normal and MG human end-plates showed that the amplitude of the elementary current event (gamma) and the average channel life-fime (tau noise) was similar at the two sites: tau noise (normal) - 1.54 +/- 0.04 msec, tau noise (MG) = 1.60 +/- 0.11 msec; gamma(normal) - 22.3 +/- 1.57 PS, gamma (MG) = 20.25 +/- 1.93 pS (Vm = - 80 mV, T = 23 degrees C). The voltage sensitivity of the channel life time, measured from end-plate current noise, was similar at normal and MG end-plates. 7. At normal human end-plates a packet of transmitter opens about 1500 channels whereas at MG end-plates a packet opens only about 600 channels. It is calculated that the size of the transmitter packets released from MG-terminals is at least as large as the packet of the ACh released from normal human nerve terminals.

Acetylcholine↗
PubMed1979

Effects of some divalent cations on synaptic transmission in frog spinal neurones.

1. Synaptic transmission between dorsal root afferents and motoneurones was studied in the isolated and hemisected spinal cord of frogs, using intracellular and extracellular recording techniques, and ionic substitutions of divalent cations in the bathing fluid. 2. Delayed components of excitatory post-synaptic potentials (e.p.s.p.s) evoked in motoneurones by dorsal root supramaximal stimuli, as well as the Ca2+-dependent slow after-hyperpolarization which follows antidromic spikes, were reversibly blocked by superfusing the cords with 'Ca2+-free' media containing Co2+ (4 mM) or Mg2+ (6-10 mM). However, short latency e.p.s.p.s persisted in these media for more than 8 hr. 3. The minimum synaptic delay of the Co2+ and Mg2+, resistant e.p.s.p.s, measured from the peak negativity of the extracellularly recorded presynaptic spike to the onset of the e.p.s.p., was 0.3 msec at 10 +/- 1 degrees C. 4. The Co2+, Mg2+-resistant e.p.s.p.s were graded, and could be elicited by stimulation of segmental or adjacent roots. Those evoked by each of two adjacent roots showed linear summation when the roots were stimulated simultaneously. 5. The Co2+, Mg2+-resistant e.p.s.p.s decreased in amplitude at stimulating frequencies between 10 and 100 Hz, and with paired stimuli at intervals shorter than 20-40 msec. These reductions in amplitude were paralleled by decreases in amplitude of the presynaptic population spike. 6. Solutions free of divalent ions, containing EGTA (2 mM) abolished the Co2+, Mg2+-resistant e.p.s.p.s. They remained blocked for a variable time after returning to Ca2+-free Ringer containing Mg2+ (8 mM). Their continued abolition at this stage is probably not due to changes in electrical properties of motoneuronal membranes. Eventually, the Mg2+-resistant e.p.s.p.s started recovering in the Ca2+-free Ringer containing Mg2+. The time of onset of this recovery depended on the duration of exposure to EGTA. 7. Sr2+ (2-11 mM), although less effective than Ca2+, restored the composite e.p.s.p.s evoked by dorsal root supramaximal stimuli, as well as the Ca2+-dependent slow after-hyperpolarization of the motoneurone. The composite e.p.s.p.s could not be restored with Ba2+ (2-10 mM). 8. The results suggest that the Co2+, Mg2+-resistant e.p.s.p is generated by electrical coupling between some afferent fibres (probably primary afferents) and motoneurones. The after-effects of EGTA treatments probably reflect uncoupling of electrotonic junctions. In contrast, the delayed components of the composite e.p.s.p.s are generated through chemical synapses whose divalent cation requirement is similar to that of the neuromuscular junction.

Action Potentials↗
PubMed1978

A re-examination of curare action at the motor endplate.

Recent evidence indicates that curare, in addition to its competitive' interference with endplate receptors, can block open ionic channels by a 'non-competitive' action on the activated acetylcholine-receptor complex. These findings called for further study of the kinetic behaviour of endplate channels and their modification by curare. Examining impulse-evoked endplate currents and acetylcholine-induced current fluctuations, it is found that the lifetime of the open channel is shortened by relatively high concentrations of curare (greater than 5 micrometer), an effect which shows up most strikingly at hyperpolarized levels of membrane potential (-130 mV and above). No shortening of this kind is observed when a neuromuscular block of equal or greater intensity is produced by a dose of alpha-bungarotoxin. Two other neuromuscular blocking agents, gallamine and pancuronium are shown to have an action on channel kinetics which cannot be explained by competitive receptor binding, but conforms to the hypothesis of rapidly repeated blocking and unblocking of individual ion channels, which had been proposed originally to account for the endplate action of local anaesthetics.

Acetylcholine↗
PubMed1978

Kainic acid and synaptic transmission in the stellate ganglion of the squid.

Kainate, a conformational analogue of glutamate, blocks synaptic transmission across the giant synapse of the squid. In the presence of blocking doses of kainate, impulses continue to propagate into the nerve terminal, but action potentials are slightly reduced in size and the subsequent hyperpolarization is greatly diminished. Kainate depolarizes the postsynaptic axon. Since the depolarizing action of kainate is confined to the postsynaptic membrane, it appears that kainate can combine with the receptors which are normally activated by the transmitter. This results in a diminished effect of the transmitter released by a presynaptic nerve impulse.

Animals↗
PubMed1978

Glutamate current noise: post-synaptic channel kinetics investigated under voltage clamp.

1. Analysis of voltage-clamped noise has been used to investigate the operation of glutamate receptors and associated channels at the locust nerve-muscle junction. Channels opened by glutamate and an agonist have been compared. 2. Glutamate-induced current fluctuations have a power spectrum with a single (1/frequency2) component which fits a simple model for the operation of channels. The form of the spectra for glutamate voltage noise and for 'background' noise has been determined. 3. The single channel conductance was estimated from the spectra, gamma glutamate = 122 +/- 0.4 (S.E.) pS. This estimate is independent of membrane potential and of the amplitude of membrane current change produced by glutamate. 4. The rate constant, alpha, for the closing of glutamate-operated channels depends exponentially on membrane potential, conforming to the equation alpha = approximately alphaeetaVm (approximately alpha = 0.26 +/- 0.014 msec-1, eta = 0.0054 +/- 0.001 msec-1); the duration of the channel lifetime (tau) decreases with hyperpolarization. Membrane potential dependence of alpha reduces as temperature is lowered. 5. For glutamate-operated channels, the temperature dependence of alpha and gamma fits the Arrhenius equation; alpha and gamma decrease exponentially as a function of T-1 (degrees K) with a descrete change in slope at about 6 degrees C, indicating a change in the activaiton energies of the respective rate processes. 6. Spectra of quisqualate-induced current fluctuations have the same form as spectra for glutamate noise. The single channel conductance was estimated from the spectra, gamma quisqualate = 120 +/- 3.9 (S.E.) pS. 7. The rate constnt, alpha, for the closing of quisqualate-induced channels depends exponentially on membrane potential. The duration of the open state for quisqualate channels was 2.2 times longer than for glutamate channels. 8. For glutamate receptors the voltage-sensitivity of the channel life-time is in the opposite direction to that of ACh receptors in vertebrate muscle. Possible explanations for the sharp change in the activation energy of the rate processes associated with the channel are discussed.

Alanine↗
PubMed1977

Isolation and characterization of presynaptically acting neurotoxins from the venom of Bungarus snakes.

1. Five presynaptic toxins have been isolated in pure form from the venom of Bungarus multicinctus and Bungarus caeruleus and named beta1, beta2, beta3, beta4, and beta-ceruleotoxin. 2. They differ in electrophoretic mobility and amino acid composition, while all have the same molecular weight (22000) and are composed of two subunits of molecular weight 9000 and 12000. 3. The toxins have phospholipase A activity when assayed with both natural and synthetic phospholipids, and this activity requires the presence of Ca2+ ions. 4. beta-Bungarotoxin (beta3) binds 1 mol of Ca2+ per mol of protein and this binding induces a conformational change as detected by fluorescence measurements in the presence of the dye 8-anilino-1-naphthalene sulfonic acid. 5. The phospholipase activity of all the toxins is lost when a critical histidine residue is modified with p-bromophenacyl bromide. 6. As a result of the modification the lethality of the toxins is greatly reduced. 7. Native toxin causes a rapid decrease in amplitude of end-plate potentials, followed by a transient increase and subsequent decrease, until transmitter release is completely abolished. The modified toxin still causes the early decrease in release but toxin action does not progress to complete block. 8. The rate of blockage of transmitter release by native toxin is reduced in the presence of modified toxin. 9. It is concluded that phospholipase activity plays an important role in the action of this class of toxins at the neuromuscular junction.

Amino Acids↗