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Biomedical subjects

R Miledi

Publications and source records attributed to R Miledi.

320 records · Page 18Linked to original sources
PubMed1967

The timing of calcium action during neuromuscular transmission.

1. When a nerve-muscle preparation is paralysed by tetrodotoxin, brief depolarizing pulses applied to a motor nerve ending cause packets of acetylcholine to be released and evoke end-plate potentials (e.p.p.s), provided calcium ions are present in the extracellular fluid.2. By ionophoretic discharge from a 1 M-CaCl(2) pipette, it is possible to produce a sudden increase in the local calcium concentration at the myoneural junction, at varying times before or after the depolarizing pulse.3. A brief application of calcium facilitates transmitter release if it occurs immediately before the depolarizing pulse. If the calcium pulse is applied a little later, during the period of the synaptic delay, it is ineffective.4. It is concluded that the utilization of external calcium ions at the neuromuscular junction is restricted to a brief period which barely outlasts the depolarization of the nerve ending, and which precedes the transmitter release itself.5. The suppressing effect of magnesium on transmitter release was studied by a similar method, with ionophoretic discharges from a 1 M-MgCl(2)-filled pipette. The results, though not quite as clear as with calcium, indicate that Mg pulses also are only effective if they precede the depolarizing pulses.

Animals↗
PubMed1967

Spontaneous synaptic potentials and quantal release of transmitter in the stellate ganglion of the squid.

1. Several kinds of synapses have been studied in the stellate ganglion of the squid.2. A small electric coupling was found between giant fibres in different stellar nerves.3. Post-synaptic potentials recorded from the cells of small axons are composite, indicating that there are converging inputs from several pre-ganglionic fibres.4. Spontaneous miniature synaptic potentials were recorded from all types of synapses. Miniature potentials in the cells of small axons had a slower time course than those in the giant fibre system.5. Tetrodotoxin abolished nerve impulses in the ganglion but did not prevent the spontaneous quantal release of transmitter from the terminals, or its action on the post-synaptic membrane; nor did it prevent the increase in rate of release produced by depolarization of the presynaptic fibre.6. Glutamate depolarized the giant fibre when applied iontophoretically to the synaptic region. Similar doses applied intracellularly were without effect.

Action Potentials↗
PubMed1967

A study of synaptic transmission in the absence of nerve impulses.

1. The axo-axonic giant synapse in the stellate ganglion of the squid has been used to study synaptic transmission.2. When nerve impulses have been eliminated with tetrodotoxin, synaptic transfer of potential changes can still be obtained by applying brief depolarizing pulses to the presynaptic terminal.3. Suitably matched pulses are as effective as the normal presynaptic spike in evoking post-synaptic potentials. The synaptic delay and the time course of the post-synaptic potential are very similar to that in the normal preparation.4. The synaptic transfer (input/output) characteristic has been studied under different experimental conditions. With brief (1-2 msec) current pulses, post-synaptic response becomes detectable when the presynaptic depolarization exceeds about 30 mV. The post-synaptic potential increases about tenfold with 10 mV increments of presynaptic depolarization.5. Calcium increases, magnesium reduces the slope of the synaptic transfer curve. The influences on this curve of (i) duration of the pulse, (ii) preceding level of membrane potential, (iii) position of recording electrode, (iv) rate of repetitive stimulation are described.6. After loading the synaptic terminal with tetraethylammonium ions, large inside-positive potentials can be produced in the terminal and maintained for many milliseconds.7. By raising the internal potential to a sufficiently high level, synaptic transfer becomes suppressed during the pulse, and the post-synaptic response is delayed until the end of the pulse.8. This observation is in accord with a prediction of the ;calcium hypothesis', viz. that inward movement of a positively charged Ca compound, or of the calcium ion itself, constitutes one of the essential links in the ;electro-secretory' coupling process of the axon terminal.

Action Potentials↗
PubMed1966

The action of calcium on neuronal synapses in the squid.

1. The isolated stellate ganglion of the squid (L. pealii) was studied with intracellular and extracellular micro-electrodes. Three or four nerve fibres in the preganglionic nerve establish synaptic relations with the giant axon in the last stellar nerve. Accordingly, 1-3 small presynaptic spikes (< 1 mV) could be recorded from within the post-synaptic axon.2. A micro-electrode was inserted in the presynaptic fibre and used to polarize and record simultaneously. In the distal (giant) synapse, hyperpolarization of the ending produced an increase in the size of the presynaptic action potential and post-synaptic potential (PSP). Depolarization had the opposite effect. These effects of polarization took more than 10 sec to develop fully, and declined with a similar time course at the end of polarization. Analogous results were obtained with two other preganglionic fibres, which make contacts in the proximal synaptic region.3. The second of a pair of preganglionic impulses evoked a PSP larger than the first. This facilitation of PSP was sometimes accompanied by a small increase in the size of the second action potential in the presynaptic axon. At some shorter intervals, the second presynaptic action potential was reduced in amplitude, but the PSP was still increased. Hyperpolarization of the presynaptic terminal increased the size of both PSPs in a pair and abolished the facilitation. With stronger hyperpolarization the second PSP was even smaller than the first.4. Removing or reducing the Ca in the bathing fluid reversibly abolished the post-synaptic response. The small presynaptic spikes remained practically unaffected. In these conditions a nerve impulse still invaded the ending and normal action potentials could be recorded from the pre-synaptic terminal. This shows that electrical coupling between pre- and post-synaptic axons is insufficient to account for synaptic transmission.5. In low-Ca solution synaptic transmission could be restored locally by extracellular ionophoretic application of Ca to a small portion of the synapse. At sensitive spots a post-synaptic current (recorded with the Ca pipette) and PSP could be detected earlier than 1 sec after commencing the application of Ca.6. Ca was ineffective when injected intracellularly into the presynaptic fibre at a spot where extracellular ionophoresis of Ca restored the PSP.7. The results indicate that synaptic transmission in the squid stellate ganglion is not electrical but due to the release of an unidentified transmitter. Release of this transmitter by the presynaptic nerve impulse requires the presence of Ca in the external medium. During the impulse Ca would combine with a ;Ca-receptor' in the membrane and initiate the reactions which lead to transmitter release. It appears that the ;Ca-receptor' is only accessible from the outside of the membrane.

Animals↗
PubMedDate not supplied

A Windows software package to record from voltage-clamped Xenopus oocytes.

We have written a software package to record, display and analyze membrane currents elicited by neurotransmitter receptors or voltage-activated channels in voltage-clamped Xenopus oocytes. This suite, which consists of 4 applications, runs on IBM-PC compatible microcomputers under Windows 3.1. The recording programs use Direct Memory Access (DMA) to access the analog-digital board. The first program, NicPulse, is aimed at studying voltage-activated channels. It delivers voltage steps to the voltage-clamp and records the resulting membrane current. The second program, NicScope, emulates a dual-trace digital oscilloscope. It operates either in continuous or triggered mode, and is used chiefly to display neurotransmitter-induced responses in oocytes. The third recording program, VRamp, automatically determines the voltage-current relationship of drug-activated responses (I/V curve), by applying a voltage ramp and recording the subsequent clamping current. The last program, NicView, is designed to analyze records taken with NicScope and NicPulse. The present paper will discuss several issues regarding the design of these programs, and will give a brief description of each application.

Animals↗
PubMedDate not supplied

Messenger RNA from human brain induces drug- and voltage-operated channels in Xenopus oocytes.

Sodium channels and receptors to serotonin and kainate were 'transplanted' from human brain into frog oocytes, by isolating messenger RNA from a fetal brain, and injecting it into Xenopus laevis oocytes. The mRNA was translated by the oocyte and induced the appearance of functional receptors and channels in its membrane. This approach renders drug- and voltage-operated channels of the human brain more amenable to detailed study.

Animals↗