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D Colquhoun

Publications and source records attributed to D Colquhoun.

At least 55 records · Page 3Linked to original sources
PubMed1993

Hypertension and heart disease. The need for clear thinking.

Hypertension is a major risk factor for vascular disease-cerebral, cardiac and peripheral. The systolic blood pressure is the most important prognostic factor. An extensive work-up searching for a cause is not indicated. Controlling hypertension has been shown to decrease incidence of stroke, heart failure, myocardial infarction and sudden death. Thiazide and beta-blockers have stood the test of time and have the best track record in preventing complications of hypertension. Surrogate endpoints of therapy, such as effect on insulin resistance, are interesting from an academic point of view. But they are no substitute for randomised clinical trials and the real endpoints of stroke, myocardial infarction and sudden death.

Adrenergic beta-Antagonists↗
PubMed1992

Single-channel conductances of NMDA receptors expressed from cloned cDNAs: comparison with native receptors.

To cast light on the subunit composition of native NMDA-type glutamate receptors, four cloned subunits of the NMDA receptor have been expressed, in pairs, in Xenopus oocytes, and their single-channel properties have been measured. The conductances of the channels, and their characteristic patterns of sublevel transitions, turn out to be useful diagnostic criteria for subunit composition. The NR1-NR2A and NR1-NR2B combinations (which have identical TM2 sequences) are very similar to each other. Both have 50 pS openings and brief 40 pS sublevels (in 1 mM external Ca2+), with similar mean lifetimes and frequencies. They also show close quantitative resemblance to the channels of hippocampal CA1 and dentate gyrus cells and of cerebellar granule cells, except that the NR1-NR2A combination has a lower glycine sensitivity than the native channels. In contrast, the NR1-NR2C combination produces a channel with 36 pS and 19 pS conductances of similar (brief) duration; these closely resemble the 38-18 pS channels that have previously been observed in large cerebellar neurons in culture (together with 50 pS channels).

Animals↗
PubMed1992

Rapid decay of averaged single-channel NMDA receptor activations recorded at low agonist concentration.

The NMDA class of glutamate receptors have the unique property of binding some agonists, including glutamate, for a very long period of time. One manifestation of this is that brief (1 ms) application of glutamate (1 mM) produces a slowly decaying current, the major component of which has a time constant of approximately 200 ms. Application of glutamate at low concentrations allows identification of groups ('superclusters') of openings in the data record that probably correspond to a single period during which one or more molecules of glutamate are bound to the receptor, i.e. a single activation of the channel. The length of such superclusters is long on average (74 ms); the longest component of the distribution has a duration of approximately 300 ms, and comprises about 25% of the area. However, aligning many superclusters to obtain an average current reveals that the decay is mainly fast; the major component has a time constant of around only 5 ms. It is shown that incorporation of a distribution of first latencies (from the time of the jump to the first opening) can explain at least part of this discrepancy.

Animals↗
PubMed1992

Asymptotic distributions of apparent open times and shut times in a single channel record allowing for the omission of brief events.

The openings and shuttings of individual ion channel molecules can be described by a Markov process with discrete states in continuous time. The predicted distributions of the durations of open times, shut times, bursts of openings, etc. are all described, in principle, by mixtures of exponential densities. In practice it is usually found that some of the open times, and the shut times, are too short to be detected reliably. If a fixed dead-time tau is assumed then it is possible to define, as an approximation to what is actually observed, an 'extended opening' or e-opening which starts with an opening of duration at least tau followed by any number of openings and shuttings, all the shut times being shorter than tau; the e-opening ends when a shut time longer than tau occurs. A similar definition is used for e-shut times. The probability densities, f(t), of these extended times have previously been obtained as expressions which become progressively more complicated, and numerically unstable to compute, as t-->infinity. In this paper we present, for the two-state model, an alternative representation as an infinite series of which a small number of terms gives a very accurate approximation of f (t) for large t. For the general model we present an asymptotic representation as a mixture of exponentials which is accurate for all except quite small values of t. Some simple model-independent corrections for missed events are discussed in relationship to the exact solutions.

Ion Channel Gating↗
PubMed1992

ATP receptor-mediated synaptic currents in the central nervous system.

Until now, the only well documented, fast excitatory neurotransmitter in the brain has been glutamate. Although there is evidence for adenosine 5'-triphosphate (ATP) acting as a transmitter in the peripheral nervous system, suggestions for such a role in the central nervous system have so far not been supported by any direct evidence. Here we report the recording of evoked and miniature synaptic currents in the rat medial habenula. The fast rise time of the currents showed that they were mediated by a ligand-activated ion channel rather than a second messenger system, thus limiting the known transmitter candidates. Evidence was found for the presence on the cells of glutamate, gamma-aminobutyric acid, acetylcholine and ATP receptors, but not for 5-hydroxytryptamine (5HT3) or glycine receptors. The evoked currents were unaffected by blockers of glutamate, gamma-aminobutyric acid or acetylcholine receptors but were blocked by the ATP receptor-blocker, suramin and the desensitizing ATP receptor-agonist alpha,beta-methylene-ATP. Our evidence identifies for the first time synaptic currents in the brain, mediated directly by ATP receptors.

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed1992

Activation of N-methyl-D-aspartate receptors by L-glutamate in cells dissociated from adult rat hippocampus.

1. Single channel recording techniques were used to study the ion channel openings resulting from activation of N-methyl-D-aspartate (NMDA) receptors by the agonist glutamate. Patches were from cells acutely dissociated from adult rat hippocampus (CA1). Channel activity was studied at low glutamate concentrations (20-100 nM) with 1 microM-glycine, in the absence of extracellular divalent cations. 2. Channel openings were to two main conductance levels corresponding to 50 pS and 40 pS openings in extracellular solution with 1 mM-Ca2+. Around 80% of openings were to the large conductance level. The single channel conductances increased as extracellular Ca2+ was reduced. 3. Distributions of channel open times were described by three exponential components of 87 microseconds, 0.91 ms and 4.72 ms (relative areas of 51, 31 and 18%). Most long openings were to the large conductance level. 4. The channel closed time distribution was complex, requiring five exponential components to describe it adequately. Of these five components, at least three, with time constants of 68 microseconds, 0.72 ms and 7.6 ms (relative areas of 38, 12 and 17%) represent gaps within single activations of the receptor. The presence of a component with a mean of 7.6 ms is notable because gaps of this length have not previously been identified as being within single NMDA receptor channel activations. 5. Channel activations were identified as including gaps underlying at least the first three closed time components. Activations consisted of clusters of channel openings. Distributions of the length of these clusters had mean time constants of 88 microseconds, 3.4 ms and 32 ms (relative areas of 45, 25 and 30%). Long clusters contained short, intermediate and long duration openings as well as subconductance openings. The open probability within clusters averaged 0.62. Three components were evident in distributions of the number of openings per cluster. These had mean values of 1.22, 3.2 and 11 openings per cluster. 6. An inverse correlation was evident between the length of adjacent open and closed times. When open intervals were separated into groups based on the length of adjacent gaps, the time constants of the exponential components in these conditional open time distributions were independent of the length of the adjacent gap. This supports the idea that the NMDA receptor channel gating has the properties of a discrete Markov process. 7. The long duration of NMDA receptor channel clusters suggests that they contribute to the slow time course of the NMDA receptor-mediated synaptic current.

Animals↗
PubMed1992

Action of brief pulses of glutamate on AMPA/kainate receptors in patches from different neurones of rat hippocampal slices.

1. Outside-out patches were isolated from granule cells of dentate gyrus and pyramidal cells of CA3 and CA1 regions of rat hippocampal slices. Patches were exposed briefly to L-glutamate using a piezo-driven double-barrelled application pipette. 2. Applications of glutamate (1 mM) of 1 ms duration activated patch currents which rose and decayed rapidly. The 20-80% rise time of these glutamate receptor (GluR)-mediated currents was usually 0.2-0.6 ms. At -50 mV the peak current varied from 10 to 500 pA in different patches. 3. The peak current-voltage relation for brief pulses of 1 mM glutamate was virtually linear in normal extracellular solution for patches from the three cell types (-100 to 60 mV). 4. The permeability of GluR channels activated at the peak to Ca2+, relative to K+, was less than 0.1 for all three cell types (under bi-ionic conditions with Ca2+ on the extracellular side and K+ on the intracellular side of the membrane). 5. The offset decay time constant of the current following 1 ms pulses of 1 mM glutamate was brief, with mean values of 3.0 +/- 0.8, 2.5 +/- 0.7, and 2.3 +/- 0.7 ms for dentate, CA3 and CA1 cell patches, respectively. Offset time constants were independent of membrane potential and independent of glutamate concentration (200 microM and 1 mM) for the three cell types. 6. Applications of 1 mM glutamate of 100 ms duration showed that glutamate responses desensitized rapidly. The time constants for desensitization were 9.4 +/- 2.7, 11.3 +/- 2.8, and 9.3 +/- 2.8 ms for patches from dentate, CA3 and CA1 cells respectively. Desensitization time constants were only weakly dependent on glutamate concentration (200 microM and 1 mM) for the three cell types. Thus offset time constants are about four times faster than desensitization time constants for both glutamate concentrations. 7. Double pulse application of glutamate indicated that even a 1 ms pulse of 1 mM glutamate causes partial (about 60%) desensitization of GluR channels. The time course of recovery from desensitization was slower in dentate gyrus granule cell patches than in CA3 or CA1 pyramidal cell patches. 8. Desensitization was studied at equilibrium by exposing patches to low glutamate concentrations for at least 15 s before a 1 ms test pulse of 1 mM glutamate.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

Glutamate activation of a single NMDA receptor-channel produces a cluster of channel openings.

Activations of the N-methyl-D-aspartate (NMDA) receptor by glutamate were studied in outside-out patches from CA1 cells in rat hippocampal slices. Very low glutamate concentrations (20-100 nM) were used so that individual receptor activations would be well separated. The shut-time distribution contained at least five components, only the longest component being obviously concentration dependent. The three briefest shut-time components had time constants of 56 microseconds, 0.68 ms and 10.1 ms; all of these were independent of glutamate concentration. An individual activation of the receptor therefore produces a long cluster of channel openings that contains longer gaps than have been reported for receptor activations by other fast neurotransmitters. In addition, (i) some activations may contain still longer (mean 78 ms) shut periods generating 'super clusters', and (ii) a significant amount of NMDA current may be carried by prolonged ('high P(open)') periods during which the channel is open for most of the time. Such periods occur intermittently even at these very low glutamate concentrations. It is suggested that the slow time course of the NMDA receptor-mediated synaptic currents may be determined mainly by the channel activation kinetics.

Animals↗
PubMed1991

Currents through single glutamate receptor channels in outside-out patches from rat cerebellar granule cells.

1. Single-channel currents evoked in outside-out membrane patches from rat cerebellar granule cells by glutamate, aspartate, N-methyl-D-aspartate (NMDA), kainate and quisqualate were studied. Each agonist produced openings to five discrete amplitude levels. At a membrane potential of -70 mV, these levels correspond to single-channel conductances of about 8, 17, 30, 40 and 50 pS. NMDA, aspartate and glutamate evoked mainly 50 pS openings and also substantial numbers of 40 pS events. Kainate evoked primarily 8 and 17 pS openings. 2. The relative proportion of openings to each conductance level showed no dependence on membrane potential. At membrane potentials negative to -100 mV, current-voltage plots for 30, 40 and 50 pS openings showed substantial inward rectification. 3. With NMDA, aspartate and glutamate, the most common type of direct transition was between the 50 pS open level and the shut level. Transitions between the 30, 40 and 50 pS levels were also relatively common. With few exceptions, 8 and 17 pS openings appeared to arise directly from, and return directly to, the shut level. The differences between granule cells and certain other central neurones, in the types of transitions associated with NMDA receptor channels, provide evidence for the existence of more than one type of NMDA receptor. 4. Four exponential components were identified consistently in the shut-time distributions that were obtained with NMDA, aspartate and glutamate. Mean time constants for the briefest two components were 30 to 65 microseconds and 0.65 to 1.00 ms. The mean duration of these 'gaps within bursts' differed for different agonists, but did not vary with membrane potential. 5. Two exponential components were distinguished in most open-time distributions for the 50 pS level (time constants, 0.9-1.2 and 3.2-3.9 ms at -100 mV), whereas open-time distributions for 30 and 40 pS events were described adequately by single exponentials with time constants below 1.0 ms. The duration of 50 pS openings decreased with hyperpolarization. 6. Mean open times for 8 and 17 pS events produced by NMDA, aspartate and glutamate were 0.3 to 0.7 ms. The longest such openings were observed with quisqualate. 7. Three exponential components were present in distributions of burst length, and of total open time per burst, that were obtained with NMDA, aspartate and glutamate. The slowest two burst-length components had mean time constants of 1.7-2.4 and 10.6-13.0 ms and originated from the kinetic behaviour of the 50 pS state.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

Single channels activated by high concentrations of GABA in superior cervical ganglion neurones of the rat.

1. Single-channel currents evoked by high concentrations of GABA (10-2000 microM) have been analysed to investigate the characteristics of GABAA receptor channels in outside-out patches from rat sympathetic neurones. When high concentrations of GABA were applied to a patch, channel openings occurred in prolonged clusters (3.8 +/- 3.7 s (mean +/- S.D.) at 50 microM-GABA) consisting, on average, of 350 apparent openings per cluster. Individual clusters were separated by long silent intervals. 2. Channel openings were to many (often ill-defined) conductance states (range 7-36 pS), but the most frequently observed conductance level was approximately 30 pS, (29.6 +/- 0.34 pS). Only these clusters during which the channel was open to this main state conductance for at least 95% of the cluster open time were used in the analysis of probability of being open. Other less frequently observed conductance levels were 15-18 and 22-23 pS, while levels of 33-36 and 7-9 pS were occasionally, but reliably, observed. 3. Bursts within clusters were defined as a series of openings separated by closed intervals shorter than some critical value, tc. At 50 microM-GABA the mean burst length was 439 +/- 434 ms (+/- S.D., tc = 50 ms). 4. The probability of being open, po, during bursts within clusters has been analysed as a function of GABA concentration. As expected, increasing the concentration of GABA resulted in an overall increase in po. However, for a given agonist concentration there was a wide spread in po, far greater than that predicted for a population of identical and independent receptor channels (demonstrated by comparison with stimulated channel activity). 5. The wide range of po values at a particular concentration of GABA was not due to inappropriate selection of tc. At 50 microM-GABA the range of po values was similar for tc of 20-1000 ms, although the overall mean po became lower (0.64 rather than 0.81). 6. On the basis of simulated channel activity, it appears that most clusters which do not contain multiple openings, arise from the activity of one individual channel. Furthermore, there was no detectable tendency for gaps between bursts to be shorter in the middle of a cluster than at its ends. Therefore it is unlikely that variability in po arose from overlapping activity of two or more channels. 7. The values for po, mean open time, and mean shut time for bursts within the same cluster, and between different clusters, were compared by a randomization test.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

Activation of ion channels in the frog endplate by several analogues of acetylcholine.

1. Single-ion-channel recording has been used to estimate the equilibrium concentration-response relationship for several acetylcholine analogues. The response, corrected for desensitization, was taken as the probability of a channel being open during clusters of openings that were separated by desensitized periods. 2. All agonists were able to block the channels which they themselves opened. Carbachol, suberyldicholine and the sulphonium analogue of acetylcholine were all found to be efficacious agonists in the sense that the results indicate that all of them, in sufficiently high concentration, would be able to open 90% or more of channels if it were not for channel block. 3. In the case of suberyldicholine the results are much as predicted by the interpretation of the fine structure of channel openings at low agonist concentrations. 4. The maximum probability of opening that could be obtained with decamethonium and with phenyltrimethylammonium was low (below 4%), and it was not possible to distinguish whether this was wholly a result of the powerful (relative to activation potency) channel-blocking action of these agonists, or whether it was to some extent attributable to their being genuine partial agonists. 5. The results suggest that, for a range of agonists, differences in equilibrium potency are usually more strongly influenced by affinity for binding to the resting state of the receptor than by ability to activate the receptor once bound, though in the case of suxamethonium (relative to acetylcholine) the contributions of each factor are similar.

Acetylcholine↗
PubMed1991

Conductance and kinetic properties of single nicotinic acetylcholine receptor channels in rat sympathetic neurones.

1. The unitary conductance of nicotinic acetylcholine (ACh) receptor channels in rat sympathetic neurones has been studied. Conductance estimates varied from 26-48 pS with a mean of 36.8 pS in 1 mM-Ca2+. The main conductance level varied from patch to patch and the presence (or absence) of additional conductance levels also varied. 2. The channels showed large open channel noise and experiments with 300 mM-NaCl in the patch pipette substantially increased the open channel noise. The appearance of detectable step-like transitions within this noise strongly suggested the existence of closely spaced discrete levels. 3. Removal of divalent cations from the external solution increased the unitary channel conductance. Altering the main permeant ion in divalent-free solutions gave the following conductance sequence: K+ (93 pS) greater than Cs+ (61 pS) greater than Na+ (51 pS) greater than Li+(23 pS). 4. Replacement of Na+ by Cs+ in the external solution considerably reduced the current evoked by ACh in whole-cell recordings and the channel-opening frequency in outside-out patches. 5. The kinetic properties of channels activated by ACh and 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) were also studied. At low concentrations of ACh and DMPP the gap distributions were complex and best fitted by the sum of four exponential components. Individual activations (bursts) were interrupted by the two shortest closed periods the briefer of which had time constants of 36 microseconds for ACh and 67 microseconds for DMPP. 6. The distribution of burst lengths had two components for each agonist, each component making up about 50% of the total area under the distribution. For ACh, the time constant of the longer component (12.2 ms) was similar to the decay time constant of excitatory postsynaptic potentials (EPSCs) at similar temperature and potential. For DMPP the time constant of the longer component was 17.6 ms. 7. The relative number of brief gaps per long burst was much larger for ACh than for DMPP. Therefore the corrected mean open time for ACh (0.86 ms) was much shorter than that for DMPP (2.3 ms). 8. In terms of receptor mechanism, the values of the channel opening equilibrium constant (beta/alpha) estimated from these numbers (ACh, 23; DMPP, 25) suggest that both agonists are efficaceous. 9. DMPP is a potent blocker of the channel with an equilibrium dissociation constant (KB) of around 50 microM and blockage gaps of around 1 ms duration. ACh also blocks the channel but with a higher KB of around 470 microM.(ABSTRACT TRUNCATED AT 400 WORDS)

Acetylcholine↗
PubMed1990

Expression of cloned receptor subunits produces multiple receptors.

Expression in Xenopus oocytes of cloned nicotinic acetylcholine receptors (alpha, beta, gamma and delta subunits of BC3H1 receptor), produced more than one sort of functional receptor. This heterogeneity was detectable neither as heterogeneity in single channel conductance, nor as heterogeneity in the burst length. It was seen most obviously as differences from patch to patch in the maximum fraction of time for which the channels are held open at high acetylcholine concentrations, and it was also detectable as differences in the shut time distributions at low acetylcholine concentrations.

Acetylcholine↗