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D Colquhoun

Publications and source records attributed to D Colquhoun.

At least 37 records · Page 2Linked to original sources
PubMed1997

Recombinant nicotinic receptors, expressed in Xenopus oocytes, do not resemble native rat sympathetic ganglion receptors in single-channel behaviour.

1. In order to establish the subunit composition of neuronal nicotinic receptors in rat superior cervical ganglia (SCG), their single-channel properties were compared with those of recombinant receptors expressed in Xenopus oocytes, using outside-out excised patch recording. 2. The mean main conductance of SCG channels from adult and 1-day-old rats was 34.8 and 36.6 pS, respectively. Less frequent openings to lower conductances occurred both as isolated bursts and as events connected to the main level by direct transitions. There was considerable interpatch variability in the values of the lower conductances. 3. Nicotinic receptors from oocytes expressing alpha3beta4 and alpha4beta4 subunits had chord conductances lower than that of SCG neurones (22 pS for alpha3beta4 and 29 pS for alpha4beta4). 4. Prolonged recording from both native and recombinant channels was precluded by 'run-down', i.e. channel activity could be elicited for only a few minutes after excision. Nevertheless, SCG channel openings were clearly seen to occur as short bursts (slowest component, 38 ms), whereas recombinant channels opened in very prolonged bursts of activity, the major component being the slowest (480 ms). 5. Addition of the alpha5 subunit to the alpha3beta4 pair produced channels with a higher conductance than those observed after injection of the pair alone (24.9 vs. 22 pS), suggesting incorporation of alpha5 into the channel. Addition of the beta2 subunit did not change alpha3beta4 single-channel properties. In one out of fourteen alpha3alpha5beta4 patches, both ganglion-like, high conductance, short burst openings and recombinant-type, low conductance, slow burst openings were observed. 6. Channels produced by expression in Xenopus oocytes of neuronal nicotinic subunits present in rat SCG as a rule differ from native ganglion receptors in single-channel conductance and gross kinetics. While it is possible that an essential nicotinic subunit remains to be cloned, it is perhaps more likely that oocytes either cannot assemble neuronal nicotinic subunits efficiently into channels with the correct composition and stoichiometry, or that they produce post-translational channel modifications which differ from those of mammalian neurones.

Acetylcholine↗
PubMed1997

A comparative study of the efficacy of simvastatin and gemfibrozil in combined hyperlipoproteinemia: prediction of response by baseline lipids, apo E genotype, lipoprotein(a) and insulin.

Combined hyperlipoproteinemia (CHL) can be difficult to treat because of the heterogeneous nature of the lipoprotein abnormalities. We compared the relative efficacies of simvastatin and gemfibrozil and sought predictors of responsiveness in terms of the baseline lipids and other potential metabolic determinants (plasma insulin, Lp(a) and apo E genotype). Sixty-six subjects entered a cross-over, randomized trial involving 12 weeks on each drug. Efficacy was assessed after 6 and 12 weeks on each treatment. Simvastatin lowered total cholesterol 24%, triglycerides 12%, LDL cholesterol 33%, raised HDL cholesterol 13% and substantially reduced the cholesterol:triglyceride ratio in VLDL and IDL. Gemfibrozil lowered total cholesterol 5%, triglycerides 44%, raised HDL 26% and reduced VLDL and IDL lipids more than simvastatin did. LDL size increased with both treatments and HDL size increased with simvastatin. Responsiveness (25% fall in cholesterol or 40% fall in triglycerides) was shown by 31/61 subjects when taking simvastatin (cholesterol-lowering) and by 44/60 taking gemfibrozil (triglyceride-lowering). Responsiveness was greatest in those with apo E2 genotype with both drugs (P < 0.05). Unexpectedly, responders to simvastatin tended to have lower baseline total cholesterol but higher triglyceride levels than those whose cholesterol or triglyceride was lowered by gemfibrozil. Nevertheless, more hypercholesterolemic subjects responded to simvastatin and more hypertriglyceridemic subjects to gemfibrozil. Lp(a) (P = 0.04) and plasma insulin concentrations (P = 0.03) were negative predictors of percentage triglyceride-lowering with gemfibrozil. The difference between the two drugs in triglyceride-lowering lessened with rising insulin and falling HDL cholesterol. Thus, the responsiveness to the two major classes of lipid lowering drugs can be partly predicted from baseline lipids and related metabolic parameters.

Anticholesteremic Agents↗
PubMed1997

Squash racquets. A review of physiology and medicine.

Squash is a moderate to high intensity sport which demands specific fitness. Squash at any level places a high demand on the aerobic system for energy delivery during play and recovery. In addition, the sport requires bursts of intense, anaerobic physical activity involving the lactic anaerobic energy system. Players must possess appropriate levels of local muscular endurance, strength, power, flexibility and speed, combined with agility, balance and co-ordination. Irrespective of the standard of play, aerobic fitness training and specific anaerobic training should be undertaken by all who play or intend to play squash. Aerobic fitness for the individual who is new to the game and has little training background can be improved using low intensity continuous running. Training sessions and matches should be preceded by warm-up and flexibility exercises which may reduce the chance of injury and enhance readiness to perform. Despite squash being an indoor sport, it is likely that play in hot and humid weather may generate significant thermal loads with the associated elevations in heart rate. Fluid losses of 2 L/min and rectal temperatures of 39 degrees C may occur, thereby increasing the cardiovascular stress of participation and the risk of heat illness. Sudden death and other manifestations of heart disease can occur in squash, therefore advice regarding the safe participation for those with, or who have the potential to develop disease appears essential. For those under 40 years of age who are well and have no known heart disease, medical clearance is not mandatory prior to taking up squash; for such individuals, regular medical monitoring may be unnecessary. For healthy individuals older than 40 years of age irrespective of health status, but particularly for those with coronary disease or relevant risk factors, a medical checkup is recommended prior to, and at least annually after taking up squash. Healthy individuals older than 40 years of age with one or more risk factors require a medical checkup prior to commencing squash for the first time and at regular intervals (every 2 years) thereafter. These individuals should also have a medically supervised exercise test. Those individuals older than 40 years of age who have a known history of heart disease, most commonly coronary artery disease, may play squash if it is demonstrated that, on examination or following therapy or surgery, they can exercise safely to a high workload. Most eye injuries which occur in squash are related to eye/ball and eye/racquet contact. The incidence of injury is very low but such injuries may be totally preventable. Ideally, all players should wear protective eye apparatus. The "ideal' protective apparatus should comply with the Australian/New Zealand Standard for eye protectors for racquet sports. It should be recognised at the outset that there is a paucity of specific data regarding squash and pregnancy. Most women with normal pregnancies may continue to exercise and play squash particularly in the early stages of pregnancy but should notify their physician of their intention to do so. Musculo-skeletal injuries to the lower limb dominate most studies and common injuries include sprains and strains to the back and ankles. Of particular interest is the development of degenerative hip disease in elite squash players necessitating retirement or curtailment of activity in the third decade. Dealing with injuries and illness that are attributed to squash requires an approach based on prevention as well as on appropriate injury management.

Adult↗
PubMed1996

Single-channel currents from recombinant NMDA NR1a/NR2D receptors expressed in Xenopus oocytes.

We have investigated the single-channel and whole-cell behaviour of recombinant N-methyl-D-aspartate (NMDA) receptors formed from NR1a and NR2D receptor subunits expressed in Xenopus oocytes. The EC50 for apparent steady-state activation of NR1a/NR2D receptors by glutamate was 450 nM, while extracellular MG2+ produced a voltage-dependent block of glutamate responses with an IC50 of 440 microM at -70 mV. At negative holding potentials glutamate-activated NR1a/NR2D single-channel currents, in 0.85 mM external Ca2+, had slope conductances of 35 pS for the main level, and 17 pS for the sublevel; direct transitions occurred between these two conductance levels. On average 35 pS events had mean open times of 1.01 +/- 0.04 ms, whereas the mean open times of 17 pS events were consistently longer (1.28 +/- 0.06 ms). In 5 mM external Ca2+ the larger conductance level was reduced to 20 pS whereas in Ca(2+)-free solutions it was increased to 50 pS. The frequency of transitions between the main and subconductance levels showed temporal asymmetry: 35-17 pS transitions were more frequent (61%) than 17-35 pS transitions. This asymmetry was not affected by alterations in the external Ca2+ concentration (up to 5 mM). In conclusion, the NR1a/NR2D channel is, like NR1a/NR2C, a 'low conductance' NMDA channel, but it can be distinguished from NR1a/NR2C channels on the basis of transition asymmetry and differences in the open times of its main and sub-conductance levels.

Animals↗
PubMed1996

Deactivation and desensitization of non-NMDA receptors in patches and the time course of EPSCs in rat cerebellar granule cells.

1. Spontaneous and evoked non-NMDA receptor-mediated EPSCs were recorded from cerebellar granule cells in slices at approximately 24 and approximately 34 degrees C. The EPSC decay was fitted with the sum of two exponential functions. 2. The time courses of non-NMDA receptor deactivation and desensitization were determined with fast concentration jumps of glutamate onto patches from cultured granule cells. Deactivation (decay time constant tau = 0.6 ms at 24 degrees C) was substantially faster than desensitization (tau = 4 ms). Both processes were fitted by single exponential functions. 3. The decay of the fast component of the spontaneous EPSC (tau EPSCfast = 0.9 ms at 23 degrees C) was marginally slower than deactivation but too fast to be determined by desensitization. Our results suggest that the decay of this component is set by both the rate of decline of transmitter concentration and channel deactivation. 4. A simple diffusion model predicts that the time course of transmitter in the cleft declines slowly during the later stages of its action. The slow phase of transmitter removal could account for the time course of the slow component of the spontaneous EPSC (tau EPSCslow = 8 ms at 23 degrees C).

Animals↗
PubMed1995

Persistence: a new statistic for characterizing ion-channel activity.

We introduce and illustrate by examples a new statistical technique, the persistence function, for characterizing ion-channel activity in a single-channel patch-clamp recording. Persistence is a function of both current and time. It is the probability that the current is at a given level (conditional on it having been at that level at an earlier time). Viewed as a function of current it exhibits the prominent conductance levels present in the recording, and viewed as a function of time for a conductance level it portrays the kinetics at that level.

Ion Channels↗
PubMed1995

Determination of NMDA NR1 subunit copy number in recombinant NMDA receptors.

Co-expression of wild-type and mutated NMDA NR1 (N598R) subunits in Xenopus oocytes has been used to determine the stoichiometry of the NMDA receptor-channel. When expressed together, wild-type NR2A and mutant NR1 (N598R) subunits produced channels with a main conductance of 2.6 pS and a sublevel of 1.2 pS. These conductances were clearly different from those obtained from wild-type NR1 and wild-type NR2A channels which gave characteristic 50 pS events with a 40 pS sublevel. When wild-type and mutant NR1 subunits were co-expressed together with NR2A subunits a different channel type with a main conductance of 15.2 pS and a sublevel of 11.4 pS was obtained, as well as the 'all wild-type' and 'all mutant' channels described above. These results indicate that there are likely to be two copies of the NR1 subunit in each NMDA receptor complex.

Animals↗
PubMed1995

Desensitization of N-methyl-D-aspartate receptors: a problem of interpretation.

The phenomenon of desensitization is universal, but its mechanism is still ill-understood and controversial. A recently published study [Lin, F. & Stevens, C. F. (1994) J. Neurosci, 14, 2153-2160] attempted to cast light on the mechanism of desensitization of N-methyl-D-aspartate (NMDA) receptors, in particular the vexed question of whether the channel must open before it can desensitize. During the desensitizing preexposure to agonist in those experiments, more desensitization was produced when channel openings were observed than when no openings were observed. The conclusion that "desensitization occurs more rapidly from the open state" unfortunately was based on a stochastic fallacy, and we present here a theoretical treatment and illustration showing that the observed behavior is predicted by a simple mechanism in which desensitization can occur only from a shut state.

Animals↗
PubMed1995

A P2X purinoceptor expressed by a subset of sensory neurons.

ATP is known to depolarize sensory neurons, and may play a role in nociceptor activation when released from damaged tissue. Here we report the molecular cloning and characterization of a new member of the P2X receptor family, P2X3, expressed by these cells. The channel transcript was present in a subset of rat dorsal-root-ganglion sensory neurons, some of which express nociceptor-associated markers; it was absent in other tissues that were tested, including sympathetic, enteric and central nervous system neurons. Moreover, when expressed in Xenopus oocytes, the channel showed an ATP-dependent cation flux. P2X3 is the only ligand-gated channel known to be expressed exclusively by a subset of sensory neurons. The remarkable selectivity of expression of the channel coupled with its sensory neuron-like pharmacology suggests that this channel may transduce ATP-evoked nociceptor activation.

Amino Acid Sequence↗
PubMed1995

Heterogeneity of neuronal nicotinic acetylcholine receptors in thin slices of rat medial habenula.

1. Neuronal nicotinic acetylcholine receptors in slices of rat medial habenula were studied using patch clamp recording techniques. 2. Whole cell current responses to cytisine could be blocked by hexamethonium, as expected for nicotinic receptors. The whole cell current-voltage relations were linear at negative membrane potentials, but showed strong inward rectification when chloride currents were minimized. 3. When 1 mM Ca2+ (0 mM Mg2+) was present in the external recording solution, the single channel conductances elicited by acetylcholine or nicotine in twenty patches were in the range 39-58 pS, with a mean of 47 pS. There appeared to be at least two groups of conductances. 4. In the open point amplitude distributions of three patches, the most common amplitude corresponded to 41 pS (81% of the area). In another four patches the most common amplitude corresponded to a mean conductance of 51 pS (83% of the area). Direct transitions between open levels were rare. 5. Channel closed times were not significantly different for the two conductance groups. However, for the four patches with predominantly 51 pS openings, the means of the distributions of open times longer than two filter rise times averaged 5.8 ms. Those patches with predominantly 41 pS openings averaged 14 ms. Also, for patches with predominantly 51 pS openings the overall mean burst length was 5.8 ms, whereas for patches with predominantly 41 pS openings it was 16.1 ms. 6. These observations suggest that 51 and 41 pS openings result from the activity of at least two, but possibly more, different receptor subtypes. We conclude that nicotinic receptors in the rat ventral medial habenula are heterogeneous.

Acetylcholine↗
PubMed1994

Comparison of neuronal nicotinic receptors in rat sympathetic neurones with subunit pairs expressed in Xenopus oocytes.

1. The agonist sensitivity of nicotinic acetylcholine receptors in rat superior cervical ganglion (SCG) neurones was compared with that of cloned receptors expressed in Xenopus oocytes by pairwise injections of alpha 3-beta 2 or alpha 3-beta 4 neuronal nicotinic subunit combinations. 2. Agonist responses in rat SCG neurones indicated that cytisine was the most potent agonist and lobeline the least potent (rank order of potency: cytisine > dimethylphenylpiperazinium iodide (DMPP) > nicotine > ACh > carbachol > lobeline). 3. Receptors expressed in oocytes by injection of alpha 3 and beta 2 subunits had a relatively high sensitivity to DMPP and low sensitivity to cytisine (rank order of potency: DMPP > ACh > lobeline > carbachol > nicotine > cytisine), whereas receptors composed of alpha 3 and beta 4 subunits had a high sensitivity to cytisine and low sensitivity to DMPP (rank-order of potency: cytisine > nicotine approximately ACh > DMPP > carbachol > lobeline). 4. With the exception of responses to DMPP, agonist sensitivity measurements suggest that nicotinic receptors in the rat SCG are composed of alpha 3 and beta 4 subunits. The results are discussed in terms of the receptor subunit mRNAs known to be expressed in the rat SCG and previous evidence of functional heterogeneity of rat SCG nicotinic acetylcholine receptors.

Alkaloids↗
PubMed1994

Single channel properties of cloned NMDA receptors in a human cell line: comparison with results from Xenopus oocytes.

1. Human embryonic kidney (HEK) 293 cells were transiently transfected with cDNAs encoding the NR1a-NMDA epsilon 1[NR2A] subunit combination of the NMDA receptor. Single channel behaviour was recorded from outside-out membrane patches, with the aim of comparing the results with those, recorded under the same conditions, from Xenopus oocytes injected with messenger RNA coding for the NR1a-NR2A combination. 2. Single channels in HEK 293 cells showed a main conductance level of 51.4 +/- 2.4 pS, compared with 50.1 +/- 1.4 pS for channels in oocytes. A subconductance level of 38.1 +/- 2.1 pS was found in HEK 293 cells, compared with 38.3 +/- 1.3 pS in oocytes. The frequencies of transitions between the shut and the two conductance levels were also very similar. 3. Distributions of shut times could be fitted with five exponential components. In HEK 293 cells the first three of these components had time constants of 39 +/- 4 microseconds, 0.54 +/- 0.04 ms and 9.94 +/- 1.3 ms; in oocytes the values were 69 +/- 35 microseconds, 0.54 +/- 0.15 ms and 6.53 +/- 4.6 ms, respectively. The relative areas of the components were also similar in the two systems. 4. The distribution of all apparent open times for the sublevels was fitted with two exponential components giving time constants of 0.18 +/- 0.02 ms and 1.31 +/- 0.17 ms (for HEK cells) or of 0.31 +/- 0.36 ms and 1.31 +/- 1.1 ms (for oocytes).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗