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M R Bennett

Publications and source records attributed to M R Bennett.

At least 181 records · Page 10Linked to original sources
PubMed1993

The effect of nitric oxide on the efficacy of synaptic transmission through the chick ciliary ganglion.

1. The effect of nitric oxide on the efficacy of synaptic transmission in the chick ciliary ganglion of post-hatched birds has been determined by use of the size of the postganglionic compound action potential resulting from chemical transmission through the ganglion as a measure of synaptic efficacy. 2. Sodium nitroprusside (100 microM) increased the synaptic efficacy by an average 26%. This is likely to be due to its ability to release nitric oxide, as potassium ferricyanide (100 microM) did not cause a potentiation. Sodium azide (100 microM), shown in sympathetic ganglia to stimulate production of cyclic GMP, did not modulate synaptic efficacy significantly. 3. 8-Br-cyclic-GMP (100 microM) increased synaptic efficacy by an average 61%. The addition of 8-Br-cyclic-AMP (100 microM) had less effect, increasing transmission by on average 46%. 4. The nitric oxide synthase blocker, NG-nitro-L-arginine methyl ester (L-NAME, 100 microM) was added prior to the tetanic stimulation of the preganglionic nerves at 30 Hz for 20 s, a procedure known to produce both post-tetanic potentiation and long-term potentiation of synaptic transmission through the ganglion. L-NAME reduced the long-term potentiation by an average of 47% but did not significantly change the post-tetanic potentiation. 5. Following the brief application of 8-Br-cyclic AMP, 8-Br-cyclic GMP and sodium nitroprusside there was an enhancement of the efficacy of synaptic transmission that persisted after the withdrawal of the drugs. The maximum increase in synaptic efficacy following the brief addition of 8-Br-cyclic GMP was 116%, sodium nitroprusside was 110% and 8-Br-cyclic AMP was 126%.6. These results suggest that nitric oxide modulates synaptic transmission through the ganglion by acting on an endogenous guanylate cyclase that produces cyclic GMP.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1993

Nitric oxide modulation of calcium-activated potassium channels in postganglionic neurones of avian cultured ciliary ganglia.

1. A study has been made of the modulation of calcium-activated potassium channels in cultured neurones of avian ciliary ganglia by sodium nitroprusside and L-arginine. 2. Sodium nitroprusside (100 microM) reduced the net outward current by 22 +/- 1% at 4.8 ms (mean +/- s.e. mean) and 25 +/- 1% at 350 ms during a test depolarization to +40 mV from a holding potential of -40 mV. The outward current remained reduced for the duration of the recording following a single application of sodium nitroprusside. These effects did not occur if the influx of calcium ions was first blocked with Cd2+ (500 microM). Application of ferrocyanide (100 microM) reduced the net outward current by only 6 +/- 3% at 350 ms during a test depolarization to +40 mV. 3. L-Arginine (270 microM) reduced the net outward current on average by 19 +/- 2% at 4.8 ms and 22 +/- 2% at 350 ms during a test depolarization to +40 mV. The current remained in this reduced state for the duration of the recording following a single application of L-arginine. These effects were reduced to 11 +/- 1% at 4.8 ms and 11 +/- 2% at 350 ms in the presence of N omega-nitro-L-arginine methyl ester (L-NAME, 100 microM). 4. In order to alleviate the dependence of calcium-activated potassium channels (Ik(Ca)) on the inward flux of calcium ions, the patch-clamp pipettes were filled with a solution containing 100 microM CaCl2, and the Ca2+ in the bathing solution was replaced with EGTA. Under these conditions sodium nitroprusside reduced the total outward current during a depolarizing pulse of + 40 mV by 9 +/_ 1% at 4.8 ms and by 36 +/- 3% at 350 ms. L-Arginine (270 microM) reduced this current under the same conditions by 9 +/- 1% at 4.8 ms and by 35 +/- 2% at 350 ms.5. Calcium-activated potassium currents were sensitive to apamin (50 nM), as this reduced the outward current by 23 +/- 3% at 350 ms when a high calcium-containing pipette was used during a depolarizing command to + 40 mV. L-Arginine still decreased the outward current in the presence of apamin(50 nM), by 5 +/- 1% at 4.8 ms and by 19 +/- 2% at 350 ms, indicating that L-arginine could reduce an apamin-insensitive Ik(Ca)6. Calcium-activated potassium currents were also sensitive to charybdotoxin (10 nM), as this reduced the outward current by 34 +/- 4% at 350 ms when a high calcium-containing pipette was used during a depolarizing command to + 40 mV. L-Arginine still decreased the outward current in the presence of charybdotoxin, by 6 +/- 1% at 4.8 ms and 12 +/- 4% at 350 ms, showing that L-arginine could reduce a charybdotoxin-insensitive Ik(Ca).7. The present results indicate that NO-synthase in ciliary ganglia can modulate Ik(Ca) by a method which is independent of the action of NO on the calcium channels. The Ik(ca) is decreased significantly at 4.8 ms into a depolarizing pulse, at a time that would decrease the rate of repolarization of the action potential. Ik(Ca) is also reduced at longer times (350 ms), indicating an affect on the inactivating process.

Action Potentials↗
PubMed1992

The effect of adenosine on spontaneous and evoked quantal secretion from different release sites of amphibian motor-nerve terminals.

The effects of adenosine on the spontaneous quantal secretion from different release sites along terminal branches of toad (Bufo marinus) motor-nerve terminals was studied. Terminal branches were visualized using 3,3-diethyloxardicarbocyanine iodide (DiOC2(5))-fluorescence to assist in the placement of extracellular electrodes along different release sites of terminal branches. The maximum rate of spontaneous secretions (fe) observed with an extracellular electrode within any 10 microns length of terminal branch declined towards the distal end of the terminal branch as does the average number of evoked quantal secretions (mE). Adenosine (1-50 microM) depressed both fe and me. The ED50 of adenosine in depressing me was 5 microM. Adenosine (10 microM) produced on average a 43% decrease in fe and a 63% decrease in me, regardless of the initial values of fe and me. It is suggested that adenosine has qualitatively similar effects on both fe and me, regardless of their initial size, at different release sites within motor-nerve terminals.

Adenosine↗
PubMed1992

Adenosine modulation of potassium currents in preganglionic nerve terminals of avian ciliary ganglia.

Potassium currents in calyciform nerve terminals of the avian ciliary ganglion were analyzed using a single microelectrode voltage clamp. The modulatory affects of adenosine on these currents was also determined. Intracellular CsCl (1 M) blocked the steady-state outward current, indicating that it is mostly carried by K+. Tetraethylammonium (TEA, 10 mM) blocked over 40% of the outward current at a command potential of -30 mV. If Ca2+ influx was blocked by CoCl2 (5 mM), the steady-state outward current was reduced by over 16% in the nerve terminals suggesting that about one fifth of the outward current passes through calcium-activated potassium channels IK(Ca). Adenosine (50 mM) decreased the outward steady-state current over a wide range of command potentials in most terminals studied but failed to decrease this current in the presence of CoCl2 (5 mM). It is concluded that adenosine blocks IK(Ca) in nerve terminals.

Adenosine↗
PubMed1992

Adenosine modulation of calcium currents in postganglionic neurones of avian cultured ciliary ganglia.

1. Calcium currents in postganglionic neurones of cultured 7- to 10-day embryonic avian ciliary ganglia were analyzed under whole-cell voltage-clamp and their modulation by 2-chloroadenosine determined. 2. In the presence of tetrodotoxin (200 nM) in the medium to block the Na+ current and CsCl (105 mM) in the patch-clamp electrode to block the K+ current, two different components of the calcium currents (transient and sustained) were identified on the basis of their voltage-dependent kinetics as well as their sensitivity to the dihydropyridine agonist Bay K 8644 and antagonist nifedipine. 3. The sustained current inactivated very slowly (tau greater than 1000 ms; for test potentials from -20 mV to +40 mV) but was reactivated at a holding potential (Vh) of -40 mV. The current was increased on average over 50% by 1 microM of Bay K 8644 at a test potential of 0 mV and decreased over 35% by 1 microM of nifedipine. 4. The transient current inactivated slowly (tau less than 200 ms; for test potentials from -20 mV to +40 mV), and could be completely reactivated at a Vh of -80 mV. This current was unaffected by Bay K 8644 (1 microM) but reduced on average by 8% with nifedipine (1 microM). 5. The sustained and transient currents were decreased more than 70% by 5 microM of omega-conotoxin and decreased more than 50% by 250 microM verapamil. 6. 2-Chloroadenosine (1 microM) decreased the transient current by over 50% and the sustained current by less than 10%. In the presence of nifedipine (1 microM), 2-chloroadenosine decreased the transient current by over 30% and the remaining sustained current by 35%.In the presence of 8-phenyltheophylline (10 microM), 2-chloroadenosine no longer decreased either the transient or sustained currents but did have a slight potentiating effect on both the transient and sustained currents.7. These observations of the effects of 2-chloroadenosine on the transient and sustained currents are discussed in relation to the different calcium channel types at preganglionic nerve terminals.

2-Chloroadenosine↗
PubMed1992

Probabilistic secretion of quanta from visualized sympathetic nerve varicosities in mouse vas deferens.

1. Sympathetic varicosities on the surface of smooth muscle cells of the mouse vas deferens were visualized with the fluorescent dye 3-3 Diethyloxardicarbocyanine iodide (DiOC2(5)) and quantal secretion recorded from these with both small diameter (4-6 microns) and large diameter (20-50 microns) microelectrodes. Small diameter electrodes were placed over one to three varicosities and large diameter electrodes over three to seven varicosities. 2. The size and distribution of varicosities along individual terminal branches was about the same when these were fluoresced with DiOC2(5) (length 1.09 +/- 0.40 microns (mean +/- S.D.); intervaricosity distance 5.53 +/- 2.68 microns) as when they were stained for catecholamines using Faglu fluorescence (length 1.05 +/- 0.43 microns; intervaricosity distance 5.12 +/- 2.79 microns) suggesting that DiOC2(5) does allow for identification of the catecholamine-containing varicosities. 3. The spontaneous excitatory junctional currents (EJCs) recorded from visualized varicosities with small diameter electrodes (amplitudes 59-67 microV) were much larger than those recorded with large diameter electrodes (amplitudes 25-29 microV). The frequency of evoked EJCs as well as the amplitude-frequency distribution of these EJCs varied greatly between sets of visualized varicosities recorded along individual branches, either with a small or large diameter electrode. These amplitude-frequency distributions typically followed Poisson statistics, in which the mean quantal content of the EJC (m) varied by over threefold for different sets of varicosities on the same branch (m was 0.07-0.21 for small electrodes whereas m was 1-3 for large electrodes). 4. Although m varied considerably for a constant number of varicosities beneath the electrode at different sites along a single branch, there was an overall correlation between m and the number of varicosities, m increasing on average 0.25 for each additional varicosity in a [Ca2+]o of 4.0 mM. 5. The frequency of evoked EJCs at visualized sets of varicosities along some branches was sufficiently high to allow binomial statistics to predict the amplitude-frequency distributions of evoked EJCs. In these cases m was again shown to vary considerably along single terminal branches, and this was primarily due to variation in the probability of secretion (p) between sets of varicosities and not to variation in binomial parameter n. 6. In one case a relatively isolated varicosity, over 3 microns from adjacent varicosities, was recorded for 30 min with a 4 microns diameter electrode. The mean and variance of the evoked EJC was similar to that of the spontaneous EJCs suggesting that this varicosity secreted at most one quantum on arrival of the nerve impulse.(ABSTRACT TRUNCATED AT 400 WORDS)

Adrenergic Fibers↗
PubMed1991

Probabilistic secretion of quanta from the release sites of nerve terminals in amphibian muscle modulated by seasonal changes.

The number of quanta secreted from release sites along visualized terminal branches at toad (Bufo marinus) neuromuscular junctions during different seasons was determined. Terminal branches were visualized by prior staining with the fluorescent dye, 3,3'-diethyloxadicarbocyanine iodide (DiOC2(5)). The average number of quanta secreted by release sites and recorded with an extracellular electrode was determined at different positions along terminal branches in winter and summer. There was a decrease in me at different positions along terminal branches in winter compared with me at similar positions in summer during each of 3 years. The observations show that the capacity of release sites to secrete quanta is modulated by seasonal changes.

Animals↗
PubMed1991

Probabilistic secretion of quanta in the central nervous system: granule cell synaptic control of pattern separation and activity regulation.

The implications of probabilistic secretion of quanta for the functioning of neural networks in the central nervous system have been explored. A model of stochastic secretion at synapses in simple networks, consisting of large numbers of granule cells and a relatively small number of inhibitory interneurons, has been analysed. Such networks occur in the input to the cerebellum Purkinje cells as well as to hippocampal CA3 pyramidal cells and to pyramidal cells in the visual cortex. In this model the input axons terminate on granule cells as well as on an inhibitory interneuron that projects to the granule cells. Stochastic secretion at these synapses involves both temporal variability in secretion at single synapses in the network as well as spatial variability in the secretion at different synapses. The role of this stochastic variability in controlling the size of the granule cell output to a level independent of the size of the input and in separating overlapping inputs has been determined analytically as well as by simulation. The regulation of granule-cell output activity to a reasonably constant value for different size inputs does not occur in the absence of an inhibitory interneuron when both spatial and temporal stochastic variability occurs at the remaining synapses; it is still very poor in the presence of such an interneuron but in the absence of stochastic variability. However, quite good regulation is achieved when the inhibitory interneuron is present with spatial and temporal stochastic variability of secretion at synapses in the network. Excellent regulation is achieved if, in addition, allowance is made for the nonlinear behaviour of the input-output characteristics of inhibitory interneurons. The capacity of granule-cell networks to separate overlapping patterns of activity on their inputs is adequate, with spatial variability in the secretion at synapses, but is improved if there is also temporal variability in the stochastic secretion at individual synapses, although this is at the expense of reliability in the network. Other factors which improve pattern separation are control of the output to very low activity levels, and a restriction on the cumulative size of the excitatory input terminals of each granule cell. Application of the theory to the input neural networks of the cerebellum and the hippocampus shows the role of stochastic variability in quantal transmission in determining the capacity of these networks for pattern separation and activity regulation.

Animals↗
PubMed1991

Treatment of severe male-factor infertility with high concentrations of motile sperm by microinsemination in embryo cryopreservation straws.

A microinsemination technique was evaluated for treating our program's most severe cases of male-factor infertility. Oocytes were inseminated with high concentrations of motile sperm (1 to 9 x 10(6)/ml) in 10 to 150 microliters within embryo cryopreservation straws. Fertilization was obtained in 20 of 29 (69%) couples treated by this technique. In the 15 patients in which only embryos generated from the straw technique were transferred, 7 clinical pregnancies resulted (46.7% per transfer). The implantation rate for couples receiving embryos from the straw technique only (12/58; 20.7%) compared favorably to that observed for other cases treated during this same time period with regular insemination techniques (111/766; 14.5%). Clinical pregnancy rates per transfer for IVF-ET, TET, and PROST were 33.0% (1/3), 0% (0/2), and 60.0% (6/10), respectively. The percentage of polyploidic embryos was significantly lower (P less than 0.0001) for male-factor patients treated by the straw technique with high sperm concentrations than for non-male-factor patients treated during this same time period with standard sperm concentrations. Normal births have resulted from straw inseminations with 3.4 x 10(6) and ongoing pregnancies with 5.0 x 10(6) motile sperm/ml. The results of this study suggest that some cases of male-factor infertility can be successfully treated by insemination with high concentrations of motile sperm in embryo cryopreservation straws. A technique of centrifuging sperm in straws was also developed to concentrate the entire fraction of washed sperm into 10 microliters. Further development of this technique may allow treatment of more severe cases of oligo/asthenospermia by microinsemination with high concentrations of motile sperm than is presently possible with standard washing techniques.

Adult↗
PubMed1991

Association of transient abnormal Q-waves during exercise testing with a stenosis of the main stem of the left coronary artery.

Transient electrocardiographic changes resembling acute myocardial infarction, with Q-waves and ST-segment elevation, have been reported in a variety of clinical situations in which evidence for acute myocardial necrosis was not apparent. Such electrocardiographic changes resolved to normal within minutes. We report a case in which exercise testing induced a painless reversible electrocardiographic abnormality identical to acute anterior myocardial infarction, and subsequent angiography revealed a severe stenosis in the proximal left coronary artery. We suggest that patients presenting with this type of electrocardiographic exercise response should proceed to urgent coronary angiography.

Constriction, Pathologic↗
PubMed1991

Fatal myocardial infarction in hypertrophic cardiomyopathy associated with non-penetrating chest trauma.

Myocardial infarction as a result of injury to the coronary arteries is a rare complication of non-penetrating chest trauma. We report a case of fatal inferior wall myocardial infarction following traumatic injury to the right coronary artery, complicated by atrioventricular dissociation, in a patient with a combination of hypertrophic cardiomyopathy and non-occlusive coronary artery disease.

Adult↗
PubMed1991

Adenosine modulation of potassium currents in postganglionic neurones of cultured avian ciliary ganglia.

1. Potassium currents in cultured postganglionic neurones of avian ciliary ganglia were analysed under whole-cell voltage clamp and their modulation by adenosine determined. 2. In the presence of tetrodotoxin (200 nM), and with moderate holding potentials (Vh = -40 mV), the steady-state current-voltage (I/V) curve was N-shaped over the range from -70 mV to +155 mV. CsCl (1 M) blocked the current, indicating that it was carried by K+. If Ca2+ influx was blocked by CdCl2 (500 microM) then the outward current was reduced and the N-shaped I-V curve lost, indicating the presence of a calcium-activated potassium current (IK(Ca)); the remaining current, due to the delayed rectifier (IK), increased with depolarization up to about a conductance of 10 nS near + 50 mV. This IK was 50% activated at about +20 mV and 50% inactivated at about -50 mV. Adenosine (10 microM) had similar affects on the N-shaped I/V curve as did CdCl2, indicating that it blocked IK(Ca). However, adenosine had little affect on the steady-state current in the presence of CdCl, indicating that it did not much affect IK. 3. In the presence of tetrodotoxin (200 nM), a large inward current occurred for large hyperpolarizations from a Vh = -50 mV. This inward rectifying current (IIR) had a reversal potential near EK and showed 50% activation at about -130 mV. Adenosine (10 microM) reduced IIR, by as much as 50% at large hyperpolarizations beyond -80 mV. 4. Relaxations of the outward current on hyperpolarization from Vh = -30mV were blocked by carbachol (10 microM), had a reversal potential near EK, and an I/V curve typical of 1M currents. These currents were little affected by adenosine (10 microM). 5. A fast transient outward current, due to depolarizing pulses from a large Vh = -110mV was observed in the presence of tetrodotoxin (200 nM). This had the characteristics of an IA current as it could be blocked with 4-aminopyridine (5 mM) and was 50% activated at about -20 mV and 50% inactivated at about -94 mV. The IA current was reduced by 42% at a depolarization of -20 mV by adenosine (10 microM). 6. Many neurones possessed a fast transient outward current that was blocked by tetrodotoxin (200nM). This current could be blocked with 4-aminopyridine (5mM); it therefore has the characteristics of a sodium-activated potassium current ('K(Na)). This IK(Na) was unaffected by adenosine (1O microM). 7. These results are discussed in relation to the role of adenosine in blocking Ca2 + channels and thereby modifying calcium-dependent components of K+ currents.

Adenosine↗
PubMed1991

Probabilistic secretion of quanta from nerve terminals in toad (Bufo marinus) muscle modulated by adenosine.

1. A study has been made of the effect of blocking endogenous adenosine on the statistics of quantal secretion at nerve terminals in toad (Bufo marinus) muscle during summer and winter. 2. Exogenous adenosine (10-50 microM) reduces the mean quantal content of the endplate potential (EPP) recorded with an intracellular microelectrode (m) by 36 +/- 6% (mean +/- S.E.M.), independent of the control value of m in both summer and winter. The variance of the EPP (S2) was reduced by adenosine in proportion to m, so that the probability of quantal secretion (p) remained relatively constant. 3. Exogenous adenosine reduces the mean quantal content of secretion recorded with an extracellular electrode (m(e)) to a similar extent (47 +/- 6%) at different relatively high secreting sites along nerve terminal branches in both summer and winter. 4. Both theophylline (20-100 microM) and adenosine deaminase (2.5 i.u./ml) increase the amplitude of the EPP in summer in a [Ca2+]o of 0.5 mM or greater; a maximum increase of about 40% is reached at a [Ca2+]o of 1.2 mM. The amplitude of the EPP in winter is usually reduced by theophylline in a [Ca2+]o of 0.5 mM, but is always increased in a [Ca2+]o greater than 0.9 mM to reach a maximum increase of about 40% at high [Ca2+]o. 5. The variance of the EPP (S2) was always increased by theophylline to a greater extent than m in summer, so that p decreased and Poisson rather than binomial statistics could be used to describe the distribution of EPP amplitudes. In winter, theophylline generally increased m and S2 to about the same extent, so that p did not change much. 6. An autocorrelation analysis of the amplitude of successive EPPs in a long train at 0.5 Hz in high [Ca2+]o showed that these are likely to be independently distributed. Adenosine secreted by one impulse is then unlikely to affect secretion by a subsequent impulse in the train. 7. These observations are discussed in terms of the hypothesis that endogenously secreted adenosine at a release site inhibits secretion by nearby release sites in summer; this has the effect of reducing m and to a greater extent S2 so that binomial rather than Poisson statistics can describe the frequency distribution of EPP amplitudes.

Acetylcholine↗
PubMed1991

Probabilistic secretion of quanta from nerve terminals in avian ciliary ganglia modulated by adenosine.

1. The effects of adenosine on the probability of secretion of acetylcholine quanta and on presynaptic and postsynaptic action potentials was examined in the post-hatched avian ciliary ganglion. 2. Adenosine (20 microM) reduced the average size of the excitatory postsynaptic potential (EPSP) by 33%. This was due to a decrease in quantal content of the EPSP (m). The effect was blocked by theophylline (50 microM). 3. Adenosine deaminase (2.5 i.u./ml) increased the size of the EPSP by 70%, suggesting that endogenous adenosine modulates synaptic transmission in the ciliary ganglion. However, theophylline (20-100 microM) did not affect the EPSP in a low [Ca2+]o of 1 mM and high [Mg2+]o of 6 mM. 4. Plateau-type action potentials with a large calcium component were generated in the ciliary neurones by bathing the ganglion in tetraethylammonium ions (TEA, 10 mM). Adenosine (20 microM) reduced the duration of these action potentials on short exposures (less than 20 min) but increased the duration on longer exposure (greater than 30 min). Adenosine did not affect the normal action potential recorded in the absence of TEA. 5. Adenosine (20 microM) hyperpolarized the nerve terminal and as a consequence increased the size of the presynaptic action potential and reduced its after-hyperpolarization. 6. Plateau-type action potentials with a large calcium component were generated in the nerve terminals using TEA (10 mM). The duration of these action potentials was significantly reduced by adenosine (20 microM). 7. Adenosines action on nerve terminals, to hyperpolarize the membrane and reduce calcium influx, may contribute to its effect in reducing m of the EPSP.

Acetylcholine↗
PubMed1991

Ebstein's anomaly associated with splenomegaly and reversible hypersplenism.

A case of Ebstein's anomaly associated with chronic right heart failure, hepatosplenomegaly, and the haematological features of hypersplenism is presented. The haematological abnormalities were corrected after tricuspid valve replacement but recurred with the reemergence of clinical features of right heart failure.

Adult↗
PubMed1990

Probabilistic secretion of quanta from nerve terminals at synaptic sites on muscle cells: non-uniformity, autoinhibition and the binomial hypothesis.

A model of the secretion of a quantum at a release site is proposed in which, following the influx of calcium ions, synaptic vesicles are made available for release by the activation of kappa phosphorylation steps with rate alpha. At any time during this process the vesicles may become unavailable for secretion at rate gamma. On completion of the kappa phosphorylation steps the vesicles participate in the formation of a fusion pore with the terminal membrane to give exocytosis at rate delta. Changes in alpha, delta and kappa are shown to produce characteristic changes in the number and timecourse of quantal secretions following a nerve impulse, which are similar to those observed following drug treatments that are thought to act selectively on each of these processes. The number of quanta secreted from nerve terminals that consist of many release sites does not fluctuate much during a low frequency train of impulses: the variance is small compared with the mean level, so secretion follows binomial rather than Poisson statistics. A theory is derived that shows that variations in the probability of secretion amongst these release sites of any particular kind fails to reduce the variance of the total secretion from the terminal; Poisson rather than binomial statistics then still apply. The theory shows that an interaction between release sites is required to reduce this variance and such an effect is provided if secretion at a site inhibits secretion at nearby sites. Simulations show that incorporating this process of autoinhibition into the model reproduces the experimental observations on the effects of calcium ions on the binomial parameters p and n as well as on the relative constancy of p during facilitation and depression of quantal secretion. Methods for estimating the timecourse of changes in the probability of secretion at release sites following an impulse, by using either the time of occurrence of first, second, third or later quantal latencies, are given. These procedures show that current methods for estimating the time-dependent probability changes are inadequate for detecting interaction between release sites, such as autoinhibition, unless this is relatively large. Therefore, estimates from third quantal latencies are used.

Animals↗
PubMed1990

The distribution of intracellular acetylcholine receptors and nuclei in avian slow muscle fibres during establishment of distributed synapses.

The distribution of intracellular acetylcholine receptor was studied by 125I-alpha-bungarotoxin autoradiography as a measure of the local acetylcholine receptor synthesis at junctional and extrajunctional sites in single fibres of the developing anterior latissimus dorsi muscle of the chicken. Large (longer than 2 microns) acetylcholine receptor clusters characteristic of synaptic contacts were localized by immunofluorescence with anti-acetylcholine receptor antibodies. The distance between acetylcholine receptor clusters at embryonic day 11 was 166 +/- 10.5 microns and this distance did not increase despite growth until after 4 days posthatch. The distance between acetylcholine receptor clusters subsequently increased proportionately with the increase in the length of fibres. Intracellular acetylcholine receptors were labelled with 125I-alpha-BGT after first blocking cell-surface acetylcholine receptor with unlabelled alpha-BGT, and treatment with saponin. Intracellular acetylcholine receptor represented about 5-15% of total cellular acetylcholine receptor. Cycloheximide experiments indicated that 80-90% of intracellular acetylcholine receptor examined represented newly synthesized acetylcholine receptor. The spatial distribution of this pool, studied by autoradiography, was determined in relation to the acetylcholine receptor clusters labelled with anti-acetylcholine receptor antibody. Between embryonic day 11 and posthatch day 14 there was a continual increase in intracellular acetylcholine receptor at both junctional and extrajunctional parts of the fibres, but with the greater increases occurring at the junctional regions. Peaks of intracellular acetylcholine receptor became associated with an increasing number of acetylcholine receptor clusters so that by posthatch day 14 there was an 80% correspondence. The accumulation of newly synthesized intracellular acetylcholine receptor under acetylcholine receptor clusters was not the result of the aggregation of nuclei at these sites, suggesting that a higher rate of acetylcholine receptor synthesis per nucleus develops at distributed synaptic sites on anterior latissimus dorsi fibres.

Animals↗