Biomedical subjects

N A Lavidis

Publications and source records attributed to N A Lavidis.

At least 37 records · Page 2Linked to original sources

PubMed1995

The effect of opiates on the terminal nerve impulse and quantal secretion from visualized amphibian nerve terminals.

1. Secretion of transmitter from amphibian motor nerve terminal release sites is intermittent, spatially non-uniform and varies considerably throughout the year and during development. The role of opioid receptors in modulating transmitter secretion from amphibian motor nerve terminals is evaluated in this study. 2. Dynorphin-A (24 microM) and morphine (500 microM) did not significantly change the shape of the nerve impulse or the consistency with which it was observed, but decreased evoked quantal secretion by more than 50%. These effects of dynorphin-A and morphine were largely reversed by naloxone (50 microM). 3. Dynorphin-A and morphine did not significantly change either the amplitude or the frequency of spontaneous quantal secretions. 4. There was a uniform decrease in evoked quantal secretion from release sites along terminal branches, irrespective of the quantal content value before drug treatment, indicating no difference in the susceptibility of proximal vs distal release sites to opiates. 5. Increasing the extracellular calcium concentration (0.3 to 0.4 mM) or trains of conditioning-test impulses (25 to 100 Hz) resulted in smaller dynorphin-A or morphine-induced decreases in evoked quantal secretion. 6. The decrease in evoked quantal secretion occurs as a result of a uniform decrease in the probability of quantal secretion from release sites without any affect on the propagation of the nerve terminal impulse. Low probability release sites become effectively silent.

Action Potentials↗

PubMed1994

Calcium dependence of quantal secretion from visualized sympathetic nerve varicosities on the mouse vas deferens.

1. The effects of calcium on the secretion of quanta recorded from single varicosities on the surface of the mouse vas deferens has been determined. 2. If recordings were made from two adjacent varicosities and the [Ca2+]o increased from a low value (1 mM), then the increase in the mean quantal content of the endplate potential (m(e)) was almost entirely due to an increase in the binomial probability for secretion (pe). At higher [Ca2+]o there was an increasing tendency for the binomial parameter (ne) to increase from 1 to 2. When ne increased there was very little change in pe, indicating that the new release site recruited from the other varicosity has a relatively low probability for secretion. 3. If recordings were restricted to single varicosities and the [Ca2+]o increased in the range from 1 to 4 mM then the increases in m(e) were almost always due to an increase in pe. The gradient relating the log of m(e) to the log of [Ca2+]o for [Ca2+]o of 1.0-1.5 mM was in the range 3.2-5.4 (mean 4.2). 4. Test impulses gave a similar proportional increase in m(e) following a conditioning impulse at all varicosities from which recordings were made. 5. Facilitation of m(e) declined exponentially with an increase in the test-conditioning interval from 0.5 to 4 s. The time constant for this decline was about 6 s. 6. The results indicate that single release sites show a similar fourth power dependency on [Ca2+]o and facilitate to about the same degree following a conditioning impulse.

Animals↗

PubMed1994

Effect of yohimbine on the contractile response of the mouse vas deferens to short, low frequency trains of nerve impulses.

A study was undertaken to ascertain why in the mouse vas deferens excitatory junction potentials facilitate whereas contractions depress during low frequency stimulation. In a set of contraction studies, where a conditioning stimulus was followed 2 s later by a test stimulus, depression in the first phase of contraction was observed only at high stimulus strengths when many nerves are activated leading to a large secretion. Low stimulus strengths did not produce depression, suggesting that the autoinhibitory effect due to released noradrenaline is absent. At supramaximal stimulus strengths short, low frequency (0.1-0.5 Hz) trains gave varying degrees of depression of the first phase at each stimulus frequency. This depression was reversed by yohimbine (10 microM), indicating that the level of autoinhibition depends on the total amount of noradrenaline secreted. In the absence of autoinhibition, facilitation could be predicted in terms of the residual Ca2+ hypothesis.

Adrenergic alpha-2 Receptor Antagonists↗

PubMed1993

Evidence that each nerve varicosity on the surface of the mouse vas deferens secretes ATP.

The secretion of ATP from visualised varicosities on the surface of the mouse vas deferens was determined. Small diameter microelectrodes (about 6 microns and 20 microns) were placed over a set of 2 to 5 varicosities visualised with DiOC2(5) in order to record excitatory junctional currents (e.j.c.s') arising from the evoked secretion of quanta. In high external calcium concentrations, [Ca2+]o, the amplitude-frequency distribution of e.j.c.s' was well described by binomial statistics in which binomial parameter n and the number of varicosities were approximately the same. Idazoxan (1 microM) had no effect on the e.j.c.s' or the binomial parameters p and n. Suramin (100 microM) blocked the e.j.c.s' as well as the spontaneous e.j.c.s' (s.e.j.c.s') in all cases, reducing n and p to zero. These results indicate that all varicosities secrete the transmitter blocked by suramin, presumably ATP.

Adenosine Triphosphate↗

PubMed1993

Sympathetic innervation of the surface of the mouse vas deferens.

The distribution of axons and axon varicosities on the surface of the mouse vas deferens has been determined following fluorescence of these structures with 3,3-diethyloxardicarbocyanide (DiOC2 (5)) to locate varicosities and FAGLU to detect catecholamine containing nerves. Small bundles of fluorescent axons treated with DiOC2(5), were shown to give rise to single or very small bundles of 2 or 3 varicose axons that passed over the surface of the muscle bundles. Varicosities had the same average diameter of 0.9 microns, length of 1.1 micron and spacing apart of 4.6 microns whether identified following DiOC2(5) fluorescence or the FAGLU method for catecholamines and this was shown statistically to imply that they came from the same population of varicosities. Serial thin sections through small axon bundles and single axons, viewed with the electron microscope, confirmed the dimensions of varicosities along axons observed following DiOC2(5) staining or use of the FAGLU method.

Animals↗

PubMed1993

Probabilistic secretion of quanta from successive sets of visualized varicosities along single sympathetic nerve terminals.

Quantal secretion was recorded from adjacent sets of varicosities along the length of single sympathetic nerve terminals that had been visualized with the fluorescent dye, 3,3-diethyloxardicarbocyanine iodide (DiOC2(5)). Recordings were made with both small diameter microelectrodes (4-6 microns) placed over 1-3 varicosities and large diameter microelectrodes (20-50 microns) placed over 3-7 varicosities. The proportion of trials in which secretion occurred from sets of varicosities declined in general for the adjacent sets of varicosities along the length of single sympathetic nerve terminals. The amplitude-frequency distributions of excitatory junctional currents (EJCs) were in general predicted by Poisson statistics and occasionally by binomial statistics. The mean quantal content of the EJC in general declined over threefold along the length of single sympathetic nerve terminals for adjacent sets of varicosities.

Animals↗

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

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 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↗

PubMed1989

The probability of quantal secretion at release sites in different calcium concentrations in toad (Bufo marinus) muscle.

1. The number of quanta secreted from visualized release sites along terminal branches at toad (Bufo marinus) neuromuscular junctions in different extracellular concentrations of calcium ions. [Ca2+]o, and during facilitation was determined. Terminal branches were visualized by prior staining with the fluorescent dye, 3-3 diethyloxardicarbocyanine iodide (DiOC2(5)). 2. Increasing [Ca2+]o between 0.25 and 0.4 mM gave a similar proportional increase in the mean quantal content of the end-plate potential recorded with an extracellular electrode (me) at all sites along terminal branches. Thus the length constant for the experimental decline in me along terminal branches (the quantal length constant) remained constant with an increase in [Ca2+]o. The increase in m with [Ca2+]o followed a fourth power relation at both proximal and distal release sites. 3. The increase in me with [Ca2+]o was almost entirely due to an increase in the binomial probability for secretion, pe, which increased as the third to fourth power of [Ca2+]o. However, at higher [Ca2+]o there was an increasing tendency for the binomial parameter ne to increase. It is shown that when ne increases by about 1 there is very little change in pe, suggesting that the new release site recruited at high [Ca2+]o has a relatively low probability for secretion. 4. Test impulses gave a similar proportional increase in me following a conditioning impulse at all sites along terminal branches. The quantal length constant remained constant for both conditioning and test values of me. The increase in me for the test impulse increased linearly with an increase in me for the conditioning impulse at all release sites. 5. Facilitation of me declined exponentially with an increase in the test-conditioning interval. The time constant for this decline (30-40 ms) was similar at both proximal and distal release sites. Changes in facilitation of me were almost entirely due to changes in pe except at very short test-conditioning intervals of about 10 ms. At these intervals ne frequently increased by about 1 and there was very little change in pe. Again, this suggests that additional release sites recruited at short intervals have relatively low probability for secretion. 6. The results indicate that relatively low probability release sites exist in close juxtaposition to relatively high probability release sites which themselves decline in probability along the length of terminal branches.

Animals↗

PubMed1989

The probability of quantal secretion at release sites of different length in toad (Bufo marinus) muscle.

1. The evoked quantal secretion recorded with an extracellular microelectrode (me) at selected sites along motor terminal branches visualized with the fluorescent dye 3-3 diethyloxardicarbocyanine iodide (DiOC2(5)) was compared with the maximum length of the synaptic contact at these release sites reconstructed from serial sections examined with the electron microscope. In addition the relationship between the binomial probability of secretion at release sites (pe) and the length of the synaptic contact was determined in an extracellular calcium. [Ca2+]o, of 0.35 mM. 2. Three of the six terminal branches studied in this way showed a decline in synaptic contact length of release sites from near the point of nerve entry (proximal) to the end of the branch (distal). The remaining three branches showed an increase in synaptic contact length near their middle, and in each case this was associated with a Schwann cell nucleus: contact length then declined to the end of the branches. 3. Both me and pe increased linearly with an increase in the maximum length of the synaptic contact over a range from 0.4 to 4.0 microns. This occurred independently of how the synaptic contact length varied along the length of terminal branches. The value of pe increased by about 0.05 for each 1 micron increase in synaptic contact length in a [Ca2+]o of 0.35 mM. 4. The decrease in synaptic contact length along the proximal parts of terminal branches, in which this occurs, is mostly due to a decrease in the length of close opposition (less than 0.2 micron) between the nerve terminal membrane and the postsynaptic membrane: the decrease in more distal parts of branches is due to the progressive encroachment of Schwann cell processes between the presynaptic and postsynaptic membranes as well as a decrease in synaptic contact length.

Animals↗

PubMed1988

Quantal secretion at release sites of nerve terminals in toad (Bufo marinus) muscle during formation of topographical maps.

1. The number of quanta secreted from selected sites along terminal branches at suppressed synapses in the developing toad (Bufo marinus) gluteus muscle has been determined. The topographical projection from segmental nerves 8 and 9 to the ventral surface of this muscle matures slowly as toads develop in size from 12 to 40 g. Terminal branches of nerves 8 and 9 were visualized by prior staining with the fluorescent dye, 3-3-diethyloxardicarbocyanine iodide (DiOC2(5]. 2. The evoked quantal release recorded with an extracellular electrode (m(e) at different positions along the length of terminal branches at synaptic sites innervated either by nerve 8 (me,8) or nerve 9 (me,9) was determined in an external Ca2+ concentration, [Ca2+]o, of 0.35-0.45 mM. For over 90% of branches longer than 80 microns, me declined along exponential curves from a relatively large value at the proximal end of branches for both nerve 8 and nerve 9 terminals; the exponent for these exponential curves gave quantal length constants that varied from 26 to 80 microns (48 +/- 4 microns, mean +/- S.E.M.) depending on the length of the branch. 3. The evoked quantal release recorded with an intracellular electrode (m) at synaptic sites dually innervated by nerve 8 and nerve 9 was nearly always (greater than 90%) greater for nerve 8 terminals than for nerve 9 terminals. At singly innervated sites the value of m per 100 microns length of terminal declined approximately exponentially with an increase in total terminal length (length constant 400 microns). However, at dually innervated sites the value of m per 100 microns length of nerve 9 terminal was very low at all total terminal lengths compared with singly innervated sites; this indicates that nerve 9 terminals were suppressed at dually innervated sites. 4. At five dually innervated sites, seven out of nine terminal branches of nerve 8 showed an exponential decline in me,8 along their length, from a relatively large value near the proximal end of the branches (length constant 35 +/- 3 microns, mean +/- S.E.M.). In contrast, all the terminal branches of nerve 9 greater than 80 microns showed a uniformly low value of me,9 along their length. 5. It is suggested that the suppression of nerve 9 terminals at dually innervated sites is primarily due to a decrease in the probability of secretion of normally highly secreting release sites at the proximal end of terminal branches.

Action Potentials↗

PubMed1987

Changes in the dimensions of release sites along terminal branches at amphibian neuromuscular synapses.

The probability of transmitter secretion from release sites declines along the length of most long terminal branches (greater than 78 micron) at toad (Bufo marinus) neuromuscular junctions; in contrast, few short terminal branches (less than 78 micron) show such a decline. The present study was carried out to see if any of the dimensions of release sites change along the length of terminal branches in a way that can be correlated with the decrease in secretion probability. The size of presynaptic release site structures was determined by examining serial transverse sections through entire terminal branches with the transmission electron microscope; the size of postsynaptic release site structures was determined by examining terminal gutters with the scanning electron microscope after the removal of terminal branches. Long terminal branches showed a significant decrease in the length of their synaptic contact and cross-sectional area (terminal size) with distance from the origin of the branch. In contrast, there was no significant difference in the length of close apposition (less than 0.2 micron) between the nerve terminal and postsynaptic muscle membrane; furthermore, neither the length of postsynaptic folds nor the frequency of the folds along the length of the terminal gutter changed. Short terminal branches showed no significant differences in the dimensions of either presynaptic or postsynaptic release site structures. The decline in the length of synaptic contacts whilst the length of close apposition remains relatively constant is due to the progressive encroachment of Schwann cell processes between presynaptic and postsynaptic membranes along the length of long terminal branches.

Animals↗

PubMed1986

Motoneurone numbers in rat neonatal lateral motor columns: use of antibody to choline acetyltransferase to identify motoneurones.

The number of motoneurones in the rat lumbosacral and brachial motor columns were determined during the neonatal period, following identification of the motoneurones with antibodies to the enzyme choline acetyltransferase. Motoneurone numbers declined by less than 15% between 21 days embryonic and 4 days postnatal. These observations suggest that there is little change in the population of motoneurones during the period of extensive loss of polyneuronal innervation of muscle.

Animals↗

PubMed1986

The probability of quantal secretion along visualized terminal branches at amphibian (Bufo marinus) neuromuscular synapses.

The number of quanta secreted from selected sites along terminal branches at toad (Bufo marinus) neuromuscular junctions was determined. Terminal branches were visualized by prior staining with the fluorescent dye, 3-3 Diethyloxadicarbocyanine iodide (DiOC2(5)); neither impulse conduction nor quantal release were affected by DiOC2(5) at concentrations less than 10 microM. The evoked quantal release recorded with an extracellular micro-electrode (me) at different sites along the length of terminal branches was determined in an external calcium concentration, [Ca]o, of 0.35-0.45 mM. For short branches (40-80 microns), me remained approximately constant for over 60% of the branches; for the rest, me declined approximately exponentially with an average length constant of 17 +/- 2 microns (mean +/- S.E. of mean). For both medium (81-120 microns) and long branches (121-160 microns), me declined in nearly all cases approximately exponentially with length constants of 39 +/- 5 and 54 +/- 8 microns respectively. These changes in me were observed at synapses having a wide range of terminal branching patterns. Some DiOC2(5)-stained branches possessed discontinuous cholinesterase staining. In general, me declined along these branches in the same way as along DiOC2(5)-stained branches with continuous cholinesterase staining. It is suggested that because of the decline in me along most medium and long terminal branches, many release sites have a very low probability for secretion in low [Ca]o. Release sites near the point of nerve entry, which have a relatively high probability, therefore make the main contribution to secretion recorded with an intracellular micro-electrode. As a consequence, transmitter secretion from the whole terminal does not fluctuate from impulse to impulse as much as expected if there were a large number of release sites, each with a low probability of secretion. Transmitter secretion then follows binomial rather than Poisson statistics.

Action Potentials↗

PubMed1985

Development of the mature distribution of synapses on fibres in the frog sartorius muscle.

Most of the fibres in mature frog sartorius muscle possess two or more synapses separated by up to one-third the length of the muscle. The aim of the present work was to determine how the relative distances between these synapses changes during development in the frog (Limnodynastes tasmaniansis), as the fibres increase in length from 2 mm (stage 56) to 20 mm (1 year postmetamorphosis). At the earliest stage investigated (fibres 2.0-4.0 mm in length; stages 56-57) about 80% of the fibres were innervated at two endplates. The percentage of fibres with two endplates then remained approximately constant with further development. The polyneuronal innervation of endplates was almost eliminated by stage 57. Muscle fibres with two endplates had each situated on average about one-third the length of the fibre from a tendinous insertion; these relative positions did not change throughout development. Thus the distance between endplates increased linearly with an increase in fibre length. The size of terminals and the complexity of their branching also increased continually throughout development, independently of the location of the terminals on the fibres. The observations suggest that the distance between terminals increases during development because of the intercalation of new plasma membrane and basal lamina associated with the increase in length and diameter of fibres.

Aging↗

PubMed1984

Segmental motor projections to rat muscles during the loss of polyneuronal innervation.

The pattern of innervation of three muscles from rat (lateral gastrocnemius, biceps brachii, pectoralis minor) has been ascertained in the neonatal period (birth to 4 days) and from two to three weeks postnatal. Contraction, electrophysiological and histological methods were used to determine the pattern of innervation. There is a change in the segmental innervation during development which establishes the topographical projection of different segmental nerves found in mature muscles. Development of such topographical projections by different segmental nerves onto the muscles studied emerges as a consequence of elimination of terminals arising from the inappropriate segmental nerve.

Animals↗