Biomedical subjects

N A Lavidis

Publications and source records attributed to N A Lavidis.

At least 19 recordsLinked to original sources

PubMed1996

Quantal secretion from visualized boutons on rat pelvic ganglion neurones.

Synaptic transmission from single preganglionic hypogastric nerves innervating monopolar pelvic ganglion neurones has been studied with intracellular electrodes to record transmission from all the boutons and with extracellular electrodes placed over boutons visualized with DiOC2(5) in order to record transmission from selected boutons. Intracellular electrodes revealed spontaneous excitatory postsynaptic potentials (EPSPs) with amplitude histograms that show increasing numbers of large EPSPs as the external calcium ([Ca2+]o) was increased from 0.15 to 1.0 mM. These histograms were in general well fitted by a Poisson mixture of gamma distributions. Extracellular electrodes placed over visualized boutons revealed evoked excitatory postsynaptic potentials (extracellular EPSPs) with amplitude histograms that were best described by single gamma distributions in most cases in low [Ca2+]o (less than 0.5 mM). The standard deviation of these gammas was not much larger than that of the electrical noise. In a minority of extracellular recordings the amplitude histogram of evoked extracellular EPSPs was best described by a gamma distribution in which the standard deviation was much greater than that of the noise. Confocal microscopy of boutons orthogradely labelled with dextran-rhodamine showed that about 30% of these formed closely apposing pairs on the surface of the neurones. These observations are discussed in terms of the hypothesis that multiquantal release at boutons occurs as a consequence of the coupled secretion from closely apposed boutons.

Animals↗

PubMed1996

Spatial relationships between sympathetic varicosities and smooth muscle cells in the longitudinal layer of the mouse vas deferens.

The spatial relationships between nerve varicosities and smooth muscle cells in the longitudinal muscle layer of the mouse vas deferens have been determined from serial section reconstructions of individual varicosities at the ultrastructural level. Bundles of up to five axons, together with single axons, occurred frequently at the surface of the muscle as well as at about 3-6 muscle cell diameters into the muscle. Varicosities within axon bundles at the muscle surface each became partially divested of Schwann cell processes. The smallest distance separating varicosity membrane from muscle cell membrane (apposition distance) was 100 nm (mean 170 nm) for varicosities contained in bundles. Varicosities from six single axons on the muscle surface were reconstructed and 11 of the 12 possessed a mean apposition distance of 48 nm. Varicosities in axon bundles at about 12 microns deep into the muscle came into an apposition distance of 50-90 nm (mean = 67 nm). All varicosities of single axons at this depth came into about 50 nm apposition (mean = 53 nm). These results indicate that the varicosities lie at varying distances from the muscle cells in the longitudinal muscle layer of the vas deferens.

Animals↗

PubMed1996

Effect of opiates on transmitter release from visualized hypogastric boutons innervating the rat pelvic ganglia.

1. The effect of opiates on neurotransmission between visualized hypogastric nerve boutons and postganglionic cell bodies has been examined using extracellular recording of nerve bouton impulses (NBIs) and excitatory postsynaptic currents (e.p.s.cs). 2. Morphine (10 to 40 microM) did not affect neurotransmission in the ganglia. Dynorphin-A (4 microM) and U50488H (1 microM) decreased quantal transmitter release and naloxone (10 microM) reversed these effects. 3. Morphine (10 microM), dynorphin-A (4 microM) and U50488H (1 microM) did not affect either the time course or consistency with which the NBI was recorded. 4. Dynorphin-A (1 to 4 microM) and U50488H (1 microM) decreased the average amplitude of e.p.s.cs by increasing the number of failures to release quanta from single or small groups of 2 to 4 boutons during continuous nerve stimulation at 0.1 Hz. 5. The decrease in quantal release induced by dynorphin-A and U50488H in 0.2 to 0.5 mM [Ca2+]zero was readily reversed by increasing the extracellular calcium ion concentration to 1 mM. 6. It was concluded that kappa-opioid receptors are located on the boutons of the hypogastric nerve and when activated by kappa-opioid receptor agonists reduce quantal release without affecting the NBI.

Animals↗

PubMed1995

Probabilistic secretion of quanta at somatic motor-nerve terminals: the fusion-pore model, quantal detection and autoinhibition.

The probability of detecting first, second, and later quanta secreted at release sites of a motor-nerve terminal during the early release period following a nerve impulse has been addressed. The possibility that early quantal release autoinhibits later quantal release during this period has also been ascertained. In this investigation, a model for the secretion of a quantum at a release site is developed in which, following the influx and diffusion of calcium ions to a release site protein associated with synaptic vesicles, kappa steps of association of the ions with the protein then occur at rate alpha. The release site protein then undergoes a conformational change which may not go on to completion if calcium ions dissociate from the protein at rate gamma. If this process does reach completion then a fusion-pore between the vesicle and the presynaptic membrane is created; this happens at rate delta. Key assumptions of this fusion-pore model are that the quantal secretions from each site are independent of each other, and that there is a large number of vesicles, each with a small probability of secretion, so that the number of secretions is Poisson in nature. These assumptions allow analytical expressions to be obtained for predicting the times at which first, second and later quanta are secreted during the early release period following an impulse. To test the model, experiments were performed in which the times of first, second and later quantal releases were determined at discrete regions along the length of visualized motor-terminal branches in toad (Bufo marinus) muscles. Estimates of model rate constants and of kappa from the times for first quantal secretions failed to give satisfactory predictions of the observed times of later secretions. Therefore, either the model fails, or the procedure used for detecting later quantal events as a consequence of their being masked by earlier quantal events is inadequate. To solve this detection problem, a two-dimensional analysis of the spread of charge following the secretion of a quantum at a random site on the motor-terminal branch has been done. This allows determination of the probability that later quanta will be detected following secretion of earlier quanta. The detection model was then incorporated into the fusion-pore model to predict the times at which second and later quanta occur during the early release period, based on the estimates of the model parameters derived from the analysis of first quantal releases. Good estimates were now obtained for the observed times of second and later quantal releases, indicating that appropriate procedures must be adopted for adequate detection of quantal secretions. Furthermore, the experiments provide support for the fusion-pore model. It has been suggested that the binomial nature of quantal release from the entire motor-nerve terminal may be explained if early quantal release inhibits later quantal release during the early quantal release phase (M. R. Bennett & J. Robinson 1990, Proc. R. Soc. Lond. B 239, 329-358). Although the fusion-pore detection error model gave good predictions of the observed times of first, second and later quantal releases, these may be improved if a model for autoinhibition is included. In this model the first quantum was taken as giving rise to an inhibition of secretion that propagates to surrounding release sites with a constant velocity, v. A combined model incorporating the fusion-pore detection error model and that for autoinhibition was then used to predict second and later quantal latencies, by using the first quantal latencies to determine the estimates for the parameters in the combined model. When this analysis was done on the times for quantal secretion at sites on thirteen different motor-nerve terminals, the value of v was estimated as zero in each case, so that no autoinhibitory effect was observed.

Animals↗

PubMed1995

Quantal secretion recorded from visualized boutons.

A method is described for recording the electrical signs of transmission at single boutons. Monopolar rat pelvic ganglion cells are electrically compact and receive an innervation from either single hypogastric or pelvic nerves that consists of a set of boutons which innervate the soma of the cells; each bouton possesses an active zone and can be observed with the confocal microscope after dextran-rhodamine orthograde labelling of the hypogastric nerves. Extracellular electrodes of about 8 microns diameter can be positioned in the loose-patch configuration over visualized boutons following their fluorescence with the vital dye, 3,3-diethyloxardicarbocyanine iodide (DiOC2(5)). Excitatory post-synaptic currents (EPSCs), due to stimulation of the hypogastric nerve, can be recorded from boutons in this way, provided that [Ca2+]o is lowered sufficiently to ensure failure of the initiation of the soma action potential by the EPSCs.

Animals↗

PubMed1995

The effect of opiates on the secretion of transmitter from amphibian motor nerve terminals.

The effects of dynorphin-A, dermorphine and morphine on the secretion of transmitter from the toad (Bufo marinus) motor nerve terminal have been determined. Intracellular recordings of miniature end plate potentials (m.e.p.p.s) and evoked end plate potentials (e.p.p.s) were used to estimate quantal content (m) and binomial parameters p and n. Dynorphin-A, and to a lesser extent morphine, decreased (m) while dermorphine had no significant effect on m. Dynorphin-A (ED50 = 24 microM) was 21 times more potent then morphine (ED50 = 510 microM) in decreasing m. The decrease in m produced by dynorphin-A and morphine was accompanied by a greater decrease in the variance (S2) of number of quanta secreted per stimulation over the recording period. The decrease in m produced by dynorphin-A, and to a lesser extent by morphine, is probably mediated by the opiates acting on kappa-opioid receptors.

Animals↗

PubMed1995

Effect of morphine on the nerve terminal impulse and transmitter release from sympathetic varicosities innervating the mouse vas deferens.

1 The effect of morphine on both the propagation of the nerve terminal impulse along the sympathetic varicose axons as well as the evoked and spontaneous transmitter release has been evaluated. 2 Morphine (1 microM) did not significantly change the shape or the regularity by which the nerve terminal impulse was recorded while evoked transmitter release was greatly reduced. 3 Morphine induced a uniform decrease in evoked transmitter release irrespective of the release probability of individual varicosities of their position along terminal branches. 4 Procedures which are thought to increase intracellular calcium concentration such as increasing the extracellular calcium concentration, stimulation of the nerve with trains of impulses and increasing the duration of the action potential with 4-aminopyridine reduced the ability of morphine to decrease evoked transmitter release. 5 Morphine had to act directly on the varicosities to induce a decrease in evoked transmitter release. 6 The decrease in evoked quantal release does not involve an affect on the nerve terminal impulse or the vesicle release process and morphine may affect the dependence of the secretory process on calcium.

Analgesics, Opioid↗

PubMed1995

Effect of chronic morphine treatment on transmitter release from sympathetic varicosities of the mouse vas deferens.

1 Transmitter release from sympathetic varicosities of mouse vasa deferentia removed from animals which were chronically treated with morphine for 7 to 9 days has been evaluated. 2 In control preparations increasing the extracellular calcium concentration ([Ca2+]o) from 1 to 2 mM increased transmitter release by 3 fold while increasing [Ca2+]o from 6 to 8 mM increased transmitter release by about 0.9 fold. Introduction of morphine (1.0 microM) produced a uniform decrease in transmitter release, shifting the relationship between transmitter release and [Ca2+]o to the right. 3 Only sympathetic varicosities with probabilities of transmitter release greater than 0.01 were chosen for this study. In these varicosities the decrease in transmitter release induced by morphine in control preparations (bathed in [Ca2+]o 2.0 mM) was not observed following 7 to 9 days of morphine treatment. When the morphine was acutely withdrawn from these preparations transmitter release was more than 6 times the average level of transmitter release from control preparations. 4 The morphine induced increase in facilitation of transmitter release while stimulating with short trains of nerve impulses was not observed when the preparations were removed from animals which had been exposed to morphine for 7 to 9 days. When these preparations were acutely withdrawn from morphine there was a further decrease in the level of facilitation and a significant increase in depression of transmitter release when compared to control. 5 The morphine induced decrease in probability of transmitter release when naive sympathetic varicosities in vitro were bathed with morphine (1 microM) was not observed following chronic morphine treatment of the animals for 7 to 9 days. When the morphine was acutely withdrawn from chronically morphine treated preparations the underlying increase in probabilities of transmitter release of sympathetic varicosities was unmasked.

Analgesics, Opioid↗

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↗