The effect of calcium ions on the binomial statistic parameters that control acetylcholine release at preganglionic nerve terminals

Synaptic Plasticity in Cardiac Innervation and Its Potential Role in Atrial Fibrillation

Synaptic plasticity is defined as the ability of synapses to change their strength of transmission. Plasticity of synaptic connections in the brain is a major focus of neuroscience research, as it is the primary mechanism underpinning learning and memory. Beyond the brain however, plasticity in peripheral neurons is less well understood, particularly in the neurons innervating the heart. The atria receive rich innervation from the autonomic branch of the peripheral nervous system. Sympathetic neurons are clustered in stellate and cervical ganglia alongside the spinal cord and extend fibers to the heart directly innervating the myocardium. These neurons are major drivers of hyperactive sympathetic activity observed in heart disease, ventricular arrhythmias, and sudden cardiac death. Both pre- and postsynaptic changes have been observed to occur at synapses formed by sympathetic ganglion neurons, suggesting that plasticity at sympathetic neuro-cardiac synapses is a major contributor to arrhythmias. Less is known about the plasticity in parasympathetic neurons located in clusters on the heart surface. These neuronal clusters, termed ganglionated plexi, or “little brains,” can independently modulate neural control of the heart and stimulation that enhances their excitability can induce arrhythmia such as atrial fibrillation. The ability of these neurons to alter parasympathetic activity suggests that plasticity may indeed occur at the synapses formed on and by ganglionated plexi neurons. Such changes may not only fine-tune autonomic innervation of the heart, but could also be a source of maladaptive plasticity during atrial fibrillation.

Differential contribution of extracellular and intracellular calcium sources to basal transmission and long-term potentiation in the sympathetic ganglion of the rat

Calcium involved in basal ganglionic transmission and long-term potentiation (LTP) can arise either by influx from the extracellular medium or release from intracellular stores. No attempts have yet been made to concurrently explore the contributions of extracellular and intracellular Ca2+ to basal ganglionic transmission or LTP. Here, we investigate this subject using the superior cervical ganglion of the rat. To explore the extracellular Ca2+ contribution, we evaluated basal transmission and LTP at different extracellular Ca2+ concentrations. To assess intracellular Ca2+ release, we explored the contribution of the calcium-induced calcium release process by overactivation or blockade of ryanodine-sensitive Ca2+ receptor channel with caffeine, and also by blocking either IP3R with Xestospongin C or the sarco(endo)plasmic reticulum Ca2+-ATPase pump with thapsigargin. Extracellular Ca2+ affected ganglionic basal transmission and LTP to different extents. While 25% of the physiological Ca2+ concentration supported 80% of basal transmission, 50% of normal Ca2+ was required to achieve 80% of LTP. Notably, disruption of intracellular Ca2+ release by all the drugs tested apparently did not affect basal ganglionic transmission but impaired LTP. We conclude that basal transmission requires only a small level of Ca2+ entry, while LTP expression not only requires more Ca2+ entry but is also dependent on Ca2+ release from intracellular stores.

Phorbol esters and adenosine affect the readily releasable neurotransmitter pool by different mechanisms at amphibian motor nerve endings

Phorbol esters and adenosine have been proposed to interact at common sites downstream of calcium entry at amphibian motor nerve endings. We thus studied the actions and interactions of phorbol esters and adenosine using electrophysiological recording techniques in conjunction with both binomial statistical analysis and high-frequency stimulation at the amphibian neuromuscular junction. To begin this study, we confirmed previous observations that synchronous evoked acetylcholine (ACh) release (reflected as endplate potentials, EPPs) is well described by a simple binomial distribution. We then used binomial analysis to study the effects of the phorbol ester phorbol dibutyrate (PDBu, 100 nM) and adenosine (50 microM) on the binomial parameters n (the number of calcium charged ACh quanta available for release) and p (the average probability of release), where the mean level of evoked ACh release (m) = np. We found that PDBu increased m by increasing the parameter n whilst adenosine reduced m by reducing n; neither agent affected the parameter p. PDBu had no effect on either the potency or efficacy of the inhibition produced by adenosine. Subtle differences between these two agents were revealed by the patterns of EPPs evoked by high-frequency trains of stimuli. Phorbol esters increased ACh release during the early phase of stimulation but not during the subsequent plateau phase. The inhibitory effect of adenosine was maximal at the beginning of the train and was still present with reduced efficacy during the plateau phase. When taken together with previous findings, these present results suggest that phorbol esters increase the immediately available store of synaptic vesicles by increasing the number of primed vesicles whilst adenosine acts at a later stage of the secretory process to decrease the number of calcium-charged primed vesicles.

Evidence for two distinct processes in the final stages of neurotransmitter release as detected by binomial analysis in calcium and strontium solutions

The statistical parameters underlying acetylcholine (ACh) release were studied using Ca(2+) and Sr(2+) ions to promote ACh secretion. Experiments were performed at frog neuromuscular junctions using electrophysiological recording techniques. Increases in asynchronous ACh release, reflected as the frequency of occurrence of miniature end-plate potentials (MEPP(f)), were evoked by high potassium depolarization in either Ca(2+) or Sr(2+) solutions. Increases in MEPP(f) mediated by Ca(2+) were of very low probability and well-described by a Poisson distribution whilst similar MEPP(f) increases mediated by Sr(2+) were best described as a simple binomial distribution. From the binomial distribution in Sr(2+) solutions, values for the average probability of release (p) and the number of releasable ACh quanta (n) may be determined (whereby mean MEPP(f) = np). In Sr(2+) solutions, values of p were independent of both bin width and of the value of n, suggesting that both n and p were stationary. Calculations of p using the simple binomial distribution in Sr(2+) solutions gave theoretical values for the third moment of the mean which were indistinguishable from the experimental distribution. These results, in conjunction with Monte Carlo simulations of the data, suggest that spatial and temporal variance do not measurably affect the analysis. Synchronous ACh release evoked by nerve impulses (end-plate potentials, EPPs) follow a simple binomial distribution in both Ca(2+) and Sr(2+) solutions. Similar mean levels of synchronous ACh release (m, where m = np) were produced by lower values of p and higher values of n in Ca(2+) as compared to Sr(2+). The statistical analyses suggest the presence of two different Ca(2+)-dependent steps in the final stages of neurotransmitter release. The results are discussed in accordance with (i) statistical models for quantal neurotransmitter release, (ii) the role of Sr(2+) as a partial agonist for evoked ACh release, and (iii) the specific loci that may represent the sites of Ca(2+) and Sr(2+) sensitivity.

The probability of quantal secretion within an array of calcium channels of an active zone

A Monte Carlo analysis has been made of calcium dynamics in submembranous domains of active zones in which the calcium contributed by the opening of many channels is pooled. The kinetics of calcium ions in these domains has been determined using simulations for channels arranged in different geometries, according to the active zone under consideration: rectangular grids for varicosities and boutons and lines for motor-nerve terminals. The effects of endogenous fixed and mobile buffers on the two-dimensional distribution of free calcium ions at these active zones are then given, together with the extent to which these are perturbed and can be detected with different affinity calcium indicators when the calcium channels open stochastically under an action potential. A Monte Carlo analysis of how the dynamics of calcium ions in the submembranous domains determines the probability of exocytosis from docked vesicles is also presented. The spatial distribution of exocytosis from rectangular arrays of secretory units is such that exocytosis is largely excluded from the edges of the array, due to the effects of endogenous buffers. There is a steeper than linear increase in quantal release with an increase in the number of secretory units in the array, indicating that there is not just a local interaction between secretory units. Conditioning action potentials promote an increase in quantal release by a subsequent action potential primarily by depleting the fixed and mobile buffers in the center of the array. In the case of two parallel lines of secretory units exocytosis is random, and diffusion, together with the endogenous calcium buffers, ensures that the secretory units only interact over relatively short distances. As a consequence of this and in contrast to the case of the rectangular array, there is a linear relationship between the extent of quantal secretion from these zones and their length, for lengths greater than a critical value. This Monte Carlo analysis successfully predicts the relationship between the size and geometry of active zones and the probability of quantal secretion at these, the existence of quantal versus multiquantal release at different active zones, and the origins of the F1 phase of facilitation in synapses possessing different active zone geometries.

Calcium channel subtypes differ at two types of cholinergic synapse in lumbar sympathetic neurones of guinea-pigs

1. The involvement of different presynaptic Ca2+ channels in transmission at 'weak' (subthreshold) and 'strong' (suprathreshold) synapses was investigated in guinea-pig paravertebral ganglia isolated in vitro. Selective Ca2+ channel antagonists were used to block excitatory synaptic currents evoked by stimulating single preganglionic axons. 2. The N-type Ca2+ channel blocker, omega-conotoxin GVIA (100 nM), reduced peak synaptic conductance by similar amounts at weak synapses (by 39 +/- 6 %) and strong synapses (34 +/- 6 %). 3. The P-type Ca2+ channel blocker, omega-agatoxin IVA (40 nM), significantly reduced transmitter release at weak synapses (by 42 +/- 6 %) but had only a small effect at strong synapses (reduced by 6 +/- 2 %). 4. Blockers of Q-, L- or T-type Ca2+ channels had no significant effects on peak synaptic conductance at either type of synapse. 5. We conclude that the two functionally distinct types of preganglionic terminal in sympathetic ganglia which synapse on the same neurone differ in their expression of particular types of voltage-dependent Ca2+ channels. Both types utilize N-type channels and channels resistant to blockade by specific antagonists, but Ca2+ entry through P-type channels makes a substantial contribution to acetylcholine release only at weak synapses.

Calcium dependence and recovery kinetics of presynaptic depression at the climbing fiber to Purkinje cell synapse

Short-term depression is a widespread form of use-dependent plasticity found in the peripheral and central nervous systems of invertebrates and vertebrates. The mechanism behind this transient decrease in synaptic strength is thought to be primarily the result of presynaptic "depletion" of a readily releasable neurotransmitter pool, which typically recovers with a time constant of a few seconds. We studied the mechanism and dynamics of recovery from depression at the climbing fiber to Purkinje cell synapse, where marked presynaptic depression has been described previously. Climbing fibers are well suited to studies of recovery from depression because they display little, if any, facilitation (even under conditions of low-release probability), which can obscure rapid recovery from depression for hundreds of milliseconds after release. We found that recovery from depression occurred in three kinetic phases. The fast and intermediate components could be approximated by exponentials with time constants of 100 msec and 3 sec at 24 degrees C. A much slower recovery phase was also present, but it was only prominent during prolonged stimulus trains. The fast component was enhanced by raising extracellular calcium and was eliminated by lowering presynaptic calcium, suggesting that, on short time scales, recovery from depression is driven by residual calcium. During regular and Poisson stimulus trains, recovery from depression was dramatically accelerated by accumulation of presynaptic residual calcium, maintaining synaptic efficacy under conditions that would otherwise deplete the available transmitter pool. This represents a novel form of presynaptic plasticity in that high levels of activity modulate the rate of recovery as well as the magnitude of depression.

Calcium in sympathetic boutons of rat superior cervical ganglion during facilitation, augmentation and potentiation

The sympathetic preganglionic nerve terminals of the rat superior cervical ganglion were loaded with the calcium indicator oregon green 488 BAPTA-1 to measure the change in calcium concentration in the terminal boutons, (delta[Ca2+]b) following short (1 or 5 impulses) and long (200 impulses) trains at 30 Hz. The delta[Ca2+]b after a single action potential or a short train declined in two phases: a fast phase with a time constant of 530+/-30 ms and a moderate phase with a time constant of 4.0+/-0.2 s. The delta[Ca2+]b following a long train eventually declined with a time constant of 127+/-34 s (slow phase). The addition of either omega-agatoxin TK (100 nM), omega-conotoxin GVIA (100 nM) or nifedipine (20 microM) to block P-type, N-type or L-type calcium channels respectively showed that the rise in delta[Ca2+ ]b in boutons was predominantly mediated by an influx of calcium through P-type (53+/-7%) and N-type (46+/-4%) calcium channels. Experiments with caffeine, ryanodine and thapsigargin indicate that intracellular caffeine-sensitive calcium stores have a small but statistically significant effect on the fast and moderate phases. The mitochondrial uncoupler carbonyl cyanide m-chlorophenyl hydrazone (CCCP; 2 microM) significantly decreased the amplitude of the slow phase of delta[Ca2+]b relaxation, and sped its time course, suggesting that mitochondria normally dump calcium during this phase. Adenosine reduced the amplitude of delta[Ca2+]b in response to single action potentials by 30+/-6%, suggesting that adenosine-mediated autoinhibition in these boutons reduces Ca2+ influx. Spontaneous increases in delta[Ca2+]b demonstrated Ca2+ coupling between adjacent boutons. The delta[Ca2+]b kinetics are compared with F2 facilitation, augmentation and post-tetanic potentiation.

Calcium dependence and recovery kinetics of presynaptic depression at the climbing fiber to Purkinje cell synapse

Short-term depression is a widespread form of use-dependent plasticity found in the peripheral and central nervous systems of invertebrates and vertebrates. The mechanism behind this transient decrease in synaptic strength is thought to be primarily the result of presynaptic "depletion" of a readily releasable neurotransmitter pool, which typically recovers with a time constant of a few seconds. We studied the mechanism and dynamics of recovery from depression at the climbing fiber to Purkinje cell synapse, where marked presynaptic depression has been described previously. Climbing fibers are well suited to studies of recovery from depression because they display little, if any, facilitation (even under conditions of low-release probability), which can obscure rapid recovery from depression for hundreds of milliseconds after release. We found that recovery from depression occurred in three kinetic phases. The fast and intermediate components could be approximated by exponentials with time constants of 100 msec and 3 sec at 24 degrees C. A much slower recovery phase was also present, but it was only prominent during prolonged stimulus trains. The fast component was enhanced by raising extracellular calcium and was eliminated by lowering presynaptic calcium, suggesting that, on short time scales, recovery from depression is driven by residual calcium. During regular and Poisson stimulus trains, recovery from depression was dramatically accelerated by accumulation of presynaptic residual calcium, maintaining synaptic efficacy under conditions that would otherwise deplete the available transmitter pool. This represents a novel form of presynaptic plasticity in that high levels of activity modulate the rate of recovery as well as the magnitude of depression.

Two excitatory motoneurons differ in quantal content of their junctional potentials in abdominal muscle fibers of the cricket, Gryllus bimaculatus

In abdominal muscles 202 and 203 of the cricket, Gryllus bimaculatus, large and small excitatory junctional potentials (l- and s-EJPs) with similar durations can be recorded from the same muscle fibers. At the normal extracellular calcium ion concentration ([Ca(2+)](o)) of 5mM, the amplitudes of l-EJPs in both muscles were larger than the threshold membrane potential for muscle action potentials, which is about -40mV. Below 0.75mM [Ca(2+)](o), the amplitudes became much smaller and were below the firing level for the action potentials. At 0.5mM, they fluctuated and decreased to 10.3 and 1.9mV in muscles 202 and 203, respectively, and at 0.25mM frequent failures occurred. The amplitudes of s-EJPs at 5mM [Ca(2+)](o) were 13.3 and 5.1mV in muscles 202 and 203, respectively, and the fluctuating amplitudes were far below the threshold for muscle action potentials. Below 0.75mM, s-EJPs were rarely observed. The relation between log(EJP amplitude) and log([Ca(2+)](o)) was linear within a certain range of [Ca(2+)](o) and the slopes of the lines for l-EJPs were about twice as steep as those for s-EJPs in both muscles. In muscle 202, the amplitude distribution of l-EJPs obtained at 0.25mM and that of s-EJPs at 0.75mM both showed peaks at once and twice the voltage at the first peak, which were coincident with the voltages at the peaks of amplitude distributions of miniature EJPs recorded simultaneously. The reversal potentials for l- and s-EJPs in muscle 202 were +1.02 and +0.22mV, respectively. In muscle 202, the decreases in amplitude of both EJPs by L-glutamate were similar and concentration-dependent. The results suggest that the difference in amplitude between l- and s-EJPs is attributable mainly to the difference in quantal contents.

Vesicle-associated proteins and calcium in nerve terminals of chick ciliary ganglia during development of facilitation

1. The developmental appearance of synaptic vesicle-associated proteins and nerve terminal calcium ([Ca2+]i) sequestering processes were determined for the chick ciliary ganglia in relation to the maturation of the different phase of increased efficacy of transmitter release following nerve impulses. The maturation phases studied were from stages 34-35, at the time of synapse formation, to stage 46 at hatching. 2. Western blots and immunohistochemical localization indicated that synaptotagmin 1 and synapsin IIa were detectable at stages 34-35 and were clearly localized at the nerve terminals by stage 37. Syntaxin was clearly localized at the nerve terminals at stage 34. 3. The relative size of the postganglionic compound action potential, used to measure the transmission efficacy through the ganglion, showed that the slope of the relationship between log efficacy and log extracellular calcium concentration ([Ca2+]o) in low [Ca2+]o was about 4 by stage 46. 4. A mature facilitatory mechanism for transmission was not present at stage 34 and did not emerge until stage 38. A mature augmentation was not present at stages 34 or 38 and was not established until stages 41-42. Post-tetanic potentiation (PTP) was not present at stage 34; it was evident at stages 37-38 and only reached maturity by stages 41-42. 5. The time course of calcium changes in the nerve terminals following trains of impulses that give rise to facilitation, augmentation and PTP was determined for different stages of development using the indicator Calcium Green-1 in the nerve terminal. The mature time course of the phases of calcium decline in the nerve terminal associated with facilitation and augmentation was observed as early as stage 38, whereas that of the PTP phase did not mature until after stage 42. 6. These results are discussed in terms of the maturation of the vesicle-associated proteins and calcium influx into the terminal following trains of impulses that give rise to the different components of increased synaptic efficacy.

Axosomatic synapses of the rat ciliary ganglion

We have identified four different types of axosomatic synapses within the rat ciliary ganglion, and present the three-dimensional relationships of both pre- and postsynaptic elements. The majority of axosomatic synapses are situated on small postsynaptic spines that simply appose the axon (termed somatic spine), or are situated within an axonal invagination (termed invaginating somatic spine). The somatic spine synapse predominates, composing 70% of the population, which may be due to simplicity of construction as it usually forms only one active zone. In contrast, the invaginating somatic spine forms multiple active zones and accounts for only 22% of the population. Synapses involving a regular nonspinous portion of the cell membrane were rarely encountered (6%; termed somatic), as were those of axon branches situated within tubular invaginations of the cell body (2%; termed tunnelling). Synapses were differentially distributed, occurring four times more frequently on that portion of neuronal cell body membrane adjacent to the glial cell perinuclear area. However, there was no preferred location by synapse type, suggesting that this unequal distribution was the result of a general mechanism. The neuronal cells of the rat ciliary ganglion apparently constitute a single population, at least on the basis of cell size, shape, and organelle content.

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.

Probabilistic secretion of quanta: spontaneous release at active zones of varicosities, boutons, and endplates

The amplitude-frequency histogram of spontaneous miniature endplate potentials follows a Gaussian distribution at mature endplates. This distribution gives the mean and variance of the quantum of transmitter. According to the vesicle hypothesis, this quantum is due to exocytosis of the contents of a single synaptic vesicle. Multimodal amplitude-frequency histograms are observed in varying degrees at developing endplates and at peripheral and central synapses, each of which has a specific active zone structure. These multimodal histograms may be due to the near synchronous exocytosis of more than one vesicle. In the present work, a theoretical treatment is given of the rise of intraterminal calcium after the stochastic opening of a calcium channel within a particular active zone geometry. The stochastic interaction of this calcium with the vesicle-associated proteins involved in exocytosis is then used to calculate the probability of quantal secretions from one or several vesicles at each active zone type. It is shown that this procedure can account for multiquantal spontaneous release that may occur at varicosities and boutons, compared with that at the active zones of motor nerve terminals.

Direct comparative effects of isoflurane and desflurane on sympathetic ganglionic transmission

Although the sympathetic ganglion is an important site of peripheral regulation, few studies have examined the effect of anesthetics on synaptic transmission. In the present study we compared the actions of desflurane with those of isoflurane on synaptic transmission and neurotransmitter release in the stellate ganglion. In the electrophysiologic group, 14 stellate ganglia were isolated from adult mongrel dogs after halothane anesthesia, desheathed, and superfused with Krebs' solution. Compound action potentials (CAP) were induced by supramaximal stimulation of the preganglionic T3-ramus at a low frequency of 0.4 Hz and were recorded from the postganglionic ventral ansa subclaviae. Each ganglion was exposed to two levels of anesthetics (equivalent to 1 and 2 minimum alveolar anesthetic concentration [MAC]), followed by an anesthetic-free washout period. While equianesthetic concentrations of isoflurane and desflurane caused essentially equipotent suppression of ganglionic transmission, desflurane was more efficacious than isoflurane, both with respect to the onset of and recovery from the inhibition of synaptic activity. In the electrochemical group, 25 ganglia were exposed to both anesthetics at a high concentration (equivalent to between 1.82 and 1.95 MAC) during maximal and submaximal current stimulations, and the release of actylcholine (ACh) in the superfusate was measured with liquid chromatography. Although desflurane and isoflurane caused a significant depression of CAP, neither anesthetic inhibited the release of ACh in the superfusate at either maximal or submaximal current stimulations. These results indicate that the suppression of ganglionic activity is equipotent for both anesthetics based on equivalent MAC values, but that desflurane is more efficacious than isoflurane with respect to onset and recovery at the higher concentrations of anesthetics.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Activity following colonic distension in enteric sensory fibres projecting to the inferior mesenteric ganglion in the guinea pig

In this investigation the characteristics of the response to colonic distension of afferent fibres projecting to the inferior mesenteric ganglion (IMG) of the guinea pig were studied in vitro. Intracellular membrane potential recordings were made from neurons in the IMG. Post-synaptic potentials arising from activity in afferent fibres were recorded in these cells following distensions of a small segment of colon placed in a separate and independently perfused organ bath. These afferent fibres showed both transient and sustained responses to distension. Application of a solution containing 0.25 mM Ca2+ and 10 mM2+ Mg2+ to the colon (but not to the IMG) reduced the overall response but did not abolish the activity in these fibres. It is concluded that some afferent fibres which respond to colonic distension project directly to the IMG.

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.

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.

Presynaptic histamine H1 and H3 receptors modulate sympathetic ganglionic synaptic transmission in the guinea-pig

1. To study the effects of histamine on the efficacy of sympathetic ganglionic synaptic transmission, extracellular recordings of the postganglionic compound action potential (CAP) and intracellular recordings of excitatory postsynaptic potentials (EPSPs) elicited by preganglionic electrical stimulation were obtained from isolated guinea-pig superior cervical ganglia (SCG). 2. In different preparations, superfusion with histamine (0.1-100 microM) either potentiated or depressed the postganglionic CAP elicited by electrical stimulation of the cervical sympathetic trunk (0.2-3.0 Hz). The direction of response produced by histamine did not depend on stimulation frequency or histamine concentration; potentiation and depression both showed concentration dependence over the range of histamine concentrations tested. 3. Experiments employing a variety of histamine receptor agonists or antagonists revealed that histamine-induced potentiation of the postganglionic CAP could be attributed to histamine H1 receptor activation, and depression to H3 receptor activation. 4. Histamine similarly potentiated or depressed the intracellularly recorded EPSP. However, these opposite effects occurred at different synapses. In agreement with the studies on the postganglionic CAP, histamine H1 antagonists prevented histamine-induced potentiation of the EPSP and H3 receptor antagonists prevented histamine-induced depression. 5. Direct quantal analyses of histamine-induced synaptic potentiation and depression were implemented to determine the pre- and postsynaptic components of these effects. Quantal size was estimated by measuring the amplitude of spontaneous miniature EPSP amplitudes. Histamine-induced potentiation and depression of the evoked EPSP were found to be accompanied by increased or decreased quantal content respectively, and unchanged quantal size, providing evidence that presynaptic mechanisms were involved in mediating both effects. 6. Some guinea-pigs were actively sensitized to ovalbumin. Subsequent exposure of the isolated SCG from these animals to the sensitizing antigen produced changes in the EPSP amplitude that correlated significantly to the response produced by exogenously applied histamine at the same synapse. 7. The correspondence between the effects of specific antigen challenge and exogenous histamine on evoked EPSPs at a synapse provides evidence that endogenous histamine released during an immunological response to antigen challenge can activate histamine H1 and H3 receptors to modulate synaptic efficacy in sympathetic ganglia.

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)

A statistical test for demonstrating a presynaptic site of action for a modulator of synaptic amplitude

A statistical technique for demonstrating a presynaptic site of action for a modulator of synaptic amplitude was developed and tested. It requires that multiple measurements of peak synaptic amplitude be made under control and test conditions. The ratio of the coefficients of variation (CV) obtained under test and control conditions is calculated. A method was developed for determining the confidence interval for the CV ratio (CVR) statistic based on the null hypothesis that the synaptic modulation is purely postsynaptic. If the measured CVR falls outside the confidence interval, this implies that the modulator of synaptic amplitude is, at least in part, acting at a presynaptic site. The effectiveness of the technique and its limitations were investigated using Monte Carlo simulations. It was found to be sensitive and reliable under a variety of realistic recording conditions. The test was effective even in the presence of simulated presynaptic rundown of the synaptic response. Conventional deconvolution analysis was also applied to the Monte Carlo simulations and was found to be an inadequate indicator of the site of synaptic modulation when the discrete amplitude components were not well resolved. The CVR technique was applied to excitatory postsynaptic currents (epsc) recorded between pairs of cultured hippocampal neurones in control and test media containing 1 mM Ca2+ and 2 mM Ca2+, respectively. Test conditions increased the average synaptic amplitude, and the statistical analysis indicated that this modulation was produced by an action at a presynaptic site.

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.

Burst-patterned stimulation promotes nicotinic transmission in isolated perfused rat sympathetic ganglia

1. Intracellular recordings of small nicotinic excitatory postsynaptic potentials (EPSPs) were made from rostral cells in superior cervical ganglia (SCG) of rats during and after test stimulation of small preganglionic fibre bundles, while perfusing the isolated ganglia via their arterial vasculature. Perfusion, in contrast to superfusion of desheathed ganglia, (a) produced much more rapid and complete equilibration of drugs and ions at synaptic sites, (b) greatly reduced depression of EPSPs during high-frequency stimulation, and (c) largely prevented slowing of conduction, presumably by minimizing accumulation of K+ in the intercellular spaces surrounding these sites. 2. Preganglionic inputs were found to fall into two major groups: those in which the EPSP amplitude during 200 pulse trains was facilitated and others in which it was depressed as stimulation frequency in the train was increased from 2 to 20 Hz or from 0.2 to 1.25 Hz. Both the facilitation and the depression were presynaptic, since they occurred without changes in miniature EPSP amplitude. 3. The maximum maintained facilitation was reached at 5-10 Hz with a value 1.26 times the 1.0 Hz control. This was associated with an increase in the binomial parameter n. While long 20 Hz trains produced a similar facilitation to an early plateau, and an increase in n, EPSP amplitude declined as the train progressed. This was associated with a decrease in the binomial parameter p. 4. Unlike the 20 Hz trains, stimulation with 0.5 s long, 20 Hz bursts given every 8 s produced a marked potentiation in facilitating units and this was maintained for as long as the stimulation was continued (3-11 min). Burst-patterned potentiation was 1.66 times larger than the facilitation evoked by tonic stimulation at the same average frequency (1.25 Hz), and more than twice that achieved with long, 200 pulse trains. The potentiation was associated with increases in both n and p in the first EPSP of the burst and mainly with an increase in n at the end of the burst. Potentiation persisted unchanged for about 30 s following the return to control 0.2 Hz stimulation, before declining to control levels over the next 2-3 min. Depressing units on average showed neither burst-patterned potentiation nor post-burst-patterned potentiation. 5. All inputs tested in Locke solution in which Ca2+ was reduced to 0.5 mM with addition of 1.2 mM-Mn2+ or 3.8 mM-MgCl2 exhibited a pronounced facilitation within each burst but no extension of potentiation into ensuing bursts. Both burst-patterned potentiation and the post-burst-patterned potentiation were abolished.(ABSTRACT TRUNCATED AT 400 WORDS)

The dependence of excitatory junction potential amplitude on the external calcium concentration in narcotic tolerant mouse vas deferens

The dependence of neurotransmitter secretion on external calcium ions during development of opiate tolerance in the mouse vas deferens was studied. The writhing response of mice to an i.p. injection of acetylcholine was inhibited by morphine. Reversal of this antinociceptive effect of morphine during chronic treatment signalled the development of tolerance. Tolerance to morphine at the neuromuscular junction was shown as a reversal of the initial shift of the size of the excitatory junction potential (e.j.p.) vs extracellular calcium concentration relationship back towards the control without any change in the power of 2.4. Facilitation in the amplitude of the e.j.p. occurs with low frequency (2 Hz) stimulation. The initial increase in facilitation induced by morphine was reversed by chronic morphine treatment without any change in the plateau e.j.p. amplitude achieved after a long low frequency train of impulses. At high frequencies (10 Hz) the initial increase in e.j.p. amplitude was followed by a depression. Acute morphine administration decreased the size of the e.j.p., this was followed by an increase in facilitation and a decrease in depression. These effects were reversed after chronic morphine treatment. Tolerance to morphine involves a counteradaptive process which restores the normal entry of calcium ions or its actions within the release sites in promoting transmitter release.

End-plate potentials in a model muscle fiber. Corrections for the effects of membrane potential on currents and on channel lifetimes

At the neuromuscular junction, the end-plate potential is generated by a conductance increase in the end-plate membrane. The end-plate depolarization brings the membrane potential toward the reversal potential, which diminishes the driving force for inward current flow. A. R. Martin (1955, J. Physiol. [Lond.]. 130:114-122) devised a simple formula to correct end-plate potential amplitudes for a diminished driving force based on a purely resistive model of the end-plate membrane. The model ignores the membrane capacity, the complexity of the equivalent circuit for a muscle fiber, the variation in channel lifetimes with changes in membrane potential, and the extension of the end plate along a length of the cable. We have developed a model that incorporates all of these features. The calculations show that Martin's correction is, in theory, quite satisfactory for a cable that has the characteristics of a muscle fiber unless the recording is made at a distance from the site of inward current flow. However, there is a discrepancy between models of the frog neuromuscular junction and the available experimental data, which suggests that the end-plate depolarization produced by a given current is greater than expected from their model.

Initial synaptic transmission at the growth cone in Xenopus nerve-muscle cultures

The excellent visibility of cultured cells allows the early events during formation of the neuromuscular junction to be suitably studied. It has been shown in various culture systems that synaptic transmission occurs early after nerve-muscle contact. Early synaptic potentials are small in amplitude and slow in time course reflecting a low acetylcholine receptor density at the site of nerve contact. Acetylcholine receptors accumulate later at the contact region. We have examined initial synaptic transmission at the growth cone-muscle contact in Xenopus nerve-muscle cultures. The approaching growth cone was observed under a phase-contrast microscope while the membrane potential of its target muscle cell was continuously monitored by using an intracellular microelectrode. The innervating neuron was stimulated extracellularly at the cell body. No synaptic potential was evoked when the growth cone was contacting the muscle only at the tip of filopodia. However, as soon as the main portion of the growth cone contacted the muscle membrane, nerve-evoked synaptic potentials were detected after stimulation of the nerve. This immediate appearance of synaptic potentials raises the possibility that acetylcholine could be released at the growth cone even prior to contact with muscle cells. As the area of contact enlarged during the observation period the amplitude of end-plate potentials also increased. Spontaneous synaptic potentials (miniature end-plate potentials) were rarely observed in these early growth cone-muscle contacts. Although there were several inherent difficulties, quantal analysis of the end-plate potentials was attempted by using binomial statistics. This analysis suggests that nerve-evoked transmitter release at the growth cone occurs in a quantal fashion.

Effects of divalent cations on responses of a sympathetic ganglion to 5-hydroxytryptamine and 1,1-dimethyl-4-phenyl piperazinium

1 The effects of raising or lowering [Ca(2+)](o) or [Mg(2+)](o) on potential changes evoked by 5-hydroxytryptamine (5-HT) and by the nicotinic agonist, 1,1-dimethyl-4-phenyl piperazinium (DMPP) have been investigated.2 Changes in membrane potential were measured at the ganglion or in postganglionic axons by the sucrose-gap technique. The ganglionic response to both 5-HT and DMPP was a depolarization followed by an after-hyperpolarization (AH). AH decayed exponentially over most of its time course; the time constant of decay for 5-HT responses was 4.4 +/- 0.3 min (mean +/- s.e.mean, rate constant 0.23 min(-1)) and that for DMPP responses was not significantly different, being 3.9 +/- 0.3 min (rate constant 0.26 min(-1)).3 Increasing [Ca(2+)](o) to 5.1 or 7.6 mM caused some hyperpolarization of the ganglion, reduced the amplitude of depolarizations evoked by 5-HT by 29% and usually potentiated responses to DMPP (average 12%). Ca-free solutions caused a depolarization of the ganglion, increased the amplitude of depolarizations evoked by 5-HT by 23% and reduced that of depolarizations to DMPP by 32%. [Mg(2+)](o) 12.7 and 25.4 mM caused depolarizations of the ganglion and reduced the amplitude of depolarizations evoked by 5-HT by 34 and 84%, respectively, and those to DMPP by 10 and 75%, respectively. Mg-free solutions or low [Mg(2+)](o) caused a slow depolarization of the ganglion and reduced the amplitude of depolarizations to both 5-HT and DMPP by approx. 20%. Ca/Mg-free solutions produced a slow depolarization of the ganglion, increased the amplitude of depolarizations evoked by 5-HT by 78% and reduced those to DMPP by 58%.4 Increasing [Ca(2+)](o) reduced the amplitude of AH evoked by 5-HT by 50% and increased that to DMPP by 73%, while prolonging AH duration and increasing the time constant of decay. Ca-free solutions had complex effects on AH evoked by 5-HT, which were increased on average by 116%, and depressed AH evoked by DMPP; in both cases there was a decrease in the time constant of decay. [Mg(2+)](o) 12.7 mM reduced the amplitude of AH evoked by 5-HT more than that evoked by DMPP, and increased the rate of decline of the exponential phase. Low Mg solutions reduced in amplitude the AH evoked by 5-HT by 56% and the AH evoked by DMPP by 38%. The time constant of decay was increased. Ca/Mg-free solutions reduced AH amplitude in both 5-HT and DMPP responses. The effects on time constant are consistent with the generation of AH by an electrogenic sodium pump, the ATP-ase of which is Mg(2+)-dependent and inhibited by Ca(2+).5 Responses to 5-HT could be recorded from postganglionic axons and consisted of a rapid depolarization, sometimes followed by an AH whose time constant of decay was smaller than that of ganglionic responses. Full dose-response curves in control and test media could be obtained. In Ca/Mg-free solutions, 5-HT depolarizations were potentiated but no significant shift in the curve was observed.6 It is suggested that divalent cations modulate the coupling between 5-HT receptor and ion channel, an increase in [Ca(2+)](o) reducing the coupling or stabilizing the ion channel in the closed conformation. Ca(2+) and Mg(2+) may compete for the same binding site. This mechanism does not appear to be involved at nicotinic receptors and their related ion channels.

Non-linear summation of end-plate potentials in the frog and mouse

1. End-plate potentials (e.p.p.s) and end-plate currents (e.p.c.s) were recorded intracellularly from muscle fibres of frog and mouse at various levels of curarization to determine the relation between the potential change and the underlying synaptic conductance change over a wide range of e.p.p. amplitudes. 2. In frog muscle fibres the e.p.p.-e.p.c. relation was linear for e.p.p. amplitudes up to about 10 mV, beyond which the rate of increase of e.p.p. amplitude became progressively smaller as the e.p.c. amplitude increased. The equation proposed by Martin (1955) to correct for this non-linearity consistently over-corrected the e.p.p. amplitudes. 3. When synaptic potentials and currents of long duration were produced by ionophoresis of ACh onto the end-plate, the voltage-current relation showed greater non-linearity than with nerve-evoked responses, and correction of the synaptic potential amplitudes resulted in a linear relation. 4. The relation between e.p.p. and e.p.c. amplitudes in mouse muscle showed a greater non-linearity than in frog muscle and over-correction by the equation was correspondingly smaller. Theoretical voltage-current relations were calculated for various membrane models and compared with the relations observed experimentally. The results from mouse muscle agreed with those expected for a point synaptic contact on an infinite cable; those from from muscle were consistent with simple resistive-capacitative model with no cable extending from the synaptic region. 6. The applicability to the experimental results of several correction factors for non-linear summation is discussed.

Segmental innervation of rotated and supernumerary axolotl hindlimbs

The segmental nerve supply to axolotl limbs was misrouted by severing the limbs at the level of the femur, rotating them 180 degrees around their long axis, and then suturing them to the intact proximal stump. Following return of the blood supply to the rotated limb by the cross-anastomosing of blood vessels, a blastema often formed to the side of the rotation site giving rise to a supernumerary limb. The muscles of both rotated and supernumerary limbs were innervated by the segmental nerves. The percentage of cells innervated by segmental nerves 16 and 17 in each muscle was determined with intracellular electrodes at 14 weeks after the operation. Despite histological evidence that nerves had been misrouted in the rotated limb, the percentage innervation of each muscle by nerves 16 and 17 was similar to that in the unoperated contralateral controls. The same results were obtained for the supernumerary limbs. In some muscles a few synaptic sites were found innervated by segmental nerves which did not innervate that muscle in the contralateral controls. These had synaptic potentials with very low quantal contents if immediately adjacent sites were innervated by the segmental nerve, which did innervate that muscle in the contralateral controls. The results suggest that the selective properties of synaptic sites are alone sufficient to determine the entire segmental innervation pattern of the muscles in a limb.

An electrophysiological analysis of the effects of morphine on the calcium dependence of neuromuscular transmission in the mouse vas deferens

1 The effects of morphine on the Ca-dependence of the synaptic potential amplitude in the mouse vas deferens have been determined. 2 The synaptic potential increased with a power factor of 2.4 for [Ca]o between 0.7 mM and 1.8 mM. Morphine (40 nM) decreased the synaptic potential, without altering the second power relationship between the synaptic potential and [Ca]o. 3 Morphine reversed the depression in the synaptic potential which develops during a short high-frequency (10 Hz) train of impulses to facilitation. Consequently the synaptic potential beyond the tenth impulse was unaffected by morphine. 4 Morphine did not alter the facilitation of the synaptic potential which develops during a short low-frequency (less than or equal to 2 Hz) train of impulses in normal [Ca]o. Consequently morphine decreased the synaptic potential for each impulse by about the same percentage amount. 5 Morphine increased the small facilitation in the synaptic potential which occurs during a short low-frequency (less than or equal to 2 Hz) train of impulses in high [Ca]o. This facilitation approximated the predictions based on the assumption that each impulse leaves residual Ca ions bound to receptors involved in transmitter release from the nerve terminal.

The effect of calcium ions on the secretion of quanta evoked by an impulse at nerve terminal release sites

A study has been made of the effects of calcium ions on the number of quanta secreted from all the release sites at an amphibian motor nerve terminal recorded with an intracellular microelectrode (m) compared with the number secreted simultaneously from a small number of release sites recorded with an extracellular microelectrode (me). If the endplate potential was made subthreshold by lowering the external calcium concentration ([Ca]o less than or equal to 0.4 mM), it was possible to find small groups of release sites for which me was comparable to m, indicating considerable nonuniformity in the probability of release of a quantum at different groups of release sites (Pe) in a given [Ca]o. Increasing [Ca]o in the range from 0.25 to 0.4 mM increased the probability of release of a quantum at groups of release sites (Pe), independent of the initial value of Pe, and the dependence of Pe on [Ca]o followed a fourth power relationship. A conditioning impulse enhanced the probability of release of a quantum by a subsequent test impulse at release sites, if Pe was less than 1.0 during the conditioning impulse. It is shown that the present observations regarding the dependence of Pe on [Ca]o and on conditioning impulses can be quantitatively predicted from previous observations regarding the dependence of the binomial parameters m, p, and n on [Ca]o and on conditioning impulses determined with intracellular electrodes, if the probability of secretion of a quantum at a release site (Pj) is different for different release sites and Pj is distributed as a beta random variable.

The regression of synapses formed by a foreign nerve in a mature axolotl striated muscle

A study has been made of the factors which determine that the terminals of a foreign flexor nerve at synaptic sites in a unrodele extensor muscle regress on return of the original extensor nerve. The quantal content (m) of the endplate potential (EPP) at flexor nerve terminal synapses, during innervation of a previously denervated extensor muscle, increased in about 8 weeks to reach the same size as at normal extensor nerve terminals; the same time was taken for m to reach normal size at extensor nerve terminals when these reinnervated their own muscle. At flexor nerve terminals, m decreased eventually to zero if the extensor nerve terminals returned within about 6 weeks of synapse formation by the flexor nerve terminals to the same or an immediately adjacent synaptic site to that occupied by these terminals. During this decrease in m at flexor nerve terminals, stimulation of the flexor nerves in the presence of horseradish peroxidase (HRP) showed HRP-labelled flexor nerve terminals present only in those regions of the extensor muscle in which the electrical signs of flexor nerve terminals were observed, indicating that the decrease in m at regressing flexor terminals was accompanied by their vacating synaptic sites. However, flexor nerve terminals failed to regress from the extensor muscle on return of the original nerve supply if the flexor nerve allowed to form synapses for more than about 10 weeks before the return of the extensor nerve.

Depression, facilitation, and mobilization of transmitter at the rat diaphragm neuromuscular junction

The characteristics of depression, facilitation, and mobilization of transmitter were examined at the rat diaphragm neuromuscular junction. Intracellular recording techniques were used to monitor end-plate potentials (EPPs), miniature end-plate potentials (MEPPs) and the muscle resting potentials. The cut-muscle technique was used to prevent muscle action potentials. Quantal release was determined by the direct method. The binomial statistical parameters, releasable store (n) and probability of release (p), were examined under various stimulating conditions to determine the basis for depression and facilitation. The present experiments demonstrate that p remains unchanged during repetitive nerve stimulation at low or moderately high frequencies. The experiments demonstrate that depression is due to a decrease in n and facilitation is due to an elevation in n. It is suggested that the increase in n during facilitation is due to a transient recruitment of inactive releasing sites. Substantial replenishment of n by mobilization occurs within a few ms after a stimulus but a slow residual rate of mobilization is needed to replenish n to resting levels.

Spontaneous multiquantal release at synapses in guinea-pig hypogastric ganglia: evidence that release can occur in bursts

1. A study was made of some properties of the spontaneous synaptic potentials recorded in cells of the hypogastric ganglia of guinea-pigs. 2. The distribution of the amplitudes of the spontaneous synaptic potentials arising from a single preganglionic fibre was found to be multimodal, with peaks at roughly integral multiples of a unit peak. 3. It was found that the amplitudes of the larger spontaneous potentials were consistent with them being the result of synchronous or near-synchronous release of two or more unit-sized quanta (multiquantal release). 4. The proportion of multiquantal potentials observed was found to be dependent on the extracellular calcium ion concentration. 5. When the stochastic properties of the spontaneous potentials were examined, it was found that the spontaneous release process was not random and independent but appeared to be clustered. Indeed, the probability of occurrence of a unit spontaneous synaptic potential was greatly enhanced during the 40--60 msec immediately following any given spontaneous synaptic potential. 6. When unit spontaneous potentials were excluded from the analysis, the multiquantal potentials were still found to be clustered although the clustering was less marked than that seen in the over-all process. 7. These results suggested that the multiquantal spontaneous potentials arose from the release of unit quanta in short high frequency bursts and it was found that a mathematical model incorporating such a mechanism could describe the spontaneous quantal release process.

Adaptive rundown of excitatory post-synaptic potentials at synapses between hair cells and eight nerve fibres in the goldfish

1. The excitatory post-synaptic potentials (e.p.s.p.s.) evoked by sound stimuli were recorded intracellularly from large afferent eight nerve fibres in the sacculus of the goldfish (S1 fibres). The fish were anaesthetized with MS-222 and spike potentials were suppressed with locally applied tetrodotoxin. 2. The e.p.s.p.s. successively evoked in response to each wound wave showed a marked rundown in size, while no reduction was observed in the microphonic potentials. The amplitude of successive e.p.s.p.s was reduced keeping approximately a fixed ratio to the preceding ones, suggesting that the rundown is attributable to a depletion of transmitter quanta from the release sites. 3. The rate of rundown of successive e.p.s.p.s, however, remained almost unchanged when the intensity of the stimulus sound was changed. It was also observed that, even after the e.p.s.p.s had been completely adapted to a continuous sound, a vigorous discharge of new e.p.s.p.s was observed when the intensity of the sound was increased. 4. These findings seem to indicate that it is the size of the readily available store and not the release fraction that is changed by a change in the sound intensity. 5. The saccular macula was superfused with solutions different in Ca and Mg ion concentrations. High Ca ion concentration brought about an increase in the size of the readily available store as well as the release fraction. 6. Mechanisms underlying these observations were discussed in terms of the quantal release mechanism as well as the morphology of the release sites.

Quantal analysis of the size of excitatory post-synaptic potentials at synapses between hair cells and afferent nerve fibres in goldfish

1. A statistical analysis has been made of the transmitter release at the hair cell afferent fibre synapse in the sacculus of the goldfish, using the amplitude of the excitatory post-synaptic potentials (e.p.s.p.s) in response to stimulus tone as a measure of the transmitter release under application of tetrodotoxin. 2. Application of binomial statistics allowed a direct calculation of the mean probability of release (p) and the readily available store (n), and the X2-test showed that the binomial predictions fitted fairly well with the observed distribution of the responses. 3. Adaptive rundown of e.p.s.p.s during sound stimulation, i.e. the successive rundown in the size of the mean quantal content (m), was found to be associated with a reduction in the size of parameter n, but not of p. 4. A marked negative correlation was demonstrated between the amplitude of two consecutive e.p.s.p.s, supporting the depletion hypothesis of the adaptive rundown of e.p.s.p.s. 5. The increase in the e.p.s.p. amplitude and the increase in the mean quantal content, m, brought about by an increase in the tone intensity were found mostly explicable in terms of an increase in the statistical parameter n. The probability parameter p was found largely in invariable, although in certain instances the increase in m was also accompanied by a slight increase in the parameter p.

The effects of calcium ions on the binomial parameters that control acetylcholine release during trains of nerve impulses at amphibian neuromuscular synapses

1. A study has been made of the effects of changing the external calcium concentration [Ca](o) on the binomial parameters p and n that control the average quantal content (m) of the end-plate potential (e.p.p.) during trains of nerve impulses at synapses in amphibian striated muscle.2. In high external calcium concentrations (0.4 mM </= [Ca](o) < 1.0 mM) the increase in m of a test impulse following a conditioning impulse at different intervals (< 100 msec) was due to an increase in the number of quanta available for release, n; the increase in m of successive e.p.p.s in a short high frequency train was primarily due to an increase in n.3. In high external calcium concentrations (1.0 mM </= [Ca](o) < 10 mM) there was a decrease in m of a test impulse following a short high frequency conditioning train (4-5 impulses, 20-100 Hz) at different intervals (200 msec < 5 sec) and this was due to a decrease in the number of quanta available for release, n; in a long high frequency train (20 impulses, 20-100 Hz) there was an increase in m for the first few successive e.p.p.s followed by a depression of m which eventually reached a steady state and these changes in m were due to changes in n; the higher the frequency the greater was the depression in n during the steady-state period.4. In high calcium concentrations, the steady-state m reached in the first 20 impulses during continual stimulation at high frequency gave way to a decline in m over several minutes until a new depressed steady-state value of m was reached and this was maintained during the longest periods of stimulation (30 min); this decline in m was primarily due to a decline in the number of quanta available for release.5. These changes in the number of quanta available for release during trains of impulses are predicted in terms of a hypothesis in which facilitation is due to the accumulation of a residual calcium-receptor complex in the nerve terminal that determines the fraction of a pool of quanta which contributes to n, and depression is due to a decrease in the number of quanta in this pool.

The effect of calcium ions and temperature on the binomial parameters that control acetylcholine release by a nerve impulse at amphibian neuromuscular synapses

1. A study has been made of the effects of changing the external calcium concentration, [Ca](o), and the temperature on both the number of quanta available for release by the nerve impulse (n) as well as the increase in release probability of a quantum p(t) during the release period (from 0 to T) following a nerve impulse at synapses in amphibian striated muscle.2. When [Ca](o) was increased in the low range from 0.25 to 0.4 mM at 18 degrees C, the average quantal content of the e.p.p. (m) increased as the fourth power of [Ca](o) and this was primarily due to a third power dependence of n on [Ca](o); the dissociation constants and power dependence of n on calcium determined in the [Ca](o) range from 0.25 to 1.0 mM were successfully used to predict the changes in size of the e.p.p. in the very high [Ca](o) range from 1 to 10 mM. When the temperature was increased from 7 to 18 degrees C in a [Ca](o) of 0.6 mM or 0.35 mM, n increased with a Q(10) of 2.5.3. When [Ca](o) was increased in the range from 0.25 to 1.0 mM at 18 degrees C, the probability that a quantum initially available for release is released during the release period (p(T)) was very sensitive to [Ca](o), increasing as the third power of [Ca](o) and with a dissociation constant of 0.13 mM. When the temperature was increased from 7 to 18 degrees C in a [Ca](o) of 0.6 mM or 0.35 mM, p(T) decreased.4. The histograms of latencies of individual quanta following a nerve impulse was very temperature dependent: the time to peak of the histograms (i.e. the interval in which most quanta fell) had a Q(10) of over 4 as did the time constant of decline of the histograms in the temperature range from 7 to 18 degrees C.5. The average number of quanta released up to time t during the release period following a nerve impulse, namely np(t), was well described by a stochastic process in which p(t) was determined by two reactions; one of these reactions released available quanta from the nerve terminal whilst the other made some of the available quanta unavailable for release by the nerve impulse.