The nicotinic activation of the denervated sympathetic neuron of the rat

Nicotinic responses to endogenous acetylcholine and to exogenously applied agonists have been studied in the intact or denervated rat sympathetic neuron in vitro, by using the two-microelectrode voltage-clamp technique. Preganglionic denervation resulted in progressive decrease of the synaptic current (excitatory postsynaptic current, EPSC) amplitude, which disappeared within 24 h. These effects were accompanied by changes in ion selectivity of the nicotinic channel (nAChR). The extrapolated EPSC null potential (equilibrium potential for acetylcholine action, E(Syn)) shifted from a mean value of -15.9+/-0.7 mV, in control, to -7.4+/-1.6 mV, in denervated neurons, indicating a decrease of the permeability ratio for the main components of the synaptic current (P(K)/P(Na)) from 1.56 to 1.07. The overall properties of AChRs were investigated by applying dimethylphenylpiperazinium or cytisine and by examining the effects of endogenous ACh, diffusing within the ganglion after preganglionic tetanization in the presence of neostigmine. The null potentials of these macrocurrents (equilibrium potential for dimethylphenylpiperazinium action, E(DMPP); and equilibrium potential for diffusing acetylcholine, E(ACh), respectively) were evaluated by applying voltage ramps and from current-voltage plots. In normal neurons, E(Syn) (-15.9+/-0.7 mV) was significantly different from E(DMPP) (-26.1+/-1.0) and E(ACh) (-31.1+/-3.3); following denervation, nerve-evoked currents displayed marked shifts in their null potentials (E(Syn)=-7.4+/-1.6 mV), whereas the amplitude and null potential of the agonist-evoked macrocurrents were unaffected by denervation and its duration (E(DMPP)=-26.6+/-1.2 mV). It is suggested that two populations of nicotinic receptors, synaptic and extrasynaptic, are present on the neuron surface, and that only the synaptic type displays sensitivity to denervation.

Biophysical properties of the silent and activated rat sympathetic neuron following denervation

A biophysical description of the denervated rat sympathetic neuron is reported, obtained by the two-electrode voltage-clamp technique in mature intact superior cervical ganglia in vitro. At membrane potential values negative to -50 mV, the normal, quiescent neuron displays voltage-dependent K and Cl conductances; following direct or synaptic stimulation (15Hz for 10 s), the neuron moves to a new resting state characterized by increased amplitude and voltage dependence of Cl conductance. Denervation produces two main effects: 1) resting Cl conductance gradually increases while its voltage-dependence decreases; by 30 days a high-conductance resting state prevails, almost independent of membrane potential in the -50/-110 mV range; 2) the increase in amplitude and voltage-dependence of Cl conductance, produced by direct stimulation in control neurons, is less marked in denervated neurons, and is observed over an increasingly small range of membrane potentials. Thirty days after denervation, the prevailing high-conductance resting state appears virtually insensitive to changes in membrane potential and stimulation. Voltage-dependent potassium currents involved in spike electrogenesis (the delayed compound potassium current and the fast transient potassium current) exhibit an early drastic decrease in peak amplitude in the denervated neuron; the effect is largely reversed after 6 days. Remarkable changes in fast transient potassium current kinetics occur following denervation: the steady-state inactivation curve shifts by up to +15 mV toward positive potential and voltage sensitivity of inactivation removal becomes more steep. A comprehensive mathematical model of the denervated neuron is presented that fits the neuron behavior under current-clamp conditions. It confirms that neuronal excitability is tuned by the conductances (mostly chloride conductance) that control the resting membrane potential level, and by fast transient potassium current. Impairment of the latter reduces both inward threshold charge for firing and spike repolarization rate, and fast transient potassium current failure cancels the voltage dependence of both processes.

A novel pattern of fast calcium oscillations points to calcium and electrical activity cross-talk in rat chromaffin cells

Slow oscillations of cytosolic calcium ion concentration - [Ca(2+)](c) - typically originate from release by intracellular stores, but in some cell types can be triggered and sustained by Ca(2+) influx as well. In this study we simultaneously monitored changes in [Ca(2+)](c) and in the electrical activity of the cell membrane by combining indo-1 and patch-clamp measurements in single rat chromaffin cells. By this approach we observed a novel type of spontaneous [Ca(2+)](c) oscillations, much faster than those previously described in these cells. These oscillations are triggered and sustained by complex electrical activity (slow action potentials and spike bursts), require Ca(2+) influx and do not involve release from intracellular stores. The possible physiological implications of this new pathway of intracellular signalling are discussed.

Neurotransmitter release: fusion or 'kiss-and-run'?

The clear synaptic vesicles of neurons release their contents at the presynaptic membrane and are then quickly retrieved. However, it is unclear whether a complete cycle of exocytosis and endocytosis is always involved or whether neurotransmitter can be released by a transient interaction. Recent findings in chromaffin and mast cells suggest that exocytosis is preceded by the formation of a pore that has similar conductance properties to ion channels. The content of the secretory organelle partially escapes at this early step, but the pore can close before the vesicle fuses fully. This article looks at the evidence that quantal release of neurotransmitter from clear synaptic vesicles may occur by a similar 'kiss-and-run' mechanism.

Cl- affects the function of the GABA cotransporter rGAT1 but preserves the mutal relationship between transient and transport currents

The effects of reducing external Cl- on the electrophysiological properties of the Na+/Cl(-)-dependent GABA transporter rGAT1 expressed in Xenopus oocytes were investigated. In agreement with a recently proposed kinetic scheme, the effects of Cl- are complex but preserve the mutual relationship that links the transport-associated current, I(tr), measured in saturating GABA concentration, and the transient current, I(pre), recorded in the absence of GABA following a voltage step from the holding potential Vh to V. In particular, I(tr) (V) - I(tr) (Vh) = r integral I(pre) (V) dt, where r is the relaxation rate of I(pre) at the same membrane potential and Cl- concentration. The model also predicts a relationship between charge relaxation rate and apparent affinity for GABA, which is also verified in the presence of lowered Na+ or Cl- concentrations. In these conditions, the binding rate of GABA to the transporter is increased. All these effects are consistent with the hypothesis that interaction of the organic substrate with rGAT1 induces a conversion from a capacitive to a conductive mode of operation without strongly altering either the amount or the rate of charge movement.

Synaptic vesicles: is kissing a matter of competence?

The "kiss-and-run" model of exocytosis and endocytosis predicts that synaptic vesicles can undergo fast and efficient recycling, after fusion with the plasmalemma, without intermixing of membranes. Evidence is mounting from several new experimental approaches that kiss-and-run occurs at synapses. Distinct vesicle pools, which initially were identified in morphological terms, are now being characterized in biochemical and functional terms. In addition, at least two functional recycling pathways, operating on different time scales (from milliseconds to tens of seconds), have been shown to coexist in the same synaptic system, and the two pathways appear to be differentially regulated. Taken together, these data suggest that kiss-and-run operates in parallel with the classical, coated-vesicle recycling. Here, we review recent evidence for kiss-and-run recycling and discuss whether it is a distinct process, dependent on the molecular organization of the fusing vesicle. We propose that vesicles undergo a process of "competence maturation". According to this view, the specific molecular make-up of the vesicles, their location and their interactions with nerve terminal proteins might determine not only the differential availability of the vesicles for fusion and neurotransmitter release but also the recycling path that they will follow.

Ca2+-dependent kinetics of hair cell Ca2+ currents resolved with the use of cesium BAPTA

Hair cells in the frog semicircular canal, studied by the whole-cell patch-clamp technique, display three distinct Ca2+ currents: two non-inactivating components (L type and R type, the latter termed R2 in the following) and a second R type current (termed R1), which runs down first and inactivates in a Ca2+-dependent fashion. Since intracellular EGTA, up to 5 mM, did not display major effects on such inactivation, we used increasing amounts of BAPTA in the patch pipette, to control [Ca2+]i more efficiently and investigate whether modifications in [Ca2+]i at the cytoplasmic side of the channel affect the inactivation of the RI component and in general the gating of all channel types. The results here reported show that (1) K+ currents heavily contaminate recordings obtained using high concentrations of BAPTA in its commercially available K+ salt form; (2) BAPTA Cs+ salt can be satisfactorily employed to obtain reliable recordings; (3) the kinetics of channel gating and R1-channel inactivation are indeed markedly affected by effectively buffering [Ca2+]i.

Mechanisms of coordination of Ca2+ signals in pancreatic islet cells

Within pancreatic islet cells, rhythmic changes in the cytosolic Ca2+ concentration have been reported to occur in response to stimulatory glucose concentrations and to be synchronous with pulsatile release of insulin. We explored the possible mechanisms responsible for Ca2+ signal propagation within islet cells, with particular regard to gap junction communication, the pathway widely credited with being responsible for coordination of the secretory activity. Using fura-2 imaging, we found that multiple mechanisms control Ca2+ signaling in pancreatic islet cells. Gap junction blockade by 18 alpha-glycyrrhetinic acid greatly restricted the propagation of Ca2+ waves induced by mechanical stimulation of cells but affected neither Ca2+ signals nor insulin secretion elicited by glucose elevation. The source of Ca2+ elevation was also different under the two experimental conditions, the first being sustained by release from inner stores and the second by nifedipine-sensitive Ca2+ influx. Furthermore, glucose-induced Ca2+ waves were able to propagate across cell-free clefts, indicating that diffusible factors can control Ca2+ signal coordination. Our results provide evidence that multiple mechanisms of Ca2+ signaling operate in beta-cells and that gap junctions are not required for intercellular Ca2+ wave propagation or insulin secretion in response to glucose.

Participation of a chloride conductance in the subthreshold behavior of the rat sympathetic neuron

The presence of a novel voltage-dependent chloride current, active in the subthreshold range of membrane potential, was detected in the mature and intact rat sympathetic neuron in vitro by using the two-microelectrode voltage-clamp technique. Hyperpolarizing voltage steps applied to a neuron held at -40/-50 mV elicited inward currents, whose initial magnitude displayed a linear instantaneous current-voltage (I-V) relationship; afterward, the currents decayed exponentially with a single voltage-dependent time constant (63.5 s at -40 mV; 10.8 s at -130 mV). The cell input conductance decreased during the command step with the same time course as the current. On returning to the holding potential, the ensuing outward currents were accompanied by a slow increase in input conductance toward the initial values; the inward charge movement during the transient ON response (a mean of 76 nC in 8 neurons stepped from -50 to -90 mV) was completely balanced by outward charge displacement during the OFF response. The chloride movements accompanying voltage modifications were studied by estimating the chloride equilibrium potential (E(Cl)) at different holding potentials from the reversal of GABA evoked currents. [Cl(-)](i) was strongly affected by membrane potential, and at steady state it was systematically higher than expected from passive ion distribution. The transient current was blocked by substitution of isethionate for chloride and by Cl(-) channel blockers (9AC and DIDS). It proved insensitive to K(+) channel blockers, external Cd(2+), intracellular Ca(2+) chelators [bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA)] and reduction of [Na(+)](e). It is concluded that membrane potential shifts elicit a chloride current that reflects readjustment of [Cl(-)](i). The cell input conductance was measured over the -40/-120-mV voltage range, in control medium, and under conditions in which either the chloride or the potassium current was blocked. A mix of chloride, potassium, and leakage conductances was detected at all potentials. The leakage component was voltage independent and constant at approximately 14 nS. Conversely, gCl decreased with hyperpolarization (80 nS at -40 mV, undetectable below -110 mV), whereas gK displayed a maximum at -80 mV (55.3 nS). Thus the ratio gCl/gK continuously varied with membrane polarization (2.72 at -50 mV; 0.33 at -110 mV). These data were forced in a model of the three current components here described, which accurately simulates the behavior observed in the "resting" neuron during membrane migrations in the subthreshold potential range, thereby confirming that active K and Cl conductances contribute to the genesis of membrane potential and possibly to the control of neuronal excitability.

The kinetics of nerve-evoked quantal secretion

Current views on quantal release of neurotransmitters hold that after the vesicle migrates towards release sites (active zones), multiple protein interactions mediate the docking of the vesicle to the presynaptic membrane and the formation of a multimolecular protein complex (the 'fusion machine') which ultimately makes the vesicle competent to release a quantum in response to the action potential. Classical biophysical studies of quantal release have modelled the process by a binomial system where n vesicles (sites) competent for exocytosis release a quantum, with probability p, in response to the action potential. This is likely to be an oversimplified model. Furthermore, statistical and kinetic studies have given results which are difficult to reconcile within this framework. Here, data are presented and discussed which suggest a revision of the biophysical model. Transient silencing of release is shown to occur following the pulse of synchronous transmitter release, which is evoked by the presynaptic action potential. This points to a schema where the vesicle fusion complex assembly is a reversible, stochastic process. Asynchronous exocytosis may occur at several intermediate stages in the process, along paths which may be differentially regulated by divalent cations or other factors. The fusion complex becomes competent for synchronous release (armed vesicles) only at appropriately organized sites. The action potential then triggers (deterministically rather than stochastically) the synchronous discharge of all armed vesicles. The existence of a specific conformation for the fusion complex to be competent for synchronous evoked fusion reconciles statistical and kinetic results during repetitive stimulation and helps explain the specific effects of toxins and genetic manipulation on the synchronization of release in response to an action potential.

A computer-driven approach to PCR-based differential screening, alternative to differential display

MOTIVATION

Polymerase chain reaction (PCR)-based RNA fingerprinting is a powerful tool for the isolation of differentially expressed genes in studies of neoplasia, differentiation or development. Arbitrarily primed RNA fingerprinting is capable of targeting coding regions of genes, as opposed to differential display techniques, which target 3' non-coding cDNA. In order to be of general use and to permit a systematic survey of differential gene expression, RNA fingerprinting has to be standardized and a number of highly efficient and selective arbitrary primers must be identified.

RESULTS

We have applied a rational approach to generate a representative panel of high-efficiency oligonucleotides for RNA fingerprinting studies, which display marked affinity for coding portions of known genes and, as shown by preliminary results, of novel ones. The choice of oligonucleotides was driven by computer simulations of RNA fingerprinting reverse transcriptase (RT)-PCR experiments, performed on two custom-generated, non-redundant nucleotide databases, each containing the complete collection of deposited human or murine cDNAs. The simulation approach and experimental protocol proposed here permit the efficient isolation of coding cDNA fragments from differentially expressed genes.

AVAILABILITY

Freely available on request from the authors.

CONTACT

fesce.riccardo@hsr.it

Mab21, the mouse homolog of a C. elegans cell-fate specification gene, participates in cerebellar, midbrain and eye development

A multitude of regulatory genes are involved in phylogenetically conserved developmental cascades required for the patterning, cell-type specification, and differentiation of specific central nervous system (CNS) structures. Here, we describe the distribution of a mouse transcript encoding a homolog of the C. elegans mab-21 gene. In the nematode tail, mab-21 is required for the short-range patterning and cell-fate determination events mediated by egl-5 and mab-18, two homeobox genes homologous to Abd-B and Pax6, respectively. In mouse midgestation embryogenesis, Mab21 is expressed at its highest levels in the rhombencephalon, cerebellum, midbrain, and prospective neural retina. Our data and the genetic interactions previously documented in the nematode suggest that Mab21 may represent a novel, important regulator of mammalian cerebellum and eye development.

The effects of perilymphatic tonicity on endolymph composition and synaptic activity at the frog semicircular canal

The effects of changes in perilymphatic tonicity on the semicircular canal were investigated by combining the measurements of transepithelial potential and endolymphatic ionic composition in the isolated frog posterior canal with the electrophysiological assessment of synaptic activity and sensory spike firing at the posterior canal in the isolated intact labyrinth. In the isolated posterior canal, the endolymph was replaced by an endolymph-like solution of known composition, in the presence of basolateral perilymph-like solutions of normal (230 mosmol/kg), reduced (105 mosmol/kg, low NaCl) or increased osmolality (550 mosmol/kg, Na-Gluconate added). Altered perilymphatic tonicity did not produce significant changes in endolymphatic ionic concentrations during up to 5 min. In the presence of hypotonic perilymph, decreased osmolality, K and Cl concentrations were observed at 10 min. In the presence of hypertonic perilymph, the endolymphatic osmolality began to increase at 5 min and by 10 min Na concentration had also significantly increased. On decreasing the tonicity of the external solution an immediate decline was observed in transepithelial potential, whereas hypertonicity produced the opposite effect. In the intact frog labyrinth, mEPSPs and spike potentials were recorded from single fibers of the posterior nerve in normal Ringer's (240 mosmol/kg) as well as in solutions with modified tonicity. Hypotonic solutions consistently decreased and hypertonic solutions consistently increased mEPSP and spike frequencies, independent of the species whose concentration was altered. These effects ensued within 1-2 min after the start of perfusion with the test solutions. In particular, when the tonicity was changed by varying Na concentration the mean mEPSP rate was directly related to osmolality. Size histograms of synaptic potentials were well described by single log-normal distribution functions under all experimental conditions. Hypotonic solutions (105 mosmol/kg) markedly shifted the histograms to the left. Hypertonic solutions (380-550 mosmol/kg, NaCl or Na-Gluconate added) shifted the histograms to the right. Hypertonic solutions obtained by adding sucrose to normal Ringer's solution (final osmolality 550 mosmol/kg) increased mEPSP and spike rates, but did not display appreciable effects on mEPSP size. All effects on spike discharge and on mEPSP rate and size were rapidly reversible. In Ca-free, 10 mM EGTA, Ringer's solution, the sensory discharge was completely abolished and did not recover on making the solution hypertonic. These results indicate that perilymphatic solutions with altered tonicity produce small and slowly ensuing changes in the transepithelial parameters which may indirectly affect the sensory discharge rate, whereas relevant, early and reversible effects occur at the cytoneural junction. In particular, the modulation of mEPSP amplitude appears to be postsynaptic; the presynaptic effect on mEPSP rate of occurrence is presumably linked to local calcium levels, in agreement with previous results indicating that calcium inflow is required to sustain basal transmitter release in this preparation.

A model of signal processing at a mammalian sympathetic neurone

A computational model has been developed for the action potential and, more generally, the electrical behaviour of the rat sympathetic neurone. The neurone is simulated as a complex system in which five voltage-dependent conductances (gNa, gCa, gKV, gA, gKCa), one Ca2+-dependent voltage-independent conductance (gAHP) and the activating synaptic conductance coexist. The individual currents are mathematically described, based on a systematic analysis obtained for the first time in a mature and intact mammalian neurone using two-electrode voltage-clamp experiments. The simulation initiates by setting the starting values of each variable and by evaluating the holding current required to maintain the imposed membrane potential level. It is then possible to simulate current injection to reproduce either the experimental direct stimulation of the neurone or the physiological activation by the synaptic current flow. The subthreshold behaviour and the spiking activity, even during long-lasting current application, can be analysed. At every time step, the program calculates the amplitude of the individual currents and the ensuing changes; it also takes into account the accompanying K+ accumulation process in the perineuronal space and changes in Ca2+ load. It is shown that the computed time course of membrane potential must be filtered, in order to reproduce the limited bandwidth of the recording instruments, if it is to be compared with experimental measurements under current-clamp conditions. The membrane potential trajectory and single current data are written in files readable by graphic software. Finally, a screen image is obtained which displays in separate graphs the membrane potential time course, the synaptic current and the six ionic current flows. The simulated action potentials are comparable to the experimental ones as concerns overshoot amplitude and rising and falling rates. Therefore, this program is potentially helpful in investigating many aspects of neurone behaviour.

Synaptic current at the rat ganglionic synapse and its interactions with the neuronal voltage-dependent currents

The membrane current activated by fast nicotinic excitation of intact and mature rat sympathetic neurons was studied at 37 degrees C, by using the two-microelectrode voltage-clamp technique. The excitatory postsynaptic current (EPSC) was modeled as the difference between two exponentials. A fast time constant (tau2; mean value 0.57 ms), which proves to be virtually voltage-independent, governs the current rise phase and a longer time constant (tau1; range 5.2-6.8 ms in 2 mM Ca2+) describes the current decay and shows a small negative voltage dependence. A mean peak synaptic conductance of 0.58 muS per neuron is measured after activation of the whole presynaptic input in 5 mM Ca2+ external solution (0.40 muS in 2 mM Ca2+). The miniature EPSCs also rise and decay with exponential time constants very similar to those of the compound EPSC recorded at the same voltage. A mean peak conductance of 4.04 nS is estimated for the unitary event. Deconvolution procedures were employed to decompose evoked macrocurrents. It is shown that under appropriate conditions the duration of the driving function describing quantal secretion can be reduced to <1 ms. The shape of the EPSC is accurately mimicked by a complete mathematical model of the sympathetic neuron incorporating the kinetic properties of five different voltage-dependent current types, which were characterized in a previous work. We show that IA channels are opened by depolarizing voltage steps or by synaptic potentials in the subthreshold voltage range, provided that the starting holding voltage is sufficiently negative to remove IA steady-state inactivation (less than -50 mV) and the voltage trajectories are sufficiently large to enter the IA activation range (greater than -65 mV). Under current-clamp conditions, this gives rise to an additional fast component in the early phase of membrane repolarization-in response to voltage pulses-and to a consistent distortion of the excitatory postsynaptic potential (EPSP) time course around its peak-in response to the synaptic signal. When the stimulation initiates an action potential, IA is shown to significantly increase the synaptic threshold conductance (up to a factor of 2 when IA is fully deinactivated), compared with that required when IA is omitted. The voltage dependence of this effect is consistent with the IA steady-state inactivation curve. It is concluded that IA, in addition to speeding up the spike repolarization process, also shunts the excitatory drive and delays or prevents the firing of the neuron action potential.

Phosphorylation-dependent effects of synapsin IIa on actin polymerization and network formation

The synapsins are a family of synaptic vesicle phosphoproteins which play a key role in the regulation of neurotransmitter release and synapse formation. In the case of synapsin I, these biological properties have been attributed to its ability to interact with both synaptic vesicles and the actin-based cytoskeleton. Although synapsin II shares some of the biological properties of synapsin I, much less is known of its molecular properties. We have investigated the interactions of recombinant rat synapsin Ila with monomeric and filamentous actin and the sensitivity of those interactions to phosphorylation, and found that: i) dephosphorylated synapsin II stimulates actin polymerization by binding to actin monomers and forming actively elongating nuclei and by facilitating the spontaneous nucleation/elongation processes; ii) dephosphorylated synapsin II induces the formation of thick and ordered bundles of actin filaments with greater potency than synapsin I; iii) phosphorylation by protein kinase A markedly inhibits the ability of synapsin II to interact with both actin monomers and filaments. The results indicate that the interactions of synapsin II with actin are similar but not identical to those of synapsin I and suggest that synapsin II may play a major structural role in mature and developing nerve terminals, which is only partially overlapping with the role played by synapsin I.

Evidence for receptor subtype cross-talk in the mitogenic action of serotonin on human small-cell lung carcinoma cells

We previously reported a significant mitogenic effect of serotonin (5-hydroxytryptamine, 5-HT) on human small-cell lung carcinoma cells (SCLC, GLC-8), mediated by both 5-HT1D and 5-HT1A receptors. Here we investigate possible interactions between the two receptor subtypes. Dose-effect curves obtained by simultaneously applying equipotent concentrations of the selective 5-HT1A agonist 8-OH-DPAT and the selective 5-HT1D receptor agonist sumatriptan are shifted to the right, although maximal effects are additive. The nonselective 5-HT antagonist metergoline displays higher potency when both receptor subtypes are activated. The 5-HT1D receptor antagonist GR127935 is markedly more potent against sumatriptan than against the sensitive portion of 5-HT effect. Indeed, both GR127935 and the 5-HT1A antagonist spiperone shift the EC50 for the residual effect of 5-HT from approximately 300 to 120-150 nM, suggesting that blocking one receptor subtype may facilitate activation of the other. Preincubation with either 8-OH-DPAT or sumatriptan suppresses the mitogenic response to the other specific receptor agonist; suppression is complete within 10 min at 37 degrees C, and is not observed when the preincubation is done at 4 degrees C. Measurements of adenylate cyclase activity do not help in interpreting the results. Conversely, measurements of MAP kinase activity reveals biphasic activation with a delayed activation at 1 h, and reproduce the suppression of the effect of the second drug by 15 min preincubation. These findings constitute the first evidence of a reciprocal negative interference between human 5-HT1A and 5-HT1D receptors, and indicate that SCLC GLC-8 cells simultaneously express both receptor subtypes. Mere reciprocal antagonism of the drugs employed cannot account for these data. We suggest that in this cell system cross-talk occurs in the transduction pathways of the two receptor subtypes.

The effect of clofilium, a K-channel blocker, on the electrogenic K secretion and the sensory discharge at the frog semicircular canal

Potassium transport by dark cells produces marked K-concentration differences between endo- and perilymphatic fluids in labyrinthine organs and generates the transepithelial potential. The ensuing electrochemical potential for K sustains the transduction current which regulates activity at the cytoneural junction. Clofilium, a compound which is known to block cardiac K channels and to decrease the endocochlear potential, was applied to the endolymphatic side of the isolated frog semicircular canal. The drug abolished the transepithelial potential and increased K outflux from the lumen to the dark cells (or the basolateral perilymph) with no apparent interference with active K secretion. When applied to the perilymphatic side in the intact labyrinth, clofilium reduced the rate of occurrence of miniature excitatory postsynaptic potentials (mEPSPs), both at rest and in response to mechanical stimulation (sinusoidal rotation at 0.1 Hz, 12.5 deg/s2 peak acceleration). This effect may be related to a reduced K-electrochemical unbalance and a decreased transduction current. The drug consistently reduced mEPSP size, although amplitude distributions remained log-normal and time intervals between successive mEPSPs remained exponentially distributed; this suggests a direct effect of clofilium on the postsynaptic membrane, in addition to any possible presynaptic effects. Spike discharge by the afferent fibre was almost completely abolished at rest and responses to mechanical stimulation were reduced by 85-90%. These effects cannot be accounted for by the mild reduction of mEPSP rates and confirm a direct action of clofilium on the afferent postsynaptic terminal.

Effects of clofilium, a K channel blocker, on electrogenic K secretion and afferent discharge at the frog semicircular canal. A preliminary report

Application of clofilium to the endolymphatic side of the isolated frog semicircular canal abolished the transepithelial potential and produced increased K and mannitol outfluxes from the lumen to the dark cells or the basolateral perilymph, with no apparent effect on active K secretion. These results suggest an increased permeability of the paracellular pathway. When applied to the perilymphatic side in the intact labyrinth, clofilium reduced the rates of quantal transmitter release (miniature EPSP frequency), an effect that might arise from a decrease in the transduction current intensity secondary to the reduced transepithelial electrochemical potential for K+. Moreover, afferent spike rates were almost completely abolished at rest as well as during mechanical stimulation. This effect together with a decreased mEPSP amplitude points to a further direct action of clofilium on the afferent postsynaptic terminal. These results suggest a multi-factorial effect of clofilium that would reduce the sensitivity of the vestibular function.

Is the use of marginal donors justified in liver transplantation? Analysis of results and proposal of modern criteria

A discrepancy exists worldwide between the number of suitable liver donors and the increasing demand for transplantation. Thus many centers have considered widening their liver donor acceptance criteria and this may increase the incidence of primary dysfunction (PD) with negative effect on the results of transplantation. In order to reduce the incidence of PD and improve patient and graft survival it becomes important to identify those risk factors associated with its occurrence. In a retrospective univariate and multivariate analysis we evaluated several donor, preservation and recipient parameters and their correlation with PD. In our Department 282 orthotopic liver transplantations (OLT) were performed on 256 adult patients over a 10-year period. Excluded were 15 cases with early vascular problems and 4 intraoperative deaths. A complete series of donor, recipient and procedure-related data were analyzed. About 30% of donors showed abnormal values. In 70 cases of PD (26%) there was a 61.4% graft failure rate compared with 15% in the group with immediate function (P < 0.05). Univariate analysis showed donor age, steatosis, ischemia time, amines, oliguria, hypotension and ICU stay to be significantly associated with PD. Multivariate analysis showed steatosis, ischemia time and amine dosage to be independent risk factors for the development of primary non function. In conclusion, the acceptance of marginal donors worsened the results of transplantation, but the rejection of these donors would reduce by about 30% our transplant activity resulting in increased mortality in the waiting list. Combinations of risk factors when possible should be avoided, and ischemia time, as the only variable that can be controlled, should be kept as short as possible.

Mitogenic effect of serotonin in human small cell lung carcinoma cells via both 5-HT1A and 5-HT1D receptors

We have recently shown that the mitogenic effect of serotonin (5-hydroxytryptamine, 5-HT) on human small lung carcinoma (SCLC) cells is at least partly due to stimulation of a 5-HT1D receptor type. We now report that the 5-HT1A receptor agonist R(+)-8-hydroxy-2-(di-n- propylamino)tetralin (8-OH-DPAT) is also capable of stimulating [3H]thymidine incorporation into SCLC GLC-8 cells, although with lower efficacy than 5-HT. The simultaneous administration of maximal doses of 8-OH-DPAT and the 5-HT1D receptor agonist sumatriptan reproduced the maximal [3H]thymidine incorporation observed with 5-HT alone. The 5-HT1A receptor antagonists spiperone and SDZ 216-525 completely abolished the effect of 8-OH-DPAT (IC50 30 nM for both drugs) behaving as pure antagonists. Accordingly, the two drugs partially inhibited the mitogenic effect of 5-HT. These data indicate that the mitogenic effect of 5-HT in SCLC cells involves both 5-HT1A and 5-HT1D receptor types.

Quantal nature of synaptic transmission at the cytoneural junction in the frog labyrinth

1. The mechanism of transmitter release at the cytoneural junction of the frog posterior canal was investigated by recording intracellularly subthreshold postsynaptic potentials (EPSPs), and performing a statistical analysis of time intervals and peak amplitudes. In single units EPSPs display highly variable size, so it is not clear whether they are generated by the release of single quanta of transmitter and whether large ones represent giant events, multiquantal events, or the random summation of independent unitary events. 2. In units with low resting EPSP rates, peak amplitudes and time intervals between EPSPs were measured directly. Peak amplitude histograms were continuous, unimodal and well fitted by log normal distributions. Time-interval histograms were well described by single exponentials. 3. At high EPSP rates (either at rest or during experimental treatments), where single events overlapped extensively, peak amplitude histograms were skewed markedly towards high values. Under these conditions, the EPSP waveform was estimated by autoregressive fit to the autocorrelation of the recorded signal. The fit was used to build a Wiener filter, for sharpening the original signal, before computing time-interval and peak amplitude histograms. This yielded consistent log normal peak amplitude distributions with no 'excess' skewness, similar to those obtained with low resting rates. 4. After sharpening by the Wiener filter, shoulders or small second peaks in amplitude distributions were observed only at the highest EPSP rates (> 300 s-1). The number of 'multiquantal' events was reduced by Wiener filtering, and was in general consistent with the expectation that more than one independent event occurred within the duration of the single event. This suggests that the events are uniquantal, random and independent, i.e. miniature EPSPs (mEPSPs). 5. In general, peak amplitude distributions obtained with modified external Ca2+ concentration ([Ca2+]o) and/or during mechanical stimulation or under efferent activation were not significantly altered with respect to those obtained in the same units at rest. Time-interval histograms were generally mono-exponential at rest as well as during mechanical or efferent stimulation, and irrespective of [Ca2+]o. Resting mEPSP rate was slightly increased by elevated [Ca2+]o and reduced by low [Ca2+]o. The increase in mEPSP rate produced by mechanical excitation was depressed by both high and low [Ca2+]o, whereas both conditions enhanced mechanical inhibition. Efferent inhibition was little affected. High [Ca2+]o hastened adaptation during efferent facilitation. Low [Ca2+]o reduced peak response during facilitation, but suppressed its warning. 6. In the presence of ATP a consistent though transient increase in resting mEPSP rate was observed in about 50% of units.(ABSTRACT TRUNCATED AT 400 WORDS)

Fluorescence approaches to the study of the actin-nucleating and bundling activities of synapsin I

Synapsin I is a neuron-specific phosphoprotein which binds to small synaptic vesicles and actin in a phosphorylation-dependent fashion. We have analyzed the ability of synapsin I to interact with actin monomers and filaments using purified proteins derivatized with fluorescent probes. Synapsin I accelerates the initial rate of actin polymerization and increases the final steady-state levels of polymerized actin. The fraction of total actin polymerized by synapsin I strongly depends on the synapsin I-actin ratio. We have visualized the actin-bundling activity of synapsin I using a non-perturbing method, video-enhanced microscopy of fluoresceinated synapsin I and actin filaments. Our findings suggest that synapsin I exerts a control on the physical characteristics of the cytoskeletal network of the nerve terminal and are consistent with the proposed role of synapsin I in mediating the interaction of synaptic vesicles with actin.

Synapsin I partially dissociates from synaptic vesicles during exocytosis induced by electrical stimulation

The distribution of the synaptic vesicle-associated phosphoprotein synapsin I after electrical stimulation of the frog neuromuscular junction was investigated by immunogold labeling and compared with the distribution of the integral synaptic vesicle protein synaptophysin. In resting terminals both proteins were localized exclusively on synaptic vesicles. In stimulated terminals they appeared also in the axolemma and its infoldings, which however exhibited a lower synapsin I/synaptophysin ratio with respect to synaptic vesicles at rest. The value of this ratio was intermediate in synaptic vesicles of stimulated terminals, and an increased synapsin I labeling of the cytomatrix was observed. These results indicate that synapsin I undergoes partial dissociation from and reassociation with synaptic vesicles, following physiological stimulation, and are consistent with the proposed modulatory role of the protein in neurotransmitter release.

Effects of the neuronal phosphoprotein synapsin I on actin polymerization. I. Evidence for a phosphorylation-dependent nucleating effect

Synapsin I is a synaptic vesicle-specific phosphoprotein which is able to bind and bundle actin filaments in a phosphorylation-dependent fashion. In the present paper we have analyzed the effects of synapsin I on the kinetics of actin polymerization and their modulation by site-specific phosphorylation of synapsin I. We found that dephosphorylated synapsin I accelerates the initial rate of actin polymerization and decreases the rate of filament elongation. The effect was observed at both low and high ionic strength, was specific for synapsin I, and was still present when polymerization was triggered by F-actin seeds. Dephosphorylated synapsin I was also able to induce actin polymerization and bundle formation in the absence of KCl and MgCl2. The effects of synapsin I were strongly decreased after its phosphorylation by Ca2+/calmodulin-dependent protein kinase II. These observations suggest that synapsin I has a phosphorylation-dependent nucleating effect on actin polymerization. The data are compatible with the view that changes in the phosphorylation state of synapsin I play a functional role in regulating the interactions between the nerve terminal cytoskeleton and synaptic vesicles in various stages of the exoendocytotic cycle.

Effects of the neuronal phosphoprotein synapsin I on actin polymerization. II. Analytical interpretation of kinetic curves

The general features of the kinetics of actin polymerization are investigated by mathematical models, with the aim of identifying the kinetically relevant parameters in the process and detecting and interpreting the alterations occurring in actin polymerization under various experimental conditions. Polymerization curves, obtained by following the increase in fluorescence of actin derivatized with N-(1-pyrenyl) iodoacetamide, are fitted using analytical equations derived from biochemical models of the actin polymerization process. Particular attention is given to the evaluation of the effects of the neuronal phosphoprotein synapsin I. The models obtained under various ionic conditions reveal that synapsin I interacts with actin in a very complex fashion, sharing some of the properties of classical nucleating proteins but displaying also actions not described previously for other actin-binding proteins. Synapsin I appears to bind G-actin with a very high stoichiometry (1:2-4), and the complex behaves as an F-actin nucleus, producing actin filaments under conditions where spontaneous polymerization is negligible. These actions of synapsin I are markedly affected by site-specific phosphorylation of the protein. An original transformation of the fluorescence data, which estimates the disappearance rate of actin monomer toward the critical concentration, is presented and shown to be of general usefulness for the study of actin-binding proteins.

Correlation between quantal secretion and vesicle loss at the frog neuromuscular junction

1. We measured the rate of occurrence of miniature endplate potentials (MEPPs) at identified endplates in frog cutaneous pectoris muscles treated with crude black widow spider venom (BWSV) or purified alpha-latrotoxin (alpha-LTX) in calcium-free solutions, and we examined the relationship between the length of the nerve terminal and the total number of quanta secreted, and the relationship between the number of quanta secreted and the number of vesicles remaining at different times. 2. The venom, or toxin, was applied in a modified Ringer solution with tetrodotoxin, 1 mM-EGTA and no divalent cations, and quantal secretion was started by applying Ca2(+)-free solutions with Mg2+. This was done to synchronize the quantal discharge at the various junctions in a muscle. Ringer solution was applied after the MEPP rate had declined to low levels, and then the muscle fibre was injected with Lucifer Yellow, the endplate stained for acetylcholinesterase and the length of the nerve terminal and the length of a sarcomere were measured on the fluorescent fibre. 3. The total number of quanta secreted by a terminal was measured under a wide variety of experimental conditions: the weights of the frogs ranged from 13 to 68 g, the temperature from 9 to 28 degrees C, and the concentration of Mg2+ from 2 to 10 mM. In one series of experiments the Mg2+ was withdrawn after 3-4 min and reapplied 35-40 min later in order to divide the total output of quanta into two approximately equal bouts of secretion that were well separated in time. 4. The total number of MEPPs recorded at a junction was loosely correlated with the length of its nerve terminal, but it was not affected by the temperature, the concentration of Mg2+ or the division of secretion into well-separated bouts of quantal release. The average total secretion per unit length was about 3700 quanta/sarcomere or about 1200 quanta/microns. 5. The average time course of quantal secretion per micrometre of terminal was determined at single junctions in muscles held at 22-23 degrees C or at 9-10 degrees C. Other muscles were fixed at various times during the course of secretion at each temperature and the number of synaptic vesicles remaining in cross-sections of the terminals were counted on electron micrographs. The number of vesicles remaining per micrometre of terminal was determined from the number per cross-section and the section thickness.(ABSTRACT TRUNCATED AT 400 WORDS)

Spontaneous [Ca2+]i fluctuations in rat chromaffin cells do not require inositol 1,4,5-trisphosphate elevations but are generated by a caffeine- and ryanodine-sensitive intracellular Ca2+ store

A considerable fraction (65%) of single rat chromaffin cells loaded with the fluorescent [Ca2+]i indicator fura-2 exhibited spontaneous rhythmic fluctuations with an average period of approximately 100 s. Parallel patch clamp experiments as well as fura-2 experiments carried out in Ca2(+)-free and other modified media in the presence of Ca2+ and Na+ channel blockers indicated an origin from intracellular stores. Appropriate concentrations of agonists (bradykinin and histamine) for receptors (B2 and H1) that trigger generation of inositol 1,4,5-trisphosphate induced increased fluctuation frequency, recruitment of silent cells, and large [Ca2+]i changes at high doses. These effects were blocked by cell pretreatment with neomycin, a drug that inhibits inositol 1,4,5-trisphosphate generation. In contrast, spontaneous fluctuations and the effects of another drug, caffeine, which also induced increased frequency and recruitment, were unaffected by neomycin. Ryanodine caused first a prolongation and then (approximately 10 min) a block of both spontaneous fluctuations and caffeine effects, where the single transients after bradykinin and histamine were maintained. Caffeine and ryanodine are known to affect selectively the process of calcium-induced Ca2+ release; this is the first demonstration of [Ca2+]i fluctuation activity arising from Ca2(+)-induced Ca2+ release in nonmuscle cells with no strict requirement for inositol 1,4,5-trisphosphate involvement.

Dual effect of potassium on transmitter exocytosis

The time course of exocytosis of quanta of acetylcholine induced by 20 mM K+ was studied at the frog neuromuscular junction. Images of vesicle fusion on freeze-fracture replicas were mostly localized at the active zones in resting preparations fixed in 20 mM K+. Fusions appeared also outside the active zones in preparations fixed after 1 min exposure to 20 mM K+ and were evenly distributed over the presynaptic membrane after 5 min in 20 mM K+ (even though secretion was prevented by withdrawing Ca2+ until 30 s before fixation). The mean densities of vesicle fusions were comparable in all conditions, as were the total number of quanta released during the fixation period. This indicates that fusions outside active zones represent ectopic exocytosis, slowly activated by potassium. Partial inactivation of K(+)-induced quantal release (time and concentration-dependent) was observed electrophysiologically; this may be related to the observed decrease in density of vesicle fusions along the active zones, with time. Consistently, after 5 min in 15 mM K+ fusion density at the active zones remained high. It is concluded that active zone-associated and ectopic fusions are two exocytotic processes activated with differential time courses and concentration-dependence by K+.

Spontaneous and nerve-evoked quantal transmission in regenerated motor terminals

Quantal acetylcholine release was studied in sciatic nerve-extensor digitorum longus muscle preparations from normal rats and in reinnervated muscles at different times following peroneus nerve crush. The efficiency of nerve-evoked transmission was regained almost completely at a time (20 days after crush) when the secretory response to increased extracellular K+ was very weak. This suggests that K+-induced secretion might be activated through mechanisms at least partially different from those involved in the response to nerve stimulation.

Static and dynamic properties of synaptic transmission at the cyto-neural junction of frog labyrinth posterior canal

The properties of synaptic transmission have been studied at the cyto-neural junction of the frog labyrinth posterior canal by examining excitatory postsynaptic potential (EPSP) activity recorded intraaxonally from the afferent nerve after abolishing spike firing by tetrodotoxin. The waveform, amplitude, and rate of occurrence of the EPSPs have been evaluated by means of a procedure of fluctuation analysis devised to continuously monitor these parameters, at rest as well as during stimulation of the semicircular canal by sinusoidal rotation at 0.1 Hz, with peak accelerations ranging from 8 to 87 deg.s-2. Responses to excitatory and inhibitory accelerations were quantified in terms of maximum and minimum EPSP rates, respectively, as well as total numbers of EPSPs occurring during the excitatory and inhibitory half cycles. Excitatory responses were systematically larger than inhibitory ones (asymmetry). Excitatory responses were linearly related either to peak acceleration or to its logarithm, and the same occurred for inhibitory responses. In all units examined, the asymmetry of the response yielded nonlinear two-sided input-output intensity functions. Silencing of EPSPs during inhibition (rectification) was never observed. Comparison of activity during the first cycle of rotation with the average response over several cycles indicated that variable degrees of adaptation (up to 48%) characterize the excitatory response, whereas no consistent adaptation was observed in the inhibitory response. All fibers appeared to give responses nearly in phase with angular velocity, at 0.1 Hz, although the peak rates generally anticipated by a few degrees the peak angular velocity. From the data presented it appears that asymmetry, adaptation, and at least part of the phase lead in afferent nerve response are of presynaptic origin, whereas rectification and possible further phase lead arise at the encoder. To confirm these conclusions a simultaneous though limited study of spike firing and EPSP activity has been attempted in a few fibers.

Synaptophysin (p38) at the frog neuromuscular junction: its incorporation into the axolemma and recycling after intense quantal secretion

Recycling of synaptophysin (p38), a synaptic vesicle integral membrane protein, was studied by the use of antisera raised against the protein purified from frog brain. When frog cutaneous pectoris muscles were fixed at rest, a bright, specific immunofluorescent signal was observed in nerve-terminal regions only if their plasma membranes had been previously permeabilized. When muscles were fixed after they had been treated for 1 h with a low dose of alpha-latrotoxin in Ca2+-free medium, an equally intense fluorescence could be observed without previous permeabilization. Under this condition, alpha-latrotoxin depletes nerve terminals of their quantal store of acetylcholine and of synaptic vesicles. These results indicate that fusion of synaptic vesicles leads to the exposure of intravesicular antigenic determinants of synaptophysin on the outer surface of the axolemma, and provide direct support for the vesicle hypothesis of neurotransmitter release. After 1 h treatment with the same dose of alpha-latrotoxin in the presence of 1.8 mM extracellular Ca2+, immunofluorescent images were obtained only after permeabilization with detergents. Under this condition, the vesicle population was maintained by an active process of recycling and more than two times the initial store of quanta were secreted. Thus, despite the active turnover of synaptic vesicles and of quanta of neurotransmitter, no extensive intermixing occurs between components of the vesicle and presynaptic plasma membrane.

The effect of potassium on exocytosis of transmitter at the frog neuromuscular junction

1. Electrophysiology and morphology have been combined to investigate the time course of the exocytosis of quanta of neurotransmitter induced by elevated concentrations of K+ at the frog neuromuscular junction. 2. Replicas of freeze-fractured resting nerve terminals fixed in the presence of 20 mM-K+ showed images of fusion of synaptic vesicles with the presynaptic axolemma which were closely associated with the active zones. After 1 min in 20 nM-K+ fusions appeared also outside the active zones, and by 5 min they became uniformly distributed over the presynaptic membrane. 3. The average total density of fusions was not significantly different at the various times examined since it decreased at the active zones while it increased over the rest of the membrane. 4. Resting terminals fixed in 20 mM-K+ released 33,000-45,000 quanta after the addition of fixative; terminals stimulated by 20 mM-K+ for 1-5 min released 50,000-100,000 quanta during fixation. The fixative potentiated K+-induced transmitter release. 5. Fusions were uniformly distributed in terminals pre-incubated for 5 min in 20 mM-K+ without added Ca2+, stimulated by adding Ca2+ for 30 s, and then fixed. Conversely, after 5 min stimulation in hypertonic Ringer solution fusions remained predominantly located near the active zones. A similar distribution was observed after 15 min stimulation by a lower concentration of K+ (15 mM). 6. At all concentrations of K+ tested (10, 15, 20, 25 mM) miniature end-plate potential (MEPP) rate attained a steady-state value within 10-15 min. Values from a single junction were generally lower at higher concentrations of K+, which indicates partial inactivation of the secretion-recycling process. 7. The data indicate that K+ initially activates exocytosis at the active zones. Subsequently, ectopic exocytosis is activated while sites at the active zones appear to undergo partial inactivation. These phenomena are not related to the intensity or to the amount of previous secretion.

Effects of black widow spider venom and Ca2+ on quantal secretion at the frog neuromuscular junction

A modification of the classical procedure of fluctuation analysis is used to measure the waveform, w(t), mean amplitude, (h), and mean rate of occurrence, (r), of miniature endplate potentials (MEPPs) at frog cutaneous pectoris neuromuscular junctions treated with black widow spider venom (BWSV). MEPP parameters are determined from the power spectrum of the fluctuating potential and the second (variance), third (skew), and fourth semi-invariants (cumulants) of high-pass-filtered records of the potential. The method gives valid results even when the mean potential undergoes slow changes unrelated to MEPPs and when the MEPP rate is not stationary; it detects changes in the distribution of MEPP amplitudes and corrects for the nonlinear summation of MEPPs. The effects of Ca2+ on BWSV-induced secretion are studied in detail. When Ca2+ is absent, the power spectrum of the fluctuations is shaped like the spectrum of w(t) and secretion is quasi-stationary; (r) rises smoothly to peak values of approximately 1,500/s and then quickly subsides to levels near 10/s. Many relatively small and some "giant" MEPPs occur at the ends of the experiments, and the distribution of MEPP amplitudes broadens. When the effects of this broadening are corrected for, we find that approximately 0.7 X 10(6) MEPPs occurred during the 30 min of intense secretion. Since BWSV depletes nerve terminals of their quanta of transmitter and their synaptic vesicles, this figure is an upper limit for the quantal store in a resting terminal. When Ca2+ is present, the noise spectrum deviates from the spectrum of w(t) and secretion is nonstationary; (r) rises to similar peak values but is sustained at levels near 400/s for up to an hour and at least 1.5 X 10(6) quanta are secreted within this period. Thus, the quantal store must have turned over at least twice under this condition. Data previously obtained at junctions treated with La3+ are corrected for nonlinear summation and for the distribution of MEPP amplitudes. The two corrections roughly compensate each other, and the corrected results confirm the previous conclusion that the number of quanta secreted from La3+-treated terminals during 1 h is not strongly dependent upon the extracellular concentration of Ca2+; approximately 2 X 10(6) quanta are released even when Ca2+ is absent.

Fluctuation analysis of nonideal shot noise. Application to the neuromuscular junction

Procedures are described for analyzing shot noise and determining the waveform, w(t), mean amplitude, (h), and mean rate of occurrence, (r), of the shots under a variety of nonideal conditions that include: (a) slow, spurious changes in the mean, (b) nonstationary shot rates, (c) nonuniform distribution of shot amplitudes, and (d) nonlinear summation of the shots. The procedures are based upon Rice's (1944. Bell Telephone System Journal. 23: 282-332) extension of Campbell's theorem to the second (variance), lambda 2, third (skew), lambda 3, and fourth, lambda 4, semi-invariants (cumulants) of the noise. It is shown that the spectra of lambda 2 and lambda 3 of nonstationary shot noise contain a set of components that are proportional to (r) and arise from w(t), and a set of components that are independent of (r) and arise from the temporal variations in r(t). Since the latter components are additive and are limited by the bandwidth of r(t), they can be removed by appropriate filters; then (r) and (h) can be determined from the lambda 2 and lambda 3 of the filtered noise. We also show that a factor related to the ratio (lambda 3)2/(lambda 2)(lambda 4) monitors the spread in the distribution of shot amplitudes and can be used to correct the estimates of (r) and (h) for the effects of that spread, if the shape of the distribution is known and if r(t) is stationary. The accuracy of the measurements of lambda 4 is assessed and corrections for the effects of nonlinear summation of lambda 2, lambda 3, and lambda 4 are derived. The procedures give valid results when they are used to analyze shot noise produced by the (linear) summation of simulated miniature endplate potentials, which are generated either at nonstationary rates or with a distribution of amplitudes.

Measurement of quantal secretion induced by ouabain and its correlation with depletion of synaptic vesicles

Ouabain (0.1 and 0.05 mM) was applied to frog cutaneous pectoris nerve-muscle preparations bathed in modified Ringer's solution containing either 1.8 mM Ca2+ (and 4 mM Mg2+) or no added Ca2+ (4 mM Mg2+ and 1 mM EGTA). During the intense quantal release of acetylcholine (ACh) induced by ouabain, the parameters of the miniature endplate potentials (mepps) were deduced from the variance, skew, and power spectra of the endplate recordings by applying a recently described modification of classical fluctuation analysis. Often the high frequency of mepps is not stationary; therefore, the signal was high-pass filtered (time constant of the resistance-capacitance filter of 2 ms) to remove the errors introduced by nonstationarity. When ouabain was applied in the presence of Ca2+, mepp frequency started to rise exponentially after a lag of 1.5-2 h, reached an average peak frequency of 1,300/s in approximately 30 min, and then suddenly subsided to low level (10/s). In Ca2+-free solution, after a shorter lag (1-1.5 h), mepp frequency rose to peak rate of 700/s in approximately 20 min and then gradually subsided. In spite of the different time course of secretion in the two experimental conditions, the cumulative quantal release was not significantly different (7.4 +/- 1.3 X 10(5) in Ca2+-containing and 8.8 +/- 2.7 X 10(5) in Ca2+-free solutions). 60 min after the peak secretion, the muscles were fixed for observation in the electron microscope. Morphometric analysis on micrographs of neuromuscular junctions revealed in both cases a profound depletion of synaptic vesicles and deep infoldings of presynaptic membrane. This rapid depletion and the lack of uptake of horseradish peroxidase suggest that ouabain impairs the recycling process that tends to conserve the vesicle population during intense secretion of neurotransmitter. The good correlation observed between the reduction in the store of synaptic vesicles and the total number of quanta of ACh secreted in the absence of a vigorous membrane recycling strongly supports the view that the secretion of a quantum of ACh requires the fusion of a synaptic vesicle with the axolemma.

Temporal coincidence between synaptic vesicle fusion and quantal secretion of acetylcholine

We applied the quick-freezing technique to investigate the precise temporal coincidence between the onset of quantal secretion and the appearance of fusions of synaptic vesicles with the prejunctional membrane. Frog cutaneous pectoris nerve-muscle preparations were soaked in modified Ringer's solution with 1 mM 4-aminopyridine, 10 mM Ca2+, and 10(-4) M d-Tubocurarine and quick-frozen 1-10 ms after a single supramaximal shock. The frozen muscles were then either freeze-fractured or cryosubstituted in acetone with 13% OsO4 and processed for thin section electron microscopy. Temporal resolution of less than 1 ms can be achieved using a quick-freeze device that increases the rate of freezing of the muscle after it strikes the chilled copper block (15 degrees K) and that minimizes the precooling of the muscle during its descent toward the block. We minimized variations in transmission time by examining thin sections taken only from the medial edge of the muscle, which was at a fixed distance from the point of stimulation of the nerve. The ultrastructure of the cryosubstituted preparations was well preserved to a depth of 5 - 10 micron, and within this narrow band vesicles were found fused with the axolemma after a minimum delay of 2.5 ms after stimulation of the nerve. Since the total transmission time to this edge of the muscle was approximately 3 ms, these results indicate that the vesicles fuse with the axolemma precisely at the same time the quanta are released. Freeze-fracture does not seem to be an adequate experimental technique for this work because in the well-preserved band of the muscle the fracture plane crosses, but does not cleave, the inner hydrophobic domain of the plasmalemma. Fracture faces may form in deeper regions of the muscle where tissue preservation is unsatisfactory and freezing is delayed.

Miniature endplate potential frequency and amplitude determined by an extension of Campbell's theorem

A method based upon an extension of Campbell's theorem is used to measure the amplitude, waveform, and frequency of occurrence of miniature endplate potentials (mepps) at rapidly secreting neuromuscular junctions of frog cutaneous pectoris muscles. Measurements of the variance, skew, and power spectrum of the fluctuations in membrane potential are used to deduce the mepp parameters. These estimates of mepp amplitude and frequency are insensitive to slow drifts in membrane potential that preclude the conventional application of Campbell's theorem, which uses the mean and variance. The new method becomes unreliable at high mepp frequencies because the distribution of the values of membrane potential approaches a Gaussian thereby reducing the accuracy of skew measurements. Frequencies approaching 10(4) s-1 can be measured, however, if the data are high-pass filtered. The method has been tested with computer simulated data and applied to junctions exposed to La3+; the effects of Ca2+ on the La3+-induced secretion have been explored. Some muscles were fixed after treatment with La3+, and changes in nerve terminal ultrastructure were assessed by morphometric analysis of electron micrographs. Horseradish peroxidase was used to obtain information about vesicle recycling.

Freeze-fracture studies of frog neuromuscular junctions during intense release of neurotransmitter. III. A morphometric analysis of the number and diameter of intramembrane particles

The intramembrane particles on the presynaptic membrane and on the membrane of synaptic vesicles were studied at freeze-fractured neuromuscular junctions of the frog. The particles on the P face of the presynaptic membrane belong to two major classes: small particles with diameters less than 9 nm and large particles with diameters between 9 and 13 nm. In addition, there were a few extralarge particles with diameters greater than 13 nm. Indirect stimulation of the muscle, or the application of black widow spider venom, decreased the concentration of small particles on the presynaptic membrane but did not change the concentration of large particles. Three similar classes of particles were found on the P face of the membrane of the synaptic vesicles. The concentrations of large and extralarge particles on the vesicle membrane were comparable to the concentrations of these particles on the presynaptic membrane, whereas the concentration of small particles on the vesicle membrane was less than than the concentration of small particles on the presynaptic membrane. These results are compatible with the idea that synaptic vesicles fuse with the presynaptic membrane when quanta of transmitter are released. However, neither the large nor the extralarge particles on the P face of the presynaptic membrane can be used to trace the movement of vesicle membrane that has been incorporated into the axolemma.