Contribution of Ca2+ inflow to quantal, phasic transmitter release from nerve terminals of frog muscle

Focal Macropatch Recordings of Synaptic Currents from the Drosophila Larval Neuromuscular Junction

Drosophila neuromuscular junction (NMJ) is an excellent model system to study glutamatergic synaptic transmission. We describe the technique of focal macropatch recordings of synaptic currents from visualized boutons at the Drosophila larval NMJ. This technique requires customized fabrication of recording micropipettes, as well as a compound microscope equipped with a high magnification, long-distance water immersion objective, differential interference contrast (DIC) optics, and a fluorescent attachment. The recording electrode is positioned on the top of a selected synaptic bouton visualized with DIC optics, epi-fluorescence, or both. The advantage of this technique is that it allows monitoring the synaptic activity of a limited number of sites of release. The recording electrode has a diameter of several microns, and the release sites positioned outside of the electrode rim do not significantly affect the recorded currents. The recorded synaptic currents have fast kinetics and can be readily resolved. These advantages are especially important for the studies of mutant fly lines with enhanced spontaneous or asynchronous synaptic activity.

Electrophysiological analysis of synaptic transmission in Drosophila

Synaptic transmission is dynamic, plastic, and highly regulated. Drosophila is an advantageous model system for genetic and molecular studies of presynaptic and postsynaptic mechanisms and plasticity. Electrical recordings of synaptic responses represent a wide-spread approach to study neuronal signaling and synaptic transmission. We discuss experimental techniques that allow monitoring synaptic transmission in Drosophila neuromuscular and central systems. Recordings of synaptic potentials or currents at the larval neuromuscular junction (NMJ) are most common and provide numerous technical advantages due to robustness of the preparation, large and identifiable muscles, and synaptic boutons which can be readily visualized. In particular, focal macropatch recordings combined with the analysis of neurosecretory quanta enable rigorous quantification of the magnitude and kinetics of transmitter release. Patch-clamp recordings of synaptic transmission from the embryonic NMJ enable overcoming the problem of lethality in mutant lines. Recordings from the adult NMJ proved instrumental in the studies of temperature-sensitive paralytic mutants. Genetic studies of behavioral learning in Drosophila compel an investigation of synaptic transmission in the central nervous system (CNS), including primary cultured neurons and an intact brain. Cholinergic and GABAergic synaptic transmission has been recorded from the Drosophila CNS both in vitro and in vivo. In vivo patch-clamp recordings of synaptic transmission from the neurons in the olfactory pathway is a very powerful approach, which has a potential to elucidate how synaptic transmission is associated with behavioral learning. WIREs Dev Biol 2017, 6:e277. doi: 10.1002/wdev.277 For further resources related to this article, please visit the WIREs website.

Depolarization amplitude and Ca2+ -inflow control the time course of quantal releases at murine motor nerve terminals

In order to test whether the time courses of quantal releases after a depolarization pulse are affected by the depolarization amplitude, time courses were measured for small depolarization pulses that elicited close to threshold releases and for large depolarizations that elicited releases approaching saturation level. Diaphragms of young mice were excised and superfused with Bretag's solution at 18 degrees C. Synaptic currents were elicited and recorded through a perfused macropatch pipette. Releases elicited by threshold depolarizations rose earlier than releases elicited by saturation depolarizations. The short delays in the rising phases of release after large depolarizations may be due to the shift of Ca(2+) currents flowing during the pulse to tail currents. After its peak, release decayed with a time constant tau. For saturation depolarizations tau was about 0.3 ms, and for threshold depolarizations tau increased up to 0.8 ms. In order to differentiate between the effects of variations in Ca(2+) inflow and in depolarization, the amplitudes of large depolarization pulses were held constant while the amount of release was depressed by halving the Ca(2+) concentration at the terminal. The time course of the lowered releases was slightly delayed while tau remained at 0.3 ms as typical for saturation depolarizations. Double pulse facilitation unexpectedly revealed a short phase of depression of release after the pulse. This depression may contribute to the rapid decay (tau) of release after large depolarizations. The dependence of tau on depolarization amplitude indicates that the final phase of the time course of release is largely controlled by the amplitude of the preceding depolarization.

The time course of transmitter release in mouse motor nerve terminals is differentially affected by activation of muscarinic M1 or M2 receptors

At endplates of mouse diaphragms the effects of activation of presynaptic muscarinic M1 and M2 autoreceptors on the time courses of monoquantal releases have been investigated at 20 degrees C. Quantal excitatory postsynaptic currents (qEPSCs) were elicited and recorded with a perfused macropatch electrode, through which control- and drug-containing solutions were applied to 10 microm phi regions of a neuromuscular junction. M2 receptors were activated with muscarine, while the M1 receptors were blocked by pirenzepine. M2 activation presented a slight, but highly significant augmentation of early releases. Analogously, M1 receptors were activated with muscarine, while M2 receptors were blocked by methoctramine. M1 activation elicited a highly significant small shift of the time course of release towards longer delays. In controls, the number of late releases decayed with a time constant of 0.3 ms. This time constant did not change appreciably when methoctramine or methoctramine + muscarine were applied. However, methoctramine + muscarine reduced the amplitude of qEPSCs and shortened their decay by a partial block of postsynaptic channels. Double blocks with pirenzepine + methoctramine allowed no presynaptic effect of muscarine, showing that the blocker concentrations were sufficient. Neither the addition of methoctramine to pirenzepine, nor the further addition of muscarine changed the time constant of decay of the number of late releases. The results are very similar to that of autoreceptor activations in the glutamatergic crayfish synapse: activation of inhibitory receptors augmented early releases, and that of facilitatory receptors depressed early releases [J. Dudel (2006a) Eur. J. Neurosci., 23, 2695-2700], which may suggest a general presynaptic mechanism.

Postfusional Control of Quantal Current Shape

Whether glutamate is released rapidly, in an all-or-none manner, or more slowly, in a regulated manner, is a matter of debate. We analyzed the time course of excitatory postsynaptic currents (EPSCs) at glutamatergic neuromuscular junctions of Drosophila and found that the decay phase of EPSCs was protracted to a variable extent. The protraction was more pronounced in evoked and spontaneous quantal EPSCs than in action potential-evoked multiquantal EPSCs; reduced in quantal EPSCs from endophilin null mutants, which maintain release via kiss-and-run; and dependent on synaptotagmin isoform, calcium, and protein phosphorylation. Our data indicate that glutamate is released from individual synaptic vesicles for milliseconds through a fusion pore. Quantal glutamate discharge time course depends on presynaptic calcium inflow and the molecular composition of the release machinery.

Pertussis toxin does not affect the time course of quantal release in crayfish and mouse muscle, but has other post- and presynaptic effects, especially on adenosine autoreceptors

While G-proteins are involved in the synaptic release machinery and also can mediate inhibition of presynaptic Ca2+ channels, we find that pertussis toxin (PTX) does not affect the amount and the time course of quantal release from motor nerve terminals on crayfish or mouse muscle. Monoquantal excitatory currents (qEPSCs) were recorded that were elicited by constant depolarisation pulses to a terminal by means of a perfused macro-patch electrode. Although presynaptic effects of PTX on output and time course of release of quanta were absent, postsynaptically the rise time of qEPCs was increased and their decay time constant reduced. Adenosine (Ad) is known to inhibit quantal release in vertebrate motor nerve terminals via PTX sensitive G-proteins, and Ad is generated during nicotinic synaptic transmission by breakdown of the co-transmitter adenosine triphosphate (ATP). As reported by others, we found in mouse muscle inhibition of quantal release after application of Ad, but in addition late facilitation. Both these effects of Ad were blocked when the muscle was pre-incubated with PTX.

Block of quantal end-plate currents of mouse muscle by physostigmine and procaine

of quantal end-plate currents of mouse muscle by physostigmine and procaine. Quantal endplate currents (qEPCs) were recorded from hemidiaphragms of mice by means of a macro-patch-clamp electrode. Excitation was blocked with tetrodotoxin, and quantal release was elicited by depolarizing pulses through the electrode. Physostigmine (Phys) or procaine (Proc) was applied to the recording site by perfusion of the electrode tip. Low concentrations of Phys increased the amplitude and prolonged the decay time constants of qEPCs from approximately 3 to approximately 10 ms, due to block of acetylcholine-esterase. With 20 microM to 2 mM Phys or Proc, the decay of qEPCs became biphasic, an initial short time constant taus decreasing to <1 ms with 1 mM Phys and to approximately 0.3 ms with 1 mM Proc. The long second time constant of the decay, taul, reached values of </=100 ms with these blocker concentrations. The blocking effects of Phys and Proc on the qEPC are due to binding to the open channel conformation. A method is described to extract the rate constants of binding (bp) from the sums 1/taus + 1/taul, and the rates of unbinding (b-p) from tau0. taus-1. taul-1 (tau0 is the decay time constant of the control EPC). For Phys and Proc bp of 1.3 and 5. 10(6) M-1 s-1 and b-p of 176 and 350 s-1, respectively, were found. Using these rate constants and a reaction scheme for the nicotinic receptor together with the respective rate constants determined before, we could model the experimental results satisfactorily.

Neuromuscular blockade by IgG antibodies from patients with Guillain-Barré syndrome: a macro-patch-clamp study

Guillain-Barré syndrome (GBS) is often associated with serum antibodies to glycoconjugates such as GM1 and GQ1b. The pathogenic role of these antibodies and other serum factors has not yet been clarified. We have investigated the effect of serum, plasma filtrate, and highly purified IgG and IgM from 10 patients with typical GBS on motor nerve terminals in the mouse hemidiaphragm. Quantal endplate currents were recorded by means of a perfused macro-patch-clamp electrode. The plasma filtrate of all GBS patients led to a 5- to 20-fold reduction of evoked quantal release within 7 to 15 minutes of continuous superfusion. In 4 patients, the amplitudes of single quanta were clearly reduced (by 10-66% of control values), indicating an additional postsynaptic action. Blocking effects could be reversed to a variable degree within 15 to 18 minutes after washout. Purified IgG was as effective as native serum, whereas a purified GBS IgM fraction did not block transmission. Sera from convalescent patients and IgG from healthy subjects were without blocking effect. The effects were complement independent and there was no link to the presence (in 6 patients) or absence (in 4 patients) of detectable antibodies to GM1 or GQ1b. In GBS, antibodies to an undetermined antigen depress the presynaptic transmitter release and, in some cases, the activation of postsynaptic channels. We suggest that weakness in the acute stage of GBS may be caused in part by circulating antibodies.

Evoked transmitter release at neuromuscular junctions in wild type and cysteine string protein null mutant larvae of Drosophila

Cysteine string proteins (CSPs) are synaptic vesicle proteins thought to be involved in neurotransmitter release. To obtain more information about the function of these proteins motor nerve terminals of wild type and CSP null mutant Drosophila larvae were depolarized and excitatory postsynaptic currents (EPSCs) were recorded with an extracellular electrode at 16-18 degrees C. At this temperature the amplitude of average EPSCs was reduced and the time constant of the exponential fit of the current decay was increased in CSP null mutant compared to wild type larvae. The number of quanta released per pulse was not different but the time course of release was distributed differently in CSP null mutant and wild type larvae. In measurements of the latency of quantal EPSCs the probability of release after a pulse reached a lower peak value and the decay after the peak was delayed in CSP null mutant compared to wild type larvae. In addition facilitation in response to twin-pulse stimulation was slightly increased at low levels of release in CSP null mutant larvae. It is concluded that CSPs are involved in neurotransmitter release and help to synchronise evoked release at nerve terminals.

Changes in MEPP frequency during depression of evoked release at the frog neuromuscular junction

1. Endplate potentials (EPPs) and miniature endplate potentials (MEPPs) were recorded from frog neuromuscular junctions bathed in Ringer solutions containing normal (1.8 mM) or high (3.6 mM) Ca2+. The peptide toxin mu-conotoxin GIIIA was added to the Ringer solution to prevent muscle action potentials and contraction. 2. The nerve was stimulated with conditioning trains of 200-4800 impulses applied at 20 impulses s-1 to characterize the effects of repetitive stimulation on changes in EPP amplitude and MEPP frequency under high quantal conditions. 3. MEPP frequency was dramatically increased during and immediately following repetitive stimulation under high quantal conditions, whereas EPP amplitude was greatly depressed. There was no effect of repetitive stimulation on MEPP amplitude. 4. Following the conditioning stimulation the increase in MEPP frequency decayed back to the control level with a time course that could be described by four exponentials. The time constants of these exponentials were very similar to those that describe the components of stimulation-induced increases in EPP amplitude and MEPP frequency observed under low quantal conditions when depression is absent. 5. The results of this study indicate that depression and the components of stimulation-induced increases in release (facilitation, augmentation and potentiation) can be present at the same time, suggesting that the mechanism of depression involves different underlying factors from the mechanism(s) responsible for increases in release. They also indicate either that depression selectively affects only those quanta destined to be released in direct response to the nerve action potential, which would suggest that EPPs and MEPPs arise from different pools of transmitter, or that depression in some way affects a step in the release process involved only in evoked release, and not asynchronous (spontaneous) release.