Control of Presynaptic Function by Axonal Dynamics

The exogenous Ca²⁺ chelator EGTA (ethylene glycol tetraacetic acid) has been widely used to probe the coupling distance between Ca²⁺ channels and vesicular Ca²⁺ sensors for neurotransmitter release. Because of its slow forward rate for binding, EGTA is thought to not capture calcium ions in very proximity to a channel, whereas it does capture calcium ions at the remote distance. However, in this study, our reaction diffusion simulations (RDSs) of Ca²⁺ combined with a release calculation using vesicular sensor models indicate that a high concentration of EGTA decreases Ca²⁺ and vesicular release in the nanodomain of single channels. We found that a key determinant of the effect of EGTA on neurotransmitter release is the saturation of the vesicular sensor. When the sensor is saturated, the reduction in the Ca²⁺ concentration by EGTA is masked. By contrast, when the sensor is in a linear range, even a small reduction in Ca²⁺ by EGTA can decrease vesicular release. In proximity to a channel, the vesicular sensor is often saturated for a long voltage step, but not for a brief Ca²⁺ influx typically evoked by an action potential. Therefore, when EGTA is used as a diagnostic tool to probe the coupling distance, care must be taken regarding the presynaptic Ca²⁺ entry duration as well as the property of the vesicular Ca²⁺ sensor.

Channel open duration affects the inhibitory effect of EGTA on vesicular release. (A) Spatiotemporal profiles of peak [Ca2+]i in response to a single channel opening under the control condition (0.2 mM ATP + EFB) and with 0.1, 1, 5, and 10 mM EGTA and 10 mM BAPTA (from top to bottom). The amplitude of the single VGCC current was 0.3 pA. Note that [Ca2+]i of >80 μM is colored yellow to magnify the concentration gradient near KD of the Ca2+ sensor. (B) Spatiotemporal profiles of Pvs calculated from Ca2+ transients in panel (A) using the five-site release model. (C) Spatiotemporal profiles of the inhibitory effect of various chelator concentrations of EGTA and 10 mM BAPTA on Pv. The graph for each chelator condition was calculated from the ratio of Pv in the chelator to that in the control.

30,682 views

14 citations

The three types of Analog signaling that have been described in the literature. Left, the orthodromic signaling, where somatodendritic voltage changes propagate to the axon. Middle, the antidromic signaling, where axonal voltage changes propagate backward to the axon initial segment and soma. Right, the local signaling, where axonal voltage changes propagate within the axon, to other varicosities, without escaping from the axonal compartment.

Mini Review

02 August 2019

Antidromic Analog Signaling

Federico F. Trigo

7,775 views

5 citations

Methods

24 July 2019

Dedicated Setup for the Photoconversion of Fluorescent Dyes for Functional Electron Microscopy

Katharine L. Dobson

, 2 more and

Vincenzo Marra

2,766 views

5 citations

Action potentials (APs) are initiated at the AIS of SNc TH-positive neurons. (A) An example recorded SNc TH-GFP neuron. (B) Dual whole-cell recording from the soma (blue) and the axon (red, 10 μm away from the soma) showing pace making activity at both recording sites. (C) Negative DC current injection to the soma prevents spontaneous firing. Positive current pulses (500 ms in duration) at the AIS cause repetitive firing at the axon and the soma. Note the absence and presence of the initial AP (arrows) in response to weak and strong current pulses (100 vs. 150 pA). (D) Injection of current pulses to the soma evoke repetitive firing at both recording sites. Also note the absence of presence of the initial AP. (E) Top, expansion of the APs indicated in B–D. Axonal APs precede the somatic APs in all cases. Bottom, the first derivative (dV/dt) of the corresponding APs. The two components at the rising phase of the somatic dV/dt trajectory (blue) correspond to the AIS and SD potentials, respectively. Note the difference of the rising phase between somatic and axonal dV/dt traces.

Original Research

10 July 2019

Biophysical Properties of Somatic and Axonal Voltage-Gated Sodium Channels in Midbrain Dopaminergic Neurons

Jun Yang

, 4 more and

Yousheng Shu

7,037 views

18 citations

Variance-Mean plots of last release number and cumulative release number in various models. A series of Monte Carlo simulations of variance-mean plots of last release number (red, open circles) and cumulative release number (gray, filled squares) (Miki et al., 2016). Synapses are subjected to trains of 8 APs at 200 Hz. In all panels, a parabola having N of 4 is shown. In (D–H), a parabolic curve having N of 8 is also shown. In (B,C,E–H), last two or three points for cumulative number were fitted with a line. Corresponding stimulus numbers for the plots of the last release number (red, open symbols) and of the cumulative release number (gray, closed symbols) are shown. (A) Simple 1-step model. There are four independent DSs that are occupied by synaptic vesicles with an initial occupancy δ = 0.8 before the 1st stimulus. Docked vesicles have a release probability p = 0.6 per DS, for each stimulus. Once docked vesicles are released, no DS replenishment occurs. (B) In the renewable 1-step model, a replenishment step is added to the 1-step model in (A). Emptied DSs are replenished from an infinite vesicle pool with a transition probability s = 0.2. (C) In the parallel model, in addition to the release from four DSs, release occurs as a Poisson process. (D) 2-step model. There are four replacement sites paired to four DSs. Once a DS becomes empty, a vesicle is supplied from the paired replacement site with a transition probability r = 0.7. There is no recruitment mechanism once a replacement site is depleted. (E) In the renewable 2-step model, a replenishment step with rate constant s is added to the 2-step model. (F) In this model (one-step model + renewable one-step model), there are four 1-step DSs and four renewable 1-step DSs. Parameters in the one-step model are the same as those in (A). (G) In another type of DSs, no release occurs upon the 1st stimulus (p21 = 0) but release occurs with a probability p22–8 = 0.4 from the 2nd stimulus. (H) 2-state model. There are two states of vesicle docking at DSs. In contrast to replacement sites, vesicles cannot occupy both states at the same time.

Perspective

21 June 2019

What We Can Learn From Cumulative Numbers of Vesicular Release Events

Takafumi Miki

3,484 views

8 citations

Ionic and capacitive components underlying propagating action potential. (A) Reconstructed action potentials at the 10th boutons in the model (left, black). Equilibrium potential of K+ ions (EK, –85 mV) was shown as a blue dotted line. When voltage-dependent K+ conductance was removed from 10th axon shaft and 10th bouton illustrating in the red, decay phase of the action potential was significantly delayed (middle, blue). Removal of voltage-dependent Na+ conductance as well largely reduced the amplitude (right, green). (B) Superimposed traces in panel (A). (C) Calculated components due to activation of voltage-dependent Na+ channels (VmNa, orange), K+ channels (VmK, purple) and the sum of them (VmNa + VmK, red). (D) Superimposed traces in C. (E–H) Similar data from panels (A–D) except for the resting potential was set at –100 mV. EK (–85 mV) was also shown as a blue dotted line in Panel (E).

Original Research

14 May 2019

Modeling Analysis of Axonal After Potential at Hippocampal Mossy Fibers

Haruyuki Kamiya

5,014 views

5 citations

Hypothesis and Theory

16 May 2019

The Slow Depolarization Following Individual Spikes in Thin, Unmyelinated Axons in Mammalian Cortex

Morten Raastad

6,199 views

2 citations

Mini Review

12 June 2019

Dynamic Factors for Transmitter Release at Small Presynaptic Boutons Revealed by Direct Patch-Clamp Recordings

Shin-ya Kawaguchi

2,631 views

5 citations

Review

24 April 2019

Past and Future of Analog-Digital Modulation of Synaptic Transmission

Mickael Zbili

and

Dominique Debanne

Modulation of AP waveform and synaptic strength by presynaptic membrane potential in invertebrates. (A) Hyperpolarization of the presynaptic element leads to an increase in the spike amplitude and the post-synaptic potential amplitude at squid giant synapse. Adapted with permission from Takeuchi and Takeuchi (1962). (B) Increasing the current applied to emit the spike leads to a decrease in presynaptic spike latency, an increase in spike amplitude and an increased EPSP amplitude at the squid giant synapse. Adapted with permission from Kusano et al. (1967). (C) Depolarization of the presynaptic cell leads to an increase in spike-evoked synaptic transmission at the cholinergic synapse of Aplysia. Adapted with permission from Shapiro et al. (1980).

Action potentials (APs) are generally produced in response to complex summation of excitatory and inhibitory synaptic inputs. While it is usually considered as a digital event, both the amplitude and width of the AP are significantly impacted by the context of its emission. In particular, the analog variations in subthreshold membrane potential determine the spike waveform and subsequently affect synaptic strength, leading to the so-called analog-digital modulation of synaptic transmission. We review here the numerous evidence suggesting context-dependent modulation of spike waveform, the discovery analog-digital modulation of synaptic transmission in invertebrates and its recent validation in mammals. We discuss the potential roles of analog-digital transmission in the physiology of neural networks.

5,043 views

40 citations

Brief Research Report

13 March 2019

Overexpression of Calretinin Enhances Short-Term Synaptic Depression

Alexey P. Bolshakov

, 4 more and

Andrei Rozov

2,110 views

1 citations

Future directions of GEVIs to explore synaptic transmission. (A) Cartoon of a synapse expressing a far-red rhodopsin-based GEVI on the membrane and a cytosolic red Ca2+ indicator. Vesicles (blue circles) contain pHluorin, a vesicular lumen-targeted pH-sensitive reporter of exocytosis (left). Theoretical traces (right) demonstrating recording of two single AP stimulations (arrows) superimposed with theoretical fluorescence traces of the Ca2+ and exocytosis response in the same cell. (B) Cartoon of a synapse expressing two spectrally separate, membrane-targeted GEVIs (left). Subthreshold events (gray box, center) are in a voltage range better detected with large fluorescence changes by a non-linear indicator than a linear indicator. A theoretical voltage trace (right, top) of a subthreshold event followed by two single AP stimulations. Theoretical fluorescence traces of the linear (middle) and non-linear (bottom) indicators demonstrate how combining indicators could help resolve both sub- and suprathreshold events in the same cell. (C) Cartoon of a synapse expressing a membrane-targeted far-red GEVI, and a mitochondrial-targeted green GEVI (left). Theoretical fluorescence traces of both indicators demonstrate simultaneous recording of mitochondrial and membrane voltage dynamics in the same cell.

Mini Review

01 March 2019

Genetically Encoded Voltage Indicators Are Illuminating Subcellular Physiology of the Axon

Lauren C. Panzera

and

Michael B. Hoppa

Everything we see and do is regulated by electrical signals in our nerves and muscle. Ion channels are crucial for sensing and generating electrical signals. Two voltage-dependent conductances, Na⁺ and K⁺, form the bedrock of the electrical impulse in the brain known as the action potential. Several classes of mammalian neurons express combinations of nearly 100 different varieties of these two voltage-dependent channels and their subunits. Not surprisingly, this variability orchestrates a diversity of action potential shapes and firing patterns that have been studied in detail at neural somata. A remarkably understudied phenomena exists in subcellular compartments of the axon, where action potentials initiate synaptic transmission. Ion channel research was catalyzed by the invention of glass electrodes to measure electrical signals in cell membranes, however, progress in the field of neurobiology has been stymied by the fact that most axons in the mammalian CNS are far too small and delicate for measuring ion channel function with electrodes. These quantitative measurements of membrane voltage can be achieved within the axon using light. A revolution of optical voltage sensors has enabled exploring important questions of how ion channels regulate axon physiology and synaptic transmission. In this review we will consider advantages and disadvantages of different fluorescent voltage indicators and discuss particularly relevant questions that these indicators can elucidate for understanding the crucial relationship between action potentials and synaptic transmission.

16,107 views

36 citations