Axonal GABAA receptors depolarize presynaptic terminals and facilitate transmitter release in cerebellar Purkinje cells

Activity-Dependent Ectopic Spiking in Parvalbumin-Expressing Interneurons of the Neocortex

Canonically, action potentials of most mammalian neurons initiate at the axon initial segment (AIS) and propagate bidirectionally: orthodromically along the distal axon and retrogradely into the soma and dendrites. Under some circumstances, action potentials may initiate ectopically, at sites distal to the AIS, and propagate antidromically along the axon. These "ectopic action potentials" (EAPs) have been observed in experimental models of seizures and chronic pain, and more rarely in nonpathological forebrain neurons. Here we report that a large majority of parvalbumin-expressing (PV+) interneurons in the upper layers of mouse neocortex, from both orbitofrontal and primary somatosensory areas, fire EAPs after sufficient activation of their somata. Somatostatin-expressing interneurons also fire EAPs, though less robustly. Ectopic firing in PV+ cells occurs in varying temporal patterns and can persist for several seconds. PV+ cells evoke strong synaptic inhibition in pyramidal neurons and interneurons and play critical roles in cortical function. Our results suggest that ectopic spiking of PV+ interneurons is common and may contribute to both normal and pathological network functions of the neocortex.

Enhanced Release Probability without Changes in Synaptic Delay during Analogue-Digital Facilitation

Neuronal timing with millisecond precision is critical for many brain functions such as sensory perception, learning and memory formation. At the level of the chemical synapse, the synaptic delay is determined by the presynaptic release probability (Pr) and the waveform of the presynaptic action potential (AP). For instance, paired-pulse facilitation or presynaptic long-term potentiation are associated with reductions in the synaptic delay, whereas paired-pulse depression or presynaptic long-term depression are associated with an increased synaptic delay. Parallelly, the AP broadening that results from the inactivation of voltage gated potassium (Kv) channels responsible for the repolarization phase of the AP delays the synaptic response, and the inactivation of sodium (Nav) channels by voltage reduces the synaptic latency. However, whether synaptic delay is modulated during depolarization-induced analogue-digital facilitation (d-ADF), a form of context-dependent synaptic facilitation induced by prolonged depolarization of the presynaptic neuron and mediated by the voltage-inactivation of presynaptic Kv1 channels, remains unclear. We show here that despite Pr being elevated during d-ADF at pyramidal L5-L5 cell synapses, the synaptic delay is surprisingly unchanged. This finding suggests that both Pr- and AP-dependent changes in synaptic delay compensate for each other during d-ADF. We conclude that, in contrast to other short- or long-term modulations of presynaptic release, synaptic timing is not affected during d-ADF because of the opposite interaction of Pr- and AP-dependent modulations of synaptic delay.

Distinct GABAergic modulation of timing-dependent LTP in CA1 pyramidal neurons along the longitudinal axis of the mouse hippocampus

Synaptic plasticity in the hippocampus underlies episodic memory formation, with dorsal hippocampus being instrumental for spatial memory whereas ventral hippocampus is crucial for emotional learning. Here, we studied how GABAergic inhibition regulates physiologically relevant low repeat spike timing-dependent LTP (t-LTP) at Schaffer collateral-CA1 synapses along the dorsoventral hippocampal axis. We used two t-LTP protocols relying on only 6 repeats of paired spike-firing in pre- and postsynaptic cells within 10 s that differ in postsynaptic firing patterns. GABAA receptor mechanisms played a greater role in blocking 6× 1:1 t-LTP that recruits single postsynaptic action potentials. 6× 1:4 t-LTP that depends on postsynaptic burst-firing unexpectedly required intact GABAB receptor signaling. The magnitude of both t-LTP-forms decreased along the dorsoventral axis, despite increasing excitability and basal synaptic strength in this direction. This suggests that GABAergic inhibition contributes to the distinct roles of dorsal and ventral hippocampus in memory formation.

Beyond a Transmission Cable-New Technologies to Reveal the Richness in Axonal Electrophysiology

The axon is a neuronal structure capable of processing, encoding, and transmitting information. This assessment contrasts with a limiting, but deeply rooted, perspective where the axon functions solely as a transmission cable of somatodendritic activity, sending signals in the form of stereotypical action potentials. This perspective arose, at least partially, because of the technical difficulties in probing axons: their extreme length-to-diameter ratio and intricate growth paths preclude the study of their dynamics through traditional techniques. Recent findings are challenging this view and revealing a much larger repertoire of axonal computations. Axons display complex signaling processes and structure-function relationships, which can be modulated via diverse activity-dependent mechanisms. Additionally, axons can exhibit patterns of activity that are dramatically different from those of their corresponding soma. Not surprisingly, many of these recent discoveries have been driven by novel technology developments, which allow for in vitro axon electrophysiology with unprecedented spatiotemporal resolution and signal-to-noise ratio. In this review, we outline the state-of-the-art in vitro toolset for axonal electrophysiology and summarize the recent discoveries in axon function it has enabled. We also review the increasing repertoire of microtechnologies for controlling axon guidance which, in combination with the available cutting-edge electrophysiology and imaging approaches, have the potential for more controlled and high-throughput in vitro studies. We anticipate that a larger adoption of these new technologies by the neuroscience community will drive a new era of experimental opportunities in the study of axon physiology and consequently, neuronal function.

GABA Increases Sensory Transmission In Monkeys

Sensory input flow is central to voluntary movements. For almost a century, GABA was believed to modulate this flow by inhibiting sensory axons in the spinal cord to sculpt neural inputs into skilled motor output. Instead, here we show that GABA can also facilitate sensory transmission in monkeys and consequently increase spinal and cortical neural responses to sensory inputs challenging our understanding of generation and perception of movement.

Axon Initial Segment GABA Inhibits Action Potential Generation throughout Periadolescent Development

Neurons are remarkably polarized structures: dendrites spread and branch to receive synaptic inputs while a single axon extends and transmits action potentials (APs) to downstream targets. Neuronal polarity is maintained by the axon initial segment (AIS), a region between the soma and axon proper that is also the site of action potential (AP) generation. This polarization between dendrites and axons extends to inhibitory neurotransmission. In adulthood, the neurotransmitter GABA hyperpolarizes dendrites but instead depolarizes axons. These differences in function collide at the AIS. Multiple studies have shown that GABAergic signaling in this region can share properties of either the mature axon or mature dendrite, and that these properties evolve over a protracted period encompassing periadolescent development. Here, we explored how developmental changes in GABAergic signaling affect AP initiation. We show that GABA at the axon initial segment inhibits action potential initiation in layer (L)2/3 pyramidal neurons in prefrontal cortex from mice of either sex across GABA reversal potentials observed in periadolescence. These actions occur largely through current shunts generated by GABAA receptors and changes in voltage-gated channel properties that affected the number of channels that could be recruited for AP electrogenesis. These results suggest that GABAergic neurons targeting the axon initial segment provide an inhibitory "veto" across the range of GABA polarity observed in normal adolescent development, regardless of GABAergic synapse reversal potential.Significance Statement GABA receptors are a major class of neurotransmitter receptors in the brain. Typically, GABA receptors inhibit neurons by allowing influx of negatively charged chloride ions into the cell. However, there are cases where local chloride concentrations promote chloride efflux through GABA receptors. Such conditions exist early in development in neocortical pyramidal cell axon initial segments (AISs), where action potentials (APs) initiate. Here, we examined how chloride efflux in early development interacts with mechanisms that support action potential initiation. We find that this efflux, despite moving membrane potential closer to action potential threshold, is nevertheless inhibitory. Thus, GABA at the axon initial segment is likely to be inhibitory for action potential initiation independent of whether chloride flows out or into neurons via these receptors.

Facilitation of sensory transmission to motoneurons during cortical or sensory-evoked primary afferent depolarization (PAD) in humans

Sensory and corticospinal tract (CST) pathways activate spinal GABAergic interneurons that have axoaxonic connections onto proprioceptive (Ia) afferents that cause long-lasting depolarizations (termed primary afferent depolarization, PAD). In rodents, sensory-evoked PAD is produced by GABAA receptors at nodes of Ranvier in Ia afferents, rather than at presynaptic terminals, and facilitates spike propagation to motoneurons by preventing branch-point failures, rather than causing presynaptic inhibition. We examined in 40 human participants whether putative activation of Ia-PAD by sensory or CST pathways can also facilitate Ia afferent activation of motoneurons via the H-reflex. H-reflexes in several leg muscles were facilitated by prior conditioning from low-threshold proprioceptive, cutaneous or CST pathways, with a similar long-lasting time course (∼200 ms) to phasic PAD measured in rodent Ia afferents. Long trains of cutaneous or proprioceptive afferent conditioning produced longer-lasting facilitation of the H-reflex for up to 2 min, consistent with tonic PAD in rodent Ia afferents mediated by nodal α5-GABAA receptors for similar stimulation trains. Facilitation of H-reflexes by this conditioning was likely not mediated by direct facilitation of the motoneurons because isolated stimulation of sensory or CST pathways did not alone facilitate the tonic firing rate of motor units. Furthermore, cutaneous conditioning increased the firing probability of single motor units (motoneurons) during the H-reflex without increasing their firing rate at this time, indicating that the underlying excitatory postsynaptic potential was more probable, but not larger. These results are consistent with sensory and CST pathways activating nodal GABAA receptors that reduce intermittent failure of action potentials propagating into Ia afferent branches. KEY POINTS: Controlled execution of posture and movement requires continually adjusted feedback from peripheral sensory pathways, especially those that carry proprioceptive information about body position, movement and effort. It was previously thought that the flow of proprioceptive feedback from Ia afferents was only reduced by GABAergic neurons in the spinal cord that sent axoaxonic projections to the terminal endings of sensory axons (termed GABAaxo neurons). Based on new findings in rodents, we provide complementary evidence in humans to suggest that sensory and corticospinal pathways known to activate GABAaxo neurons that project to dorsal parts of the Ia afferent also increase the flow of proprioceptive feedback to motoneurons in the spinal cord. These findings support a new role for spinal GABAaxo neurons in facilitating afferent feedback to the spinal cord during voluntary or reflexive movements.

Acceleration of GABA-switch after early life stress changes mouse prefrontal glutamatergic transmission

Early life stress (ELS) alters the excitation-inhibition-balance (EI-balance) in various rodent brain areas and may be responsible for behavioral impairment later in life. The EI-balance is (amongst others) influenced by the switch of GABAergic transmission from excitatory to inhibitory, the so-called "GABA-switch". Here, we investigated how ELS affects the GABA-switch in mouse infralimbic Prefrontal Cortex layer 2/3 neurons, using the limited-nesting-and-bedding model. In ELS mice, the GABA-switch occurred already between postnatal day (P) 6 and P9, as opposed to P15-P21 in controls. This was associated with increased expression of the inward chloride transporter NKCC1, compared to the outward chloride transporter KCC2, both of which are important for the intracellular chloride concentration and, hence, the GABA reversal potential (Erev). Chloride transporters are not only important for regulating chloride concentration postsynaptically, but also presynaptically. Depending on the Erev of GABA, presynaptic GABA_A receptor stimulation causes a depolarization or hyperpolarization, and thereby enhanced or reduced fusion of glutamate vesicles respectively, in turn changing the frequency of miniature postsynaptic currents (mEPSCs). In accordance, bumetanide, a blocker of NKCC1, shifted the Erev GABA towards more hyperpolarized levels in P9 control mice and reduced the mEPSC frequency. Other modulators of chloride transporters, e.g. VU0463271 (a KCC2 antagonist) and aldosterone -which increases NKCC1 expression-did not affect postsynaptic Erev in ELS P9 mice, but did increase the mEPSC frequency. We conclude that the mouse GABA-switch is accelerated after ELS, affecting both the pre- and postsynaptic chloride homeostasis, the former altering glutamatergic transmission. This may considerably affect brain development.

GABA facilitates spike propagation through branch points of sensory axons in the spinal cord

Movement and posture depend on sensory feedback that is regulated by specialized GABAergic neurons (GAD2⁺) that form axo-axonic contacts onto myelinated proprioceptive sensory axons and are thought to be inhibitory. However, we report here that activating GAD2⁺ neurons directly with optogenetics or indirectly by cutaneous stimulation actually facilitates sensory feedback to motor neurons in rodents and humans. GABA_A receptors located at or near nodes of Ranvier of sensory axons cause this facilitation by preventing spike propagation failure at the many axon branch points, which is otherwise common without GABA. In contrast, GABA_A receptors are generally lacking from axon terminals and so cannot inhibit transmitter release onto motor neurons, unlike GABA_B receptors that cause presynaptic inhibition. GABAergic innervation near nodes and branch points allows individual branches to function autonomously, with GAD2⁺ neurons regulating which branches conduct, adding a computational layer to the neuronal networks generating movement and likely generalizing to other central nervous system axons.

Neurogenesis and neuronal differentiation in the postnatal frontal cortex in Down syndrome

Although Down syndrome (DS), the most common developmental genetic cause of intellectual disability, displays proliferation and migration deficits in the prenatal frontal cortex (FC), a knowledge gap exists on the effects of trisomy 21 upon postnatal cortical development. Here, we examined cortical neurogenesis and differentiation in the FC supragranular (SG, II/III) and infragranular (IG, V/VI) layers applying antibodies to doublecortin (DCX), non-phosphorylated heavy-molecular neurofilament protein (NHF, SMI-32), calbindin D-28K (Calb), calretinin (Calr), and parvalbumin (Parv), as well as β-amyloid (APP/Aβ and Aβ_1-42) and phospho-tau (CP13 and PHF-1) in autopsy tissue from age-matched DS and neurotypical (NTD) subjects ranging from 28-weeks (wk)-gestation to 3 years of age. Thionin, which stains Nissl substance, revealed disorganized cortical cellular lamination including a delayed appearance of pyramidal cells until 44 wk of age in DS compared to 28 wk in NTD. SG and IG DCX-immunoreactive (-ir) cells were only visualized in the youngest cases until 83 wk in NTD and 57 wk DS. Strong SMI-32 immunoreactivity was observed in layers III and V pyramidal cells in the oldest NTD and DS cases with few appearing as early as 28 wk of age in layer V in NTD. Small Calb-ir interneurons were seen in younger NTD and DS cases compared to Calb-ir pyramidal cells in older subjects. Overall, a greater number of Calb-ir cells were detected in NTD, however, the number of Calr-ir cells were comparable between groups. Diffuse APP/Aβ immunoreactivity was found at all ages in both groups. Few young cases from both groups presented non-neuronal granular CP13 immunoreactivity in layer I. Stronger correlations between brain weight, age, thionin, DCX, and SMI-32 counts were found in NTD. These findings suggest that trisomy 21 affects postnatal FC lamination, neuronal migration/neurogenesis and differentiation of projection neurons and interneurons that likely contribute to cognitive impairment in DS.

Postnatal Cytoarchitecture and Neurochemical Hippocampal Dysfunction in Down Syndrome

Although the prenatal hippocampus displays deficits in cellular proliferation/migration and volume, which are later associated with memory deficits, little is known about the effects of trisomy 21 on postnatal hippocampal cellular development in Down syndrome (DS). We examined postnatal hippocampal neuronal profiles from autopsies of DS and neurotypical (NTD) neonates born at 38-weeks’-gestation up to children 3 years of age using antibodies against non-phosphorylated (SMI-32) and phosphorylated (SMI-34) neurofilament, calbindin D-_28k (Calb), calretinin (Calr), parvalbumin (Parv), doublecortin (DCX) and Ki-67, as well as amyloid precursor protein (APP), amyloid beta (Aβ) and phosphorylated tau (p-tau). Although the distribution of SMI-32-immunoreactive (-ir) hippocampal neurons was similar at all ages in both groups, pyramidal cell apical and basal dendrites were intensely stained in NTD cases. A greater reduction in the number of DCX-ir cells was observed in the hippocampal granule cell layer in DS. Although the distribution of Calb-ir neurons was similar between the youngest and oldest NTD and DS cases, Parv-ir was not detected. Conversely, Calr-ir cells and fibers were observed at all ages in DS, while NTD cases displayed mainly Calr-ir fibers. Hippocampal APP/Aβ-ir diffuse-like plaques were seen in DS and NTD. By contrast, no Aβ_1–42 or p-tau profiles were observed. These findings suggest that deficits in hippocampal neurogenesis and pyramidal cell maturation and increased Calr immunoreactivity during early postnatal life contribute to cognitive impairment in DS.

Unique Actions of GABA Arising from Cytoplasmic Chloride Microdomains

Developmental, cellular, and subcellular variations in the direction of neuronal Cl- currents elicited by GABAA receptor activation have been frequently reported. We found a corresponding variance in the GABAA receptor reversal potential (EGABA) for synapses originating from individual interneurons onto a single pyramidal cell. These findings suggest a similar heterogeneity in the cytoplasmic intracellular concentration of chloride ([Cl-]i) in individual dendrites. We determined [Cl-]i in the murine hippocampus and cerebral cortex of both sexes by (1) two-photon imaging of the Cl--sensitive, ratiometric fluorescent protein SuperClomeleon; (2) Fluorescence Lifetime IMaging (FLIM) of the Cl--sensitive fluorophore MEQ (6-methoxy-N-ethylquinolinium); and (3) electrophysiological measurements of EGABA by pressure application of GABA and RuBi-GABA uncaging. Fluorometric and electrophysiological estimates of local [Cl-]i were highly correlated. [Cl-]i microdomains persisted after pharmacological inhibition of cation-chloride cotransporters, but were progressively modified after inhibiting the polymerization of the anionic biopolymer actin. These methods collectively demonstrated stable [Cl-]i microdomains in individual neurons in vitro and in vivo and the role of immobile anions in its stability. Our results highlight the existence of functionally significant neuronal Cl- microdomains that modify the impact of GABAergic inputs.SIGNIFICANCE STATEMENT Microdomains of varying chloride concentrations in the neuronal cytoplasm are a predictable consequence of the inhomogeneous distribution of anionic polymers such as actin, tubulin, and nucleic acids. Here, we demonstrate the existence and stability of these microdomains, as well as the consequence for GABAergic synaptic signaling: each interneuron produces a postsynaptic GABAA response with a unique reversal potential. In individual hippocampal pyramidal cells, the range of GABAA reversal potentials evoked by stimulating different interneurons was >20 mV. Some interneurons generated postsynaptic responses in pyramidal cells that reversed at potentials beyond what would be considered purely inhibitory. Cytoplasmic chloride microdomains enable each pyramidal cell to maintain a compendium of unique postsynaptic responses to the activity of individual interneurons.

Calcium-dependent docking of synaptic vesicles

The concentration of calcium ions in presynaptic terminals regulates transmitter release, but underlying mechanisms have remained unclear. Here we review recent studies that shed new light on this issue. Fast-freezing electron microscopy and total internal reflection fluorescence microscopy studies reveal complex calcium-dependent vesicle movements including docking on a millisecond time scale. Recordings from so-called 'simple synapses' indicate that calcium not only triggers exocytosis, but also modifies synaptic strength by controlling a final, rapid vesicle maturation step before release. Molecular studies identify several calcium-sensitive domains on Munc13 and on synaptotagmin-1 that are likely involved in bringing the vesicular and plasma membranes closer together in response to calcium elevation. Together, these results suggest that calcium-dependent vesicle docking occurs in a wide range of time domains and plays a crucial role in several phenomena including synaptic facilitation, post-tetanic potentiation, and neuromodulator-induced potentiation.

GABAA receptor agonist cinazepam and its active metabolite 3-hydroxyphenazepam act differently at the presynaptic site

Cinazepam C₁₉H₁₄BrClN₂O₅, ("Levana^Ⓡ ІC") a partial GABA_A receptor agonist, and its active metabolite 3-hydroxyphenazepam C₁₅H₁₀BrClN₂O₂ were comparatively assessed in vitro using nerve terminals isolated from rat cortex (synaptosomes). At the presynaptic site, cinazepam (100 and 200 µM) facilitated synaptosomal transporter-mediated [³H]GABA uptake by enhancing both the initial rate and accumulation, and decreased the ambient level and transporter-mediated release of [³H]GABA. Whereas, 3-hydroxyphenazepam decreased the uptake and did not change the ambient synaptosomal level and transporter-mediated release of [³H]GABA. To exclude GABA transporter influence, NO-711, the transporter blocker, was applied and it was found that exocytotic release of [³H]GABA decreased, whereas tonic release of [³H]GABA was not changed in the presence of both cinazepam or 3-hydroxyphenazepam after treatment of synaptosomes with NO-711. In fluorimetric studies using potential- and pH-sensitive dyes rhodamine 6G and acridine orange, respectively, it was found that cinazepam hyperpolarized the synaptosomal plasma membrane, and increased synaptic vesicle acidification, whereas, 3-hydroxyphenazepam demonstrated opposite effects on these parameters. Therefore, action of cinazepam and its active metabolite 3-hydroxyphenazepam on GABAergic neurotransmission was different. Therapeutic effects of cinazepam can be associated with its ability to hyperpolarize the plasma membrane, to increase synaptic vesicle acidification and capacity of its active metabolite 3-hydroxyphenazepam to inhibit GABA transporter functioning.

Axonal Modulation of Striatal Dopamine Release by Local γ-Aminobutyric Acid (GABA) Signalling

Striatal dopamine (DA) release is critical for motivated actions and reinforcement learning, and is locally influenced at the level of DA axons by other striatal neurotransmitters. Here, we review a wealth of historical and more recently refined evidence indicating that DA output is inhibited by striatal γ-aminobutyric acid (GABA) acting via GABAA and GABAB receptors. We review evidence supporting the localisation of GABAA and GABAB receptors to DA axons, as well as the identity of the striatal sources of GABA that likely contribute to GABAergic modulation of DA release. We discuss emerging data outlining the mechanisms through which GABAA and GABAB receptors inhibit the amplitude as well as modulate the short-term plasticity of DA release. Furthermore, we highlight recent data showing that DA release is governed by plasma membrane GABA uptake transporters on striatal astrocytes, which determine ambient striatal GABA tone and, by extension, the tonic inhibition of DA release. Finally, we discuss how the regulation of striatal GABA-DA interactions represents an axis for dysfunction in psychomotor disorders associated with dysregulated DA signalling, including Parkinson's disease, and could be a novel therapeutic target for drugs to modify striatal DA output.

Axonal mechanisms mediating γ-aminobutyric acid receptor type A (GABA-A) inhibition of striatal dopamine release

Axons of dopaminergic neurons innervate the striatum where they contribute to movement and reinforcement learning. Past work has shown that striatal GABA tonically inhibits dopamine release, but whether GABA-A receptors directly modulate transmission or act indirectly through circuit elements is unresolved. Here, we use whole-cell and perforated-patch recordings to test for GABA-A receptors on the main dopaminergic neuron axons and branching processes within the striatum of adult mice. Application of GABA depolarized axons, but also decreased the amplitude of axonal spikes, limited propagation and reduced striatal dopamine release. The mechanism of inhibition involved sodium channel inactivation and shunting. Lastly, we show the positive allosteric modulator diazepam enhanced GABA-A currents on dopaminergic axons and directly inhibited release, but also likely acts by reducing excitation from cholinergic interneurons. Thus, we reveal the mechanisms of GABA-A receptor modulation of dopamine release and provide new insights into the actions of benzodiazepines within the striatum.

Differentially poised vesicles underlie fast and slow components of release at single synapses

In several types of central mammalian synapses, sustained presynaptic stimulation leads to a sequence of two components of synaptic vesicle release, reflecting the consecutive contributions of a fast-releasing pool (FRP) and of a slow-releasing pool (SRP). Previous work has shown that following common depletion by a strong stimulation, FRP and SRP recover with different kinetics. However, it has remained unclear whether any manipulation could lead to a selective enhancement of either FRP or SRP. To address this question, we have performed local presynaptic calcium uncaging in single presynaptic varicosities of cerebellar interneurons. These varicosities typically form "simple synapses" onto postsynaptic interneurons, involving several (one to six) docking/release sites within a single active zone. We find that strong uncaging laser pulses elicit two phases of release with time constants of ∼1 ms (FRP release) and ∼20 ms (SRP release). When uncaging was preceded by action potential-evoked vesicular release, the extent of SRP release was specifically enhanced. We interpret this effect as reflecting an increased likelihood of two-step release (docking then release) following the elimination of docked synaptic vesicles by action potential-evoked release. In contrast, a subthreshold laser-evoked calcium elevation in the presynaptic varicosity resulted in an enhancement of the FRP release. We interpret this latter effect as reflecting an increased probability of occupancy of docking sites following subthreshold calcium increase. In conclusion, both fast and slow components of release can be specifically enhanced by certain presynaptic manipulations. Our results have implications for the mechanism of docking site replenishment and the regulation of synaptic responses, in particular following activation of ionotropic presynaptic receptors.

Incomplete vesicular docking limits synaptic strength under high release probability conditions

Central mammalian synapses release synaptic vesicles in dedicated structures called docking/release sites. It has been assumed that when voltage-dependent calcium entry is sufficiently large, synaptic output attains a maximum value of one synaptic vesicle per action potential and per site. Here we use deconvolution to count synaptic vesicle output at single sites (mean site number per synapse: 3.6). When increasing calcium entry with tetraethylammonium in 1.5 mM external calcium concentration, we find that synaptic output saturates at 0.22 vesicle per site, not at 1 vesicle per site. Fitting the results with current models of calcium-dependent exocytosis indicates that the 0.22 vesicle limit reflects the probability of docking sites to be occupied by synaptic vesicles at rest, as only docked vesicles can be released. With 3 mM external calcium, the maximum output per site increases to 0.47, indicating an increase in docking site occupancy as a function of external calcium concentration.

Excitability Tuning of Axons by Afterdepolarization

The axon provides a sole output of the neuron which propagates action potentials reliably to the axon terminal and transmits neuronal information to the postsynaptic neuron across the synapse. A classical view of neuronal signaling is based on these two processes, namely binary (all or none) signaling along the axon and graded (tunable) signaling at the synapse. Recent studies, however, have revealed that the excitability of the axon is subject to dynamic tuning for a short period after axonal action potentials. This was first described as post-spike hyperexcitability, as measured by the changes in stimulus threshold for a short period after an action potential. Later on, direct recordings from central nervous system (CNS) axons or axon terminals using subcellular patch-clamp recording showed that axonal spikes are often followed by afterdepolarization (ADP) lasting for several tens of milliseconds and has been suggested to mediate post-spike hyperexcitability. In this review article, I focused on the mechanisms as well as the functional significance of ADP in fine-scale modulation of axonal spike signaling in the CNS, with special reference to hippocampal mossy fibers, one of the best-studied CNS axons. As a common basic mechanism underlying axonal ADP, passive propagation by the capacitive discharge of the axonal membrane as well as voltage-dependent K⁺ conductance underlies the generation of ADP. Small but prolonged axonal ADP lasting for several tens of milliseconds may influence the subsequent action potential and transmitter release from the axon terminals. Both duration and amplitude of axonal spike are subject to such modulation by preceding action potential-ADP sequence, deviating from the conventional assumption of digital nature of axonal spike signaling. Impact on the transmitter release is also discussed in the context of axonal spike plasticity. Axonal spike is subject to dynamic control on a fine-scale and thereby contributes to the short-term plasticity at the synapse.

Antidromic Analog Signaling

Analog signaling describes the use of graded voltage changes as signals in the axonal compartment. Analog signaling has been described originally in invertebrates but more recent work has established its presence in the mammalian brain (Alle and Geiger, 2006; Shu et al., 2006). In recent years, many different groups have contributed to the understanding of the physiological significance of analog signaling from a cellular perspective (for a recent review the reader may take a look at the work by Zbili and Debanne, 2019 in this Frontiers in Neuroscience Special Issue). The great majority of the experimental work related to analog signaling, however, concerns the propagation of subthreshold voltage changes from the soma to the axon. Much less attention has been paid to the propagation of subthreshold voltage changes in the opposite direction, from the axon to the soma, or to the propagation of local signals within the axon. In this mini review we will describe these other variants of analog signaling that we call here “antidromic” coupling and “local” coupling.

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

Small size of an axon and presynaptic structures have hindered direct functional analysis of axonal signaling and transmitter release at presynaptic boutons in the central nervous system. However, recent technical advances in subcellular patch-clamp recordings and in fluorescent imagings are shedding light on the dynamic nature of axonal and presynaptic mechanisms. Here I summarize the functional design of an axon and presynaptic boutons, such as diversity and activity-dependent changes of action potential (AP) waveforms, Ca²⁺ influx, and kinetics of transmitter release, revealed by the technical tour de force of direct patch-clamp recordings and the leading-edge fluorescent imagings. I highlight the critical factors for dynamic modulation of transmitter release and presynaptic short-term plasticity.

Modeling Analysis of Axonal After Potential at Hippocampal Mossy Fibers

Action potentials reliably propagate along the axons, and after potential often follows the axonal action potentials. After potential lasts for several tens of millisecond and plays a crucial role in regulating excitability during repetitive firings of the axon. Several mechanisms underlying the generation of after potential have been suggested, including activation of ionotropic autoreceptors, accumulation of K⁺ ions in the surrounding extracellular space, the opening of slow voltage-dependent currents, and capacitive discharge of upstream action potentials passively propagated through axon cable. Among them, capacitive discharge is difficult to examine experimentally, since the quantitative evaluation of a capacitive component requires simultaneous recordings from at least two different sites on the connecting axon. In this study, a series of numerical simulation of the axonal action potential was performed using a proposed model of the hippocampal mossy fiber where morphological as well as electrophysiological data are accumulated. To evaluate the relative contribution of the capacitive discharge in axonal after potential, voltage-dependent Na⁺ current as well as voltage-dependent K⁺ current was omitted from a distal part of mossy fiber axons. Slow depolarization with a similar time course with the recorded after potential in the previous study was left after blockade of Na⁺ and K⁺ currents, suggesting that a capacitive component contributes substantially in axonal after potential following propagating action potentials. On the other hand, it has been shown that experimentally recorded after potential often showed clear voltage-dependency upon changes in the initial membrane potential, obviously deviating from voltage-independent nature of the capacitive component. The simulation revealed that activation of voltage-dependent K⁺ current also contributes to shape a characteristic waveform of axonal after potential and reconstitute similar voltage-dependency with that reported for the after potential recorded from mossy fiber terminals. These findings suggest that the capacitive component reflecting passive propagation of upstream action potential substantially contributes to the slow time course of axonal after potential, although voltage-dependent K⁺ current provided a characteristic voltage dependency of after potential waveform.

Past and Future of Analog-Digital Modulation of Synaptic Transmission

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.

Direction of action of presynaptic GABAA receptors is highly dependent on the level of receptor activation

Many synapses, including parallel fiber synapses in the cerebellum, express presynaptic GABA_A receptors. However, reports of the functional consequences of presynaptic GABA_A receptor activation are variable across synapses, from inhibition to enhancement of transmitter release. We find that presynaptic GABA_A receptor function is bidirectional at parallel fiber synapses depending on GABA concentration and modulation of GABA_A receptors in mice. Activation of GABA_A receptors by low GABA concentrations enhances glutamate release, whereas activation of receptors by higher GABA concentrations inhibits release. Furthermore, blocking GABA_B receptors reduces GABA_A receptor currents and shifts presynaptic responses toward greater enhancement of release across a wide range of GABA concentrations. Conversely, enhancing GABA_A receptor currents with ethanol or neurosteroids shifts responses toward greater inhibition of release. The ability of presynaptic GABA_A receptors to enhance or inhibit transmitter release at the same synapse depending on activity level provides a new mechanism for fine control of synaptic transmission by GABA and may explain conflicting reports of presynaptic GABA_A receptor function across synapses. NEW & NOTEWORTHY GABA_A receptors are widely expressed at presynaptic terminals in the central nervous system. However, previous reports have produced conflicting results on the function of these receptors at different synapses. We show that presynaptic GABA_A receptor function is strongly dependent on the level of receptor activation. Low levels of receptor activation enhance transmitter release, whereas higher levels of activation inhibit release at the same synapses. This provides a novel mechanism by which presynaptic GABA_A receptors fine-tune synaptic transmission.

Sodium Channel-Dependent and -Independent Mechanisms Underlying Axonal Afterdepolarization at Mouse Hippocampal Mossy Fibers

Action potentials propagating along axons are often followed by prolonged afterdepolarization (ADP) lasting for several tens of milliseconds. Axonal ADP is thought to be an important factor in modulating the fidelity of spike propagation during repetitive firings. However, the mechanism as well as the functional significance of axonal ADP remain unclear, partly due to inaccessibility to small structures of axon for direct electrophysiological recordings. Here, we examined the ionic and electrical mechanisms underlying axonal ADP using whole-bouton recording from mossy fiber terminals in mice hippocampal slices. ADP following axonal action potentials was strongly enhanced by focal application of veratridine, an inhibitor of Na⁺ channel inactivation. In contrast, tetrodotoxin (TTX) partly suppressed ADP, suggesting that a Na⁺ channel–dependent component is involved in axonal ADP. The remaining TTX-resistant Na⁺ channel–independent component represents slow capacitive discharge reflecting the shape and electrical properties of the axonal membrane. We also addressed the functional impact of axonal ADP on presynaptic function. In paired-pulse stimuli, we found that axonal ADP minimally affected the peak height of subsequent action potentials, although the rising phase of action potentials was slightly slowed, possibly due to steady-state inactivation of Na⁺ channels by prolonged depolarization. Voltage clamp analysis of Ca²⁺ current elicited by action potential waveform commands revealed that axonal ADP assists short-term facilitation of Ca²⁺ entry into the presynaptic terminals. Taken together, these data show that axonal ADP maintains reliable firing during repetitive stimuli and plays important roles in the fine-tuning of short-term plasticity of transmitter release by modulating Ca²⁺ entry into presynaptic terminals.

Targeting light-gated chloride channels to neuronal somatodendritic domain reduces their excitatory effect in the axon

Light-gated chloride channels are emerging as promising optogenetic tools for inhibition of neural activity. However, their effects depend on the transmembrane chloride electrochemical gradient and may be complex due to the heterogeneity of this gradient in different developmental stages, neuronal types, and subcellular compartments. Here we characterized a light-gated chloride channel, GtACR2, in mouse cortical neurons. We found that GtACR2 activation inhibited the soma, but unexpectedly depolarized the presynaptic terminals resulting in neurotransmitter release. Other light-gated chloride channels had similar effects. Reducing the chloride concentrations in the axon and presynaptic terminals diminished the GtACR2-induced neurotransmitter release, indicating an excitatory effect of chloride channels in these compartments. A novel hybrid somatodendritic targeting motif reduced the GtACR2-induced neurotransmitter release while enhancing the somatic photocurrents. Our results highlight the necessity of precisely determining the effects of light-gated chloride channels under specific experimental conditions and provide a much-improved light-gated chloride channel for optogenetic inhibition.

Signal propagation along the axon

Axons link distant brain regions and are usually considered as simple transmission cables in which reliable propagation occurs once an action potential has been generated. Safe propagation of action potentials relies on specific ion channel expression at strategic points of the axon such as nodes of Ranvier or axonal branch points. However, while action potentials are generally considered as the quantum of neuronal information, their signaling is not entirely digital. In fact, both their shape and their conduction speed have been shown to be modulated by activity, leading to regulations of synaptic latency and synaptic strength. We report here newly identified mechanisms of (1) safe spike propagation along the axon, (2) compartmentalization of action potential shape in the axon, (3) analog modulation of spike-evoked synaptic transmission and (4) alteration in conduction time after persistent regulation of axon morphology in central neurons. We discuss the contribution of these regulations in information processing.