Biology subjects

Brocard, F.

Publications and source records attributed to Brocard, F..

8 recordsLinked to original sources

bioRxiv2026

SK2/3 CHANNELS COUPLE WITH T-TYPE CA2+ CHANNELS TO GATE SPINAL LOCOMOTOR RHYTHM GENERATION

Initiating locomotion requires central pattern generator (CPG) interneurons to transition from tonic firing to persistent sodium current (INaP)-dependent bursting. While INaP provides the rhythmogenic drive, the conductances gating this transition remain unclear. Here, we show that functional coupling between small-conductance calcium-activated potassium (SK2/3) channels and low-threshold T-type Ca2+ channels, notably Cav3.2, gates locomotor rhythm generation. Pharmacological or genetic disruption of this SK-T-type axis triggers intrinsic bursting in Hb9 interneurons, a genetically identified rhythmogenic population of the locomotor CPG, and initiates fictive locomotion, whereas SK activation silences ongoing rhythmic output. Immunohistochemical co-expression of SK2, SK3 and Cav3.2 in Hb9 interneurons provides an anatomical basis for this functional coupling. Simulation-based inference further shows that, beyond this gating mechanism, burst diversity is primarily determined by the balance between INaP and M-type potassium conductances. Together, these findings identify SK-T-type coupling as a tunable brake on CPG activation, defining a biophysical module that controls the initiation and termination of locomotor rhythmic activity, with potential relevance for rhythmogenic circuits beyond locomotion.

neuroscience↗

bioRxiv2025

Ionic Mechanisms Underlying Bistability in Spinal Motoneurons: Insights from a Computational Model

Spinal motoneurons are the final output of spinal circuits that engage skeletal muscles to generate motor behaviors. Many motoneurons exhibit bistable behavior, alternating between a quiescent resting state and a self-sustained firing mode, classically attributed to plateau potentials driven by persistent inward currents. This intrinsic property is important for normal movement control, but can become dysregulated, causing motor function deficits, like spasticity. Here we use a conductance-based single-compartment model, together with mouse spinal slice recordings,to investigate the ionic interactions underlying motoneuron bistability. We show that synergistic interactions among high-voltage-activated L-type Ca2+ current (ICaL), calcium-induced calcium release (CICR) and the Ca2+-activated non-specific cation current (ICAN) constitute a minimal mechanistic core that produces plateau potentials and bistable firing. Within this framework, the persistent sodium current (INaP) promotes plateau generation, in contrast to the Ca2+-dependent K+ current (IKCa) which opposes it. These results delineate ionic dependencies at the level of interactions rather than spatial localisation and provide a tractable basis for interpreting altered motoneuron excitability in disease. Key PointsO_LIWe investigated how spinal motoneurons, critical for skeletal muscle control, exhibit bistability, switching between quiet and self-sustained firing. This property stabilizes motor functions like postural control, and its dysregulation contributes to disorders such as spasticity. Using a single-compartment computational model and mouse spinal slice recordings, we explored the ionic interactions driving bistability. C_LIO_LIOur findings reveal that a calcium-activated cation non-specific current and calcium-induced calcium release form a core mechanism supporting the plateau depolarization essential for bistable firing. Within this framework, the persistent sodium current facilitates plateau generation, while the calcium-dependent potassium current counteracts it. Pharmacological manipulations in slices yielded results consistent with these current roles. C_LIO_LIOur study delineates the ionic dependencies of motoneuron bistability based on interactions, not spatial location. This offers a concise framework for interpreting excitability changes observed in normal conditions and following spinal cord injury, providing valuable insights into motor function and neurological disorders. C_LI

neuroscience↗

bioRxiv2024

Functional contribution of astrocytic Kir4.1 channels to spasticity after spinal cord injury

Spasticity, a prevalent motor issue characterized by network hyperexcitability, causes pain and discomfort, with existing treatments offering limited relief. While past research has focused on neuronal factors, the role of astrocytes in spasticity has been overlooked. This study explores the potential of restoring astrocytic potassium (K+) uptake to reduce spasticity following SCI. Astrocytes buffer extracellular K+ via Kir4.1 channels, preventing neuronal hyperexcitability. Following spinal cord injury (SCI), Kir4.1 levels decrease at the injury site, though the consequences and mechanisms of this reduction within the motor output area have not been investigated. Utilizing advanced techniques, we demonstrate that lumbar astrocytes in a juvenile thoracic SCI mouse model switch to reactive phenotype, displaying morpho-functional and pro-inflammatory changes. These astrocytes also experience NBCe1-mediated intracellular acidosis, leading to Kir4.1 dysfunction and impaired K+ uptake. Enhancing Kir4.1 function reduces spasticity in SCI mice, revealing new therapeutic targets for neurological diseases associated with neuronal hyperexcitability. HighlightsO_LILumbar astrocytes adopt a reactive phenotype following a thoracic SCI C_LIO_LINBCe1-mediated acidosis in astrocytes disrupts Kir4.1 function post-SCI. C_LIO_LIImpaired K+ uptake leads to motoneuron hyperexcitability post-SCI. C_LIO_LIEnhanced astroglial Kir4.1 function reduces spastic-like symptoms in SCI mice. C_LI

neuroscience↗

bioRxiv2023

A size principle for bistability in mouse spinal motoneurons

Bistability in spinal motoneurons supports tonic spike activity in the absence of excitatory drive. Earlier work in adult preparations suggested that smaller motoneurons innervating slow antigravity muscle fibers are more likely to generate bistability for postural maintenance. However, whether large motoneurons innervating fast-fatigable muscle fibers display bistability related to postural tone is still controversial. To address this, we examined the relationship between soma size and bistability in lumbar ventrolateral -motoneurons of ChAT-GFP and Hb9-GFP mice across different developmental stages: neonatal (P2-P7), young (P7-P14) and mature (P21-P25). We found that as neuron size increases, the prevalence of bistability rises. Smaller -motoneurons lack bistability, while larger fast -motoneurons (MMP-9+/Hb9+) with a soma area [≥] 400{micro}m2 exhibit significantly higher bistability. Ionic currents associated with bistability, including the persistent Nav1.6 current, thermosensitive Trpm5 Ca2+-activated Na+ current and the slowly inactivating Kv1.2 current, also scale with cell size. Serotonin evokes full bistability in large motoneurons with partial bistable properties, but not in small motoneurons. Our study provides important insights into the neural mechanisms underlying bistability and how motoneuron size dictates this process. New and NoteworthyBistability is not a common feature of all mouse spinal motoneurons. It is absent in small, slow motoneurons but present in most large, fast motoneurons. This difference results from differential expression of ionic currents that enable bistability, which are highly expressed in large motoneurons but small or absent in small motoneurons. These results support a possible role for fast motoneurons in maintenance of tonic posture in addition to their known roles in fast movements.

neuroscience↗

bioRxiv2023

Persistent Nav1.6 current drives spinal locomotor functions through nonlinear dynamics

Persistent sodium current (INaP) in the spinal locomotor network promotes two distinct nonlinear firing patterns: a self-sustained spiking triggered by a brief excitation in bistable motoneurons and bursting oscillations in interneurons of the central pattern generator (CPG). Here, we identified the NaV channels responsible for INaP and their role in motor behaviors. We report the axonal Nav1.6 as the main molecular player for INaP in lumbar motoneurons. The motoneuronal inhibition of Nav1.6, but not of Nav1.1, impairs INaP, bistability, postural tone and locomotor performance. In interneurons of the CPG region, Nav1.6 with Nav1.1 equally mediate INaP and the inhibition of both channels is required to abolish oscillatory bursting activities and the locomotor rhythm. Overall, Nav1.6 plays a significant role both in posture and locomotion by governing INaP-dependent bistability in motoneurons and working in tandem with Nav1.1 to provide INaP-dependent rhythmogenic properties of the CPG.

neuroscience↗

bioRxiv2022

Astrocytes regulate locomotion by orchestrating neuronal rhythmicity in the spinal network via potassium clearance

SO_SCPLOWUMMARYC_SCPLOWNeuronal rhythmogenesis in the spinal cord is correlated with variations in extracellular K+ levels ([K+]e). Astrocytes play important role in[K+]e homeostasis and compute neuronal information. Yet it is unclear how neuronal oscillations are regulated by astrocytic K+ homeostasis. Here we identify the astrocytic inward-rectifying K+ channel Kir4.1 (a.k.a. Kcnj10) as a key molecular player for neuronal rhythmicity in the spinal central pattern generator (CPG). By combining two-photon calcium imaging with electrophysiology, immunohistochemistry and genetic tools, we report that astrocytes display Ca2+ transients before and during oscillations of neighbouring neurons. Inhibition of astrocytic Ca2+ transients with BAPTA decreases the barium-sensitive Kir4.1 current responsible of K+ clearance. Finally, we show in mice that Kir4.1 knockdown in astrocytes progressively prevents neuronal oscillations and alters the locomotor pattern resulting in lower motor performances in challenging tasks. These data identify astroglial Kir4.1 channels as key regulators of neuronal rhythmogenesis in the CPG driving locomotion. SO_SCPLOWIGNIFICANCEC_SCPLOWO_SCPCAP C_SCPCAPO_SCPLOWSTATEMENTC_SCPLOWDespite decades of research, the cellular mechanisms responsible of the synchronized rhythmic oscillations driving locomotion remain elusive. To gain insight into the function of the spinal locomotor network, numerous studies have characterized diverse classes of locomotor-related neurons to determine their role in generating rhythmic movements during locomotion. In contrast, studies investigating non-neuronal components of the spinal cord are sparse. Our study represents a significant breakthrough by identifying astrocytic K+ uptake as a key regulator of neuronal rhythmicity synchronization and locomotor pattern at the cellular, microcircuit and system levels. These data provide mechanistic insights into the neuroglial dialogue at play during rhythmogenesis and point to a novel astroglial target for restoring normal neuronal network excitability in brain disorders and neurodegenerative diseases.

neuroscience↗

bioRxiv2021

Altered action potential waveform and shorter axonal initial segment in hiPSC-derived motor neurons with mutations in VRK1

We recently described new pathogenic variants in VRK1, in patients affected with distal Hereditary Motor Neuropathy associated with upper motor neurons signs. Specifically, we provided evidences that hiPSC-derived Motor Neurons (hiPSC-MN) from these patients display Cajal bodies (CBs) disassembly and defects in neurite outgrowth and branching. We here focused on the Axonal Initial Segment (AIS) and the related firing properties of hiPSC-MNs from these patients. We found that the patients Action Potential (AP) was smaller in amplitude, larger in duration, and displayed a more depolarized threshold while the firing patterns were not altered. These alterations were accompanied by a decrease in the AIS length measured in patients hiPSC-MNs. These data indicate that mutations in VRK1 impact the AP waveform and the AIS organization in MNs and may ultimately lead to the related motor neuron disease. HighlightsO_LIhiPSC-MNs are functional and sustain firing patterns, typical of spinal MNs C_LIO_LIhiPSC-MNs from patients with VRK1 mutations have altered Action Potential C_LIO_LIAxonal Initial Segment is shorter in hiPSC-MNs from patients with mutated VRK1 C_LIO_LIhiPSC-MNs are a useful platform to study Motor Inherited Peripheral Neuropathies C_LI eTOC BlurbIn human spinal Motor Neurons derived from induced Pluripotent Stem Cells from patients with VRK1 -related distal Hereditary Motor Neuropathy, Bos, Rihan et al. show that the mutations in VRK1 affect the electrical properties of these neurons: they display defects in the initiation of the Action Potential due to a shortening of the Axonal Initial Segment.

neuroscience↗

bioRxiv2020

It takes two to tango: M-current swings with the persistent sodium current to set the speed of locomotion.

The central pattern generator (CPG) for locomotion is set of pacemaker neurons endowed with inherent bursting driven by the persistent sodium current (INaP). How they proceed to regulate the locomotor rhythm remained unknown. Here, in neonatal rodents, we identified a persistent potassium current, critical in regulating pacemakers and locomotion speed. This current recapitulates features of the M-current (IM); a subthreshold non-inactivating outward current blocked by XE991 and enhanced by ICA73. Immunostaining and mutant mice highlight an important role of axonal Kv7.2 channels in mediating IM. Pharmacological modulation of IM regulates the emergence and the frequency regime of both pacemaker and CPG activities, and controls the speed of locomotion. Computational models captured these results and show howed an interplay between IM and INaP that endows the locomotor CPG with rhythmogenic properties. Overall, this study provides fundamental insights into how IM and INaP work in tandem to set the speed of locomotion.

neuroscience↗