Michaël Russier

Impact of neonatal asphyxia and hind limb immobilization on musculoskeletal tissues and S1 map organization: implications for cerebral palsy.

Cerebral palsy (CP) is a complex disorder of locomotion, posture and movements resulting from pre-, peri- or postnatal damage to the developing brain. In a previous study (Strata, F., Coq, J.O., Byl, N.N., Merzenich, M.M., 2004. Comparison between sensorimotor restriction and anoxia on gait and motor cortex organization: implications for a rodent model of cerebral palsy. Neuroscience 129, 141-156.), CP-like movement disorders were more reliably reproduced in rats by hind limb sensorimotor restriction (disuse) during development rather than perinatal asphyxia (PA). To gain new insights into the underpinning mechanisms of CP symptoms we investigated the long-term effects of PA and disuse on the hind limb musculoskeletal histology and topographical organization in the primary somatosensory cortex (S1) of adult rats. Developmental disuse (i.e. hind limb immobilization) associated with PA induced muscle fiber atrophy, extracellular matrix changes in the muscle, and mild to moderate ankle and knee joint degeneration at levels greater than disuse alone. Sensorimotor restricted rats with or without PA exhibited a topographical disorganization of the S1 cortical hind limb representation with abnormally large, multiple and overlapping receptive fields. This disorganization was enhanced when disuse and PA were associated. Altered cortical neuronal properties included increased cortical responsiveness and a decrease in neuronal selectivity to afferent inputs. These data support previous observations that asphyxia per se can generate the substrate for peripheral tissue and brain damage, which are worsened by aberrant sensorimotor experience during maturation, and could explain the disabling movement disorders observed in children with CP.

Peripheral and central changes combine to induce motor behavioral deficits in a moderate repetition task

Repetitive motion disorders, such as carpal tunnel syndrome and focal hand dystonia, can be associated with tasks that require prolonged, repetitive behaviors. Previous studies using animal models of repetitive motion have correlated cortical neuroplastic changes or peripheral tissue inflammation with fine motor performance. However, the possibility that both peripheral and central mechanisms coexist with altered motor performance has not been studied. In this study, we investigated the relationship between motor behavior changes associated with repetitive behaviors and both peripheral tissue inflammation and cortical neuroplasticity. A rat model of reaching and grasping involving moderate repetitive reaching with negligible force (MRNF) was used. Rats performed the MRNF task for 2 h/day, 3 days/week for 8 weeks. Reach performance was monitored by measuring reach rate/success, daily exposure, reach movement reversals/patterns, reach/grasp phase times, grip strength and grooming function. With cumulative task exposure, reach performance, grip strength and agility declined while an inefficient food retrieval pattern increased. In S1 of MRNF rats, a dramatic disorganization of the topographic forepaw representation was observed, including the emergence of large receptive fields located on both the wrist/forearm and forepaw with alterations of neuronal properties. In M1, there was a drastic enlargement of the overall forepaw map area, and of the cortex devoted to digit, arm-digits and elbow-wrist responses. In addition, unusually low current amplitude evoked digit movements. IL-1 beta and TNF-alpha increased in forearm flexor muscles and tendons of MRNF animals. The increases in IL-1 beta and TNF-alpha negatively correlated with grip strength and amount of current needed to evoke forelimb movements. This study provides strong evidence that both peripheral inflammation and cortical neuroplasticity jointly contribute to the development of chronic repetitive motion disorders.

Impact of prenatal ischemia on behavior, cognitive abilities and neuroanatomy in adult rats with white matter damage

Early brain damage, such as white matter damage (WMD), resulting from perinatal hypoxia-ischemia in preterm and low birth weight infants represents a high risk factor for mortality and chronic disabilities, including sensory, motor, behavioral and cognitive disorders. In previous studies, we developed a model of WMD based on prenatal ischemia (PI), induced by unilateral ligation of uterine artery at E17 in pregnant rats. We have shown that PI reproduced some of the main deficits observed in preterm infants, such as white and gray matter damage, myelination deficits, locomotor, sensorimotor, and short-term memory impairments, as well as related musculoskeletal and neuroanatomical histopathologies [1-3]. Here, we determined the deleterious impact of PI on several behavioral and cognitive abilities in adult rats, as well as on the neuroanatomical substratum in various related brain areas. Adult PI rats exhibited spontaneous exploratory and motor hyperactivity, deficits in information encoding, and deficits in short- and long-term object memory tasks, but no impairments in spatial learning or working memory in watermaze tasks. These results were in accordance with white matter injury and damage in the medial and lateral entorhinal cortices, as detected by axonal degeneration, astrogliosis and neuronal density. Although there was astrogliosis and axonal degeneration in the fornix, hippocampus and cingulate cortex, neuronal density in the hippocampus and cingulate cortex was not affected by PI. Levels of spontaneous hyperactivity, deficits in object memory tasks, neuronal density in the medial and lateral entorhinal cortices, and astrogliosis in the fornix correlated with birth weight in PI rats. Thus, this rodent model of WMD based on PI appears to recapitulate the main neurobehavioral and neuroanatomical human deficits often observed in preterm children with a perinatal history of ischemia.

Neuroanatomical, sensorimotor and cognitive deficits in adult rats with white matter injury following prenatal ischemia

Perinatal brain injury including white matter damage (WMD) is highly related to sensory, motor or cognitive impairments in humans born prematurely. Our aim was to examine the neuroanatomical, functional and behavioral changes in adult rats that experienced prenatal ischemia (PI), thereby inducing WMD. PI was induced by unilateral uterine artery ligation at E17 in pregnant rats. We assessed performances in gait, cognitive abilities and topographical organization of maps, and neuronal and glial density in primary motor and somatosensory cortices, the hippocampus and prefrontal cortex, as well as axonal degeneration and astrogliosis in white matter tracts. We found WMD in corpus callosum and brainstem, and associated with the hippocampus and somatosensory cortex, but not the motor cortex after PI. PI rats exhibited mild locomotor impairments associated with minor signs of spasticity. Motor map organization and neuronal density were normal in PI rats, contrasting with major somatosensory map disorganization, reduced neuronal density, and a marked reduction of inhibitory interneurons. PI rats exhibited spontaneous hyperactivity in open‐field test and short‐term memory deficits associated with abnormal neuronal density in related brain areas. Thus, this model reproduces in adult PI rats the main deficits observed in infants with a perinatal history of hypoxia‐ischemia and WMD.

Mild musculoskeletal and locomotor alterations in adult rats with white matter injury following prenatal ischemia

Early brain injury including white matter damage (WMD) appears strongly correlated to perinatal hypoxia‐ischemia and adverse neurological outcomes in preterm survivors. Indeed, WMD has been widely associated with subtle to major motor disturbances, sensory, behavioral and cognitive impairments in preterm infants who afterward develop cerebral palsy (CP). Prenatal ischemia (PI) has been shown to reproduce the main features of WMD observed in preterm infants. The present study was aimed at determining in adult rats the impact of PI on brain axons, musculoskeletal histology and locomotor activity. PI was induced by unilateral intrauterine artery ligation at E17 in pregnant rats. We found axonal degeneration and reactive astrogliosis in several white matter regions of adult PI rats. We found mild myopathic and secondary joint changes, including increased variability in myofiber size in several hind limb muscles, decreased myofibers numbers but increased Pax 7 cells and myofiber size in the gastrocnemius, and mild knee and ankle chondromalacia. Although treadmill locomotion appeared normal, several kinematic parameters, such as stride length, amplitude, velocity and leg joint angles were altered in adult PI rats compared to shams. Using intra‐ and inter‐group variability of kinematic parameters, PI seemed to impair the maturation of locomotion on the treadmill. In addition, PI rats exhibited spontaneous hyperactivity in open‐field test. Musculoskeletal changes appeared concomitant with mild impairments in gait and posture. Our rodent model of WMD based on PI reproduces the mild motor deficits and musculoskeletal changes observed in many preterm infants with a perinatal history of hypoxia‐ischemia, and contributes towards a better understanding of the interplay between brain injury, musculoskeletal histopathology and gait disturbances encountered subsequently.

LGI1 tunes intrinsic excitability by regulating the density of axonal Kv1 channels

Significance Leucine-rich glioma-inactivated 1 (LGI1) is a secreted neuronal protein associated in a multiprotein complex with Kv1 channels. Natural mutations in this protein are found in the early-adulthood–onset disorder “autosomal dominant epilepsy with auditory features” and animal models with Lgi1 genetic deletion display spontaneous seizures. We investigated the potential link between LGI1 depletion and intrinsic neuronal excitability. We found that LGI1 determines neuronal excitability in CA3 pyramidal neurons through the control of axonal Kv1-channel expression. Genetic deletion of Lgi1 induces a robust diminution in Kv1 channels, especially in axons where they colocalize with LGI1, very likely contributing to epileptogenesis. These results strongly suggest that LGI1 controls neuronal excitability by regulating Kv1-channel expression. Autosomal dominant epilepsy with auditory features results from mutations in leucine-rich glioma-inactivated 1 (LGI1), a soluble glycoprotein secreted by neurons. Animal models of LGI1 depletion display spontaneous seizures, however, the function of LGI1 and the mechanisms by which deficiency leads to epilepsy are unknown. We investigated the effects of pure recombinant LGI1 and genetic depletion on intrinsic excitability, in the absence of synaptic input, in hippocampal CA3 neurons, a classical focus for epileptogenesis. Our data indicate that LGI1 is expressed at the axonal initial segment and regulates action potential firing by setting the density of the axonal Kv1.1 channels that underlie dendrotoxin-sensitive D-type potassium current. LGI1 deficiency incurs a >50% down-regulation of the expression of Kv1.1 and Kv1.2 via a posttranscriptional mechanism, resulting in a reduction in the capacity of axonal D-type current to limit glutamate release, thus contributing to epileptogenesis.

Early movement restriction leads to enduring disorders in muscle and locomotion: Enduring impact of sensorimotor restriction

Motor control and body representation in the central nervous system (CNS) as well as musculoskeletal architecture and physiology are shaped during development by sensorimotor experience and feedback, but the emergence of locomotor disorders during maturation and their persistence over time remain a matter of debate in the absence of brain damage. By using transient immobilization of the hind limbs, we investigated the enduring impact of postnatal sensorimotor restriction (SMR) on gait and posture on treadmill, age‐related changes in locomotion, musculoskeletal histopathology and Hoffmann reflex in adult rats without brain damage. SMR degrades most gait parameters and induces overextended knees and ankles, leading to digitigrade locomotion that resembles equinus. Based on variations in gait parameters, SMR appears to alter age‐dependent plasticity of treadmill locomotion. SMR also leads to small but significantly decreased tibial bone length, chondromalacia, degenerative changes in the knee joint, gastrocnemius myofiber atrophy and muscle hyperreflexia, suggestive of spasticity. We showed that reduced and atypical patterns of motor outputs, and somatosensory inputs and feedback to the immature CNS, even in the absence of perinatal brain damage, play a pivotal role in the emergence of movement disorders and musculoskeletal pathologies, and in their persistence over time. Understanding how atypical sensorimotor development likely contributes to these degradations may guide effective rehabilitation treatments in children with either acquired (ie, with brain damage) or developmental (ie, without brain injury) motor disabilities.

How do electrical synapses regulate their strength

Neuronal communication in the central nervous system is ensured by synapses through which neuronal events can be transmitted from one cell to the next. Classically, two major classes of synapses can be distinguished: (i) chemical synapses that use a chemical transmitter to activate or inhibit the postsynaptic neuron and (ii) electrical synapses that transmit information to the next cell by passive transmission of voltage in an analogue way (i.e. they do not require an action potential). While the basic function and plasticity of chemical synapses is relatively well established today, much less is known about the mechanism of activity-dependent plasticity at electrical synapses. Electrical synapses connect two adjacent neurons through intercellular channels that form gap junctions. They are widely expressed in the central nervous system of mammals and are particularly abundant in inhibitory interneurons (Pereda, 2014). Unlike chemical synapses, electrical synapses are bidirectional, reliable and conduct almost instantaneously. Functionally, electrical synapses are involved in many features of the network activity. Because of their ohmic nature, they can transmit excitation as well as inhibition, and it has been shown that they are involved in synchronous oscillatory activity. The thalamic reticular nucleus (TRN) contains a homogeneous population of parvalbumin-positive γ-aminobutyric acid (GABA)-releasing neurons that surround the dorsal thalamus. TRN neurons inhibit thalamic relay cells and thus control the switch of their discharge from bursting to tonic mode occurring during the transition from sleep to wakefulness. These interneurons communicate essentially through gap junctions constituted of connexin36 (Cx36), the main connexin found in the mammalian brain. Long-term depression of electrical coupling (eLTD) at electrical synapses in the TRN has been shown to occur when afferent cortical input to electrically coupled neurons is tetanized at 100 Hz (Landisman & Connors, 2005). This eLTD is mediated by activation of the metabotropic glutamate receptor (mGluR) and it can be induced by the sole stimulation of group I mGluR (Wang et al. 2015). Alternatively, eLTD can be induced at TRN electrical synapses with a physiological protocol based on the synchronous activation of both neurons (Haas et al. 2011). Functionally, modification of electrical coupling in TRN neurons is thought to modulate temporal and spatial transmission of information within the thalamo-cortical system. Today, the mechanisms underlying both forms of eLTD remain unclear. In this issue of The Journal of Physiology, Sevetson and co-workers addressed this important issue by showing that, while mGluR-dependent eLTD and burst-induced eLTD occlude each other, Ca2+ entry through T-type calcium channels is required for the induction of burst-induced eLTD but not for mGluR-dependent eLTD (Sevetson et al. 2017). In fact, while the Ca2+ chelator BAPTA or the T-type calcium channel antagonist TTA-A2 blocked burst-induced eLTD, these compounds were found to have no effect on mGluR-dependent eLTD. Induction of burst-induced eLTD was blocked by caffeine or ryanodine, indicating that Ca2+ influx recruits intracellular pools of Ca2+. Furthermore, blocking activation of the calcium-activated protein phosphatase calcineurin with FK-506 or cyclosporin A occluded induction of burst-induced eLTD. This paper is important because it opens several interesting perspectives. First, the findings reported in Sevetson et al. (2017) indicate that there are at least two forms of eLTD at TRN electrical synapses that are independent in both their induction and expression mechanisms. While mGluR-dependent eLTD corresponds to a global phenomenon in which glutamatergic stimulation affects many neighbouring electrical synapses, the burst-induced eLTD is initiated by activity in pairs of coupled neurons and may thus correspond to a more local phenomenon. Second, the involvement of calcineurin in eLTD suggests that other forms of plasticity at electrical synapses might be induced. In fact, calcineurin controls activity of calcium/calmodulin-dependent protein kinase II (CaMKII), which is involved in NMDA receptor-dependent long-term synaptic plasticity (long-term potentiation; LTP) in cortical chemical synapses but also in some forms of LTP of electrical coupling (eLTP) (Pereda, 2014; Turecek et al. 2014). Moreover, Cx36, interacts with and is phosphorylated by CaMKII in a way similar to CaMKII interaction with glutamate receptors. eLTP has been shown to be induced in TRN neurons following stimulation of group II metabotropic glutamate receptors (Wang et al. 2015). However, no physiological induction of mGluR-dependent eLTP has been described so far nor the conditions to induce burst-dependent eLTP. At chemical synapses, the degree of synchrony between preand postsynaptic activity determines the polarity of synaptic modification (Debanne et al. 1994). Since eLTD is induced by synchronous bursting activity in electrically coupled neurons, it is tempting to suggest, by analogy with what we know about plasticity at chemical synapses, that eLTP might be induced by asynchronous bursting in weakly coupled TRN neurons. To complete the library of plasticity rules at electrical synapses, one must also explore the existence of homeostatic plasticity (i.e. compensatory plastic changes to maintain the global network activity constant) at electrical synapses. There is no doubt that sooner or later both induction mechanisms of physiological eLTP and induction mechanisms of homeostatic plasticity will be elucidated at electrical synapses.

Plasticity of intrinsic neuronal excitability

Long-term synaptic modification is not the exclusive mode of memory storage, and persistent regulation of voltage-gated ion channels also participates in memory formation. Intrinsic plasticity is expressed in virtually all neuronal types including principal cells and interneurons. Activation of synaptic glutamate receptors initiates long-lasting changes in neuronal excitability at presynaptic and postsynaptic side. As synaptic plasticity, intrinsic plasticity is bi-directional and expresses a certain level of input-specificity or cell-specificity. We discuss here the nature of the learning rules shared by intrinsic and synaptic plasticity and the impact of intrinsic plasticity on temporal processing.

PERSPECTIVES: Axonal propagation: does the spike stop here?

Nerve cells in the brain are characterized by a complex axon which may project locally and distally to several thousands of postsynaptic targets. The main function of the axon is to conduct the presynaptic action potential from the cell body to the nerve terminals, thus allowing a spread of activity over long distances in the brain (Fig. 1). Axonal propagation is driven by sodium channels and prevented by activation of potassium channels. Thus, the axon is generally considered to be a very reliable transmission cable in which stable propagation occurs once an action potential is generated. However, the high divergence of the axonal tree (i.e. the presence of branch points) and the presence of small varicosities along axons are parameters that decrease the safety factor for axonal conduction. Several theoretical studies have highlighted the role of these local perturbations in axonal geometry on propagation (Goldstein & Rall, 1974). In addition, the biophysical properties of ion channels may confer some filtering properties to the axon (Debanne et al. 1997; Kopysova & Debanne, 1998). However, direct experimental assay of axonal propagation is difficult because the small diameter of the axon prevents electrophysiological recording beyond the axonal initial segment. More recently, studies using optical imaging of calcium transients associated with spike propagation have indicated a high fidelity of axonal transmission (Mackenzie et al. 1996; Cox et al. 2000; Koester & Sakmann, 2000). These sophisticated methods may still, however, overestimate propagation, and the consequences of calcium buffering by the fluorescent probe have not yet been precisely evaluated.