Early Movement Restriction Affects FNDC5/Irisin and BDNF Levels in Rat Muscle and Brain.

Interaction with the environment appears necessary for the maturation of sensorimotor and cognitive functions in early life. In rats, a model of sensorimotor restriction (SMR) from postnatal day 1 (P1) to P28 has shown that low and atypical sensorimotor activities induced the perturbation of motor behavior due to muscle weakness and the functional disorganization of the primary somatosensory and motor cortices. In the present study, our objective was to understand how SMR affects the muscle-brain dialogue. We focused on irisin, a myokine secreted by skeletal muscles in response to exercise. FNDC5/irisin expression was determined in hindlimb muscles and brain structures by Western blotting, and irisin expression in blood and cerebrospinal fluid was determined using an ELISA assay at P8, P15, P21 and P28. Since irisin is known to regulate its expression, Brain-Derived Neurotrophic Factor (BDNF) levels were also measured in the same brain structures. We demonstrated that SMR increases FNDC5/irisin levels specifically in the soleus muscle (from P21) and also affects this protein expression in several brain structures (as early as P15). The BDNF level was increased in the hippocampus at P8. To conclude, SMR affects FNDC5/irisin levels in a postural muscle and in several brain regions and has limited effects on BDNF expression in the brain.

Early movement restriction affects the acquisition of neurodevelopmental reflexes in rat pups.

Childhood is a period of construction of the organism, during which interactions with the environment and regular physical activity are necessary for the maturation of the neuronal networks. An atypical sensorimotor activity during childhood (due to bed-rest or neurodevelopmental disorders) impacts the development of the neuromuscular system. A model of sensorimotor restriction (SMR) developed in rats has shown that casting pups' hind limbs from postnatal day 1 (P1) to P28 induced a severe perturbation of motor behavior, due to muscle weakness as well as disturbances within the central nervous system. In the present study, our objective was to determine whether SMR affects the early postnatal ontogenesis. We explored the neuromuscular development through the determination of the age for achievement of the main neurodevelopmental reflexes, which represent reliable indicators of neurological and behavioral development. We also evaluated the maturation of postural control. Our results demonstrate that SMR induces a delay in the motor development, illustrated by a several days delay in the acquisition of a mature posture and in the acquisition reflexes: hind limb grasping, righting, hind limb placing, cliff avoidance, negative geotaxis. In conclusion, impaired physical activity and low interactions with environment during early development result in altered maturation of the nervous system.

Early movement restriction deteriorates motor function and soleus muscle physiology.

Children with low physical activity and interactions with environment experience atypical sensorimotor development and maturation leading to anatomical and functional disorganization of the sensorimotor circuitry and also to enduring altered motor function. Previous data have shown that postnatal movement restriction in rats results in locomotor disturbances, functional disorganization and hyperexcitability of the hind limb representations in the somatosensory and motor cortices, without apparent brain damage. Due to the reciprocal interplay between the nervous system and muscle, it is difficult to determine whether muscle alteration is the cause or the result of the altered sensorimotor behavior (Canu et al., 2019). In the present paper, our objectives were to evaluate the impact of early movement restriction leading to sensorimotor restriction (SMR) during development on the postural soleus muscle and on sensorimotor performance in rats, and to determine whether changes were reversed when typical activity was resumed. Rats were submitted to SMR by hind limb immobilization for 16 h / day from birth to postnatal day 28 (PND28). In situ isometric contractile properties of soleus muscle, fiber cross sectional area (CSA) and myosin heavy chain content (MHC) were studied at PND28 and PND60. In addition, the motor function was evaluated weekly from PND28 to PND60. At PND28, SMR rats presented a severe atrophy of soleus muscle, a decrease in CSA and a force loss. The muscle maturation appeared delayed, with persistence of neonatal forms of MHC. Changes in kinetic properties were moderate or absent. The Hoffmann reflex provided evidence for spinal hyperreflexia and signs of spasticity. Most changes were reversed at PND60, except muscle atrophy. Functional motor tests that require a good limb coordination, i.e. rotarod and locomotion, showed an enduring alteration related to SMR, even after one month of 'typical' activity. On the other hand, paw withdrawal test and grip test were poorly affected by SMR whereas spontaneous locomotor activity increased over time. Our results support the idea that proprioceptive feedback is at least as important as the amount of motor activity to promote a typical development of motor function. A better knowledge of the interplay between hypoactivity, muscle properties and central motor commands may offer therapeutic perspectives for children suffering from neurodevelopmental disorders.

Sensorimotor Perturbation Induces Late and Transient Molecular Synaptic Proteins Activation and Expression Changes.

Plasticity of the cerebral cortex following a modification of the sensorimotor experience takes place in several steps that can last from few hours to several months. Among the mechanisms involved in the dynamic modulation of the cerebral cortex in adults, it is commonly proposed that short-term plasticity reflects changes in synaptic connections. Here, we were interested in the time-course of synaptic plasticity taking place in the somatosensory primary cortex all along a 14-day period of sensorimotor perturbation (SMP), as well as during a recovery phase up to 24 h. Activation and expression level of pre- (synapsin 1, synaptophysin, synaptotagmin 1) and postsynaptic (AMPA and NMDA receptors) proteins, postsynaptic density scaffold proteins (PSD-95 and Shank2), and cytoskeletal proteins (neurofilaments-L and M, ß3-tubulin, synaptopodin, N-cadherin) were determined in cortical tissue enriched in synaptic proteins. During the SMP period, most changes were observed as soon as D7 in the presynaptic compartment and were followed, at D14, by changes in the postsynaptic compartment. These changes persisted at least until 24 h of recovery. Proteins involved in synapse structure (scaffolding, adhesion, cytoskeletal) were mildly affected and almost exclusively at D14. We concluded that experience-dependent reorganization of somatotopic cortical maps is accompanied by changes in synaptic transmission with a very close time-course.

Optimization of 2-DE and multiplexed detection of O-GlcNAcome, phosphoproteome and whole proteome protocol of synapse-associated proteins within the rat sensorimotor cortex.

BACKGROUND: Several studies have shown the importance of phosphorylation, O-GlcNAcylation and their interplay in neuronal processes. NEW METHOD: To get understanding about molecular mechanisms of synaptic plasticity, we performed a preparation of synaptic protein-enriched fraction on a small sample of rat sensorimotor cortex. We then optimized a multiplexed proteomic strategy to detect O-GlcNAcylated proteins, phosphoproteins, and the whole proteome within the same bidimensional gel. We compared different protocols (solubilisation buffer, reticulation and composition of the gel, migration buffer) to optimize separating conditions for 2D-gel electrophoresis of synaptic proteins. The O-GlcNAcome was revealed using Click chemistry and the azide-alkyne cycloaddition of a fluorophore on O-GlcNAc moieties. The phosphoproteome was detected by Phospho-Tag staining, while the whole proteome was visualized through SYPRORuby staining. RESULTS: This method permitted, after sequential image acquisition, the direct in-gel detection of O-GlcNAcome, phosphoproteome, and whole proteome of synapse-associated proteins. CONCLUSION: This original method of differential proteomic analysis will permit to identify key markers of synaptic plasticity that are O-GlcNAcylated and/or phosphorylated, and their molecular regulations in neuronal processes.

From cerebral palsy to developmental coordination disorder: Development of preclinical rat models corresponding to recent epidemiological changes.

Cerebral palsy (CP) is a complex syndrome of various sensory, motor and cognitive deficits. Its prevalence has recently decreased in some developed countries and its symptoms have also shifted since the 1960s. From the 1990s, CP has been associated with prematurity, but recent epidemiologic studies show reduced or absent brain damage, which recapitulates developmental coordination disorder (DCD). In previous studies, we developed a rat model based on mild intrauterine hypoperfusion (MIUH) that recapitulated the diversity of symptoms observed in preterm survivors. Briefly, MIUH led to early inflammatory processes, diffuse brain damage, minor locomotor deficits, musculoskeletal pathologies, neuroanatomical and functional disorganization of the primary somatosensory (S1) cortex but not in the motor cortex (M1), delayed sensorimotor reflexes, spontaneous hyperactivity, deficits in sensory information processing, and memory and learning impairments in adult rats. Adult MIUH rats also exhibited changes in muscle contractile properties and phenotype, enduring hyperreflexia and spasticity, as well as hyperexcitability in the sensorimotor cortex. We recently developed a rat model of DCD based on postnatal sensorimotor restriction (SMR) without brain damage. Briefly, SMR led to digitigrade locomotion (i.e., "toe walking") related to ankle-knee overextension, degraded musculoskeletal tissues (e.g., gastrocnemius atrophy), and lumbar hyperreflexia. The postnatal SMR then led to secondary degradation of the hind-limb maps in S1 and M1 cortices, altered cortical response properties and cortical hyperexcitability, but no brain damage. Thus, our 2 rat models appear to recapitulate the diversity of symptoms ranging from CP to DCD and contribute to understanding the emergence and mechanisms underlying the corresponding neurodevelopmental disorders. These preclinical models seem promising for testing strategies of rehabilitation based on both physical and cognitive training to promote adaptive brain plasticity and to improve physical body conditions.

Interplay between hypoactivity, muscle properties and motor command: How to escape the vicious deconditioning circle?

Activity-dependent processes addressing the central nervous system (CNS) and musculoskeletal structures are critical for maintaining motor performance. Chronic reduction in activity, whether due to a sedentary lifestyle or extended bed rest, results in impaired performance in motor tasks and thus decreased quality of life. In the first part of this paper, we give a narrative review of the effects of hypoactivity on the neuromuscular system and behavioral outcomes. Motor impairments arise from a combination of factors including altered muscle properties, impaired afferent input, and plastic changes in neural structure and function throughout the nervous system. There is a reciprocal interplay between the CNS and muscle properties, and these sensorimotor loops are essential for controlling posture and movement. As a result, patients under hypoactivity experience a self-perpetuating cycle, in with sedentarity leading to decreased motor activity and thus a progressive worsening of a situation, and finally deconditioning. Various rehabilitation strategies have been studied to slow down or reverse muscle alteration and altered motor performance. In the second part of the paper, we review representative protocols directed toward the muscle, the sensory input and/or the cerebral cortex. Improving an understanding of the loss of motor function under conditions of disuse (such as extended bed rest) as well as identifying means to slow this decline may lead to therapeutic strategies to preserve quality of life for a range of individuals. The most efficient strategies seem multifactorial, using a combination of approaches targeting different levels of the neuromuscular system.

Early movement restriction leads to maladaptive plasticity in the sensorimotor cortex and to movement disorders.

Motor control and body representations in the central nervous system are built, i.e., patterned, during development by sensorimotor experience and somatosensory feedback/reafference. Yet, early emergence of locomotor disorders remains a matter of debate, especially in the absence of brain damage. For instance, children with developmental coordination disorders (DCD) display deficits in planning, executing and controlling movements, concomitant with deficits in executive functions. Thus, are early sensorimotor atypicalities at the origin of long-lasting abnormal development of brain anatomy and functions? We hypothesize that degraded locomotor outcomes in adulthood originate as a consequence of early atypical sensorimotor experiences that induce developmental disorganization of sensorimotor circuitry. We showed recently that postnatal sensorimotor restriction (SMR), through hind limb immobilization from birth to one month, led to enduring digitigrade locomotion with ankle-knee overextension, degraded musculoskeletal tissues (e.g., gastrocnemius atrophy), and clear signs of spinal hyperreflexia in adult rats, suggestive of spasticity; each individual disorder likely interplaying in self-perpetuating cycles. In the present study, we investigated the impact of postnatal SMR on the anatomical and functional organization of hind limb representations in the sensorimotor cortex and processes representative of maladaptive neuroplasticity. We found that 28 days of daily SMR degraded the topographical organization of somatosensory hind limb maps, reduced both somatosensory and motor map areas devoted to the hind limb representation and altered neuronal response properties in the sensorimotor cortex several weeks after the cessation of SMR. We found no neuroanatomical histopathology in hind limb sensorimotor cortex, yet increased glutamatergic neurotransmission that matched clear signs of spasticity and hyperexcitability in the adult lumbar spinal network. Thus, even in the absence of a brain insult, movement disorders and brain dysfunction can emerge as a consequence of reduced and atypical patterns of motor outputs and somatosensory feedback that induce maladaptive neuroplasticity. Our results may contribute to understanding the inception and mechanisms underlying neurodevelopmental disorders, such as DCD.

Mild Intrauterine Hypoperfusion Leads to Lumbar and Cortical Hyperexcitability, Spasticity, and Muscle Dysfunctions in Rats: Implications for Prematurity.

Intrauterine ischemia-hypoxia is detrimental to the developing brain and leads to white matter injury (WMI), encephalopathy of prematurity (EP), and often to cerebral palsy (CP), but the related pathophysiological mechanisms remain unclear. In prior studies, we used mild intrauterine hypoperfusion (MIUH) in rats to successfully reproduce the diversity of clinical signs of EP, and some CP symptoms. Briefly, MIUH led to inflammatory processes, diffuse gray and WMI, minor locomotor deficits, musculoskeletal pathologies, neuroanatomical and functional disorganization of the primary somatosensory and motor cortices, delayed sensorimotor reflexes, spontaneous hyperactivity, deficits in sensory information processing, memory and learning impairments. In the present study, we investigated the early and long-lasting mechanisms of pathophysiology that may be responsible for the various symptoms induced by MIUH. We found early hyperreflexia, spasticity and reduced expression of KCC2 (a chloride cotransporter that regulates chloride homeostasis and cell excitability). Adult MIUH rats exhibited changes in muscle contractile properties and phenotype, enduring hyperreflexia and spasticity, as well as hyperexcitability in the sensorimotor cortex. Taken together, these results show that reduced expression of KCC2, lumbar hyperreflexia, spasticity, altered properties of the soleus muscle, as well as cortical hyperexcitability may likely interplay into a self-perpetuating cycle, leading to the emergence, and persistence of neurodevelopmental disorders (NDD) in EP and CP, such as sensorimotor impairments, and probably hyperactivity, attention, and learning disorders.

Synaptic protein changes after a chronic period of sensorimotor perturbation in adult rats: a potential role of phosphorylation/O-GlcNAcylation interplay.

In human, a chronic sensorimotor perturbation (SMP) through prolonged body immobilization alters motor task performance through a combination of peripheral and central factors. Studies performed on a rat model of SMP have shown biomolecular changes and a reorganization of sensorimotor cortex through events such as morphological modifications of dendritic spines (number, length, functionality). However, underlying mechanisms are still unclear. It is well known that phosphorylation regulates a wide field of synaptic activity leading to neuroplasticity. Another post-translational modification that interplays with phosphorylation is O-GlcNAcylation. This atypical glycosylation, reversible, and dynamic, is involved in essential cellular and physiological processes such as synaptic activity, neuronal morphogenesis, learning, and memory. We examined potential roles of phosphorylation/O-GlcNAcylation interplay in synaptic plasticity within rat sensorimotor cortex after a SMP period. For this purpose, sensorimotor cortex synaptosomes were separated by sucrose gradient, in order to isolate a subcellular compartment enriched in proteins involved in synaptic functions. A period of SMP induced plastic changes at the pre- and post-synaptic levels, characterized by a reduction in phosphorylation (synapsin1, α-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid receptors (AMPAR) GluA2) and expression (synaptophysin, PSD-95, AMPAR GluA2) of synaptic proteins, as well as a decrease in MAPK/ERK42 activation. Expression levels of O-GlcNAc transferase/O-GlcNAcase enzymes was unchanged but we observed a specific reduction of synapsin1 O-GlcNAcylation in sensorimotor cortex synaptosomes. The synergistic regulation of synapsin1 phosphorylation/O-GlcNAcylation could affect pre-synaptic neurotransmitter release. Associated with other pre- and post-synaptic changes, synaptic efficacy could be impaired in somatosensory cortex of SMP rat. Thus, phosphorylation/O-GlcNAcylation interplay appears to be involved in synaptic plasticity by finely regulating neural activity.

Early movement restriction leads to enduring disorders in muscle and locomotion.

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.

Reorganization of motor cortex and impairment of motor performance induced by hindlimb unloading are partially reversed by cortical IGF-1 administration.

Immobilization, bed rest, or sedentary lifestyle, are known to induce a profound impairment in sensorimotor performance. These alterations are due to a combination of peripheral and central factors. Previous data conducted on a rat model of disuse (hindlimb unloading, HU) have shown a profound reorganization of motor cortex and an impairment of motor performance. Recently, our interest was turned towards the role of insulin-like growth factor 1 (IGF-1) in cerebral plasticity since this growth factor is considered as the mediator of beneficial effects of exercise on the central nervous system, and its cortical level is decreased after a 14-day period of HU. In the present study, we attempted to determine whether a chronic subdural administration of IGF-1 in HU rats could prevent deleterious effects of HU on the motor cortex and on motor activity. We demonstrated that HU induces a shrinkage of hindlimb cortical representation and an increase in current threshold to elicit a movement. Administration of IGF-1 in HU rats partially reversed these changes. The functional evaluation revealed that IGF-1 prevents the decrease in spontaneous activity found in HU rats and the changes in hip kinematics during overground locomotion, but had no effect of challenged locomotion (ladder rung walking test). Taken together, these data clearly indicate the implication of IGF-1 in cortical plastic mechanisms and in behavioral alteration induced by a decreased in sensorimotor activity.

A new device combining mechanical stimulation of plantar sole and Achilles' tendon to alleviate the consequences of muscle deconditioning.

Limb immobilization or confinement to bed results in a severe atrophy and weakness of lower leg muscles. Full recovery of muscle strength and physical function is rare and may impact the patient's outcome. Studies performed on rodents have demonstrated that the deleterious structural and functional adaptations which occur during muscle deconditioning can be counteracted through adequate physiological stimuli. Thus, based on this fundamental work, we developed a device that combines mechanical stimulation of proprioceptors located in the plantar sole and Achilles' tendon. The device is adapted to patients immobilized and confined to bed. Stimulations can be applied on muscle in passive state. The protocol is non-invasive and is well accepted by patients. This paper presents the technical features of the device, as well as preliminary results of the first clinical study. This device might allow considering new therapeutic strategies for prevention of atrophy in many pathologies.

Role of IGF-1 in cortical plasticity and functional deficit induced by sensorimotor restriction.

In the adult rat, sensorimotor restriction by hindlimb unloading (HU) is known to induce impairments in motor behavior as well as a disorganization of somatosensory cortex (shrinkage of the cortical representation of the hindpaw, enlargement of the cutaneous receptive fields, decreased cutaneous sensibility threshold). Recently, our team has demonstrated that IGF-1 level was decreased in the somatosensory cortex of rats submitted to a 14-day period of HU. To determine whether IGF-1 is involved in these plastic mechanisms, a chronic cortical infusion of this substance was performed by means of osmotic minipump. When administered in control rats, IGF-1 affects the size of receptive fields and the cutaneous threshold, but has no effect on the somatotopic map. In addition, when injected during the whole HU period, IGF-1 is interestingly implied in cortical changes due to hypoactivity: the shrinkage of somatotopic representation of hindlimb is prevented, whereas the enlargement of receptive fields is reduced. IGF-1 has no effect on the increase in neuronal response to peripheral stimulation. We also explored the functional consequences of IGF-1 level restoration on tactile sensory discrimination. In HU rats, the percentage of paw withdrawal after a light tactile stimulation was decreased, whereas it was similar to control level in HU-IGF-1 rats. Taken together, the data clearly indicate that IGF-1 plays a key-role in cortical plastic mechanisms and in behavioral alterations induced by a decrease in sensorimotor activity.

Hypoactivity affects IGF-1 level and PI3K/AKT signaling pathway in cerebral structures implied in motor control.

A chronic reduction in neuromuscular activity through prolonged body immobilization in human alters motor task performance through a combination of peripheral and central factors. Studies performed in a rat model of sensorimotor restriction have shown functional and biochemical changes in sensorimotor cortex. However, the underlying mechanisms are still unclear. Interest was turned towards a possible implication of Insulin-like Growth Factor 1 (IGF-1), a growth factor known to mediate neuronal excitability and synaptic plasticity by inducing phosphorylation cascades which include the PI3K-AKT pathway. In order to better understand the influence of IGF-1 in cortical plasticity in rats submitted to a sensorimotor restriction, we analyzed the effect of hindlimb unloading on IGF-1 and its main molecular pathway in structures implied in motor control (sensorimotor cortex, striatum, cerebellum). IGF-1 level was determined by ELISA, and phosphorylation of its receptor and proteins of the PI3K-AKT pathway by immunoblot. In the sensorimotor cortex, our results indicate that HU induces a decrease in IGF-1 level; this alteration is associated to a decrease in activation of PI3K-AKT pathway. The same effect was observed in the striatum, although to a lower extent. No variation was noticed in the cerebellum. These results suggest that IGF-1 might contribute to cortical and striatal plasticity induced by a chronic sensorimotor restriction.

Dendritic spine remodeling induced by hindlimb unloading in adult rat sensorimotor cortex.

A sensorimotor restriction, for instance in patients confined to bed, induces an impairment in motor function, which could be due to structural and functional reorganization of the sensorimotor cortex. Hindlimb unloading (HU) is a rodent model used to reproduce the chronic weightless bearing and reduction in hindlimb movement. In this study, we determined whether a 14-day period of HU in adult rats leads to dendritic spine plasticity. For this purpose, we visualized a large number of spines on pyramidal neurons located in superficial and deep layers of the cortex within the hindpaw representation area, by means of confocal microscopy. Spines were classified according to their shape, as stubby, thin, mushroom, or filopodium. Spine density was increased (+26%) after HU. The increase concerned mainly filopodium spines (+82%) and mushrooms (+33%), whereas no change was noticed for stubby and thin spines. Spine length was decreased, whatever their shape. Head diameter evolved differently depending on the layer: it was increased in superficial layers and decreased in deeper ones. These results indicate that morphological changes accompany functional reorganization of motor cortex in response to a decrease in sensorimotor function during adulthood.

Hindlimb unloading affects cortical motor maps and decreases corticospinal excitability.

A chronic reduction in neuromuscular activity through prolonged body immobilization of humans results in muscle atrophy and weakness as well as motor tasks performance impairment, which is correlated to a change in corticospinal excitability. In rats, hindlimb unloading (HU) is commonly used to mimic the effects of confinement to bed in patients. Several studies have reported changes in the representation of the somatosensory cortex in rodents submitted to HU or sensorimotor restriction by casting: remapping and enlargement of receptive fields, changes in the response of layer 4 neurons to peripheral stimulation. However, we have no data about motor cortical maps in rats submitted to a period of motor restriction during adulthood. Therefore, the objectives of the present study were twofold: to determine, in control rats and in rats submitted to a 14-day period of HU, the size and organization of hindlimb representation in the M1 cortex and to evaluate the overall excitability of M1 cortex by determining the stimulation thresholds. HU led to a dramatic decrease in the hindlimb representation on the M1 cortex (-61%, p<0.01). In addition, current thresholds for eliciting a movement were increased. The toes were less strongly affected by HU than other joint. Our main conclusion is that HU dramatically affects the organization and functioning of cortical motor maps and decreases corticospinal excitability.

ERK is involved in the reorganization of somatosensory cortical maps in adult rats submitted to hindlimb unloading.

Sensorimotor restriction by a 14-day period of hindlimb unloading (HU) in the adult rat induces a reorganization of topographic maps and receptive fields. However, the underlying mechanisms are still unclear. Interest was turned towards a possible implication of intracellular MAPK signaling pathway since Extracellular-signal-Regulated Kinase 1/2 (ERK1/2) is known to play a significant role in the control of synaptic plasticity. In order to better understand the mechanisms underlying cortical plasticity in adult rats submitted to a sensorimotor restriction, we analyzed the time-course of ERK1/2 activation by immunoblot and of cortical reorganization by electrophysiological recordings, on rats submitted to hindlimb unloading over four weeks. Immunohistochemistry analysis provided evidence that ERK1/2 phosphorylation was increased in layer III neurons of the somatosensory cortex. This increase was transient, and parallel to the changes in hindpaw cortical map area (layer IV). By contrast, receptive fields were progressively enlarged from 7 to 28 days of hindlimb unloading. To determine whether ERK1/2 was involved in cortical remapping, we administered a specific ERK1/2 inhibitor (PD-98059) through osmotic mini-pump in rats hindlimb unloaded for 14 days. Results demonstrate that focal inhibition of ERK1/2 pathway prevents cortical reorganization, but had no effect on receptive fields. These results suggest that ERK1/2 plays a role in the induction of cortical plasticity during hindlimb unloading.

Activity-dependent changes in the electrophysiological properties of regular spiking neurons in the sensorimotor cortex of the rat in vitro.

Sensorimotor performance is highly dependent on the level of physical activity. For instance, a period of disuse induces an impairment of motor performance, which is the result of combined muscular, spinal and supraspinal mechanisms. Concerning this latter origin, our hypothesis was that intrinsic properties and input/output coupling of cells within the sensorimotor cortex might participate to the alteration in cortical motor control. The aim of the present study was thus to examine the basic electrophysiological characteristics of cortical cells in control rats and in animals submitted to 14 days of hindlimb unloading, a model of sensorimotor deprivation. Intracellular recordings were obtained in vitro from coronal slices from cortical hindpaw representation area. We have also made an attempt to determine the morphological characteristics as well as the location of the investigated neurons by biocytin labelling. Passive properties of neurons were affected by hindlimb unloading: input resistance and time constant were decreased (-20%), the rheobase was increased (+34%), whereas the resting potential was unchanged. The frequency-current relationships were also modified, the curve being shifted towards right. The size of body area of recorded neurons was unchanged in unloaded rats. Taken together, these data reflect a decrease in excitability of cortical cells in response to a decreased cortical activation.