Functional mapping of the lower urinary tract by epidural electrical stimulation of the spinal cord in decerebrated cat model.

Several neurologic diseases including spinal cord injury, Parkinson's disease or multiple sclerosis are accompanied by disturbances of the lower urinary tract functions. Clinical data indicates that chronic spinal cord stimulation can improve not only motor function but also ability to store urine and control micturition. Decoding the spinal mechanisms that regulate the functioning of detrusor (Detr) and external urethral sphincter (EUS) muscles is essential for effective neuromodulation therapy in patients with disturbances of micturition. In the present work we performed a mapping of Detr and EUS activity by applying epidural electrical stimulation (EES) at different levels of the spinal cord in decerebrated cat model. The study was performed in 5 adult male cats, evoked potentials were generated by EES aiming to recruit various spinal pathways responsible for LUT and hindlimbs control. Recruitment of Detr occurred mainly with stimulation of the lower thoracic and upper lumbar spinal cord (T13-L1 spinal segments). Responses in the EUS, in general, occurred with stimulation of all the studied sites of the spinal cord, however, a pronounced specificity was noted for the lower lumbar/upper sacral sections (L7-S1 spinal segments). These features were confirmed by comparing the normalized values of the slope angles used to approximate the recruitment curve data by the linear regression method. Thus, these findings are in accordance with our previous data obtained in rats and could be used for development of novel site-specific neuromodulation therapeutic approaches.

Transient expression of heavy-chain neurofilaments in the perigeniculate nucleus of cats.

The perigeniculate nucleus (PGN) is a visual part of the thalamic reticular nucleus modulating the information transfer between the lateral geniculate nucleus and the visual cortex. This study focused on the postnatal development of the PGN in cats, using the SMI-32 antibody, which recognizes non-phosphorylated heavy-chain neurofilaments responsible for neuronal structural maturation and is also used as a marker for motion processing, or Y, stream. We questioned whether transient neuronal populations exist in the PGN and can they possibly be related to the Y processing stream. We uncovered a transient, robust SMI-32 staining in the PGN of kittens aged 0-34 days with the significant decline in the cellular density of labeled cells in older animals. According to the double-labeling, in all examined age groups, perigeniculate SMI-32-immunopositive cells are part of the main parvalbumin-positive population. The maximal cellular density of the double-stained cells appeared in animals aged 10-28 days. We also revealed that the most significant growth of perigeniculate cells's soma occurred at three postnatal weeks. The possible link of our data to the development of the Y visual processing stream and to the heterogeneity of the perigeniculate neuronal population is also discussed.

Activation of the spinal and brainstem locomotor networks during free treadmill stepping in rats lacking dopamine transporter.

Dopamine is extremely important for the multiple functions of the brain and spinal cord including locomotor behavior. Extracellular dopamine levels are controlled by the membrane dopamine transporter (DAT), and animals lacking DAT (DAT-KO) are characterized by hyperdopaminergia and several alterations of locomotion including hyperactivity. Neuronal mechanisms of such altered locomotor behavior are still not fully understood. We believe that in hyperdopaminergic animals both the spinal and brain neuronal networks involved in locomotion are modified. Using the c-fos technique, we studied activated neuronal networks of the spinal cord and two brainstem structures related to locomotor control and being under the strong dopaminergic influence, the cuneiform nucleus (CnF) and ventrolateral periaqueductal gray (VLPAG), in wild-type (DAT-WT) and DAT-KO rats. In the spinal cord, most c-fos-positive cells were located in the dorsal laminae II-IV and in the central gray matter (laminae V-VI). No differences were revealed for the central areas. As for the dorsal areas, in the DAT-WT group, labeled cells mostly occupied the lateral region, whereas, in the DAT-KO group, c-fos-positive cells were observed in both medial and lateral regions in some animals or in the medial regions in some animals. In the brainstem of the DAT-WT group, approximately the same number of labeled cells were found in the CnF and VLPAG, but in the DAT-KO group, the VLPAG contained a significantly smaller number of c-fos-positive cells compared to the CnF. Thereby, our work indicates an imbalance in the sensorimotor networks located within the dorsal horns of the spinal cord as well as a disbalance in the activity of brainstem networks in the DAT-deficient animals.

Calcium-binding protein parvalbumin in the spinal cord and dorsal root ganglia.

Parvalbumin is one of the calcium-binding proteins. In the spinal cord, it is mainly expressed in inhibitory neurons; in the dorsal root ganglia, it is expressed in proprioceptive neurons. In contrast to in the brain, weak systematization of parvalbumin-expressing neurons occurs in the spinal cord. The aim of this paper is to provide a systematic review of parvalbumin-expressing neuronal populations throughout the spinal cord and the dorsal root ganglia of mammals, regarding their mapping, co-expression with some functional markers. The data reviewed are mostly concerning rodentia species because they are predominantly presented in literature.

Development of neurochemical labeling in the intermediolateral nucleus of cats' spinal cord.

NeuN is a neuron-specific nuclear protein expressed in most mature neuronal cell types, with some exceptions. These exceptions are known mainly for the brain but not for the spinal cord or the spinal visceral networks for which only scarce information is available. One of the most defined visceral structures in the spinal cord is the sympathetic intermediolateral nucleus located within the thoracolumbar segments. We investigated the NeuN staining in the intermediolateral nucleus and compared it with the staining for two neurochemical markers of visceral neurons: nitric oxide synthase and calcium-binding protein calretinin in adult cats and in kittens aged 0, 14, and 35 days. A clear NeuN-immunonegativity was obtained for intermediolateral neurons labeled for nitric oxide synthase for both adult cats and kittens. In contrast, a matched immunopositivity for the NeuN and calretinin was obtained, showing an age-dependent degree of this colocalization, which was high in newborn kittens, decreased on postnatal 14 and 35 days and persisted at a moderate level up to adulthood. Perhaps our data displayed a heterogeneity of the intermediolateral neurons.

Postnatal growth of the lumbosacral spinal segments in cat: Their lengths and positions in relation to vertebrae.

Cat is a prominent model for investigating neural networks of the lumbosacral spinal cord that control locomotor and visceral activity. We previously proposed an integral function, establishing the topographical relationship between the spinal cord segments and vertebrae in adult animals. Here, we investigated the dynamic of this topographical relationship through early and middle periods of development in kittens. We calculated the length of each vertebra relative to the total length of the region from 13th thoracic (T) to the 7th lumbar (L) vertebrae (V) as well as the length of each segment relative to the total region from T13 to the three-dimensional sacral (S) segment. As in our previous work, the length and position of VL2 were used to establish relationships between the characteristics of the segments and vertebrae. Cubic regression reliably approximates the lengths of segments relative to VL2 length. As the cat aged, the relative length of VT13 and VL1 decreased while the relative length of VL5 increased. The relative length of the T13 and L3 segments increased while the relative length of the S1-S2 segments decreased. The T13-L2 segments are descended monotonically relative to the VL1-VL2 border. The L3-S1 segments are also descended, though with more complex dynamics. The positions of the S2-S3 segments remained unchanged. To conclude, different spinal segments displayed different developmental dynamics. The revealed relationship between vertebrae and lumbosacral spinal segments may be helpful for clearly defining stimulation regions to invoke particular functions, both in experimental studies on the spinal cord and clinical treatment.

Neurochemical atlas of the cat spinal cord.

The spinal cord is a complex heterogeneous structure, which provides multiple vital functions. The precise surgical access to the spinal regions of interest requires precise schemes for the spinal cord structure and the spatial relation between the spinal cord and the vertebrae. One way to obtain such information is a combined anatomical and morphological spinal cord atlas. One of the widely used models for the investigation of spinal cord functions is a cat. We create a single cell-resolution spinal cord atlas of the cat using a variety of neurochemical markers [antibodies to NeuN, choline acetyltransferase, calbindin 28 kDa, calretinin, parvalbumin, and non-phosphorylated heavy-chain neurofilaments (SMI-32 antibody)] allowing to visualize several spinal neuronal populations. In parallel, we present a map of the spatial relation between the spinal cord and the vertebrae for the entire length of the spinal cord.

Transient neurochemical features of the perigeniculate neurons during early postnatal development of the cat.

The thalamic reticular nucleus receives axons from the thalamic sensory nuclei and the cerebral cortex. The visual part of this nucleus in carnivores is the perigeniculate nucleus located dorsal to the lateral geniculate nucleus. The perigeniculate nucleus participates in the modulation of visual processing and in the transition of synchronized slow rhythmicity during sleep into desynchronized high-frequency activity during arousal and consists of inhibitory neurons. The main neurochemical markers for perigeniculate neurons are glutamic acid decarboxylase and Ca2+ -binding protein parvalbumin. Previous studies of postnatal development focused on the morphological features of the perigeniculate nucleus; however, its neurochemistry remains poorly understood. In this study, we focused on the postnatal development of perigeniculate neurons using immunohistochemical labeling of parvalbumin, two related Ca2+ -binding proteins (calretinin and calbindin), glutamic acid decarboxylase, and a common neuronal protein, NeuN, in kittens that were 0-123 days old and in adult cats. In parallel with the well-known dominant neuronal populations expressing parvalbumin and GAD67 and persisting until adulthood, transient populations expressing calretinin and calbindin were observed. The calbindin-positive neurons were similar to the main perigeniculate population and showed close morphological features and parvalbumin coexpression. In contrast, the calretinin-positive neurons differed in their morphological characteristics and did not express GAD67, thus distinguishing them from the majority of perigeniculate neurons. A possible link between these populations was revealed, and the development of thalamocortical processing is discussed.

Trace Amine-Associated Receptor 2 Is Expressed in the Limbic Brain Areas and Is Involved in Dopamine Regulation and Adult Neurogenesis.

Trace amines are a group of biogenic amines that are structurally and functionally close to classical monoamine neurotransmitters. Trace amine-associated receptors (TAARs) are emerging as promising targets for treating neuropsychiatric disorders. It has been documented that all TAARs, apart from TAAR1, function as olfactory receptors involved in sensing innate odors encoded by volatile amines. However, recently, brain expression and function of TAAR5 were also demonstrated. In this study, we assessed the behavior, brain neurochemistry, and electrophysiology changes in knock-out mice lacking Trace amine-associated receptor 2 (TAAR2) but expressing beta-Galactosidase mapping expression of TAAR2 receptors. As expected, we detected beta-Galactosidase staining in the glomerular layer of the olfactory bulb. However, we also found staining in the deeper layers of the olfactory bulb and several brain regions, including the hippocampus, cerebellum, cortex, raphe nuclei, hypothalamus, and habenula, indicating that TAAR2 receptors are not only expressed in the olfactory system but are also present in the limbic brain areas that receive olfactory input. In behavioral experiments, TAAR2 knock-out (TAAR2-KO) mice showed increased locomotor activity and less immobility in the forced swim test, with no changes in anxiety level. Furthermore, TAAR2-KO mice showed alterations in brain electrophysiological activity-particularly, decreased spectral power of the cortex and striatum in the 0, 9-20 Hz range. TAAR2-KO mice also had elevated tissue dopamine levels in the striatum and an increased dopaminergic neuron number in the Substantia Nigra. In addition, an increased brain-derived neurotrophic factor (BDNF) mRNA level in the striatum and Monoamine Oxidase B (MAO-B) mRNA level in the striatum and midbrain was found in TAAR2-KO mice. Importantly, TAAR2-KO mice demonstrated an increased neuroblast-like and proliferating cell number in the subventricular and subgranular zone, indicating increased adult neurogenesis. These data indicate that in addition to its role in the innate olfaction of volatile amines, TAAR2 is expressed in limbic brain areas and regulates the brain dopamine system, neuronal electrophysiological activity, and adult neurogenesis. These findings further corroborated observations in TAAR1-KO and TAAR5-KO mice, indicating common for TAAR family pattern of expression in limbic brain areas and role in regulating monoamine levels and adult neurogenesis, but with variable involvement of each subtype of TAAR receptors in these functions.

Differences in backward and forward treadmill locomotion in decerebrated cats.

Locomotion in different directions is vital for animal life and requires fine-adjusted neural activity of spinal networks. To compare the levels of recruitability of the locomotor circuitry responsible for forward and backward stepping, several electromyographic and kinematic characteristics of the two locomotor modes were analysed in decerebrated cats. Electrical epidural spinal cord stimulation was used to evoke forward and backward locomotion on a treadmill belt. The functional state of the bilateral spinal networks was tuned by symmetrical and asymmetrical epidural stimulation. A significant deficit in the backward but not forward stepping was observed when laterally shifted epidural stimulation was used but was not observed with central stimulation: only half of the cats were able to perform bilateral stepping, but all the cats performed forward stepping. This difference was in accordance with the features of stepping during central epidural stimulation. Both the recruitability and stability of the EMG signals as well as inter-limb coordination during backward stepping were significantly decreased compared with those during forward stepping. The possible underlying neural mechanisms of the obtained functional differences of backward and forward locomotion (spinal network organisation, commissural communication and supraspinal influence) are discussed.

SMI-32 labeling in Cajal-Retzius cells of feline primary visual cortex.

Cajal-Retzius cells are one of the transient elements of the developing cerebral cortex. These cells express some characteristic molecules. One of them, heavy-chain neurofilaments, participating in the construction of the mature cerebral networks, are believed to be a specific feature of the human's Cajal-Retzius cells. Using histochemical stain for SMI-32 antibody to the non-phosphorylated heavy-chain neurofilaments, large neurons having horizontally oriented soma and bipolar processes were labeled in the molecular layer of the primary visual cortex of cats aged 0-2 postnatal days. Using DiI technique, similar neurons having a well-developed system of parallel vertical branches coming from the two horizontal processes were visualized in these areas. The location and general morphology of these neurons were similar to the Cajal-Retzius cells allowing to suppose for the carnivores to share similar with primates developmental mechanisms of the corticogenesis.

The role of load-dependent sensory input in the control of balance during gait in rats.

Locomotor activity requires fine balance control that strongly depends on the afferent input from the load receptors. Following hindlimb unloading (HU), the kinematic and EMG activity of the hindlimbs is known to change significantly. However, the effects of HU on the integrative control mechanisms of posture and locomotion are not clear. The goal of the present study was to evaluate the center of mass (CoM) dynamic stabilization and associated adaptive changes in the trunk and hindlimb muscle activity during locomotion after 7 days of HU. The EMG signals from the muscles of the low lumbar trunk [m. longissimus dorsi (VERT)] and the hind limb [m. tibialis anterior (TA), m. semitendinosus (ST), m. soleus (SOL)] were recorded together with the hindquarter kinematics during locomotion on a treadmill in six rats before and after HU. The CoM lateral shift in the step cycle significantly increased after HU and coincided with the enhanced activity of the VERT. The mean EMG of the TA and the ST flexor activity increased significantly with reduction of their burst duration. These data demonstrate the disturbances of body balance after HU that can influence the basic parameters of locomotor activity. The load-dependent mechanisms resulted in compensatory adjustments of flexor activity toward a faster gait strategy, such as a trot or gallop, which presumably have supraspinal origin. The neuronal underpinnings of these integrative posture and locomotion mechanisms and their possible reorganization after HU are discussed.

Rostrocaudal Distribution of the C-Fos-Immunopositive Spinal Network Defined by Muscle Activity during Locomotion.

The optimization of multisystem neurorehabilitation protocols including electrical spinal cord stimulation and multi-directional tasks training require understanding of underlying circuits mechanisms and distribution of the neuronal network over the spinal cord. In this study we compared the locomotor activity during forward and backward stepping in eighteen adult decerebrated cats. Interneuronal spinal networks responsible for forward and backward stepping were visualized using the C-Fos technique. A bi-modal rostrocaudal distribution of C-Fos-immunopositive neurons over the lumbosacral spinal cord (peaks in the L4/L5 and L6/S1 segments) was revealed. These patterns were compared with motoneuronal pools using Vanderhorst and Holstege scheme; the location of the first peak was correspondent to the motoneurons of the hip flexors and knee extensors, an inter-peak drop was presumably attributed to the motoneurons controlling the adductor muscles. Both were better expressed in cats stepping forward and in parallel, electromyographic (EMG) activity of the hip flexor and knee extensors was higher, while EMG activity of the adductor was lower, during this locomotor mode. On the basis of the present data, which showed greater activity of the adductor muscles and the attributed interneuronal spinal network during backward stepping and according with data about greater demands on postural control systems during backward locomotion, we suppose that the locomotor networks for movements in opposite directions are at least partially different.

Heterochrony of postnatal accumulation of nonphosphorylated heavy-chain neurofilament by neurons of the cat dorsal lateral geniculate nucleus.

Accumulation of the heavy-chain neurofilaments reflects the maturation status of neuronal structures. The spatial distribution and postnatal developmental dynamic of neurons expressing nonphosphorylated heavy-chain neurofilaments (labeled by SMI-32 antibody) were analyzed in the dorsal lateral geniculate nucleus (LGNd) of the cat. Both interlaminar and intralaminar differences in the dynamic of SMI-32 staining were observed. The following results were obtained: (a) Ascending dorsoventral gradient in the density of SMI-32 immunopositive (SMI-32(+)) neurons (the greatest neuronal density in layer Cm, the minor in the top sublayer of layer A). This gradient was most prominent at the earliest stages of postnatal development (1st-2nd weeks) and slowly flattened up to adulthood; (b) Layer A1 exhibits increases in SMI-32-positive cells earlier than layer A; (c) The general transient increment in the number and density of SMI-32(+) neurons around 2-5 postnatal weeks. Since SMI-32 antibody is considered to be a putative marker for Y cells forming a motion processing stream, we suggest that peculiarities of SMI-32 staining at geniculate level could reflect the heterogeneity of Y cell subpopulations and the heterochrony of their postnatal maturation.

Dynamic Foot Stimulations During Short-Term Hindlimb Unloading Prevent Dysregulation of the Neurotransmission in the Hippocampus of Rats.

Spaceflight and simulated microgravity both affect learning and memory, which are mostly controlled by the hippocampus. However, data about molecular alterations in the hippocampus in real or simulated microgravity conditions are limited. Adult Wistar rats were recruited in the experiments. Here we analyzed whether short-term simulated microgravity caused by 3-day hindlimb unloading (HU) will affect the glutamatergic and GABAergic systems of the hippocampus and how dynamic foot stimulation (DFS) to the plantar surface applied during HU can contribute in the regulation of hippocampus functioning. The results demonstrated a decreased expression of vesicular glutamate transporters 1 and 2 (VGLUT1/2) in the hippocampus after 3 days of HU, while glutamate decarboxylase 67 (GAD67) expression was not affected. HU also significantly induced Akt signaling and transcriptional factor CREB that are supposed to activate the neuroprotective mechanisms. On the other hand, DFS led to normalization of VGLUT1/2 expression and activity of Akt and CREB. Analysis of exocytosis proteins revealed the inhibition of SNAP-25, VAMP-2, and syntaxin 1 expression in DFS group proposing attenuation of excitatory neurotransmission. Thus, we revealed that short-term HU causes dysregulation of glutamatergic system of the hippocampus, but, at the same time, stimulates neuroprotective Akt-dependent mechanism. In addition, most importantly, we demonstrated positive effect of DFS on the hippocampus functioning that probably depends on the regulation of neurotransmitter exocytosis.

Increased dopamine transmission and adult neurogenesis in trace amine-associated receptor 5 (TAAR5) knockout mice.

Trace amine-associated receptors (TAARs) are a class of sensory G protein-coupled receptors that detect biogenic amines, products of decarboxylation of amino acids. The majority of TAARs (TAAR2-TAAR9) have been described mainly in the olfactory epithelium and considered to be olfactory receptors sensing innate odors. However, there is recent evidence that one of the members of this family, TAAR5, is expressed also in the limbic brain areas receiving projection from the olfactory system and involved in the regulation of emotions. In this study, we further characterized a mouse line lacking TAAR5 (TAAR5 knockout, TAAR5-KO mice) that express beta-galactosidase mapping TAAR5 expression. We found that in TAAR5-KO mice the number of dopamine neurons, the striatal levels of dopamine and its metabolites, as well as striatal levels of GDNF mRNA, are elevated indicating a potential increase in dopamine neuron proliferation. Furthermore, an analysis of TAAR5 beta-galactosidase expression revealed that TAAR5 is present in the major neurogenic areas of the brain such as the subventricular zone (SVZ), the subgranular zone (SGZ) and the less characterized potentially neurogenic zone surrounding the 3rd ventricle. Direct analysis of neurogenesis by using specific markers doublecortin (DCX) and proliferating cell nuclear antigen (PCNA) revealed at least 2-fold increase in the number of proliferating neurons in the SVZ and SGZ of TAAR5-KO mice, but no such markers were detected in mutant or control mice in the areas surrounding the 3rd ventricle. These observations indicate that TAAR5 involved not only in regulation of emotional status but also adult neurogenesis and dopamine transmission. Thus, future TAAR5 antagonists may exert not only antidepressant and/or anxiolytic action but may also provide new treatment opportunity for neurodegenerative disorders such as Parkinson's disease.

Refractory density model of cortical direction selectivity: Lagged-nonlagged, transient-sustained, and On-Off thalamic neuron-based mechanisms and intracortical amplification.

A biophysically detailed description of the mechanisms of the primary vision is still being developed. We have incorporated a simplified, filter-based description of retino-thalamic visual signal processing into the detailed, conductance-based refractory density description of the neuronal population activity of the primary visual cortex. We compared four mechanisms of the direction selectivity (DS), three of them being based on asymmetrical projections of different types of thalamic neurons to the cortex, distinguishing between (i) lagged and nonlagged, (ii) transient and sustained, and (iii) On and Off neurons. The fourth mechanism implies a lack of subcortical bias and is an epiphenomenon of intracortical interactions between orientation columns. The simulations of the cortical response to moving gratings have verified that first three mechanisms provide DS to an extent compared with experimental data and that the biophysical model realistically reproduces characteristics of the visual cortex activity, such as membrane potential, firing rate, and synaptic conductances. The proposed model reveals the difference between the mechanisms of both the intact and the silenced cortex, favoring the second mechanism. In the fourth case, DS is weaker but significant; it completely vanishes in the silenced cortex.DS in the On-Off mechanism derives from the nonlinear interactions within the orientation map. Results of simulations can help to identify a prevailing mechanism of DS in V1. This is a step towards a comprehensive biophysical modeling of the primary visual system in the frameworks of the population rate coding concept.

Rapid prototyping of soft bioelectronic implants for use as neuromuscular interfaces.

Neuromuscular interfaces are required to translate bioelectronic technologies for application in clinical medicine. Here, by leveraging the robotically controlled ink-jet deposition of low-viscosity conductive inks, extrusion of insulating silicone pastes and in situ activation of electrode surfaces via cold-air plasma, we show that soft biocompatible materials can be rapidly printed for the on-demand prototyping of customized electrode arrays well adjusted to specific anatomical environments, functions and experimental models. We also show, with the monitoring and activation of neuronal pathways in the brain, spinal cord and neuromuscular system of cats, rats and zebrafish, that the printed bioelectronic interfaces allow for long-term integration and functional stability. This technology might enable personalized bioelectronics for neuroprosthetic applications.

Site-Specific Neuromodulation of Detrusor and External Urethral Sphincter by Epidural Spinal Cord Stimulation.

Impairments of the lower urinary tract function including urine storage and voiding are widely spread among patients with spinal cord injuries. The management of such patients includes bladder catheterization, surgical and pharmacological approaches, which reduce the morbidity from urinary tract-related complications. However, to date, there is no effective treatment of neurogenic bladder and restoration of urinary function. In the present study, we examined neuromodulation of detrusor (Detr) and external urethral sphincter by epidural electrical stimulation (EES) of lumbar and sacral regions of the spinal cord in chronic rats. To our knowledge, it is the first chronic study where detrusor and external urethral sphincter signals were recorded simultaneously to monitor their neuromodulation by site-specific spinal cord stimulation (SCS). The data obtained demonstrate that activation of detrusor muscle mainly occurs during the stimulation of the upper lumbar (L1) and lower lumbar (L5-L6) spinal segments whereas external urethral sphincter was activated predominantly by sacral stimulation. These findings can be used for the development of neurorehabilitation strategies based on spinal cord epidural stimulation for autonomic function recovery after severe spinal cord injury (SCI).

Comparison of operation of spinal locomotor networks activated by supraspinal commands and by epidural stimulation of the spinal cord in cats.

KEY POINTS: Epidural electrical stimulation (ES) of the spinal cord restores/improves locomotion in patients. ES-evoked locomotor movements differ to some extent from the normal ones. Operation of the locomotor network during ES is unknown. We compared the activity of individual spinal neurons during locomotion initiated by signals from the brainstem and by ES. We demonstrated that the spinal network generating locomotion under each of the two conditions is formed by the same neurons. A part of this network operates similarly under the two conditions, suggesting that it is essential for generation of locomotion under both conditions. Another part of this network operates differently under the two conditions, suggesting that it is responsible for differences in the movement kinematics observed under the two conditions. ABSTRACT: Locomotion is a vital motor function for both animals and humans. Epidural electrical stimulation (ES) of the spinal cord is used to restore/improve locomotor movements in patients. However, operation of locomotor networks during ES has never been studied. Here we compared the activity of individual spinal neurons recorded in decerebrate cats of either sex during locomotion initiated by supraspinal commands (caused by stimulation of the mesencephalic locomotor region, MLR) and by ES. We found that under both conditions, the same neurons had modulation of their activity related to the locomotor rhythm, suggesting that the network generating locomotion under the two conditions is formed by the same neurons. About 40% of these neurons had stable modulation (i.e. small dispersion of their activity phase in sequential cycles), as well as a similar phase and shape of activity burst in MLR- and ES-evoked locomotor cycles. We suggest that these neurons form a part of the locomotor network that operates similarly under the two conditions, and are critical for generation of locomotion. About 23% of the modulated neurons had stable modulation only during MLR-evoked locomotion. We suggest that these neurons are responsible for some differences in kinematics of MLR- and ES-evoked locomotor movements. Finally, 25% of the modulated neurons had unstable modulation during both MLR- and ES-evoked locomotion. One can assume that these neurons contribute to maintenance of the excitability level of locomotor networks necessary for generation of stepping, or belong to postural networks, activated simultaneously with locomotor networks by both MLR stimulation and ES.