[Role of Ca(2+) in the pacemaker-like property of spinal motoneurons]

Preclinical evidence supporting the clinical development of central pattern generator-modulating therapies for chronic spinal cord-injured patients

Ambulation or walking is one of the main gaits of locomotion. In terrestrial animals, it may be defined as a series of rhythmic and bilaterally coordinated movement of the limbs which creates a forward movement of the body. This applies regardless of the number of limbs-from arthropods with six or more limbs to bipedal primates. These fundamental similarities among species may explain why comparable neural systems and cellular properties have been found, thus far, to control in similar ways locomotor rhythm generation in most animal models. The aim of this article is to provide a comprehensive review of the known structural and functional features associated with central nervous system (CNS) networks that are involved in the control of ambulation and other stereotyped motor patterns-specifically Central Pattern Generators (CPGs) that produce basic rhythmic patterned outputs for locomotion, micturition, ejaculation, and defecation. Although there is compelling evidence of their existence in humans, CPGs have been most studied in reduced models including in vitro isolated preparations, genetically-engineered mice and spinal cord-transected animals. Compared with other structures of the CNS, the spinal cord is generally considered as being well-preserved phylogenetically. As such, most animal models of spinal cord-injured (SCI) should be considered as valuable tools for the development of novel pharmacological strategies aimed at modulating spinal activity and restoring corresponding functions in chronic SCI patients.

Central pattern generator for locomotion: anatomical, physiological, and pathophysiological considerations

This article provides a perspective on major innovations over the past century in research on the spinal cord and, specifically, on specialized spinal circuits involved in the control of rhythmic locomotor pattern generation and modulation. Pioneers such as Charles Sherrington and Thomas Graham Brown have conducted experiments in the early twentieth century that changed our views of the neural control of locomotion. Their seminal work supported subsequently by several decades of evidence has led to the conclusion that walking, flying, and swimming are largely controlled by a network of spinal neurons generally referred to as the central pattern generator (CPG) for locomotion. It has been subsequently demonstrated across all vertebrate species examined, from lampreys to humans, that this CPG is capable, under some conditions, to self-produce, even in absence of descending or peripheral inputs, basic rhythmic, and coordinated locomotor movements. Recent evidence suggests, in turn, that plasticity changes of some CPG elements may contribute to the development of specific pathophysiological conditions associated with impaired locomotion or spontaneous locomotor-like movements. This article constitutes a comprehensive review summarizing key findings on the CPG as well as on its potential role in Restless Leg Syndrome, Periodic Leg Movement, and Alternating Leg Muscle Activation. Special attention will be paid to the role of the CPG in a recently identified, and uniquely different neurological disorder, called the Uner Tan Syndrome.

Locomotion after spinal cord injury depends on constitutive activity in serotonin receptors

Following spinal cord injury (SCI) neurons caudal to the injury are capable of rhythmic locomotor-related activity that can form the basis for substantial functional recovery of stepping despite the loss of crucial brain stem-derived neuromodulators like serotonin (5-HT). Here we investigated the contribution of constitutive 5-HT(2) receptor activity (activity in the absence of 5-HT) to locomotion after SCI. We used a staggered hemisection injury model in rats to study this because these rats showed a robust recovery of locomotor function and yet a loss of most descending axons. Immunolabeling for 5-HT showed little remaining 5-HT below the injury, and locomotor ability was not correlated with the amount of residual 5-HT. Furthermore, blocking 5-HT(2) receptors with an intrathecal (IT) application of the neutral antagonist SB242084 did not affect locomotion (locomotor score and kinematics were unaffected), further indicating that residual 5-HT below the injury did not contribute to generation of locomotion. As a positive control, we found that the same application of SB242084 completely antagonized the muscle activity induced by exogenous application of the 5-HT(2) receptor agonists alpha-methyl-5-HT (IT). In contrast, blocking constitutive 5-HT(2) receptor activity with the potent inverse agonist SB206553 (IT) severely impaired stepping as assessed with kinematic recordings, eliminating most hindlimb weight support and overall reducing the locomotor score in both hind legs. However, even in the most severely impaired animals, rhythmic sweeping movements of the hindlimb feet were still visible during forelimb locomotion, suggesting that SB206553 did not completely eliminate locomotor drive to the motoneurons or motoneuron excitability. The same application of SB206553 had no affect on stepping in normal rats. Thus while normal rats can compensate for loss of 5-HT(2) receptor activity, after severe spinal cord injury rats require constitutive activity in these 5-HT(2) receptors to produce locomotion.