Effects of intrathecal alpha1- and alpha2-noradrenergic agonists and norepinephrine on locomotion in chronic spinal cats

Initiating or blocking locomotion in spinal cats by applying noradrenergic drugs to restricted lumbar spinal segments

After an acute low thoracic spinal transection (T13), cats can be made to walk with the hindlimbs on a treadmill with clonidine, an alpha2-noradrenergic agonist. Because previous studies of neonatal rat spinal cord in vitro suggest that the most important lumbar segments for rhythmogenesis are L1-L2, we investigated the role of various lumbar segments in the initiation of walking movements on a treadmill of adult cats spinalized (T13), 5-6 d earlier. The locomotor activities were evaluated from electromyographic and video recordings. The results show that: (1) localized topical application of clonidine in restricted baths over either the L3-L4 or the L5-L7 segments was sufficient to induce walking movements. Yohimbine, an alpha2-noradrenergic antagonist, could block this locomotion when applied over L3-L4 or L5-L7; (2) microinjections of clonidine in one or two lumbar segments from L3 to L5 could also induce locomotion; (3) after an intravenous injection of clonidine, locomotion was blocked by microinjections of yohimbine in segments L3, L4, or L5 but not if the injection was in L6; (4) locomotion was also blocked in all cases by additional spinal transections at L3 or L4. These results show that it is possible to initiate walking in the adult spinal cat with a pharmacological stimulation of a restricted number of lumbar segments and also that the integrity of the L3-L4 segments is necessary to sustain the locomotor activity.

New perspectives on spinal motor systems

The production and control of complex motor functions are usually attributed to central brain structures such as cortex, basal ganglia and cerebellum. In traditional schemes the spinal cord is assigned a subservient function during the production of movement, playing a predominantly passive role by relaying the commands dictated to it by supraspinal systems. This review challenges this idea by presenting evidence that the spinal motor system is an active participant in several aspects of the production of movement, contributing to functions normally ascribed to 'higher' brain regions.

Activation of locomotion in adult chronic spinal rats is achieved by transplantation of embryonic raphe cells reinnervating a precise lumbar level

Traumatic lesions of the spinal cord yield a loss of supraspinal control of voluntary locomotor activity, although the spinal cord contains the necessary circuitry to generate the basic locomotor pattern. In spinal rats, this network, known as central pattern generator (CPG), was shown to be sensitive to serotonergic pharmacological stimulation. In previous works we have shown that embryonic raphe cells transplanted into the sublesional cord of adult rats can reinnervate specific targets, restore the lesion-induced increase in receptor densities of neurotransmitters, promote hindlimb weight support, and trigger a locomotor activity on a treadmill without any other pharmacological treatment or training. With the aim of discriminating whether the action of serotonin on CPG is associated to a specific level of the cord, we have transplanted embryonic raphe cells at two different levels of the sublesional cord (T9 and T11) and then performed analysis of the kinematic and EMG activity synchronously recorded during locomotion. Locomotor performances were correlated to the reinnervated level of the cord and compared to that of intact and transected nontransplanted animals. The movements expressed by T11 transplanted animals correspond to a well defined locomotor pattern comparable to that of the intact animals. On the contrary, T9 transplanted animals developed limited and disorganized movements as those of nontransplanted animals. The correlation of the locomotor performances with the level of reinnervation of the spinal cord suggests that serotonergic reinnervation of the L1-L2 level constitutes a key element in the genesis of this locomotor rhythmic activity. This is the first in vivo demonstration that transplanted embryonic raphe cells reinnervating a specific level of the cord activate a locomotor behavior.

Noradrenergic control of locomotor networks in the in vitro spinal cord of the neonatal rat

In this paper, we address the role of noradrenaline (NA) in the control of locomotor networks in the neonatal rat. Using an in vitro isolated spinal cord preparation, we observed that bath application of NA elicited an extremely slow alternating motor pattern (period around 80-90 s) alternating between contralateral sides, which was recorded in the lumbar ventral roots but not between flexor and extensor units. These effects of NA were mimicked by the alpha1 agonists methoxamine and phenylephrine, whereas alpha2 and beta receptors bath-applied alone did not elicit any activity. NA slightly affected the locomotor-like activity induced by the activation of NMDA receptors, whereas the alpha1 agonists speeded up the locomotor activity. Both the activation of alpha2 and beta agonist receptors slowed down the motor rhythm while simultaneously increasing the burst amplitude in the case of the beta agonists. NA depolarized the motoneurones and increased their input membrane resistance. It was concluded that NA does not trigger locomotor activity, but modulates the activity of the locomotor networks. This multimodal control is mediated by the activation of a variety of receptor types.

Effects of noradrenaline on locomotor rhythm-generating networks in the isolated neonatal rat spinal cord

We have studied the effects of the biogenic amine noradrenaline (NA) on motor activity in the isolated neonatal rat spinal cord. The motor output was recorded with suction electrodes from the lumbar ventral roots. When applied on its own, NA (0.5-50 microM) elicited either no measurable root activity, or activity of a highly variable nature. When present, the NA-induced activity consisted of either low levels of unpatterned tonic discharges, or an often irregular, slow rhythm that displayed a high degree of synchrony between antagonistic motor pools. Finally, in a few cases, NA induced a slow locomotor-like rhythm, in which activity alternated between the left and right sides, and between rostral and caudal roots on the same side. As shown previously, stable locomotor activity could be induced by bath application of N-methyl-D-aspartate (NMDA; 4-8.5 microM) and/or serotonin (5-HT; 4-20 microM). NA modulated this activity by decreasing the cycle frequency and increasing the ventral root burst duration. These effects were dose dependent in the concentration range 1-5 microM. In contrast, at no concentration tested did NA have consistent effects on burst amplitudes or on the background activity of the ongoing rhythm. Moreover, NA did not obviously affect the left/right and rostrocaudal alternation of the NMDA/5-HT rhythm. The NMDA/5-HT locomotor rhythm sometimes displayed a time-dependent breakdown in coordination, ultimately resulting in tonic ventral root activity. However, the addition of NA to the NMDA/5-HT saline could reinstate a well-coordinated locomotor rhythm. We conclude that exogenously applied NA can elicit tonic activity or can trigger a slow, irregular and often synchronous motor pattern. When NA is applied during ongoing locomotor activity, the amine has a distinct slowing effect on the rhythm while preserving the normal coordination between flexors and extensors. The ability of NA to "rescue" rhythmic locomotor activity after its time-dependent deterioration suggests that the amine may be important in the maintenance of rhythmic motor activity.

Tapping into spinal circuits to restore motor function

Motivated by the challenge of improving neuroprosthetic devices, the authors review current knowledge relating to harnessing the potential of spinal neural circuits, such as reflexes and pattern generators. If such spinal interneuronal circuits could be activated, they could provide the coordinated control of many muscles that is so complex to implement with a device that aims to address each participating muscle individually. The authors' goal is to identify candidate spinal circuits and areas of research that might open opportunities to effect control of human limbs through electrical activation of such circuits. David McCrea's discussion of the ways in which hindlimb reflexes in the cat modify motor activity may help in developing optimal strategies for functional neuromuscular stimulation (FNS), by using knowledge of how reflex actions can adapt to different conditions. Michael O'Donovan's discussion of the development of rhythmogenic networks in the chick embryo may provide clues to methods of generating rhythmic activity in the adult spinal cord. Serge Rossignol examines the spinal pattern generator for locomotion in cats, its trigger mechanisms, modulation and adaptation, and suggests how this knowledge can help guide therapeutic approaches in humans. Hugues Barbeau applies the work of Rossignol and others to locomotor training in human subjects who have suffered spinal cord injury (SCI) with incomplete motor function loss (IMFL). Michel Lemay and Warren Grill discuss some of the technical challenges that must be addressed by engineers to implement a neuroprosthesis using electrical stimulation of the spinal cord, particularly the control issues that would have to be resolved.

Hindlimb locomotor and postural training modulates glycinergic inhibition in the spinal cord of the adult spinal cat

Adult spinal cats were trained initially to perform either bipedal hindlimb locomotion on a treadmill or full-weight-bearing hindlimb standing. After 12 wk of training, stepping ability was tested before and after the administration (intraperitoneal) of the glycinergic receptor antagonist, strychnine. Spinal cats that were trained to stand after spinalization had poor locomotor ability as reported previously, but strychnine administration induced full-weight-bearing stepping in their hindlimbs within 30-45 min. In the cats that were trained to step after spinalization, full-weight-bearing stepping occurred and was unaffected by strychnine. Each cat then was retrained to perform the other task for 12 wk and locomotor ability was retested. The spinal cats that were trained initially to stand recovered the ability to step after they received 12 wk of treadmill training and strychnine was no longer effective in facilitating their locomotion. Locomotor ability declined in the spinal cats that were retrained to stand and strychnine restored the ability to step to the levels that were acquired after the step-training period. Based on analyses of hindlimb muscle electromyographic activity patterns and kinematic characteristics, strychnine improved the consistency of the stepping and enhanced the execution of hindlimb flexion during full-weight-bearing step cycles in the spinal cats when they were trained to stand but not when they were trained to step. The present findings provide evidence that 1) the neural circuits that generate full-weight-bearing hindlimb stepping are present in the spinal cord of chronic spinal cats that can and cannot step; however, the ability of these circuits to interpret sensory input to drive stepping is mediated at least in part by glycinergic inhibition; and 2) these spinal circuits adapt to the specific motor task imposed, and that these adaptations may include modifications in the glycinergic pathways that provide inhibition.

Recovery of locomotion after ventral and ventrolateral spinal lesions in the cat. II. Effects of noradrenergic and serotoninergic drugs

The effects of serotoninergic and noradrenergic drugs (applied intrathecally) on treadmill locomotion were evaluated in two adult cats subjected to a ventral and ventrolateral spinal lesion (T13). Despite the extensive spinal lesion, severely damaging important descending pathways such as the reticulo- and vestibulospinal tracts, both cats recovered quadrupedal voluntary locomotion. As detailed in a previous paper, the locomotor recovery occurred in three stages defined as early period, when the animal could not walk with its hindlimbs, recovery period, when progressive improvement occurred, and plateau period, when a more stable locomotor performance was observed. At this latter stage, the cats suffered from postural and locomotor deficits, such as poor lateral stability, irregular stepping of the hindlimbs, and inconsistent homolateral fore- and hindlimb coupling. The present study aimed at evaluating the potential of serotoninergic and/or noradrenergic drugs to improve the locomotor abilities in the early and late stages. Both cats were implanted chronically with an intrathecal cannula and electromyographic (EMG) electrodes, which allowed determination, under similar recording conditions, of the locomotor performance pre- and postlesion and comparisons of the effects of different drugs. EMG and kinematic analyses showed that norepinephrine (NE) injected in early and plateau periods improved the regularity of the hindlimb stepping and stabilized the interlimb coupling, permitting to maintain constant locomotion for longer periods of time. Methoxamine, the alpha1-agonist (tested only at the plateau period), had similar effects. In contrast, the alpha2-agonist, clonidine, deteriorated walking. Serotoninergic drugs, such as the neurotransmitter itself, serotonin (5HT), the precursor 5-hydroxytryptophan (5HTP), and the agonist quipazine improved the locomotion by increasing regularity of the hindlimb stepping and by increasing the step cycle duration. In contrast, the 5HT1A agonist 8-hydroxy-dipropylaminotetralin (DPAT) caused foot drag in one of the cats, resulting in frequent stumbling. Injection of combination of methoxamine and quipazine resulted in maintained, regular stepping with smooth movements and good lateral stability. Our results show that the effects of drugs can be integrated to the residual voluntary locomotion and improve some of its postural aspects. However, this work shows clearly that the effects of drugs (such as clonidine) may depend on whether or not the spinal lesion is complete. In a clinical context, this may suggest that different classes of drugs could be used in patients with different types of spinal cord injuries. Possible mechanisms underlying the effect of noradrenergic and serotoninergic drugs on the locomotion after partial spinal lesions are discussed.

A novel method for locomotion training

This article describes a novel therapeutic system for locomotion training and learning for patients with a wide range of neurological and musculoskeletal disorders. The technique embraces the notion that locomotion therapy should be goal oriented and task specific. The task specificity includes a partial weight-bearing device that permits the posture/equilibrium, movement, and weight-bearing components of gait function to operate concurrently, even in patients with serious deficits. In addition, it allows interaction with therapists and others to facilitate locomotion control, particularly during the early stages of gait therapy. Neurobiological bases for this technique and early clinical results are discussed, and two case studies of patients with traumatic brain injury (TBI) are presented. Although well-designed efficacy studies are needed, clearly this therapeutic approach to locomotor disorders among TBI patients meets the various criteria for recovery of gait function established in this article.

Regeneration in the spinal cord

Important advances have been made in our understanding of conditions that influence the intrinsic capacity of mature CNS neurons to initiate and maintain a regrowth response. The combination of exogenous neurotrophic support with strategies to alter the terrain at the injury site itself suggests that there are important interactions between them that lead to increased axonal regeneration. The ability of chronically injured neurons to initiate a regeneration response is unexpected. Our view of the role that inhibitors play in restricting axonal growth has also expanded. The findings indicate that the windows of opportunity for enhancing growth after spinal cord injury may be more numerous than previously thought.

Pharmacological activation and modulation of the central pattern generator for locomotion in the cat

Pharmacological agents have been shown to be capable of inducing a pattern of rhythmic activity recorded in muscle nerves or motoneurons of paralyzed spinal cats that closely resembles the locomotor pattern seen in intact cats. Further work, using intraperitoneal or intrathecal injections, suggests that different neurotransmitters may be involved in various aspects of locomotor control, e.g., initiation and modulation of the pattern. Although precursors, agonists or the neurotransmitters themselves of several systems have been investigated (noradrenergic, dopaminergic, serotonergic, glutamatergic), the noradrenergic system seems the most efficient in triggering locomotion in complete spinal cats, with the alpha-2 agonists (clonidine, tizanidine, oxymetazoline) being more potent than the alpha-1 agonist, methoxamine. Moreover, the potency of the drugs may depend on the time of application after the spinal lesion. In chronic spinal cats capable of spontaneous walking on hindlimbs on the treadmill, all neurotransmitters appear to exert distinct recognizable effects on the locomotor pattern. More recent work also suggests that the effects of drugs may differ significantly depending on the type of spinal lesion. For instance, clonidine further reduces the level of weight support during quadrupedal locomotion of cats with lesions of the ventral-ventrolateral funiculi, possibly due to an interference of clonidine with essential compensatory mechanisms used by these animals to walk. Such considerations as the type of drugs, type of lesions, and the time after the lesion will be important for future studies in spinal cord injured patients.