Induction of fictive locomotion by sulphur-containing amino acids in an in vitro newborn rat preparation

And yet it moves: Recovery of volitional control after spinal cord injury

Preclinical and clinical neurophysiological and neurorehabilitation research has generated rather surprising levels of recovery of volitional sensory-motor function in persons with chronic motor paralysis following a spinal cord injury. The key factor in this recovery is largely activity-dependent plasticity of spinal and supraspinal networks. This key factor can be triggered by neuromodulation of these networks with electrical and pharmacological interventions. This review addresses some of the systems-level physiological mechanisms that might explain the effects of electrical modulation and how repetitive training facilitates the recovery of volitional motor control. In particular, we substantiate the hypotheses that: (1) in the majority of spinal lesions, a critical number and type of neurons in the region of the injury survive, but cannot conduct action potentials, and thus are electrically non-responsive; (2) these neuronal networks within the lesioned area can be neuromodulated to a transformed state of electrical competency; (3) these two factors enable the potential for extensive activity-dependent reorganization of neuronal networks in the spinal cord and brain, and (4) propriospinal networks play a critical role in driving this activity-dependent reorganization after injury. Real-time proprioceptive input to spinal networks provides the template for reorganization of spinal networks that play a leading role in the level of coordination of motor pools required to perform a given functional task. Repetitive exposure of multi-segmental sensory-motor networks to the dynamics of task-specific sensory input as occurs with repetitive training can functionally reshape spinal and supraspinal connectivity thus re-enabling one to perform complex motor tasks, even years post injury.

Rhythmic motor activity evoked by NMDA in the spinal zebrafish larva

We have examined the localization and activity of the neural circuitry that generates swimming behavior in developing zebrafish that were spinalized to isolate the spinal cord from descending brain inputs. We found that addition of the excitatory amino acid agonist N-methyl-d-aspartate (NMDA) to spinalized zebrafish at 3 days in development induced repeating episodes of rhythmic tail beating activity reminiscent of slow swimming behavior. The neural correlate of this activity, monitored by extracellular recording comprised repeating episodes of rhythmic, rostrocaudally progressing peripheral nerve discharges that alternated between the two sides of the body. Motoneuron recordings revealed an activity pattern comprising a slow oscillatory and a fast synaptic component that was consistent with fictive swimming behavior. Pharmacological and voltage-clamp analysis implicated glycine and glutamate in generation of motoneuron activity. Contralateral alternation of motor activity was disrupted with strychnine, indicating a role for glycine in coordinating left-right alternation during NMDA-induced locomotion. At embryonic stages, while rhythmic synaptic activity patterns could still be evoked in motoneurons, they were typically lower in frequency. Kinematic recordings revealed that prior to 3 days in development, NMDA was unable to reliably generate rhythmic tail beating behavior. We conclude that NMDA induces episodes of rhythmic motor activity in spinalized developing zebrafish that can be monitored physiologically in paralyzed preparations. Therefore as for other vertebrates, the zebrafish central pattern generator is intrinsic to the spinal cord and can operate in isolation provided a tonic source of excitation is given.

L-DOPA-induced air-stepping in the preweanling rat: electromyographic and kinematic analyses

Ontogenetic changes in intralimb coordination may result from maturation of the central pattern for locomotion, maturation of peripheral efferents, changes in afferent modulation of the centrally generated pattern, interactions with the substrate, biomechanical changes within the limb itself, or a combination of these. Electromyograms obtained from three hindlimb extensors of rats on Postnatal Days (PND) 5, 10, 15, or 20, during episodes of coordinated L-DOPA-induced air-stepping, showed that muscle activation preceded extension of the corresponding joints at all ages. The delay between the onset of extensor activity and the onset of joint extension increased during ontogeny and was greatest at PND 20. Ontogenetic changes in the relative timing of muscle activity and corresponding joint movements probably resulted from changes in biomechanical factors, changes in afferent modulation of central motor output, or both.

Ubiquity of motor networks in the spinal cord of vertebrates

In a recent paper, we found that it is possible to record motor activity in sacral segments in the in vitro neonatal rat spinal cord preparation. This motor activity recorded in segments that are not innervating hindlimbs is driven by the lumbar locomotor network. Indeed, compartimentalizations of the cord with Vaseline walls or section experiments, reveals that the sacral segments possess their own rhythmogenic capabilities but that in an intact spinal cord they are driven by the lumbar locomotor network. In this review, these recent findings are placed in the context of spinal motor network interactions. As previously suspected, the motor networks do not operate in isolation but interact with each other according to behavioural needs. These interactions provide some insight into the discrepancies observed in several studies dealing with the localization of the lumbar locomotor network in the neonatal rat spinal cord. In conclusion, the spinal cord of quadrupeds appears as an heterogeneous structure where it is possible to identify neuronal networks that are crucial for the genesis of locomotor-related activities.

GABAergic control of spinal locomotor networks in the neonatal rat

We studied the GABAergic control of the spinal locomotor network using an isolated brain stem/spinal cord from newborn rats, in which locomotor-like activity was recorded. We demonstrate that endogenously released GABA controls the locomotor network, by decreasing or completely abolishing all locomotor-like activity. At first, we investigated the role played by GABA in the control of the locomotor period. By separately superfusing various compartments of the lumbar cord, we identified the targets of GABA. When bath-applied on the upper lumbar segments (L1/L2), GABA or its agonists (muscimol, baclofen) modulated the locomotor period, whereas it had no effects when bath-applied on the caudal lumbar cord (L3/L6). In the second step we studied how GABA may presynaptically control the locomotor drive arising from the locomotor network located in L1/L2. By use of the partitioned spinal cord, intracellular recordings from the caudal pool motoneurons (L4/L5) were performed, while initiating locomotor-like activity in L1/L2. We found that GABA or its agonists decreased the monosynaptic locomotor drive that the motoneurons received from the L1/L2 network, and we found a presynaptic effect exerted through the activation of GABAB receptors. In conclusion, this study emphasizes the role played by GABA at various levels in the control of the locomotor network in mammals.

Reorganization of locomotor activity during development in the prenatal rat

Development of neuronal circuits generating locomotor activity was studied using an isolated lumbar spinal cord preparation from fetal and neonatal rats. Bath application of N-methyl-D-aspartate (NMDA) or 5-HT evoked patterned motor activity resembling that seen during normal fictive locomotion on embryonic day (E) 20.5. Glycine-mediated inhibition was essential to the formation of this coordinated motor activity. In preparations from fetuses at the earlier stages (E14.5-E16.5), we observed spontaneous motoneuronal activity and chemically induced rhythmic bursts, which were synchronized on the two sides in the corresponding ventral roots. The spontaneous activity was not blocked by kynurenate, the glutamate receptor blocker, although it was completely abolished by strychnine, the glycine receptor antagonist. A brief application of glycine evoked excitatory responses resembling the spontaneous bursts in both time course and amplitude. It is concluded that glycine functions transiently as excitatory transmitters at these stages. These results suggest that functional change in glycine-induced responses during development plays an important role in differentiation of the neuronal circuits generating locomotion.

Spinal pattern generation

Recent research in the field of spinal pattern generation has concentrated on three main areas: the effects of various transmitters on spinal rhythmic patterns in reduced preparations (neonatal rats, chick embryos, tadpole embryos, lampreys); the changes in membrane properties of different elements of the generating circuits; and the interactions between central generating mechanisms and afferent inputs. The important message is that new properties of neural membranes, as well as new reflex responses, have been identified that could not have been predicted in the absence of such rhythmic activity.