Treadmill training of rats after sciatic nerve graft does not alter accuracy of muscle reinnervation

Background and purpose

After peripheral nerve lesions, surgical reconstruction facilitates axonal regeneration and motor reinnervation. However, functional recovery is impaired by aberrant reinnervation.

Materials and methods

We tested whether training therapy by treadmill exercise (9 × 250 m/week) before (run-idle), after (idle-run), or both before and after (run-run) sciatic nerve graft improves the accuracy of reinnervation in rats. Female Lewis rats (LEW/SsNHsd) were either trained for 12 weeks (run) or not trained (kept under control conditions, idle). The right sciatic nerves were then excised and reconstructed with 5 mm of a congenic allograft. One week later, training started in the run-run and idle-run groups for another 12 weeks. No further training was conducted in the run-idle and idle-idle groups. Reinnervation was measured using the following parameters: counting of retrogradely labeled motoneurons, walking track analysis, and compound muscle action potential (CMAP) recordings.

Results

In intact rats, the common fibular (peroneal) and the soleus nerve received axons from 549 ± 83 motoneurons. In the run-idle group, 94% of these motoneurons had regenerated 13 weeks after the nerve graft. In the idle-run group, 81% of the normal number of motoneurons had regenerated into the denervated musculature and 87% in both run-run and idle-idle groups. Despite reinnervation, functional outcome was poor: walking tracks indicated no functional improvement of motion in any group. However, in the operated hindlimb of run-idle rats, the CMAP of the soleus muscle reached 11.9 mV (normal 16.3 mV), yet only 6.3-8.1 mV in the other groups.

Conclusion

Treadmill training neither altered the accuracy of reinnervation nor the functional recovery, and pre-operative training (run-idle) led to a higher motor unit activation after regeneration.

Noradrenergic Components of Locomotor Recovery Induced by Intraspinal Grafting of the Embryonic Brainstem in Adult Paraplegic Rats

Intraspinal grafting of serotonergic (5-HT) neurons was shown to restore plantar stepping in paraplegic rats. Here we asked whether neurons of other phenotypes contribute to the recovery. The experiments were performed on adult rats after spinal cord total transection. Grafts were injected into the sub-lesional spinal cord. Two months later, locomotor performance was tested with electromyographic recordings from hindlimb muscles. The role of noradrenergic (NA) innervation was investigated during locomotor performance of spinal grafted and non-grafted rats using intraperitoneal application of α₂ adrenergic receptor agonist (clonidine) or antagonist (yohimbine). Morphological analysis of the host spinal cords demonstrated the presence of tyrosine hydroxylase positive (NA) neurons in addition to 5-HT neurons. 5-HT fibers innervated caudal spinal cord areas in the dorsal and ventral horns, central canal, and intermediolateral zone, while the NA fiber distribution was limited to the central canal and intermediolateral zone. 5-HT and NA neurons were surrounded by each other’s axons. Locomotor abilities of the spinal grafted rats, but not in control spinal rats, were facilitated by yohimbine and suppressed by clonidine. Thus, noradrenergic innervation, in addition to 5-HT innervation, plays a potent role in hindlimb movement enhanced by intraspinal grafting of brainstem embryonic tissue in paraplegic rats.

Contribution of 5-HT2 Receptors to the Control of the Spinal Locomotor System in Intact Rats

Applying serotonergic (5-HT) agonists or grafting of fetal serotonergic cells into the spinal cord improves locomotion after spinal cord injury. Little is known about the role of 5-HT receptors in the control of voluntary locomotion, so we administered inverse agonists of 5-HT₂ (Cyproheptadine; Cypr), 5-HT_2A neutral antagonist (Volinanserin; Volin), 5-HT_2C neutral antagonist (SB 242084), and 5-HT_2B/2C inverse agonist (SB 206553) receptors intrathecally in intact rats and monitored their effects on unrestrained locomotion. An intrathecal cannula was introduced at the low thoracic level and pushed caudally until the tip reached the L2/L3 or L5/L6 spinal segments. Locomotor performance was evaluated using EMG activity of hindlimb muscles during locomotion on a 2 m long runway. Motoneuron excitability was estimated using EMG recordings during dorsi- and plantar flexion at the ankle. Locomotion was dramatically impaired after the blockage of 5-HT_2A receptors. The effect of Cypr was more pronounced than that of Volin since in the L5/L6 rats Cypr (but not Volin) induced significant alteration of the strength of interlimb coordination followed by total paralysis. These agents significantly decreased locomotor EMG amplitude and abolished or substantially decreased stretch reflexes. Blocking 5-HT_2B/2C receptors had no effect either on locomotion or reflexes. We suggest that in intact rats serotonin controls timing and amplitude of muscle activity by acting on 5-HT_2A receptors on both CPG interneurons and motoneurons, while 5-HT_2B/2C receptors are not involved in control of the locomotor pattern in lumbar spinal cord.

LFP Oscillations in the Mesencephalic Locomotor Region during Voluntary Locomotion

Oscillatory rhythms in local field potentials (LFPs) are thought to coherently bind cooperating neuronal ensembles to produce behaviors, including locomotion. LFPs recorded from sites that trigger locomotion have been used as a basis for identification of appropriate targets for deep brain stimulation (DBS) to enhance locomotor recovery in patients with gait disorders. Theta band activity (6–12 Hz) is associated with locomotor activity in locomotion-inducing sites in the hypothalamus and in the hippocampus, but the LFPs that occur in the functionally defined mesencephalic locomotor region (MLR) during locomotion have not been determined. Here we record the oscillatory activity during treadmill locomotion in MLR sites effective for inducing locomotion with electrical stimulation in rats. The results show the presence of oscillatory theta rhythms in the LFPs recorded from the most effective MLR stimulus sites (at threshold ≤60 μA). Theta activity increased at the onset of locomotion, and its power was correlated with the speed of locomotion. In animals with higher thresholds (>60 μA), the correlation between locomotor speed and theta LFP oscillations was less robust. Changes in the gamma band (previously recorded in vitro in the pedunculopontine nucleus (PPN), thought to be a part of the MLR) were relatively small. Controlled locomotion was best achieved at 10–20 Hz frequencies of MLR stimulation. Our results indicate that theta and not delta or gamma band oscillation is a suitable biomarker for identifying the functional MLR sites.

Serotonin controls initiation of locomotion and afferent modulation of coordination via 5-HT7 receptors in adult rats

KEY POINTS

Experiments on neonatal rodent spinal cord showed that serotonin (5-HT), acting via 5-HT₇ receptors, is required for initiation of locomotion and for controlling the action of interneurons responsible for inter- and intralimb coordination, but the importance of the 5-HT system in adult locomotion is not clear. Blockade of spinal 5-HT₇ receptors interfered with voluntary locomotion in adult rats and fictive locomotion in paralysed decerebrate rats with no afferent feedback, consistent with a requirement for activation of descending 5-HT neurons for production of locomotion. The direct control of coordinating interneurons by 5-HT₇ receptors observed in neonatal animals was not found during fictive locomotion, revealing a developmental shift from direct control of locomotor interneurons in neonates to control of afferent input from the moving limb in adults. An understanding of the afferents controlled by 5-HT during locomotion is required for optimal use of rehabilitation therapies involving the use of serotonergic drugs.

ABSTRACT

Serotonergic pathways to the spinal cord are implicated in the control of locomotion based on studies using serotonin type 7 (5-HT₇ ) receptor agonists and antagonists and 5-HT₇ receptor knockout mice. Blockade of these receptors is thought to interfere with the activity of coordinating interneurons, a conclusion derived primarily from in vitro studies on isolated spinal cord of neonatal rats and mice. Developmental changes in the effects of serotonin (5-HT) on spinal neurons have recently been described, and there is increasing data on control of sensory input by 5-HT₇ receptors on dorsal root ganglion cells and/or dorsal horn neurons, leading us to determine the effects of 5-HT₇ receptor blockade on voluntary overground locomotion and on locomotion without afferent input from the moving limb (fictive locomotion) in adult animals. Intrathecal injections of the selective 5-HT₇ antagonist SB269970 in adult intact rats suppressed locomotion by partial paralysis of hindlimbs. This occurred without a direct effect on motoneurons as revealed by an investigation of reflex activity. The antagonist disrupted intra- and interlimb coordination during locomotion in all intact animals but not during fictive locomotion induced by stimulation of the mesencephalic locomotor region (MLR). MLR-evoked fictive locomotion was transiently blocked, then the amplitude and frequency of rhythmic activity were reduced by SB269970, consistent with the notion that the MLR activates 5-HT neurons, leading to excitation of central pattern generator neurons with 5-HT₇ receptors. Effects on coordination in adults required the presence of afferent input, suggesting a switch to 5-HT₇ receptor-mediated control of sensory pathways during development.

Thoracic Hemisection in Rats Results in Initial Recovery Followed by a Late Decrement in Locomotor Movements, with Changes in Coordination Correlated with Serotonergic Innervation of the Ventral Horn

Lateral thoracic hemisection of the rodent spinal cord is a popular model of spinal cord injury, in which the effects of various treatments, designed to encourage locomotor recovery, are tested. Nevertheless, there are still inconsistencies in the literature concerning the details of spontaneous locomotor recovery after such lesions, and there is a lack of data concerning the quality of locomotion over a long time span after the lesion. In this study, we aimed to address some of these issues. In our experiments, locomotor recovery was assessed using EMG and CatWalk recordings and analysis. Our results showed that after hemisection there was paralysis in both hindlimbs, followed by a substantial recovery of locomotor movements, but even at the peak of recovery, which occurred about 4 weeks after the lesion, some deficits of locomotion remained present. The parameters that were abnormal included abduction, interlimb coordination and speed of locomotion. Locomotor performance was stable for several weeks, but about 3-4 months after hemisection secondary locomotor impairment was observed with changes in parameters, such as speed of locomotion, interlimb coordination, base of hindlimb support, hindlimb abduction and relative foot print distance. Histological analysis of serotonergic innervation at the lumbar ventral horn below hemisection revealed a limited restoration of serotonergic fibers on the ipsilateral side of the spinal cord, while on the contralateral side of the spinal cord it returned to normal. In addition, the length of these fibers on both sides of the spinal cord correlated with inter- and intralimb coordination. In contrast to data reported in the literature, our results show there is not full locomotor recovery after spinal cord hemisection. Secondary deterioration of certain locomotor functions occurs with time in hemisected rats, and locomotor recovery appears partly associated with reinnervation of spinal circuitry by serotonergic fibers.

Cholinergic mechanisms in spinal locomotion-potential target for rehabilitation approaches

Previous experiments implicate cholinergic brainstem and spinal systems in the control of locomotion. Our results demonstrate that the endogenous cholinergic propriospinal system, acting via M₂ and M₃ muscarinic receptors, is capable of consistently producing well-coordinated locomotor activity in the in vitro neonatal preparation, placing it in a position to contribute to normal locomotion and to provide a basis for recovery of locomotor capability in the absence of descending pathways. Tests of these suggestions, however, reveal that the spinal cholinergic system plays little if any role in the induction of locomotion, because MLR-evoked locomotion in decerebrate cats is not prevented by cholinergic antagonists. Furthermore, it is not required for the development of stepping movements after spinal cord injury, because cholinergic agonists do not facilitate the appearance of locomotion after spinal cord injury, unlike the dramatic locomotion-promoting effects of clonidine, a noradrenergic α-2 agonist. Furthermore, cholinergic antagonists actually improve locomotor activity after spinal cord injury, suggesting that plastic changes in the spinal cholinergic system interfere with locomotion rather than facilitating it. Changes that have been observed in the cholinergic innervation of motoneurons after spinal cord injury do not decrease motoneuron excitability, as expected. Instead, the development of a “hyper-cholinergic” state after spinal cord injury appears to enhance motoneuron output and suppress locomotion. A cholinergic suppression of afferent input from the limb after spinal cord injury is also evident from our data, and this may contribute to the ability of cholinergic antagonists to improve locomotion. Not only is a role for the spinal cholinergic system in suppressing locomotion after SCI suggested by our results, but an obligatory contribution of a brainstem cholinergic relay to reticulospinal locomotor command systems is not confirmed by our experiments.

The upright posture improves plantar stepping and alters responses to serotonergic drugs in spinal rats

Recent studies on the restoration of locomotion after spinal cord injury have employed robotic means of positioning rats above a treadmill such that the animals are held in an upright posture and engage in bipedal locomotor activity. However, the impact of the upright posture alone, which alters hindlimb loading, an important variable in locomotor control, has not been examined. Here we compared the locomotor capabilities of chronic spinal rats when placed in the horizontal and upright postures. Hindlimb locomotor movements induced by exteroceptive stimulation (tail pinching) were monitored with video and EMG recordings. We found that the upright posture alone significantly improved plantar stepping. Locomotor trials using anaesthesia of the paws and air stepping demonstrated that the cutaneous receptors of the paws are responsible for the improved plantar stepping observed when the animals are placed in the upright posture.We also tested the effectiveness of serotonergic drugs that facilitate locomotor activity in spinal rats in both the horizontal and upright postures. Quipazine and (±)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide (8-OH-DPAT) improved locomotion in the horizontal posture but in the upright posture either interfered with or had no effect on plantar walking. Combined treatment with quipazine and 8-OH-DPAT at lower doses dramatically improved locomotor activity in both postures and mitigated the need to activate the locomotor CPG with exteroceptive stimulation. Our results suggest that afferent input from the paw facilitates the spinal CPG for locomotion. These potent effects of afferent input from the paw should be taken into account when interpreting the results obtained with rats in an upright posture and when designing interventions for restoration of locomotion after spinal cord injury.

Changes of the force-frequency relationship in the rat medial gastrocnemius muscle after total transection and hemisection of the spinal cord

The relationships between the stimulation frequency and the force developed by motor units (MUs) of the medial gastrocnemius muscle were compared between intact rats and animals after total transection or hemisection of the spinal cord at the low thoracic level. The experiments on functionally isolated MUs were carried out 14, 30, 90, and 180 days after the spinal cord injury. Axons of investigated MUs were stimulated with trains of pulses at 10 progressively increased frequencies (from 1 to 150 Hz), and the force-frequency curves were plotted. Spinal cord hemisection resulted in a considerable leftward shift of force-frequency curves in all types of MUs. After the total transection, a leftward shift of the curve was observed in fast MUs, whereas there was a rightward shift in slow MUs. These changes coincided with a decrease of stimulation frequencies necessary to evoke 60% of maximal force. Moreover, the linear correlation between these stimulation frequencies and the twitch contraction time observed in intact rats was disrupted in all groups of animals with spinal cord injury. The majority of the observed changes reached the maximum 1 mo after injury, whereas the effects evoked by spinal cord hemisection were significantly smaller and nearly constant in the studied period. The results of this study can be important for the prediction of changes in force regulation in human muscles after various extends of spinal cord injury and in evaluation of the frequency of functional electrical stimulation used for training of impaired muscles.

Time-related changes of motor unit properties in the rat medial gastrocnemius muscle after the spinal cord injury. II. Effects of a spinal cord hemisection

The contractile properties of motor units (MUs) were investigated in the medial gastrocnemius (MG) muscle in rats after the spinal cord hemisection at a low thoracic level. Hemisected animals were divided into 4 groups: 14, 30, 90 and 180 days after injury. Intact rats formed a control group. The mass of the MG muscle did not change significantly after spinal cord hemisection, hind limb locomotor pattern was almost unchanged starting from two weeks after injury, but contractile properties of MUs were however altered. Contraction time (CT) and half-relaxation time (HRT) of MUs were prolonged in all investigated groups of hemisected rats. The twitch-to-tetanus ratio (Tw/Tet) of fast MUs after the spinal cord hemisection increased. For slow MUs Tw/Tet values did not change in the early stage after the injury, but significantly decreased in rats 90 and 180 days after hemisection. As a result of hemisection the fatigue resistance especially of slow and fast resistant MU types was reduced, as well as fatigue index (Fat I) calculated for the whole examined population of MUs decreased progressively with the time. After spinal cord hemisection a reduced number of fast MUs presented the sag at frequencies 30 and 40 Hz, however more of them revealed sag in 20 Hz tetanus in comparison to control group. Due to considerable changes in twitch contraction time and disappearance of sag effect in unfused tetani of some MUs in hemisected animals, the classification of MUs in all groups of rats was based on the 20 Hz tetanus index (20 Hz Tet I) but not on the standard criteria usually applied for MUs classification. MU type differentiations demonstrated some clear changes in MG muscle composition in hemisected animals consisting of an increase in the proportion of slow MUs (likely due to an increased participation of the studied muscle in tonic antigravity activity) together with an increase in the percentage of fast fatigable MUs.

Time-related changes of motor unit properties in the rat medial gastrocnemius muscle after spinal cord injury. I. Effects of total spinal cord transection

The contractile properties of motor units (MUs) were electrophysiologically investigated in the medial gastrocnemius (MG) muscle in 17 Wistar three-month-old female rats: 14, 30, 90 and 180 days after the total transection of the thoracic spinal cord and compared to those in intact (control) rats. A sag phenomenon, regularly observed in unfused tetani of fast units in intact animals at 40 Hz stimulation, almost completely disappeared in spinal rats. Therefore, the MUs of intact and spinal rats were classified as fast or slow types basing on 20 Hz tetanus index, the value of which was lower or equal 2.0 for fast and higher than 2.0 for slow MUs. The MUs composition of MG muscle changed with time after the spinal cord transection: an increasing proportion of fast fatigable (FF) units starting one month after injury and a disappearance of slow (S) units within the three months were observed. In all MUs investigated the twitch contraction and half-relaxation time were significantly prolonged after injury (p<0.01, Mann-Whitney U-test). Moreover, a decrease of the fatigue index for fast resistant (FR) and slow MUs was observed in subsequent groups of spinal rats. No significant changes were found between twitch forces in all MU types of spinal animals (p>0.05). However, due to a decrease of the maximal tetanic force, a significant rise of the twitch-to-tetanus ratio of all MUs in spinal rats was detected (p<0.01). The considerable reduction of ability to potentiate the force was noticed for fast, especially FF type MUs. In conclusion, the spinal cord transection leads to changes in the proportion of the three MU types in rat MG muscle. The majority of changes in MUs' contractile properties were developed progressively with time after the spinal cord injury. However, the most intensive alterations of twitch-time parameters were observed in rats one month after the transection.

Division of motor units into fast and slow of the basis of profile of 20 Hz unfused tetanus

In the medial gastrocnemius muscle of intact rats, division of motor units (MUs) into slow (S) or fast (F) types is typically based on presence of a sag phenomenon in 40 Hz unfused tetanic contraction. MUs with sag are classified as F, while those without sag as S. However, in rats one month after spinal cord injury this phenomenon almost completely disappears and cannot be used as a basis for MUs differentiation, whereas the twitch contraction time increased significantly. Analysis of myosin heavy chain (MHC) isoform composition confirmed transformational changes of muscle fibres after spinal cord transection and indicated unchanged proportion of type I MHC isoforms, disappearance of type IIa MHC isoforms, and increase of type IIb MHC isoforms. We proposed an additional method for division of MUs into types when standard criteria are not applicable. It was observed that relative effectiveness of force summation during 20 Hz tetanus, described as a ratio of the force of the last contraction of this tetanus to the force of the first contraction, did not change after spinal cord injury. This ratio for S MUs both in intact and spinal rats exceeded 2.0, whereas for F units was lower than 2.0. Calculations of this ratio made for better fused tetani, evoked by 30 Hz or 40 Hz stimulation, showed overlapping values. We conclude that this 20 Hz tetanus index appears to be an alternative method useful for division of motor units into S and F types.

Overground Locomotion after Incomplete Spinal Lesions in the Rat: Quantitative Gait Analysis

In rats with incomplete low thoracic spinal cord lesions of different extents, the basic indices of gait such as locomotor velocity, step and stance phase duration and the duty factor (i.e., the relative duration of the stance phase) during overground runway locomotion were analyzed using contact electrodes on each paw for data recording. In animals with lesions confined to the dorsal columns (DC), tested 3 weeks postsurgery, these gait indices were essentially unchanged compared to the preoperative period. After the same recovery period, rats with larger lesions, comprising the dorsal columns plus a major part of the dorsolateral funiculi (DL), showed a transient increase in the hindlimb stance phase duration and the duty factor. More extensive injuries, with additional damage to parts of the ventrolateral and ventral funiculi (VL), produced increments in the stance phase duration and duty factor much above that which would be expected from changes in step cycle duration due to slowing down of locomotion. These changes, which lasted for at least 3 months, were more conspicuous in animals with extensive spinal cord injuries and were due to an altered relationship between the stance phase and step cycle duration. It is suggested that the excessive increment in the hindlimb stance phase and the duty factor constitute a reliable indicator of impairment in locomotor movements, which is correlated with the extent of spinal cord injury.

Locomotor recovery after thoracic spinal cord lesions in cats, rats and humans

More than a hundred years of extensive studies have led to the development of clinically valid animal models of spinal cord injury (SCI) used to investigate neurophysiological mechanisms, pathology and potential therapies. The cat and rat models of SCI were found particularly useful due to several behavioral responses that correspond to clinical symptoms seen in patients. This review concentrates on recovery of motor behavior in the rat and cat models of thoracic spinal cord injury. At the beginning an outline of the general concept of neural control of locomotion: the existence of a spinal network producing the locomotor activity and the supraspinal and sensory inputs that influence this network is presented. Next, the severity of functional impairment in relation to the extent and precise location of lesions at the thoracic level in cats and rats is described. Finally, the impact of animal studies on the treatment of SCI patients and the possibility that a spinal network producing the locomotor activity also exists in humans is discussed.

Intrathecal administration of yohimbine impairs locomotion in intact rats

The effects of upper lumbar level intrathecal injection of yohimbine, an alpha2-noradrenergic antagonist, on overground locomotion in intact rats was studied. This treatment caused dose-dependent impairment of hindlimb locomotor movement, which varied from transient hindlimb paralysis at a dose of 200 microg/20 microl to transient trunk instability at 50 microg/20 microl. Repetitive (every 48 h) injections of yohimbine at high (200 microg/20 microl) and medium (100 microg/20 microl) doses caused tachyphylaxis, which usually led to a lack of reaction to the third injection. This phenomenon was not observed after repetitive injections of the low (50 microg/20 microl) dose of the drug. These results show that the noradrenergic system is involved in the control of locomotion, since intrathecal administration of a specific antagonist affects this activity in intact rats.

Changes in contractile properties of motor units of the rat medial gastrocnemius muscle after spinal cord transection

The effects of complete transection of the spinal cord at the level of Th9/10 on contractile properties of the motor units (MUs) in the rat medial gastrocnemius (MG) muscle were investigated. Our results indicate that 1 month after injury the contraction time (time-to-peak) and half-relaxation time were prolonged and the maximal tetanic force in most of the MUs in the MG muscle of spinal rats was reduced. The resistance to fatigue also decreased in most of the MUs in the MG of spinal animals. Moreover, the post-tetanic potentiation of twitches in MUs diminished after spinal cord transection. Criteria for the division of MUs into three types, namely slow (S), fast fatigue resistant (FR) and fast fatigable (FF), applied in intact animals, could not be directly used in spinal animals owing to changes in contractile properties of MUs. The 'sag' phenomenon observed in unfused tetani of fast units in intact animals essentially disappeared in spinal rats and it was only detected in few units, at low frequencies of stimulation only. Therefore, the MUs in spinal rats were classified as fast or slow on the basis of an adjusted borderline of 20 ms, instead of 18 ms as in intact animals, owing to a slightly longer contraction time of those fast motor units with the 'sag'. We conclude that all basic contractile properties of rat motor units in the medial gastrocnemius muscle are significantly changed 1 month after complete spinal cord transection, with the majority of motor units being more fatigable and slower than those of intact rats.

Serotonin-related enhancement of recovery of hind limb motor functions in spinal rats after grafting of embryonic raphe nuclei

Recently, we demonstrated improvements in hind limb locomotor-like movements following grafting of embryonic raphe nuclei cells into the spinal cord below the level of total transection in adult rats. The purpose of the present study was to clarify whether this improvement was due to newly established serotonergic innervation between the graft and the host. Two months after intraspinal grafting of the embryonic raphe nuclei, the spinalized rats, when put on a treadmill, could be induced to walk with regular alternating hind limb movements with the plantar contact with the ground during the stance phase, and ankle dorsiflexion during the swing phase of each step cycle. In the same situation the spinal rats, that did not receive the graft, were not able to initiate the dorsiflexion of the ankle joint during the swing phase and very often the dorsal surface of the foot was dragged along the ground. Intraperitoneal application of directly acting 5-HT2 antagonist Cyproheptadine (1 mg/kg) impaired reversibly the hind limb locomotor-like movements in grafted rats. This impairment lasted for 2-3 h. The same procedure in control rats did not markedly alter the hind limb locomotor-like movements. The effect of Cyproheptadine in grafted rats was reversed by i.p. injections of the 5-HT2 agonist Quipazine (0.5 mg/kg). These results show that the graft-induced restitution of hind limb locomotor abilities in adult spinal rats is brought about by the new serotonergic innervation of the host spinal cord circuitry from the grafted neurons and is mediated by 5-HT2 receptors.

Intrathecal application of cyproheptadine impairs locomotion in intact rats

In intact adult rats, cyproheptadine, a 5-HT2 antagonist, administered intrathecally at the midlumbar segments was found to impair hindlimb locomotor movements during overground locomotion. These effects were dose-dependent; they varied from transient complete hindlimb paraplegia seen at doses of 300 microg/20 microl, to short-lasting trunk instability at doses of 100 microg/20 microl. After the return of overground locomotion, transient abduction of one of the hindlimbs was observed in some animals. These findings demonstrate that the blockade of 5-HT2 receptors affects locomotion in intact rats. Our results provide support for the hypothesis of serotonergic involvement in rat locomotion, which, so far, has been based mainly on the effects of 5-HT2 agonists on the recovery of locomotion in spinal rats.

Recovery of hindlimb motor functions after spinal cord transection is enhanced by grafts of the embryonic raphe nuclei

In this study, a piece of embryonic tissue from the raphe nucleus was transplanted into the spinal cord below the lesion 1 month after transection. Two months later the recovery of hindlimb motor function in rats which had received a transplant of neural tissue (ST rats) was much better than in spinal control animals without the graft (SC rats). Analysis of the electromyographic (EMG) activity showed that the timing of muscle activity during locomotor-like movement of hindlimbs in ST rats was more regular than in SC rats. In SC rats the relationships between EMG burst duration (soleus, tibialis anterior) and step cycle duration were significantly altered. The restoration of hindlimb motor function of ST rats was also reflected in the better interlimb coordination during locomotor-like hindlimb movements. The results of several behavioural tests demonstrated that the responses to stimulation of various receptors, such as tactile or proprioceptive, in ST rats were more complex than in SC rats. Additionally, unlike in SC animals, in ST rats long-lasting spontaneous episodes of air stepping movement of hindlimbs accompanied by a relatively high amplitude of EMG activity were obtained. These results confirm that grafted embryonic raphe nuclei which contain serotoninergic cells are likely to increase the excitability of neuronal circuitry in the injured spinal cord. Moreover, transplantation of embryonic raphe nuclei encourages the recovery of hindlimb motor function in adult rats even when the grafting is carried out several weeks after spinal cord injury.

Overground locomotion in intact rats: interlimb coordination, support patterns and support phases duration

The interlimb coordination during overground locomotion was analysed in intact rats, using the method of contact electrodes (Górska et al. 1998). It was found that in animals moving with a speed ranging from 10 to 78 cm/s (step cycles 685 to 215 ms, respectively) the interlimb coordination was characterized by homologous phase shifts close to 0.5 and much shorter diagonal than lateral phase shifts. These features corresponded to symmetrical gait with diagonal sequence and diagonal couplets (Hildebrandt 1976). Shortening the step cycle changed the gait from a walking trot (duty factor > 0.5) into a running trot (duty factor < 0.5). Correspondingly, the support patterns in the four-legged step cycles, i.e., the sequence of phases of support on various limbs changed: the support on diagonal limbs persisted but the three-limb support was replaced by one-limb support and the support on homolateral limbs by phases of flight. For each phase of support the relationship between its absolute and relative durations and the step cycle duration is being described. The paper explains the variability of support patterns described in the literature. The picture of locomotion obtained in intact rats will be used as a template for studying locomotor control deficits after CNS lesions.

Overground locomotion in intact rats: contact electrode recording

The aim of the experiments was to check the validity of the method of contact electrodes for studying overground locomotion in the rat. The basic indices of locomotion, obtained in 7 intact rats with at least 100 steps recorded in each, were analysed and compared with those described by other authors using different methods of movement recording. It was found that the method of contact electrodes gives reproducible and reliable results and may thus be used in further experiments of rat locomotion after CNS lesions.

Different forms of impairment of the fore-hindlimb coordination after partial spinal lesions in cats

Effects of large low thoracic (T10-T11) partial spinal lesions involving either the ventral quadrants of the spinal cord and, to a different extent the dorsolateral funiculi, or different extent of the lateral funiculi and/or the dorsal columns, on the fore-hindlimb coordination were examined in cats walking overground at moderate speeds. In both groups of operated cats, except those in which the lesion was essentially confined to dorsal columns, three different forms of impairment of fore-hindlimb coordination were observed, depending on the extent of lesion: (1) a change of locomotion towards pacing with preservation of the equality rhythms in the fore- and the hindlimbs; (2) episodes of fore- and hindlimb rhythm dissociation and (3) a permanent dissociation of the fore- and hindlimb rhythms. A comparison of the results obtained in these two groups of operated cats points to the more important role played by the lateral funiculi, than by other parts of the spinal white matter, in controlling the fore-hindlimb coordination in cats.

Different patterns of fore-hindlimb coordination during overground locomotion in cats with ventral and lateral spinal lesions

The effect of large, low thoracic (T10-T11), partial spinal lesions involving the ventral quadrants of the spinal cord and, to a different extent, the dorsolateral funiculi, on fore-hindlimb coordination was examined in cats walking overground at moderate speeds (40-100 cm/s). Three different forms of impairment of fore-hindlimb coordination depending on the extent of the lesions, were observed. Lesions sparing the dorsolateral or the ventral funiculus on one side preserved the equality of the fore- and hindlimb locomotor rhythms but changed the coupling between the movements of both girdles as compared to intact animals. Larger lesions in which, in addition to the ventral quadrants of the spinal cord, also major parts of the dorsolateral funiculi were destroyed elicited episodes of rhythm oscillations in both girdles, which appeared at the background of a small difference in these rhythms. Lesions destroying almost the whole spinal cord induced a permanent difference (about 200 ms) in the step cycle duration of the fore- and the hindlimbs. However, even in these animals some remnant form of fore-hindlimb coordination was found. The results suggest that dorsolateral funiculi play a major role in preserving the equality of rhythms in the fore- and the hindlimbs, while lesions of the ventral quadrants change the coupling between limbs.

Unrestrained walking in intact cats

In five freely moving cats walking with speeds of 0.4-1.0 m/s several parameters of locomotion were investigated. Special attention was paid to the analysis of support patterns and the duration of support phases. The animals used almost exclusively (in 88 to 99% of steps) the 3-2-3-2-3-2-3-2 support pattern in which phases of support on three limbs alternated with phases of support on two limbs, homolateral and diagonal. The relative duration of support phases showed a tendency to decrease with increased locomotor velocity, except for the supports on diagonal limbs which slightly increased. The mean duration of the majority of support phases was similar and ranged between 12.2 and 14.5% of the step cycle. Phases of support on both hind- and one forelimb were somewhat (about 5%) shorter. It is concluded that the relative stability of support patterns and of the duration of support phases during walking observed in the present experiment may serve as a template for comparing changes in the gait produced by various CNS lesions.

Unrestrained walking in cats with partial spinal lesions

In four cats with partial spinal lesions, performed at a low thoracic level, involving ventral quadrants and, to a different extent, the dorsolateral funiculi, several parameters of locomotion were analyzed during unrestrained walking at moderate speed (0.3-1.0 m/s). Special attention was paid to the analysis of support patterns and the durations of support phases in step cycles. The operated subjects displayed a much greater variability of support patterns than intact cats as well as changes in the relative duration of some support phases. The most striking difference was an increase in the relative duration of support on two homolateral limbs accompanied by a reduction of support on diagonal limbs. These changes were mainly due to an impairment of fore-hindlimb coordination as shown by an increase in the phase shifts between the movements of diagonal limbs. Other parameters of locomotion were essentially unaltered, except for cats in which the lesion destroyed bilaterally major portions of the dorsolateral funiculi.