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1Departments of Physiological Science and 2Neurobiology and 3Brain Research Institute, University of California, Los Angeles, California; and 4Pavlov Institute of Physiology, St. Petersburg, Russia
Submitted 26 July 2007; accepted in final form 7 September 2007
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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; Iwahara et al. 1991), as well as in chronically spinalized cats (Gerasimenko et al. 2003) and rats (Ichiyama et al. 2005). These studies showed that ES can activate the intraspinal neural networks that coordinate and recruit the motoneuronal pools with the precision required for stepping.
The features of the stepping pattern induced by ES in decerebrated, and in acute and chronically spinalized animals, however, differ substantially. In decerebrated cats, ES elicits a well-organized stepping pattern with weight-bearing and plantar foot placement. In contrast, the stepping induced by ES in acute spinal cord transected cats is not weight bearing, although the durations of the EMG bursts before and after transection are similar (Iwahara et al. 1991). ES elicits rhythmic alternating hindlimb movements in chronic spinal cats and rats, but these movements are not weight bearing, and plantar foot placement occurs rarely (Gerasimenko et al. 2002; Ichiyama et al. 2005). These observations highlight two important points: 1) ES can initiate rhythmic hindlimb movements in the absence of any supraspinal control and 2) a tonic drive within the spinal cord seems to be necessary to generate weight-bearing stepping during ES.
The concept of the importance of tonic descending drive in maintaining posture has been reviewed (Deliagina et al. 2006). How can a tonic drive be generated in spinal animals when supraspinal control is absent? One means of accomplishing this is by the use of pharmacological agents, in particular through monoamines such as clonidine and quipazine. For example, Jankowska et al. (1967) showed that the intravenous administration of L-DOPA induced locomotor-like bursting activity in the nerve filaments to the flexor and extensor muscles of the hindlimb in acute spinal cats in the absence of cyclic afferent input. Subsequently, a large number of experiments have shown differential modulation of flexor and extensor motor pools during locomotion in response to monoamines (Rossignol et al. 1998; for review, see Schmidt and Jordan 2000).
Recently, we have shown that stepping in complete spinal mice can be markedly facilitated when quipazine administration is combined with step training (Fong et al. 2005). Here we hypothesize that epidural spinal cord stimulation after quipazine (a nonspecific 5-HT2 agonist) administration will differentially modulate the excitability of flexor- and extensor-related intraspinal neural networks in qualitatively unique, but complementary, ways while facilitating locomotion in spinal cord–injured rats. Furthermore, the data suggest that the quality of the locomotor response to ES and quipazine treatment in chronic spinally transected rats is a function of their training state. Preliminary results have been published (Gerasimenko et al. 2005a, 2006b).
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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6 wk while being stimulated in the presence of quipazine during each training session. All procedures followed the National Institute of Health Guide for the Care and Use of Laboratory Animals and were approved by the Animal Use Committee at the University of California, Los Angeles. ST and electrode implantation procedures
All surgical procedures were performed under aseptic conditions. The rats were anesthetized with a combination of ketamine (100 mg/kg) and xylazine (10 mg/kg) administered intraperitoneally and maintained at a deep level of anesthesia with supplemental doses of ketamine as needed. The spinal cord was exposed by a partial laminectomy at T8–T9 vertebral level, the dura was incised longitudinally, and the spinal cord was completely transected at T9 spinal cord level using microscissors as described previously (Talmadge et al. 2002). Gelfoam was inserted into the gap created by the transection as a coagulant and to separate the cut ends of the spinal cord. Partial laminectomies were performed to expose spinal cord segments L2 and S1 for implantation of epidural stimulating electrodes. The stimulation electrodes were made by removing a small portion (1-mm notch) of the Teflon coating to expose the stainless steel wire (AS632, 50-µm gauge; Cooner Wire, Chatsworth, CA) on the surface facing the spinal cord and secured in position as close to the midline as possible by suturing the wire to the dura mater above and below the segment of interest as described previously (Ichiyama et al. 2005). Proper location of the epidural electrodes was verified postmortem.
Bipolar intramuscular EMG electrodes were inserted into the following muscles bilaterally: rectus femoris (RF), vastus lateralis (VL), semitendinosus (St), medial gastrocnemius (MG), and tibialis anterior (TA) as described previously (Roy et al. 1991). The electrodes for recording EMG signals were made by removing 0.5–1.0 mm of insulation from each wire, and each wire was secured with a suture at its entry and exit from the muscle. A common ground electrode (a wire with 1–2 cm of Teflon insulation removed at the distal end) was inserted subcutaneously in the mid-back region. All wires were attached to a headplug secured firmly on the skull as described previously (Lavrov et al. 2006). Proper location of the EMG electrodes was verified postmortem. Postsurgical care and maintenance procedures were similar to those described previously for spinal cord–injured cats and rats (Lavrov et al. 2006; Roy et al. 1992).
Stimulation and recording procedures
ES was achieved through wires chronically implanted at the L2 and S1 spinal cord segments using a Grass S88 stimulator. Rectangular pulses (200-µs duration) were delivered through a constant voltage stimulus isolation unit (Grass SIU5) at 40 Hz (Ichiyama et al. 2005). The intensity of stimulation ranged from 1 to 13 V. EMG signals were differentially amplified (AM System, model 1700) using a band-pass filter of 30 Hz to 10 kHz. Playback data were digitized at 2 kHz using a National Instrument A/D board and analyzed with computer programs developed with the LabView package. A four-camera system with retro-reflective markers placed on bony landmarks at the antero-posterior border of the iliac crest, greater trochanter, lateral condyle, lateral malleolus, and the metatarsal joint of the fifth digit on both legs was used to record the kinematics of the hip, knee, and ankle joints (Gerasimenko et al. 2006a; Ichiyama et al. 2005).
Data analysis
Video analysis of the kinematics of the stepping movements was performed using a Peak Motion Analysis System (Peak Performance Co.). This arrangement allowed us to perform a three-dimensional (3-D) reconstruction of the synchronized movements of both legs. Stick diagrams and trajectories of knee ankle were calculated using Motus (Peak System) software.
The raw EMG signals were passed through a butterworth band-pass filter with a low cut-off of 30 Hz and high cut-off of 500 Hz and rectified. Cycle period was calculated as the time between the onset of an EMG burst and the onset of the following EMG burst. Burst duration was determined as the time between the onset and the offset of an EMG burst. Plots of the relationship between the burst duration of a flexor (TA) and an extensor (MG) muscle versus cycle period were constructed. The maximal amplitude of each rectified EMG burst was calculated from base line to maximal value. For all step-related parameters 
10 steps were analyzed unless stated otherwise. Spectral analysis of the EMG activity was performed using fast Fourier transformation (FFT) procedures (Cohen 1995; O'Neal et al. 2005). FFT analysis of separate EMG bursts during bipedal stepping was performed in all rats. FFT analysis of separate EMG bursts induced by ES in the presence and absence of quipazine was performed in ST and ST-Tr rats. Spectral peaks in the frequency range from 0 to 500 Hz were calculated.
Quipazine treatment
We performed a dose–response study (0.1–1 mg/kg) and found that the optimum dose of quipazine in combination with ES to facilitate coordinated, alternating plantar stepping on a treadmill belt was 0.3 mg/kg delivered intraperitoneally (unpublished observations). As noted previously, we observed that quipazine facilitated stepping beginning 5 min after administration and that this effect lasted for at least 1 h. Based on these results, this dose of quipazine was administered 10 min before each step training or testing session in ST and ST-Tr rats.
Training and testing procedures
The rats in the control group were tested while stepping bipedally on a motor-driven rodent treadmill belt at varying speeds (9–21 cm/s). A cylindrical tube made of soft fabric was secured to a body weight support device (Ahn et al. 2005). The rats climbed into the tube such that the head and forelimbs remained inside the cylinder. The cylinder was connected to a weight-supporting device that was positioned over a treadmill belt and used to maintain the rat's body at 45° relative to the surface of the treadmill belt. Using this system, the rats were allowed to bear the maximum amount of their body weight while maintaining relatively normal bipedal stepping kinematics. The duration of each training session was between 15 and 20 min.
For the rats in the ST groups, a body harness support system was used during bipedal treadmill stepping. An automated body weight support system provided the specific amount of body weight support that each rat needed to perform the bipedal locomotor task (de Leon et al. 2002). The rats in the ST-Tr group were administered quipazine 10 min before each stepping session and were step trained daily (7 days/wk, for 20 min) for 6 wk. In addition, continuous bipolar ES at 40 Hz was applied to either the L2 or S1 spinal cord segment during the stepping sessions, i.e., at the frequency of epidural stimulation that has been shown to be the most effective for inducing stepping movements (Ichiyama et al. 2005). At the beginning of each session, the cathode was located at L2 and the anode at S1. After 10 min, the polarity was reversed, i.e., the cathode was located at S1 and the anode at L2. All stimulation for testing was done with the cathode at S1. Recording sessions to evaluate the locomotor ability of all spinal rats were performed weekly for 6 wk, starting 1 wk after surgery. During these sessions, the hindlimb stepping patterns induced by ES at S1 were determined before and 10 min after quipazine administration. The last recording of EMG activity in all spinal rats with ES and quipazine administration was performed 35–40 days after surgery. Control rats were tested two times 1 wk apart with no ES or quipazine treatment.
Statistical analysis
Data are presented as means ± SE. Statistical significance was assessed using paired t-tests in prelesion vs. postlesion comparisons. Unpaired t-tests were used when comparing across groups. Statistical significance was determined at P < 0.05.
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RESULTS |
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The results presented in the following sections were obtained during the last recording session, i.e., between 4 and 6 wk after spinal cord transection. Figures 1 and 2 show the effects of increasing ES (40 Hz) intensity at S1 on the locomotor behavior of ST-Tr rats that were administered quipazine 10 min before testing. Hindlimb stepping movements usually were initiated at a stimulation intensity of 4 V (Fig. 1, A–C). An increase in ES intensity (up to 5 or 6 V) during stepping at a constant treadmill speed (9 cm/s) increased the EMG burst amplitudes (Figs. 1, A–C, and 2A). It also shortened the step cycle period and EMG burst durations (Figs. 1, A–C, and 2B) in the RF, St, MG, and TA, showing a centrally derived modulation of stepping frequency.
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During low-intensity ES (4 V), the TA EMG bursts occurred between consecutive MG EMG bursts (see expanded EMG bursts at the bottom of Fig. 1C). With ES at 5 V, two EMG bursts became more apparent in the TA with an initial low-amplitude burst and a second high-amplitude burst (marked by asterisks in the bottom traces in Fig. 1C). In contrast, only relatively short, high-amplitude TA EMG bursts were observed when stimulated at high intensity (6 V). At the 6-V intensity, the bursting pattern in the MG and TA remained distinct compared with the St and RF, i.e., the periodic bursting was clearly sustained in the MG and TA but not the RF and St (Fig. 1C).
The hindlimb locomotor activity occurring in ST-Tr rats after ES alone (Fig. 3A) and with ES combined with quipazine administration (Fig. 3B) differed markedly. In the absence of quipazine administration, ES applied at S1 initially evoked rhythmic EMG bursting activity that was seen most clearly in the extensor muscle (MG) (Fig. 3A). When ES was combined with quipazine, the initial response was in the flexor motor pools, with the St and TA evoking two, and sometimes three, synchronous relatively high-amplitude, short EMG bursts (Fig. 3B). After these initial short bursts, locomotor-like EMG bursting activity occurred in all muscles. The MG EMG bursting activity, in general, appeared later and frequently the first EMG burst was short and of a low amplitude. In contrast, the high-amplitude flexor bursts observed initially in the presence of quipazine were followed by consistent bursting with a lower amplitude and longer cycle period. An additional difference between the effects of ES on the initiation of stepping in the presence, compared with that in the absence, of quipazine was a much clearer on:off bursting pattern among the flexor motor pools in the presence of quipazine. The patterns of flexor and extensor activation shown in Fig. 3 were typical for the two rats that had received step training.
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The stepping pattern in ST-Tr rats in the presence of ES and quipazine was highly coordinated with plantar foot placement and with partial weight bearing. The mean cycle duration (1.50 ± 0.02 s) and the mean burst duration of the TA (0.50 ± 0.02 s) were higher in ST-Tr than control rats, whereas the mean burst duration of the MG was similar to control rats (Fig. 4A). The mean EMG amplitude in both the MG (0.25 ± 0.02 mV) and TA (0.60 ± 0.03 mV) were higher in St-Tr than control rats. In summary, the rhythm of the stepping pattern in ST-Tr rats was slower than in control and ST rats and the activity of the flexor muscle (TA) during stepping was dominant.
The effects of quipazine in ST and ST-Tr rats with ES on cycle period duration and EMG burst duration and amplitude are shown in Fig. 5. The mean cycle period duration was >5 times longer after than before quipazine administration in the ST-Tr compared with ST rats. Similarly, the increase in EMG burst duration of the MG was longer after than before quipazine in the ST-Tr compared with ST rats (Fig. 5, A and B). Only after quipazine administration could a TA burst be identified in ST rats (Fig. 5C). As shown in Fig. 4, the effect of quipazine on burst amplitude was greater in the TA than the MG in the ST-Tr rats (Fig. 5D).
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In general, the step cycle period and the duration and amplitude of extensor (MG) and flexor (TA) EMG bursts changed similarly as a function of the treadmill speed in control (Fig. 8, A and D), ST (Fig. 8, B and E), and ST-Tr (Fig. 8, C and F) rats. Cycle period and MG EMG burst duration decreased as the treadmill speed increased from 9 to 21 cm/s in all three groups (Fig. 8, A–C). At the same time, the EMG burst duration of the TA was unaffected in control (Fig. 8A) and ST (Fig. 8B) rats, and increased in the ST-Tr (Fig. 8C) rats. Note the relatively large EMG amplitudes in the ST-Tr (Fig, 8F) rats compared with control (Fig. 8D) and ST (Fig. 8E) rats, consistent with that shown in Fig. 4.
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The differences in the spectral characteristics of the EMG between flexor and extensor motor pools during stepping in the presence of ES and quipazine were readily apparent (Fig. 9B). To examine this quantitatively, we used FFT analysis. Between six and eight EMG bursts for each MG and TA muscle were analyzed by FFT for each condition in each rat. The typical spectral composition for an MG and TA burst are shown in Fig. 9. In control rats, we did not observe dominant spectral peaks in either the TA or MG. The spectral composition of EMG bursts from both the MG and TA ranged from 1 to 500 Hz.
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In ST-Tr rats, ES (at 40 Hz) alone elicited bursting activity in both the extensor (MG) and flexor (TA) muscles (Fig. 10A). After quipazine administration, the burst durations of both muscles were prolonged compared with the durations during ES alone (Fig. 10B). During ES alone, the spectral peaks of MG activity were integer harmonics of the fundamental frequency of 40 Hz (Fig. 10C). After quipazine administration, the spectral composition of the MG did not change, i.e., the dominant spectral peak was still harmonics of the 40-Hz signal, although it was approximately two- to fourfold larger than that observed during ES alone (Fig. 10C). In contrast, quipazine administration had a significant effect on the spectral composition in the TA muscle. During ES in the presence of quipazine, facilitation of the responses was evident in the higher frequency range (i.e., >75 Hz; Fig. 10D).
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5 ms from the stimulus, indicating a monosynaptic response (Gerasimenko et al. 2006a; Lavrov et al. 2006). The amplitude of the average potential after quipazine was higher compared with the potential before quipazine (Fig. 10E). This also suggests that there was monosynaptic modulation of the responses in the MG as indicated by the high-amplitude bursts after quipazine (cf. Fig. 10, A, burst 1, and B, burst 2). The mean amplitude of the averaged potentials from MG EMG bursts in ST (0.51 ± 0.04 mV) and in ST-Tr (3.17 ± 0.30 mV) rats after quipazine administration increased to 1.40 ± 0.11 mV in ST and to 4.43 ± 0.42 mV in ST-Tr rats, respectively (Fig. 10G). The averaged potentials of the individually evoked responses within the EMG burst of the TA before (burst 3) and after (burst 4) quipazine are shown in Fig. 10F. Before quipazine administration, the amplitude of the potential was 10 times smaller than the amplitude of the potential averaged from MG EMG burst. After quipazine administration, this potential almost disappeared (Fig. 10F). Thus it seems that the polysynaptic activity in the TA EMG burst inhibits the monosynaptic component of the evoked potentials. |
DISCUSSION |
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Changing the physiological state of the spinal cord with quipazine facilitates weight-bearing stepping induced by ES in spinal rats
We previously showed that ES applied to different spinal cord segments of the lumbar enlargement can induce rhythmic alternating activity in hindlimb muscles in chronically spinalized adult rats (Ichiyama et al. 2005). However, these stepping-like movements had poor plantar foot placement and weight-bearing capabilities. A similar pattern of stepping of the hindlimbs was observed in spinal cats during ES (Gerasimenko et al. 2002). Although these data show that ES can facilitate locomotor-like movements in spinal animals, it seems that ES alone is not sufficient for generating weight-bearing stepping in rats as occurs in decerebrated cats (Gerasimenko et al. 2001; Iwahara et al. 1991). Recently, it was shown that bilateral locomotion in spinal cats can be induced by intraspinal microstimulation (Barthélemy et al. 2007; Guevremont et al. 2006).
Two of the neurotransmitter systems involved in supraspinal control of locomotion are the serotonergic and noradrenergic systems. Evidence suggests that descending 5-HT fibers can influence the electrophysiological properties of 
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-motoneurons of flexor and extensor muscles (Ahlman et al. 1971; Myslinski and Anderson 1978). Although in chronic spinal cats the 5-HT agonist quipazine can modulate the locomotor pattern on a moving treadmill belt (Barbeau and Rossignol 1990), at moderate dosages, it may facilitate rather than induce locomotion. Jacobs and Fornel (1993) hypothesized that 5-HT neurons can facilitate central pattern generation and the activation of -motoneurons and inhibit sensory information processing. Each of these studies is consistent with the conclusion that 5-HT is an important, although complex, neuromodulator of locomotion, probably involving a variety of populations of neurons and of 5-HT receptor subtypes (Schmidt and Jordan 2000). The present data suggest that a combination of ES and quipazine administration provides conditions that are sufficient for generating weight-bearing stepping in spinal animals.
Differential features of flexor and extensor activation in response to ES and quipazine administration in spinal rats
An increase in step length and in the EMG amplitude of hindlimb extensors and flexors in spinal cats after the administration of 5-HT agonists has been observed (Barbeau and Rossignol 1990). Similarly, treatment with quipazine results in coordinated rhythmic movements when pinching the tail of spinal rats (Antri et al. 2002; Feraboli-Lohnherr et al. 1999). Weak extension during stepping in spinal animals has been attributed to a loss of the bistable properties of the motor units in the extensor muscles involved in postural control (Lee and Heckman 1998). In this study, we did not observe this extensor deficit when ES was combined with quipazine and step training. Instead, we found the EMG activity of both the extensors and flexors to be robust with flexor activity being more dominant. This response may reflect a similar effect as the facilitation of the flexor reflex by quipazine in spinal rats observed by Maj et al. (1976). Barbeau and Rossignol (1990) also reported that 5-HT agonists increase flexor motoneuron output and the excitability of cutaneous pathways resulting in a greater flexion withdrawal in spinal cats. These findings are consistent with our observations of an enhanced flexor EMG amplitude during stepping after quipazine administration in spinal rats.
The spectral composition of the EMG burst signals of the MG and TA muscles differed in response to ES at 40 Hz. When ES was applied to the S1 segment to induce a locomotor pattern, the spectral composition of the EMG burst signals for the MG and TA muscles differed. The dominant spectral peaks in the MG were integer harmonics of the fundamental frequency of 40 Hz, indicating the MG activation was a response to the stimulation. The potential averaged from the MG EMG burst during ES had a latency of 5 ms from the stimulus onset and was likely a monosynaptic reflex (Gerasimenko et al. 2006a; Lavrov et al. 2006). After quipazine administration, the spectral composition of the MG caused by ES did not change, i.e., the dominant spectral peaks were still at integer harmonics of the 40-Hz frequency (Fig. 10). There was, however, an increase in the power of these peaks and in the amplitude of the potentials that were averaged from the MG EMG burst, i.e., there was a facilitatory effect of quipazine on the output properties of the monosynaptic neural circuit associated with the MG motor pool during ES-elicited stepping.
In the flexor muscles, the spectral composition of the bursts during ES was different from that in the extensor muscles, i.e., there were no predominant spectral peaks (Fig. 10). We suggest that the EMG bursting activity of flexors during ES reflects amplitude modulation of mainly polysynaptic and monosynaptic pathways, although to a lesser extent. A similar effect has been observed during ES in paraplegic patients (Gerasimenko et al. 2001; Minassian et al. 2001, 2004).
The differences in the spectral characteristics noted above suggest some fundamentally different mechanisms in the generation of EMG bursting activity in extensor and flexor muscles during ES-elicited stepping in spinal rats. In extensor muscles, this process seems to be dominated by modulation at a common frequency. The flexor EMG modulation reflects a broader range of frequencies and has a peak frequency at 40 Hz. In ST-Tr and ST rats, quipazine administration increased monosynaptic excitability in both extensor and flexor muscles. However, a more substantial quipazine effect was observed in the flexor muscle in the high-frequency range of the spectrum. This part of the spectrum might reflect a greater role of interneurons and polysynaptic reflex systems in flexors compared with extensors.
Significance of sensory input in the regulation of locomotor activity during ES and quipazine administration in spinal rats
We showed that sensory input played a critical role in the regulation of spinal locomotion induced by ES and quipazine administration in ST-Tr rats. For example, a coordinated stepping pattern could be elicited only when the limbs were placed on the moving treadmill belt. In other words, the levels of ES and quipazine used were insufficient to evoke rhythmic movements when the legs were suspended in a non–weight-bearing position. In addition, a change in treadmill speed during locomotion in the presence of ES and quipazine administration resulted in an appropriate change in stepping pattern, i.e., a change in cycle duration and in the amplitude and temporal features of extensor and flexor muscle activity.
Shik et al. (1966) showed that the locomotor pattern of a decerebrated cat could be changed from stepping to galloping by increasing the intensity of stimulation of the mesencephalic locomotor region and/or increasing treadmill speed (Shik et al. 1966). We also could change the frequency of the locomotor pattern and the duty cycle of extensor/flexor recruitment in spinal rats by increasing the intensity of ES at S1 and/or by quipazine administration, while maintaining a constant treadmill speed (9 cm/s). Alternating activity of the TA and MG was observed even during high-intensity stimulation (6 V) when the cycle duration was 0.62 ± 0.02 s and MG and TA burst durations were 0.27 ± 0.01 and 0.26 ± 0.01 s, respectively (Fig. 1). These data show the flexibility in the spinal circuitry and its responsive to sensory input, i.e., the spinal circuitry itself can produce a high-frequency stepping rhythm, but the details of the stepping pattern is the prerogative of sensory input.
Most successful spinal locomotion induced by ES and quipazine administration was observed in step-trained rats
In this study, the effects of ES and quipazine treatment on the stepping performance of the two spinal rats that were step trained were marked. The ST-Tr rats showed excellent bipedal stepping with partial weight bearing in the presence of ES and quipazine administration. In contrast, the stepping movements in ST rats were not typically weight bearing, and the cycle period duration (at a treadmill speed of 9 cm/s) was significantly shorter than in the ST-Tr rats. Similarly, the mean duration of the TA EMG bursts was more than two times longer in the ST-Tr than ST rats. Also, the amplitude differences were much more dramatic in ST-Tr than ST rats. Although we did not determine the effects of step training alone in this study, we observed without exception that adult complete spinal rats cannot be trained to generate plantar-placed steps consistently without some additional facilitating intervention. This poor plantar placement despite step training differs from observations in adult spinal cats (de Leon et al. 1998, 2002; Edgerton et al. 1991, 2004; Lovely et al. 1986, 1990) and in neonatally spinalized rats (Kubasak et al. 2005; Stelzner et al. 1975).
Pharmacological treatment of spinal rats by daily administration of a 5-HT2 agonist for 1 mo also indicated a positive effect in locomotor recovery (Feraboli-Lohnherr et al. 1999; Antri et al. 2002, 2005). Even after 5-HT2 treatment, however, the spinal rats were able to perform coordinated stepping movements only when the tail was pinched. Also the stepping pattern was described as abnormal because of weak activity of the extensor muscles. Fong et al. (2005) showed that quipazine administration before each robotically assisted step training session improved stepping in adult spinal mice. Both of these observations suggest that a general increase in the excitability of the spinal circuitry can facilitate stepping and promote chronic alterations in the spinal circuit properties when administered in association with use-dependent factors. The results of this study are consistent with this concept in that improvement in stepping can be observed when ES is combined with quipazine treatment and step training, particularly in the ability to perform weight-bearing plantar-placed steps.
In summary, these data showed a clear, selective, and qualitatively different response between flexor and extensor motor pools to ES and quipazine, particularly in step-trained spinal rats. We hypothesize that the differential responses reflect muscle-specific peripheral afferent activity during stepping and the associated processing of locomotor-related proprioceptive information that selectively projects to flexor and extensor motor pools. These differential effects of ES and quipazine provide the opportunity to use two different, but largely complementary, mechanisms to enhance locomotor potential in spinal animals. These two mechanisms, in turn, enable further complementary effects to be manifested when they are combined with locomotor training.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Address for reprint requests and other correspondence: V. R. Edgerton, Dept. of Physiological Science, Univ. of California, Los Angeles, CA 90095 (E-mail: vre{at}ucla.edu)
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