Does elimination of afferent input modify the changes in rat motoneurone properties that occur following chronic spinal cord transection?

Low oral dose of 4-methylumbelliferone reduces glial scar but is insufficient to induce functional recovery after spinal cord injury

Spinal cord injury (SCI) induces the upregulation of chondroitin sulfate proteoglycans (CSPGs) at the glial scar and inhibits neuroregeneration. Under normal physiological condition, CSPGs interact with hyaluronan (HA) and other extracellular matrix on the neuronal surface forming a macromolecular structure called perineuronal nets (PNNs) which regulate neuroplasticity. 4-methylumbelliferone (4-MU) is a known inhibitor for HA synthesis but has not been tested in SCI. We first tested the effect of 4-MU in HA reduction in uninjured rats. After 8 weeks of 4-MU administration at a dose of 1.2 g/kg/day, we have not only observed a reduction of HA in the uninjured spinal cords but also a down-regulation of CS glycosaminoglycans (CS-GAGs). In order to assess the effect of 4-MU in chronic SCI, six weeks after Th8 spinal contusion injury, rats were fed with 4-MU or placebo for 8 weeks in combination with daily treadmill rehabilitation for 16 weeks to promote neuroplasticity. 4-MU treatment reduced the HA synthesis by astrocytes around the lesion site and increased sprouting of 5-hydroxytryptamine fibres into ventral horns. However, the current dose was not sufficient to suppress CS-GAG up-regulation induced by SCI. Further adjustment on the dosage will be required to benefit functional recovery after SCI.

The electrophysiological properties of hindlimb motoneurons do not differ between male and female rats

Several studies have reported differences in the morphological characteristics of motoneurons and the contractile properties of motor units of male and female rats. However, differences in spinal motoneuron activity between the sexes are not well understood. This study investigates the electrophysiological properties of spinal α- motoneurons in male and female Wistar rats under pentobarbital anesthesia. Fast and slow types of tibial motoneurons were recorded intracellularly in 15 male and 15 female rats, and the measured parameters were compared statistically using 2-way ANOVA and Tukey post-hoc tests. The membrane properties, action potential parameters and firing characteristics were not different between sexes, though significant differences were observed in the properties of fast and slow motoneuron types within both sex groups. We conclude that the sex-related differences observed in motor performance between male and female rats are largely due to differences in muscle mass, the proportion of muscle fibre types and the related motor unit contractile properties, while the mechanisms of motor control dependent on the electrophysiological activity of motoneurons are similar between the sexes. These findings are significant, as they indicate that results of experiments investigating electrophysiological properties can be reliably compared between sexes.

Absence of inspiratory premotor potentials during quiet breathing in cervical spinal cord injury

A premotor potential, or Bereitschaftspotential (BP), is a low-amplitude negativity in the electroencephalographic activity (EEG) of the sensorimotor cortex. It begins ~1 s prior to the onset of inspiration in the averaged EEG. Although normally absent during quiet breathing in healthy, younger people, inspiration-related BPs are present in people with respiratory disease and healthy, older people, indicating a cortical contribution to quiet breathing. People with tetraplegia have weak respiratory muscles and increased neural drive during quiet breathing, indicated by increased inspiratory muscle activity. Therefore, we hypothesized that BPs would be present during quiet breathing in people with tetraplegia. EEG was recorded in 17 people with chronic tetraplegia (14M, 3 female; 22-51 yr; C3-C7, American Spinal Injury Association Impairment Scale A-D; >1 yr postinjury). They had reduced lung function and respiratory muscle weakness [FEV₁: 54 ± 19% predicted, FVC: 59 ± 22% predicted and MIP: 56 ± 24% predicted (mean ± SD)]. Participants performed quiet breathing and voluntary self-paced sniffs (positive control condition). A minimum of 250 EEG epochs during quiet breathing and 60 epochs during sniffs, time-locked to the onset of inspiration, were averaged to determine the presence of BPs at Cz, FCz, C3, and C4. Fifteen participants (88%) had a BP for the sniffs. Of these 15 participants, only one (7%) had a BP in quiet breathing, a rate similar to that reported during quiet breathing in young able-bodied participants (12%). The findings suggest that, as in young able-bodied people, a cortical contribution to quiet breathing is absent in people with tetraplegia despite higher neural drive.NEW & NOTEWORTHY People with tetraplegia have weak respiratory muscles, increased neural drive during quiet breathing, and a high incidence of sleep-disordered breathing. Using electroencephalographic recordings, we show that inspiratory premotor potentials are absent in people with chronic tetraplegia during quiet breathing. This suggests that cortical activity is not present during resting ventilation in people with tetraplegia who are awake and breathing independently.

Meta-analysis of biological variables' impact on spinal motoneuron electrophysiology data

Experimental, methodological, and biological variables must be accounted for statistically to maximize accuracy and comparability of published neuroscience data. However, accounting for all variables is nigh impossible. Thus we aimed to identify particularly influential variables within published neurological data, from cat, rat, and mouse studies, via a robust statistical process. Our goal was to develop tools to improve rigor in the collection and analysis of data. We strictly constrained experimental and methodological variables and then assessed four key biological variables within motoneuron research: species, age, sex, and cell type. We quantified intraexperimental and interexperimental variances in 11 commonly reported electrophysiological properties of spinal motoneurons. We first assessed variances without accounting for biological variables and then reassessed them while accounting for all four variables. We next assessed variances with all possible combinations of these four variables. We concluded that some motoneuron properties have low intraexperimental, but high interexperimental, variance; that individual motoneuron properties are impacted differently by biological variables; and that some unexplained variances still remain. We report here the optimal combinations of biological variables to reduce interexperimental variance for all 11 parameters. We also rank each parameter by intra- and interexperimental consistency. We expect these results to assist with design of experimental and analytical methods, and to support accuracy in simulations. Furthermore, although demonstrated on spinal motoneuron electrophysiology literature, our approach is applicable to biological data from all fields of neuroscience. This approach represents an important aid to experimental design, comparison of reported data, and reduction of unexplained variance in neuroscience data.NEW & NOTEWORTHY Our meta-analysis shows the impact of species, age, sex, and cell type on lumbosacral motoneuron electrophysiological properties by thoroughly quantifying variances across literature for the first time. We quantify the variances of 11 motoneuron properties with consideration of biological variables, thus providing specific insights for motoneuron modelers and experimenters, and providing a general methodological template for the quantification of variance in neurological data with the consideration of any experimental, methodological, or biological variables of interest.

Understanding exercise-dependent plasticity of motoneurons using intracellular and intramuscular approaches

Spinal motoneurons (MN) exhibit exercise-dependent adaptations to increased activity, such as exercise and locomotion, as well as decreased activity associated with disuse, spinal cord injury, and aging. The development of several experimental approaches, in both human and animal models, has contributed significantly to our understanding of this plasticity. The purpose of this review is to summarize how intracellular recordings in an animal model and motor unit recordings in a human model have, together, contributed to our current understanding of exercise-dependent MN plasticity. These approaches and techniques will allow neuroscientists to continue to advance our understanding of MN physiology and the plasticity of the "final common path" of the motor system, and to design experiments to answer the critical questions that are emerging in this field.

Motoneuron firing properties are modified by trans-spinal direct current stimulation in rats

Spinal polarization evoked by direct current stimulation [trans-spinal direct current stimulation (tsDCS)] is a novel method for altering spinal network excitability; however, it remains not well understood. The aim of this study was to determine whether tsDCS influences spinal motoneuron activity. Twenty Wistar rats under general pentobarbital anesthesia were subjected to 15 min anodal (n = 10) or cathodal (n = 10) tsDCS of 0.1 mA intensity, and the electrophysiological properties of their motoneurons were intracellularly measured before, during, and after direct current application. The major effects of anodal intervention included increased minimum firing frequency and the slope of the frequency-current (f-I) relationship, as well as decreased rheobase and currents evoking steady-state firing (SSF). The effects of cathodal polarization included decreased maximum SSF frequency, decreased f-I slope, and decreased current evoking the maximum SSF. Notably, the majority of observed effects appeared immediately after the current onset, developed during polarization, and outlasted it for at least 15 min. Moreover, the effects of anodal polarization were generally more pronounced and uniform than those evoked by cathodal polarization. Our study is the first to present polarity-dependent, long-lasting changes in spinal motoneuron firing following tsDCS, which may aid in the development of more safe and accurate application protocols in medicine and sport. NEW & NOTEWORTHY Trans-spinal direct current stimulation induces significant polarity-dependent, long-lasting changes in the threshold and firing properties of spinal motoneurons. Anodal polarization potentiates motoneuron firing whereas cathodal polarization acts mainly toward firing inhibition. The alterations in rheobase and rhythmic firing properties are not restricted to the period of current application and can be observed long after the current offset.

Spinal Hyper-Excitability and Altered Muscle Structure Contribute to Muscle Hypertonia in Newborns After Antenatal Hypoxia-Ischemia in a Rabbit Cerebral Palsy Model

Rabbit kits after global antenatal hypoxic-ischemic injury exhibit motor deficits similar to humans with cerebral palsy. We tested several mechanisms previously implicated in spinal hyper-excitability after perinatal brain injury that may explain muscle hypertonia in newborns. Stiffness of hind limb muscles during passive stretch, electromyogram, and spinal excitability by Hoffman reflex, were assessed in rabbit kits with muscle hypertonia after global hypoxic-ischemic brain injury and naïve controls. Affected muscle architecture, motoneuron morphology, primary afferents density, gliosis, and KCC2 expression transporter in the spinal cord were also examined. Decrease knee stiffness after anesthetic administration was larger, but residual stiffness was higher in hypertonic kits compared to controls. Hypertonic kits exhibited muscle shortening and atrophy, in both agonists and antagonists. Sarcomere length was longer in tibialis anterior in hypertonic kits than in controls. Hypertonic kits had decreased rate dependent depression and increased H_max/M_max in H-reflex. Motor neuron soma sizes, primary afferent density were not different between controls and hypertonic kits. Length of dendritic tree and ramification index were lower in hypertonic group. Gene expression of KCC2 was lower in hypertonic kits, but protein content was not different between the groups. In conclusion, while we found evidence of decreased supraspinal inhibitory control and increased excitability by H-reflex that may contribute to neuronal component in hypertonia, increased joint resistance to stretch was explained predominantly by changes in passive properties of muscles and joints. We did not find structural evidence of increased sensory afferent input or morphological changes in motoneurons that might explain increased excitability. Gliosis, observed in spinal gray matter, may contribute to muscle hypertonia.

Ih contributes to increased motoneuron excitability in restless legs syndrome

KEY POINTS

Restless legs patients complain about sensory and motor symptoms leading to sleep disturbances. Symptoms include painful sensations, an urge to move and involuntary leg movements. The responsible mechanisms of restless legs syndrome are still not known, although current studies indicate an increased neuronal network excitability. Reflex studies indicate the involvement of spinal structures. Peripheral mechanisms have not been investigated so far. In the present study, we provide evidence of increased hyperpolarization-activated cyclic nucleotide-gated (HCN) channel-mediated inward rectification in motor axons. The excitability of sensory axons was not changed. We conclude that, in restless legs syndrome, an increased HCN current in motoneurons may play a pathophysiological role, such that these channels could represent a valuable target for pharmaceutical intervention.

ABSTRACT

Restless legs syndrome is a sensorimotor network disorder. So far, the responsible pathophysiological mechanisms are poorly understood. In the present study, we provide evidence that the excitability of peripheral motoneurons contributes to the pathophysiology of restless legs syndrome. In vivo excitability studies on motor and sensory axons of the median nerve were performed on patients with idiopathic restless legs syndrome (iRLS) who were not currently on treatment. The iRLS patients had greater accommodation in motor but not sensory axons to long-lasting hyperpolarization compared to age-matched healthy subjects, indicating greater inward rectification in iRLS. The most reasonable explanation is that hyperpolarization-activated cyclic nucleotide-gated (HCN) channels open at less hyperpolarized membrane potentials, a view supported by mathematical modelling. The half-activation potential for HCN channels (Bq) was the single best parameter that accounted for the difference between normal controls and iRLS data. A 6 mV depolarization of Bq reduced the discrepancy between the normal control model and the iRLS data by 92.1%. Taken together, our results suggest an increase in the excitability of motor units in iRLS that could enhance the likelihood of leg movements. The abnormal axonal properties are consistent with other findings indicating that the peripheral system is part of the network involved in iRLS.

Improvements in bladder, bowel and sexual outcomes following task-specific locomotor training in human spinal cord injury

OBJECTIVE

Locomotor training (LT) as a therapeutic intervention following spinal cord injury (SCI) is an effective rehabilitation strategy for improving motor outcomes, but its impact on non-locomotor functions is unknown. Given recent results of our labs' pre-clinical animal SCI LT studies and existing overlap of lumbosacral spinal circuitries controlling pelvic-visceral and locomotor functions, we addressed whether LT can improve bladder, bowel and sexual function in humans at chronic SCI time-points (> two years post-injury).

STUDY DESIGN

Prospective cohort study; pilot trial with small sample size.

METHODS

Eight SCI research participants who were undergoing 80 daily one-hour sessions of LT on a treadmill using body-weight support, or one-hour of LT and stand training on alternate days, as part of another research study conducted at the Kentucky Spinal Cord Injury Research Center, University of Louisville, were enrolled in this pilot trial. Urodynamic assessments were performed and International Data Set questionnaire forms completed for bladder, bowel and sexual functions at pre-and post-training time points. Four usual care (non-trained; regular at-home routine) research participants were also enrolled in this study and had the same assessments collected twice, at least 3 months apart.

RESULTS

Filling cystometry documented significant increases in bladder capacity, voiding efficiency and detrusor contraction time as well as significant decreases in voiding pressure post-training relative to baseline. Questionnaires revealed a decrease in the frequency of nocturia and urinary incontinence for several research participants as well as a significant decrease in time required for defecation and a significant increase in sexual desire post-training. No significant differences were found for usual care research participants.

CONCLUSIONS

These results suggest that an appropriate level of sensory information provided to the spinal cord, generated through task-specific stepping and/or loading, can positively benefit the neural circuitries controlling urogenital and bowel functions.

TRIAL REGISTRATION

ClinicalTrials.gov NCT03036527.

Retracing your footsteps: developmental insights to spinal network plasticity following injury

During development of the spinal cord, a precise interaction occurs between descending projections and sensory afferents, with spinal networks that lead to expression of coordinated motor output. In the rodent, during the last embryonic week, motor output first occurs as regular bursts of spontaneous activity, progressing to stochastic patterns of episodes that express bouts of coordinated rhythmic activity perinatally. Locomotor activity becomes functionally mature in the 2nd postnatal wk and is heralded by the onset of weight-bearing locomotion on the 8th and 9th postnatal day. Concomitantly, there is a maturation of intrinsic properties and key conductances mediating plateau potentials. In this review, we discuss spinal neuronal excitability, descending modulation, and afferent modulation in the developing rodent spinal cord. In the adult, plastic mechanisms are much more constrained but become more permissive following neurotrauma, such as spinal cord injury. We discuss parallel mechanisms that contribute to maturation of network function during development to mechanisms of pathological plasticity that contribute to aberrant motor patterns, such as spasticity and clonus, which emerge following central injury.

α-Motoneurons maintain biophysical heterogeneity in obesity and diabetes in Zucker rats

Small-diameter sensory dysfunction resulting from diabetes has received much attention in the literature, whereas the impact of diabetes on α-motoneurons (MN) has not. In addition, the chance of developing insulin resistance and diabetes is increased in obesity. No study has examined the impact of obesity or diabetes on the biophysical properties of MN. Lean Zucker rats and Zucker diabetic fatty (ZDF) rats were separated into lean, obese (ZDF fed standard chow), and diabetic (ZDF fed high-fat diet that led to diabetes) groups. Glass micropipettes recorded hindlimb MN properties from identified flexor and extensor MN. MN were separated within their groups on the basis of input conductance, which created high- and low-input conductance subpopulations for each. A significant shorter (20%) afterhyperpolarization half-decay (AHP_1/2) was found in low-conductance MN for the diabetic group only, whereas AHP½ tended to be shorter in the obese group (19%). Significant positive correlations were found among rheobase and input conductance for both lean and obese animals. No differences were found between the groups for afterhyperpolarization amplitude (AHP_amp), input conductance, rheobase, or any of the rhythmic firing properties (frequency-current slope and spike-frequency adaptation index). MN properties continue to be heterogeneous in obese and diabetic animals. Obesity does not seem to influence lumbar MN. Despite the resistance of MN to the impact of diabetes, the reduced AHP_1/2 decay and the tendency for a reduction in AHP_amp may be the first sign of change to MN function.NEW & NOTEWORTHY Knowledge about the impact of obesity and diabetes on the biophysical properties of motoneurons is lacking. We found that diabetes reduces the duration of the afterhyperpolarization and that motoneuron function is unchanged by obesity. A reduced afterhyperpolarization may impact discharge characteristics and may be the first sign of change to motoneuron function.

Characterization of Involuntary Contractions after Spinal Cord Injury Reveals Associations between Physiological and Self-Reported Measures of Spasticity

Correlations between physiological, clinical and self-reported assessments of spasticity are often weak. Our aims were to quantify functional, self-reported and physiological indices of spasticity in individuals with thoracic spinal cord injury (SCI; 3 women, 9 men; 19–52 years), and to compare the strength and direction of associations between these measures. The functional measure we introduced involved recording involuntary electromyographic activity during a transfer from wheelchair to bed which is a daily task necessary for function. High soleus (SL) and tibialis anterior (TA) F-wave/M-wave area ratios were the only physiological measures that distinguished injured participants from the uninjured (6 women, 13 men, 19–67 years). Hyporeflexia (decreased SL H/M ratio) was unexpectedly present in older participants after injury. During transfers, the duration and intensity of involuntary electromyographic activity varied across muscles and participants, but coactivity was common. Wide inter-participant variability was seen for self-reported spasm frequency, severity, pain and interference with function, as well as tone (resistance to imposed joint movement). Our recordings of involuntary electromyographic activity during transfers provided evidence of significant associations between physiological and self-reported measures of spasticity. Reduced low frequency H-reflex depression in SL and high F-wave/M-wave area ratios in TA, physiological indicators of reduced inhibition and greater motoneuron excitability, respectively, were associated with long duration SL and biceps femoris (BF) electromyographic activity during transfers. In turn, participants reported high spasm frequency when transfers involved short duration TA EMG, decreased co-activation between SL and TA, as well as between rectus femoris (RF) vs. BF. Thus, the duration of muscle activity and/or the time of agonist-antagonist muscle coactivity may be used by injured individuals to count spasms. Intense electromyographic activity and high tone related closely (possibly from joint stabilization), while intense electromyographic activity in one muscle of an agonist-antagonist pair (especially in TA vs. SL, and RF vs. BF) likely induced joint movement and was associated with severe spasms. These data support the idea that individuals with SCI describe their spasticity by both the duration and intensity of involuntary agonist-antagonist muscle coactivity during everyday tasks.

Rehabilitation Strategies after Spinal Cord Injury: Inquiry into the Mechanisms of Success and Failure

Body-weight supported locomotor training (BWST) promotes recovery of load-bearing stepping in lower mammals, but its efficacy in individuals with a spinal cord injury (SCI) is limited and highly dependent on injury severity. While animal models with complete spinal transections recover stepping with step-training, motor complete SCI individuals do not, despite similarly intensive training. In this review, we examine the significant differences between humans and animal models that may explain this discrepancy in the results obtained with BWST. We also summarize the known effects of SCI and locomotor training on the muscular, motoneuronal, interneuronal, and supraspinal systems in human and non-human models of SCI and address the potential causes for failure to translate to the clinic. The evidence points to a deficiency in neuronal activation as the mechanism of failure, rather than muscular insufficiency. While motoneuronal and interneuronal systems cannot be directly probed in humans, the changes brought upon by step-training in SCI animal models suggest a beneficial re-organization of the systems' responsiveness to descending and afferent feedback that support locomotor recovery. The literature on partial lesions in humans and animal models clearly demonstrate a greater dependency on supraspinal input to the lumbar cord in humans than in non-human mammals for locomotion. Recent results with epidural stimulation that activates the lumbar interneuronal networks and/or increases the overall excitability of the locomotor centers suggest that these centers are much more dependent on the supraspinal tonic drive in humans. Sensory feedback shapes the locomotor output in animal models but does not appear to be sufficient to drive it in humans.

Effects of exercise training on urinary tract function after spinal cord injury

Spinal cord injury (SCI) causes dramatic changes in the quality of life, including coping with bladder dysfunction which requires repeated daily and nightly catheterizations. Our laboratory has recently demonstrated in a rat SCI model that repetitive sensory information generated through task-specific stepping and/or loading can improve nonlocomotor functions, including bladder function (Ward PJ, Herrity AN, Smith RR, Willhite A, Harrison BJ, Petruska JC, Harkema SJ, Hubscher CH. J Neurotrauma 31: 819-833, 2014). To target potential underlying mechanisms, the current study included a forelimb-only exercise group to ascertain whether improvements may be attributed to general activity effects that impact target organ-neural interactions or to plasticity of the lumbosacral circuitry that receives convergent somatovisceral inputs. Male Wistar rats received a T9 contusion injury and were randomly assigned to three groups 2 wk postinjury: quadrupedal locomotion, forelimb exercise, or a nontrained group. Throughout the study (including preinjury), all animals were placed in metabolic cages once a week for 24 h to monitor water intake and urine output. Following the 10-wk period of daily 1-h treadmill training, awake cystometry data were collected and bladder and kidney tissue harvested for analysis. Metabolic cage frequency-volume measurements of voiding and cystometry reveal an impact of exercise training on multiple SCI-induced impairments related to various aspects of urinary tract function. Improvements in both the quadrupedal and forelimb-trained groups implicate underlying mechanisms beyond repetitive sensory information from the hindlimbs driving spinal network excitability of the lumbosacral urogenital neural circuitry. Furthermore, the impact of exercise training on the upper urinary tract (kidney) underscores the health benefit of activity-based training on the entire urinary system within the SCI population.

Decrease of mRNA Editing after Spinal Cord Injury is Caused by Down-regulation of ADAR2 that is Triggered by Inflammatory Response

We recently showed that spinal cord injury (SCI) leads to a decrease in mRNA editing of serotonin receptor 2C (5-HT_2CR) contributing to post-SCI spasticity. Here we study post-SCI mRNA editing and global gene expression using massively parallel sequencing. Evidence is presented that the decrease in 5-HT_2CR editing is caused by down-regulation of adenosine deaminase ADAR2 and that editing of at least one other ADAR2 target, potassium channel Kv1.1, is decreased after SCI. Bayesian network analysis of genome-wide transcriptome data indicates that down-regulation of ADAR2 (1) is triggered by persistent inflammatory response to SCI that is associated with activation of microglia and (2) results in changes in neuronal gene expression that are likely to contribute both to post-SCI restoration of neuronal excitability and muscle spasms. These findings have broad implications for other diseases of the Central Nervous System and could open new avenues for developing efficacious antispastic treatments.

Extensor motoneurone properties are altered immediately before and during fictive locomotion in the adult decerebrate rat

This study examined motoneurone properties during fictive locomotion in the adult rat for the first time. Fictive locomotion was induced via electrical stimulation of the mesencephalic locomotor region in decerebrate adult rats under neuromuscular blockade to compare basic and rhythmic motoneurone properties in antidromically identified extensor motoneurones during: (1) quiescence, before and after fictive locomotion; (2) the 'tonic' period immediately preceding locomotor-like activity, whereby the amplitude of peripheral flexor (peroneal) and extensor (tibial) nerves are increased but alternation has not yet occurred; and (3) locomotor-like episodes. Locomotion was identified by alternating flexor-extensor nerve activity, where the motoneurone either produced membrane oscillations consistent with a locomotor drive potential (LDP) or did not display membrane oscillation during alternating nerve activity. Cells producing LDPs were referred to as such, while those that did not were referred to as 'idle' motoneurones. LDP and idle motoneurones during locomotion had hyperpolarized spike threshold (Vth ; LDP: 3.8 mV; idle: 5.8 mV), decreased rheobase and an increased discharge rate (LDP: 64%; idle: 41%) during triangular ramp current injection even though the frequency-current slope was reduced by 70% and 55%, respectively. Modulation began in the tonic period immediately preceding locomotion, with a hyperpolarized Vth and reduced rheobase. Spike frequency adaptation did not occur in spiking LDPs or firing generated from sinusoidal current injection, but occurred during a sustained current pulse during locomotion. Input conductance showed no change. Results suggest motoneurone modulation occurs across the pool and is not restricted to motoneurones engaged in locomotion.

Motor unit firing rates during spasms in thenar muscles of spinal cord injured subjects

Involuntary contractions of paralyzed muscles (spasms) commonly disrupt daily activities and rehabilitation after human spinal cord injury (SCI). Our aim was to examine the recruitment, firing rate modulation, and derecruitment of motor units that underlie spasms of thenar muscles after cervical SCI. Intramuscular electromyographic activity (EMG), surface EMG, and force were recorded during thenar muscle spasms that occurred spontaneously or that were triggered by movement of a shoulder or leg. Most spasms were submaximal (mean: 39%, SD: 33 of the force evoked by median nerve stimulation at 50 Hz) with strong relationships between EMG and force (R (2) > 0.69). Unit recruitment occurred over a wide force range (0.2-103% of 50 Hz force). Significant unit rate modulation occurred during spasms (frequency at 25% maximal force: 8.8 Hz, 3.3 SD; at maximal force: 16.1 Hz, 4.1 SD). Mean recruitment frequency (7.1 Hz, 3.2 SD) was significantly higher than derecruitment frequency (5.4 Hz, 2.4 SD). Coactive unit pairs that fired for more than 4 s showed high (R (2) > 0.7, n = 4) or low (R (2):0.3-0.7, n = 12) rate-rate correlations, and derecruitment reversals (21 pairs, 29%). Later recruited units had higher or lower maximal firing rates than lower threshold units. These discrepant data show that coactive motoneurons are drive both by common inputs and by synaptic inputs from different sources during muscle spasms. Further, thenar motoneurons can still fire at high rates in response to various peripheral inputs after SCI, supporting the idea that low maximal voluntary firing rates and forces in thenar muscles result from reduced descending drive.

Locomotor training modifies soleus monosynaptic motoneuron responses in human spinal cord injury

The objective of this study was to assess changes in monosynaptic motoneuron responses to stimulation of Ia afferents after locomotor training in individuals with chronic spinal cord injury (SCI). We hypothesized that locomotor training modifies the amplitude of the soleus monosynaptic motoneuron responses in a body position-dependent manner. Fifteen individuals with chronic clinical motor complete or incomplete SCI received an average of 45 locomotor training sessions. The soleus H-reflex and M-wave recruitment curves were assembled using data collected in both the right and left legs, with subjects seated and standing, before and after training. The soleus H-reflexes and M-waves, measured as peak-to-peak amplitudes, were normalized to the maximal M-wave (M(max)). Stimulation intensities were normalized to 50% M(max) stimulus intensity. A sigmoid function was also fitted to the normalized soleus H-reflexes on the ascending limb of the recruitment curve. After training, soleus H-reflex excitability was increased in both legs in AIS C subjects, and remained unchanged in AIS A-B and AIS D subjects during standing. When subjects were seated, soleus H-reflex excitability was decreased after training in many AIS C and D subjects. Changes in reflex excitability coincided with changes in stimulation intensities at H-threshold, 50% maximal H-reflex, and at maximal H-reflex, while an interaction between leg side and AIS scale for the H-reflex slope was also found. Adaptations of the intrinsic properties of soleus motoneurons and Ia afferents, the excitability profile of the soleus motoneuron pool, oligosynaptic inputs, and corticospinal inputs may all contribute to these changes. The findings of this study demonstrate that locomotor training impacts the amplitude of the monosynaptic motoneuron responses based on the demands of the motor task in people with chronic SCI.

Human spinal cord injury: motor unit properties and behaviour

Spinal cord injury (SCI) results in widespread variation in muscle function. Review of motor unit data shows that changes in the amount and balance of excitatory and inhibitory inputs after SCI alter management of motoneurons. Not only are units recruited up to higher than usual relative forces when SCI leaves few units under voluntary control, the force contribution from recruitment increases due to elevation of twitch/tetanic force ratios. Force gradation and precision are also coarser with reduced unit numbers. Maximal unit firing rates are low in hand muscles, limiting voluntary strength, but are low, normal or high in limb muscles. Unit firing rates during spasms can exceed voluntary rates, emphasizing that deficits in descending drive limit force production. SCI also changes muscle properties. Motor unit weakness and fatigability seem universal across muscles and species, increasing the muscle weakness that arises from paralysis of units, motoneuron death and sensory impairment. Motor axon conduction velocity decreases after human SCI. Muscle contractile speed is also reduced, which lowers the stimulation frequencies needed to grade force when paralysed muscles are activated with patterned electrical stimulation. This slowing does not necessarily occur in hind limb muscles after cord transection in cats and rats. The nature, duration and level of SCI underlie some of these species differences, as do variations in muscle function, daily usage, tract control and fibre-type composition. Exploring this diversity is important to promote recovery of the hand, bowel, bladder and locomotor function most wanted by people with SCI.

Motor unit

Movement is accomplished by the controlled activation of motor unit populations. Our understanding of motor unit physiology has been derived from experimental work on the properties of single motor units and from computational studies that have integrated the experimental observations into the function of motor unit populations. The article provides brief descriptions of motor unit anatomy and muscle unit properties, with more substantial reviews of motoneuron properties, motor unit recruitment and rate modulation when humans perform voluntary contractions, and the function of an entire motor unit pool. The article emphasizes the advances in knowledge on the cellular and molecular mechanisms underlying the neuromodulation of motoneuron activity and attempts to explain the discharge characteristics of human motor units in terms of these principles. A major finding from this work has been the critical role of descending pathways from the brainstem in modulating the properties and activity of spinal motoneurons. Progress has been substantial, but significant gaps in knowledge remain.

Motoneurone afterhyperpolarisation time-course following stroke

OBJECTIVE

Our aim was to investigate any changes in the estimated time-course of the afterhyperpolarisation (AHP) in motoneurones innervating the tibialis anterior following stroke, with a secondary objective to compare the results from two different AHP estimation techniques.

METHODS

Motor units from tibialis anterior on the paretic and non-paretic sides of 15 subjects with chronic stroke were recorded using intramuscular electrodes during voluntary isometric contraction. Participants varied the motor unit firing rate from its lowest rate to approximately 10 Hz. The AHP duration was estimated using the interval death rate (IDR) and transition point methods.

RESULTS

The AHP decay time-constant was significantly different between sides (paretic: 41.7 ± 8.5 ms, non-paretic: 36.2 ± 6.4 ms). Additionally, the paretic AHP time-constant was significantly longer in participants with low motor recovery (45.9 ± 9.1 ms) than with high motor recovery (39.3 ± 10.0 ms) as measured by CMSA score. The AHP estimates from the two techniques were correlated (r=0.78).

CONCLUSIONS

The AHP time-course prolongation on the paretic side of people with chronic stroke is more pronounced in people with low motor recovery.

SIGNIFICANCE

Changes in the motoneurone AHP time course post-stroke were related to muscle function and may play a role in the commonly-observed reduction of motor unit discharge rate during voluntary contractions following stroke.

The use of poly(N-[2-hydroxypropyl]-methacrylamide) hydrogel to repair a T10 spinal cord hemisection in rat: a behavioural, electrophysiological and anatomical examination

There have been considerable interests in attempting to reverse the deficit because of an SCI (spinal cord injury) by restoring neural pathways through the lesion and by rebuilding the tissue network. In order to provide an appropriate micro-environment for regrowing axotomized neurons and proliferating and migrating cells, we have implanted a small block of pHPMA [poly N-(2-hydroxypropyl)-methacrylamide] hydrogel into the hemisected T10 rat spinal cord. Locomotor activity was evaluated once a week during 14 weeks with the BBB rating scale in an open field. At the 14th week after SCI, the reflexivity of the sub-lesional region was measured. We also monitored the ventilatory frequency during an electrically induced muscle fatigue known to elicit the muscle metaboreflex and increase the respiratory rate. Spinal cords were then collected, fixed and stained with anti-ED-1 and anti-NF-H antibodies and FluoroMyelin. We show in this study that hydrogel-implanted animals exhibit: (i) an improved locomotor BBB score, (ii) an improved breathing adjustment to electrically evoked isometric contractions and (iii) an H-reflex recovery close to control animals. Qualitative histological results put in evidence higher accumulation of ED-1 positive cells (macrophages/monocytes) at the lesion border, a large number of NF-H positive axons penetrating the applied matrix, and myelin preservation both rostrally and caudally to the lesion. Our data confirm that pHPMA hydrogel is a potent biomaterial that can be used for improving neuromuscular adaptive mechanisms and H-reflex responses after SCI.

The influence of a 5-wk whole body vibration on electrophysiological properties of rat hindlimb spinal motoneurons

The study aimed at determining the influence of a whole body vibration (WBV) on electrophysiological properties of spinal motoneurons. The WBV training was performed on adult male Wistar rats, 5 days a week, for 5 wk, and each daily session consisted of four 30-s runs of vibration at 50 Hz. Motoneuron properties were investigated intracellularly during experiments on deeply anesthetized animals. The experimental group subjected to the WBV consisted of seven rats, and the control group of nine rats. The WBV treatment induced no significant changes in the passive membrane properties of motoneurons. However, the WBV-evoked adaptations in excitability and firing properties were observed, and they were limited to fast-type motoneurons. A significant decrease in rheobase current and a decrease in the minimum and the maximum currents required to evoke steady-state firing in motoneurons were revealed. These changes resulted in a leftward shift of the frequency-current relationship, combined with an increase in slope of this curve. The functional relevance of the described adaptive changes is the ability of fast motoneurons of rats subjected to the WBV to produce series of action potentials at higher frequencies in a response to the same intensity of activation. Previous studies proved that WBV induces changes in the contractile parameters predominantly of fast motor units (MUs). The data obtained in our experiment shed a new light to possible explanation of these results, suggesting that neuronal factors also play a substantial role in MU adaptation.

HCN4 subunit expression in fast-spiking interneurons of the rat spinal cord and hippocampus

Hyperpolarisation-activated (I_h) currents are considered important for dendritic integration, synaptic transmission, setting membrane potential and rhythmic action potential (AP) discharge in neurons of the central nervous system. Hyperpolarisation-activated cyclic nucleotide-gated (HCN) channels underlie these currents and are composed of homo- and hetero-tetramers of HCN channel subunits (HCN1–4), which confer distinct biophysical properties on the channel. Despite understanding the structure–function relationships of HCN channels with different subunit stoichiometry, our knowledge of their expression in defined neuronal populations remains limited. Recently, we have shown that HCN subunit expression is a feature of a specific population of dorsal horn interneurons that exhibit high-frequency AP discharge. Here we expand on this observation and use neuroanatomical markers to first identify well-characterised neuronal populations in the lumbar spinal cord and hippocampus and subsequently determine whether HCN4 expression correlates with high-frequency AP discharge in these populations. In the spinal cord, HCN4 is expressed in several putative inhibitory interneuron populations including parvalbumin (PV)-expressing islet cells (84.1%; SD: ±2.87), in addition to all putative Renshaw cells and Ia inhibitory interneurons. Similarly, virtually all PV-expressing cells in the hippocampal CA1 subfield (93.5%; ±3.40) and the dentate gyrus (90.9%; ±6.38) also express HCN4. This HCN4 expression profile in inhibitory interneurons mirrors both the prevalence of I_h sub-threshold currents and high-frequency AP discharge. Our findings indicate that HCN4 subunits are expressed in several populations of spinal and hippocampal interneurons, which are known to express both I_h sub-threshold currents and exhibit high-frequency AP discharge. As HCN channel function plays a critical role in pain perception, learning and memory, and sleep as well as the pathogenesis of several neurological diseases, these findings provide important insights into the identity and neurochemical status of cells that could underlie such conditions.

Differential effects of brain-derived neurotrophic factor and neurotrophin-3 on hindlimb function in paraplegic rats

We compared the effect of viral administration of brain-derived neurotrophic factor (BDNF) or neurotrophin 3 (NT-3) on locomotor recovery in adult rats with complete thoracic (T10) spinal cord transection injuries, in order to determine the effect of chronic neurotrophin expression on spinal plasticity. At the time of injury, BDNF, NT-3 or green fluorescent protein (GFP) (control) was delivered to the lesion via adeno-associated virus (AAV) constructs. AAV–BDNF was significantly more effective than AAV–NT-3 in eliciting locomotion. In fact, AAV–BDNF-treated rats displayed plantar, weight-supported hindlimb stepping on a stationary platform, that is, without the assistance of a moving treadmill and without step training. Rats receiving AAV–NT-3 or AAV–GFP were incapable of hindlimb stepping during this task, despite provision of balance support. AAV–NT-3 treatment did promote the recovery of treadmill-assisted stepping, but this required continuous perineal stimulation. In addition, AAV–BDNF-treated rats were sensitized to noxious heat, whereas AAV–NT-3-treated and AAV–GFP-treated rats were not. Notably, AAV–BDNF-treated rats also developed hindlimb spasticity, detracting from its potential clinical applicability via the current viral delivery method. Intracellular recording from triceps surae motoneurons revealed that AAV–BDNF significantly reduced motoneuron rheobase, suggesting that AAV–BDNF promoted the recovery of over-ground stepping by enhancing neuronal excitability. Elevated nuclear c-Fos expression in interneurons located in the L2 intermediate zone after AAV–BDNF treatment indicated increased activation of interneurons in the vicinity of the locomotor central pattern generator. AAV–NT-3 treatment reduced motoneuron excitability, with little change in c-Fos expression. These results support the potential for BDNF delivery at the lesion site to reorganize locomotor circuits.

Plasticity of rat motoneuron rhythmic firing properties with varying levels of afferent and descending inputs

Hindlimb motoneuron excitability was compared among exercise-trained (E), sedentary (S), and spinal cord transected (T) Sprague-Dawley rats by examining the slope of the frequency-current (F/I) relationship with standard intracellular recording techniques in rats anesthetized with ketamine-xylazine. The T group included spinal transected and spinal isolated rats; the E animals were either spontaneously active (exercise wheel) or treadmill trained; and rats in the S group were housed in pairs. An analysis of motoneuron initial [1st interspike interval (ISI)], early (mean of 1st three ISIs), and steady-state (mean of last 3 ISIs) discharge rate slopes resulting from increasing and decreasing 500-ms injected square-wave depolarizing current pulses was used to describe rhythmic motoneuron properties. The steepest slope occurred in the S group (55.3 ± 22.2 Hz/nA), followed by the T group (35.5 ± 15.3 Hz/nA), while the flattest slope was found in the E group (25.4 ± 10.9 Hz/nA). The steepest steady-state slope occurred in the S group but was found to be similar between the T and E groups. Furthermore, a spike-frequency adaptation (SFA) index revealed a slower adaptation in motoneurons of the E animals only (∼40% lower). Finally, evidence for a secondary range of firing existed more frequently in the T group (41%) compared with the S (12%) and E (31%) groups. The lower F/I slope and lower SFA index of motoneurons for E rats may be a result of an increase in Na(+) conductance at the initial segment. The results show that motoneuronal rhythmic firing behavior is plastic, depending on the volume of daily activation and on intact descending pathways.

Polysynaptic excitatory postsynaptic potentials that trigger spasms after spinal cord injury in rats are inhibited by 5-HT1B and 5-HT1F receptors

Sensory afferent transmission and associated spinal reflexes are normally inhibited by serotonin (5-HT) derived from the brain stem. Spinal cord injury (SCI) that eliminates this 5-HT innervation leads to a disinhibition of sensory transmission and a consequent emergence of unusually long polysynaptic excitatory postsynaptic potentials (EPSPs) in motoneurons. These EPSPs play a critical role in triggering long polysynaptic reflexes (LPRs) that initiate muscles spasms. In the present study we examined which 5-HT receptors modulate the EPSPs and whether these receptors adapt to a loss of 5-HT after chronic spinal transection in rats. The EPSPs and associated LPRs recorded in vitro in spinal cords from chronic spinal rats were consistently inhibited by 5-HT(1B) or 5-HT(1F) receptor agonists, including zolmitriptan (5-HT(1B/1D/1F)) and LY344864 (5-HT(1F)), with a sigmoidal dose-response relation, from which we computed the 50% inhibition (EC(50)) and potency (-log EC(50)). The potencies of 5-HT receptor agonists were highly correlated with their binding affinity to 5-HT(1B) and 5-HT(1F) receptors, and not to other 5-HT receptors. Zolmitriptan also inhibited the LPRs and general muscle spasms recorded in vivo in the awake chronic spinal rat. The 5-HT(1B) receptor antagonists SB216641 and GR127935 and the inverse agonist SB224289 reduced the inhibition of LPRs by 5-HT(1B) agonists (zolmitriptan). However, when applied alone, SB224289, SB216641, and GR127935 had no effect on the LPRs, indicating that 5-HT(1B) receptors do not adapt to chronic injury, remaining silent, without constitutive activity. The reduction in EPSPs with zolmitriptan unmasked a large glycine-mediated inhibitory postsynaptic current (IPSC) after SCI. This IPSC and associated chloride current reversed at -73 mV, slightly below the resting membrane potential. Zolmitriptan did not change motoneuron properties. Our results demonstrate that 5-HT(1B/1F) agonists, such as zolmitriptan, can restore inhibition of sensory transmission after SCI without affecting general motoneuron function and thus may serve as a novel class of antispastic drugs.

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.

Vitamin D₃ improves respiratory adjustment to fatigue and H-reflex responses in paraplegic adult rats

We previously demonstrated that vitamin D₂ (ergocalciferol) triggers axon regeneration in a rat model of peripheral nerve transection. In order to confirm the regenerative potential of this neuroactive steroid, we performed a study in which vitamin D₃ (cholecalciferol) was delivered at various doses to paralytic rats. After spinal cord compression at the T10 level, rats were given orally either vehicle or vitamin D₃ at the dose of 50 IU/kg/day or 200 IU/kg/day. Three months later, M and H-waves were recorded from rat Tibialis anterior muscle in order to quantify the maximal H-reflex (H(max)) amplitude. We also monitored the ventilatory frequency during an electrically induced muscle fatigue known to elicit the muscle metaboreflex and an increase in respiratory rate. Spinal cords were then collected, fixed and immunostained with an anti-neurofilament antibody. We show here that vitamin D-treated animals display an increased number of axons within the lesion site. In addition, rats supplemented with vitamin D₃ at the dose of 200 IU/kg/day exhibit (i) an improved breathing when hindlimb was electrically stimulated; (ii) an H-reflex depression similar to control animals and (iii) an increased number of axons within the lesion and in the distal area. Our data confirm that vitamin D is a potent molecule that can be used for improving neuromuscular adaptive mechanisms and H-reflex responses.

Locomotor training maintains normal inhibitory influence on both alpha- and gamma-motoneurons after neonatal spinal cord transection

Spinal cord injuries lead to impairments, which are accompanied by extensive reorganization of neuronal circuits caudal to the injury. Locomotor training can aid in the functional recovery after injury, but the neuronal mechanisms associated with such plasticity are only sparsely known. We investigated ultrastructurally the synaptic inputs to tibialis anterior motoneurons (MNs) retrogradely labeled in adult rats that had received a complete midthoracic spinal cord transection at postnatal day 5. A subset of the injured rats received locomotor training. Both γ- and α-MNs were studied. The total number of boutons apposing γ-MNs, but not α-MNs, was reduced after neonatal spinal cord transection. The proportion of inhibitory to excitatory boutons, however, was increased significantly in both α-MNs and γ-MNs in spinally transected rats, but with locomotor training returned to levels observed in intact rats. The specific densities and compositions of synaptic boutons were, however, different between all three groups. Surprisingly, we observed the atypical presence of both C- and M-type boutons apposing the somata of γ-MNs in the spinal rats, regardless of training status. We conclude that a neonatal spinal cord transection induces significant reorganization of synaptic inputs to spinal motoneurons caudal to the site of injury with a net increase in inhibitory influence, which is associated with poor stepping. Spinal cord injury followed by successful locomotor training, however, results in improved bipedal stepping and further synaptic changes with the proportion of inhibitory and excitatory inputs to the motoneurons being similar to that observed in intact rats.

Adrenergic receptors modulate motoneuron excitability, sensory synaptic transmission and muscle spasms after chronic spinal cord injury

The brain stem provides most of the noradrenaline (NA) present in the spinal cord, which functions to both increase spinal motoneuron excitability and inhibit sensory afferent transmission to motoneurons (excitatory postsynaptic potentials; EPSPs). NA increases motoneuron excitability by facilitating calcium-mediated persistent inward currents (Ca PICs) that are crucial for sustained motoneuron firing. Spinal cord transection eliminates most NA and accordingly causes an immediate loss of PICs and emergence of exaggerated EPSPs. However, with time PICs recover, and thus the exaggerated EPSPs can then readily trigger these PICs, which in turn produce muscle spasms. Here we examined the contribution of adrenergic receptors to spasms in chronic spinal rats. Selective activation of the α(1A) adrenergic receptor with the agonists methoxamine or A61603 facilitated Ca PIC and spasm activity, recorded both in vivo and in vitro. In contrast, the α(2) receptor agonists clonidine and UK14303 did not facilitate Ca PICs, but did decrease the EPSPs that trigger spasms. Moreover, in the absence of agonists, spasms recorded in vivo were inhibited by the α(1) receptor antagonists WB4010, prazosin, and REC15/2739, and increased by the α(2) receptor antagonist RX821001, suggesting that both adrenergic receptors were endogenously active. In contrast, spasm activity recorded in the isolated in vitro cord was inhibited only by the α(1) antagonists that block constitutive receptor activity (activity in the absence of NA; inverse agonists, WB4010 and prazosin) and not by the neutral antagonist REC15/2739, which only blocks conventional NA-mediated receptor activity. RX821001 had no effect in vitro even though it is an α(2) receptor inverse agonist. Our results suggest that after chronic spinal cord injury Ca PICs and spasms are facilitated, in part, by constitutive activity in α(1) adrenergic receptors. Additionally, peripherally derived NA (or similar ligand) activates both α(1) and α(2) adrenergic receptors, controlling PICs and EPSPs, respectively.

Motoneuron excitability and muscle spasms are regulated by 5-HT2B and 5-HT2C receptor activity

Immediately after spinal cord injury (SCI), a devastating paralysis results from the loss of brain stem and cortical innervation of spinal neurons that control movement, including a loss of serotonergic (5-HT) innervation of motoneurons. Over time, motoneurons recover from denervation and function autonomously, exhibiting large persistent calcium currents (Ca PICs) that both help with functional recovery and contribute to uncontrolled muscle spasms. Here we systematically evaluated which 5-HT receptor subtypes influence PICs and spasms after injury. Spasms were quantified by recording the long-lasting reflexes (LLRs) on ventral roots in response to dorsal root stimulation, in the chronic spinal rat, in vitro. Ca PICs were quantified by intracellular recording in synaptically isolated motoneurons. Application of agonists selective to 5-HT(2B) and 5-HT(2C) receptors (including BW723C86) significantly increased the LLRs and associated Ca PICs, whereas application of agonists to 5-HT(1), 5-HT(2A), 5-HT(3), or 5-HT(4/5/6/7) receptors (e.g., 8-OH-DPAT) did not. The 5-HT(2) receptor agonist-induced increases in LLRs were dose dependent, with doses for 50% effects (EC(50)) highly correlated with published doses for agonist receptor binding (K(i)) at 5-HT(2B) and 5-HT(2C) receptors. Application of selective antagonists to 5-HT(2B) (e.g., RS127445) and 5-HT(2C) (SB242084) receptors inhibited the agonist-induced increase in LLR. However, antagonists that are known to specifically be neutral antagonists at 5-HT(2B/C) receptors (e.g., RS127445) had no effect when given by themselves, indicating that these receptors were not activated by residual 5-HT in the spinal cord. In contrast, inverse agonists (such as SB206553) that block constitutive activity at 5-HT(2B) or 5-HT(2C) receptors markedly reduced the LLRs, indicating the presence of constitutive activity in these receptors. 5-HT(2B) or 5-HT(2C) receptors were confirmed to be on motoneurons by immunolabeling. In summary, 5-HT(2B) and 5-HT(2C) receptors on motoneurons become constitutively active after injury and ultimately contribute to recovery of motoneuron function and emergence of spasms.

Characteristics and organization of discharge properties in rat hindlimb motoneurons

The discharge properties of hindlimb motoneurons in ketamine-xylazine anesthetized rats were measured to assess contributions of persistent intrinsic currents to these characteristics and to determine their distribution in motoneuron pools. Most motoneurons (30/37) responded to ramp current injections with adapting patterns of discharge and the frequency-current (f-I) relations of nearly all motoneurons included a steep subprimary range of discharge. Despite the prevalence of adapting f-I relations, responses included indications that persistent inward currents (PICs) were activated, including increased membrane noise and prepotentials before discharge, as well as counterclockwise hysteresis and secondary ranges in f-I relations. Examination of spike thresholds and afterhyperpolarization (AHP) trajectories during repetitive discharge revealed systematic changes in threshold and trajectory within the subprimary, primary, and secondary f-I ranges. These changes in the primary and secondary ranges were qualitatively similar to those described previously for cat motoneurons. Within the subprimary range, AHP trajectories often included shallow approaches to threshold following recruitment and slope of the AHP ramp consistently increased until the subprimary range was reached. We suggest that PICs activated near recruitment contributed to these slope changes and formation of the subprimary range. Discharge characteristics were strongly correlated with motoneuron size, using input conductance as an indicator of size. Discharge adaptation, recruitment current, and frequency increased with input conductance, whereas both subprimary and primary f-I gains decreased. These results are discussed with respect to potential mechanisms and their functional implications.

Persistent currents and discharge patterns in rat hindlimb motoneurons

We report here the first direct measurements of persistent inward currents (PICs) in rat hindlimb motoneurons, obtained from ketamine-xylazine anesthetized rats during slow voltage ramps performed by single-electrode somatic voltage clamp. Most motoneurons expressed PICs and current-voltage (I-V) relations often contained a negative-slope region (NSR; 13/19 cells). PICs activated at -52.7 ± 3.89 mV, 9 mV negative to spike threshold. NSR onset was -44.2 ± 4.1 mV. PIC amplitudes were assessed by maximum inward currents measured relative to extrapolated leak current and to NSR-onset current. PIC conductance at potentials just positive to activation was assessed by the relative change in slope conductance (g(in)/g(leak)). PIC amplitudes varied widely; some exceeded 5 and 10 nA relative to current at NSR onset or leak current, respectively. PIC amplitudes did not vary significantly with input conductance, but PIC amplitudes normalized by recruitment current decreased with increasing input conductance. Similarly, g(in)/g(leak) decreased with increasing input conductance. Currents near resting potential on descending limbs of I-V relations were often outward, relative to ascending-limb currents. This residual outward current was correlated with increases in leak conductance on the descending limb and with input conductance. Excluding responses with accommodation, residual outward currents matched differences between recruitment and derecruitment currents, suggesting a role for residual outward current in frequency adaptation. Comparison of potentials for PIC activation and NSR onset with interspike trajectories during discharge demonstrated correspondence between PIC activation and frequency-current (f-I) range boundaries. Contributions of persistent inward and outward currents to motoneuron discharge characteristics are discussed.

Recovery of motoneuron and locomotor function after spinal cord injury depends on constitutive activity in 5-HT2C receptors

Muscle paralysis after spinal cord injury is partly caused by a loss of brainstem-derived serotonin (5-HT), which normally maintains motoneuron excitability by regulating crucial persistent calcium currents. Here we examine how over time motoneurons compensate for lost 5-HT to regain excitability. We find that, months after a spinal transection in rats, changes in post-transcriptional editing of 5-HT2C receptor mRNA lead to increased expression of 5-HT2C receptor isoforms that are spontaneously active (constitutively active) without 5-HT. Such constitutive receptor activity restores large persistent calcium currents in motoneurons in the absence of 5-HT. We show that this helps motoneurons recover their ability to produce sustained muscle contractions and ultimately enables recovery of motor functions such as locomotion. However, without regulation from the brain, these sustained contractions can also cause debilitating muscle spasms. Accordingly, blocking constitutively active 5-HT2C receptors with SB206553 or cyproheptadine, in both rats and humans, largely eliminates these calcium currents and muscle spasms, providing a new rationale for antispastic drug therapy.

Multiple Patterns and Components of Persistent Inward Current With Serotonergic Modulation in Locomotor Activity–Related Neurons in Cfos-EGFP Mice

Using CFos-EGFP transgenic mice (P6–P12), we targeted persistent inward current (PIC) in the spinal interneurons activated by locomotion. Following a locomotor task, whole cell patch-clamp recordings were obtained from ventral EGFP+ neurons in spinal cord slices (200–250 μm from T13–L4). PIC was recorded by a family of 10 s voltage bi-ramps starting from −70 mV with 30 mV steps. PIC could be classified as ascending and descending forms based on the rising and falling phases of the bi-ramps. Multiple patterns of PIC with various hystereses were found in EGFP+ neurons. A novel form of PIC, single PIC crossing both phases of the bi-ramps, was described in this study. PIC was found in 82% of EGFP+ neurons ( n = 129) with no significant difference in laminar distribution. PIC activated at −56.7 ± 8 mV with an amplitude of 85.3 ± 59 pA and time constant of 657.0 ± 272 ms ( n = 63). PIC in lamina VIII neurons activated significantly lower (−60.2 ± 7 mV) than in lamina VII (−54.8 ± 6 mV) and lamina X (−55.8 ± 9 mV) neurons. PIC could be differentiated as calcium dependent (Ca-PIC) by bath application of 1–5 μM TTX or sodium dependent (Na-PIC) by administration of 20–30 μM dihydropyridine. Ca-PIC activated at −40.2 ± 19 mV ( n = 49), whereas Na-PIC activated at −46.8 ± 16 mV ( n = 17). Composite-, Ca-, and Na-PICs were significantly different in activation but not amplitude and time constant. Bath application of 5-HT (10–30 μM) enhanced PIC ( n = 32) by hyperpolarizing onset (4.2 ± 6 mV) and increasing amplitude (16%). 5-HT–increased amplitude seemed to be significantly larger in lamina VII neurons (32%) than VIII (6%) and X (14%) neurons. 5-HT enhancement of Ca-PIC ( n = 6) and Na-PICs ( n = 4) was also observed in EGFP+ neurons. This study unveiled unique properties of PICs in EGFP+ neurons. The lamina-related PIC activation and variable effects of 5-HT on PIC amplitude provides insight into the ionic basis on which locomotion could be generated.

Intrinsic properties of mouse lumbar motoneurons revealed by intracellular recording in vivo

We have developed an in vivo model for intracellular recording in the adult anesthetized mouse using sharp microelectrode electrodes as a basis for investigations of motoneuron properties in transgenic mouse strains. We demonstrate that it is possible to record postsynaptic potentials underlying identified circuits in the spinal cord. Forty-one motoneurons with antidromic spike potentials (>50 mV) from the sciatic nerve were investigated. We recorded the intrinsic properties of the neurons, including input resistance (mean: 2.4 +/- 1.2 MOmega), rheobase (mean: 7.1 +/- 5.9 nA), and the duration of the afterhyperpolarization (AHP; mean: 55.3 +/- 14 ms). We also measured the minimum firing frequencies (F(min), mean 23.5 +/- 5.7 SD Hz), the maximum firing frequencies (F(max); >300 Hz) and the slope of the current-frequency relationship (f-I slope) with increasing amounts of current injected (mean: 13 +/- 5.7 Hz/nA). Signs of activation of persistent inward currents (PICs) were seen, such as accelerations of firing frequency or jumps in the membrane potential with increasing amounts of injected current. It is likely that the particular anesthetic regime with a mixture of Hypnorm and midazolam is essential for the possibility to evoke PICs. The data demonstrate that mouse spinal motoneurons share many of the same properties that have been demonstrated previously for cat, rat, and human motoneurons. The shorter AHP duration, steeper f-I slopes, and higher F(min) and F(max) than those in rats, cats, and humans are likely to be tailored to the characteristics of the mouse muscle contraction properties.

Activity-dependent plasticity of spinal locomotion: implications for sensory processing

The lumbosacral spinal cord of mammals contains the neural circuitry capable of generating full weight-bearing locomotion of the hind limbs without any supraspinal input. One or more interventions, for example, pharmacological, epidural stimulation, and/or locomotor training, however, are necessary to gain access to and modulate the properties of this circuitry and to facilitate recovery of full weight-bearing locomotion after spinal cord injury.

Motoneuron excitability: the importance of neuromodulatory inputs

The excitability of spinal motoneurons is both fundamental for motor behavior and essential in diagnosis of neural disorders. There are two mechanisms for altering this excitability. The classic mechanism is mediated by synaptic inputs that depolarize or hyperpolarize motoneurons by generating postsynaptic potentials. This "ionotropic" mechanism works via neurotransmitters that open ion channels in the cell membrane. In the second mechanism, neurotransmitters bind to receptors that activate intracellular signaling pathways. These pathways modulate the properties of the voltage-sensitive channels that determine the intrinsic input-output properties of motoneurons. This "neuromodulatory" mechanism usually does not directly activate motoneurons but instead dramatically alters the neuron's response to ionotropic inputs. We present extensive evidence that neuromodulatory inputs exert a much more powerful effect on motoneuron excitability than ionotropic inputs. The most potent neuromodulators are probably serotonin and norepinephrine, which are released by axons originating in the brainstem and can increase motoneuron excitability fivefold or more. Thus, the standard tests of motoneuron excitability (H-reflexes, tendon taps, tendon vibration and stretch reflexes) are strongly influenced by the level of neuromodulatory input to motoneurons. This insight is likely to be profoundly important for clinical diagnosis and treatment.

Caloric Restriction Does Not Offset Age-Associated Changes in the Biophysical Properties of Motoneurons

Age-associated changes in neuromuscular function may be due to a loss of motor neurons as well as changes in their biophysical properties. Neuronal damage imposed by reactive oxygen species may contribute to age-related deficits in CNS function. Thus we hypothesized that aging would alter the functional properties of motoneurons and that caloric-restriction would offset these changes. Intracellular recordings were made from lumbar motoneurons of old Fisher Brown Norway (FBN) fed ad libitum (oldAL, 30.8 ± 1.3 mo) or on a fortified calorie-restricted diet from 14 wk of age (oldCR, 31.0 ± 1.8 mo). Basic and rhythmic firing properties recorded from these aged motoneurons (MNs) were compared with properties recorded from young FBN controls (young, 8.4 ± 4.6 mo). Compared with young MNs, old MNs had a 104% greater ( P < 0.001) afterhyperpolarization potential (AHP), a 21.1% longer AHP half-decay time ( P < 0.05), 28.7% lower rheobase ( P < 0.001), 49.7% greater ( P < 0.001) input resistance, 21.1% ( P < 0.0001) less spike frequency adaptation, lower minimal (30.2%, P < 0.0001) and maximal (16.7%, P < 0.0001) steady-state firing frequencies, a lower (35.5%, P < 0.0001) frequency-current slope, and an increased incidence of persistent inward current. Because basic properties became more diverse in old MNs and the slope of the frequency-current relationship, which is normally similar for high- and low-threshold MNs, was lower in the old group, we conclude that aging alters the biophysical properties of MNs in a fashion that cannot be simply attributed to a loss of high-threshold MNs. Surprisingly, caloric restriction, which is known to attenuate aging-associated changes in hindlimb muscles, had no effect on the progress of aging in the innervating MNs.

Up-regulation of slow K(+) channels in peripheral motor axons: a transcriptional channelopathy in multiple sclerosis

Spinal lesions produce plastic changes in motoneuron properties. We have documented the excitability of motor axons in the median nerve of 12 patients with multiple sclerosis and 50 normal subjects, hypothesizing that plastic changes in the properties of spinal motoneurons might be reflected in the properties of peripheral motor axons and be demonstrable in vivo. In the patients, there were changes in physiological measures of axonal excitability attributable to increased slow K(+) channel activity. Other measures were within control limits. These changes could be modelled by an 11% increase in slow K(+) current, with compensatory changes in membrane potential, suggesting increased expression of the responsible channels. The changes cannot be explained solely by changes in membrane potential and are not those expected if peripheral nerve axons were involved in the inflammatory process of multiple sclerosis. They probably represent a transcriptional channelopathy, due to up-regulation of channel expression. The abnormalities do not imply that peripheral nerve function has been significantly compromised, but they do suggest that the properties of the parent motoneurons have changed. This study thus provides evidence for plasticity in motoneuronal properties at a molecular level, the first such evidence for intact human subjects.