Robust upregulation of serotonin 2A receptors after chronic spinal transection of rats: an immunohistochemical study

Covert actions of epidural stimulation on spinal locomotor circuits

Spinal circuitry produces the rhythm and patterning of locomotion. However, both descending and sensory inputs are required to initiate and adapt locomotion to the environment. Spinal cord injury (SCI) disrupts descending controls of the spinal cord, producing paralysis. Epidural stimulation (ES) is a promising clinical therapy for motor control recovery and is capable of reactivating the lumbar spinal locomotor networks, yet little is known about the effects of ES on locomotor neurons. Previously, we found that both sensory afferent pathways and serotonin exert mixed excitatory and inhibitory actions on lumbar interneurons involved in the generation of the locomotor rhythm, identified by the transcription factor Shox2. However, after chronic complete SCI, sensory afferent inputs to Shox2 interneurons become almost exclusively excitatory and Shox2 interneurons are supersensitive to serotonin. Here, we investigated the effects of ES on these SCI-induced changes. Inhibitory input from sensory pathways to Shox2 interneurons was maintained and serotonin supersensitivity was not observed in SCI mice that received daily sub-motor threshold ES. Interestingly, the effects of ES were maintained for at least three weeks after the ES was discontinued. In contrast, the effects of ES were not observed in Shox2 interneurons from mice that received ES after the establishment of the SCI-induced changes. Our results demonstrate mechanistic actions of ES at the level of identified spinal locomotor circuit neurons and the effectiveness of early treatment with ES on preservation of spinal locomotor circuitry after SCI, suggesting possible therapeutic benefits prior to the onset of motor rehabilitation.

Molecular Identification of Pro-Excitogenic Receptor and Channel Phenotypes of the Deafferented Lumbar Motoneurons in the Early Phase after SCT in Rats

Spasticity impacts the quality of life of patients suffering spinal cord injury and impedes the recovery of locomotion. At the cellular level, spasticity is considered to be primarily caused by the hyperexcitability of spinal α-motoneurons (MNs) within the spinal stretch reflex circuit. Here, we hypothesized that after a complete spinal cord transection in rats, fast adaptive molecular responses of lumbar MNs develop in return for the loss of inputs. We assumed that early loss of glutamatergic afferents changes the expression of glutamatergic AMPA and NMDA receptor subunits, which may be the forerunners of the developing spasticity of hindlimb muscles. To better understand its molecular underpinnings, concomitant expression of GABA and Glycinergic receptors and serotoninergic and noradrenergic receptors, which regulate the persistent inward currents crucial for sustained discharges in MNs, were examined together with voltage-gated ion channels and cation-chloride cotransporters. Using quantitative real-time PCR, we showed in the tracer-identified MNs innervating extensor and flexor muscles of the ankle joint multiple increases in transcripts coding for AMPAR and 5-HTR subunits, along with a profound decrease in GABAAR, GlyR subunits, and KCC2. Our study demonstrated that both MNs groups similarly adapt to a more excitable state, which may increase the occurrence of extensor and flexor muscle spasms.

The Development of Hindlimb Postural Asymmetry Induced by Focal Traumatic Brain Injury Is Not Related to Serotonin 2A/C Receptor Expression in the Spinal Cord

Brain injury and stroke are leading causes of adult disability. Motor deficits are common problems, and their underlying pathological mechanisms remain poorly understood. The serotoninergic system is implicated in both functional recovery from and the occurrence of spasticity after injuries to the central nervous system. This study, which was conducted on rats, investigated the development of limb postural changes and their relationship to the expression of serotonin (5-HT) 2A and 2C receptors in the spinal cord in the 4 weeks after focal traumatic brain injury (TBI) to the right hindlimb sensorimotor cortex. The limb motor deficits were assessed by measuring gait pattern changes during walking and hindlimb postural asymmetry at different time intervals (3−28 days) after surgery. The expressions of the 5-HT2A and 2C receptors in the lumbar spinal cord were investigated using immunohistochemistry. The results showed that all the rats with TBI, independently of the duration of the interval, displayed postural asymmetry with flexion on the contralateral (left) side (>2 mm), while the sham-operated rats showed no apparent postural asymmetry. The TBI rats also had longer stride lengths during walking in both their hindlimbs and their forelimbs compared with the sham rats. For both the TBI and the sham rats, the hind-paw placement angles were larger on the contralateral side in some of the groups. Compared to the sham-operated rats, the 5-HT2A and 2C receptor expression did not significantly change on either side of the lumbar spinal cords of the TBI rats in any of the groups. These results suggest that focal TBI can induce motor deficits lasting a relatively long time, and that these deficits are not related to the expression of the 5-HT2A and 2C receptors in the spinal cord.

Role of Descending Serotonergic Fibers in the Development of Pathophysiology after Spinal Cord Injury (SCI): Contribution to Chronic Pain, Spasticity, and Autonomic Dysreflexia

As the nervous system develops, nerve fibers from the brain form descending tracts that regulate the execution of motor behavior within the spinal cord, incoming sensory signals, and capacity to change (plasticity). How these fibers affect function depends upon the transmitter released, the receptor system engaged, and the pattern of neural innervation. The current review focuses upon the neurotransmitter serotonin (5-HT) and its capacity to dampen (inhibit) neural excitation. A brief review of key anatomical details, receptor types, and pharmacology is provided. The paper then considers how damage to descending serotonergic fibers contributes to pathophysiology after spinal cord injury (SCI). The loss of serotonergic fibers removes an inhibitory brake that enables plasticity and neural excitation. In this state, noxious stimulation can induce a form of over-excitation that sensitizes pain (nociceptive) circuits, a modification that can contribute to the development of chronic pain. Over time, the loss of serotonergic fibers allows prolonged motor drive (spasticity) to develop and removes a regulatory brake on autonomic function, which enables bouts of unregulated sympathetic activity (autonomic dysreflexia). Recent research has shown that the loss of descending serotonergic activity is accompanied by a shift in how the neurotransmitter GABA affects neural activity, reducing its inhibitory effect. Treatments that target the loss of inhibition could have therapeutic benefit.

Deciphering Spinal Endogenous Dopaminergic Mechanisms That Modulate Micturition Reflexes in Rats with Spinal Cord Injury

Spinal neuronal mechanisms regulate recovered involuntary micturition after spinal cord injury (SCI). It was recently discovered that dopamine (DA) is synthesized in the rat injured spinal cord and is involved in lower urinary tract (LUT) activity. To fully understand the role of spinal DAergic machinery in micturition, we examined urodynamic responses in female rats during pharmacological modulation of the DA pathway. Three to four weeks after complete thoracic SCI, the DA precursor L-DOPA administered intravenously during bladder cystometrogram (CMG) and external urethral sphincter (EUS) electromyography (EMG) reduced bladder overactivity and increased the duration of EUS bursting, leading to remarkably improved voiding efficiency. Apomorphine (APO), a non-selective DA receptor (DR) agonist, or quinpirole, a selective DR₂ agonist, induced similar responses, whereas a specific DR₂ antagonist remoxipride alone had only minimal effects. Meanwhile, administration of SCH 23390, a DR₁ antagonist, reduced voiding efficiency by increasing tonic EUS activity and shortening the EUS bursting period. Unexpectedly, SKF 38393, a selective DR₁ agonist, increased EUS tonic activity, implying a complicated role of DR₁ in LUT function. In metabolic cage assays, subcutaneous administration of quinpirole decreased spontaneous voiding frequency and increased voiding volume; L-DOPA and APO were inactive possibly because of slow entry into the CNS. Collectively, tonically active DR₁ in SCI rats inhibit urine storage and enhance voiding by differentially modulating EUS tonic and bursting patterns, respectively, while pharmacologic activation of DR₂, which are normally silent, improves voiding by enhancing EUS bursting. Thus, enhancing DA signaling achieves better detrusor-sphincter coordination to facilitate micturition function in SCI rats.

Spinal Cord Injury Alters Spinal Shox2 Interneurons by Enhancing Excitatory Synaptic Input and Serotonergic Modulation While Maintaining Intrinsic Properties in Mouse

Neural circuitry generating locomotor rhythm and pattern is located in the spinal cord. Most spinal cord injuries (SCIs) occur above the level of spinal locomotor neurons; therefore, these circuits are a target for improving motor function after SCI. Despite being relatively intact below the injury, locomotor circuitry undergoes substantial plasticity with the loss of descending control. Information regarding cell type-specific plasticity within locomotor circuits is limited. Shox2 interneurons (INs) have been linked to locomotor rhythm generation and patterning, making them a potential therapeutic target for the restoration of locomotion after SCI. The goal of the present study was to identify SCI-induced plasticity at the level of Shox2 INs in a complete thoracic transection model in adult male and female mice. Whole-cell patch-clamp recordings of Shox2 INs revealed minimal changes in intrinsic excitability properties after SCI. However, afferent stimulation resulted in mixed excitatory and inhibitory input to Shox2 INs in uninjured mice which became predominantly excitatory after SCI. Shox2 INs were differentially modulated by serotonin (5-HT) in a concentration-dependent manner in uninjured conditions but following SCI, 5-HT predominantly depolarized Shox2 INs. 5-HT7 receptors mediated excitatory effects on Shox2 INs from both uninjured and SCI mice, but activation of 5-HT2B/2C receptors enhanced excitability of Shox2 INs only after SCI. Overall, SCI alters sensory afferent input pathways to Shox2 INs and 5-HT modulation of Shox2 INs to enhance excitatory responses. Our findings provide relevant information regarding the locomotor circuitry response to SCI that could benefit strategies to improve locomotion after SCI.SIGNIFICANCE STATEMENT Current therapies to gain locomotor control after spinal cord injury (SCI) target spinal locomotor circuitry. Improvements in therapeutic strategies will require a better understanding of the SCI-induced plasticity within specific locomotor elements and their controllers, including sensory afferents and serotonergic modulation. Here, we demonstrate that excitability and intrinsic properties of Shox2 interneurons, which contribute to the generation of the locomotor rhythm and pattering, remain intact after SCI. However, SCI induces plasticity in both sensory afferent pathways and serotonergic modulation, enhancing the activation and excitation of Shox2 interneurons. Our findings will impact future strategies looking to harness these changes with the ultimate goal of restoring functional locomotion after SCI.

Exercise-Induced Plasticity in Signaling Pathways Involved in Motor Recovery after Spinal Cord Injury

Spinal cord injury (SCI) leads to numerous chronic and debilitating functional deficits that greatly affect quality of life. While many pharmacological interventions have been explored, the current unsurpassed therapy for most SCI sequalae is exercise. Exercise has an expansive influence on peripheral health and function, and by activating the relevant neural pathways, exercise also ameliorates numerous disorders of the central nervous system (CNS). While the exact mechanisms by which this occurs are still being delineated, major strides have been made in the past decade to understand the molecular underpinnings of this essential treatment. Exercise rapidly and prominently affects dendritic sprouting, synaptic connections, neurotransmitter production and regulation, and ionic homeostasis, with recent literature implicating an exercise-induced increase in neurotrophins as the cornerstone that binds many of these effects together. The field encompasses vast complexity, and as the data accumulate, disentangling these molecular pathways and how they interact will facilitate the optimization of intervention strategies and improve quality of life for individuals affected by SCI. This review describes the known molecular effects of exercise and how they alter the CNS to pacify the injury environment, increase neuronal survival and regeneration, restore normal neural excitability, create new functional circuits, and ultimately improve motor function following SCI.

Buspirone for functional improvement after acute traumatic spinal cord injury: a propensity score-matched cohort study

STUDY DESIGN

Retrospective analysis of treated inpatients compared to expected neurorecovery from a propensity score-matched national database cohort.

OBJECTIVE

Evaluate the effectiveness of buspirone on clinical neurorecovery following traumatic SCI when started during acute inpatient rehabilitation.

SETTING

University-based hospital in Boston, USA.

METHODS

Chart review yielded thirty-one individuals with acute, traumatic SCI treated with buspirone during inpatient rehabilitation from 2011-2017. Propensity score matching to a cohort of individuals from the spinal cord injury model systems (SCIMS) national database was completed. Changes in upper extremity motor score (UEMS), lower extremity motor score (LEMS), American Spinal Injury Association Impairment Scale (AIS), neurological level of injury (NLI), and functional impairment measure (FIM) from admission to discharge and discharge to 1 year were computed and compared between matched pairs (buspirone and mean national SCIMs cohort). A local control cohort not treated with buspirone was similarly compared to a matched mean national SCIMs group to identify location-specific effects.

RESULTS

From admission to discharge from inpatient rehabilitation, 95% confidence intervals of changes in UEMS (-2.43 to +2.78), LEMS (-1.02 to +6.02), AIS (-0.04 to +0.35), NLI (-0.42 to +1.08), and FIM (-4.42 to +6.40) were not significantly different between those individuals who received buspirone and their propensity-matched SCIMS cohort. Similarly, changes in these metrics were not significantly different at 1-year follow up. Buspirone group individuals with initial clinically complete SCI demonstrated a higher 1-year conversion rate to incomplete injury (6 out of 14; 42.9%) compared to the matched national SCIMS cohort (14 out of 70; 21.2%, p = 0.047) though this was not significantly different from non-buspirone local controls (p = 0.25).

CONCLUSIONS

Retrospective analysis shows no statistically significant difference in gross markers of neurorecovery following acute traumatic SCI when buspirone is initiated indiscriminately during acute inpatient rehabilitation. In individuals with clinically complete SCI, findings suggest possible increased rates of 1-year conversion to incomplete injury.

Delivery of the 5-HT2A Receptor Agonist, DOI, Enhances Activity of the Sphincter Muscle during the Micturition Reflex in Rats after Spinal Cord Injury

Simple Summary

Spinal cord injury often disrupts connections between the brain and spinal cord leading to a plethora of health complications, including bladder dysfunction. Spinal cord injured patients are left with symptoms such as a leaky bladder (the inability to hold their urine), frequent urinary tract infections, and potential kidney failure. However, previous studies have shown that manipulation of serotoninergic receptors can improve urinary performance following spinal cord injury. In the current study, we sought to explore how stimulation of a specific serotonergic receptor subtype can significantly enhance bladder function in spinal cord injured rats. To do so, we utilized spinal cord injured female rats that underwent various bladder performance evaluations combined with pharmacological intervention of a specific serotonergic subtype. Additionally, the primary site of action was investigated to determine effects elicited during various administration routes (e.g., directly into the cord, into the femoral vein, or into the skin). Stimulation of this receptor subtype, regardless of delivery route, improved activity of the external urethral sphincter and detrusor-sphincter coordination in spinal cord injured rats. Collectively, the results of these experiments have the potential to provide vital guidance for the development of therapeutic strategies to alleviate urinary dysfunction following spinal cord injury.

Abstract

Traumatic spinal cord injury (SCI) interrupts spinobulbospinal micturition reflex pathways and results in urinary dysfunction. Over time, an involuntary bladder reflex is established due to the reorganization of spinal circuitry. Previous studies show that manipulation of serotonin 2A (5-HT_2A) receptors affects recovered bladder function, but it remains unclear if this receptor regulates the activity of the external urethral sphincter (EUS) following SCI. To elucidate how central and peripheral serotonergic machinery acts on the lower urinary tract (LUT) system, we employed bladder cystometry and EUS electromyography recordings combined with intravenous or intrathecal pharmacological interventions of 5-HT_2A receptors in female SCI rats. Three to four weeks after a T10 spinal transection, systemic and central blockage of 5-HT_2A receptors with MDL only slightly influenced the micturition reflex. However, delivery of the 5-HT_2A receptor agonist, DOI, increased EUS tonic activity and elicited bursting during voiding. Additionally, subcutaneous administration of DOI verified the enhancement of continence and voiding capability during spontaneous micturition in metabolic cage assays. Although spinal 5HT_2A receptors may not be actively involved in the recovered micturition reflex, stimulating this receptor subtype enhances EUS function and the synergistic activity between the detrusor and sphincter to improve the micturition reflex in rats with SCI.

Differential effects of spinal cord transection on glycinergic and GABAergic synaptic signaling in sub-lesional lumbar motoneurons

This review takes stock on the impact of complete spinal cord transection (SCT) on the plasticity of inhibitory synaptic transmission on sub-lesional lumbar motoneurons (MNs), differentiating between studies in neonate and adult rats. After neonatal SCT, normal maturational up-regulation of glycine receptors was observed. On the other hand, the developmental downregulation of the GABAA receptors, as well as the up-regulation of the co-transporter KCC2 were prevented, but not the normal decrease of NKCC1. In adult SCT rats, glycinergic synaptic transmission, which is the major contributor to spinal MNs inhibition in adulthood, had normal control levels 2 months post-injury. On the other hand, the GABAergic transmission was altered through an up-regulation of the pre-signaling levels and a down-regulation in the density of post synaptic receptors. KCC2 membrane expression was down-regulated at all post-injury times tested (24h to 4 months), thereby depolarizing the Cl- equilibrium potential and reducing the strength of postsynaptic inhibition. The preservation of glycinergic pre- and post signaling is probably a key factor in the success of locomotor rehabilitation programs in adult SCT rats. However, these data highlight the need to develop strategies to restore KCC2 levels in lumbar MNs, to stabilize the excitation/inhibition balance, which is essential to the effective control of skeletal muscle activity.

Activation of 5-HT2A Receptors Restores KCC2 Function and Reduces Neuropathic Pain after Spinal Cord Injury

Downregulation of the potassium chloride cotransporter type 2 (KCC2) after a spinal cord injury (SCI) disinhibits motoneurons and dorsal horn interneurons causing spasticity and neuropathic pain, respectively. We showed recently (Bos et al., 2013) that specific activation of 5-HT2A receptors by TCB-2 [(4-bromo-3,6-dimethoxybenzocyclobuten-1-yl)methylamine hydrobromide] upregulates KCC2 function, restores motoneuronal inhibition and reduces SCI-induced spasticity. Here, we tested the potential analgesic effect of TCB-2 on central (thoracic hemisection) and peripheral [spared nerve injury (SNI)] neuropathic pain. We found mechanical and thermal hyperalgesia reduced by an acute administration of TCB-2 in rats with SCI. This analgesic effect was associated with an increase in dorsal horn membrane KCC2 expression and was prevented by pharmacological blockade of KCC2 with an intrathecal injection of DIOA [(dihydroindenyl)oxy]alkanoic acid]. In contrast, the SNI-induced neuropathic pain was not attenuated by TCB-2 although there was a slight increase of membrane KCC2 expression in the dorsal horn ipsilateral to the lesion. Up-regulation of KCC2 function by targeting 5-HT2A receptors, therefore, has therapeutic potential in the treatment of neuropathic pain induced by SCI but not by SNI.

Intraspinal Grafting of Serotonergic Neurons Modifies Expression of Genes Important for Functional Recovery in Paraplegic Rats

Serotonin (5-hydroxytryptamine; 5-HT) plays an important role in control of locomotion, partly through direct effects on motoneurons. Spinal cord complete transection (SCI) results in changes in 5-HT receptors on motoneurons that influence functional recovery. Activation of 5-HT_2A and 5-HT₇ receptors improves locomotor hindlimb movements in paraplegic rats. Here, we analyzed the mRNA of 5-HT_2A and 5-HT₇ receptors (encoded by Htr2a and Htr7 genes, resp.) in motoneurons innervating tibialis anterior (TA) and gastrocnemius lateralis (GM) hindlimb muscles and the tail extensor caudae medialis (ECM) muscle in intact as well as spinal rats. Moreover, the effect of intraspinal grafting of serotonergic neurons on Htr2a and Htr7 gene expression was examined to test the possibility that the graft origin 5-HT innervation in the spinal cord of paraplegic rats could reverse changes in gene expression induced by SCI. Our results indicate that SCI at the thoracic level leads to changes in Htr2a and Htr7 gene expression, whereas transplantation of embryonic serotonergic neurons modifies these changes in motoneurons innervating hindlimb muscles but not those innervating tail muscles. This suggests that the upregulation of genes critical for locomotor recovery, resulting in limb motoneuron plasticity, might account for the improved locomotion in grafted animals.

Serotonin receptor and dendritic plasticity in the spinal cord mediated by chronic serotonergic pharmacotherapy combined with exercise following complete SCI in the adult rat

Severe spinal cord injury (SCI) damages descending motor and serotonin (5-HT) fiber projections leading to paralysis and serotonin depletion. 5-HT receptors (5-HTRs) subsequently upregulate following 5-HT fiber degeneration, and dendritic density decreases indicative of atrophy. 5-HT pharmacotherapy or exercise can improve locomotor behavior after SCI. One might expect that 5-HT pharmacotherapy acts on upregulated spinal 5-HTRs to enhance function, and that exercise alone can influence dendritic atrophy. In the current study, we assessed locomotor recovery and spinal proteins influenced by SCI and therapy. 5-HT, 5-HT_2AR, 5-HT_1AR, and dendritic densities were quantified both early (1 week) and late (9 weeks) after SCI, and also following therapeutic interventions (5-HT pharmacotherapy, bike therapy, or a combination). Interestingly, chronic 5-HT pharmacotherapy largely normalized spinal 5-HTR upregulation following injury. Improvement in locomotor behavior was not correlated to 5-HTR density. These results support the hypothesis that chronic 5-HT pharmacotherapy can mediate recovery following SCI, despite acting on largely normal spinal 5-HTR levels. We next assessed spinal dendritic plasticity and its potential role in locomotor recovery. Single therapies did not normalize the loss of dendritic density after SCI. Groups displaying significantly atrophied dendritic processes were rarely able to achieve weight supported open-field locomotion. Only a combination of 5-HT pharmacotherapy and bike therapy enabled significant open-field weigh-supported stepping, mediated in part by restoring spinal dendritic density. These results support the use of combined therapies to synergistically impact multiple markers of spinal plasticity and improve motor recovery.

On Denny-Brown's 'spastic dystonia' - What is it and what causes it?

In this review, we will work around two simple definitions of two different entities, which most often co-exist in patients with lesions to central motor pathways: Spasticity is "Enhanced excitability of velocity-dependent responses to phasic stretch at rest", which will not be the subject of this review, while Spastic dystonia is tonic, chronic, involuntary muscle contraction in the absence of any stretch or any voluntary command (Gracies, 2005). Spastic dystonia is a much less well understood entity that will be the subject this review. Denny-Brown (1966) observed involuntary sustained muscle activity in monkeys with lesions restricted to the motor cortices . He further observed that such involuntary muscle activity persisted following abolition of sensory input to the spinal cord and concluded that a central mechanism rather than exaggerated stretch reflex activity had to be involved. He coined the term spastic dystonia to describe this involuntary tonic activity in the context of otherwise exaggerated stretch reflexes. Sustained involuntary muscle activity in the absence of any stretch or any voluntary command contributes to burdensome and disabling body deformities in patients with spastic paresis. Yet, little has been done since Denny-Brown's studies to determine the pathophysiology of this non- stretch or effort related sustained involuntary muscle activity following motor lesions and there is a clear need for research studies in order to improve current therapy. The purpose of the present review is to discuss some of the possible mechanisms that may be involved in the hope that this may guide future research. We discuss the existence of persistent inward currents in spinal motoneurones and present the evidence that the channels involved may be upregulated following central motor lesions. We also discuss a possible contribution from alterations in synaptic inputs from surviving or abnormally branched sensory and descending fibres leading to over-activity and lack of motor coordination. We finally discuss evidence of alterations in motor cortical representational maps and basal ganglia lesions.

Production of Dopamine by Aromatic l-Amino Acid Decarboxylase Cells after Spinal Cord Injury

Aromatic l-amino acid decarboxylase (AADC) cells are widely distributed in the spinal cord, and their functions are largely unknown. We have previously found that AADC cells in the spinal cord could increase their ability to produce serotonin (5-hydroxytryptamine) from 5-hydroxytryptophan after spinal cord injury (SCI). Because AADC is a common enzyme catalyzing 5-hydroxytryptophan to serotonin and l-3,4-dihydroxyphenylalanine (l-dopa) to dopamine (DA), it seems likely that the ability of AADC cells using l-dopa to synthesize DA is also increased. To prove whether or not this is the case, a similar rat sacral SCI model and a similar experimental paradigm were adopted as that which we had used previously. In the chronic SCI rats (> 45 days), no AADC cells expressed DA if there was no exogenous l-dopa application. However, following administration of a peripheral AADC inhibitor (carbidopa) with or without a monoamine oxidase inhibitor (pargyline) co-application, systemic administration of l-dopa resulted in ∼94% of AADC cells becoming DA-immunopositive in the spinal cord below the lesion, whereas in normal or sham-operated rats none or very few of AADC cells became DA-immunopositive with the same treatment. Using tail electromyography, spontaneous tail muscle activity was increased nearly fivefold over the baseline level. When pretreated with a central AADC inhibitor (NSD-1015), further application of l-dopa failed to increase the motoneuron activity although the expression of DA in the AADC cells was not completely inhibited. These findings demonstrate that AADC cells in the spinal cord below the lesion gain the ability to produce DA from its precursor in response to SCI. This ability also enables the AADC cells to produce 5-HT and trace amines, and likely contributes to the development of hyperexcitability. These results might also be implicated for revealing the pathological mechanisms underlying l-dopa-induced dyskinesia in Parkinson's disease.

Preclinical models of muscle spasticity: valuable tools in the development of novel treatment for neurological diseases and conditions

Poor validity of preclinical animal models is one of the most commonly discussed explanations for the failures to develop novel drugs in general and in neuroscience in particular. However, there are several areas of neuroscience such as injury-induced spasticity where etiological factor can be adequately recreated and models can focus on specific pathophysiological mechanisms that likely contribute to spasticity syndrome in humans (such as motoneuron hyperexcitability and spinal hyperreflexia). Methods used to study spasticity in preclinical models are expected to have a high translational value (e.g., electromyogram (EMG)-based electrophysiological tools) and can efficiently assist clinical development programs. However, validation of these models is not complete yet. First, true predictive validity of these models is not established as clinically efficacious drugs have been used to reverse validate preclinical models while newly discovered mechanisms effective in preclinical models are yet to be fully explored in humans (e.g., 5-HT2C receptor inverse agonists, fatty acid amid hydrolase inhibitors). Second, further efforts need to be invested into cross-laboratory validation of study protocols and tools, adherence to the highest quality standards (blinding, randomization, pre-specified study endpoints, etc.), and systematic efforts to replicate key sets of data. These appear to be readily achievable tasks that will enable development not only of symptomatic but also of disease-modifying therapy of spasticity, an area that seems to be currently not in focus of research efforts.

Two-step production of monoamines in monoenzymatic cells in the spinal cord: a different control strategy of neurotransmitter supply?

Monoamine neurotransmitters play an important role in the modulation of sensory, motor and autonomic functions in the spinal cord. Although traditionally it is believed that in mammalian spinal cord, monoamine neurotransmitters mainly originate from the brain, accumulating evidence indicates that especially when the spinal cord is injured, they can also be produced in the spinal cord. In this review, I will present evidence for a possible pathway for two-step synthesis of dopamine and serotonin in the spinal cord. Published data from different sources and unpublished data from my own ongoing projects indicate that monoenzymatic cells expressing aromatic L-amino acid decarboxylase (AADC), tyrosine hydroxylase (TH) or tryptophan hydroxylase (TPH) are present in the spinal cord and that these TH and THP cells often lie in close proximity to AADC cells. Prompted by the above evidence, I hypothesize that dopamine and serotonin could be synthesized sequentially in two monoenzymatic cells in the spinal cord via a TH-AADC and a TPH-AADC cascade respectively. The monoamines synthesized through this pathway may compensate for lost neurotransmitters following spinal cord injury and also may play specific roles in the recovery of sensory, motor and autonomic functions.

A combination therapy of neural and glial restricted precursor cells and chronic quipazine treatment paired with passive cycling promotes quipazine-induced stepping in adult spinalized rats

INTRODUCTION

In order to develop optimal treatments to promote recovery from complete spinal cord injury (SCI), we examined the combination of: (1) a cellular graft of neural and glial restricted precursor (NRP/GRP) cells, (2) passive exercise, and (3) chronic quipazine treatment on behavioral outcomes and compared them with the individual treatment elements. NRP/GRP cells were transplanted at the time of spinalization.

METHODS

Daily passive exercise began 1 week after injury to give sufficient time for the animals to recover. Chronic quipazine administration began 2 weeks after spinalization to allow for sufficient receptor upregulation permitting the expression of its behavioral effects. Behavioral measures consisted of the Basso, Beattie, and Bresnahan (BBB) locomotor score and percent of weight-supported steps and hops on a treadmill.

RESULTS

Rats displayed an increased response to quipazine (BBB ≥ 9) beginning at 8 weeks post-injury in all the animals that received the combination therapy. This increase in BBB score was persistent through the end of the study (12 weeks post-injury).

CONCLUSION

Unlike the individual treatment groups which never achieved weight support, the combination therapy animals were able to perform uncoordinated weight-supported stepping without a body weight support system while on a moving treadmill (6.5 m per minute) and were capable of supporting their own weight in stance during open field locomotion testing. No regeneration of descending serotonergic projections into and through the lesion cavity was observed. Furthermore, these results are a testament to the capacity of the lumbar spinal cord, when properly stimulated, to sustain functioning locomotor circuitry following complete SCI.

Spinal Cord Hemisection Facilitates Aromatic L-Amino Acid Decarboxylase Cells to Produce Serotonin in the Subchronic but Not the Chronic Phase

Neuromodulators, such as serotonin (5-hydroxytryptamine, 5-HT) and noradrenalin, play an essential role in regulating the motor and sensory functions in the spinal cord. We have previously shown that in the rat spinal cord the activity of aromatic L-amino acid decarboxylase (AADC) cells to produce 5-HT from its precursor (5-hydroxytryptophan, 5-HTP) is dramatically increased following complete spinal cord transection. In this study, we investigated whether a partial loss of 5-HT innervation could similarly increase AADC activity. Adult rats with spinal cord hemisected at thoracic level (T11/T12) were used with a postoperation interval at 5 days or 60 days. Using immunohistochemistry, first, we observed a significant reduction in the density of 5-HT-immunoreactive fibers in the spinal cord below the lesion on the injured side for both groups. Second, we found that the AADC cells were similarly expressed on both injured and uninjured sides in both groups. Third, increased production of 5-HT in AADC cells following 5-HTP was seen in 5-day but not in 60-day postinjury group. These results suggest that plastic changes of the 5-HT system might happen primarily in the subchronic phase and for longer period its function could be compensated by plastic changes of other intrinsic and/or supraspinal modulation systems.

Inducing hindlimb locomotor recovery in adult rat after complete thoracic spinal cord section using repeated treadmill training with perineal stimulation only

Although a complete thoracic spinal cord section in various mammals induces paralysis of voluntary movements, the spinal lumbosacral circuitry below the lesion retains its ability to generate hindlimb locomotion. This important capacity may contribute to the overall locomotor recovery after partial spinal cord injury (SCI). In rats, it is usually triggered by pharmacological and/or electrical stimulation of the cord while a robot sustains the animals in an upright posture. In the present study we daily trained a group of adult spinal (T7) rats to walk with the hindlimbs for 10 wk (10 min/day for 5 days/wk), using only perineal stimulation. Kinematic analysis and terminal electromyographic recordings revealed a strong effect of training on the reexpression of hindlimb locomotion. Indeed, trained animals gradually improved their locomotion while untrained animals worsened throughout the post-SCI period. Kinematic parameters such as averaged and instant swing phase velocity, step cycle variability, foot drag duration, off period duration, and relationship between the swing features returned to normal values only in trained animals. The present results clearly demonstrate that treadmill training alone, in a normal horizontal posture, elicited by noninvasive perineal stimulation is sufficient to induce a persistent hindlimb locomotor recovery without the need for more complex strategies. This provides a baseline level that should be clearly surpassed if additional locomotor-enabling procedures are added. Moreover, it has a clinical value since intrinsic spinal reorganization induced by training should contribute to improve locomotor recovery together with afferent feedback and supraspinal modifications in patients with incomplete SCI.

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.

Serotonin receptor and KCC2 gene expression in lumbar flexor and extensor motoneurons posttransection with and without passive cycling

Sacrocaudal motoneuron gene expression is altered following a spinal transection. Of interest here is the regulation of serotonin (5-HT) receptors (R), glutamate receptor, metabotropic 1 (mGluR1), and potassium-chloride cotransporter (KCC2), which mediate motoneuron excitability, locomotor recovery, and spasticity posttransection. The examination of these genes in lumbar motoneurons posttransection has not been studied, which is necessary for developing potential pharmacological interventions aimed at restoring locomotion and/or reducing spasticity. Also, if activity is to be used to promote recovery or reduce spasticity postinjury, a further examination of neuromuscular activity on gene expression posttransection is warranted. The purpose of this study was to examine motoneuronal gene expression of 5-HT receptors, KCC2, and mGluR1 at 3 mo following a complete thoracic spinal cord transection, with and without the inclusion of daily passive cycling. Physiological hindlimb extensor and flexor motoneurons were differentially identified with two retrograde fluorescent tracers, allowing for the identification and separate harvesting of extensor and flexor motoneurons with laser capture microdissection and the subsequent examination of mRNA content using quantitative RT-PCR analysis. We demonstrate that posttransection 5-HT1AR, 5-HT2CR, and mGluR1 expression was downregulated, whereas the 5-HT2AR was upregulated. These alterations in gene expression were observed in both flexor and extensor motoneurons, whereas passive cycling influenced gene expression in extensor but not flexor motoneurons. Passive cycling in extensor motoneurons further enhanced 5-HT2AR expression and increased 5-HT7R and KCC2 expression. Our results demonstrate that passive cycling influences serotonin receptor and KCC2 gene expression and that extensor motoneurons compared with flexor motoneurons may be more plastic to activity-based interventions posttransection.

The role of the serotonergic system in locomotor recovery after spinal cord injury

Serotonin (5-HT), a monoamine neurotransmitter synthesized in various populations of brainstem neurons, plays an important role in modulating the activity of spinal networks involved in vertebrate locomotion. Following spinal cord injury (SCI) there is a disruption of descending serotonergic projections to spinal motor areas, which results in a subsequent depletion in 5-HT, the dysregulation of 5-HT transporters as well as the elevated expression, super-sensitivity and/or constitutive auto-activation of specific 5-HT receptors. These changes in the serotonergic system can produce varying degrees of locomotor dysfunction through to paralysis. To date, various approaches targeting the different components of the serotonergic system have been employed to restore limb coordination and improve locomotor function in experimental models of SCI. These strategies have included pharmacological modulation of serotonergic receptors, through the administration of specific 5-HT receptor agonists, or by elevating the 5-HT precursor 5-hydroxytryptophan, which produces a global activation of all classes of 5-HT receptors. Stimulation of these receptors leads to the activation of the locomotor central pattern generator (CPG) below the site of injury to facilitate or improve the quality and frequency of movements, particularly when used in concert with the activation of other monoaminergic systems or coupled with electrical stimulation. Another approach has been to employ cell therapeutics to replace the loss of descending serotonergic input to the CPG, either through transplanted fetal brainstem 5-HT neurons at the site of injury that can supply 5-HT to below the level of the lesion or by other cell types to provide a substrate at the injury site for encouraging serotonergic axon regrowth across the lesion to the caudal spinal cord for restoring locomotion.

Spinal cord injury enables aromatic L-amino acid decarboxylase cells to synthesize monoamines

Serotonin (5-HT), an important modulator of both sensory and motor functions in the mammalian spinal cord, originates mainly in the raphe nuclei of the brainstem. However, following complete transection of the spinal cord, small amounts of 5-HT remain detectable below the lesion. It has been suggested, but not proven, that this residual 5-HT is produced by intraspinal 5-HT neurons. Here, we show by immunohistochemical techniques that cells containing the enzyme aromatic l-amino acid decarboxylase (AADC) occur not only near the central canal, as reported by others, but also in the intermediate zone and dorsal horn of the spinal gray matter. We show that, following complete transection of the rat spinal cord at S2 level, AADC cells distal to the lesion acquire the ability to produce 5-HT from its immediate precursor, 5-hydroxytryptophan. Our results indicate that this phenotypic change in spinal AADC cells is initiated by the loss of descending 5-HT projections due to spinal cord injury (SCI). By in vivo and in vitro electrophysiology, we show that 5-HT produced by AADC cells increases the excitability of spinal motoneurons. The phenotypic change in AADC cells appears to result from a loss of inhibition by descending 5-HT neurons and to be mediated by 5-HT1B receptors expressed by AADC cells. These findings indicate that AADC cells are a potential source of 5-HT at spinal levels below an SCI. The production of 5-HT by AADC cells, together with an upregulation of 5-HT2 receptors, offers a partial explanation of hyperreflexia below a chronic SCI.

Swimming against the tide: investigations of the C-bouton synapse

C-boutons are important cholinergic modulatory loci for state-dependent alterations in motoneuron firing rate. m2 receptors are concentrated postsynaptic to C-boutons, and m2 receptor activation increases motoneuron excitability by reducing the action potential afterhyperpolarization. Here, using an intensive review of the current literature as well as data from our laboratory, we illustrate that C-bouton postsynaptic sites comprise a unique structural/functional domain containing appropriate cellular machinery (a “signaling ensemble”) for cholinergic regulation of outward K⁺ currents. Moreover, synaptic reorganization at these critical sites has been observed in a variety of pathologic states. Yet despite recent advances, there are still great challenges for understanding the role of C-bouton regulation and dysregulation in human health and disease. The development of new therapeutic interventions for devastating neurological conditions will rely on a complete understanding of the molecular mechanisms that underlie these complex synapses. Therefore, to close this review, we propose a comprehensive hypothetical mechanism for the cholinergic modification of α-MN excitability at C-bouton synapses, based on findings in several well-characterized neuronal systems.

5-HT₂ and 5-HT₇ receptor agonists facilitate plantar stepping in chronic spinal rats through actions on different populations of spinal neurons

There is considerable evidence from research in neonatal and adult rat and mouse preparations to warrant the conclusion that activation of 5-HT₂ and 5-HT_1A/7 receptors leads to activation of the spinal cord circuitry for locomotion. These receptors are involved in control of locomotor movements, but it is not clear how they are implicated in the responses to 5-HT agonists observed after spinal cord injury. Here we used agonists that are efficient in promoting locomotor recovery in paraplegic rats, 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OHDPAT) (acting on 5-HT_1A/7 receptors) and quipazine (acting on 5-HT₂ receptors), to examine this issue. Analysis of intra- and interlimb coordination confirmed that the locomotor performance was significantly improved by either drug, but the data revealed marked differences in their mode of action. Interlimb coordination was significantly better after 8-OHDPAT application, and the activity of the extensor soleus muscle was significantly longer during the stance phase of locomotor movements enhanced by quipazine. Our results show that activation of both receptors facilitates locomotion, but their effects are likely exerted on different populations of spinal neurons. Activation of 5-HT₂ receptors facilitates the output stage of the locomotor system, in part by directly activating motoneurons, and also through activation of interneurons of the locomotor central pattern generator (CPG). Activation of 5-HT_7/1A receptors facilitates the activity of the locomotor CPG, without direct actions on the output components of the locomotor system, including motoneurons. Although our findings show that the combined use of these two drugs results in production of well-coordinated weight supported locomotion with a reduced need for exteroceptive stimulation, they also indicate that there might be some limitations to the utility of combined treatment. Sensory feedback and some intraspinal circuitry recruited by the drugs can conflict with the locomotor activation.

Quantitative expression profiling in mouse spinal cord reveals changing relationships among channel and receptor mRNA levels across postnatal maturation

Neural networks ultimately arrive at functional output via interaction of the excitability of individual neurons and their synaptic interactions. We investigated the relationships between voltage-gated ion channel and neurotransmitter receptor mRNA levels in mouse spinal cord at four different postnatal time points (P5, P11, P17, and adult) and three different adult cord levels (cervical, thoracic, and lumbosacral) using quantitative reverse-transcription polymerase chain reaction (qRT-PCR). Our analysis and data visualization are novel in that we chose a focal group of voltage-gated channel subunits and transmitter receptor subunits, performed absolute quantitation of mRNA copy number for each gene from a sample, and used multiple correlation analyses and correlation matrices to detect patterns in correlated mRNA levels across all genes of interest. These correlation profiles suggest that postnatal maturation of the spinal cord includes changes among channel and receptor subunits that proceed from widespread co-regulation to more refined and distinct functional relationships.

Inter- and intralimb adaptations to a sensory perturbation during activation of the serotonin system after a low spinal cord transection in neonatal rats

Activation of the serotonin system has been shown to induce locomotor activity following a spinal cord transection. This study examines how the isolated spinal cord adapts to a sensory perturbation during activation of the serotonergic system. Real-time and persistent effects of a perturbation were examined in intact and spinal transected newborn rats. Rats received a spinal surgery (sham or low thoracic transection) on postnatal day 1 and were tested 9 days later. At test, subjects were treated with the serotonergic receptor agonist quipazine (3.0 mg/kg) to induce stepping behavior. Half of the subjects experienced range of motion (ROM) restriction during stepping, while the other half did not. Differences in stepping behavior (interlimb coordination) and limb trajectories (intralimb coordination) were found to occur in both intact and spinal subjects. Adaptations were seen in the forelimbs and hindlimbs. Also, real-time and persistent effects of ROM restriction (following removal of the perturbation) were seen in ROM-restricted subjects. This study demonstrates the sensitivity of the isolated spinal cord to sensory feedback in conjunction with serotonin modulation.

Isolating specific cell and tissue compartments from 3D images for quantitative regional distribution analysis using novel computer algorithms

BACKGROUND

Isolating specific cellular and tissue compartments from 3D image stacks for quantitative distribution analysis is crucial for understanding cellular and tissue physiology under normal and pathological conditions. Current approaches are limited because they are designed to map the distributions of synapses onto the dendrites of stained neurons and/or require specific proprietary software packages for their implementation.

NEW METHOD

To overcome these obstacles, we developed algorithms to Grow and Shrink Volumes of Interest (GSVI) to isolate specific cellular and tissue compartments from 3D image stacks for quantitative analysis and incorporated these algorithms into a user-friendly computer program that is open source and downloadable at no cost.

RESULTS

The GSVI algorithm was used to isolate perivascular regions in the cortex of live animals and cell membrane regions of stained spinal motoneurons in histological sections. We tracked the real-time, intravital biodistribution of injected fluorophores with sub-cellular resolution from the vascular lumen to the perivascular and parenchymal space following a vascular microlesion, and mapped the precise distributions of membrane-associated KCC2 and gephyrin immunolabeling in dendritic and somatic regions of spinal motoneurons.

COMPARISON WITH EXISTING METHODS

Compared to existing approaches, the GSVI approach is specifically designed for isolating perivascular regions and membrane-associated regions for quantitative analysis, is user-friendly, and free.

CONCLUSIONS

The GSVI algorithm is useful to quantify regional differences of stained biomarkers (e.g., cell membrane-associated channels) in relation to cell functions, and the effects of therapeutic strategies on the redistributions of biomolecules, drugs, and cells in diseased or injured tissues.

Daily passive cycling attenuates the hyperexcitability and restores the responsiveness of the extensor monosynaptic reflex to quipazine in the chronic spinally transected rat

Activity-based interventions such as locomotor training or passive cycling have a positive influence on the spinal circuitry and recovery following a spinal cord injury (SCI). The use of quipazine in combination with exercise training has demonstrated a greater functional recovery than has exercise training alone. However, the influence of exercise or training on the responsiveness of the spinal cord to quipazine has not been examined following a chronic spinal transection. The purpose of this study was to characterize the flexor and extensor monosynaptic reflex (MSR) response pre- and post-quipazine in chronic complete spinally transected rats that either underwent daily passive cycling for 3 months or did not receive passive cycling. Following a chronic spinal transection, the extensor MSR demonstrated a hyperreflexive response (fivefold increase) to afferent stimuli, and did not respond to quipazine injection. With daily passive cycling, the extensor MSR hyperexcitability was attenuated, and the MSR amplitude increased 72% following quipazine injection (p<0.004), which was comparable to the extensor MSR response (94%) in the control group. For both chronic spinal transection groups, the flexor MSR amplitudes were not altered following quipazine injection, whereas in the control group the flexor MSR amplitude increased 86% in response to quipazine (p<0.004). These results demonstrate that passive cycling attenuates the hyperreflexive response of the extensor MSR following a chronic SCI, and restores the MSR response to quipazine.

The time course of serotonin 2C receptor expression after spinal transection of rats: an immunohistochemical study

In the spinal cord serotonin (5-HT) systems modulate the spinal network via various 5-HT receptors. Serotonin 2A receptor and serotonin 2C receptor (5-HT2A and 2C receptors) are likely the most important 5-HT receptors for enhancing the motoneuron excitability by facilitating the persistent inward current (PIC), and thus play an important role for the pathogenesis of spasticity after spinal cord injury. In conjunction with our 5-HT2A receptor study, using a same sacral spinal transection rat model we have in this study examined 5-HT2C receptor immunoreactivity (5-HT2CR-IR) changes at seven different time intervals after spinal injury. We found that 5-HT2CR-IR was widely distributed in different regions of the spinal gray matter and was predominantly located in the neuronal somata and their dendrites although it seemed also present in axonal fibers in the superficial dorsal horn. 5-HT2CR-IR in different regions of the spinal gray matter was seen to be increased at 14days after transection (with an average ∼1.3-fold higher than in sham-operated group) but did not reach a significant level until at 21days (∼1.4-fold). The increase sustained thereafter and a plateau level was reached at 45days (∼1.7-fold higher), a value similar as that at 60days. When 5-HT2CR-IR analysis was confined to the ventral horn motoneuron somata (including a proportion of proximal dendrites) a significant increase was not detected until 45days post-operation. 5-HT2CR upregulation in the spinal gray matter is confirmed with Western blot in the rats 60days post-operation. The time course of 5-HT2CR upregulation in the spinal gray matter and motoneurons was positively correlated with the development of tail spasticity (clinical scores). This indicates that 5-HT2CR is probably an important factor underlying this pathophysiological development by increasing the excitability of both motoneurons and interneurons.

Activation of 5-HT2A receptors upregulates the function of the neuronal K-Cl cotransporter KCC2

In healthy adults, activation of γ-aminobutyric acid (GABA)(A) and glycine receptors inhibits neurons as a result of low intracellular chloride concentration ([Cl(-)](i)), which is maintained by the potassium-chloride cotransporter KCC2. A reduction of KCC2 expression or function is implicated in the pathogenesis of several neurological disorders, including spasticity and chronic pain following spinal cord injury (SCI). Given the critical role of KCC2 in regulating the strength and robustness of inhibition, identifying tools that may increase KCC2 function and, hence, restore endogenous inhibition in pathological conditions is of particular importance. We show that activation of 5-hydroxytryptamine (5-HT) type 2A receptors to serotonin hyperpolarizes the reversal potential of inhibitory postsynaptic potentials (IPSPs), E(IPSP), in spinal motoneurons, increases the cell membrane expression of KCC2 and both restores endogenous inhibition and reduces spasticity after SCI in rats. Up-regulation of KCC2 function by targeting 5-HT(2A) receptors, therefore, has therapeutic potential in the treatment of neurological disorders involving altered chloride homeostasis. However, these receptors have been implicated in several psychiatric disorders, and their effects on pain processing are controversial, highlighting the need to further investigate the potential systemic effects of specific 5-HT(2A)R agonists, such as (4-bromo-3,6-dimethoxybenzocyclobuten-1-yl)methylamine hydrobromide (TCB-2).

Serotonergic pharmacotherapy promotes cortical reorganization after spinal cord injury

Cortical reorganization plays a significant role in recovery of function after injury of the central nervous system. The neural mechanisms that underlie this reorganization may be the same as those normally responsible for skilled behaviors that accompany extended sensory experience and, if better understood, could provide a basis for further promoting recovery of function after injury. The work presented here extends studies of spontaneous cortical reorganization after spinal cord injury to the role of rehabilitative strategies on cortical reorganization. We use a complete spinal transection model to focus on cortical reorganization in response to serotonergic (5-HT) pharmacotherapy without any confounding effects from spared fibers left after partial lesions. 5-HT pharmacotherapy has previously been shown to improve behavioral outcome after SCI but the effect on cortical organization is unknown. After a complete spinal transection in the adult rat, 5-HT pharmacotherapy produced more reorganization in the sensorimotor cortex than would be expected by transection alone. This reorganization was dose dependent, extended into intact (forelimb) motor cortex, and, at least in the hindlimb sensorimotor cortex, followed a somatotopic arrangement. Animals with the greatest behavioral outcome showed the greatest extent of cortical reorganization suggesting that the reorganization is likely to be in response to both direct effects of 5-HT on cortical circuits and indirect effects in response to the behavioral improvement below the level of the lesion.

Different effects of spinalization and locomotor training of spinal animals on cholinergic innervation of the soleus and tibialis anterior motoneurons

Cholinergic input modulates excitability of motoneurons and plays an important role in the control of locomotion in both intact and spinalized animals. However, spinal cord transection in adult rats affects cholinergic innervation of only some hindlimb motoneurons, suggesting that specificity of this response is related to functional differences between motoneurons. Our aim was therefore to compare cholinergic input to motoneurons innervating the soleus (Sol) and tibialis anterior (TA) motoneurons following spinal cord transection at a low-thoracic level. The second aim was to investigate whether deficits in cholinergic input to these motoneurons could be modified by locomotor training. The Sol and TA motoneurons were identified by retrograde labelling with fluorescent dyes injected intramuscularly. Cholinergic terminals were detected using anti-vesicular acetylcholine transporter (VAChT) antibody. Overall innervation of motoneurons was evaluated with anti-synaptophysin antibody. After spinalization we found a decrease in the number of VAChT-positive boutons apposing perikarya of the Sol (to 49%) but not TA motoneurons. Locomotor training, resulting in moderate functional improvement, partly reduced the deficit in cholinergic innervation of Sol motoneurons by increasing the number of VAChT-positive boutons. However, the optical density of VAChT-positive boutons terminating on various motoneurons, which decreased after spinalization, continued to decrease despite the training, suggesting an impairment of acetylcholine availability in the terminals. Different effects of spinal cord transection on cholinergic innervation of motoneurons controlling the ankle extensor and flexor muscles point to different functional states of these muscles in paraplegia as a possible source of activity-dependent signaling regulating cholinergic input to the motoneurons.

Alterations of serotonin 2C and 2A receptors in response to T10 spinal cord transection in rats

Recent studies reported that a recovery of motoneurons after spinal cord transection at the sacrocaudal level may depend on adaptive alterations of the serotonin 2C (5-HT(2C)R) and 2A (5-HT(2C)R) receptor function via changes in mRNA editing or protein expression, respectively. It has been suggested that depletion of serotonergic input may drive these adaptations. Here, mRNA editing and/or expression of 5-HT(2C)R and 5-HT(2A)R was evaluated in rats that sustained a complete transection at the thoracic (T10) level. While 5-HT(2A)R mRNA expression was upregulated below the site of spinal cord injury (SCI), no changes in 5-HT(2C)R mRNA editing or expression were detected. These findings argue against the hypothesis that 5-HT(2C)R editing is regulated by extracellular serotonin levels. Rather, it appears that the editing process is just one of the ways in which excitability of motor neurons can be restored following SCI. To this end, the influence of excitatory locomotor circuits on motor neurons in the thoracic spinal cord of rats requires further exploration.

Contribution of 5-HT to locomotion - the paradox of Pet-1(-/-) mice

Serotonin (5-HT) plays a critical role in locomotor pattern generation by modulating the rhythm and the coordinations. Pet-1, a transcription factor selectively expressed in the raphe nuclei, controls the differentiation of 5-HT neurons. Surprisingly, inactivation of Pet-1 (Pet-1(-/-) mice) that causes a 70% reduction in the number of 5-HT-positive neurons in the raphe does not impair locomotion in adult mice. The goal of the present study was to investigate the operation of the locomotor central pattern generator (CPG) in neonatal Pet-1(-/-) mice. We first confirmed, by means of immunohistochemistry, that there is a marked reduction of 5-HT innervation in the lumbar spinal cord of Pet-1(-/-) mice. Fictive locomotion was induced in the in vitro neonatal mouse spinal cord preparation by bath application of N-methyl-d,l-Aspartate (NMA) alone or together with dopamine and 5-HT. A locomotor pattern characterized by left-right and flexor-extensor alternations was observed in both conditions. Increasing the concentration of 5-HT from 0.5 to 5 μm impaired the pattern in Pet-1(-/-) mice. We tested the role of endogenous 5-HT in the NMA-induced fictive locomotion. Application of 5-HT(2) or 5-HT(7) receptor antagonists affected the NMA-induced fictive locomotion in both heterozygous and homozygous mice although the effects were weaker in the latter strain. This may be, at least partly, explained by the reduced expression of 5-HT(2A) R as observed by means of immunohistochemistry. These results suggest that compensatory mechanisms take place in Pet-1(-/-) mice that make locomotion less dependent upon 5-HT.

The time course of serotonin 2A receptor expression after spinal transection of rats: an immunohistochemical study

Hyperexcitability of motoneurons is one of the key mechanism that has been demonstrated to underlie the pathogenesis of spasticity after spinal injury. Serotonin (5-HT) denervation supersensitivity is one of the mechanisms underlying this increased motoneuron excitability. In this study, to examine whether the supersensitivity is caused by 5-HT receptor upregulation we investigated changes in levels of 5-HT2A receptor immunoreactivity (5-HT2AR-IR) following a spinal transection in the sacral spinal cord of rats at seven different time points post injury: 2, 8, 16 h, and 1, 2, 7 and 28 days, respectively. 5-HT2AR-IR density was analyzed in motoneurons (regions containing their somata and dendrites) in the spinal segments below the lesion. The results showed no significant changes in 5-HT2AR-IR in the motoneurons up to 16 h following the transection. After 1-day, however the levels of 5-HT2AR-IR increased in the motoneurons and their dendrites, with the density level being 3.4-fold higher in spinalized rats than in sham-operated rats. The upregulation increased progressively until a maximal level was reached at 28 days post-injury. We also investigated 5-HT and 5-HT transporter expressions at five different post injury time points: 1, 2, 7, 21 and 60 days and they showed concurrent down-regulation changes after 2 days. These results suggest that the upregulation of 5-HT2ARs may at least partly underlie the development of 5-HT denervation supersensitivity in spinal motoneurons following spinal injury and thereby implicates their involvement in the pathogenesis of the subsequent development of spasticity.

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.