Novel repair strategies to restore bladder function following cauda equina/conus medullaris injuries

Axonal Injury Induces ATF3 in Specific Populations of Sacral Preganglionic Neurons in Male Rats

Compared to other neurons of the central nervous system, autonomic preganglionic neurons are unusual because most of their axon lies in the periphery. These axons are vulnerable to injury during surgical procedures, yet in comparison to peripheral neurons and somatic motor neurons, the impact of injury on preganglionic neurons is poorly understood. Here, we have investigated the impact of axotomy on sacral preganglionic neurons, a functionally diverse group of neurons required for micturition, defecation, and sexual function. We have previously observed that after axotomy, the injury-related transcription factor activating transcription factor-3 (ATF3) is upregulated in only half of these neurons (Peddie and Keast, 2011: PMID: 21283532). In the current study, we have investigated if this response is constrained to particular subclasses of preganglionic neurons that have specific functions or signaling properties. Seven days after unilateral pelvic nerve transection, we quantified sacral preganglionic neurons expressing ATF3, many but not all of which co-expressed c-Jun. This response was independent of soma size. Subclasses of sacral preganglionic neurons expressed combinations of somatostatin, calbindin, and neurokinin-1 receptor, each of which showed a similar response to injury. We also found that in contrast to thoracolumbar preganglionic neurons, the heat shock protein-25 (Hsp25) was not detected in naive sacral preganglionic neurons but was upregulated in many of these neurons after axotomy; the majority of these Hsp25 neurons expressed ATF3. Together, these studies reveal the molecular complexity of sacral preganglionic neurons and their responses to injury. The simultaneous upregulation of Hsp25 and ATF3 may indicate a distinct mechanism of regenerative capacity after injury.

A ventral root avulsion injury model for neurogenic underactive bladder studies

Detrusor underactivity (DU) is defined as a contraction of reduced strength and/or duration during bladder emptying and results in incomplete and prolonged bladder emptying. The clinical diagnosis of DU is challenging when present alone or in association with other bladder conditions such as detrusor overactivity, urinary retention, detrusor hyperactivity with impaired contractility, aging, and neurological injuries. Several etiologies may be responsible for DU or the development of an underactive bladder (UAB), but the pathobiology of DU or UAB is not well understood. Therefore, new clinically relevant and interpretable models for studies of UAB are much needed in order to make progress towards new treatments and preventative strategies. Here, we review a neuropathic cause of DU in the form of traumatic injuries to the cauda equina (CE) and conus medullaris (CM) portions of the spinal cord. Lumbosacral ventral root avulsion (VRA) injury models in rats mimic the clinical phenotype of CM/CE injuries. Bilateral VRA injuries result in bladder areflexia, whereas a unilateral lesion results in partial impairment of lower urinary tract and visceromotor reflexes. Surgical re-implantation of avulsed ventral roots into the spinal cord and pharmacological strategies can augment micturition reflexes. The translational research need for the development of a large animal model for UAB studies is also presented, and early studies of lumbosacral VRA injuries in rhesus macaques are discussed.

The effects of minocycline or riluzole treatment on spinal root avulsion-induced pain in adult rats

Spinal root avulsion produces tactile and thermal hypersensitivity, neurodegeneration, and microglial and astrocyte activation in both the deafferented and the adjacent intact spinal cord segments. Following avulsion of the fifth lumbar spinal root, immediate and prolonged treatment with riluzole or minocycline for 2 weeks altered the development of behavioral hypersensitivity. Riluzole delayed the onset of thermal and tactile hypersensitivity and partially reversed established pain behavior. Minocycline effectively prevented and reversed both types of behavioral change. Histologic analysis revealed that both drugs reduced microglial staining in the spinal cord, with minocycline being more effective than riluzole. Astrocyte activation was ameliorated to a lesser extent. Surprisingly, neither drug provided a neuroprotective effect on avulsed motoneurons.

PERSPECTIVE

Immediate treatment of spinal root avulsion injuries with minocycline or riluzole prevents the onset of evoked pain hypersensitivity by reducing microglial cell activation. When treatment is delayed, minocycline, but not riluzole, reverses pre-established hypersensitivity. Thus, these drugs may provide a new translational treatment option for chronic avulsion injury pain.

Long-term effects of a lumbosacral ventral root avulsion injury on axotomized motor neurons and avulsed ventral roots in a non-human primate model of cauda equina injury

Here, we have translated from the rat to the non-human primate a unilateral lumbosacral injury as a model for cauda equina injury. In this morphological study, we have investigated retrograde effects of a unilateral L6-S2 ventral root avulsion (VRA) injury as well as the long-term effects of Wallerian degeneration on avulsed ventral roots at 6-10 months post-operatively in four adult male rhesus monkeys. Immunohistochemistry for choline acetyl transferase and glial fibrillary acidic protein demonstrated a significant loss of the majority of the axotomized motoneurons in the affected L6-S2 segments and signs of an associated astrocytic glial response within the ventral horn of the L6 and S1 spinal cord segments. Quantitative analysis of the avulsed ventral roots showed that they exhibited normal size and were populated by a normal number of myelinated axons. However, the myelinated axons in the avulsed ventral roots were markedly smaller in caliber compared to the fibers of the intact contralateral ventral roots, which served as controls. Ultrastructural studies confirmed the presence of small myelinated axons and a population of unmyelinated axons within the avulsed roots. In addition, collagen fibers were readily identified within the endoneurium of the avulsed roots. In summary, a lumbosacral VRA injury resulted in retrograde motoneuron loss and astrocytic glial activation in the ventral horn. Surprisingly, the Wallerian degeneration of motor axons in the avulsed ventral roots was followed by a repopulation of the avulsed roots by small myelinated and unmyelinated fibers. We speculate that the small axons may represent sprouting or axonal regeneration by primary afferents or autonomic fibers.

Segmental spinal root avulsion in the adult rat: a model to study avulsion injury pain

Road traffic accidents are the most common cause of avulsion injury, in which spinal roots are torn from the spinal cord. Patients suffer from a loss of sensorimotor function, intractable spontaneous pain, and border-zone hypersensitivity. The neuropathic pains are particularly difficult to treat because the lack of a well-established animal model of avulsion injury prevents identifying the underlying mechanisms and hinders the development of efficacious drugs. This article describes a hindlimb model of avulsion injury in adult rats where the L5 dorsal and ventral spinal root are unilaterally avulsed (spinal root avulsion [SRA]), leaving the adjacent L4 spinal root intact. SRA produced a significant ipsilateral hypersensitivity to mechanical and thermal stimulation by 5 days compared with sham-operated or naïve rats. This hypersensitivity is maintained for up to 60 days. No autotomy was observed and locomotor deficits were minimal. The hypersensitivity to peripheral stimuli could be temporarily ameliorated by administration of amitriptyline and carbamazepine, drugs that are currently prescribed to avulsion patients. Histological assessment of the L4 ganglion cells revealed no significant alterations in calcitonin gene-related peptide (CGRP), IB4, transient receptor potential cation channel subfamily V member 1 (TrpV1), or N52 staining across groups. Immunohistochemistry of the spinal cord revealed a localized glial response, phagocyte infiltration, and neuronal loss within the ipsilateral avulsed segment. A comparable response from glia and phagocytes was also found in the intact L4 spinal cord, supporting the role for central mechanisms within the L4-5 spinal cord in contributing to the generation of the pain-related behavior. The SRA model provides a platform to investigate possible new pharmacological treatments for avulsion injuries.

Selective plasticity of primary afferent innervation to the dorsal horn and autonomic nuclei following lumbosacral ventral root avulsion and reimplantation in long term studies

Previous studies involving injuries to the nerves of the cauda equina and the conus medullaris have shown that lumbosacral ventral root avulsion in rat models results in denervation and dysfunction of the lower urinary tract, retrograde and progressive cell death of the axotomized motor and parasympathetic neurons, as well as the emergence of neuropathic pain. Root reimplantation has also been shown to ameliorate several of these responses, but experiments thus far have been limited to studying the effects of lesion and reimplantation local to the lumbosacral region. Here, we have expanded the region of investigation after lumbosacral ventral root avulsion and reimplantation to include the thoracolumbar sympathetic region of the spinal cord. Using a retrograde tracer injected into the major pelvic ganglion, we were able to define the levels of the spinal cord that contain sympathetic preganglionic neurons innervating the lower urinary tract. We have conducted studies on the effects of the lumbosacral ventral root avulsion and reimplantation models on the afferent innervation of the dorsal horn and autonomic nuclei at both thoracolumbar and lumbosacral levels through immunohistochemistry for the markers calcitonin gene-related peptide (CGRP) and vesicular glutamate transporter 1 (VGLUT1). Surprisingly, our experiments reveal a selective and significant decrease of CGRP-positive innervation in the dorsal horn at thoracolumbar levels that is partially restored with root reimplantation. However, no similar changes were detected at the lumbosacral levels despite the injury and repair targeting efferent neurons, and being performed at the lumbosacral levels. Despite the changes evident in the thoracolumbar dorsal horn, we find no changes in afferent innervation of the autonomic nuclei at either sympathetic or parasympathetic segmental levels by CGRP or VGLUT1. We conclude that even remote, efferent root injuries and repair procedures can have an effect on remote and non-lesioned sensory systems sharing common peripheral ganglia.

Sophisticated models and methods for studying neurogenic bladder dysfunction

AIM

To describe how the use of new and established animal models and methods can generate vital and far reaching experimental data in the study of mechanism underlying neurogenic bladder overactivity.

METHODS

Bladder and colonic irradiated mice and those with upper and lower motor neuron lesions were used to study neurogenic bladder overactivity. Methods included cystometry, tension measurements, afferent nerve recordings and optical mapping of action potentials and intracellular Ca(2+) transients. Recordings were made in a number of innovative preparations including in-line cultured cells, bladder-urethra sheets and cross-sections, spinal cord slices and the cerebral cortex.

RESULTS

The animal models and methods used allow for the study of peripheral and central mechanisms of neurogenic overactivity. While colonic irradiation results in solely neurogenic dysfunction, spinal cord lesions also induce non-neural changes resulting in increased spontaneous detrusor contractions that can directly stimulate afferent nerves. Imaging of cultured bladder interstitial cells reveals spontaneous firing that could contribute to detrusor overactivity, while optical imaging of the spinal cord and brain could identify changes in central pathways that underlie lower urinary tract dysfunction.

CONCLUSIONS

The animal models and methods described allow for the study of neurogenic overactivity at the peripheral, spinal and cortical levels. This may lead to greater understanding of sensory and motor mechanisms involved in incontinence, the contributions of interstitial cells and spontaneous detrusor contractions, and the involvement of the cortex.

Transdifferentiation of bone marrow stromal cells into cholinergic neuronal phenotype: a potential source for cell therapy in spinal cord injury

BACKGROUND AIMS

Cholinergic neurons are very important cells in spinal cord injuries because of the deficits in motor, autonomic and sensory neurons. In this study, bone marrow stromal cells (BMSC) were evaluated as a source of cholinergic neurons in a rat model of contusive spinal cord injury.

METHODS

BMSC were isolated from adult rats and transdifferentiated into cholinergic neuronal cells. The BMSC were pre-induced with beta-mercaptoethanol (BME), while the induction was done with nerve growth factor (NGF). Neurofilament (NF)-68, -160 and -200 immunostaining was used for evaluating the transdifferentiation of BMSC into a neuronal phenotype. NeuroD expression, a marker for neuroblast differentiation, and Oct-4 expression, a marker for stemness, were evaluated by reverse transcriptase (RT)-polymerase chain reaction (PCR). Choline acetyl transferase (ChAT) immunoreactivity was used for assessing the cholinergic neuronal phenotype. Anti-microtubule-associated protein-2 (MAP-2) and anti-synapsin I antibodies were used as markers for the tendency for synptogenesis. Finally, the induced cells were transplanted into the contused spinal cord and locomotion was evaluated with the Basso-Beattie-Bresnahan (BBB) test.

RESULTS

At the induction stage, there was a decline in the expression of NF-68 associated with a sustained increase in the expression of NF-200, NF-160, ChAT and synapsin I, whereas MAP-2 expression was variable. Transplanted cells were detected 6 weeks after their injection intraspinally and were associated with functional recovery.

CONCLUSIONS

The transdifferentiation of BMSC into a cholinergic phenotype is feasible for replacement therapy in spinal cord injury.

Surgical implantation of avulsed lumbosacral ventral roots promotes restoration of bladder morphology in rats

Injuries to the cauda equina and conus medullaris of the spinal cord commonly result in paraplegia, sensory deficits, neuropathic pain, as well as bladder, bowel, and reproductive dysfunctions. In a recently developed lower motoneuron model for cauda equina injury and repair, we have demonstrated that an acute surgical implantation of avulsed lumbosacral ventral roots into the conus medullaris is neuroprotective, promotes regeneration of efferent spinal cord axons into the implanted roots, and may result in functional reinnervation of the lower urinary tract. Here, we investigated the effects of a bilateral lumbosacral ventral root avulsion (VRA) injury and re-implantation on the morphology of the rat bladder at twelve weeks post-operatively. We demonstrated a VRA-induced overall thinning of the bladder wall, which exhibited reduced thickness of both the lamina propria and smooth muscle. In contrast, the bladder epithelium markedly increased its thickness in the injured series. Quantitative immunohistochemical studies showed a selective increase in CGRP immunoreactivity in the lamina propria after the VRA injury. Interestingly, the injury-induced changes in bladder wall morphology were ameliorated by an acute implantation of the lesioned roots into the conus medullaris. Specifically, bladders of the implanted group showed a partial restoration of the thickness of the lamina propria and epithelium as well as a return of CGRP immunoreactivity to baseline levels in the lamina propria. Our results support the notion that surgical implantation of severed ventral roots into the spinal cord may promote the recovery of a normal morphological phenotype in peripheral end organs.

Minocycline protects motor but not autonomic neurons after cauda equina injury

Conus medullaris/cauda equina injuries typically result in loss of bladder, bowel, and sexual functions, partly as a consequence of autonomic and motor neuron death. To mimic these injuries, we previously developed a rodent lumbosacral ventral root avulsion (VRA) injury model, where both autonomic and motor neurons progressively die over several weeks. Here, we investigate whether minocycline, an antibiotic with putative neuroprotective effects, may rescue degenerating autonomic and motor neurons after VRA injury. Adult female rats underwent lumbosacral VRA injuries followed by a 2-week treatment with either minocycline or vehicle injected intraperitoneally. The sacral segment of the spinal cord was studied immunohistochemically using choline acetyltransferase (ChAT) and activated caspase-3 at 4 weeks post-operatively. Minocycline increased the survival of motoneurons but not preganglionic parasympathetic neurons (PPNs). Further investigations demonstrated that a larger proportion of motoneurons expressed activated caspase-3 compared to PPNs after VRA injury and indicated an association with minocycline's differential neuroprotective effect. Our findings suggest that minocycline may protect degenerating motoneurons and expand the therapeutic window of opportunity for surgical repair of proximal root lesions affecting spinal motoneurons.

Re-established micturition reflexes show differential activation patterns after lumbosacral ventral root avulsion injury and repair in rats

Previous studies have demonstrated that an acute implantation of lesioned lumbosacral ventral roots into the rat conus medullaris (CM) results in functional reinnervation of the lower urinary tract (LUT). Although the root implantation procedure results in a return of reflexive micturition, voiding efficiency (VE) remains incompletely recovered. Here, we performed a detailed urodynamic analysis of cystometry and external urethral sphincter (EUS) electromyography (EMG) recordings to determine underlying mechanisms for the incompletely recovered VE. For this purpose, adult female rats were studied at 12 weeks after a bilateral L5-S2 ventral root avulsion injury followed by an acute surgical implantation of the avulsed L6 and S1 ventral roots into the CM (n=6). Age-matched sham-operated rats (n=6) were included for control purposes. Compared to sham-operated controls, rats of the implanted series showed 1) reflex bladder contractions with a significantly shortened urine expulsion phase, 2) markedly decreased phasic EUS EMG activity during micturition, and 3) a pronounced bladder-sphincter dys-coordination, as demonstrated by a significantly delayed onset of the switch from low-amplitude tonic EUS EMG activity to either phasic EUS EMG activity or a large-amplitude tonic EUS EMG activity during the urine expulsion phase. Our findings provide a mechanistic explanation for the incomplete recovery of the VE following implantation of avulsed ventral roots into the spinal cord. Our future studies will aim to increase successful axonal regeneration in attempts to augment the recovery of the LUT after cauda equina injury and repair.

Functional reinnervation of the rat lower urinary tract after cauda equina injury and repair

Conus medullaris and/or cauda equina forms of spinal cord injury commonly result in a permanent loss of bladder function. Here, we developed a cauda equina injury and repair rodent model to investigate whether surgical implantation of avulsed lumbosacral ventral roots into the spinal cord can promote functional recovery of the lower urinary tract. Adult female rats underwent sham surgery (n = 6), bilateral L5-S2 ventral root avulsion (VRA) injury (n = 5), or bilateral L5-S2 VRA followed by an acute implantation of the avulsed L6 and S1 ventral roots into the conus medullaris (n = 6). At 12 weeks after operation, the avulsed group demonstrated urinary retention, absence of bladder contractions and external urethral sphincter (EUS) electromyographic (EMG) activation during urodynamic recordings, increased bladder size, and retrograde death of autonomic and motoneurons in the spinal cord. In contrast, the implanted group showed reduced urinary retention, return of reflexive bladder voiding contractions coincident with EUS EMG activation, anatomical reinnervation of the EUS demonstrated by retrograde neuronal labeling, normalization of bladder size, and a significant neuroprotection of both autonomic and motoneurons. In addition, a positive correlation between motoneuronal survival and voiding efficiency was observed in the implanted group. Our results show that implantation of avulsed lumbosacral ventral roots into the spinal cord promotes reinnervation of the urinary tract and return of functional micturition reflexes, suggesting that this surgical repair strategy may also be of clinical interest after conus medullaris and cauda equina injuries.

Acute implantation of an avulsed lumbosacral ventral root into the rat conus medullaris promotes neuroprotection and graft reinnervation by autonomic and motor neurons

Trauma to the conus medullaris and cauda equina may result in autonomic, sensory, and motor dysfunctions. We have previously developed a rat model of cauda equina injury, where a lumbosacral ventral root avulsion resulted in a progressive and parallel death of motoneurons and preganglionic parasympathetic neurons, which are important for i.e. bladder control. Here, we report that an acute implantation of an avulsed ventral root into the rat conus medullaris protects preganglionic parasympathetic neurons and motoneurons from cell death as well as promotes axonal regeneration into the implanted root at 6 weeks post-implantation. Implantation resulted in survival of 44+/-4% of preganglionic parasympathetic neurons and 44+/-4% of motoneurons compared with 22% of preganglionic parasympathetic neurons and 16% of motoneurons after avulsion alone. Retrograde labeling from the implanted root at 6 weeks showed that 53+/-13% of surviving preganglionic parasympathetic neurons and 64+/-14% of surviving motoneurons reinnervated the graft. Implantation prevented injury-induced atrophy of preganglionic parasympathetic neurons and reduced atrophy of motoneurons. Light and electron microscopic studies of the implanted ventral roots demonstrated a large number of both myelinated axons (79+/-13% of the number of myelinated axons in corresponding control ventral roots) and unmyelinated axons. Although the diameter of myelinated axons in the implanted roots was significantly smaller than that of control roots, the degree of myelination was appropriate for the axonal size, suggesting normal conduction properties. Our results show that preganglionic parasympathetic neurons have the same ability as motoneurons to survive and reinnervate implanted roots, a prerequisite for successful therapeutic strategies for autonomic control in selected patients with acute conus medullaris and cauda equina injuries.

A single re-implanted ventral root exerts neurotropic effects over multiple spinal cord segments in the adult rat

Spinal cord injuries, particularly traumatic injuries to the conus medullaris and cauda equina, are typically complex and involve multiple segmental levels. Implantation of avulsed ventral roots into the spinal cord as a repair strategy has been shown to be neuroprotective and promote axonal regeneration by spinal cord neurons into an implanted root. However, it is not well known over what distance in the spinal cord an implanted ventral root can exert its neurotropic effect. Here, we investigated whether an avulsed L6 ventral root acutely implanted into the rat spinal cord after a four level (L5-S2) unilateral ventral root avulsion injury may exert neurotropic effects on autonomic and motor neurons over multiple spinal cord segments at 6 weeks postoperatively. Using retrograde labeling techniques and stereological quantification methods, we demonstrate that autonomic and motor neurons from all four lesioned spinal cord segments, spanning more than an 8 mm rostro-caudal distance, reinnervated the one implanted root. The rostro-caudal distribution suggested a gradient of neurotropism, where the axotomized neurons closest to the implanted site had the highest probability of root reinnervation. These results suggest that implantation of a single ventral root may provide neurotropic effects to injured neurons at the site of lesion as well as in the adjacent spinal cord segments. Our findings may be of translational research interest for the development of surgical repair strategies after multi-level conus medullaris and cauda equina injuries, in which fewer ventral roots than spinal cord segments may be available for implantation.