Effects of acute and chronic midthoracic spinal cord injury on neural circuits for male sexual function. II. Descending pathways

Kinematic analysis of penile reflexes in a rat model of spinal cord injury

The ex-copula penile dorsiflexion reflex (PDFR) is an established measure of sexual dysfunction in male rat models of spinal cord injury. Although the PDFR after complete spinal transection is well described, information regarding the more clinically relevant incomplete spinal contusion injury model is limited. This study examined, using two-dimensional (2D) kinematic analysis, the relationship between the PDFR and degree of white matter sparing (WMS). Male Wistar rats received a T9 contusion with varying degrees of impactor forces. Weekly kinematic recordings of the PDFR were made 3–8 weeks postinjury. Sexual reflex components examined included maximum angle of penile dorsiflexion, total penile event duration, and penile ascent speed. Post hoc comparison between animals grouped based upon injury severity (moderate–severe: 13.33%–17.15% WMS vs moderate: 20.85%–33.50% WMS) indicated PDFR effects. Specifically, the numbers of animals with more moderate contusions having data points above the median in both maximum angle of penile dorsiflexion and penile ascent speed were significantly lower than animals with more severe injuries. Total penile event duration was also affected but only at more chronic time points (6–8 weeks). Thus, 2D kinematic analysis of the PDFR allows for more consistent and quantifiable analysis of the subtle differences that can occur between injury severity groups in the rat contusion model.

Severing the ventral funiculus in chronic spinal cord injury has the most deteriorating effect on spermatogenesis in rats

Sexual dysfunction, following spinal cord injury (SCI), is highly dependent on the extent of injury. SCI disrupts the supraspinal innervation of the reproductive organs; resulting in structural and functional deficits. Relating the extent of SCI to these changes could eventually improve diagnoses and treatment planning of sexual dysfunction following SCI. In the present study, following chronic SCI of different severities (1/3 dorsal SCI (1/3 SCI), 2/3 dorsal SCI (2/3 SCI), and complete transection (Tx)) at T8 spinal level, histological changes of seminiferous tubules parameters in testis were examined. The diameter of seminiferous tubules (DST) and epithelial height of seminiferous tubules (HST) were significantly decreased in all SCI groups compared to control and sham. In addition, DST in 2/3 SCI and Tx groups and HST in Tx group were significantly decreased in comparison with 1/3 SCI animals. Nonetheless, the diameter of seminiferous tubules' lumen decreased significantly in 2/3 SCI and Tx compared to control, sham, and 1/3 SCI groups. Concerning cellular component, the number of spermatocytes and spermatids layers significantly decreased in both 1/3 and 2/3 SCI in comparison to normal. However, Tx had the most prominent deteriorating effect on these layers; indicating impairment in the process of spermatogenesis. These results show that the spinal tracts are part of the neural circuitries innervating the testis and responsible for their structural support. These tracts are mainly distributed between the lateral and ventral funiculi at T8 spinal level. Consequently, sparing ventral funiculi in the SCI prevents the severe decline in spermatogenesis.

Recommendations for evaluation of bladder and bowel function in pre-clinical spinal cord injury research

Objective: In order to encourage the inclusion of bladder and bowel outcome measures in preclinical spinal cord injury (SCI) research, this paper identifies and categorizes 1) fundamental, 2) recommended, 3) supplemental and 4) exploratory sets of outcome measures for pre-clinical assessment of bladder and bowel function with broad applicability to animal models of SCI.Methods: Drawing upon the collective research experience of autonomic physiologists and informed in consultation with clinical experts, a critical assessment of currently available bladder and bowel outcome measures (histological, biochemical, in vivo functional, ex vivo physiological and electrophysiological tests) was made to identify the strengths, deficiencies and ease of inclusion for future studies of experimental SCI.Results: Based upon pre-established criteria generated by the Neurogenic Bladder and Bowel Working Group that included history of use in experimental settings, citations in the literature by multiple independent groups, ease of general use, reproducibility and sensitivity to change, three fundamental measures each for bladder and bowel assessments were identified. Briefly defined, these assessments centered upon tissue morphology, voiding efficiency/volume and smooth muscle-mediated pressure studies. Additional assessment measures were categorized as recommended, supplemental or exploratory based upon the balance between technical requirements and potential mechanistic insights to be gained by the study.Conclusion: Several fundamental assessments share reasonable levels of technical and material investment, including some that could assess bladder and bowel function non-invasively and simultaneously. Such measures used more inclusively across SCI studies would advance progress in this high priority area. When complemented with a few additional investigator-selected study-relevant supplemental measures, they are highly recommended for research programs investigating the efficacy of therapeutic interventions in preclinical animal models of SCI that have a bladder and/or bowel focus.

Telemetric monitoring of penile pressure during mating in rats after chronic spinal cord injury

In men with a spinal cord injury (SCI), erectile function, ejaculation, and fertility are severely impaired. The present study utilized a telemetric pressure transducer implanted into the corpus cavernosum of the penis to examine sexual function during awake mating behavior in a rat contusion model with a range of Infinite Horizon Impactor forces distributed between 150 and 215 kdyn. The mating behavior paradigm included examination of the counts, average pressure, and average duration for mounts, intromissions, and ejaculations. Male Wistar rats were mated with receptive females in 30-min sessions preinjury (sexual acclimation) and once per week for 6 wk beginning after a 2-wk recovery period post-SCI. All SCI animals had significant deficits in sexual function in the parameters measured. These deficiencies were more prevalent in a subset having less than 20% white matter sparing, likely a reflection of the extent of bilateral spino-bulbo-spinal sexual circuitry disruption at the lesion epicenter. The resulting discoordination of the autonomic and somatic reflex control of erection and ejaculation recorded using telemetry devices in an awake, behaving animal model provides an effective means of gauging sexual function deficits after SCI and could have utility for quantifying recovery after a therapeutic intervention.

Activity-Based Training Alters Penile Reflex Responses in a Rat Model of Spinal Cord Injury

INTRODUCTION

Multisystem functional gains have been reported in males with spinal cord injury (SCI) after undergoing activity-based training (ABT), including increases in scoring of sexual function and reports of improved erectile function.

AIM

This study aims to examine the effect of daily 60-minute locomotor training and exercise in general on sexual function in a rat SCI contusion model.

METHODS

Male Wistar rats received a T9 contusion SCI. Animals were randomized into 4 groups: a quadrupedal stepping group (SCI + QT), a forelimb-only exercise group (SCI + FT), a non-trained harnessed group (SCI + NT), and a home cage non-trained group (SCI + HC). The 2 non-trained groups were combined (SCI) post hoc. Daily training sessions were 60 minutes in duration for 8 weeks. Urine samples were collected during bi-weekly 24-hour metabolic cage behavioral testing. Latency, numbers of penile dorsiflexion, and glans cupping were recorded during bi-weekly penile dorsiflexion reflex (PDFR) testing. Terminal electromyography (EMG) recordings of the bulbospongiosus muscle (BSM) were recorded in response to stimulation of the dorsal nerve of the penis (DNP).

OUTCOMES

ABT after SCI had a significant effect on PDFR, as well as BSM EMG latency and burst duration.

RESULTS

SCI causes a significant decrease in the latency to onset of PDFR. After 8 weeks of ABT, SCI + QT animals had a significantly increased latency relative to the post-SCI baseline. BSM EMG response to DNP stimulation had a significantly decreased latency and increase in average and maximum amplitude in SCI + QT animals. SCI animals had a significantly longer burst duration than trained animals. Time between PDFR events, penile dorsiflexion, glans cupping, and urine testosterone were not affected by ABT.

CLINICAL IMPLICATIONS

ABT has a positive influence on sexual function and provides a potential therapy to enhance the efficacy of current sexual dysfunction therapies in the male SCI population.

STRENGTHS AND LIMITATIONS

Several significant small improvements in sexual function were found in a clinically relevant rat model of SCI using a readily available rehabilitative therapy. The limited findings could reflect insensitivity of the PDFR as a measure of erectile function.

CONCLUSIONS

These results indicate that task-specific stepping and/or loading provide sensory input to the spinal cord impacting the neural circuitry responsible for sexual function. Steadman CJ, Hoey RF, Montgomery LR, et al. Activity-Based Training Alters Penile Reflex Responses in a Rat Model of Spinal Cord Injury. J Sex Med 2019; 16:1143-1154.

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

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

The effect of spinal cord injury on the neurochemical properties of vagal sensory neurons

The vagus nerve is composed primarily of nonmyelinated sensory neurons whose cell bodies are located in the nodose ganglion (NG). The vagus has widespread projections that supply most visceral organs, including the bladder. Because of its nonspinal route, the vagus nerve itself is not directly damaged from spinal cord injury (SCI). Because most viscera, including bladder, are dually innervated by spinal and vagal sensory neurons, an impact of SCI on the sensory component of vagal circuitry may contribute to post-SCI visceral pathologies. To determine whether SCI, in male Wistar rats, might impact neurochemical characteristics of NG neurons, immunohistochemical assessments were performed for P2X3 receptor expression, isolectin B4 (IB4) binding, and substance P expression, three known injury-responsive markers in sensory neuronal subpopulations. In addition to examining the overall population of NG neurons, those innervating the urinary bladder also were assessed separately. All three of the molecular markers were represented in the NG from noninjured animals, with the majority of the neurons binding IB4. In the chronically injured rats, there was a significant increase in the number of NG neurons expressing P2X3 and a significant decrease in the number binding IB4 compared with noninjured animals, a finding that held true also for the bladder-innervating population. Overall, these results indicate that vagal afferents, including those innervating the bladder, display neurochemical plasticity post-SCI that may have implications for visceral homeostatic mechanisms and nociceptive signaling.

Spinal cord regeneration

Three theories of regeneration dominate neuroscience today, all purporting to explain why the adult central nervous system (CNS) cannot regenerate. One theory proposes that Nogo, a molecule expressed by myelin, prevents axonal growth. The second theory emphasizes the role of glial scars. The third theory proposes that chondroitin sulfate proteoglycans (CSPGs) prevent axon growth. Blockade of Nogo, CSPG, and their receptors indeed can stop axon growth in vitro and improve functional recovery in animal spinal cord injury (SCI) models. These therapies also increase sprouting of surviving axons and plasticity. However, many investigators have reported regenerating spinal tracts without eliminating Nogo, glial scar, or CSPG. For example, many motor and sensory axons grow spontaneously in contused spinal cords, crossing gliotic tissue and white matter surrounding the injury site. Sensory axons grow long distances in injured dorsal columns after peripheral nerve lesions. Cell transplants and treatments that increase cAMP and neurotrophins stimulate motor and sensory axons to cross glial scars and to grow long distances in white matter. Genetic studies deleting all members of the Nogo family and even the Nogo receptor do not always improve regeneration in mice. A recent study reported that suppressing the phosphatase and tensin homolog (PTEN) gene promotes prolific corticospinal tract regeneration. These findings cannot be explained by the current theories proposing that Nogo and glial scars prevent regeneration. Spinal axons clearly can and will grow through glial scars and Nogo-expressing tissue under some circumstances. The observation that deleting PTEN allows corticospinal tract regeneration indicates that the PTEN/AKT/mTOR pathway regulates axonal growth. Finally, many other factors stimulate spinal axonal growth, including conditioning lesions, cAMP, glycogen synthetase kinase inhibition, and neurotrophins. To explain these disparate regenerative phenomena, I propose that the spinal cord has evolved regenerative mechanisms that are normally suppressed by multiple extrinsic and intrinsic factors but can be activated by injury, mediated by the PTEN/AKT/mTOR, cAMP, and GSK3b pathways, to stimulate neural growth and proliferation.

Effects of lateral funiculus sparing, spinal lesion level, and gender on recovery of bladder voiding reflexes and hematuria in rats

Deficits in bladder function are complications following spinal cord injury (SCI), severely affecting quality of life. Normal voiding function requires coordinated contraction of bladder and urethral sphincter muscles dependent upon intact lumbosacral reflex arcs and integration of descending and ascending spinal pathways. We previously reported, in electrophysiological recordings, that segmental reflex circuit neurons in anesthetized male rats were modulated by a bilateral spino-bulbo-spinal pathway in the mid-thoracic lateral funiculus. In the present study, behavioral measures of bladder voiding reflexes and hematuria (hemorrhagic cystitis) were obtained to assess the correlation of plasticity-dependent recovery to the degree of lateral funiculus sparing and mid-thoracic lesion level. Adult rats received mid-thoracic-level lesions at one of the following severities: complete spinal transection; bilateral dorsal column lesion; unilateral hemisection; bilateral dorsal hemisection; a bilateral lesion of the lateral funiculi and dorsal columns; or a severe contusion. Voiding function and hematuria were evaluated by determining whether the bladder was areflexic (requiring manual expression, i.e., "crede maneuver"), reflexive (voiding initiated by perineal stroking), or "automatic" (spontaneous voiding without caretaker assistance). Rats with one or both lateral funiculi spared (i.e., bilateral dorsal column lesion or unilateral hemisection) recovered significantly faster than animals with bilateral lateral funiculus lesions, severe contusion, or complete transection. Bladder reflex recovery time was significantly slower the closer a transection lesion was to T10, suggesting that proximity to the segmental sensory and sympathetic innervation of the upper urinary tract (kidney, ureter) should be avoided in the choice of lesion level for SCI studies of micturition pathways. In addition, hematuria duration was significantly longer in males, compared to females, despite similar bladder reflex onset times. We conclude that the sparing of the mid-thoracic lateral funiculus on one side is required for early recovery of bladder reflex voiding function and resolution of hematuria.

Maladaptive spinal plasticity opposes spinal learning and recovery in spinal cord injury

Synaptic plasticity within the spinal cord has great potential to facilitate recovery of function after spinal cord injury (SCI). Spinal plasticity can be induced in an activity-dependent manner even without input from the brain after complete SCI. A mechanistic basis for these effects is provided by research demonstrating that spinal synapses have many of the same plasticity mechanisms that are known to underlie learning and memory in the brain. In addition, the lumbar spinal cord can sustain several forms of learning and memory, including limb-position training. However, not all spinal plasticity promotes recovery of function. Central sensitization of nociceptive (pain) pathways in the spinal cord may emerge in response to various noxious inputs, demonstrating that plasticity within the spinal cord may contribute to maladaptive pain states. In this review we discuss interactions between adaptive and maladaptive forms of activity-dependent plasticity in the spinal cord below the level of SCI. The literature demonstrates that activity-dependent plasticity within the spinal cord must be carefully tuned to promote adaptive spinal training. Prior work from our group has shown that stimulation that is delivered in a limb position-dependent manner or on a fixed interval can induce adaptive plasticity that promotes future spinal cord learning and reduces nociceptive hyper-reactivity. On the other hand, stimulation that is delivered in an unsynchronized fashion, such as randomized electrical stimulation or peripheral skin injuries, can generate maladaptive spinal plasticity that undermines future spinal cord learning, reduces recovery of locomotor function, and promotes nociceptive hyper-reactivity after SCI. We review these basic phenomena, how these findings relate to the broader spinal plasticity literature, discuss the cellular and molecular mechanisms, and finally discuss implications of these and other findings for improved rehabilitative therapies after SCI.

Persistent polyuria in a rat spinal contusion model

Polyuria contributes to bladder overdistention, which confounds both lower and upper urinary tract management in individuals having a spinal cord injury (SCI). Bladder overdistention post-SCI is one of the most common triggers for autonomic dysreflexia, a potentially life-threatening condition. Post-SCI polyuria is thought to result from loss of vascular tone in the lower extremities, leading to edema and subsequent excess fluid, resulting in polyuria. Mild SCIs that have near complete recovery would therefore be expected to have little to no polyuria, while severe injuries resulting in flaccid limbs and lower extremity edema would be expected to exhibit severe polyuria. Since interventions that may decrease lower extremity edema are recommended to lessen the severity of polyuria, step training (which promotes vascular circulation) was evaluated as a therapy to reduce post-SCI polyuria. In the present study, polyuria was evaluated in mild, moderate, and severe contusive SCI in adult male rats. The animals were housed in metabolic cages for 24-hour periods pre- and post-SCI (to 6 weeks). Urine, feces, food, water, and body weights were collected. Other assessments included residual expressed urine volumes, locomotor scoring, in-cage activity, and lesion histology. SCI produced an immediate increase in 24-hour urine collection, as early as 3 days post-SCI. Approximately 2.6-fold increases in urine collection occurred from weeks 1-6 post-SCI for all injury severities. Even with substantial gains in locomotor and bladder function following a mild SCI, polyuria remained severe. Step training (30 min/day, 6 days/week) did not alleviate polyuria in the moderate SCI contusion group. These results indicate that (1) mild injuries retaining weight-bearing locomotion that should have mild, if any, edema/loss of vascular tone still exhibit severe polyuria, and (2) step training was unable to reduce post-SCI polyuria. Taken together, these results indicate that the current mechanistic hypothesis of post-SCI polyuria may be incomplete.

Bilateral bulbospinal projections to pudendal motoneuron circuitry after chronic spinal cord hemisection injury as revealed by transsynaptic tracing with pseudorabies virus

Complications of spinal cord injury in males include losing brainstem control of pudendal nerve-innervated perineal muscles involved in erection and ejaculation. We previously described, in adult male rats, a bulbospinal pathway originating in a discrete area within the medullary gigantocellularis (GiA/Gi), and lateral paragigantocellularis (LPGi) nuclei, which when electrically microstimulated unilaterally, produces a bilateral inhibition of pudendal motoneuron reflex circuitry after crossing to the contralateral spinal cord below T8. Microstimulation following a long-term lateral hemisection, however, revealed reflex inhibition from both sides of the medulla, suggesting the development or unmasking of an injury-induced bulbospinal pathway crossing the midline cranial to the spinal lesion. In the present study, we investigated this pathway anatomically using the transsynaptic neuronal tracer pseudorabies virus (PRV) injected unilaterally into the bulbospongiosus muscle in uninjured controls, and ipsilateral to a chronic (1-2 months) unilateral lesion of the lateral funiculus. At 4.75 days post-injection, PRV-labeled cells were found bilaterally in the GiA/Gi/LPGi with equal side-to-side labeling in uninjured controls, and with significantly greater labeling contralateral to the lesion/injection in lesioned animals. The finding of PRV-labeled neurons on both sides of the medulla after removing the mid-thoracic spinal pathway on one side provides anatomical evidence for the bilaterality in both the brainstem origin and the lumbosacral pudendal circuit termination of the spared lateral funicular bulbospinal pathway. This also suggests that this bilaterality may contribute to the quick functional recovery of bladder and sexual functions observed in animals and humans with lateral hemisection injury.

Sex and hormonal variations in the development of at-level allodynia in a rat chronic spinal cord injury model

The development of central neuropathic pain varies among patients with spinal cord injury (SCI). The factors contributing to the development and perpetuation of segmental pain (at-level allodynia) has been the focus of ongoing experiments in our laboratory. One such factor is hormonal status. We have shown previously, using a male rat model of SCI, that a severe contusion injury is necessary for the development of allodynia in trunk regions at and just above the level of a T8 injury. In this study, we examined at-level sensitivity for SCI ovariectomized (ovx) and cycling female rats as well as for SCI males implanted with either a placebo pellet or one that slowly releases 17beta-estradiol. The proportion of ovx SCI female rats and placebo-treated SCI males displaying pain-like behaviors to touch/pressure of at-level dermatomes up to 6 weeks post-injury (67% and 75%, respectively) was similar to our previous studies on SCI males (69%). In contrast, significantly fewer cycling SCI female rats and 17beta-estradiol treated SCI male rats showed sensitivity to touch at-level (26% and 30%, respectively). These results implicate 17beta-estradiol as a potential target that can readily be modulated to prevent segmental pain following SCI.

Select spinal lesions reveal multiple ascending pathways in the rat conveying input from the male genitalia

The specific white matter location of all the spinal pathways conveying penile input to the rostral medulla is not known. Our previous studies using rats demonstrated the loss of low but not high threshold penile inputs to medullary reticular formation (MRF) neurons after acute and chronic dorsal column (DC) lesions of the T8 spinal cord and loss of all penile inputs after lesioning the dorsal three-fifths of the cord. In the present study, select T8 lesions were made and terminal electrophysiological recordings were performed 45-60 days later in a limited portion of the nucleus reticularis gigantocellularis (Gi) and Gi pars alpha. Lesions included subtotal dorsal hemisections that spared only the lateral half of the dorsal portion of the lateral funiculus on one side, dorsal and over-dorsal hemisections, and subtotal transections that spared predominantly just the ventromedial white matter. Electrophysiological data for 448 single unit recordings obtained from 32 urethane-anaesthetized rats, when analysed in groups based upon histological lesion reconstructions, revealed (1) ascending bilateral projections in the dorsal, dorsolateral and ventrolateral white matter of the spinal cord conveying information from the male external genitalia to MRF, and (2) ascending bilateral projections in the ventrolateral white matter conveying information from the pelvic visceral organs (bladder, descending colon, urethra) to MRF. Multiple spinal pathways from the penis to the MRF may correspond to different functions, including those processing affective/pleasure/motivational, nociception, and mating-specific (such as for erection and ejaculation) inputs.

Urinary bladder irritation alters efficacy of vagal stimulation on rostral medullary neurons in chronic T8 spinalized rats

The presence of pelvic visceral inputs to neurons in the rostral medulla that are responsive to electrical stimulation of the abdominal branches of the vagus nerve (VAG-abd) was investigated in a complete chronic T8 spinal transection rat model. Using extracellular electrophysiological recordings from single medullary reticular formation (MRF) neurons, 371 neurons in 15 rats responsive to pinching the ear (search stimulus) were tested for somato-visceral and viscero-visceral convergent responses to stimulation of the following nerves/territories: VAG-abd, dorsal nerve of the penis, pelvic nerve, distention of urinary bladder and colon, penile stimulation, urethral infusion, and touch/pinch of the entire body surface. In addition to these mechanical and electrical stimuli, a chemical stimulus applied to the bladder was assessed as well. Of the total neurons examined, 205 were tested before and 166 tested beginning 20 min after application of a chemical irritant (2% acetic acid) to the urinary bladder (same rats used pre/post irritation). As with intact controls, many ear-responsive MRF neurons responded to the electrical stimulation of VAG-abd. Although MRF neuron responses failed to be evoked with direct (mechanical and electrical nerve) pelvic visceral stimuli, acute chemical irritation of the urinary bladder produced a significant increase in the number of MRF neurons responsive to stimulation of VAG-abd. The results of this study indicate a central effect that potentially relates to some of the generalized below level pelvic visceral sensations that have been documented in patients with complete spinal cord injury.

Loss and spontaneous recovery of forelimb evoked potentials in both the adult rat cuneate nucleus and somatosensory cortex following contusive cervical spinal cord injury

Varying degrees of neurologic function spontaneously recovers in humans and animals during the days and months after spinal cord injury (SCI). For example, abolished upper limb somatosensory potentials (SSEPs) and cutaneous sensations can recover in persons post-contusive cervical SCI. To maximize recovery and the development/evaluation of repair strategies, a better understanding of the anatomical locations and physiological processes underlying spontaneous recovery after SCI is needed. As an initial step, the present study examined whether recovery of upper limb SSEPs after contusive cervical SCI was due to the integrity of some spared dorsal column primary afferents that terminate within the cuneate nucleus and not one of several alternate routes. C5-6 contusions were performed on male adult rats. Electrophysiological techniques were used in the same rat to determine forelimb evoked neuronal responses in both cortex (SSEPs) and the cuneate nucleus (terminal extracellular recordings). SSEPs were not evoked 2 days post-SCI but were found at 7 days and beyond, with an observed change in latencies between 7 and 14 days (suggestive of spared axon remyelination). Forelimb evoked activity in the cuneate nucleus at 15 but not 3 days post-injury occurred despite dorsal column damage throughout the cervical injury (as seen histologically). Neuroanatomical tracing (using 1% unconjugated cholera toxin B subunit) confirmed that upper limb primary afferent terminals remained within the cuneate nuclei. Taken together, these results indicate that neural transmission between dorsal column primary afferents and cuneate nuclei neurons is likely involved in the recovery of upper limb SSEPs after contusive cervical SCI.

Activation of lumbosacral 5-HT2C receptors induces bursts of rhythmic activity in sympathetic nerves to the vas deferens in male rats

1. We previously demonstrated that p-chloroamphetamine (PCA) intravenously (i.v.) evokes a specific patterned bursting response in the vas deferens nerve (VDN) of anaesthetised male rats that is associated with contraction of the vas deferens, and ejaculation and contraction of the bulbospongiosus muscles. The present study used selective 5-HT agonists to induce similar rhythmic bursting responses in the VDN in order to reveal the 5-HT receptor subtypes involved. 2. The 5-HT(2C) receptor agonist (1.0 mg kg(-1) Ro600175 i.v.) evoked the characteristic bursting pattern responses in the VDN. The 5-HT(1A) receptor agonist (1.0 mg kg(-1) 8-OH-DPAT i.v.) failed to elicit any responses. However, 8-OH-DPAT coadministered in combination with Ro600175 induced a potentiation of the responses. 3. Responses were also evoked in rats with a mid-thoracic spinalisation, with a more predictable response being observed following the combination of agonists. This suggests an action of both agonists in the lumbosacral spinal cord. 4. Responses were blocked by 0.5 mg kg(-1) SB206553 i.v. (5-HT(2B/C) receptor antagonist) or 0.5 mg kg(-1) WAY100635 i.v. (5-HT(1A) receptor antagonist), but not 0.1 or 1.0 mg kg(-1) SB269970 i.v. (5-HT(7) receptor antagonist). 5. We suggest that activation of 5-HT(2C) and 5-HT(1A) receptor subtypes synergistically elicits contraction of the vas deferens through the activation of sympathetic preganglionic neurones in the spinal cord. 6. These data support the idea of a proejaculatory action of 5-HT(2C) receptors in the lumbosacral spinal cord, suggesting a descending 5-HT excitatory pathway in addition to a 5-HT inhibitory pathway. An excitatory action of 8-OH-DPAT at lumbosacral sites is also evident.

Descending pathways modulating the spinal circuitry for ejaculation: effects of chronic spinal cord injury

Sexual dysfunction is a common complication in men with chronic spinal cord injury. In particular, ejaculation is severely compromised or absent and the resulting infertility issues are important to this group of predominantly young men. To investigate the neural circuits and descending spinal pathways involved in ejaculation, animal models have been developed in normal and spinal cord-injured preparations. Primarily through studies in rats, spinal ejaculatory circuits have been described including (i) autonomic circuits at the thoracolumbar and lumbosacral levels mediating the emission phase of ejaculation, (ii) somatic circuits at the lumbosacral level controlling the expulsion phase of ejaculation through sequential and rhythmic contraction of perineal striated muscles (e.g. bulbospongiosus), and (iii) a proposed ejaculatory pattern generator in the lumbar cord. Midthoracic incomplete chronic spinal cord injury has revealed the dependency of spinal ejaculatory circuits on bilateral spinal pathways from the brainstem via modulation of pudendal motor neuron reflexes and pudendal nerve autonomic fibers. Accordingly, sensory input from the dorsal nerve of the penis, required to trigger the ejaculatory response in animals and humans, is no longer inhibited from the lateral paragigantocellularis nucleus in the ventrolateral medulla. This inhibitory effect, likely presynaptic through a serotonergic pathway, is thought to be necessary to provide the rhythmic, bursting, and sequential contractions of the perineal muscles during ejaculation. Chronic lateral hemisection injury, which severs half of the descending lateral funiculus-located pathways, results in new functional connections of the pudendal reflex inhibitory and pudendal sympathetic activation pathways across the midline, above and below the lesion, respectively. Clinical correlations in spinal cord-injured men have demonstrated the validity of the rodent animal for the study of ejaculatory dysfunction after chronic injury.

Alterations in eliminative and sexual reflexes after spinal cord injury: defecatory function and development of spasticity in pelvic floor musculature

Spinal cord injury often results in loss of normal eliminative and sexual functions. This chapter is focused on defecatory function, although aspects of micturition and erectile function will be covered as well due to the overlap in anatomical organization and response to injury. These systems have both autonomic and somatic components, and are organized in the thoracolumbar (sympathetic), lumbosacral (somatic), and sacral (parasympathetic) spinal cord. Loss of supraspinal descending control and plasticity-mediated alterations at the level of the spinal cord, result in loss of voluntary control and in abnormal functioning of these systems including the development of dyssynergies and spasticity. There are several useful models of spinal cord injury in rodents that exhibit many of the autonomic dysfunctions observed after spinal cord injury in humans. Numerous studies involving these animal models have demonstrated development of abnormalities in bladder, external anal sphincter, and erectile function, such as detrusor-sphincter-dyssynergia and external anal sphincter hyperreflexia. Here we review many of these studies and show some of the anatomical alterations that develop within the spinal cord during the development of these hyperreflexias. Furthermore, we show that spasticity develops in other pelvic floor musculature as well, such as the bulbospongiosus muscle, which results in increased duration and magnitude of pressures developed during erectile events and increased duration of micturition. Advances and continued improvement in the use of current animal models of spinal cord injury should encourage and increase the laboratory work devoted to this relatively neglected area of experimental spinal cord injury.

Telemetric monitoring of corpus spongiosum penis pressure in conscious rats for assessment of micturition and sexual function following spinal cord contusion injury

Disruption of bladder function and sexual reflexes are major complications following spinal cord injury (SCI). We examined the use of telemetric monitoring of corpus spongiosum penis (CSP) pressures for assessment of micturition and erectile events following SCI in rats. Pressure catheters were implanted in the bulb of the CSP of seven male Long-Evans hooded rats, subjected to a standardized weight drop SCI (10 g x 12.5 mm) at T10. CSP pressures were analyzed for spontaneously occurring micturition and erectile events, and during ex copula reflex erection tests until 25 days after SCI. Urine volume was determined until 21 days after SCI. Results show initial loss of bladder function after SCI with gradual return of reflex micturition. When compared to baseline (BL), micturition pressure characteristics after SCI included prolonged duration, increased area under the curve (AUC), increased mean pressures, increased number of pressure peaks, and increased peak frequency. At 21 days after SCI, the urine volume per micturition was significantly increased. The number of full erectile events decreased significantly following SCI. Pressure wave analyses demonstrated increased AUC, increased maximum pressures, increased suprasystolic peak duration, increased AUC of the suprasystolic peaks, and increased maximum pressures of the suprasystolic peaks during recovery. The number of partial erectile events decreased significantly following SCI. Ex copula reflex erection testing demonstrated significantly decreased latency. The study demonstrates that telemetric monitoring of CSP pressures in conscious rats is a valuable and reliable method for assessing recovery of autonomic function following SCI.

Serotonergic fiber sprouting to external anal sphincter motoneurons after spinal cord contusion

The present study analyzed the anatomical plasticity of serotonergic immunoreactive projections to external anal sphincter (EAS) motoneurons, and the behavioral plasticity of EAS reflexes, penile erection, and locomotion in rats with spinal contusion injury (SCI) or complete spinal cord transection (TX). Electromyographic activity of the EAS, penile erection latency, and BBB locomotor score exhibited parallel recovery over the 6-week recovery period after contusion SCI. This pattern of recovery was not observed in TX animals. While locomotor scores demonstrated a small increase after TX, erectile and anorectal function remained at abnormal levels established immediately after injury. Serotonergic immunofluorescent (5-HT-IF) staining at the lesion site identified a small number of fibers spared after SCI that may provide a substrate for functional recovery. Pixel density measurements of 5-HT-IF in the vicinity of retrogradely labeled EAS and unlabeled pudendal motoneurons necessary for penile erection provide indirect evidence of serotonergic sprouting that parallels the observed functional recovery in animals with SCI. No 5-HT-IF was detected caudal to the injury site in TX animals. These studies indicate: (1) lumbosacral eliminative and reproductive reflexes provide a valid means of studying the mechanisms of post-SCI plasticity; (2) the similar recovery curves suggest similar return of descending control, perhaps through sprouting of descending serotonergic fibers; (3) the observed deficits after TX likely represent the permanent removal of descending inhibition and reflect reorganization of segmental circuitry.

Targeting recovery: priorities of the spinal cord-injured population

In the United States alone, there are more than 200,000 individuals living with a chronic spinal cord injury (SCI). Healthcare for these individuals creates a significant economic burden for the country, not to mention the physiological, psychological, and social suffering these people endure everyday. Regaining partial function can lead to greater independence, thereby improving quality of life. To ascertain what functions are most important to the SCI population, in regard to enhancing quality of life, a novel survey was performed in which subjects were asked to rank seven functions in order of importance to their quality of life. The survey was distributed via email, postal mail, the internet, interview, and word of mouth to the SCI community at large. A total of 681 responses were completed. Regaining arm and hand function was most important to quadriplegics, while regaining sexual function was the highest priority for paraplegics. Improving bladder and bowel function was of shared importance to both injury groups. A longitudinal analysis revealed only slight differences between individuals injured <3 years compared to those injured >3 years. The majority of participants indicated that exercise was important to functional recovery, yet more than half either did not have access to exercise or did not have access to a trained therapist to oversee that exercise. In order to improve the relevance of research in this area, the concerns of the SCI population must be better known and taken into account. This approach is consistent with and emphasized by the new NIH roadmap to discovery.

Effects of Chronic Dorsal Column Lesions on Pelvic Viscerosomatic Convergent Medullary Reticular Formation Neurons

Single medullary reticular formation (MRF) neurons receive multiple somatovisceral convergent inputs originating from many different spinal and cranial nerves, including the pelvic nerve (PN), dorsal nerve of the penis (DNP), and the abdominal branches of the vagus. In a previous study, the input to MRF from the male genitalia was shown to be eliminated with chronic 30-day dorsal hemisection at the T8 spinal level. In this study, the effect of a smaller chronic lesion [dorsal column lesion (DCx)] on MRF neuronal responses was examined. Responses to bilateral electrical stimulation of the DNP remained. MRF neuronal responses to non-noxious (touch/stroke) levels of penile stimulation, however, were eliminated; only responses to noxious pinch remained. No differences were found for the number of neurons responding to noxious distention of the colon between the DCx and control groups. Although no differences were found across these groups for the percent MRF responses to vagal stimulation, the mean response latency for the DCx group was twice the sham-DCx/intact control group. Taken together, these results indicate that the MRF receives at least some of its input from the male genitalia via pathways located within the dorsal columns at the mid-thoracic spinal level.

An anatomical and electrophysiological study of the genitofemoral nerve and some of its targets in the male rat

Anatomical descriptions of the genitofemoral nerve (GFn) innervating the lower pelvic area are contradictory. Here we re-examine its origin and innervation by its various branches of principal target organs in the male rat. Using gross dissection, electrophysiological techniques and retrograde tracing of motoneurones with horseradish peroxidase, we confirm that the GFn originates from lumbar spinal nerves 1 and 2, and that at the level of the common iliac artery it divides into a lateral femoral and a medial genital branch. In contrast to previous studies, we report that the genital and not the femoral branch innervates the abdominal-inguinal skin, and not only the genital but also the femoral branch innervates the cremaster muscle (Cm) surrounding the testes. Motoneurones innervating the Cm proper are located in the ventral nucleus of L1 and L2, and those innervating the muscular transition region of the rostral Cm are located in the ventral nucleus in L1 and the ventrolateral nucleus in L2. The GFn may contribute to male reproductive performance by transmitting cutaneous information during copulation and, via contraction of the Cm to promote ejaculation, the protective displacement of the testes into the abdominal cavity during fighting and as a sperm-protecting thermoregulatory measure.

Differential effects of chronic spinal hemisection on somatic and visceral inputs to caudal brainstem

The medullary reticular formation (MRF) receives convergent inputs from multiple somatic and pelvic/visceral territories. The effects of chronic 30-day lateral hemisections at T8 on the responses of single MRF neurons to noxious mechanical stimulation of both hindpaws was examined in urethane-anesthetized male rats. Neuronal responses on both sides of the MRF to pinching of the hindpaw on the side opposite the lesion (intact-side) were found either to be completely absent or if present, weak (i.e. hindpaw was hyposensitive). The presence or absence of intact-side responses appeared to be dependent on the lesion extent. In contrast, bilateral MRF responses to pinching the lesion-side hindpaw were present; however, responses were greater in magnitude (lower thresholds) relative to surgical sham controls suggesting hypersensitivity. Responses to lesion-side hindpaw stimulation on both sides of the MRF indicated that whereas the ascending projections are primarily crossed below the level of lesion, they are both crossed and uncrossed above. These findings are in contrast with our previous data on ascending projections from the bilaterally organized male urogenital tract. The results presented for the hindpaws correlate with clinical observations of patients with similar incomplete spinal cord injuries (Brown-Séquard syndrome).

Dissociation of the effects of nucleus raphe obscurus or rostral ventrolateral medulla lesions on eliminatory and sexual reflexes

Rat preparations were used to investigate long-term changes in external anal sphincter (EAS) contractions and reflexive penile erection following electrolytic lesions of the nucleus raphe obscurus (nRO) or the rostral ventrolateral medulla. EAS contractions were measured electromyographically (EAS EMG) following distention of the EAS with a 5-mm probe. Penile erections were measured using a standard ex copula reflex testing paradigm. At 48 h postlesion, 100% of nRO-lesioned animals displayed reflexive erections and the magnitude of EAS EMG was significantly greater in lesioned animals than in sham controls. These results suggested EAS hyperreflexia following destruction of the nRO. By 14 days postlesion, EAS responsiveness in nRO-lesioned animals had returned to levels comparable to nonlesioned animals. No measures of penile erection were affected by nRO lesions. In animals with nucleus gigantocellularis (Gi) and lateral nucleus paragigantocellularis (Gi-lPGi) lesions, no significant changes to EAS reflexes were observed at any time point. At 48 h postoperative, Gi-lPGi lesions significantly reduced the latency to first erection and increased the number of erections elicited relative to controls. Similar facilitation of erection latency was observed at 14 days postlesion, while erection number and flip total were no longer significantly different from controls. These and previous studies suggest that the nRO regulates defecatory reflexes in the rat. These data further suggest that the comingled EAS and bulbospongiosus (BS) motoneurons are controlled by discrete and separate brainstem circuits and that increases in EAS and penile reflexes after spinal cord lesions are mediated by loss of different descending inputs.