Spinal neuronal mechanisms regulate recovered involuntary micturition after spinal cord injury (SCI). It is 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 DA-ergic 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, L-DOPA administered intravenously during bladder cystometrogram and external urethral sphincter (EUS) electromyography reduced bladder overactivity and increased the duration of EUS bursting, leading to remarkably improved voiding efficiency. Apomorphine, a non-selective dopamine receptor (DR) agonist, or quinpirole, a selective DR 2 agonist, induced similar responses whereas a specific DR 2 antagonist remoxipride alone only had minimal effects. Meanwhile, administration of SCH 23390, a DR 1 antagonist, reduced voiding efficiency by increasing tonic EUS activity and shortening the EUS bursting period. Unexpectedly, SKF 38393, a selective DR 1 agonist, increased EUS tonic activity, implying a complicated role of DR 1 in LUT function.In metabolic cage assays, subcutaneous administration of quinpirole decreased spontaneous voiding frequency and increased voiding volumes; while L-DOPA and apomorphine were inactive possibly due to slow entry into the CNS. Collectively, tonically active DR 1 in SCI rats inhibits urine storage and enhances voiding by differentially modulating the EUS tonic and bursting patterns, respectively; while pharmacologic activation of DR 2 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.

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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-HT2A) 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-HT2A receptors in female SCI rats. Three to four weeks after a T10 spinal transection, systemic and central blockage of 5-HT2A receptors with MDL only slightly influenced the micturition reflex. However, delivery of the 5-HT2A 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 5HT2A 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.

The diencephalic A11 nuclei are the primary source of spinal dopamine (DA). Neurons in this region project to all levels of the spinal cord. Traumatic spinal cord injury (SCI) often interrupts descending and ascending neuronal pathways and further elicits injury-induced neuronal plasticity. However, it is unknown how A11 neurons and projections respond to SCI-induced axotomy. Based on preliminary observation, we hypothesized that A11 DA-ergic neurons rostral to the lesion site might change their capacity to synthesize DA after SCI. Adult rats received a complete spinal cord transection at the 10th thoracic (T10) level. After 3 or 8 weeks, rostral (T5) and caudal (L1) spinal cord tissue was collected to measure mRNA levels of DA-related genes. Meanwhile, A11 neurons in the brain were explicitly isolated by laser capture microdissection, and single-cell qPCR was employed to evaluate mRNA levels in the soma. Histological analysis was conducted to assess the number of A11 DA-ergic neurons. The results showed that, compared to naïve rats, mRNA levels of tyrosine hydroxylase (TH), dopamine decarboxylase (DDC), and D2 receptors in the T5 spinal segment had a transient decrease and subsequent recovery. However, dopamine-β-hydroxylase (DBH), D1 receptors, and DA-associated transcription factors did not change following SCI. Furthermore, axon degeneration below the lesion substantially reduced mRNA levels of TH and D2 in the L1 spinal segment. However, DDC transcript underwent only a temporary decrease. Similar mRNA levels of DA-related enzymes were detected in the A11 neuronal soma between naïve and SCI rats. In addition, immunostaining revealed that the number of A11 DA neurons did not change after SCI, indicating a sustention of capacity to synthesize DA in the neuroplasm. Thus, impaired A11 diencephalospinal pathways following SCI may transiently reduce DA production in the spinal cord rostral to the lesion but not in the brain.

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Diaphragm dysfunction occurs following high cervicall spinal cord injury (SCI). As the diaphragm is a primary respiratory muscle its dysfunction often necessitates assited‐ventiation, dramatically impacting quality of life and increasing risk of mortality in injured individuals. Despite these devastating outcomes, functional plasticity and recovery of diaphragm activity, albeit limited, can occur spontaneously over the time. Understanding of the mechanisms underlying this recovery will facilitate developing new therapeutic strategies to treat respiratory impairment post‐SCI. In the current study, a C2 cervical spinal cord hemisection (C2Hx) was used to explore spontaneous plasticity ‐ known as the spontaneous crossed phrenic phenomenon – in awake behaving animals. While previous studies have assessed functional under anesthesia, the present study monitored changes in diaphragm activity using chronically implanted diaphragm EMG electrodes in behaving rats up to 8 weeks. Our EMG recording data show that recovery of diaphragm activity in awake animal occurs in one‐two weeks post injury, but inspiratory bursts on the injured side differ from the contralateral. Even when the amplitude of ipsilateral bursts is comparable with uninjured side, intraburst spike frequency is significantly lower especially during quiet breathing. Recording diaphragm EMG activity under anesthesia (isoflurane or ketamine) can eliminate ipsilateral diaphragm activity up to 100% indicating that recovery seen in behaving rats is mostly attributed to NMDA receptor activation. Ongoing studies are assessing this progressive functional recovery in more detail.Support or Funding InformationSupported by Conquer Paralysis Now (CPN) and the Edward Jekkal Muscular Dystrophy Association Fellowship (Drexel).

Traumatic spinal cord injury (SCI) often leads to urinary dysfunction. Although an involuntary micturition reflex can be established to elicit voiding with time, complications arise in the form of bladder hyper-reflexia and detrusor-sphincter dyssynergia that cause incontinence and inefficient expulsion of urine. To date, the neuronal mechanisms that underlie regulation of micturition after SCI are not well understood. We recently observed an increase of a population of tyrosine hydroxylase (TH) + cells in the rat lumbosacral cord post-SCI, which contribute to the sustention of a low level of dopamine that modulates the recovered bladder reflex. To identify whether spinal TH + cells are involved in the micturition reflex pathway post-SCI, two isoforms of the trans-synaptic retrograde tracer, pseudorabies virus encoding green fluorescent protein (GFP; PRV-152) or red fluorescent protein (RFP; PRV-614), were injected into the bladder detrusor or the external urethral sphincter (EUS), respectively, 3 weeks after a spinal cord transection at the 10th thoracic level (T10) in rats. Immunohistochemistry was performed to examine infected TH + cells in the caudal cord at both 48 and 72 h post-injection. As a result, double-labeled TH + /GFP + and TH + /RFP + cells could be found in the superficial dorsal horn, parasympathetic nuclei, and dorsal gray commissure (lamina X) at both time points. More importantly, a shared population of TH + interneurons (TH + /GFP + /RFP + ) exists between bladder and EUS circuitry. These results suggest that spinal TH + interneurons may coordinate activity of the bladder and EUS that occurs during micturition reflexes post-SCI.