Dopamine is produced in the rat spinal cord and regulates micturition reflex after spinal cord injury

Highlights

• Dopamine (DA) neurons reside in the rat spinal cord and constitute a local, spinal source of DA.

• Spinal DA-ergic mechanisms regulate micturition reflex following spinal cord injury.

• Stimulating spinal D2-like receptors facilitates urinary voiding in spinal cord-injured rats.

Abstract

Dopamine (DA) neurons in the mammalian central nervous system are thought to be restricted to the brain. DA-mediated regulation of urinary activity is considered to occur through an interaction between midbrain DA neurons and the pontine micturition center. Here we show that DA is produced in the rat spinal cord and modulates the bladder reflex. We observed numerous tyrosine hydroxylase (TH)⁺ neurons in the autonomic nuclei and superficial dorsal horn in L6–S3 spinal segments. These neurons are dopamine-β-hydroxylase (DBH)⁻ and some contain detectable dopamine decarboxylase (DDC), suggesting their capacity to produce DA. Interestingly, following a complete thoracic spinal cord injury (SCI) to interrupt supraspinal projections, more TH⁺ neurons emerged in the lumbosacral spinal cord, coincident with a sustained, low level of DA expression there and a partially recovered micturition reflex. Non-selective blockade of spinal DA receptors reduced bladder activity whereas activation of spinal D₂-like receptors increased bladder activity and facilitated voiding. Additionally, depletion of lumbosacral TH⁺ neurons with 6-hydroxydopamine (6-OHDA) decreased bladder non-voiding contractions and voiding efficiency. Furthermore, injecting the transsynaptic neuronal tracer pseudorabies virus (PRV) into the bladder detrusor labeled TH⁺ cells in the lumbosacral cord, confirming their involvement in spinal micturition reflex circuits. These results illustrate that DA is synthesized in the rat spinal cord; plasticity of lumbosacral TH⁺ neurons following SCI may contribute to DA expression and modulate the spinal bladder reflex. Thus, spinally-derived DA and receptors could be a novel therapeutic target to improve micturition recovery after SCI.

Introduction

Dopamine (DA) is an important neurotransmitter modulating a broad range of body behaviors. Disrupting DA signaling results not only in motor dysfunction but also in autonomic disorders (Shulman, L.M., et al., 2001, Sakakibara, R., et al., 2011). For instance, irritable hyperactive bladder symptoms, such as urinary urgency, frequency, and incontinence, often occur when midbrain DA neurons are damaged in Parkinson's disease (PD) (Winge and Fowler, 2006). Although the underlying mechanism behind bladder hyperreflexia in PD patients is not completely understood, the degeneration of DA neurons in the substantia nigra and resulting deficiency of D₁-mediated action upon the pontine micturition center (PMC) are a fundamental cause (Sakakibara, R., et al., 2002, Yoshimura, N., et al., 2003). Interestingly, D₂-mediated facilitation of micturition has been reported in normal, cerebrally-infarcted, and chemically-induced Parkinsonian animals (Yoshimura, N., et al., 1998, Yokoyama, O., et al., 1999, Seki, S., et al., 2001, Kitta, T., et al., 2012), indicating that DA in regions other than the brain may modulate micturition.

Recent studies revealed that autonomic neurons in the rat lower spinal cord express DA receptors (Gladwell, S.J., et al., 1999, Stafford, S.A. and Coote, J.H., 2006), suggesting that DA released within the cord helps regulate autonomic function. Though DA neurons are known to reside in the spinal cord of non-mammalian species, e.g. birds and fish (Roberts, B.L. and Meredith, G.E., 1987, Acerbo, M.J., et al., 2003), they are thought to be restricted to the brain in mammals (Bjorklund and Dunnett, 2007). Thus, DA in the spinal cord is assumed to come from diencephalospinal pathways that originate mainly from the A11 cell group (Skagerberg, G., et al., 1982, Taniguchi, W., et al., 2011, Sharples, S.A., et al., 2014). Nevertheless, Mouchet and colleagues observed tyrosine hydroxylase (TH)⁺ cells in the rat spinal cord (Mouchet et al., 1986). Since TH is expressed in multiple neuron types, including DA-ergic and adrenergic ones, it is not clear if these neurons synthesize neurotransmitter DA. Moreover, the function of these neurons is unknown. Here, we perceived a similar distribution of TH⁺ cells in the rat spinal cord, confirming the previous observation. The majority of these cells are aggregated in the lumbosacral segments, particularly within the autonomic region and superficial dorsal horn. Importantly, some of them display typical DA-ergic characteristics.

The location of TH⁺ cells in the lumbosacral cord suggests their involvement in pelvic visceral activity, such as micturition. However, the presence of A11 DA-ergic and other descending catecholaminergic projections that contain DA as a precursor precludes us from determining what function the spinal TH⁺ neurons have. To specifically identify if these neurons play a role in urinary function, we used a complete spinal cord injury (SCI) model to remove descending control and retain only spinal micturition neural circuitry. Interruption of supraspinal micturition pathways causes acute areflexic bladder paralysis. Yet, over a few weeks, there is usually a partial recovery of urinary function via involuntary bladder and urethral reflexes (Fowler, C.J., et al., 2008, de Groat, W.C. and Yoshimura, N., 2012). In the present study, we observed remarkable plasticity of lumbosacral TH⁺ neurons after SCI that contributed to a low level of sustained, local spinal DA expression. Furthermore, spinal DA receptors regulating bladder reflex are active, indicating that this spinally-derived DA modulates the recovered micturition function.