Characterization of supraspinal vasomotor pathways and autonomic dysreflexia after spinal cord injury in F344 rats

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Abstract

Cardiovascular dysfunction usually occurs after high thoracic and cervical spinal cord injury (SCI). The disruption of supraspinal vasomotor pathways (SVPs) results in the loss of bulbospinal regulation of sympathetic preganglionic neurons, leading to hypotension and compensatory tachycardia at rest. Episodic autonomic dysreflexia can develop upon sensory stimulation below the level of injury. In rodents, the precise spatial distribution of SVPs in the spinal cord originating from the rostral ventrolateral medulla (RVLM) has not been fully defined. To facilitate future studies of axon regeneration to regain cardiovascular control, we injected biotinylated dextran amine (BDA) bilaterally into the RVLM to anterogradely trace SVPs in Fischer 344 (F344) rats. Three weeks later, BDA-labeled descending projections were predominantly localized within the dorsolateral funiculus throughout the cervical and thoracic spinal segments as expected. Additionally, BDA-labeled fibers were also observed in ventral white matter. After a T4 dorsal hemisection to interrupt the dorsolateral funiculus, BDA labeled terminals originating from the ventral white matter as well as serotonergic projections were still detected in regions of autonomic nuclei below the injury. Based on these results, we examined cardiovascular responses after different lesions at spinal level T4, including lateral or dorsal hemisection, dorsolateral or complete transection. Hemodynamic dysfunction and autonomic dysreflexia were only elicited in rats with complete T4 transections when all SVPs were disrupted. Hence, F344 rats with complete T4 transections provide a reliable model for investigating means to improve cardiovascular functional recovery after SCI.

Introduction

The autonomic nervous system is in charge of cardiovascular, gastrointestinal and urinary function, respiration, thermoregulation, sudomotor activity and sexual behavior (Appenzeller and Oribe, 1997). Normal autonomic functioning is critically dependent on the interaction of autonomic components within the spinal cord and supraspinal centers. Descending projections from the brainstem and hypothalamus control spinal sympathetic and parasympathetic outflow (Caverson et al., 1983, Furlan et al., 2003, Maiorov et al., 1998). Spinal cord injury (SCI) often interrupts supraspinal connections including pre-sympathetic descending projections. If a severe injury occurs above level T6, most of the sympathetic and the entire sacral parasympathetic outflow are isolated from cerebral control. As a result, not only devastating paralysis develops but autonomic function is also compromised. In the cardiovascular system, the loss of inhibitory control from higher levels is believed to alter the vasomotor tone generated in the spinal cord, causing abnormality of hemodynamics (Furlan and Fehlings, 2008, Inskip et al., 2009, Laird et al., 2006, Teasell et al., 2000). Particularly, life-threatening autonomic dysreflexia defined by episodic hypertension and baroreflex-mediated bradycardia in response to noxious stimuli below the injury level can develop due to a sudden, massive discharge of sympathetic preganglionic neurons (SPNs) (Karlsson, 1999, Lindan et al., 1980).

Although injury-induced unmyelinated pelvic afferent and intraspinal plasticity are related to the development of chronic autonomic dysreflexia (Cameron et al., 2006, Hou et al., 2008, Hou et al., 2009), the disruption of bulbospinal inhibitory pathways is the fundamental underlying cause of cardiovascular abnormalities. Over the last two decades, great progress has been made in understanding the mechanisms of autonomic regulation. The five main supraspinal regions providing input to SPNs in the thoracolumbar spinal cord include the rostral ventrolateral medulla (RVLM), the rostral ventromedial medulla, the caudal raphe nuclei, the A5 region, and the paraventricular nucleus of the hypothalamus (Calaresu and Yardley, 1988, Chalmers et al., 1994, Jansen et al., 1995, Llewellyn-Smith, 2009). Among these autonomic centers, the RVLM is considered to play a key role in regulating cardiovascular function (Dampney et al., 2000, Krassioukov and Fehlings, 1999, Schramm et al., 1993). In rodents, descending supraspinal vasomotor pathways (SVPs) originating in the RVLM and other sites have been identified within the dorsolateral aspects of the spinal cord using electrophysiological techniques (Dampney et al., 2000, Krassioukov, 2006, Ruggiero et al., 1989, Seyedabadi et al., 2006) or retrograde tract tracing (Fontes et al., 2001, Jeske and McKenna, 1992, Schramm et al., 1993, Strack et al., 1989). Although spinal segmental innervation of RVLM-derived SVPs has been examined by anterograde tract tracing (Pyner and Coote, 1998), little is known about their precise spatial distribution in the spinal cord of rodents. A detailed characterization of SVPs is a prerequisite to investigate axon regeneration as a means to ameliorate cardiovascular dysfunction.

The Fischer 344 (F344) rat, an inbred strain, is widely used as a SCI model to study motor and sensory axon regeneration and plasticity allowing for syngeneic cell or tissue transplantation (Bonner et al., 2010, Brock et al., 2010, Lu et al., 2007), yet the hemodynamic characteristics after SCI have never been characterized. Indeed, strain-specific variations in cardiovascular responses to SCI have been reported. For example, blood pressure changes after SCI and visceral pain are more pronounced in Wistar rats than in Sprague Dawley rats (Landrum et al., 1998). Whether F344 rats can be used as a reliable and reproducible SCI model to study neural regeneration and functional recovery associated with cardiovascular function is unknown.

In the present study, we characterized the spatial localization of SVPs originating from the RVLM region in the brainstem by anterograde tracing in F344 rats. Our data indicate that in addition to the dorsolateral funiculus, RVLM-derived projections in the ventral white matter also innervate SPNs. Consistent with the anatomical localization of SVPs, lesions that include only the dorsolateral components of SVPs at high thoracic level are insufficient to influence cardiovascular parameters. Only rats with complete spinal cord transections show significant cardiovascular dysfunction at rest and persistent inducible autonomic dysreflexia in response to noxious visceral stimulation.

Section snippets

Animals

A total of 42 adult female F344 rats weighing 150–200 g were used. Institutional Animal Care and Use Committee and Society for Neuroscience guidelines on animal care were strictly followed to minimize the number of rats used and any potential suffering. Rats were anesthetized intraperitoneally with a combination (2 ml/kg) of ketamine (25 mg/ml; Fort Dodge Animal Health, Fort Dodge, IA), xylazine (1.3 mg/ml; Butler, Columbus, OH), and acepromazine (0.25 mg/ml; Boehringer Ingelheim Vetmedica, St.

Anterograde tracing of RVLM projections

BDA injection sites in the brainstem were verified by histochemical labeling in coronal sections of the rostral medulla. BDA labeling was largely restricted to the ventrolateral aspect overlapping with the RVLM regions according to the Paxinos rat brainstem atlas (Paxinos et al., 1999) (Fig. 2A). Throughout cervical and thoracic segments, BDA-labeled supraspinal projections descended predominantly in the dorsolateral funiculus of the white matter as expected (Fig. 2B, C). Labeled fibers could

Discussion

The present study characterizes for the first time parts of SVPs and cardiovascular dysfunction after different spinal cord lesions in F344 rats. Vasomotor projections from the RVLM are not only found in the dorsolateral funiculus but also in the ventral white matter in the cervical and thoracic spinal cord. Partial transection of these and other SVPs by dorsal hemisection, or unilateral hemisection or bilateral dorsolateral funiculus lesions at high thoracic level do not cause severe

Acknowledgments

Supported by grants from NIH/NINDS (NS054883) to A. B. and Craig H. Neilsen Foundation (161456) to S. H.

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