Author ORCID Identifier

https://orcid.org/0000-0003-1674-2386

Year of Publication

2019

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Medicine

Department

Physiology

First Advisor

Dr. Alexander G. Rabchevsky

Abstract

Vital autonomic and cardiovascular functions are susceptible to dysfunction after spinal cord injury (SCI), with cardiovascular dysregulation contributing to morbidity and mortality in the SCI population. Autonomic dysreflexia (AD) is a condition that develops after injury to the sixth thoracic spinal segment or higher and is characterized by potentially dangerous and volatile surges in arterial pressure often accompanied with irregular heart rate, headache, sweating, flushing of the skin, and nasal congestion. These symptoms occur in response to abnormal outflow of sympathetic activity from the decentralized spinal cord typically triggered by noxious, yet unperceived nociceptive stimulation beneath the level of lesion. Maladaptive plasticity of primary afferents and spinal interneurons influencing sympathetic preganglionic neurons is known to contribute to the development of AD. However, there are currently no treatments capable of targeting this underlying pathophysiology. The goal of this work was to test pharmacological agents for their potential to modify intraspinal plasticity associated with AD in order to prophylactically prevent the development of this condition altogether.

We first tested whether the drug rapamycin (RAP), a well-studied inhibitor of the growth promoting kinase “mammalian target of rapamycin” (mTOR), could prevent aberrant sprouting of primary c-fiber afferents in association with reduced indices of AD severity. Naïve and T4-transected rats undergoing 24/7 cardiovascular monitoring were treated with rapamycin (i.p.) for 4 weeks before tissue collection. RAP attenuated intraspinal mTOR activity after injury, however it also caused toxic weight loss. RAP treated SCI rats developed abnormally high blood pressure both at rest and during colorectal distension (CRD) induced AD, as well as more frequent bouts of spontaneous AD (sAD). These cardiovascular alterations occurred without altered intraspinal c-fiber sprouting. Our finding that rapamycin exacerbates cardiovascular dysfunction after SCI underscores the importance of screening potential pharmacological agents for cardiovascular side effects and suggests that the mTOR pathway plays a limited or dispensable role in c-fiber sprouting after SCI.

We next examined the effects of the antinociceptive drug gabapentin (GBP) on AD development. Our previous work demonstrated that a single acute administration of GBP can reduce the severity of AD. The mechanism of action, however, remains unclear. Emerging evidence suggests that GBP may act by blocking de novo synaptogenesis. We investigated whether continuous GBP treatment could attenuate the development of AD by modifying synaptic connectivity between primary afferents and ascending propriospinal neurons. SCI rats were treated with GBP every six hours for four weeks. We found that GBP reduced blood pressure during CRD stimulation and prevented bradycardia typically observed during AD. However, GBP treated rats also had a higher sAD frequency and failed to return to pre-injury body weight. Moreover, SCI reduced the density of putative excitatory (VGLUT2+) and inhibitory (VGAT+) synaptic puncta in the lumbosacral cord, although GBP did not alter these parameters. Our results suggest that continuous GBP treatment alters hemodynamic control after SCI and that decreased synaptic connectivity may contribute to the development of AD.

These studies demonstrate the need for further research to better understand the cellular signaling driving maladaptive plasticity after SCI as well as the complex and dynamic changes in intraspinal synaptic connectivity contributing to the development of AD. Moreover, GBP treatment may offer clinical benefit by reducing blood pressure during AD, however the optimal dosage must be identified to avoid undesired side-effects.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2019.064

Funding Information

This research was supported by the National Institutes of Health grant T32 5T32NS077889 and the Kentucky Spinal Cord and Head Injury Research Trust grant #10-10.

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