Paradoxical effects of continuous high dose gabapentin treatment on autonomic dysreflexia after complete spinal cord injury

Highlights

• Chronic high-dose GBP after SCI reduces AD evoked by noxious colorectal stimulation.

• High-dose GBP impairs weight gain, promotes splenomegaly and increase sAD events.

• Reduced synaptic densities after SCI are unaltered by high-dose GBP treatment.

• GBP alters induced AD and sAD events distinctly, perhaps acting on vasculature.

• Low- or high-dose acute GBP may be relevant in controlled settings to reduce AD.

Abstract

Spinal cord injury (SCI) can have profound effects on the autonomic and cardiovascular systems, notably with injuries above high-thoracic levels that result in the development of autonomic dysreflexia (AD) characterized by volatile hypertension in response to exaggerated sympathetic reflexes triggered by afferent stimulation below the injury level. Pathophysiological changes associated with the development of AD include sprouting of both nociceptive afferents and ascending propriospinal ‘relay’ neurons below the injury, as well as dynamic changes in synaptic inputs onto sympathetic preganglionic neurons. However, it remains uncertain whether synapse formation between sprouted c-fibers and propriospinal neurons contributes to the development of exaggerated sympathetic reflexes produced during AD. We previously reported that once daily treatment with the anti-epileptic and neuropathic pain medication, gabapentin (GBP), at low dosage (50 mg/kg) mitigates experimentally induced AD soon after injections, likely by impeding glutamatergic signaling. Since much higher doses of GBP are reported to block the formation of excitatory synapses, we hypothesized that continuous, high dosage GBP treatment after SCI might prevent the formation of aforementioned aberrant synapses and, accordingly, reduce the incidence and severity of AD. Adult female rats implanted with aortic telemetry probes for hemodynamic monitoring underwent T4-transection SCI and immediately received 100 mg/kg (i.p.) of GBP and then every six hours (400 mg/kg/day) for 4-weeks after injury. We assessed daily body weight, mean arterial pressure, heart rate, frequency of spontaneous AD, and hemodynamic changes during colorectal distension (CRD) to establish whether high dose GBP treatment prophylactically mitigates both AD and associated aberrant synaptic plasticity. This regimen significantly reduced both the absolute blood pressure reached during experimentally induced AD and the time required to return to baseline afterwards. Conversely, GBP prevented return to pre-injury body weights and paradoxically increased the frequency of spontaneously occurring AD. While there were significant decreases in the densities of excitatory and inhibitory pre-synaptic markers in the lumbosacral dorsal horn following injury alone, they were unaltered by continuous GBP treatment. This indicates distinct mechanisms of action for acute GBP to mitigate induced AD whereas chronic GBP increases non-induced AD frequencies. While high dose prophylactic GBP is not recommended to treat AD, acute low dose GBP may hold therapeutic value to mitigate evoked AD, notably during iatrogenic procedures under controlled clinical conditions.

Introduction

Along with sensory and motor impairments, traumatic spinal cord injury (SCI) results in a constellation of cardiovascular and autonomic dysfunctions (Hou and Rabchevsky, 2014; Krassioukov et al., 2003; Weaver et al., 2012), notably with injuries above the sixth thoracic (T6) segment that frequently result in the development of a condition termed autonomic dysreflexia (AD). This syndrome is characterized by episodes of volatile and potentially lethal hypertension in response to exaggerated sympathetic reflexes triggered by unperceived afferent stimuli below the injury level. Affected individuals may experience cardiac arrhythmia, pounding headache, anxiety, flushing of the skin and profuse sweating above the lesion during an episode of AD. This episodic disorder can occur frequently throughout the day due to regular filling of the bowel and bladder creating noxious stimuli, or by irregular and less predictable triggers such as pressure sores or ingrown toe nails (Karlsson, 1999). Because of the unpleasant and potentially dangerous manifestations, prevention or effective treatment of AD is one of the highest priorities in the SCI community for enhancing quality of life (Anderson, 2004).

Mechanisms known to contribute to the development of AD include the loss of descending vasomotor modulatory pathways, hyperreactivity of peripheral vasculature to adrenergic stimulation, and a number of maladaptive changes within spinal circuitry influencing sympathetic outflow below the lesion (Krassioukov et al., 1999; Schramm, 2006). Intraspinal changes include sprouting of both unmyelinated afferent c-fibers and ascending propriospinal ‘relay’ projections towards sympathetically-correlated interneurons in the thoracic spinal cord, as well as dynamic alterations of synaptic inputs to decentralized sympathetic preganglionic neurons (SPN) in the thoracolumbar spinal cord. Synapses derived from descending supraspinal projections onto SPN are eliminated within one week after complete spinal transection at the fourth thoracic (T4) segment, but by two weeks after injury there is restoration of synaptic terminals onto SPN derived from spinal interneurons and/or primary afferents below the injury (Llewellyn-Smith and Weaver, 2001). Because the development of AD occurs over weeks after SCI in rodents, it has been suggested that injury induced synaptogenesis onto decentralized SPN contributes to AD pathophysiology (Weaver et al., 1997). Based on seminal electrophysiological studies (Krassioukov et al., 2002), a model of AD development has emerged in which the loss of supraspinal control, coupled with convergence from both primary afferents and propriospinal neurons onto sympathetically correlated interneurons, enhances the transmission of noxious stimuli below the injury to SPN, triggering unrestricted sympathetic reflexes during AD (Rabchevsky, 2006); however, this has yet to be thoroughly examined.

We have reported that weeks following complete T4 spinal transection in rats, the administration of the anticonvulsant neuropathic pain medication gabapentin (GBP) significantly reduces the magnitude of colorectal distension (CRD) induced AD and tail spasticity shortly after treatment, lasting several hours (Rabchevsky et al., 2011; Rabchevsky et al., 2012). The mechanism of this clinically valuable effect, however, remains uncertain. GBP was first developed for the treatment of epilepsy and gained subsequent traction as a treatment for neuropathic pain (reviewed in Sirven, 2010). Despite being a structural analog of y-aminobutyric-acid (GABA) with enhanced blood-brain barrier penetration (Crawford et al., 1987), most studies suggest that its actions are independent of GABAergic modulation. GBP's high-affinity binding site is the L-type calcium channel α₂δ subunits on presynaptic terminals (Gee et al., 1996), blocking it decreases intraspinal glutamatergic neurotransmission (Coderre et al., 2007), and large daily doses of GBP are reported to prevent the formation of excitatory glutamatergic synapses in the developing CNS by binding to the α₂δ₁ calcium channel subunit (Eroglu et al., 2009). However, we have found that injured rats treated with GBP an hour prior to CRD weeks later showed significantly reduced AD, whether or not they were treated once daily with saline versus low-dose (50 mg/kg, i.p.) GBP after SCI (Rabchevsky et al., 2012). Because this appears to favor the notion that acute GBP treatment transiently blocks intraspinal glutamatergic neurotransmission which underlies AD (Maiorov et al., 1997), but not synaptogenesis in the injured spinal cord, per se, we sought to determine whether chronic high-dose GBP treatment beginning immediately after SCI alters the development of AD by modulating synaptogenesis of aforementioned intraspinal pathways.