Elsevier

Neuroscience

Volume 311, 17 December 2015, Pages 499-507
Neuroscience

Antihyperalgesic effect of tetrodotoxin in rat models of persistent muscle pain

https://doi.org/10.1016/j.neuroscience.2015.10.059Get rights and content

Highlights

  • While persistent muscle pain is very common and disabling it lacks effective therapy.

  • Tetrodotoxin is a promising analgesic but has not been evaluated for muscle pain.

  • Tetrodotoxin did not increase muscle nociceptive threshold in control naïve rats.

  • Tetrodotoxin attenuated mechanical hyperalgesia in models of persistent inflammatory and ergonomic muscle pain.

Abstract

Persistent muscle pain is a common and disabling symptom for which available treatments have limited efficacy. Since tetrodotoxin (TTX) displays a marked antinociceptive effect in models of persistent cutaneous pain, we tested its local antinociceptive effect in rat models of muscle pain induced by inflammation, ergonomic injury and chemotherapy-induced neuropathy. While local injection of TTX (0.03–1 μg) into the gastrocnemius muscle did not affect the mechanical nociceptive threshold in naïve rats, exposure to the inflammogen carrageenan produced a marked muscle mechanical hyperalgesia, which was dose-dependently inhibited by TTX. This antihyperalgesic effect was still significant at 24 h. TTX also displayed a robust antinociceptive effect on eccentric exercise-induced mechanical hyperalgesia in the gastrocnemius muscle, a model of ergonomic pain. Finally, TTX produced a small but significant inhibition of neuropathic muscle pain induced by systemic administration of the cancer chemotherapeutic agent oxaliplatin. These results indicate that TTX-sensitive sodium currents in nociceptors play a central role in diverse states of skeletal muscle nociceptive sensitization, supporting the suggestion that therapeutic interventions based on TTX may prove useful in the treatment of muscle pain.

Introduction

For many decades tetrodotoxin (TTX), which is present in many poisonous animals including fishes from the Tetraodontidae family such as the pufferfish, has been used as a pharmacological tool to selectively block a subset of inward sodium currents (TTX-S INa) in electrophysiological recordings (Narahashi, 2008). Indeed, in vitro studies have shown that TTX is able to inhibit the conduction of action potentials in isolated nerve preparations (Muroi et al., 2011) and to block INa in neurons from sensory ganglia (Blair and Bean, 2002, Muroi et al., 2011). The current subsets identified by TTX have been demonstrated to depend on specific voltage-gated sodium channels (VGSC): TTX-sensitive (TTX-S) sodium channels, such as α-subunit of mammalian voltage-gated sodium channels (Nav) Nav1.1, Nav.1.3, Nav1.6 and Nav1.7 which are blocked by TTX at nanomolar concentrations, and TTX-resistant (TTX-R) sodium channels, such as Nav1.8 and Nav1.9 which are blocked by TTX only at micromolar concentrations (Dib-Hajj et al., 2009). This potent sodium channel block can explain the classical local symptoms of exposure to this toxin (e.g., fugu poisoning), including oral numbness, tingling and anesthesia (Bane et al., 2014, You et al., 2015). These properties are consistent with the strong antinociceptive effect exhibited by TTX in a number of in vivo pre-clinical (Lyu et al., 2000, Marcil et al., 2006, Nieto et al., 2008) and clinical (Hagen et al., 2008, Hagen et al., 2011, Shi et al., 2009, Song et al., 2011) studies. Importantly, while the expression of VGSC varies between sensory neurons contributing to different pain symptoms (Minett et al., 2014), the antinociceptive effects of TTX have, however, been mainly studied in models of cutaneous pain.

While chronic muscle pain is an extremely common and disabling group of syndromes, which lack effective therapy, it has received much less attention than cutaneous pain. This is probably due to the fact that clinical entities related to chronic muscle pain, such as neuropathic muscle pain, are still not well characterized. Because of this scarcity of preclinical muscle pain models, most of the preclinical screening of new analgesic drugs is performed in models assessing cutaneous nociception. TTX-S VGSC have been reported to be present in dorsal root ganglion (DRG) nociceptors innervating the skeletal muscle (Ramachandra et al., 2012, Ramachandra and Elmslie, 2014), and nociceptive spinal monosynaptic reflexes are attenuated after exposure of sensory fibers innervating the skeletal muscle to TTX as observed in in vivo preparations (Schomburg et al., 2012). Furthermore, large-diameter sensory neurons, likely innervating the skeletal muscle, exhibit de novo expression of TTX-S VGSC after spinal nerve injury (Fukuoka et al., 2015). However, whether TTX is able to produce antinociceptive effects in models of persistent muscle pain remains to be determined. Thus, given the clinical and societal importance of persistent muscle pain and the promising profile of TTX as a putative analgesic, we explored its antinociceptive effects in models of nociceptive inflammatory, ergonomic and neuropathic muscle pain.

Section snippets

Animals

Adult male Sprague–Dawley rats (initial weight 250–300 g; Charles River, Hollister, CA, USA) were used in these experiments. They were housed in the Laboratory Animal Resource Center facility at the University of California San Francisco, under environmentally controlled conditions (lights on 07:00–19:00 h; room temperature 21–23 °C) with food and water available ad libitum. Upon completion of experiments, rats were euthanized by CO2 induced asphyxia followed by bilateral thoracotomy. Animal care

Effect of TTX on muscle nociceptive threshold

While the presence of TTX-S sodium channels in primary afferents innervating skeletal muscle has been demonstrated (Ramachandra et al., 2012), whether the local administration of TTX is able to modify the baseline mechanical nociceptive threshold in the skeletal muscle remains to be established. To test this we evaluated the effect of cumulative incremental doses of TTX (0.03, 0.1, 0.3, and 1 μg/20 μl) on nociceptive threshold in normal control rats. We found that, while the TTX doses of 0.1 and

Discussion

The safe and effective treatment of muscle pain remains a major problem in clinical medicine, with most of the currently used analgesics lacking in adequate efficacy and safety. Since persistent pain is, in most cases, dependent on activity in primary afferent nociceptors, it has long been a goal of translational pain research to develop effective therapies that would normalize nociceptor function (Richards and McMahon, 2013). Toward this end we explored whether TTX affected the protective

Conclusions

In summary, we provide evidence that TTX displays important antihyperalgesic effects on rat models of persistent muscle pain, without interfering with the nociceptor function to signal for further potentially harmful stimuli. Given its safety and efficacy profiles exhibited in patients affected by cancer pain (Hagen et al., 2008, Hagen et al., 2011) or withdrawal symptoms in heroin addicts (Shi et al., 2009, Song et al., 2011) and the present data, TTX might be a useful therapeutic agent in the

Acknowledgments

Authors thank Lindsay Conner for excellent technical assistance. This work was support by a grant from the National Institutes of Health (NIH) AR063312 and a grant from the UCSF Academic Senate Committee on Research. Authors report no conflicts of interest.

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