tetrodotoxin and Burns

Burn injuries have been identified as the primary cause of injury in 5% of U.S. military personnel evacuated from Operations Iraqi Freedom and Enduring Freedom. Severe burn-associated pain is typically treated with opioids such as fentanyl, morphine, and methadone. Side effects of opioids include respiratory depression, cardiac depression, decrease in motor and cognitive function, as well as the development of hyperalgesia, tolerance and dependence. These effects have led us to search for novel analgesics for the treatment of burn-associated pain in wounded combat service members. Tetrodotoxin (TTX) is a selective voltage-gated sodium channel blocker currently in clinical trials as an analgesic. A phase 3 clinical trial for cancer-related pain has been completed and phase 3 clinical trials on chemotherapy-induced neuropathic pain are planned. It has also been shown in mice to inhibit the development of chemotherapy-induced neuropathic pain. TTX was originally identified as a neurotoxin in marine animals but has now been shown to be safe in humans at therapeutic doses. The antinociceptive effects of TTX are thought to be due to inhibition of Na(+) ion influx required for initiation and conduction of nociceptive impulses. One TTX sensitive sodium channel, Nav1.7, has been shown to be essential in lowering the heat pain threshold after burn injuries. To date, the analgesic effect of TTX has not been tested in burn-associated pain. Male Sprague-Dawley rats were subjected to a full thickness thermal injury on the right hind paw. TTX (8 μg/kg) was administered once a day systemically by subcutaneous injection beginning 3 days post thermal injury and continued through 7 days post thermal injury. Thermal hyperalgesia and mechanical allodynia were assessed 60 and 120 min post injection on each day of TTX treatment. TTX significantly reduced thermal hyperalgesia at all days tested and had a less robust, but statistically significant suppressive effect on mechanical allodynia. These results suggest that systemic TTX may be an effective, rapidly acting analgesic for battlefield burn injuries and has the potential for replacing or reducing the need for opioid analgesics.

Topics: Analgesics; Analgesics, Opioid; Animals; Burns; Hot Temperature; Hyperalgesia; Male; Morphine; Pain; Physical Stimulation; Rats, Sprague-Dawley; Tetrodotoxin

Marked hypersensitivity to heat and mechanical (pressure) stimuli develop after a burn injury, but the neural mechanisms underlying these effects are poorly understood. In this study, we establish a new mouse model of focal second-degree burn injury to investigate the molecular and cellular basis for burn injury-induced pain. This model features robust injury-induced behavioral effects and tissue-specific altered cytokine profile, but absence of glial activation in spinal dorsal horn. Three voltage-gated sodium channels, Na(v)1.7, Na(v)1.8, and Na(v)1.9, are preferentially expressed in peripheral somatosensory neurons of the dorsal root ganglia (DRGs) and have been implicated in injury-induced neuronal hyperexcitability. Using knock-out mice, we provide evidence that Na(v)1.7 selectively contributes to burn-induced hypersensitivity to heat, but not mechanical, stimuli. After burn model injury, wild-type mice display increased sensitivity to heat stimuli, and a normally non-noxious warm stimulus induces activity-dependent Fos expression in spinal dorsal horn neurons. Strikingly, both effects are absent in Na(v)1.7 conditional knock-out (cKO) mice. Furthermore, burn injury increases density and shifts activation of tetrodotoxin-sensitive currents in a hyperpolarized direction, both pro-excitatory properties, in DRG neurons from wild-type but not Na(v)1.7 cKO mice. We propose that, in sensory neurons damaged by burn injury to the hindpaw, Na(v)1.7 currents contribute to the hyperexcitability of sensory neurons, their communication with postsynaptic spinal pain pathways, and behavioral thresholds to heat stimuli. Our results offer insights into the molecular and cellular mechanisms of modality-specific pain signaling, and suggest Na(v)1.7-blocking drugs may be effective in burn patients.

Topics: Activating Transcription Factor 3; Analysis of Variance; Animals; Biophysics; Burns; Calcitonin Gene-Related Peptide; Calcium; Cells, Cultured; Cytokines; Disease Models, Animal; Edema; Electric Stimulation; Functional Laterality; Ganglia, Spinal; Glycoproteins; Hot Temperature; Hyperalgesia; Membrane Potentials; Mice; Mice, Inbred C57BL; Mice, Transgenic; NAV1.7 Voltage-Gated Sodium Channel; NAV1.8 Voltage-Gated Sodium Channel; NAV1.9 Voltage-Gated Sodium Channel; Neuralgia; Neuroglia; Pain Threshold; Patch-Clamp Techniques; Proteins; RNA, Messenger; RNA, Untranslated; Sensory Receptor Cells; Sodium Channel Blockers; Sodium Channels; Tetrodotoxin; Transfection