Although axons within neuromas have been shown to produce inappropriate spontaneous ectopic discharges, the molecular basis for pain in patients with neuromas is still not fully understood. Because sodium channels are known to play critical roles in neuronal electrogenesis and hyperexcitability, we examined the expression of all the neuronal voltage-gated sodium channels (Nav1.1, Nav1.2, Nav1.3, Nav1.6, Nav1.7, Nav1.8, and Nav1.9) within human painful neuromas. We also examined the expression of two mitogen-activated protein (MAP) kinases, activated p38 and extracellular signal-regulated kinases 1 and 2 (ERK1/2), which are known to contribute to chronic pain, within these human neuromas.
We used immunocytochemical methods with specific antibodies to sodium channels Nav1.1, Nav1.2, Nav1.3, Nav1.6, Nav1.7, Nav1.8, and Nav1.9, and to activated MAP kinases p38 and ERK1/2 to study by confocal microscopy control and painful neuroma tissue from five patients with well-documented pain.
We demonstrate upregulation of sodium channel Nav1.3, as well as Nav1.7 and Nav1.8, in blind-ending axons within human painful neuromas. We also demonstrate upregulation of activated p38 and ERK1/2 MAP kinases in axons within these neuromas.
These results demonstrate that multiple sodium channel isoforms (Nav1.3, Nav1.7, and Nav1.8), as well as activated p38 and ERK1/2 MAP kinases, are expressed in painful human neuromas, indicating that these molecules merit study as possible therapeutic targets for the treatment of pain associated with traumatic neuromas.
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