Three functionally distinct classes of C-fibre nociceptors in primates.
Level 5 - mechanism / opinion, no new human data
Preclinical animal/bench neurophysiology and imaging study
PubMed 24947823 · doi:10.1038/ncomms5122
What was done
Researchers evaluated primate mechanically sensitive C-fibre polymodal nociceptors categorized as quick-responding (QC) or slow-responding (SC) to heat stimuli. They compared their responses to mild burn, heat sensitization, conductive properties, and chemosensitivity following application of capsaicin and the MrgprD agonist β-alanine. Mechanically insensitive C-fibres (C-MIAs) were tested with β-alanine, capsaicin, and histamine. Calcium imaging of primate dorsal root ganglion neurons was performed to examine cellular activation patterns.
What was found
All QC fibres responded vigorously to capsaicin and β-alanine. Only 40% of SC fibres responded to β-alanine, exhibiting half the response magnitude of QCs. C-MIAs did not respond to β-alanine but responded vigorously to both capsaicin and histamine with distinct discharge patterns. Calcium imaging demonstrated that β-alanine and histamine activated separate subpopulations of capsaicin-responsive dorsal root ganglion neurons. Absolute sample sizes, variances, and statistical significance values were not reported in the abstract.
Why it matters
The findings identify three functionally separate classes of primate C-fibre nociceptors and provide evidence that histamine-induced itch and capsaicin-induced pain are segregated in peripheral C-MIAs, identifying a primate equivalent to murine MrgprD-expressing afferents.
Limits
The abstract reports on non-human primate neurophysiology and in vitro cellular imaging, which may not fully reflect human in vivo perception. Sample sizes for animals, recorded single units, and imaged neurons are omitted from the abstract.
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- supports The paresthesia/tingling caused by beta-alanine is mediated by sensory receptor activation rather than a histamine dump.