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1Department of Oral Surgery and Diagnostic Sciences, Division of Neuroscience, J. H. Miller Health Center, University of Florida College of Dentistry and McKnight Brain Institute, 2Department of Physiological Sciences, College of Veterinary Medicine and University of Florida, McKnight Brain Institute, and 3Department of Neuroscience, College of Medicine and University of Florida, McKnight Brain Institute, Gainesville, Florida
Submitted 17 August 2005; accepted in final form 20 December 2005
We contrasted the physiology and peripheral targets of subclassified nociceptive and nonnociceptive afferents that express acid-sensing ion channel (ASIC)–like currents. The threshold for current activation was similar in eight distinct cell subclasses regardless of functional modality (pH 6.8). When potency was determined from concentration–response curves, nonnociceptors exhibited currents with significantly greater potency than that of all but one class of nociceptors (pH50 = 6.54 and 6.75 vs. 6.20–6.34). In nonnociceptive cells, acid transduction was also confined to a very narrow range (0.1–0.3 vs. 0.8–1.4 pH units for nociceptors). Simultaneous whole cell recording and ratiometric imaging of three peptidergic nociceptive classes were consistent with the expression of Ca2+-permeable ASICs. Sensitivity to psalmotoxin and flurbiprofen indicated the presence of Ca2+-permeable ASIC1a. Immunocytochemistry on these subclassified populations revealed a differential distribution of five ASIC proteins consistent with Ca2+ permeability and differential kinetics of proton-gated currents (type 5: ASIC1a, 1b, 2a, 2b, 3; type 8a: ASIC1a, 1b, 3; type 8b: ASIC1a, 1b, 2a, 2b, 3). Using DiI tracing, we found that nociceptive classes had discrete peripheral targets. ASIC-expressing types 8a and 9 projected to hairy skin, but only types 8a and 13 projected to glabrous skin. Non-ASIC–expressing types 2 and 4 were present only in hairy skin. We conclude that ASIC-expressing nociceptors differ from ASIC-expressing nonnociceptors mainly by range of proton reactivity. ASIC- as well as non-ASIC–expressing nociceptors have highly distinct cutaneous targets, and only one class was consistent with the existence of a generic C polymodal nociceptor (type 8a).
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