Chemosensitivity and mechanosensitivity of nociceptors from incised rat hindpaw skin.
BACKGROUND
The authors have demonstrated a decrease in pH in the incisional wound environment, suggesting a possible contribution of low pH to postsurgical pain. In this study, the authors characterized the acid-responsiveness of nociceptors innervating the plantar aspect of the rat hind paw 1 day after plantar incision and compared this to plantar skin from unincised control rats.


METHODS
Using the rat glabrous in vitro skin-tibial nerve preparation, afferent nerve activities from single mechanosensitive nociceptors were recorded. Differences in mechanosensitivity, spontaneous activity, and chemosensitivity of units were evaluated. For chemosensitivity, acid-responsiveness of nociceptors to lactic acid (pH 5.5 to 6.5) was studied.


RESULTS
C-fibers showed dose-dependent, sustained responses to lactic acid. A greater proportion of C-fibers from 2 mm or less from the incision was activated by pH 6.0 lactic acid (52.9%) compared to control (14.3%). Total evoked potentials during acid exposure were greater in C-fibers innervating 2 mm or less from the incision compared to those in unincised skin. The prevalence of acid responses and total evoked potentials during acid exposure in C-fibers innervating more than 2 mm from the incision were not different from control. Few A-fibers responded to lactic acid, with a range of pH 5.5 to 6.5 in both incision and control groups. Increased spontaneous activity and mechanosensitivity were also evident.


CONCLUSIONS
C-fibers in the vicinity of the incision showed qualitatively and quantitatively greater chemosensitivity to pH 6.0 lactic acid compared to control. This change was localized to 2 mm or less from the incision, suggesting increased chemosensitivity of nociceptive C-fibers 1 day after plantar incision.
Kang
Sinyoung
S
Department of Anesthesia, University of Iowa Hospitals and Clinics, Iowa City, Iowa 52242, USA. sinyoung-kang@uiowa.edu
Brennan
Timothy J
TJ
D000818:Animals; D002628:Chemoreceptor Cells / Q000502:physiology*; D006614:Hindlimb / Q000294:innervation / Q000502:physiology*; D008297:Male; D008465:Mechanoreceptors / Q000502:physiology*; D036421:Nerve Fibers, Unmyelinated / Q000502:physiology; D009619:Nociceptors / Q000502:physiology*; D010147:Pain Measurement / Q000379:methods; D051381:Rats; D017207:Rats, Sprague-Dawley; D012879:Skin Physiological Phenomena