Postoperative pain is characterized by spontaneous pain at the surgical site and increased pain due to movements. To study postoperative pain mechanisms, we investigated discharge properties of mechano-heat sensitive C-fiber afferents innervating the rat glabrous hindpaw skin 1 day after plantar incision. Behaviors indicating spontaneous pain, heat and mechanical hyperalgesia were present 1 day after incision. Recording of afferents using in vitro glabrous skin-nerve preparation showed that more C-fibers from the incision had spontaneous discharge than control rats. The spontaneously discharging fibers from incised rats had lower heat response threshold compared with fibers without spontaneous activity. In all fibers less than 2 mm from the incision, an increased percentage responded to lower temperatures (35-41°C), the mean heat response threshold was 3.1°C less, the stimulus-response function for heat evoked response was shifted to the left and the total number of impulses in response to a 33-48°C heat stimulus was increased. Heat responses of C-fibers more than 2 mm from the incision, however, were not different from control. The mean mechanical response thresholds, measured by a servo force-controlled stimulator, were not different between groups. The total spikes evoked at supra-threshold mechanical stimulation were not increased in afferents from the incision. In conclusion, 1 day after incision, when behaviors indicating spontaneous pain, heat and mechanical hyperalgesia are present, C-fibers close to incision showed spontaneous discharge and sensitization to heat but not to mechanical stimuli, in vitro.
Bibliographical noteFunding Information:
The authors thank Peter W. Reeh, Jerry Gebhart, Kazue Mizumura and Andrew Strassman for help in the preparation of our experimental set-up. This work was supported by the Department of Anesthesia at the University of Iowa and by National Institutes of Health, Bethesda, Maryland, grant GM 55831 and GM 067762 to T.J.B.
- Plantar incision
- Primary afferent neuron sensitization
- in vitro preparation