Using their rat model, Zahn et al. designed a set of experiments for human postoperative pain to determine whether mechanically insensitive areas of the receptive field of the dorsal horn neuron could be converted to areas responsive to weak mechanical stimuli. The researchers induced anesthesia with 4% halothane in 59 adult male Sprague-Dawley rats, prepared for intravenous administration of fluids and drugs, and then performed limited laminectomies at the cervical and thoracolumbar level. Electrodes were inserted into the C1 segment for antidromic stimulation, and action potentials of the dorsal horn neurons were recorded. Cells were classified as wide dynamic range (WDR) or high threshold (HT) depending on responses to brushing or pinching of the rat's left foot. HT cells responded to pinch only, whereas WDR neurons responded to brushing with von Frey filaments and even more strongly to pinch stimuli.
Of 50 neurons recorded (29 WDR and 21 HT cells), only 9 showed a sustained increase in background activity after incision. In 9 of 28 WDR neurons, there was a marked decrease in threshold to von Frey filaments applied adjacent to the wound. A blunt mechanical stimulus (a 5-mm plastic disk) applied directly on the incision activated 18 of 22 WDR neurons, but HT cells were not excited by this stimulus after incision. The expansion of pinch receptive fields outside the injury area was common and similar in both WDR and HT neurons. The data suggest that conversion of mechanically insensitive areas of WDR neurons to areas responsive to weak mechanical stimuli could contribute to pain behaviors caused by punctate and blunt mechanical stimuli, but do not explain secondary hyperalgesia, the reasons for which must be examined further.