Changes in Response Properties and Receptive Fields of Spinal Dorsal Horn Neurons in Rats after Surgical Incision in Hairy Skin

Background: Mechanical hyperalgesia and allodynia associated with chemical irritant application are mediated by spinal high-threshold (HT) as well as wide-dynamic-range neurons as a result of “central sensitization.” Because the pathophysiology of pain is thought to differ depending on the type of injury and may vary between hairy and glabrous skin, the authors examined changes in properties of spinal dorsal horn neurons after surgical incisions in hairy skin of rats to obtain insights into the mechanisms of postoperative pain. Methods: Withdrawal responses to punctate mechanical stimulation and gentle brushing were measured in awake rats in an area adjacent to the injured site (primary area) and in an area 2 cm from the injured site (secondary area) after 1-cm longitudinal incisions through the hairy skin, fascia, and muscle had been made in the hindquarters. In a separate study, responses of spinal wide-dynamic-range, HT, and low-threshold neurons to nonnoxious and noxious stimuli were recorded before and after similar incisions had been made in the centers of their receptive fields. Effects of spinal application of the γ-aminobutyric acid A receptor antagonist bicuculline (15 μg) on responses of HT neurons were then studied. Results: Awake rats showed primary and secondary hyperalgesia to punctate mechanical stimulation 30 min after the incision and thereafter for 4 days and 1 day, respectively. Mechanical allodynia associated with brush stimulation was only seen in the primary area 30 min after the incision and thereafter for 1 day. The incision resulted in increases in activity of wide-dynamic-range neurons (receptive field sizes and responses to both innocuous and noxious stimuli). HT neurons did not respond to innocuous stimulation and showed very small increases or no changes in receptive field size and responses to noxious stimuli after the incision. However, the majority of HT neurons began to respond to innocuous stimuli after application of bicuculline (15 μg/50 μl) to the spinal cord. Conclusions: The results suggest that wide-dynamic-range neurons are responsible for behavioral hyperexcitability after surgical incision but that HT neurons are not involved in the hyperexcitability, despite the fact that HT neurons are capable of responding to innocuous stimuli by reversal of γ-aminobutyric acid–mediated inhibition.