Activity of the descending noradrenergic pathway after surgery in rats

Background: Previous studies have shown that activation of the descending noradrenergic inhibition pathway results in analgesia after surgery. However, the time course of activity of the descending noradrenergic pathway after surgery has not been examined previously. Here, we investigated the spinal release of noradrenaline (NA) in the post-operative period in a freely moving rat model of incisional pain. Methods: Loop microdialysis catheters were implanted subarachnoidally via the atlanto-occipital membrane in Sprague-Dawley rats. Twelve healthy rats without neural deficits were divided into two groups, Group A and Group B, following 5 days of recovery. A plantar incision in the right hind paws of rats in Group A was performed under 1.2% isoflurane. All rats in Group B were only anesthetized by 1.2% isoflurane for the same duration. The microdialysate samples for NA determination were collected before anesthesia, 3 h and 1, 2 and 3 days after incision (or isoflurane anesthesia in Group B) in both groups. The cumulative pain scores were assessed at the above time points. Results: The spinal release of NA increased gradually, peaked at 2 days after the incision and remained at the peak level up to the third day after the incision. The cumulative pain scores peaked 3 h after the incision, and gradually decreased afterwards and returned to the baseline values 3 days after the incision. Conclusions: The descending NA tone might be apparently more active in the post-operative period. The descending noradrenergic inhibitory pathway plays an important role in post-operative neuroplasticity.