D2-like receptors in the descending dopaminergic pathway are not involved in the decreased postoperative nociceptive threshold induced by plantar incision in adult rats

Abstract

BackgroundApproximately half of all patients who undergo surgery develop postoperative pain, the mechanisms of which are not well understood by anesthesiologists. D2-like receptors in the descending dopaminergic pathway play an important role in regulation of pain transmission in the spinal cord. Impairment of inhibitory neurons in the spinal cord is suggested as part of the mechanism for neuropathic pain, which is one component of postoperative pain. The purpose of this study was to investigate whether impairment of D2-like receptors in the descending dopaminergic pathway in the spinal cord is involved in the decreased postoperative nociceptive threshold in rats.MethodsMale Sprague-Dawley rats (250–300 g) were anesthetized with sevoflurane and an intrathecal (IT) catheter was implanted. Six days later, a plantar incision was made. On the following day, saline, a D2-like receptor agonist (quinpirole), or a D2-like receptor antagonist (sulpiride) was administered intrathecally. Thermal and mechanical nociceptive responses were assessed by exposure to infrared radiant heat and the von Frey filament test before and after plantar incision.ResultsPlantar incision decreased both thermal latency and the mechanical nociceptive threshold. IT administration of quinpirole inhibited the nociceptive responses induced by plantar incision, but sulpiride had no effect.ConclusionA D2-like receptor agonist had antinociceptive effects on the hypersensitivity response triggered by a surgical incision, but a D2-like receptor antagonist had no effect on this response. These results suggest that impairment and/or modification of D2-like receptors in the descending dopaminergic pathway in the spinal cord is not involved in the postoperative decrease in nociceptive threshold.

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“…Thermal hypersensitivity was evaluated using a plantar test apparatus, as previously reported [22]. Briefly, an infrared radiant heat source was focused on the middle of the plantar surface of the hind paw, and time taken for the paw with-Journal of Behavioral and Brain Science drawal was measured automatically.…”

“…Mechanical nociceptive thresholds were evaluated using an automated Von Frey-type dynamic plantar esthesiometer, as described before [22] [23]. Briefly, each rat was placed in a plastic cage with a wire mesh floor.…”