Changes in cardiovascular parameters and plasma norepinephrine level in rats after chronic constriction injury on the sciatic nerve

To evaluate whether neuropathic pain affects autonomic nervous activities, we investigated daily change in cardiovascular parameters and plasma norepinephrine (NE) in free-moving rats after chronic constriction injury (CCI) on the sciatic nerve. Arterial blood pressure (BP), heart rate (HR), and the power spectrum of pulse interval variability were analyzed. Daily change in motor activity and nociceptive behavior was also measured from some CCI rats. In others, NE from daily blood samples was quantified and spontaneous pain was evaluated by daily monitoring of foot guarding behavior. We identified three stages in the daily change of cardiovascular parameters and plasma NE level over 3 weeks following CCI. The first stage (up to 3 days after the surgery) was characterized by increased MAP and HR, especially in the daytime, even though plasma NE was unchanged and motor activity decreased. The second stage (mid first to mid second postoperative weeks) was characterized by increased daytime MAP and HR, and the animals developed punctate hyperalgesia in the affected hindpaw. An NE surge that may have been related to spontaneous pain was present 3-5 days after CCI. The third stage, which appeared after the second postoperative week, was characterized by normalized MAP and decreased HR, and increased high-frequency (0.8-3.0Hz) power in pulse interval variability, which is an index of cardiac parasympathetic tone. These results demonstrated that cardiovascular function was kept high through sympathetic and non-sympathetic activity for 2 weeks after CCI, followed by a predominance of parasympathetic tone.