Baroreceptor reflex is suppressed in rats that develop hyperalgesia behavior after nerve injury

The baroreceptor reflex buffers autonomic changes by decreasing sympathetic activity and increasing vagal activity in response to blood pressure elevations, and by the reverse actions when the blood pressure falls. Because of the many bidirectional interactions of pain and autonomic function, we investigated the effect of painful nerve injury by spinal nerve ligation (SNL) on heart rate (HR), blood pressure (BP) and their regulation by the baroreceptor reflex. Rats receiving SNL were separated into either a hyperalgesic group that developed sustained lifting, shaking and grooming of the foot after plantar punctate nociceptive stimulation by pin touch or a group of animals that failed to show this hyperalgesic behavior after SNL. SNL produced no effect on resting BP recorded telemetrically in unrestrained rats compared to control rats receiving either skin incision or sham SNL. However, two tests of baroreceptor gain showed depression only in animals that developed sustained hyperalgesia after SNL. The animals that failed to develop hyperalgesia after SNL were found to have elevations in HR both before and for the first 4 days after SNL, and HR variability analysis gave indications of decreased vagal control of resting HR and elevated sympatho-vagal balance at these same time intervals. In human patients, other research has shown that blunted baroreceptor reflex sensitivity predicts poor outcome during conditions such as hypertension, congestive heart failure, myocardial infarction, and stroke. If baroreceptor reflex suppression is also found in human subjects during chronic neuropathic pain, this may adversely affect survival.