Hypoalgesia and hyperalgesia with inherited hypertension in the rat

Many studies indicate that blood pressure control systems can attenuate pai n (hypoalgesia) of short duration; however, we recently found exaggerated n ociceptive responses (hyperalgesia) of persistent duration in the spontaneo usly hypertensive rat (SHR). Here, we used SHR, Dahl Salt-Sensitive (SS), a nd normotensive control rats to evaluate the contribution of sustained elev ations in arterial pressure to nociceptive responses. Compared with Sprague -Dawley and/or Wistar-Kyoto controls, SHR were 1) hypoalgesic in the hot pl ate test and 2) hyperalgesic in longer latency tail and paw-withdrawal test s and in two models of inflammatory nociception. These differences were not observed between SS and salt-resistant controls fed a high-salt diet. Infl ammatory hyperalgesia in SHR was correlated with neither paw edema nor the number of Fos-positive spinal cord neurons. Our results indicate that "pain " phenotype of the SHR is not restricted to hypoalgesia. This phenotype is related to genetic factors or to the autonomic systems that control blood p ressure and not to sustained elevations in blood pressure, differences in s pinal neuron activity, or inflammatory edema.