The relationship between cardiovascular and pain regulatory systems.

An increasing amount of anatomical, physiological, and pharmacological evidence suggest that pain inhibitory circuitry is linked with cardiovascular regulatory systems in man and laboratory animals. Induction of hypertension in rats by different methods (mineralocorticoid treatment, stenosis of renal artery, or social deprivation) is associated with reduced responsiveness to noxious thermal stimuli (hot-plate) or to noxious mechanical stimuli (paw pressure). Genetically hypertension-prone rats derived from the SABRA strain and spontaneously hypertensive rats derived from Wistar/Kyoto strain also display a similar hypoalgesia. Acute increases in blood pressure are associated with reduced sensitivity to painful stimuli. Additionally, the interaction between blood pressure and pain perception has also been supported by the demonstration that various experimental interventions that diminish the magnitude of hypertension also attenuate the hypoalgesia. Recent clinical findings are also in agreement with the laboratory animal findings since sensory and pain thresholds have been shown to be significantly higher in unmedicated essential hypertensive subjects compared to normotensive controls. Thus, the human data corroborate animal data and suggest that a relation between blood pressure and pain sensitivity is likely to be a general phenomenon. It is unlikely that damage to peripheral pain fibers caused by a change in blood pressure contributes to the observed hypoalgesia. Naloxone, which has no effect on blood pressure, returns the pain sensitivity to normal levels. Behavioral tests (open field and motor activity cage) of normotensive and of renal and genetically (SBH and SHR) hypertensive rats exclude the possibility of a general motor deficit in hypertensive rats. Endogenous opioid peptides in central and peripheral nervous systems as well as in endocrine organs are implicated, although non-opioid mechanisms are also evident. Activation of baroreceptor afferents by acute or chronic increases in arterial or venous blood pressure may play an important role in the somatosensory responses associated with the increase in blood pressure. Coordinated cardiovascular-pain regulatory responses may be part of an adaptive mechanism that helps the body to face stressful events.