Chronic unpredictable stress inhibits nociception in male rats.

Chronic stress elicits remarkable alterations to the structure and function of several areas of the central nervous system. Nociception is known to be affected by chronic stress and age, although the observations are contradictory. Herein we report that both young and old rats submitted to a chronic unpredictable stress paradigm have reduced nociception in the tail-flick nociceptive test. Moreover, stressed animals show an increase in nociceptive threshold after three successive exposures to noxious stimulation (within a 2 min interval). While the sustained stress-induced analgesia is usually attributed to the resulting hypercorticalism, the immediate exacerbation of tolerance to pain displayed by stressed animals is most likely mediated through other mechanisms due to the very rapid antinociceptive effect observed.