Negative affect, pain and sex: The role of endogenous opioids

Opioid neurotransmission modulates pain and negative affect during psychological stress. To determine whether these effects differ between men and women, the opioid receptor antagonist naltrexone or placebo was administered double-blind to 21 men and 22 women before they completed 30 min of difficult mental arithmetic. To heighten negative affect, participants received seven moderately noxious electric shocks during the math task, which were believed to be contingent upon performance. Before and after the math task, participants rated pain intensity and unpleasantness while their left hand was immersed in 2 °C water for up to 4 min. Anxiety, discouragement and anger were also rated before, during and after the math task. Tolerance of cold-induced pain was greater in men, whereas discouragement during the math task was greater in women. Opioid blockade did not influence ratings of negative affect, which increased in line with the intensity and unpleasantness of shock-induced pain. The intensity and unpleasantness of cold-induced pain increased after the math task only in women administered naltrexone. Within the naltrexone condition, pain ratings increased most in the most discouraged subjects. However, this relationship was absent in placebo recipients, implying that the hyperalgesic effect of psychological distress was tempered by opioid release. Greater stress-evoked discouragement in women than men may explain why cold-induced pain increased after the math task only in women administered naltrexone.