Epinephrine Suppresses Stress-Induced Increases in Plasma Immunoreactive β-Endorphin in Humans

The present study evaluated the hypothesis that increased plasma levels of epinephrine (EPI) stimulate immunoreactive β-endorphin (iβEND) secretion in humans experiencing a mild stress. The stressor consisted of intraoral injections of a local anesthetic solution (with or without EPI) just before the surgical extraction of impacted third molars in 26 awake unsedated patients. The EPI group experienced a 30-fold increase in plasma EPI levels by 2 min after injection; these concentrations were physiologically active, as evidenced by increased pulse rate and systolic blood pressure. However, compared to a no EPI control group the EPI group had a significantly reduced iβEND response to the stressor, as evaluated by comparison of plasma levels at individual time points, maximal increases in plasma iβEND levels, and areas under the timeresponse curve. Whereas there was no association between plasma levels of EPI and iβEND in the EPI group (r = 0.119; P = NS), EPI and iβEND levels were strongly related in the no EPI group (r = 0.82; P < 0.001). These results do not support the hypothesis of a stimulatory effect for EPI on iβEND release and, instead, suggest that an inhibitory relationship may exist in humans experiencing stress. The association between EPI and iβEND responses observed in the control group during this form of stress appears to be due to activation of a common central neural element.