Modulatory role of endogenous adrenaline in propofol-related nociceptive responses in rats

Propofol is a short-acting intravenous hypnotic anesthetic widely used for sedation and the induction and maintenance of general anesthesia. Although it reliably produces unconsciousness, its intrinsic analgesic efficacy remains a subject of debate. Accumulating evidence indicates that propofol suppresses catecholaminergic activity, and inhibition of endogenous adrenaline may therefore limit its ability to modulate nociceptive processing. Accordingly, this study aimed to investigate the nociceptive suppressions of subhypnotic and anesthetic doses of propofol in rats and their relationship with endogenous adrenaline levels.

Thirty-six male Wistar rats were randomly allocated into six experimental groups: healthy healthy control, subhypnotic-dose propofol (25 mg/kg; PRO-25, intraperitoneally), anesthetic-dose propofol (50 mg/kg; PRO-50, intraperitoneally), adrenaline-alone (0.3 mg/kg; ADRG, intraperitoneally), and adrenaline combined with subhypnotic or anesthetic-dose propofol (PRAD-25 and PRAD-50). Mechanical paw withdrawal thresholds, anesthetic duration, behavioral responses to standardized scalpel incision, and plasma adrenaline levels were evaluated using statistical analyses.

Propofol administered alone failed to produce significant nociceptive suppressionat either dose and induced dose-dependent suppression of circulating adrenaline, whereas adrenaline alone exerted no significant effect on nociceptive thresholds. In contrast, combined administration significantly increased pain thresholds at subhypnotic doses and abolished responses to surgical incision at anesthetic doses despite comparable anesthetic duration. Notably, co-administration restored serum adrenaline levels toward physiological values at lower, but not higher, propofol doses.

These findings provide experimental results suggesting that nociceptive responses observed during propofol exposure may be modulated by endogenous adrenergic activity. This interaction may contribute to explaining the dissociation between anesthetic depth and nociceptive suppression and points to a catecholaminergic influence on anesthetic-nociceptive dynamics that has not been sufficiently investigated previously.