Effects of Epinephrine and Cortisol on the Analgesic Activity of Metyrosine in Rats

Some endogenous hormones (epinephrine and cortisol) can change an individual's pain threshold. Propranolol is a non-selective beta adrenergic receptor blocker which antagonises the anti-inflammatory effect of non-steroidal anti-inflammatory drugs via the beta 1 and beta 2 adrenergic receptors. The roles of epinephrine and cortisol were investigated in the analgesic activity of metyrosine in rats with reduced epinephrine levels induced by metyrosine. Pain threshold measurement was performed using an analgesimeter with different doses and the single or combined usage of metyrosine, prednisolone, metyrapone and propranolol in rats. Epinephrine and corticosterone levels were measured by high-performance liquid chromatography in metyrosine-administered rats. Metyrosine reduces the epinephrine levels without affecting the corticosterone levels, thereby creating an analgesic effect. It was determined that prednisolone did not have an analgesic effect in rats with normal epinephrine levels, but its analgesic activity increased with a parallel decrease in the epinephrine levels. Similarly, the combined use of prednisolone and metyrosine provided a stronger analgesic effect than that rendered by metyrosine alone. The strongest analgesic effect, however, was observed in the group of rats with the lowest epinephrine level in whom the metyrosine + prednisolone combination was administered. The findings of this study may be useful in severe pain cases in which the available analgesics are unable to relieve the individual's pain.