The mineralocorticoid receptor antagonist spironolactone enhances morphine antinociception.

Aims Spironolactone, an antimineralocorticoid, has been reported to potentiate the cataleptic effect of morphine in the rat. Since no previous research exists on the matter and the interaction might be clinically significant, the effects of spironolactone on morphine antinociception and pharmacokinetics in the rat were investigated. Methods Male SD rats were used to assess the effects of spironolactone on acute morphine-induced antinociception, development of morphine tolerance, and established morphine tolerance in the tail-flick and hot plate tests. Spironolactone was also administered with loperamide to assess whether spironolactone enhances the brain distribution of the acknowledged P-glycoprotein substrate across the blood-brain barrier. Results Spironolactone had no antinociceptive effects of its own but when co-administrated with morphine the antinociceptive effect of morphine was greatly enhanced. Morphine concentrations in the brain were increased fourfold in the spironolactone co-administrated group. Spironolactone did not inhibit the formation of pro-nociceptive morphine-3-glucuronide, nor did inhibit the development of tolerance. The peripherally restricted opioid, loperamide, had no antinociceptive effects by itself, but co-administration with spironolactone produced a clear change in the hot plate test. Conclusions Although mineralocorticoids have been proposed to take part in pain signaling, in our setting spironolactone did not have antinociceptive properties of its own. The increased antinociceptive effect of morphine is apparently caused by the increased morphine brain concentrations. We suggest this to be due to P-glycoprotein inhibition, as indicated by the loperamide assay. The clinical relevance of P-glycoprotein inhibition by spironolactone should be studied.