Ketanserin potentiates morphine-induced antinociception mediated by kappa-receptor activation

How can we treat patients with reduced morphine doses without loosing the pain killing effect or morphine antinociceptive effects (MAE)? To address this question, we hypothesized that serotonin (5-HT2) receptor antagonism could enhance MAE mediated by kappa-opioid receptors. We pretreated mice with ketanserin, a 5-HT2 receptor antagonist, and measured the morphine dose required to observe analgesia. The morphine dose effective in 50% of animals (ED(50)) was reduced from 4.7 to 1.3mg/kg, and the morphine dose effective in 100% of animals (ED(max)) from 13.7 to 2.5mg/kg. Ketanserin has a similar enhancer effect when morphine, which has a dual role via mu and kappa receptors, was substituted by the antinociceptive spiradoline, a selective κ-opioid agonist. At a morphine dose of 3.5mg/kg, 30% of the mice showed antinociceptive behaviour, rising to 100% when ketanserin was co-administered and then reduced to 20% in the presence of nor-binaltorphimine, a kappa-opioid receptor antagonist. Our data strongly suggests a serotonergic inhibition of the kappa-opioid component of MAE and the possibility that this serotonergic inhibition could be reversed through 5-HT2 receptor antagonism.