Effect of chronic pain on morphine-induced respiratory depression in mice

▶Morphine (MOR)-induced respiratory depression (RD) was not observed in nerve-ligated mice. ▶This attenuation was reversed by 5-HT4a receptor antagonist. ▶5-HT4 receptor agonist, mosapride, antagonized the MOR-induced RD. ▶Expression of 5-HT4a receptor in brainstem was enhanced in nerve-ligated mice. ▶5-HT4a receptor up-regulation under chronic pain causes decrease in MOR-induced RD.