Morphine has a dual concentration-dependent effect on K(+)-evoked substance P release from rat peripheral airways

Our previous investigations of possible lung mechanisms underlying the effectiveness of nebulized morphine for the relief of dyspnoea, have shown a high density of non-conventional opioid binding sites in rat airways with similar binding characteristics (opioid alkaloid-sensitive, opioid peptide-insensitive) to that of putative mu 3-opioid receptors on immune cells. To investigate whether these lung opioid binding sites are functional receptors, this study was designed to determine (using superfusion) whether morphine modulates the K(+)-evoked release of the pro-inflammatory neuropeptide, substance P (SP), from rat peripheral airways. Importantly, K(+)-evoked SP release was Ca(2+)-dependent, consistent with vesicular release. Submicromolar concentrations of morphine (1 and 200 nM) inhibited K(+)-evoked SP release from rat peripheral airways in a naloxone (1 microM) reversible manner. By contrast, 1 microM morphine enhanced K(+)-evoked SP release and this effect was not reversed by 1 microM naloxone. However, 100 microM naloxone not only antagonized the facilitatory effect of 1 microM morphine on K(+)-evoked SP release from rat peripheral airways but it inhibited release to a similar extent as 200 nM morphine. It is possible that these latter effects are mediated by non-conventional opioid receptors located on mast cells, activation of which causes naloxone-reversible histamine release that in turn augments the release of SP from sensory nerve terminals in the peripheral airways. Clearly, further studies are required to investigate this possibility.