The effects of morphine on the release of noradrenaline from the cat isolated nictitating membrane and the guinea-pig ileum myenteric plexus-longitudinal muscle preparation.

1. Electrical field stimulation of either the cat isolated nictitating membrane or the guinea-pig ileum myenteric plexus-longitudinal muscle preparation caused the release of noradrenaline into the bathing medium. 2. In the cat nictitating membrane, the output per pulse of noradrenaline was constant at frequencies of stimulation from 0.5 to 15 Hz. In the guinea-pig myenteric plexus preparation the output per pulse of noradrenaline increased as the frequency of stimulation was increased from 2 to 16 Hz. 3. Phenoxybenzamine (29.3 muM) caused a marked increase in the noradrenaline output from both the cat nictitating membrane and guinea-pig myenteric plexus preparations. 4. Morphine (0.13-8 muM) inhibited the contractions of the cat nictitating membrane caused by electrical stimulation. This effect was greater at low (1Hz) than at high (15Hz) frequencies of stimulat The site of action is at the nerve-smooth muscle junction. 5. The action of narcotic analgesic drugs on the cat nictitating membrane showed stereospecificity. Naloxone (0.1 muM) reversed the inhibition caused by normorphine (3.2 muM). 6. Morphine (3 muM) reduced the noradrenaline output from the cat nictitating membrane stimulated at 1 Hz but not at 15 Hz. At 1 Hz, the inhibition of noradrenaline output by normorphine (muM) was reversed by naloxone (0.25 muM). 7. Morphine (1.5 muM) did not alter the noradrenaline output from the guinea-pig myenteric plexus preparation stimulated at 2 or 16 Hz.