Both the cholinergic and non-cholinergic components of airway excitation are inhibited by morphine in the guinea-pig.

In order to investigate the effect of morphine on the excitatory and inhibitory non-adrenergic, non-cholinergic (NANC) neurotransmission and cholinergic neurotransmission in airways, ring preparations from guinea-pig trachea and bronchi were studied. The preparations were stimulated by electrical field stimulation to elicit contractions and relaxations. Both cholinergic and NANC contractions were reduced by morphine in a dose-dependent way, while the inhibitory NANC neurotransmission was unaffected. A morphine concentration of 10(-4) M reduced the NANC contractions to 20.8 +/- 1.5% of the control contractions and the cholinergic contractions to 49.7 +/- 4.2%. The effect of morphine on cholinergic neurotransmission was completely inhibited by naloxone (10(-5) M) while the effect of morphine on NANC contractions only was reduced to 67.6 +/- 5.6% of the control contractions. It is suggested that morphine influences the excitatory neurotransmission via pre-junctional sites, since morphine had no effect on the acetylcholine-induced contractions.