Action of morphine on the neuro‐effector transmission in the guinea‐pig ileum and in the mouse vas deferens.

Abstract

Effects of morphine on the neuro‐effector junction of the guinea‐pig ileum or mouse vas deferens were investigated by the micro‐electrode and double sucrose gap methods. 1. Morphine (10(‐8)‐10(‐5) M) did not change the membrane potential, membrane resistance and electrical threshold required to produce the action potential of smooth muscle cells of guinea‐pig ileum or mouse vas deferens. 2. Morphine (10(‐8)‐10(‐7) M) markedly suppressed the amplitude of excitatory junction potential (e.j.p.) of ileum or that of vas deferens. However the same concentration of morphine did not suppress the inhibitory junction potential recorded from the guinea‐pig ileum or the facilitation phenomena observed with repetitive stimulation in the mouse vas deferens. In addition, this opiate (3.5 x 10(‐5) M) did not alter the amplitude or the frequency of miniature excitatory junction potential recorded from the mouse vas deferens. 3. Naloxone (3.5 x 10(‐7) M), itself, exerted no effect on the membrane potential and amplitude of the e.j.p. After pretreatment with naloxone, however, the inhibitory action of morphine on the e.j.p. was suppressed. 4. The extracellularly recorded action potential from the small nerve bundle innervating the mouse vas deferens was not affected by morphine (3.5 x 10(‐5) M). 5. The amplitude of the e.j.p. of the guinea‐pig ileum was dependent on the concentration of [Ca]o. When [Ca]o and the relative amplitude of e.j.p. were plotted on a double logarithmic scale, the above relation yielded a straight line with a slope of 1.7. Application of morphine resulted in a reduction in the slope of the straight line to 1.0. 6. These results indicate that morphine probably suppresses the influx of Ca during spike electrogenesis in the nerve terminal, however, there is no modification of the action of Ca in nerve terminals.