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Excitatory effects of morphine on toad spinal cord in vitro

Hu Guoyuan

Shanghai Institute of Materia Medica, Academia Sinica. Shanghai, China

Abstract

The effects of morphine on the electrical reflex activities of toad spinal cord in vitro were studied. Morphine 10~(-5)-10~(-3) M enhanced the ventral root reflex response evoked by dorsal root stimulation (DR-VRR). The effect of morphine was not antagonized by naloxone which itself enhanced DR-VRR slightly at a concentration of 10~(-4) M. The phenanthrene ring containing opium alkaloids, codeine and thebaeine (a compound without opiate activity) exerted more potent excitatory effects than morphine. However, these 2 compounds as well as morphine depressed or abolished DR-VRR at still higher concentrations. Methadone, pethidine and FK33-824, an enkephain analogue, which do not contain a phenanthrene ring, did not enhance, but depress DR-VRR. Although morphine blocked spinal presynaptic inhibition, and abolished dorsal root potential evoked by ventral root stimulation (VR-DRP) that can be reversed by naloxone,the enhancement of DR-VRR.by morphine did not seem to be attributed to this disinhibitory effect. The results suggested that excitation of toad spinal cord by morphine was unrelated to opiate receptors but appeared to be characteristic of the alkaloids containing a phenanthrene ring.

Key words: Toad spinal cord in vitro;Ventral root reflex;Response evoked by dorsal root stimulation;Morphine;Codeine

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Citing This Article：

Hu Guoyuan. Excitatory effects of morphine on toad spinal cord in vitro. Acta Physiol Sin 1985; 37 (3): (in Chinese with English abstract).