Effect of morphine on the pre- and postsynaptic inhibitions in the spinal cord

Abstract

The effect of morphine on spinal inhibitory mechanisms was investigated in intact and spinal cats. A relatively small dose (5 mg/kg) of morphine facilitated post-synaptic inhibition of the monosynaptic reflex in intact cats, but not in spinal cats. This result suggested that small doses of morphine may primarily facilitate the supraspinal inhibitory system from which the descending inhibitory effect is derived. In spinal cats, however, large doses (10 mg/kg) of morphine facilitated post-synaptic inhibition. Presynaptic inhibition of the monosynaptic reflex or the dorsal root potential was not augmented by morphine. On the basis of analogy of synaptic activities between the motoneurone and the sensory neurone, it was suggested that enhancement by morphine of the central descending inhibitory influences on the spinal sensory transmission may be exerted post-synaptically.