Inhibitory action of morphine on the release of a bradykinin-like substance after sciatic nerve stimulation.

Effects of morphine on the release of a bradykinin-like substance into the subcutaneous perfusate of the rat paw, elicited by pinching and heating of the foot instep and by electrical stimulation of the sciatic nerve, were investigated. Morphine (5 mg/kg i.m.) inhibited the release of a bradykinin-like substance only when sciatic nerve stimulation was applied. The release due to sciatic nerve stimulation was not inhibited by gallamine (8 mg/kg i.p.), but significantly by physostigmine (1 mg/kg i.p.). The release was also observed by adrenergic agents and was inhibited by adrenergic alpha-blocking agents. No release of a bradykinin-like substance due to sciatic nerve stimulation was observed in reserpinized and 6-hydroxydopaminized rats. Dopa restored the releasing ability due to sciatic nerve stimulation in the reserpinized rats, but not in the 6-hydroxydopaminized ones. The results suggest that the inhibitory action of morphine can be explained by an inhibition of the release of noradrenaline.