Does morphine produce presynaptic inhibition in the spinal cord?

Inhibition of the release of substance P in trigeminal slices by opiates (Jessel & Iversen, 1977) provides support for the hypothesis that opiates produce presynaptic inhibition. The authors have used a modification of the excitability testing method (see Schmidt, 1971) to measure possible long-term opiate-produced polarization changes at cutaneous myelinated (A) and unmyelinated (C) afferent fiber terminals in the spinal cord. The results thus show that morphine might act at intraspinal terminals of cutaneous C fibers. However, the excitability changes observed do not reflect primary afferent depolarization as the underlying mechanism. Alternative possibilities for a presynaptic inhibitory action of morphine are discussed.