Presynaptic excitability changes induced by morphine in single cutaneous afferent C- and A-fibers.

Intraspinal microstimulation was used to test the excitability of single cutaneous (sural) A- and C-fibers at their central terminals in the cat. Systemic morphine (1.0-2.6 mg/kg) increased the antidromic threshold to 110% (mean value) of control in C-fibers. In some of the C-fibers this effect was reversed by naloxone. A-fibers were not consistently affected by mophine.