Morphine inhibits antidromic vasodilatation without affecting the excitability of C-polymodal nociceptors in the skin of the rat.

The effect of morphine was examined under identical conditions on (a) antidromic vasodilatation, an 'efferent' function of C-nociceptors in the skin, and (b) afferent responses to heat and pressure of C-polymodal nociceptors. Morphine caused a large, naloxone-reversible, fall in peak antidromic vasodilatation (ADV). However it caused no significant change in heat or mechanical excitability of C-polymodal nociceptors in normal or mildly inflamed skin. The mechanisms by which morphine might affect efferent, but not afferent, functions of C-nociceptors are discussed.