Morphine inhibits herpetic allodynia through mu-opioid receptors induced in Abeta-fiber neurons.

Peripheral action of mu-opioid receptor agonist inhibits mechanical allodynia in mice with herpetic pain. Mechanical allodynia is mainly mediated by Abeta fibers, whereas mu-opioid receptors are present in C and Adelta fibers. This study was conducted to address this discrepancy. Neonatal capsaicin treatment, which almost abolished aversive response to capsaicin, did not affect herpetic allodynia and antiallodynic effect of local injection of morphine. Although mu-opioid receptor was chiefly expressed in small-sized and medium-sized sensory neurons in naive mice, it was induced in large sensory neurons in mice with herpetic pain. Viral propagation in the sensory ganglion may induce mu-opioid receptor expression in Abeta fibers, which may be responsible for the inhibitory action of local opioids on mechanical allodynia in mice with herpetic pain.