Morphine inhibits herpetic allodynia through μ-opioid receptors induced in Aβ-fiber neurons

Peripheral action of μ-opioid receptor agonist inhibits mechanical allodynia in mice with herpetic pain. Mechanical allodynia is mainly mediated by Aβ fibers, whereas μ-opioid receptors are present in C and Aδ fibers. This study was conducted to address this dis-crepancy. Neonatal capsaicin treatment, which almost abolished aversive response to capsaicin, did not affect herpetic allodynia and antiallodynic effect of local injection of morphine. Although μ-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 μ-opioid receptor expression in Aβ fibers, which may be responsible for the inhibitory action of local opioids on mechanical allodynia in mice with herpetic pain.