Roles of α-Adrenoceptors and Sympathetic Nerve in Acute Herpetic Pain Induced by Herpes Simplex Virus Inoculation in Mice

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Percutaneous inoculation with herpes simplex virus type-1 brings about herpes zoster-like skin lesions, tactile allodynia, and mechanical hyperalgesia in mice. This study was conducted to determine whether the sympathetic nervous system and α-adrenoceptors would be involved in these pain-related responses and whether the α₂-adrenoceptor agonist clonidine would suppress these responses. The adrenergic neuron blocker guanethidine and the non-selective α-adrenoceptor antagonist phentolamine did not affect the pain-related responses, although these agents suppressed the pain-related responses induced by partial ligation of the sciatic nerve. The pain-related responses induced by herpetic inoculation was suppressed by intraperitoneal and intrathecal injections, but not by intraplantar and intracerebroventricular injections, of clonidine. The suppressive effect of an intraperitoneal injection of clonidine (0.1 mg/kg) was antagonized by intrathecal injections of phentolamine and the α₂-adrenoceptor antagonist yohimbine, but not the α₁-adrenoceptor antagonist prazosin. The results suggest that sympathetic nerves and α-adrenoceptors are not involved in the pain-related responses induced by herpetic infection. Clonidine suppresses the responses probably through the action on α₂-adrenoceptors in the dorsal horn.