Attenuating effect of mexiletine hydrochloride on herpetic pain in mice infected with herpes simplex virus

The influence of mexiletine hydrochloride on herpes-related pain responses was examined using mice infected with herpes virus. BALB/c mice were inoculated with herpes simplex virus (HSV; 1 x 10(6) plaque-forming units) on the right hind paw, and the contralateral hind paw was without inoculation. The changes in nociceptive threshold were examined using electric von fray meter. BALB/c mice inoculated with HSV showed a decrease in nociceptive threshold. Intraperitoneal administration of mexiletine prevented the decrease in nociceptive threshold dose-dependently in HSV-inoculated mice, which was firstly observed at a dose of 15.0 mg kg(-1), and peaked at doses more than 17.5 mg kg(-1). This antinociceptive effect of mexiletine attained peaks at 60-90 min after administration and declined gradually to non-treated levels by 150 min. Intraperitoneal administration of mexiletine at a dose of 17.5 mg kg(-1) (but not 10.0 mg kg(-1)) caused significant increase in beta-endorphin levels in the mid brain and hypothalamus of HSV-inoculated mice. However, mexiletine scarcely affected noradrenaline (norepinephrine) levels in the pons and medulla oblongata, even when HSV-inoculated mice were treated with 17.5 mg kg(-1) mexiletine. These results strongly suggested that mexiletine exerts antinociceptive effects on herpes-related pain through enhancement of beta-endorphin levels in the central nervous system in HSV-inoculated mice. It is also suggested that mexiletine will be a good candidate for an antinociceptive drug in the treatment of acute herpetic pain in man.