Effect of centrally acting muscle relaxants on the morphine-induced Straub tail reaction in mice.

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Abstract
This investigation was carried out in an attempt to ascertain whether all narcotic analgesics produce the Straub tail reaction (STR) and whether the morphine-induced STR could be depressed by centrally acting muscle relaxants. Morphine, oxycodone and levorphanol elicited the STR dose-dependently; codeine, pethidine and pentazocine gave only slight STR. The peak time of the STR was 30 min after the administration of morphine. Intracerebral injection of morphine evoked the STR, but not markedly. Naloxone and pentazocine depressed the morphine-induced STR considerably. Chlordiazepoxide, diazepam, oxazepam, carisoprodol, chlormezanone, chlorzoxazone, meprobamate, orphenadrine, chlorpromazine, perphenazine and haloperidol also reduced the morphine-induced STR markedly. The morphine-induced STR was noticeably inhibited by intracerebral or systemic injection of baclofen and difenamizole. The intracerebral dose of baclofen required for this inhibition was as low as approximately 1/1000 of that by systemic administration. These findings suggest that the morphine-induced STR is positively depressed by centrally acting muscle relaxants.