Abstract
The effects of the benzodiazepines diazepam and midazolam on the rigidity produced by systemic administration of morphine (15 mg/kg i.p.) were studied in rats. The rigidity was recorded as a tonic activity from the gastrocnemius-soleus muscle of non-anesthetized rats in the electromyogram. Both diazepam (1–5 mg/kg i.p.) and midazolam (2.5 and 5 mg/kg i.p.), when administered 45 min after morphine, temporarily antagonized the muscular rigidity. When the competitive antagonist Ro 15-1788 (ethyl-8-fluoro-5, 6-dihydro-5-methyl-6-oxo-4H-imidazo [1, 5a]-[1, 4] benzodiazepine-3-carboxylate) (5 mg/kg i.p.) was administered together with diazepam (5 mg/kg i.p.), it prevented the effect of diazepam, although by itself it did not affect the morphine-induced muscular rigidity. Our results suggest that diazepam, by acting on benzodiazepine receptors, can antagonize the morphine-induced muscular rigidity. The latter phenomenon might serve as a model for some types of muscular rigidity observed in humans.
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