Evaluation of oral skeletal muscle relaxants in the morphine-induced straub tail test in mice

Morphine is known to produce a characteristic and reproducible elevation of the tail in mice (Straub tail response). The morphine-induced Straub tail response in mice has been used to evaluate skeletal muscle relaxant (SMR) activity of compounds administered intraperitoneally (i.p.). This model was used to evaluate the oral (p.o.) efficacy of a number of SMRs and other pharmacological agents. Male mice (n ≥ 5) were given test drugs p.o. followed by morphine sulfate [15 mg/kg subcutaneously (s.c.)] 15 min later. The mice were scored all or none for a Straub tail reaction 45 min later. Graded doses of active compounds were further evaluated for ED₅₀ estimation by probit analysis. ED₅₀ values (mg/kg) were estimated for the following compounds: baclofen (6.4), chlorpromazine HCl (3.8), cyclobenzaprine HCl (24.6), dantrolene Na (14.4), diazepam (8.3), haloperidol (6.2), naloxone HCl (8.7), phenoxybenzamine HCl (47.6), phentolamine HCl (265), and trifluoperazine HCl (25.4). These ED₅₀ values appeared to correlate with initial adult human daily oral doses for muscle relaxation. The following compounds inhibited the Straub tail response in ≤ 40% of the mice tested at the doses indicated (mg/kg): carisoprodol (300), lidocaine (100), mephenesin (300), phenytoin (100), procainamide HCl (100), procaine HCl (100), propranolol HCl (100), quinidine sulfate (100); and all selected calcium-channel blockers (30), antidepressants (30), and neuromuscular blocking agents (> 10 times the literature i.p. ED₅₀ values with the exception of gallamine triethiodide at 30 mg/kg) that were tested. Pentobarbital Na was active only at doses that impaired the righting reflex. This animal model was thus determined to be useful for evaluating SMR efficacy and in predicting.