Muscle relaxant action of MS-322, a new centrally acting muscle relaxant, in rats.

1. The pharmacological properties of (R)-(+)-3-phenyl-5-¿2-(1-pyrrolidinylmethyl)-butyryl¿isoxazole hydrochloride (MS-322) were investigated and compared with those of other muscle relaxants. 2. MS-322 (3-12 mg/kg, IV) reduced spinal reflex potentials in acutely spinalized rats dose dependently. Its inhibitory effects on polysynaptic and dorsal root reflexes were more marked than those on the monosynaptic reflex. 3. MS-322 (1.5-12.0 mg/kg, IV) depressed the afferent discharges of de-efferented muscle spindles in anesthetized rats dose dependently. 4. The inhibitory effect of MS-322 on this afferent activity was considered to be due to its membrane stabilizing action. 5. In addition to the central effects of MS-322, its effect on muscle spindles may contribute to its muscle relaxant activity.