Investigation of the muscle relaxant activity of nitrazepam.

: Administered intravenously in decerebrate cats nitrazepam or diazepam (0.0625 to 0.5 mg/kg) produced dose-related inhibition of the ipsilateral extensor reflex. Nitrazepam (0.25 mg/kg i.v.) produced a significantly greater (P less than 0.001) inhibition than that produced by diazepam (0.25 mg/kg i.v.). Nitrazepam or diazepam (0.0625-4 mg/kg i.v.) had no effect on the contractions of directly stimulated (120 V, 5 msec, 0.1 Hz) quadriceps femoris muscle and on the contractions of tibialis anterior muscle produced by stimulating the cut peripheral end of the lateral popliteal nerve (8 V, 1.5 msec, 0.1 Hz). Nitrazepam or diazepam (0.125-0.5 mg/kg i.v.) produced dose-related depressor responses in cats anaesthetized with chloralose or pentobarbitone. Nitrazepam produced a depressor response at 0.0625 mg/kg dose while diazepam did not. The drugs did not appear to have any deleterious effect on the veins removed 6 hr after the first exposure to the drugs as evidenced by lack of histological changes. It is concluded that nitrazepam and diazepam produce central muscle relaxation by inhibiting polysynaptic pathways in the spinal cord. The potency of nitrazepam appears to be greater than that of diazepam. Definitive evidence has been provided that the peripheral neuromuscular or direct muscular actions are not involved in the muscular relaxation produced by the two drugs.