Antinociceptive effects of tizanidine, diazepam and eperisone in isolated spinal cord-tail preparations of newborn rat

Antinociceptive effects of three centrally acting muscle relaxants, tizanidine, diazepam and eperisone, were studied using an isolated newborn rat spinal cord-tail preparation. Potentials were recorded from a lumbar ventral root (L3–L5) cxtracellularly using a suction electrode. Pinch stimulation applied to the tail elicited a depolarizing response in the ventral root lasting 15/2–30 sec, referred to as the tail-pinch potential. Electrical stimulation of the ipsilateral dorsal root of the same segment with a single shock induced depolarizing responses in the ventral root. The responses consisted of monosynaptic and polysynaptic reflexes with a fast time course, followed by a slow depolarizing response lasting about 20 sec. The latter slow response was designated the ipsilateral slow ventral root potential (VRP). Both the tail-pinch potential and the ipsilateral slow VRP were depressed by application to the spinal cord of tizanidine (2–3 μM), diazepam (2 μM) and eperisone (100–200 μM). The effect of tizanidine was reversed by α 2-adrenoreceptor antagonists, yohimbine and idazoxan, but not by an α 1-antagonist, prazosin. The effect of diazepam was reversed by the benzodiazepine antagonist, flumazenil.