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) extracellularly using a suction electrode. Pinch stimulation applied to the tail elicited a depolarizing response in the ventral root lasting 15-30 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 microM), diazepam (2 microM) and eperisone (100-200 microM). The effect of tizanidine was reversed by alpha 2-adrenoreceptor antagonists, yohimbine and idazoxan, but not by an alpha 1-antagonist, prazosin. The effect of diazepam was reversed by the benzodiazepine antagonist, flumazenil.