Spasticity in rats with sacral spinal cord injury

We have investigated sacral spinal cord lesions in rats with the goal of developing a rat model of muscular spasticity that is minimally disruptive, not interfering with bladder, bowel, or hindlimb locomotor function. Spinal transections were made at the S2 sacral level and, thus, only affected the tail musculature. After spinal transection, the muscles of the tail were inactive for 2 weeks. Following this initial period, hypertonia, hyperreflexia, and clonus developed in the tail, and grew more pronounced with time. These changes were assessed in the awake rat, since the tail is readily accessible and easy to manipulate. Muscle stretch or cutaneous stimulation of the tail produced muscle spasms and marked increases in muscle tone, as measured with force and electromyographic recordings. When the tail was unconstrained, spontaneous or reflex induced flexor and extensor spasms coiled the tail. Movement during the spasms often triggered clonus in the end of the tail. The tail hair and skin were extremely hyperreflexive to light touch, withdrawing quickly at contact, and at times clonus could be entrained by repeated contact of the tail on a surface. Segmental tail muscle reflexes, e.g., Hoffman reflexes (H-reflexes), were measured before and after spinalization, and increased significantly 2 weeks after transection. These results suggest that sacral spinal rats develop symptoms of spasticity in tail muscles with similar characteristics to those seen in limb muscles of humans with spinal cord injury, and thus provide a convenient preparation for studying this condition.