MOTONEURONS INNERVATING PARTIALLY DENERVATED RAT HINDLIMB MUSCLES REMAIN SUSCEPTIBLE TO AXOTOMY-INDUCED CELL-DEATH

Tibialis anterior and extensor digitorum longus muscles were partially denervated by cutting the L4 spinal nerve in three-day-old rats. The ultrastructure of the intact axons to these muscles in the LS spinal n erve was examined in nine-day-old rats. In the control L5 spinal nerve , myelinated and unmyelinated axons were intermingled throughout the c ross-section of the nerve, while on the operated side the nerve contai ned areas with predominantly small unmyelinated immature axons. The nu mber of motoneurons innervating the partially denervated muscles was e stablished by retrograde labelling with Diamidino Yellow. In nine-and 21-day-old rats, the number of labelled motoneurons on the partially d enervated side, expressed as a percentage of the control side, was 26. 1+/-5.5% and 20.7+/-3.0%, respectively. The response of these uninjure d motoneurons to axotomy was tested. The axons of the motoneurons to t he partially denervated muscles were crushed at nine days and the numb ers of labelled motoneurons in the spinal cord of these rats counted a t 21 days of age. Only 4.9+/-2.0% labelled motoneurons were seen on th e operated side, as opposed to 20.7+/-3.0% present in animals without sciatic nerve injury. In normal animals, nerve injury at nine days doe s not cause motoneuron death. Thus, motoneurons to partially denervate d muscles (i) have axons with several immature features and (ii) remai n susceptible to axotomy-induced death for much longer than normal. (C ) 1998 IBRO. Published by Elsevier Science Ltd.