Motorneuron protection by N-acetyl-cysteine after ventral root avulsion and ventral rhizotomy

Abstract

Motor recovery after proximal nerve injury remains extremely poor, despite advances in surgical care. Several neurobiological hurdles are implicated, the most fundamental being extensive cell death within the motorneuron pool. N-acetyl-cysteine almost completely protects sensory neurons after peripheral axotomy, hence its efficacy in protecting motorneurons after ventral root avulsion/rhizotomy was investigated. In adult rats, the motorneurons supplying medial gastrocnemius were unilaterally pre-labelled with retrograde tracer (true-blue/fluoro-gold), prior to L5 and 6 ventral root avulsion, or rhizotomy. Groups received either intraperitoneal N-acetyl-cysteine (ip, 150 or 750 mg/kg/day), immediate or delayed intrathecal N-acetyl-cysteine treatment (it, 2.4 mg/day), or saline; untreated animals served as controls. Either 4 (avulsion model) or 8 (rhizotomy model) weeks later, the pre-labelled motorneurons' mean soma area and survival were quantified. Untreated controls possessed markedly fewer motorneurons than normal due to cell death (avulsion 53% death; rhizotomy 26% death, P<0.01 vs. normal). Motorneurons were significantly protected by N-acetyl-cysteine after avulsion (ip 150 mg/kg/day 40% death; it 30% death, P<0.01 vs. no treatment), but particularly after rhizotomy (ip 150 mg/kg/day 17% death; ip 750 mg/kg/day 7% death; it 5% death, P<0.05 vs. no treatment). Delaying intrathecal treatment for 1 week after avulsion did not impair neuroprotection, but a 2-week delay was deleterious (42% death, P<0.05 vs. 1-week delay, 32% death). Treatment prevented the decrease in soma area usually found after both types of injury. N-acetyl-cysteine has considerable clinical potential for adjuvant treatment of major proximal nerve injuries, including brachial plexus injury, in order that motorneurons may survive until surgical repair facilitates regeneration.