Expression and Role of Low-Affinity Nerve Growth Factor Receptor (p75) in Spinal Motor Neurons of Aged Rats following Axonal Injury

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Abstract
Expression of low-affinity nerve growth factor receptor (p75) and its regulation in spinal motor neurons of aged rats following axonal injury were investigated by immunocytochemical staining with antibody against p75. Under normal conditions, approximately 60% of spinal motor neurons expressed p75 in aged rats whereas no p75 expression was observed in spinal motor neurons of young adult rats. We examined the effects of spinal motor neuron injury on aged rats by two approaches, i.e. distal axotomy and spinal nerve root avulsion. A 20% increase in the number of p75-positive motor neurons was observed in aged rats 2 weeks after distal axotomy after which it returned to normal by 8 weeks post-injury and remained constant. Following root avulsion, a transient and slight up-regulation of p75 expression was observed in injured motor neurons. The number of p75-positive motor neurons decreased quickly to below normal levels 1 week after lesion and progressively declined with time post-injury, 40% by 2 weeks, 33% by 4 weeks, 23% by 8 weeks, and 5.8% by 12 weeks compared with the normal controls. This study demonstrates that p75 is re-expressed in aged spinal motor neurons. Following axonal injury in aged rats, up-regulation of p75 seems to coincide with the survival of injured motor neurons. Potential roles of re-expression of p75 in aged motor neurons are discussed.