Differences in the synaptic complement of thoracic motoneurons of adult and ageing cats after permanent or reversible axotomy

We have compared the effects of intercostal nerve crush (reversible axotomy) or nerve transection with proximal ligation (permanent axotomy) on the somatic synaptic terminals of thoracic motoneurons of adult (1–2 years) and ageing (10–15 years) cats. Retrograde axonal transport of horseradish peroxidase (HRP) was used to identify axotomised motoneurons; control motoneurons were labelled by the intramuscular injection of HRP. Synaptic frequency and cover of control motoneurons in adult and ageing cats was similar. In adults, 8–16 days following both types of axotomy, synaptic cover was halved without any significant change in synaptic frequency. By 32–64 days following reversible axotomy, synaptic frequency and cover were not significantly different from controls. By contrast, 32–64 days following permanent axotomy synaptic frequency and cover were reduced to 30–50% of adult control values. In ageing cats 2 months following reversible axotomy, synaptic frequency and cover were reduced to 40% and 33% of ageing control values, respectively, while no significant change from controls was found 2 months following permanent axotomy. The long‐term synaptic response of axotomised motoneurons in ageing cats is therefore opposite to that of adults. Synapse 28:176–184, 1998. © 1998 Wiley‐Liss, Inc.