Figure 6. Synaptic protection in Wld^s mice depends on synaptic maturity and not the age of the animal.

A, schematic representation of the experimental protocol used to assess the role of synaptic maturity versus chronological age on nerve terminal preservation following axotomy. The Wld^s mice used were > 7 months old, therefore permitting a test of the responses to axotomy at immature (i.e. 2–4-week-old), regenerated synapses, in mature mice. B, confocal micrograph of an immunocytochemically labelled 7 month Wld^s FDB muscle, 3 days post axotomy. One of the few remaining terminals from this preparation is shown, with a characteristic bloated appearance, surrounded by three vacated endplates. C, confocal micrograph of a regenerated synapse from a 14 month Wld^s FDB muscle, 5 days post axotomy. Axons and motor nerve terminals were labelled immunocytochemically (NF and SV2; FITC) and acetylcholine receptors were labelled with TRITC-conjugated α-BTX. Note that all endplates shown are occupied and that the uppermost instance shows evidence of partial occupancy. D, electrophysiological recording from a 14 month Wld^s FDB muscle fibre with regenerated synaptic connections, 5 days post axotomy, showing robust synaptic transmission. E, graph showing the percentage of fibres with synaptic innervation, as assessed by electrophysiological (EP) and immunocytochemical (IM) techniques, in old (non-regenerated) Wld^s FDB muscle fibres at 3 days post axotomy; in reinnervated muscle fibres 8 weeks after the first lesion but before a second lesion; and in reinnervated endplates at 3–5 days after the second lesion. Note the rapid and almost complete loss of innervation following axotomy at the axotomised ‘old’ synapses (compare with Figure 4) and the retention of > 30 % of regenerated synaptic connections for up to 5 days post axotomy.