Synapse elimination from polyneuronally innervated muscle fibres is regulated by activity-dependent and -independent mechanisms (Costanzo et al. 1999, 2000). An unresolved issue is whether synapse withdrawal is prompted wholly by local interactions at motor endplates, or whether mechanisms intrinsic to motor axons also initiate synapse withdrawal (Brown et al. 1976). We have addressed this by examining the complement of motor nerve terminals in single motor units expressing Yellow Fluorescent Protein (YFP), in mice with slow Wallerian degeneration. Axotomy induces withdrawal – rather than degeneration – of synapses from motor endplates in these mice, but hitherto it was unclear whether this occurred synchronously or asynchronously within motor units (Gillingwater et al. 2002).

We mated double-mutant Wld^S/Tg-4836 line mice with mice expressing YFP in a sub-set of motor axons (Mack et al. 2000; Keller-Peck et al. 2001). Mice in the F1 generation show the same phenoype as Wld^S homozygous mutant mice. At age 4-6 weeks, six of these mice were anaesthetized with ketamine-xylazine (100 mg kg^-1 Ketanest; 5 mg kg^-1 Rompun; I.P.) and the sciatic nerve sectioned unilaterally. After 5 days, the mice were humanely killed by cervical dislocation and hind-foot lumbrical muscles were isolated and examined for fluorescence of YFP. A single YFP-expressing motor unit was present in four lumbrical muscles out of 24 dissected from these six mice. One of these units was reconstructed from a complete confocal z-series taken through the muscle. Out of 58 motor end-plates supplied by this unit, 19 were almost fully occupied (> 80 % occupancy), whereas five terminals occupied less than 20 % of their motor end-plates. The distribution of fractional occupancies was not statistically different from a random distribution (P > 0.05; Kolmogorov-Smirnov test). We therefore conclude that a single, synchronised disruption of connectivity between a motoneurone cell body and its terminals can result in asynchronous nerve terminal withdrawal. This suggests that asynchronous withdrawal of synapses from motor units undergoing postnatal remodelling could reflect selective trafficking of soma-derived maintenance factors into nerve terminals, as well as competition for extracellular factors in the vicinity of motor end-plates.

This work was supported by the Wellcome Trust.