Wallerian degeneration describes the characteristic degeneration of the severed distal segments of axons and their terminals after they have been separated from their cell bodies. In the slow Wallerian degeneration (Wld^S) mutant mouse, this active process is delayed by expression of a chimeric Nmnat/Ube4b gene: disconnected axons are preserved for up to 3 weeks, rather than 24-48 h characteristic of wild-type mice (Coleman, 2005). However, synaptic terminals are less well protected by expression of the chimeric protein. For instance, motor nerve terminals appear to withdraw from motor endplates over a 3-10 day period (Gillingwater et al. 2002). This suggests that neurodegenerative mechanisms may be compartmentalised in neurones and that presynaptic terminals represent an independent neurodegenerative compartment (Gillingwater & Ribchester, 2001). To gain further insights into the pattern and mechanism of neuromuscular synaptic degeneration, we have developed a minimally invasive in vivo imaging protocol based on confocal microendoscopy. This methodology has potential application to analysis covert neuromuscular phenotypes, for example, following random germ-line mutations induced by ethylnitrosourea (ENU) mouse mutants (Nolan et al. 2002). Wld^S mice were first crossbred with two transgenic lines expressing Yellow Fluorescent Protein in neurones: thy1-YFP16 which expresses the fluorochrome in all motor neurones, and thy1-YFPH in which only about 5% of motor neurone expresses fluorescent protein (Feng et al. 2000). The offspring were backcrossed to Wld^S mice to generate fluorescent lines that were homozygous for Wld^S. Next, we transected the sciatic nerve in these mice under 3% halothane/40-50% N₂0/40-50% O₂ anaesthesia. Mice were inspected daily. All of them compensated within 24 h for their unilateral lower hind-limb paralysis. Following recovery 1-7 days later, we reanaesthetised the mice (ketamine/xylazine, 100/10 mg/kg, I.P.). We visualised the severed distal axons in the tibial nerve and superficial neuromuscular junctions in the gastrocnemius muscle via small incisions in the skin, using Proflex S-1500, S-650 and prototype Z-1000 fibre-optic probes connected to a Cell Vizio confocal microendoscope (Mauna Kea Technologies, Paris). In two mice from control transgenic YFP lines, axons and neuromuscular junctions had visibly undergone fragmentation and degeneration within 48 h. In the YFP-Wld^S variants (n=4), however, intact axons and neuromuscular synapses were visible for at least 4 days after sciatic nerve section, proving that the Cell Vizio system can resolve intact and degenerating synapses in vivo. We are currently adapting this methodology to carry out a) repeated, semi-continuous visualisation of axonal and synaptic degeneration in vivo with minimal surgical invasion; b) high-throughput screening of ENU mutants (Nolan et al. 2002) for novel phenotypes showing slow synaptic degeneration.