Activity-dependent degeneration of axotomized neuromuscular synapses in Wld S mice.
Bottom Line: Periodic high-frequency nerve stimulation (100 Hz: 1s/100s) reduced synaptic protection in Wld(S) preparations by about 50%.This effect was abolished in reduced Ca(2+) solutions.Together, the data suggest that vulnerability of mature neuromuscular synapses to axotomy, a potent neurodegenerative trigger, may be enhanced bimodally, either by disuse or by hyperactivity.
Affiliation: Euan MacDonald Centre for Motor Neurone Disease Research, Hugh Robson Building, University of Edinburgh, George Square, Edinburgh EH8 9XD, UK.Show MeSH
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Mentions: In preliminary experiments we found that persistence of synaptic function over 12–48h ex vivo was strongly temperature dependent. Surprisingly, and by contrast with the rates of synaptic degeneration observed in vivo, incubating preparations in MPS at 37 °C rendered synaptic degeneration as rapid in preparations from WldS mice as in WT preparations. Conversely, culturing the explanted muscles at 25 °C slowed degeneration of synaptic terminals in WT mice to such an extent that the effect of WldS could not be distinguished at this temperature either (data not shown, but see also (Tsao et al., 1999)). However, when muscles from homozygous WldS mice were incubated at 32 °C, many NMJ’s were still functional 24 h later, compared with WT muscles in which most did not respond to nerve stimulation (Fig. 4B, C, F). Specifically, in preparations from WldS mice, 89.61 ± 2.93% of fibers (n = 11 muscles) responded with nerve-evoked EPPs at 5–15 h. After 15–25 h, there was no significant change (95.88 ± 1.15% responsive fibers; n = 17) and most NMJs (70.42 ± 7.06%, n = 8) still showed significant persistence of synaptic function after 25–30 h ex vivo (Fig. 4F). The number responding at 35–45 h was reduced to 45.72 ± 11.90% (n = 4). By 45–50 h, substantial numbers of fibers had become denervated but 2.50 ± 1.71% (n = 5) still remained functional. By contrast, in WT muscles neuromuscular synapses degenerated much more rapidly: only 36.39 ± 16.02% of NMJs responded to stimulation at 15 h (n = 6 muscles), and only 10.77 ± 5.30% at 15–25 h (n = 13 muscles). By 25 h, only 1.25 ± 1.25% of fibers produced evoked synaptic responses (n = 4 muscles). Thus, 32-h incubation of these nerve muscle preparations ex vivo constituted a suitable checkpoint for determining the rate of synaptic degeneration following stimulation in WldS muscles, as approximately 50% of synapses were innervated, meaning that potential positive or detrimental effects of stimulation could be distinguished.
Affiliation: Euan MacDonald Centre for Motor Neurone Disease Research, Hugh Robson Building, University of Edinburgh, George Square, Edinburgh EH8 9XD, UK.