REGULAR AND PHOTODAMAGE-ENHANCED REMODELING IN VITALLY STAINED FROG AND MOUSE NEUROMUSCULAR-JUNCTIONS

Repeated in vivo observations of vitally stained neuromuscular junctio ns allow direct monitoring of ongoing structural changes, although, no rmally occurring changes (remodelling) and those inflicted by the illu mination itself (photodamage) need to be dissociated. In frog cutaneus pectoris muscles, stained in vivo with 4Di-2ASP twice within four to five weeks, growth only was observed in 14 out of 92 junctions (in 18 muscles), retraction only in 19, and both features simultaneously in 2 2 junctions, while 37 junctions showed no changes. The summed growth i n a junction amounted to 5-42 mum, retraction to 5-52 mum, while overa ll changes in synaptic length were absent. This and the simultaneous o ccurrence of both, growth and retraction within a single junction, ind icates junctional remodelling. Similar amounts of remodelling were obs erved in junctions illuminated for 180, 60 or 10-30 s, indicating that within this range and with the given optical system, blue light and 4 Di-2ASP fluorescence were not harmful. Remodelling was not induced by the experimental procedure per se, since in in vitro preparations sign s of sprouting (and retraction) were equally frequent in junctions of totally untreated muscles, in junctions vitally stained four to five w eeks previously or vitally stained but not light-exposed in the same m uscle. In endplates of mouse gluteus maximus muscles double stained wi th 4Di-2ASP and rhodamine-alpha-bungarotoxin, unusually large (>10 mum ) terminal sprouts and retraction with gain and loss of ACh-receptors became prominent 9-29 days following illumination for more than 60 s. Frequently also, muscle fibre damage with local contracture and acute loss of stainability of axon terminals occurred; often followed by mus cle fibre denervation. In contrast, no such changes occurred after low er illumination intensities (12% emission filter for green) or shorter exposure times (<60 s); even at a fourth exposure performed within 53 days. Previously reported smaller changes indicating regular remodell ing were not investigated here.