INJURY-INDUCED REMODELING AND REGENERATION OF THE RIBBON PRESYNAPTIC TERMINAL IN-VITRO

The neuronal response to axonal injury may relate to the type of insul t incurred. Recently, neuritic and presynaptic varicosity regeneration by isolated adult salamander photoreceptors was demonstrated. We have used this system to compare the rod photoreceptor response to two typ es of injury: denervation/detargeting, the removal of pre- and postsyn aptic partners from the axon terminal, and axotomy, the removal of the axon terminal itself. Cells were followed with time-lapse video micro scopy for 24-48 h in culture and immunolabelled for SV2 or synaptophys in to identify synaptic vesicle-containing varicosities. Although all injured cells responded with regenerative growth, denervated/detargete d photoreceptors (i.e. neurons which retain their axon terminal) grew 80% more processes and fourfold more presynaptic varicosities than axo tomized neurons. In cells which retained their original axon and termi nal, varicosity formation generally began with axon retraction. Retrac tion was followed by elaboration of a lamellipodium and, by 48 h, deve lopment of varicosity-bearing neurites from the lamellipodium. Synapti c vesicle protein localization in denervated/detargeted cells parallel ed axon terminal reorganization. Axotomized cells, in contrast, lacked synaptic vesicle protein immunoreactivity during this period. To dete ct synaptic protein synthesis, photoreceptors were examined for coloca lization of synaptic vesicle protein with rab6, a Golgi marker, by con focal microscopy. As expected, synaptic vesicle protein staining was p resent in the Golgi complex during regeneration; however, in cells wit h an axon, new synaptic vesicle protein-labelled varicosities were fou nd at early stages, prior to the appearance of immunolabel in the Golg i complex. The data demonstrate remarkable plasticity in the ribbon sy napse, and suggest that in adult rod cells with an intact axon termina l, synaptic vesicle protein synthesis is not a prerequisite for the fo rmation of new presynaptic-like terminals. We propose that preexisting axonal components are reutilized to expedite presynaptic renewal as a n early response to denervation/detargeting.