PROCESS OUTGROWTH AND SYNAPTIC VARICOSITY FORMATION BY ADULT PHOTORECEPTORS IN-VITRO

To assess the regenerative capability of the photoreceptor synapse, we have isolated and cultured photoreceptors from the mature salamander retina. Both rod and cone photoreceptors were able to regenerate proce sses within 3 d of plating. Cells extended numerous actin- containing filopodia as well as a few neuritic processes. The neurites contained microtubules and formed synaptic vesicle-filled varicosities, as shown by immunostaining for tubulin and synaptic vesicle proteins and by el ectron microscopy. Furthermore, regenerated varicosities were capable of depolarization-induced vesicle labeling, suggesting that they can r ecycle synaptic vesicles and release neurotransmitter by synaptic vesi cle exocytosis. Differences were observed between rod and cone cell sy naptic regeneration in vitro, which resembled structural differences b etween their synaptic terminals in situ: rod cells formed multiple syn aptic vesicle-filled varicosities-along neurites at a distance from th e soma, whereas cone cells tended to accumulate synaptic vesicles with in the soma. The regeneration of neurites and synaptic vesicle-filled varicosities was abolished by microtubule depolymerizing agents, sugge sting a role for microtubule-based vesicle transport in the formation of varicosities. Finally, process outgrowth and varicosity formation w ere independent of cell-cell contact and, indeed, proceeded in the com plete absence of other cells. These findings suggest not only that dif ferentiated photoreceptors are capable of synaptic renewal but that th e regeneration of presynaptic-like terminals is an intrinsic ability o f rod and cone cells.