Electrophysiological evidence for transient topographic organization of retinotectal projections during optic nerve regeneration in the lizard, Ctenophorus ornatus

In the lizard, Ctenophorus ornatus, anatomical studies have revealed that o ptic axons regenerate to visual centers within 2 months of nerve crush but that, from the outset, the regenerated projections lack topographic order ( Beazley et al., 1997; Dunlop et al., 1997b). Here we assess the functional topography of the regenerated retinotectal projections by electrophysiologi cal recording of extracellular multiunit responses to visual stimulation an d by observing the lizards' ability to capture live prey. At the completion of the electrophysiology, DiI was applied locally to the retina and the to pography of the rectal projection later assessed. Electrophysiology reveale d that, at 2-4.2 months, responses were weak and habituated readily; no ret inotopic order was detected. Between 4.5-6 months, responses were more reli able and the majority of lizards displayed a crude retinotopic order, espec ially in the ventro-temporal to dorso-nasal retinal axis. Although response s were variable between 6-9 months, they tended to be more reliable again t hereafter. However, from 6-18 months, the projection consistently lacked to pography with many retinal regions projecting to each tectal locus. Lizards , including those with electrophysiological evidence of crude retinotopy, w ere consistently unable to capture live prey using the experimental eye. La belling with DiI confirmed the absence of anatomical retinotopy throughout. Taken together, the electrophysiological and anatomical data indicate that retinotopically appropriate axon terminals (or parts thereof) are transien tly active whilst inappropriately located ones are silent. Presumably in li zard map-making cues fade with time and/or the mechanisms are lacking to st abilize and refine the ephemeral map. Moreover, the transient retinotopy is insufficient for useful visual function.