Reinnervation of the superior colliculus delays down-regulation of ephrin A2 in neonatal rat

Although the adult mammalian optic nerve does not regenerate following lesi on, in the neonatal rat, retinal ganglion cell (RGC) axons retain the capac ity to grow across lesion sites in the brain. Following a brachial lesion a t postnatal day 2 (P2), some RGC axons, together with ingrowing cortico-tec tal axons, cross the lesion to reinnervate the superior colliculus (SC). He re we use immunohistochemistry to examine expression of the guidance cue ep hrin A2 following a brachial lesion. Normal animals show a steady decrease in ephrin A2 immunoreactivity between P5 and P31, with a low rostral to hig h caudal gradient being evident only at P5. By contrast, after brachial les ion values are significantly elevated rostrally at P5 an caudally at P12; m oreover, a steep rostro-caudal gradient is present at both ages. By P31 val ues fall to normal levels. Following unilateral enucleation at P2, levels a re not significantly different from normal. Our results show that innervati on but not denervation triggers increased ephrin A2 expression after a brac hial lesion. (C) 2001 Academic Press.