Survival of axotomized retinal ganglion cells in adult mammals.

As a nature of central neurons, retinal ganglion cells (RGCs) of adult mammals cannot regenerate spontaneously, but rather, degenerate retrogradely after optic nerve (ON) transection. Recent studies have clarified that some RGCs survive axotomy and regrow their axons through the graft when the peripheral nerve is transplanted to the cut stump. However, the numbers of regenerated fibers are still quite small and a great majority of RGCs die after axotomy. Studies on the rodent and cat retinas have revealed that vulnerability to axotomy differs among RGC types. Especially in the cat retina, X/beta cells are more vulnerable to axotomy than other cells. Several neurotrophic factors such as brain-derived neurotrophic factor (BDNF) and neurotrophin-4 or 5 (NT-4/5) are effective to rescue RGCs from retrograde cell death, and promote growth of intraretinal axons. Localization of their receptors is also demonstrated in RGCs. On the other hand, evidence has been accumulated that regenerated ON fibers reform functional synapses with central visual neurons and thereby axotomized RGCs survive longer. These reformed retino-collicular synapses enabled the animals to recover the ability of light-dark discrimination.