GDNF and neoplasm: Additionally, while GDNF is clearly protective, GDNF has been reported to cause aberrant sprouting and negative feedback effects on neurotransmitter homeostasis88,98,99 and high doses of GDNF plasmid led to a significant reduction of scotopic b-wave amplitude and photoreceptor thickness.100 A long-term tumor/ toxicology study is next required to determine long-term cell function and toxicity.