Enhancement of Neurite Outgrowth and Aspartate‐Glutamate Uptake Systems in Retinal Explants Cultured with Chick Gizzard Extract

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Abstract
Chicken gizzard extract promoted a long and radially directed neurite outgrowth from retinal explants of 8‐day‐old chick embryo in cultures of 2–3 days. The neurite outgrowth from retinal explants cultured in the absence of gizzard extract was short and restricted to the explant perimeter. The neurite outgrowth promoted by gizzard extract depended strictly on several factors. (a) Fetal calf serum and polycationic substratum were required in this culture system, (b) Pretreatment of the polyornithine‐coated substratum with gizzard extract allowed the retinal explants to extend neurites even in the absence of gizzard extract in the medium. (c) Maximal neurite outgrowth was observed in retinal explants dissected from 8‐day embryos, but thereafter the explants’response to gizzard extract rapidly declined and was almost lost at the 12th day. As a biochemical parameter of differentiation of cultured neuroretina, uptake systems for neurotransmitter candidates were examined in homogenates of retinal explants cultured in the absence or presence of gizzard extract. After 3 days in culture with gizzard extract, the uptake increased for aspartate and glutamate 1.6‐ to 1.8‐fold and for γ‐aminobutyric acid to a lesser degree when examined at a concentration for high‐affinity uptake (10‐6M). In contrast, the uptake capacity for glycine, choline, and dopamine was not altered in explants cultured with or without gizzard extract. Kinetic analysis showed that the enhanced capacity to accumulate aspartate was not due to an alteration of Km, but to an increase of Vmax. The results suggest that one or several factors in chick gizzard muscle promote not only neurite outgrowth but also the aspartate‐glutamate uptake systems in the developing neuroretina, probably related to ganglion cells.