Development of muscimol binding sites in chick embryo neural retina in vivo and in vitro: Regulatory effects of cyclic AMP

We report here the development in the chick embryo retina of binding sites for [3H]muscimol, a potent agonist of GABA receptors. In vivo studies were carried out with isolated neural retinas from different stages of development. High‐affinity binding sites were absent before embryonic day (E) 8, but increased conspicuously between E10 and E16. Scatchard analysis indicated that this rise was due to an increase in the number of binding sites. Kinetic parameters of the embryonic binding sites were consistent with those typically found for mature muscimol receptors. Measurements of the low‐affinity binding site showed a relatively similar developmental pattern although a pronounced decrease in binding to the low‐affinity site was observed between E12 and E14. In vitro studies were carried out using glial‐free, purified monolayers of retinal neurons, starting at E8. Cultured retinal neurons showed a developmental pattern for high‐affinity muscimol binding sites resembling that observed in ovo. These binding sites were susceptible to regulation by cyclic AMP analogues. Increases of 100 to 200% in muscimol binding could be induced by a 24 hr treatment with dibutyryl cyclic AMP, 8‐bromo cyclic AMP, or the phosphodiesterase inhibitor IBMX. Scatchard analysis showed that this increase was due to a change in receptor affinity. No effects were found with either butyric acid or with adenosine 5′‐monophosphate. These results raise the possibility that cyclic AMP may be involved in the regulation of components of the GABA system.