Heterogeneity of muscarinic cholinergic receptors in the developing chick embryo retina

Heterogeneity of muscarinic cholinergic receptors was investigated in chick embryo retina throughout development and in chicks immediately after hatching. The presence of a homogeneous receptor population was evidenced by antagonist binding. The affinity of antagonists increased up to day 14 of incubation, when synaptogenesis occurs. After this stage, it remained substantially unchanged. The number of receptors increased in embryos until hatching. On the contrary, agonists, such as acetylcholine and carbachol, bound to two (high- and low-affinity) binding sites. Through development, the affinity of both significantly increased until day 14, further substantiating the hypothesis of a maturation of the receptor pattern which precedes synapse formation. Muscarinic cholinergic binding seems to identify 3 critical steps in retinal neuronal development. The first is between 7 and 9 days of incubation, the second when synaptogenesis occurs and the third after initiation of function.