The binding properties and regional ontogeny of receptors for alpha-bungarotoxin in chick brain

The subcellular distribution, binding kinetics, pharmacologic properties and regional ontogeny of binding sites for alpha-[125I]bungarotoxins (alpha-[125I]BT) were studied in preparations of chick brain. Almost 95% of the specific binding was found in a 20,000 X g pellet. Two binding components were found, one with high (half-saturation at 1.5 X 10(-9) M) and another with low affinity for the toxin. The rate constants of association and dissociation of the toxin for the high-affinity site were found to be 3.5 X 10(4) M-1 - sec-1 and 3.8 X 10(-5) sec-1 (tau1/2 = 5.1 h, respectively). These data yielded a K alpha value of 1.1 X 10(-9) M. Nicotinic, but not muscarinic ligands were potent inhibitors of toxin binding at the high-affinity site. Thus, as in muscle, the binding site for toxin in chick brain appears to have the properties of a nicotinic acetylcholine (ACh) receptor. The ontogeny of the alpha-BT/ACh receptor was studied using homogenates of both whole brain and of discrete brain regions. In brain stem, optic lobe and cerebellum specific binding reached maximum levels in ovo and then decreased by maturity to values of 10,22 and 4.5 fmoles/mg protein, respectively. In contrast, whole brain and cerebral hemispheres reached plateau levels in ovo of 17 and 10 fmoles/mg protein, respectively. Major increases in total receptor number per region appeared in ovo between days 12 and 19 in brain stem and cerebellum, days 12 and 15 in optic lobes, and days 15 and 19 in hemispheres. The hemispheres were the only region to show a continued increase in total binding after hatching (3-fold increase by maturity). These findings are interpreted with respect to the development of chick brain and a model is suggested for the relationship between receptor ontogeny and neuronal differentiation and maturation.