Kainic acid binding sites in the developing chick optic tectum

A membrane fraction from embryonic or newborn chick tectum has been found to bind [(3)H]kainic acid in a specific, saturable manner. A similar membrane preparation from chick liver does not show any appreciable specific binding under the same experimental conditions. Specific [(3)H]kainic acid binding to tectal membranes is displaced by suitable concentrations of unlabelled kainic acid, l-glutamate and other excitatory amino acid analogues, both agonists and antagonists. The appearance and accumulation of kainic acid binding sites in chick optic tectum has been studied between embryonic day 8 and postnatal day 20. Although by hatching time a sizable amount of binding can be measured in tectal membranes, most of the receptor accumulation takes place after birth. Saturation kinetic studies of kainic acid binding, before and after birth, show one single binding mode with an apparent dissociation constant K (d) ? 160 nM and a binding capacity increasing from 0.83 pmol/mg of protein, at embryonic day 16, to 6.45 pmol/mg of protein at day 6 after birth. Changing levels of visual input (both deprivation and stimulation) modulate the postnatal rate of receptor accumulation, with a higher density of binding sites in visually deprived optic lobes and a lower density in stimulated lobes, as compared with tectal lobes in normally raised chicks.