Cholinergic binding sites in rat hippocampal formation: properties and ontogenesis

The hippocampal formation of the rat contains two types of membrane-bound cholinergic binding sites, as revealed by specific binding of [3H]quinuclidinyl benzilate (QNB) or of a-[125I]bungarotoxin (a-Btx). The sites differ in pharmacological profile, sensitivity to detergents and ontogenesis. The major binding site (about 17 pmol per adult hippocampus) is of a muscarinic nature, and binds [3H]QNB with an on-rate of 2 x 10(6) M-1 sec-1 and an apparent KD of 0.4 nM. This binding is displaced by low concentrations of muscarinic ligands but not of nicotinic ligands. The earliest increase in binding level is detected at about day 4 postnatal and a sharp increase in total binding takes place between days 10 and 15. Total binding continues to increase gradually about 3-fold until an age of about 7 weeks, at a rate resembling that of acetylcholinesterase. a-Btx-binding sites (about 0.6 pmol per adult hippocampus) display a nicotinic profile with an on-rate constant for a-[125I]Btx of 6 x 10(4) M-1 sec-1 and an apparent KD of 2 nM. Ontogenesis of these sites clearly differs from that of muscarinic sites and acetylcholinesterase. Absolute binding reaches mature levels at an age of 12--14 days postnatal, and binding per tissue protein is higher during the first postnatal days than at maturity. It appears that the level of toxin-binding sites attains mature values before the major synaptogenetic events in the area are completed.