ONTOGENIC DISTRIBUTION OF MUSCARINIC RECEPTORS AND ACETYLCHOLINESTERASE IN THE RABBIT HIPPOCAMPUS

Ontogenic development of muscarinic receptors was examined in the hipp ocampus of rabbits (from P2 to P60) using radioautographic method. Mus carinic sites were labelled with (3H)-quinuclinidyl-benzilate and phar macologically defined M1 and M2 receptor subtypes with (3H)-pirenzepin e and (3H)-oxotremorine, respectively. The distribution of binding sit es was compared to acetylcholinesterase (AChE) staining in adjacent hi ppocampal sections. The two cholinergic components are progressively s et up in the hippocampus during the first three postnatal weeks. The A ChE staining was very low in all hippocampal fields in P2 rabbits. At P8 and after, the AChE staining was more pronounced in CA3 and CA4 tha n in CA1 and CA2. On the contrary, the MI muscarinic binding sites wer e more abundant in CA1 and CA2 hippocampal fields than in CA3 and CA4 at all ages studied. M2 muscarinic binding sites were only distinguish able at P45 and have a relatively homogeneous distribution. This study shows a differential developmental evolution in the distribution of A ChE and muscarinic M1 receptors, and no obvious correspondence between these two cholinergic markers was observed.