Postnatal Development of Cholinergic Enzymes and Receptors in Mouse Brain

Abstract: The developmental profiles for the cholinergic enzymes acetylcholinesterase and choline acetyltransferase, and the muscarinic and nicotinic receptors were determined in whole mouse brain. The enzyme activities (per milligram of protein) increased steadily from birth, reaching adult levels at 20 days of age. These increases were primarily due to increases in V_max. Muscarinic receptor numbers, measured by [³H]quinuclidinyl benzilate binding, also increased from birth to 25 days of age. Brain nicotinic receptors were measured with the ligands L-[³H]nicotine and α-[¹²⁵I]-bungarotoxin. Neonatal mouse brain had approximately twice the number of a-bungarotoxin binding sites found in adult mouse brain. Binding site numbers rose slightly until 10 days of age, after which they decreased to adult values, which were reached at 25 days of age. The nicotine binding site was found in neonatal brain at concentrations comparable to those at the a-bungarotoxin site followed by a steady decline in nicotine binding until adult values were reached. Thus, brain nicotinic and muscarinic systems develop in totally different fashions; the quantity of muscarinic receptors increases with age, while the quantity of nicotinic receptors decreases. It is conceivable that nicotinic receptors play an important role in directing the development of the cholinergic system.