Age-dependent changes in the laminar distribution of cholinergic markers in rat visual cortex

The laminar distribution of muscarinic acetylcholine receptors, choline acetyltransferase, and sodium-dependent high-affinity choline uptake sites was studied in individual layers of rat visual cortex at 10, 15, 25 and 90 days of age, using a cryocut technique to separate the different cortical layers. Muscarinic acetylcholine receptor density in layer I rises sharply from postnatal day 10 to day 25, and then it continues to increase more gradually until adulthood. In layers II-V, receptor density increases slightly from day 10 to day 15, then it rises sharply until day 25 and continues decreasing up to day 90. In layer VI, muscarinic receptor binding increases markedly from postnatal day 10 to day 25, at which age the adult level is reached. Only slight developmental changes in the dissociation constants were observed in layers I-VI. The activity of the choline acetyltransferase rises in all cortical layers slightly from day 10 to day 15 followed by a sharp increase until day 25, at which age already the adult activity level is reached. In all visual cortical layers, the highest [(3)H]hemicholinium-3 binding to choline uptake sites during the postnatal period studied is already detectable at 10 days of age, then binding decreases sharply until day 25 at which age it nearly equals the value found in the adult. No essential developmental changes in the laminar pattern of choline acetyltransferase activity and choline uptake sites in rat visual cortex are observed, whereas the laminar distribution of muscarinic acetylcholine receptor density at 25 days of age is significantly different to that detectable in adulthood. The developmental changes in the laminar pattern of muscarinic receptors which represent the target structures that receive cholinergic input, might reflect changes of the cholinergic activity in the visual cortical layers during postnatal development.