Transient increase in the number of cholinergic neurons in the developing rat dentate gyrus.

Using a monoclonal antibody against choline acetyltransferase (ChAT), we have examined the distribution of cholinergic neurons in the rat dentate gyrus during development. ChAT-positive neurons were occasionally detected in the hilus on postnatal day 2 (P2). There was a transient abrupt increase in the number and density of ChAT-positive neurons between P15 and P20 and then a decline to the adult level with few ChAT-immunoreactive neurons. A few ChAT-positive varicose fibers and punctae were first seen at P5. They increased in number and density until P20 when they reached the adult level and distribution. These observations suggest the occurrence of a transient expression of cholinergic markers in the hippocampus.