The development of basal forebrain projections to the rat visual cortex.

The development of the basal forebrain projections to the visual cortex of the rat were studied using retrograde tracer techniques. Injections of wheat germ agglutinin-horseradish peroxidase placed in the visual cortex of newborn animals resulted in labelling of neurons throughout the basal forebrain nuclei. Although at this time the overall distribution of retrogradely labelled cells within the basal forebrain appeared similar to that seen in the adult, cells were smaller and weakly stained. It was only at the end of the second postnatal week that the somata of stained neurons showed sizes and staining intensity comparable to the adult. This precedes or coincides with the reported significant increases in cortical and basal forebrain ChAT activity and the first detection of ChAT-labelled fibres in this cortical area. These data suggest an important developmental point around the end of the second postnatal week that may correspond to the time when a significant number of cholinergic axons first appear within the visual cortex. They also suggest that the cholinergic projections to the visual cortex develop late in comparison with the thalamic and other subcortical afferents in this cortical area.