Cortical neurons expressing the cholecystokinin gene in the rat: distribution in the adult brain, ontogeny, and some of their projections.

Recent studies of neuronal cholecystokinin (CCK) expression performed with more sensitive techniques have demonstrated that the distribution of the expression of this peptide is more widespread than previously thought. In the present study, hybridization histochemistry was used to map cortical neurons expressing the CCK gene in adult and developing rats. Retrograde tracing with Fluorogold in combination with hybridization histochemistry was used to demonstrate some of the projections of these neurons. Neurons expressing the CCK gene were found in all areas of the neo- and allocortices. They were of several morphological types, including pyramidal neurons, and were found in almost all layers, albeit at different relative numbers and with different levels of expression. Generally, layers II and III, deep layer V, and layer VI had many neurons expressing CCK mRNA. Cortical CCK expression was first detected on the 15th day of gestation in the primordial plexiform layer. Expression developed thereafter in a regular and continuous fashion until an adult-like pattern was present on the 21st day after birth. Cortical neurons containing CCK mRNA were found in almost all the projections studied. Many neurons in both neo- and allocortical areas with cortico-cortical, associational, and commissural pathways contained CCK mRNA. Similarly, numerous corticostriatal neurons contained CCK mRNA; however, only a few corticothalamic neurons expressed CCK mRNA. These results demonstrate that in the rat cortex the distribution of projection neurons expressing CCK is much more widespread than had been previously shown and will stimulate further investigations into the role of CCK in these neurons.