Cholecystokinin neurons in the developing visual cortex of normal and dark reared rats. Comparison with other peptidergic populations.

The laminar distribution and the density of cholecystokinin-like (CCK) neurons in the developing visual cortical areas (17, 18 and 18 a) were studied in Wistar rats that were reared under normal lighting conditions or in complete darkness. Immunocytochemistry on paraffin sections at postnatal days (P) 7, 14, 21, 30 and 60 showed that the density of CCK neurons in all visual areas of both groups of animals progressively increases from P7 to P21 but declines thereafter, to reach 50-75% of peak densities at P30. From P30 to P60 the density of CCK neurons in the visual cortex remains relatively unchanged in non-deprived animals, but increases slightly again in areas 17 and 18 a of dark reared rats. Fluctuations in the density of CCK neurons reflect changes in the number of immunoreactive neurons mainly present in layers IV-VI. It is concluded that the development of the CCK neurons in the rat visual cortex is minimally affected by light deprivation.