Postnatal development of neuropeptide Y-containing neurons in the visual cortex of normal- and dark-reared rats

The postnatal development of neuropeptide Y (NPY)-immunoreactive neurons in the visual cortical areas (17, 18 and 18a) has been studied in Wistar rats reared under normal lighting conditions or in complete darkness. Immunohistochemistry on paraffin sections at postnatal days (P)7, 14, 21, 30 and 60 showed an overall similarity in laminar distribution of NPY neurons in all 3 visual areas of both normal- and dark-reared animals. The pattern of development of NPY neurons was characterized by an increase in their density from P7 to reach a peak at P21 followed by a decline to 37-47% of peak levels at P60. However, this diminution was not so great in dark-reared rats as in the normal, since the density only decreased to 62-78% of peak levels. At P60 the resulting differences in neuron density were marked in areas 17 and 18, where the dark-reared had 75% more cells than normal, and moderate in area 18a (30% more than normal). These results suggest that the normal decline in NPY neurons is not entirely mediated by visual experience since it takes place, albeit to a modified extent, in its total absence.