DEVELOPMENT OF NADPH-DIAPHORASE ACTIVITY IN THE RAT NEOCORTEX

The activity of nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d), an enzyme related to the synthesis of nitric oxide (NO), w as studied histochemically in rat neocortex from the day of birth (PO) to young adulthood. At birth, NADPH-d containing neurons were already identifiable, sparsely distributed in the deep half of the pallium of the cerebrum. In addition, weakly stained bands of NADPH-d positive n europil were detectable in layers VI and deep V and the cortical plate (CP). During the first postnatal week, NADPH-d positive neurons incre ased markedly, especially in CP and the superficial layers. By P7 the cells were mainly in layers VIb and II/III. Differential NADPH-d activ ity in the neuropil during this period appeared as higher activity gra dually moving upwards until the highest intensity localised in layers II and upper III. In the caudal part of the cortex, the higher activit y covered the whole of the supragranular layers. By the end of the sec ond postnatal week, both the number and laminar distribution of NADPH- d neurons were adult-like, still mainly in layers VIb and II/III. The staining intensity in the neuropil was generally reduced but the bandi ng pattern seen at P7 was still present. No detectable changes in the patterns of NADPH-d positive cells and neuropil in the neocortex occur ed after the second postnatal week. The rostral part of the cortex mat ured slightly earlier than the caudal part. These results show that th e development of NADPH-d activity correlates with the laminar differen tiation and suggest that NADPH-d or NO may play an important role in t he maturation of cortical neurons including the establishment of funct ional connections.