Maturation of NADPH-d Activity in the Rat's Barrel-Field Cortex and Its Relationship to Cytochrome Oxidase Activity

Histochemical detection of NADPH-d activity in rat barrel-field cortex reveals four types of distributions. (i) A transient, diffuse neuropil staining is visible in the cortical plate and in deeper layers until postnatal day (P) 4. Thereafter, until P15, it is segregated in whisker-specific patches in layer IV, then the pattern gradually disappears, becoming virtually indistinct by P21. This transient patterning of diffuse NADPH-d activity in layer IV disappears after cortical injections of kainic acid and is affected by neonatal damage to the contralateral snout. An intense labeling (ii) of scattered cells and (iii) of a plexus of fibers is present. With maturation, the cells become localized mostly in layers II/III, in the lower part of layer V, and in layer VI. They are sparse in layer I, in upper layer V, and in layer IV where their somata are located primarily in the interbarrel septa. (iv) Light staining of cortical neurons is detected mostly in layers II-IV but occasionally also in layers V-VI. Cytochrome c oxidase (CO)-positive patches associated with barrels are first detected in layer IV around P4-P5; their staining density increases with development, then stays high. In the adult, CO activity is moderate in supragranular layers, highest in the barrels in layer IV, low in upper layer V, medium dense in the deeper half of layer V, and low in lamina VI. Thus, NADPH-d and CO activities are not necessarily colocalized in the rodent barrel-field cortex. The varied (transient and long-lasting) distributions of NADPH-d activity indicate that the enzyme and its associated production of NO serve multiple roles in developing and adult barrel-field cortex.