TRANSIENT EXPRESSION OF NADPH DIAPHORASE ACTIVITY IN THE MOUSE WHISKER TO BARREL FIELD PATHWAY

Development of the topographic map of the somatosensory cortex of rode nts appears to depend on fine-tuned patterns of neuronal activity. Nit ric oxide (NO) has been described as a potent messenger in the modulat ion of neural activity associated with synaptic plasticity. To evaluat e the role of NO in the murine somatosensory pathway, we investigated NO synthase activity by NADPH diaphorase histochemistry at crucial dev elopmental stages. At birth, NADPH diaphorase activity was detected in the cortical plate of the developing somatosensory cortex. At day 3, diffuse NADPH diaphorase activity increased within the emerging layer 4 in the future barrel field hollows. This staining was most intense a t day 6 in the barrel field hollows and became undetectable by the end of the second postnatal week. The appearance of the diffuse NADPH dia phorase staining pattern was also observed in a similar time course an d topography in the ascending relays of the somatosensory cortex, spec ifically in the barreloids within the ventrobasal nucleus of the thala mus and the barrelettes of the trigeminal nucleus of the brainstem. Le sioning the C row of whiskers at day 1 (i.e. during the critical perio d of barrel formation) led to fused C barrels of diffuse NADPH diaphor ase activity in the barrel fields. In addition, highly NADPH diaphoras e activity-positive individual cells present in the deeper layers of t he somatosensory cortex at days 0 and 3 became visible in the upper la yers at day 6 and remained until day 15. In layer 4, these cells were predominantly localized in the septa at day 6 and 9. No positive indiv idual cells were detected in barrelettes or barreloids at any age. We conclude that NADPH diaphorase activity is present during experience-d ependent consolidation of synaptic contacts in the somatosensory pathw ay.