Late postnatal changes in rat somatosensory cortex

Summary Temporal and spatial developmental relationships between AChE and GABA-T reactivity in the sensory-motor cortex of rat were evaluated histochemically. Special attention was given to the barrels in layer IV of SmI that stain intensely for both enzymes. In the first and second postnatal weeks very low levels of diffuse GABA-T reactivity are seen in cortex, although cells and neuropil in the neostriatum are already clearly positive neonatally. There is little change until 16–18 days postnatal (dpn), when a steady increase in overall cortical reactivity for GABA-T has begun. In layer IV of SmI cortex, relatively intense foci of GABA-T staining begin to appear then, that overlap the barrel centers. GABA-T reactive non-pyramidal neurons at the periphery of these stained foci have distinctly stained processes that may enter the barrel centers. The intensity of GABA-T staining increases until 28 dpn when adult levels were reached. In contrast, AChE staining in cortex begins to appear at 2–3 dpn with prominent barrel staining seen by 6 dpn. When adult AChE reactivity levels are being achieved throughout all regions of neocortex, between 16–19 dpn, the barrel centers progressively lose demonstrable AChE-staining of their fiber plexus. Hence, the onset of GABA-T staining in the barrels during the third week postnatally coincides with the initiation of the progressive reduction in intensity of AChE staining in the same cortical zones. These observations on GABA-T correlate well with biochemical information on the time course of the maturation of the cortical GABAergic system. We should, therefore, consider the possibility that GABA-T staining may reflect the development of the GABAergic system in neocortex cerebri. The temporal linkage between AChE and GABA-T staining, raises the question of a possible developmental interaction between the non-cholinergic AChE-rich thalamic inputs to somatosensory cortex and the maturation of other more tardily developing components of the barrels in SmI. If the temporal relationship is only coincidental then the data still suggest that an intricate series of sequentially timed events characterize the ontogenesis of neocortical circuitry with relatively major developmental events occurring late in the postnatal period.