Postnatal development of GABA neurons in the rat somatosensory barrel cortex: a quantitative study

As an estimate of the numerical importance of GABA-containing neurons during development, their quantitative distribution was analysed in the primary somatosensory cortex of rats between postnatal days (P) 5 and 60, using the dissector method and GABA postembedding immunocytochemistry. In relation to the overall number of neurons in the barrel field cortex, the proportion of GABA neurons showed an early significant decrease between P5 and P10 from 14 to 11%, most likely due to termination of transient expression of GABA by some cells. It then remained stable until P20, after which it started slowly but steadily to increase, reaching 14% of the total at P60. The absolute number of GABA neurons also increased by nearly 50% during that period, whereas the number of all neurons remained constant. These changes are seemingly due to a subpopulation of neurons, shown to be of small size, which express GABA late in development. Thus, anatomical adjustments of the cortical GABA system may be observed at least until the end of the second postnatal month, reflecting both delayed maturation and adaptation of this inhibitory circuitry. We suggest the existence of three subpopulations of cortical GABA neurons depending on the time of onset and the regulation of their GABA expression: (i) neurons which express GABA before completion of migration and thus provide for its neurotrophic influence, (ii) neurons which express GABA immediately after completion of migration and build up the cortical inhibitory circuitry, and (iii) neurons which express GABA later in development and represent a substrate of experience-dependent plasticity.