Quantitative aspects of synaptogenesis in the rat barrel field cortex with special reference to GABA circuitry

The postnatal establishment of cortical connectivity was studied by estimating the number (numerical density, synapse-to-neuron ratio, and total number) of the overall synaptic population and its distribution into gamma-aminobutyric acid (GABA)-immunopositive and GABA-immunonegative synaptic contacts in the developing rat somatosensory cortex. These numerical data were obtained using the unbiased disector method in combination with GABA postembedding immunocytochemistry. The numerical density of both synaptic populations was low in the early postnatal period (postnatal days 5 and 10, P5, P10) after which it abruptly increased between P10 and P15 to approach adult values. However, since cortical volume continues to increase after this age, the number of synapses per neuron and the total number of synapses reached adult values only by P30. There was no evidence of overproduction of either GABA or non-GABA synapses. Direct comparison between the two synaptic populations revealed a similar developmental pattern with the exception of the period around P20 when the production of GABA synapses slowed down. Thus, while the formation of non-GABA synapses proceeded in a continuous manner throughout the first month of life, GABA synapse production was accomplished in two consecutive waves. We suggest that the second delayed wave of GABA synapse formation is related to the great developmental plasticity of the cortical inhibitory system.