Inhibitory synaptogenesis in mouse somatosensory cortex

It is widely believed that inhibitory synapses are not present or presentin only small numbers in the rodent cerebral cortex during the earlypostnatal period when the cortex is being innervated by thalamocorticalfibers. Quantitative electron microscopy was carried out on theposteromedial barrel subfield of mouse somatosensory cortex from postnatalday 4 (P4) when thalamocortical innervation of the barrels is becomingestablished, through to sexual maturity (>P32), and in adulthood. Bothasymmetrical (putatively excitatory) and symmetrical (putativelyinhibitory) synapses were present in all layers from P4. The symmetricalsynapses were immunoreactive for GABA at all ages. There was a progressiveincrease in both asymmetrical and symmetrical synapses up to P32, densityin all layers increasing 16-fold, with the production of asymmetricalsynapses leading and greatly outstripping that of symmetrical. From P32 toP120, the oldest age studied, synaptic numbers declined by 18% to 13 timesthe P4 level, but this affected predominantly layers II/III, IV and V, andmainly involved asymmetrical synapses. The relative percentage ofasymmetrical to symmetrical synapses from P4 to P8 was 57%/43% but at P32it was 89.5%/10.5% and in adulthood 85.4%/14.6%. These data indicate thatinhibitory synaptogenesis in the rodent cortex begins earlier thanpreviously thought, a basis for inhibition being present from the earliestperiod. Pruning of all synapses occurs well after thalamocorticalinnervation is established and inhibitory synapses are less affected by thepruning process.