Synaptogenesis in the adult CNS—Hippocampus

The adult dentate gyrus generates granule cells that make thousands of new synapses in the preexisting circuitry. Synapses from GABAergic and glutamatergic neurons along the somatodendritic compartments of new cells develop at variable rates. GABAergic inputs are formed early but develop at slow speed, while glutamatergic inputs display late onset and rapid maturation. This asynchrony defines discrete functional stages with differential capacity for signal integration in new neurons. Glutamatergic excitation reaches sufficient strength to trigger spiking, while inhibitory GABA responses are still weak and immature, resulting in hyperactive cells with facilitated activity-dependent modification of excitatory synapses. The late maturation of GABAergic connections reduces excitability and defines the end of neuronal development. At the output level, synaptogenesis also displays distinct kinetics depending on the nature of the target neurons (principal cells or interneurons) and the region (hilus or CA3). All of these phenomena are modulated by activity, behavior, and the physiological status of the brain.