Delayed integration of somatostatin interneurons into visual circuits.

In primary visual cortex (V1), neuronal receptive fields are generally thought to be fully established prior to eye-opening, with subsequent experience-dependent refinement controlled by GABAergic inhibition and regulated by homeostatic mechanisms. However, GABAergic interneurons (INs) are diverse and relatively little is known about the early postnatal roles of dendrite-targeting interneurons. Surprisingly, we find that somatostatin-expressing interneurons (SST-INs) in mouse V1 are not visually responsive at eye opening, instead developing visual sensitivity during the third postnatal week. Over the same period, SST-INs exhibit a rapid increase in excitatory innervation without compensatory synaptic scaling. Simultaneous imaging and optogenetic manipulation in juvenile animals reveals that SST-INs largely exert a multiplicative modulation of nearby excitatory neuron responses at all ages, but this effect increases over time. Our results identify a uniquely delayed developmental window for maturation of this inhibitory circuit and its contribution to visual gain normalization.