Alteration of this balance through interference with the function of local inhibitory circuits determines the character-istics and spacing of input segregation for ocular dominance columns formation and also controls the onset of critical periods by regulating perisomatic GABA responses. The level of inhibition present in developing cortical networks plays therefore an important role in fine-tuning cortical circuitry to experience. In line,AZ 960 functional deficits in neurodevelopmental disorders, such as the Down and the Rett syndrome, or autism spectrum disorders have been proposed to be linked to a shift in the balance between excitation and inhibition in the CNS. The majority of currently used general anesthetics potentiates neurotransmission via the GABAA receptor complex and/or inhibit glutamatergic signaling via the blockade of NMDA receptors. Given the important role of GABAergic and glutamatergic signaling during brain maturation,AZD2281 an intrigu-ing possibility is that exposure to general anesthetics during critical periods of development might interfere with neural circuitry assembly. We tested here this hypothesis by examining spine density and dynamics following application of anesthetics or by applying antagonists of excitatory receptors. Using in vivo and in vitro analyses, we find that these pharmacological approaches lead to a rapid regulation of spine and synapse number during critical periods of cortical development. We show that this effect is produced through an enhanced rate of spine and filopodia growth and a better long-term stabilization of newly formed spines, is lasting and results in the formation of functional synapses. Altogether, these results reveal that general anesthetics-induced modulation of neural activity initiates substantial changes in synapse number and dynamics, shaping thereby cortical connec-tivity during critical periods of development. Importantly, these new data also raise essential questions with regard to the debate about the safety and cognitive consequences of administering anesthetics to young infants.