The proportion of neurons receiving fast excitatory cholinergic or serotonergic input depends on postnatal age. A larger fraction of tested cells showed cholinergic EPSCs compared with serotonergic events during the entire developmental period investigated. A, B, The percentage of cells showing these events increased after P14, which corresponds to the ingrowth of thalamic afferents. The mean frequencies of both spontaneous cholinergic and serotonergic EPSCs increased after P30 (C, D), i.e., after eye opening. n, Number of cells included; error bars represent SEM. Spontaneous activity was recorded for a period of several minutes in the presence of glutamate and GABA receptor antagonists, and synaptic events were pharmacologically identified by subsequent addition of DHβE and ICS in each cell included in the plots.