Somatostatin (Som), one of the most concentrated neuropeptides in the brain, is 
highly expressed in the olfactory bulb (OB). However, the temporal profile by 
which OB somatostatin-expressing (Som+) interneurons are produced and the 
molecular mechanisms controlling this profile are totally unknown. In the 
present study, we found that all the Som+ interneurons in the mouse external 
plexiform layer (EPL) and the rat glomerular layer (GL) express the 
transcription factor Sp8.Using the 5-bromo-2'-deoxyuridine (BrdU) birth dating 
method, combined with immunostaining, we showed that the generation of Som+ 
interneurons in the mouse and rat OB is confined to the later embryonic and 
earlier postnatal stages. Within the mouse OB, the production of Som+ 
interneurons is maximal during late embryogenesis and decreases after birth, 
whereas the generation of Som+ interneurons is low during embryogenesis and 
increases gradually after birth in the rat OB. Interestingly, genetic ablation 
of Sp8 by cre/loxP-based recombination severely reduces the number of Som+ 
interneurons in the EPL of the mouse OB. Taken together, these results suggest 
that Sp8 is required for the normal production of Som+ interneurons in the EPL 
of the mouse OB.