Prior to our research on the relationships between odorant chemical structure and the spatial distribution of glomerular responses, the Leon laboratory was engaged for many years in research on the effects of early experience with odors on the structure and function of the rat olfactory bulb. The majority of this work focused on natural types of differential experience that might be responsible for the early learning of maternal odors by rat pups. Although many aspects of this research have been reviewed elsewhere (Leon, 1987; Johnson and Leon, 2001), we will summarize it again briefly here for those who might only now be becoming aware of the work. 

In 1984, Coopersmith and Leon showed that three-week old rat pups that had received daily presentations of peppermint odor paired with tactile stimulation displayed higher levels of 2-deoxyglucose uptake in peppermint-activated glomeruli located in a lateral portion of the olfactory bulb compared to pups that had not had experience with the odor of peppermint extract. The effect lasted into adulthood, even without additional days of exposure (Coopersmith and Leon, 1986). The peppermint-activated glomeruli were not affected by prior experience with cyclohexanone, which stimulated glomeruli in a different bulbar area (Coopersmith et al., 1986). The effect required the pairing of tactile stimulation with the odor: neither odor nor tactile stimulation alone was effective and backward conditioning also was ineffective (Sullivan and Leon, 1986). The effect also was observed for natural odor mixtures such as maternal odor (Sullivan et al., 1990). 

Following the daily experience with peppermint odor, increases in mid-lateral glomerular activity also were observed by way of immunohistochemistry using antibodies to c-Fos (Johnson et al., 1995). C-Fos-like immunoreactivity did not increase in more ventral glomeruli activated by peppermint odor (Johnson et al., 1995). To explore this apparent spatial heterogeneity in the effect of experience, we mapped peppermint-evoked 2-DG uptake across the glomerular layer in the anterior part of the olfactory bulb in rats with or without prior odor experience. We found that the largest changes in glomerular 2-DG uptake indeed were associated with mid-lateral foci of peppermint-evoked activity, although smaller increases were distributed across large regions of the glomerular layer (Johnson and Leon, 1996). Experience with peppermint odor also decreased beta-adrenergic receptor binding in the mid-lateral portion of the glomerular layer, while not having as great an effect on more ventral parts of the bulb (Woo and Leon, 1995). 

The changes in bulbar function associated with early odor experience also were associated with changes in structure. The glomerular layer increased in width in the activated area of the bulb (Woo et al., 1987). This width was associated with both an increase in glomerular diameter and an increase in the number of juxtaglomerular neurons surrounding the activated glomeruli (Woo and Leon, 1991). Activated glomeruli also exhibited an increased density of glial processes after the early odor experience (Matsutani and Leon, 1993).  

The effects of odor experience were different for laminae of the olfactory bulb other than the glomerular layer. Daily odor experience decreased c-Fos like immunoreactivity in the superficial granule cell layer underneath peppermint-activated glomeruli (Woo et al., 1996). Mitral cells displayed more suppressive responses to peppermint odor following prior experience (Wilson et al., 1987), and the numbers of granule cells and mitral cells beneath activated glomeruli did not change with prior odor experience (McCollum et al., 1997).