A code for complex objects: Emerging principles for natural odor representation in the brain.

摘要

Recognition of individual body odors is analogous to human face recognition in that it provides information about identity. Individual body odors determined by differences at the major histocompatibility complex (MHC or H-2 in mice) have been shown to influence mate choice, pregnancy block and maternal behavior in mice. Unfortunately, the mechanism and extent of the main olfactory bulb (MOB) and accessory olfactory bulb (AOB) involvement in the discrimination of animals according to H-2-type has remained ambiguous. Odorous quality is hypothetically represented by a combinatorial code—activation of distinct, but overlapping subsets of olfactory receptors resulting in activation of a distinct subset of glomeruli (structures analogous to cortical barrels of the somatosensory system) in the olfactory bulb—the first central processing station.; This thesis investigates the neuronal activation patterns evoked in the MOB in different individuals upon exposure to these complex, biologically meaningful sensory stimuli. In order to accomplish this, a new mapping method to generate two- and three-dimensional maps of the glomerular sheet in the olfactory bulbs of mice is presented. Next, we demonstrate that body odors from congenic mice (H-2k and H-2b; differ at all their class I genes) evoke overlapping but distinct maps of neuronal activation in the MOB. Then, we show that modification of a single gene (the K gene of the MHC locus in mouse), which results in a change in the odoriferous quality of urine barely detectable by conspecifics, causes a small, but significant change in the glomerular activation pattern in the MOB. The spatial patterns of odor-evoked activity are sufficient to predict H-2 identity. These results provide functional evidence for discrimination of H-2 determined body odors in the MOB, but do not preclude a role for the AOB. Lastly, evidence is provided that shows the magnitude of disparity between urine evoked spatial activity patterns is predictive of the extent of genetic difference among the donor's and the receiver's ability to discriminate them. Thus, we demonstrate that spatial activity patterns are tightly linked to behavior. These data show that the combinatorial code applies to complex odors such as urine and further our understanding of the neural strategies employed to decode socially relevant odors.