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FIGURE 6.26 Odorant receptor protein is expressed on both dendrites and axons of olfactory sensory
neurons. A.-C. Staining of mouse olfactory epithelium with A. an antibody to extracellular epitope on a par-
ticular odorant receptor, B. an antibody to the cytoplasmic epitope of the same receptor, and C. an antibody
to a different receptor. Scale bars, 10 mm. D-F . Staining of mouse olfactory bulb with the same antibodies.
(From Barnea et al., 2004)
the transmembrane domains. When these are inter-
changed between two parent receptors, the axons
expressing the hybrid receptors may map to the same
glomerulus as the axons that express one of the parent
molecules they may map or to a different glomerulus
altogether. Whatever they do, all the axons that express
the chimeric proteins seem to behave the same way.
These results suggest that odorant receptor molecules
expressed on axons and growth cones may mediate
homotypic adhesion between like axons. In agreement
with this idea is the finding that the majority of axons
that express the same receptor fasciculate with each
other before they enter the glomerular neuropil (Potter
et al., 2001). However, some axons always seem to
follow more tortuous courses without fasciculating
before they terminate in the target glomerulus. This
indicates that homotypic adhesion cannot be the whole
story and that there is also is a direct axon to glomeru-
lus-specific interaction mediated by the olfactory
receptor molecule. If the odorant receptor molecule is
used in target recognition in this way, the next ques-
tion is what these receptors on the growth cone are
sensing and whether particular regions of the olfactory
bulb “smell” different to these axons.
Interestingly, activity may play a critical role in the
refinement of the target selection of odorant sensory
axons. In the section on topographic mapping above, we
discussed a role for such a mechanism in the refinement
of somatosensory axons in the mouse to specific
barrel fields in the cortex. Could something similar be
happening here? Certainly, it is true that axons; that are
active at the same time tend to terminate together; this is
part of the refinement process, and olfactory neurons
with the same receptor molecule would respond the
same way to odorants. However, the capacity of cells
expressing a particular receptor to converge to discrete
glomeruli in the olfactory bulb appears to be uncompro-
mised or minimally affected in transgenic or mutant
mice in which the activity patterns of olfactory neurons
are altered, and elimination of all synaptic transmission
from olfactory sensory neurons appears to leave the
initial targeting process initially intact (Belluscio et al.,
1998; Lin et al., 2000; Zheng et al., 2000). However, if
olfactory sensory neurons express a mutation that
makes them electrically unresponsive to odorants,
though they initially map correctly to their target
glomeruli, they then fail to compete with active axons
expressing the same receptor and are eliminated (Zhao
and Reed, 2001). Interestingly, these axons can be
rescued by nose plugs, which cause the potentially
active axons to be odorant deprived. These studies
suggest that competitive activity is necessary to stabilize
specific connections in the bulb. The idea of competition
is strengthened by the discovery that very early in the
innervation of the bulb, some glomeruli are innervated
by more than one receptor type of axon, but this sorts out
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