Cell, Vol. 79, 951-991, December 16, 1994, Copyright 0 1994 by Cell Press
Topographic Organization of Sensory Projection
to the Olfactory Bulb
Robert Vassar, Steve K. Chao, Raquel Sitcheran,”
ennifer M. Nuiiez, Leslie B. Vosshall,
and Richard Axel
Department of Biochemistry and Molecular Biophysics
and Howard Hughes Medical Institute
Columbia University College of Physicians and Surgeons
ew York, New York 10032
The detection of odorant receptor mRNAs within the
axon terminals of sensory neurons has permitted us
to ask whether neurons expressing
reject their axons to common
olfactory bulb. In situ hybridization
receptor probes demonstrates
rons expressing a given receptor converge on one,
or at most, a few glomeruli
Moreover, the position of specific glomeruli is bilater-
ally symmetric and is constant in different individuals
within a species. These data support a model in which
exposure to a given odorant may result in the stimula-
tion of a spatially restricted set of glomeruli, such that
the individual odorants would be associated with spe-
cific topographic patterns of activity within the olfac-
a given receptor
with five different
that axons from neu-
within the olfactory bulb.
in vertebrate sensory systems, peripheral neurons receive
information from the environment and transmit this infor-
mation to the brain, where it is processed to provide an
internal representation of the external world. Mammals
possess an o!factory system of enormous discriminatory
power. Humans, for example, are thought to be capable
of distinguishing among thousands of discrete odors. This
diversity of odor recognition is coupled with remarkable
specificity, such that subtle alterations in the molecular
structure of an odor can often lead to profound changes
in perceived odor quality.
The initial step in olfactory discrimination requires the
interaction of odorous ligands with a family of seven trans-
membrane domain receptors on olfactory sensory neu-
rons. The repertoire of mammalian olfactory receptors is
extremely large and consists of about 1000 different genes
(Buck and Axel, 1991; Levy et al., 1991; Parmentier et
al., 1992; Ben-Arie et al., 1994). Discrimination among
odorants requires that the brain determine which of the
numerous receptors have been activated. In situ hybridiza-
tion with olfactory receptor genes suggests that each cell
expresses only one or a small number of receptor genes,
such that individual olfactory neurons are functionally dis-
tinct (Ngai et al., 1993a; Ressler et al., 1993). The problem
“Present address: Department of Physiology, University of California,
San Francisco, San Francisco, California 94143.
of distinguishing which receptor has been activated can
therefore be reduced to a problem of distinguishing which
neurons have been activated.
How does the brain determine which of the functionally
distinct sensory neurons have been activated? In other
sensory systems, defined spatial patterns of sensory neu-
rons or their projections are used to indicate the quality
of the stimulus. In the somatosensorysystem,
the different somatic sensorysubmodalities
oception, nociception, and thermoreception)
the activation of distinct sensory cells that project to spe-
cific regions of the brain via topographically segregated
pathways (Mountcastle, 1957; Perl et al., 1962; Dykes et
al., 1982; Berkley, 1985). The olfactory system, therefore,
may also use spatial segregation of sensory input to en-
code the identity of the odorant stimulus.
What features of the vertebrate olfactory apparatus
might form the anatomic basis for a spatial map of olfactory
information? Odorant stimuli are received from the envi-
ronment by receptors on olfactory sensory neurons in the
olfactory epithelium (Figure 1). Each olfactory neuron pro-
jects a single unbranched axon. As the collection of axons
emerge from ihe olfactory mucosa, they fasciculate to form
the olfactory nerve. The axons of the olfactory neurons
synapse with dendrites of the mitral and tufted cells in the
olfactory bulb, the first relay station for olfacicry signaling
in the brain. The mitral and tufted cells of the olfactory
bulb in turn project axons to higher cortical centers via the
olfactory tract. Thus, the anatomy of the olfactory system
affords the opportunity for spatial segregation of afferent
sensory input at all levels from the peripheral epithelium
to the olfactory cortex (reviewed by Shepherd, 1991; Res-
sler et al., 1994).
We have used receptor genes as molecular probes to
map the position of individual sensory neurons in the epi-
thelium as well as their projections to the olfactory bulb.
In situ hybridization with specific receptor probes in fish
demonstrates that neurons expressing individual recep-
tors are randomly distributed in the epithelium (Ngai et
al., 1993a). In mammals, neurons expressing a specific
receptor segregate within one of four broad but circum-
scribed zones within the epithelium (Ressler et ai., 1993;
Vassar et al., 1993). However, within a given zone, neu-
rons expressing a specific receptor appear to be randomly
distributed. Despite the restriction of specific receptors to
one of a small number of broad domains, the most im-
portant feature of this sort of organization is the random
distribution of receptor expression within a given domain.
These observations suggest that if spatial segregation
is employed to encode odor quality, neurons expressing a
given receptor, although randomly distributed throughout
,the epithelium, must project their axons to one or a subset
of spatially defined glomeruli in the olfactory bulb. Such
a model is consistent with anatomic considerations indi-
cating that the number of glomeruli, lOOO-3000 in mam-
mals (Meisami, 1979; Royet et ai., 1988) and 80 in fish
(Baier and Korsching, 1994), approximates the number of
Figure 1. Cellular Organization
Sensory neurons in the olfactory epithelium project axons to spherical
regionsof neuropil in theolfactorybulbcalledglomeruli.
Ius represents a discrete unit of synaptic contact comprising the termi-
nals of approximately 3000 sensory axons and the dendrites of mitral,
tufted, and periglomerular cells, The principal output neurons of the
bulb are the mitral and tufted cells, which project axons to the cortexvia
the olfactorytract. Granule and periglomerular
interneurons. (Adapted from Kandel et al., 1991.)
of the Olfactory Bulb
cells are iocal inhibitory
receptor genes in each organism (Buck and Axel, 1991;
Ngai et al., 19936). This model is in accord with previous
experiments demonstrating that different odorants elicit
spatially defined patterns of glomerular activity in the olfac-
tory bulb. Optical recordings (Kauer et al., 1987), meta-
bolic labeling (Stewart et al., 1979; Lancet et al., 1982)
and electrophysiological studies (Imamura at al., 1992;
Katoh et al., 1993) demonstrate that individual glomeruli
are differentially responsive to distinct odors. These obser-
vations suggest that neurons responsive to a given odor-
ant project to spatially defined glomeruli within the olfac-
In this study, we provide physical evidence that neurons
expressing a given receptor project their axons to one or
a small number of discrete glomeruli within the olfactory
bulb. We have exploited the observation that receptor
mRNA can be detected in the terminals of sensory axons
in the olfactory bulb to map the projections of specific
sensory neurons. We observe that neurons expressing a
given receptor project their axons to one or a small number
of topographically fixed glomeruli. The positions of specific
glomeruli are bilaterally symmetric and are conserved in
the brains of all animals within a species. These data sup-
port a model ofolfactorycoding
of odor quality would result from the detection of specific
spatial patterns of glomerular activity within the olfactory
in which the
in the Olfactory Bulb
Experiments were designed to ask whether neurons ex-
pressing a given receptor converge upon a discrete subset
of glomeruli within the olfactory bulb. Sensory neurons
expressing a given receptor are randomly distributed
within zones of the olfactory epithelium. To define a spatial
map of the projections of the olfactory neurons within the
bulb, it is necessary to identify markers that distinguish
the axons of neurons expressing different receptors. The
specific receptors or their respective mRNAs would pro-
vide obvious markers if they were expressed in axons.
mRNA in neurons is largely localized to the cell body; occa-
sionally, specific RNAs are observed in dendrites, but
mRNA is rarely found in axonal projections (Jirikowski et
al., 1990; Brunet et al., 1991; Mohr et al., 1991; Land-y
et al., 1992; reviewed by Steward and Banker, 1992). The
mRNA encoding specific odorant receptors is abundant
within the neuronal cell bodies in the sensory epithelium.
Estimates from in situ hybridization analyses suggest that
an olfactory sensory neuron expresses about 1000 copies
of the mRNA for a single, specific receptor (data not
shown). Each glomerulus in the bulb results from the con-
vergence of axonal projections from about 3000 neurons
(Meisami, 1979, 1989). If a receptor mRNA molecule is
infrequently found at the axon terminal and neurons ex-
pressing a given receptor indeed synapse on a single glo-
merulus, this convergence should permit the detection of
specific receptor mRNAs in individual glomeruli by in situ
Control experiments were performed using the mRNA
encoding the olfactory marker protein (OMP), which is ex-
pressed in all olfactory sensory neurons (Farbman and
Margolis, 1980) at a level ten times higher than that of
receptor mRNA (R. V. and R. A., unpublished data). Previ-
ous studies have detected OMP mRNA in the olfactory
bulb by RNA blot hybridization (Rogers et al., 1987; Ehrlich
et al., 1990). We therefore performed in situ hybridization
experiments to ask whether OMP mRNA was detectable
within axon terminals in the glomeruli of the olfactory bulb.
Sensory axons from the olfactory epithelium enter the OS-
factory nerve layer of the bulb and synapse with the den-
drites of the mitral and tufted cells to form discrete spheri-
cal glomeruli. The axons of the mitral and tufted cells form
the major output tract of the bulb that projects to the olfac-
tory cortex (Figures 1, 2A, and 25).
Coronal sections through the olfactory bulb were an-
nealed with a33P-labeled OMP probe and exposed to emul-
sion for 1 day. Strong hybridization signals are apparent
throughout the olfactory nerve layer and in the vast major-
ity of the glomeruli (Figure 2C). Hybridization is not ob-
served in the mitral cell layer, nor in the granule cell layer.
This pattern contrasts with in situ hybridization with a
of a Topographic
of Sensory Axons in the Olfactory Bulb
probe detecting the glutamate receptor subunit GluRl,
which identifies the bulbar neurons including the periglo-
merular, mitral, and granule ceils (Figure 2D; Hollmann
et al., 1989; Keinanen et al., 1990; Nakanishi et al., 1990).
These data suggest that the OMP mRNA detected is not
derived from bulbar neurons, but rather results from RNA
present within the axon terminals of olfactory sensory
The observation that OMP mRNA could be detected in
sensory axon terminals prompted us to ask whether similar
experiments could be used to detect receptor mRNA
within individual glomeruli after significantly longer expo-
sures. A 33P-labeled RNA probe for the rat F12 receptor
genes was used in in situ hybridization experiments on
coronal sections of the bulb (Figure 3). The F12 receptor
probe detects five genes in the genome (Buck and Axel,
1991). In situ hybridization with the F12 probe on serial
sections through the entire olfactory bulb reveals hybrid-
ization signals over only five discrete glomeruli. The five
sections that exhibit the positive glomeruli are shown in
Figures 3A-3E. Importantly, each of these five glomeruli is
bilaterally symmetric; positive glomeruli occupy the same
relative position in the right and the left bulb. These results
provided initial evidence that sensory neurons expressing
a given receptor project to one or a small number of glo-
meruli that are spatially defined within the olfactory bulb.
We have established a more complete topographic map
by performing in situ hybridization with probes to four addi-
tional receptor subfamilies (Figure 4). Hybridization with
a probe for the single copy receptor I7 identifies only two
giomeruli. One glomerulus resides in the ventrolateral re-
gion of the bulb (Figure 4C), whereas the second is more
Figure 2. Anatomy of the Pat Olfactory Bulb
(A) Coronal section of rat olfactory bulb follow-
ing in situ hybridization with a 33P-labeled OMP
antisense RNA probe and exposure to emul-
sion for 1 day. In this bright-field micrograph,
silver grains appear light gray and are ob-
served over the olfactory nerve layer and glo-
meruli. Neuronal cell bodies are stained blue.
(6) Drawing of the section in (A), identifying
the different layers of the olfactory bulb (GL,
glomerular layer; EPL, external plexiform layer;
MCL, mitral cell layer; ONL, olfactory nerve
layer; GCL, granule cell layer; D, dorsal; V,
(C) Dark-field micrograph of oifactory bulb sec-
tion in (A). Strong OMP probe hybridization
apparent in the olfactory nerve !ayer and the
mRNA in olfactory sensory axons and axon ter-
minals Signal is not observed over the cell bod-
ies of olfactory bulb neurons.
(D) Coronal section of rat olfactory
nealed with a33P-labeled GluAl antisense RNA
probe, exposed to emuision for one day, and
photographed in dark field. Dense silver grain
clusters appear over the cell bodies of mitral,
periglomerular, and granule celis, while no hy-
bridization is observed in the olfactory nerve
layer or in the neuropil of glomeruli. Scale bar
in (D) equals 760 urn in (A), (C), and (D)
the presence of OMP
medially situated (data not shown).
meruli are bilaterally symmetric. In situ hybridization with
the F5 probe, which anneals with a subfamily of 11 recep-
tor genes, identifies about seven positive glomeruli (Fig-
ures 4A and 4D). The F6 subfamily includes three genes,
and F6 probes identify three glomeruli within the right and
left olfactory bulbs (Figures 48 and 4E). The J7 probe
hybridizes to two receptor genes and identifies two glomer-
uli (Figure 4F). Representative examples of a subset of
glomeruli hybridizing with each probe are shown in Figures
4A-4F. The vast majority of positive glomeruli are bilater-
ally symmetric. However, the two symmetric glomeruli are
not always observed in the same section, but may be dis-
placed a few sections apart due to skewing of the bulb
during embedding or sectioning procedures. In most in-
stances, axons converge on an isolated glomerulus; oc-
casionally, we observe that neurons expressing a given
receptor converge on a small cluster of contiguous
It is not possible to estimate accurately the percentage
of axons from neurons expressing a given receptor that
project to an individual glomerulus. At one extreme, it is
possible that all of the axons from a given class of sensory
neuron project to only a single glomerulus. Alternatively,
only a subpopulation of axons may converge on a single
glomerulus, with the remainder broadly distributed through-
out the olfactory bulb. The convergent axons would be
readily detected by in situ hybridization, whereas the dis-
tributive projections would be below the level of detection
in our assays. This notwithstanding, our data with five dif-
probes, which hybridize to 22 genes and
identify only 19ofthe3000glomeruli, stro~~~y~uggestt~at
Figure 3. Localization of Odorant Receptor F12 RNA lo Discrete Glomeruli within the Olfactory Bulb
Coronal sections of rat olfactory bulb were annealed with a 33P-labeled F12 receptor antisense probe, washed under high stringency
and exposed to emulsion for 4 weeks.
(A-E) Dark-field micrographs of sections that demonstrate the five pairs of discrete glomeruli hybridizing with the F1.2 receptor probe. Sections
are ordered in an anterior-to-posterior fashion. Arrows indicate the positions of weakly positive glomeruli. Note that pasitive glomeruli are bilaterally
symmetric. Occasionally, glomeruli did not occur within the same section, a result most likely due to misalignment
procedures. In these cases, frames including each glomerulus of a pair were spliced together as in (A), (D), and (E). The F12 receptor probe
recognizes five genes in the rat genome.
(F) High magnification bright-field micrograph of the positive glomerulus on the right half of the section in (C). The cell bodies of periglomerular
cells (stained blue) indicate the boundaries of individual glomeruli. Note that silver grains are observed over a single glomerulus
whereas neighboring glomeruli are negative. Scale bar in (F) equals 760 brn in (A)-(E) and 150 pm in (F).
of the bulb during embedding
a significant subpopulation of axons from sensory neurons
expressing a given receptor converge on one or a small
number of glomeruli within the olfactory bulb.
Rather than Bulbar Neurons
We have performed several experiments that indicate that
hybridization to discrete glomeruli is the consequence of
mRNA present within the axons of sensory neurons, rather
than mRNA expressed in the cell bodies or processes of
olfactory bulb neurons. First, hybridization with specific
receptor probes is restricted to the glomeruli. Hybridiza-
tion is never observed over the cell bodies of any of the
olfactory bulb neurons. Moreover, we have performed con-
trol experiments with the glutamate receptor subunit
GluRl (Hollmann et al., 1989; Nakanishi et al., 1990).
GluFil is expressed by neurons within the olfactory bulb,
but is not expressed by the presynaptic sensory cells (Kei-
n&en et al., 1990). Hybridization with GluRl is apparent
over the mitral cell bodies and in the periglomerular cells
Detects mRNA in Sensory Axons
of the bulb that outline the glomeruli (see Figure 2D). This
pattern contrasts with the restricted glomerular labeling
observed upon in situ hybridization with odorant receptor
probes. Receptor mRNA is abundant in the cell bodies of
the sensory neurons, but no hybridization is apparent
within the cell bodies of mitral cells. These data suggest
that the receptor hybridization observed in glomeruli re-
sults from receptor mRNA present within the axons of sen-
sory neurons rather than from receptor mRNA within the
dendrites of a specific subpopulation of mitral cells.
Further evidence that the hybridization to discrete glo-
meruli reflects the presence of receptor mRNA in the ax-
ons of sensory neurons derives from the frequent detec-
tion of receptor mRNA in axon fascicles entering specific
glomeruli. Receptor probes such as F5, which exhibit
strong hybridization signals, reveal labeling not only over
the glomeruli, but also in the fasciculated sensory axons
entering the glomeruli (Figure 5). Serial sections reveal
labeling over axon bundles in the olfactory nerve layer
that then enter specific F5 glomeruli. These observations
provide strong evidence that the mRNA detected within
g;gnvergence of Sensory Axons in the Olfactory Bulb
Figure 4. Locaiization of Additional Receptor RNAs to Specific Glomeruli
Dark-field micrographs showing coronal sections of rat olfactory bulbs following in situ hybridization with 32P-or =P-labeled antisense RNA probes
for receptor subfamilies F5 (A and D), F6 (B and E), 17 (C), and J7 (F). Representative
giomeruli are shown for each probe. Arrows indicate the positions of weakly positive glomeruli. The number of glomeruli identified and the number
of genes recognized by a given probe, respectively, are noted for each probe: F5, 7 and 11; F6, 3 and 3; 17, 2 and 1; J7, 2 and 2. Scale bar in
(F) equals 760 w.m for (A)-(F).
examples of a subset of positive, bilaterally symmetric
individual giomeruli reflects the presence of receptor
mRNA in sensory axon terminals.
Finally, we have demonstrated that the receptor hybrid-
ization to glomeruli disappears after sensory axons are
destroyed (Figure 6). When the sensory epithelium is ex-
posed to the detergent Triton X-100, olfactory neurons
die and the sensory axons degenerate within 1-2 weeks
(Margolis and Grillo, 1978; Nadi et al., 1981; Verhaagen
et al., 1990). If the hybridization to individual glomeruli
results from the presence of mRNA within sensory axon
terminals, we predict that deafferentation would result in the
loss of glomerular hybridization signals. If, however, the
hybridization signals derive from mRNA present within
the dendrites of a specific subpopulation of mitral cells,
we would expect that deafferentation should not alter the
Chemical treatment produces a range of deafferentation
varying from animal to animal. We therefore monitored
the extent of axonal destruction in individual animals by
examining the thickness of the olfactory nerve layer and
the persistence of OMP mRNA in axon terminals. To pro-
vide an internal control for receptor hybridization, we
chose an animal in which degeneration appeared to be
severe on the right side and relatively mild on the left (Fig-
ures SA and (33). This degeneration correlated with de-
creased signal for OMP mRNA in the right olfactory epithe-
lium compared with the left (data not shown). Hybridization
“with the OMP probe is significantly diminished on the right
side of the bulb, consistent with the degeneration of sen-
sory axons (Figure 6A). Degeneration in this animal was
most severe on the lateral aspect of the right olfactory
bulb. Hybridization with the F12 receptor probe reveals
no positive glomeruli in the right olfactory bulb, whereas
positive glomeruli were apparent in sections throughout
the left bulb (Figure 6B). A similar result was obtained with
the F6 receptor probe hybridized to alternate sections of
the same olfactory bulb (data not shown).
As a control, we demonstrated that bulbar neurons re-
main intact despite the degeneration of the sensory axons.
The patternsof hybridization forGluR1 mRNA, which iden-
tify the bulbar neurons, appear normal in an animal that
received multipie chemical treatments and sustained se-
vere deafferentation (Figure 6C).
These experiments provide strong evidence that the hy-
bridization to individual glomeruli is the consequence of
receptor mRNA present within the axon terminals of sen-
sory neurons. Thus, the pattern of hybridization with re-
ceptor probes in the olfactory bulb is likely to reflect accu-
rately the sites of convergence of axons from neurons
expressing a specific odorant receptor.
Figure 5. Localization of Cdorant Receptor RNA to Sensory Axons within the Olfactory Bulb
(A-D) Dark-field micrographs showing serial coronal sections of a rat olfactory bulb following in situ hybridization
antisense RNA probe. Sections are ordered in anterior-to-posterior
(6 and C). Labeling is also apparent over sensory axon fascicles in the olfactory nerve layer that enter the positive glomeruli (A-Q).
(E) High magnification bright-field micrograph of an olfactory bulb section from a different animal hybridized with the F5 probe. Silver grains are
observed over a sensory axon fascicle entering a single glomerulus. Scale bar in (E) equals 760 pm in (A)-(D) and 150 pm in (E).
with 32P-iabeled receptor F5
fashion. Intense hybridization is observed over bilaterally symmetric glomeruli
The Positions of Positive Glomeruli Are
Constant in All Individuals
We next asked whether the convergent sites of projections
from sensory neurons expressing a given receptor are
topographically fixed, or whether the positions of these
sites differ in different individuals. Each of the five receptor
probes was therefore annealed to sections from at least
three individuals, and the relative positions of the positive
glomeruli were determined. In one particularly clear exam-
ple, we identified the position of one of the five F-i 2 glomer-
within a Species
uli in four different animals (Figure 7). This Ft.2 glomerulus
is bilaterally symmetric and maintains the same relative
position in the olfactory bulbs of all four animals. A more
complete analysis of the relative position of 19 glomeruli
confirms the constancy of glomerular position in different
animals. It is difficult in these experiments to determine
precisely the position of a specific glomerulus in different
bulbs owing to the inherent variation in the size and shape
of the olfactory bulb in different individuals. Nonetheless,
most glomeruli can be identified in the same relative posi-
Figure 6. In Situ Hybridization
The nasal cavities of adult rats were exposed to Triton X-100 causing preferential
Procedures). Adjacent bulb sections from a treated animal were hybridized with either OMP (A) or receptor F12 (8) 32P-labeled antisense RNA
probes and micrographed in dark field. In a separate experiment, olfactory bulb sections from a treated animal were hybridized with a33P-labeled
GluRl antisense RNA probe (C). Sensory axon degeneration was most severe on the lateral aspect of the right bulb as measured by the thickness
of the olfactory nerve layer (compare right and left bulbs in [A)). The intensity of OMP labeling is substantially
the right bulb (A), correlating with sensory axon degeneration. Hybridization
positive glomeruli in the denervated right bulb, whereas positive glomeruli are apparent in the left bulb (arrow in IS]). In contrast, the pattern of
GluRl mRNA expression in the olfactory bulb is unchanged following deafferentation
of Olfactory Bulb Sections Following Sensory Axon Degeneration
deafferentation of the right otfactory bulb (see Experimental
decreased on the lateral aspect of
of adjacent bulb sections with the F12 receptor probe (8) reveals no
(C). Scale bar in (C) equals 760 nm for (A)-(C).
of Sensory Axons in the Olfactory Butb
Figure 7. The Position of individual Glomeruli
Is Constant in Different Animals
tions from four individuals following in situ hy-
bridization with the F12 receptor probe. The
four panels show the position of one of the five
F12-positive glomeruli in four different individu-
als. Note that the F12 glomerulus is bilaterally
symmetric and maintains the same relative po-
sition in all four olfactory bulbs. Scale bar in
(D) equals 760 &rn in (A)-(D).
of olfactory bulb sec-
tion in all individuals analyzed. These observations dem-
onstrate that neurons expressing a given receptor project
to topographically distinct glomeruli. Moreover, the posi-
tions of the specific glomeruli are not random; rather, they
remain relatively constant in all individuals in a species.
?he initial event in the perception of odors involves the
interaction of odorous ligands with receptors on sensory
neurons. Discrimination among odors therefore requires
a mechanism by which the brain can discern which of
the numerous receptors have been activated. Individual
olfactory sensory neurons are likely to express only one
a small number of receptor genes (Ngai et al., 1993a;
ssler et al., 1993). The problem of distinguishing which
receptors have been activated therefore reduces to a prob-
lem of distinguishing which neurons have been activated.
A topographic map defining the positions of specific neu-
rons in the olfactory epithelium, or the positions of their
axons in the olfactory bulb, may be employed to determine
which of the neurons have been activated. In this manner,
the olfactory system may use spatial segregation of sen-
sory input to encode the quality of an olfactory stimulus.
Previous studies have examined the spatial distribution
of neurons expressing specific odorant receptors in the
sensory epithelium (Nef et al., 1992; Strotmann et al.,
1992; Ngai et al., 1993a; Raming et al., 1993; Ressler
et al., 1993; Vassar et al., 1993). In mammals, neurons
expressing a specific receptor are distributed within one of
four broad but circumscribed zones within the epithelium.
Within a given zone, however, neurons expressing a spe-
cific receptor are not spatially segregated, but rather are
Map Encoding Odor Quality
randomly dispersed within the epithelium (Ressler et al.,
1993; Vassar et al., 1993). We have therefore asked
whether neurons expressing a given receptor project their
axons to common glomeruli within the olfactory bulb. The
observation that individual neurons express about 1000
receptor mRNA molecules suggested an experimental ap-
proach to map axonal projections based upon the pres-
ence of mRNA at the axon terminal. A single glomerulus,
about 200 pm in diameter, represents the site of conver-
gence of about 3000 sensory axons. If the neurons that
project to a single glomerulus each express the same re-
ceptor mRNA and a small number of mRNA molecules
are present at the axon terminal, then this convergence
of axons might permit the identification of individual glo-
meruli by in situ hybridization
probes. In this study, we demonstrate that the neurons
expressing a given receptor project to one, or at most, a
few glomerular targets among the thousands of glomeruli
within the olfactory bulb. Our data with five different recep-
tor probes, which hybridize to 22 genes and identify only
19 of the 3000 glomeruli, strongly suggest that a significant
subpopulation of axons from sensory neurons expressing
a given receptor converge on one or a small number of
glomeruli within the olfactory bulb. Moreover, these pro-
jections define specific glomeruli that maintain a fixed po-
sition in the brains of all animals within a species.
Preliminary analysis indicates that the axons of neurons
from one topographic zone in the epithelium tend to con-
verge on glomeruli that are localized to a discrete region
of the olfactory bulb. For example, neurons expressing
?he F6, J7, and F5 receptor families are expressed in the
same epithelial zone and project to glomeruii that segre-
gate within a ventromedial strip of the bulb (Figure 8, right).
These observations are consistent with i-etro
with specific receptor
Figure 8. Summary of Specific Glomerular Positions within the Olfac-
Schematic representations of the lateral (left) and medial (right) sur-
faces of the right olfactory bulb. The relative positions of 19 glomeruli
identified with five different odorant receptor probes are summarized
Glomeruli labeled with specific probes are color coded as follows: F5,
green; F12, red; F6, blue; 17, black; J7, yellow. The majority of F5,
F6, and J7 glomeruli segregate within an anterior-posterior
the ventromedial surface of the olfactory bulb (right; shaded gray). (A,
anterior; P, posterior; D, dorsal; V, ventral.)
experiments demonstrating that neurons within defined
regions of the epithelium project to broad but circum-
scribed regions of the olfactory bulb (Astic and Saucier,
1986; Saucier and Astic, 1986; Astic et al., 1987; Schoen-
feld et al., 1994). Thus, the zonal organization of receptor
expression in the epithelium may be preserved in the pro-
jection of sensory axons to the bulb. Compartmentaliza-
tion of the epithelium and the bulb into anatomically and
functionally discrete units of lesser complexity may reduce
the problems inherent in regulating the expression of spe-
cific receptors and may facilitate the segregation of axons
within the olfactory bulb.
Our data are consistent with previous studies sug-
gesting that glomeruli represent functional units, such that
specific odorants elicit spatially defined patterns of glo-
merular activity in the olfactory bulb. For example, after
exposure to a single odorant, enhanced 2-deoxyglucose
uptake is observed over a restricted group of glomeruli.
Moreover, this pattern of P-deoxyglucose labeling of spe-
cific glomeruli is distinct for different odors (Stewart et al.,
1979; Jourdan et al., 1980; Lancet et al., 1982; Royet et
al., 1987). Optical recordings of the olfactory bulb using
voltage-sensitive dyes similarly reveal diffuse yet circum-
scribed patterns of activity in response to single odors
(Kauer et al., 1987). These patterns are distinct but over-
lapping for different odors.
Electrophysiologicstudies provide independent support
for a model of olfactory discrimination in which individual
glomeruli are differentially responsive to different odors.
Extracellular recording of spike responses from single mi-
tral cells of a glomerulus indicate that individual glomeruli
are “tuned” to discrete molecular structures (Mori et al.,
1992; lmamuraetal., 1992; Katoh et al., 1993). The individ-
ual mitral cells of a given glomerulus might all be maximally
responsive to o&o-xylene, whereas the mitral cells of a
topographically distinct glomerulus respond most strongly
to para-xylene (Katoh et al., 1993).
These experimental approaches demonstrate the acti-
vation of specific glomeruli with specific odors, but cannot
relate the activation of specific sensory neurons with spe-
cific patterns of glomerular activation. Our data provide
direct anatomic evidence that neurons expressing a given
receptor, and therefore responsive to a given odorant,
project with precision to one or a small number of glomeruli
within the olfactory bulb. Since the positions of the individ-
ual glomeruli are topographically defined, the olfactory
bulb provides a two-dimensional map that identifies which
of the numerous receptors have been activated within the
sensory epithelium. One important implication of such a
model is that exposure to a given odorant would result in
the stimulation of a spatially restricted set of glomeruli,
such that individual odorants would be associated with
bulb. The quality of an olfactory stimulus would therefore
be encoded by the specific combination of glomeruli acti-
vated by a given odorant.
How does the olfactory cortex decode this two-dimensional
map within the bulb to allow the discrimination among a
diverse array of odors? A given neuron is likely to express
only one or a small number of receptors from the repertoire
of 1000 genes (Ngai et al., 1993% Ressler et al., 1993).
Since the organism can detect far greater than 1000 dis-
crete odors, this implies that a given odorant will interact
with multiple receptors, and a single receptor will interact
with multiple odorous ligands. In this manner, an odorant,
even one that consists of a single molecular species, will
interact with a unique combination of receptors, which in
turn results in the activation of specific combinations of
glomeruli. The maintenance of a fixed map of glomeruli
in the bulb therefore provides a mechanism to relay the
information resulting from the activation of receptors in
the periphery to the brain.
How is such a combinatorial code of glomerular activa-
tion interpreted by the cortex’? In one model, the topo-
graphic map in the olfactory bulb could project directly to
olfactory cortex, providing a more central representation
of the glomerular map. Cortical loci activated by a given
odorant might then converge on higher centers to facilitate
odor recognition. Alternatively, it is possible that each of
the multiple glomeruli activated by a given odor would
project fibers that converge on a single locus in the olfac-
tory cortex. Each locus in the olfactory cortex would reflect
the convergence of output fibers from a unique combina-
tion of glomeruli, such that the activation of a cortical locus
would encode a given odor quality. In this model, each
odor would result in the activation of a unique combination
of glomeruli, and each combination would be encoded in
a specific locus in the cortex. This model is consistent
cells from a single glomerulus project to multiple loci in
the olfactory cortex (Schwab and Price, 1978; Ojima et
al., 1984; Buonviso et al., 1991). Moreover, retrograde
tracing studies demonstrate that a cortical locus receives
input from multiple glomeruli (Haberly and Price, 1977;
Scott et al., 1980; Luskin and Price, 1982).
In either model, the ability of an odorant to activate a
combination of glomeruli allows for the discrimination of
for Sensory Processing
indicating that mitral
&wergence of Sensory Axons in the Olfactory Bulb
tom and their associated glomeruli. Despite the diversity
afforded by combinatorial activation of glomeruli, the or-
ganism is likely to be capable of discriminating only an
extremely small subset of potential odors. As in other sen-
sory systems, the olfactory system only perceives a mea-
ger image of the sensory information in the environment.
This restricted array of recognizable odors will be a func-
tion of the repertoire of receptors expressed by the organ-
ism as well as the nature of the glomerular combinations
that are encoded in the cortex. Those odors that are per-
ceived from the universe of potential odorants presumably
reflect selection for that subset of odorants that are biologi-
cally important for only the species. In this manner, individ-
uals within a species will maintain a repertoire of receptor
genes and a combination of cortical connections that per-
mit the detection of those odors important for its survival
This model of olfactory perception shares several basic
features with perception in other sensory systems. The
brain analyzes avisual image by interpreting the individual
components of the image: form, location, movement, color
(Livingstone and Hubel, 1987; Zeki and Shipp, 1988). The
unity of an image is accomplished by several parallel pro-
cessing pathways, such thatthe image is initially dissected
into tractable components
higher visual centers of the cortex. A similar logic may
be employed to discriminate olfactory stimuli. Olfactory
processing initially requires that the odorant molecule is
analyzed by “dissecting” its structural features, such that
each of its structural elements will activate a unique subset
of receptors, which in turn results in the activation of a
unique subset of glomeruli. The odorous stimulus is then
reconstructed by the olfactory cortex, which determines
which of the numerous glomeruli have been activated.
and then reconstructed in
in the Olfactory Bulb
The establishment of a topographic map of sensory projec-
tions might initially involve the expression of receptors by
sensory ceils. The choice of a given receptor would then
be linked to the expression of guidance molecules, such
that neurons expressing a given receptor project axons
to a single glomerulus. Alternatively, it is possible that im-
mature neurons, not yet expressing receptor, randomly
project axons to any one of multiple glomeruli within the
olfactory bulb. Contact with an individual glomerulus may
then elicit a retrograde signal that directs the expression
of specific receptor genes. In this manner, receptor gene
expression would be regulated by the ultimate position of
its axon terminal within the olfactory bulb. This model is
unlikely, since we observe receptor expression in sensory
neurons early in embryogenesis (rat embryonic day 14)
prior to the birth of mitral cells and the presence of glomer-
uli (C. Dulac and R. A., unpublished data). Moreover, anal-
ysis of receptor gene expression in individual neurons sug-
gests that the choice of receptor expressed by a neuron is
not regulated, but rather is stochastic (Chess et al., 1994).
Thus, it is likely that neurons stochastically express one
receptor gene prior to synapse formation. Neurons ex-
of a Topographic Map
pressing a given receptor will then form synapses with
one or a small number of topographically fixed glomeruli.
How is this precise topographic map established? One
attractive model combines guidance mechanisms during
development with activity-dependent refinement to achieve
the precision of connections between the sensory epithe-
lium and olfactory bulb. It is possible, for example, that
axonsof neuronsexpressing asubpopulation of the recep-
tor repertoire are restricted in their projections to broad
but defined regions of the olfactory bulb. Neuronal growth
cones may, for example, be guided to their appropriate
target by a gradient of recognition molecules. This guid-
ance process could establish a coarse topographic map.
Activity-dependent mechanisms, as a consequence of the
synchronous firing of neurons responsive to a given odor-
ant, could refine the pattern of synaptic connections within
this restricted region of the bulb to establish a relatively
precise topographic map. Such a model draws heavily on
observations in the visual system indicating that retinal
axons first use activity-independent mechanisms to form
a coarse retinotopic map. Activity-dependent
progressively refine this coarse map to provide a level of
precision required to perceive the visual image (reviewed
by ConstantinePaton et al., 1990; Goodman and Shatz,
4 993). Alternatively, mechanisms involving multiple guid-
ance cues that provide each glomerulus with a unique
identity, coupled with multiple receptors on the different
sensory neurons, may provide the precision of bulbar con-
The nature of the putative guidance cues in the target,
as well as the recognition molecules on the sensory axons,
remains elusive. One particularly attractive possibility is
that the odorant receptors themselves may play a role in
atterning the projections of olfactory sensory axons. The
presence of receptor mRNA in sensory axons may result
in the functional expression of receptors on axonal projec-
tions. These presynaptic receptors may be activated by
guidance cues expressed by the postsynaptic mitral cells.
in this manner, the odorant receptors may function in the
dendrite in the recognition of odors and may exhibit a dis-
tinct function in the axon in the establishment of a topo-
graphic map that encodes odor quality In the olfactory
In Situ Hybridization
In situ hybridization
iabeled RNA probes were used. Probes were synthesized
reaction with either 500 uCi of [32P]CTP or 330 t&i of [33P]LlTP (3000
Cilmmole and 1000 CVmmole, respectively, Amersham Life Sciences)
using T3 or T7 RNA polymerases (Melton et al., 1984). Clones encod-
ing full-length cDNAs (constructed
were used as templates to synthesize
kb in size) from the following odorant receptor sequences:
P12, and 17. The sequence encompassing
mately 0.7 kb) of receptor J7 was isolated by the polymerase
reaction (L. Suck and R. A., unpublished
sponding to the BamHI-Asp-718 fragment of OMP cDNA (Rogers et
al., 1987) were used to synthesize
full-length GluFil cDNA clone (Hollmann et al., 1989; Nakanishi et al.,
1990) was used to synthesize a 1.4 kb probe.
was performed essentially
1993), except that =P- or =P-
in a 10 pl
in pSluescript II SK(+); Stratagene)
probes (ranging from 1 .O-2.0
TM4 through TM7 (approxi-
corre- data). Sequences
a 1 kb 3’ OMP probe. Finally, a
(1 l-13 weeks old) were oriented for coronal sections and cut to 15-
30 urn thickness. Serial sections of entire bulbs were allowed to dry
and then were fixed for 10 min in 4% paraformaldehyde
buffered saline (PBS) at room temperature.
PBS, sections were incubated in 0.25% acetic anhydride, 0.1 M trietha-
nolamine (pH 8) washed again in PBS, and prehybridized
at room temperature (Schaeren-Wiemers
Probes were diluted to a concentration
in hybridization buffer (Wilkinson et al., 1987a, 1987b), and 100 ui
was applied to each slide. Following a 14-20 hr incubation at 72OC,
sections were washed in 0.2x SSC for I hr at 72OC, treated with 2
Kg/ml RNase A for 30 min at 37OC, and washed again in 0.2 x SSC
at 72’C. After dehydration, slides were dipped in undiluted NTB-3
emulsion (Kodak) and allowed to expose at 4OC. Slides hybridized
with OMP and GluRl probes were developed after 1 day, whereas
slides hybridized with odorant receptor probes were exposed to the
emulsion for 4 weeks. After development,
toluidine blue 0, dehydrated, and mounted in Accumount 280 (Baxter).
Sections were photographed using a Nikon Labophot-2 microscope
with 2 x and 10 x objectives. Dark-field illumination was with a vertical
Darklite attachment (MicroVideo Instruments).
olfactory bulbs of adult female Sprague-Dawley rats
After several rinses in
for 2-4 hr
x lOa cpmlml
of 1 x IO’-2
sections were stained with
dures described for the mouse (Margolis and Grillo, 1978; Nadi et al.,
1981; Verhaagen et al., 1990; F. L. Margolis, personal communica-
thetized with an intraperitoneal injection of ketamine (40 mglkg). Ap
proximately 0.5 ml of a 0.7% Triton X-100, 0.9% sodium chloride
solution was injected into the right nostril of each animal using a
blunted 23-gauge needle. Animals were sacrificed 2 weeks after this
procedure, and the bulbs prepared for sectioning as described above.
In other experiments, we applied 1 ml of 1% Triton X-100, 0.9%
sodium chloride, allowed the animals to recover for 2 days, and re-
peated the procedure for a total of four or five treatments.
were sacrificed after a recovery period of 2 weeks following the last
In pilotexperiments, weaddeda bluedyetotheTriton
and examined the extent of staining in the left and right nasal cavities
10 min after treatment. We found extensive staining of olfactory epithe-
lium on both right and left sides in some animals and preferential
accumulation on one side in others. This variability was apparent in
the extent of sensory axon degeneration
animals. Some animals had uniformly
whereas others had more extreme axonal degeneration
only. This appeared to be independent of the number of treatments,
although deafferentation was usually more complete in animals receiv-
ing multiple treatments.
of rat olfactory bulb was adapted from proce-
seen in the experimental
denervated olfactory bulbs,
on one side
We thank Tom Jessell, Eric Kandel, John Ngai, Andrew Tomlinson,
and the members of the Axel Lab for helpful discussions
reading of the manuscript. We also thank Phyllis J. Kisloff for assis-
tance in preparing the manuscript and David Rosenzweig for illustra-
tions This work was funded by the Howard Hughes Medical Institute.
R. V. is also a fellow of the Helen Hay Whitney Foundation
S. K. C. is supported by a grant from the National institutes of Health
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