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The Molecular Logic of Smell
Msmmals can recognize thousands ofodors, some of which prompt
powerful responses. Recent experiments illuminate
how the nose and brain may perceive scents
by Richard Axel
mell is perhaps our most evocative sense. In Marcel Proust's novel Remembrance of Things Pasc,
the nostalgic flavor and fragra nce of a
madeleine, a delicate pastry, evokes a
description of taste and smell, the senses that "alone. more fragile but more enduring, more unsubstantial, more persistent. .. bear unflinchingly, in the tiny
and almost impalpable drop of their
essence, th e vast structure of recollection." Humans often view smell as an
aestheti c sense, yet for most animals
smell is the primal sense. one they rely
on to iden tify food, predators and
mates. Indeed, for many organ-
S
isms, odors arc their most cfficient means of communicating wi th others and
interpreting their surroundings. Innate behavior in response to smell
is essential to these organisms' survi val and
most likely result s from
nonconscious perception
of odors.
Each individual has a
unique, genetically deter-
mined scent. This olfactory identiry is
coupled with a remarkable ability to distinguish a diversity of odors. Humans.
for instance, canrecognize approximate-
ly 10,000 scents , ranging from the pleasurable scent of freshly cut Dowers to
the aversive smell of an angry skunk.
Many animals have an even greater sensitivity to odors than humans do: bloodhounds, for example, are legendary for
their extraordinary abiliry to dlscrimlnatescents.
The wide spectru m of odors that humans consciously detect prompt varied
emotional and cognitive responses. But
do humans recognize other smells without a conscious awareness of this perception, and do such odo rs elicit innate
behavioral responses? How does the
perception of specific odors lead to appropriate thoughts, memories and behaviors? Whether smell is primal or aesthetic to a species, all organisms must
have developed in the cour se of evolution mechanisms to recogni ze various
odors and transmit this olfactory information from the nose to the brain,
where it is decoded to provide an inter-
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OLFAClORY
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nal representation o f the external world.
As molecular biologists stu dying perceptio n, my colleagues and I have reduced these questions to the level of
genes and prot eins. We have used these
molerules to examine how animals recognizc such a diverse array o f scents
and how the recognition of odors in the
nose is translated into a map of odo r
qu ality in the brain.
The ba sic anato my of the nose and
olfactory system has been underst ood
for so me time. In mammals, for example, the initial detection of odo rs takes
place at the posterior of the nose, In
the small region known as the olfactory
epithelium. A scanni ng electro n micrograph of the area reveals two interest ing types of cells. In this region, millions of neuron s, the signaling cells of
se nso ry sys tems, provide a direct physical connec tion between the exte rnal
world and the brain. From o ne end of
each neuron , hairlike sensors called cilia extend o utward and are in direct
co ntact wi th the air. At the o ther end
of the cell, a fiber known as a n axon
run s into the brain. In additlon, the olfactory epithelium co ntains neuron al
stem cells, which genera te olfacto ry
BRAIN
neurons throughout the life of the organism. Unlike mo st neurons, which
die and are never repla ced , the olfactory sensory neurons are continually regenera ted.
When an animal inhales odorous molecules , these st ruc tures hiod to specialize d proteins, knO\\TI as recep tor prorein s, that extend fro m the cilia . The
bindin g of odors to these receptor s ininat es an electrical signal that travels
along th e axons to the olfactory bul b,
which is located in the front of the
brain , right behind the nose Itself. The
olfacto ry bulb serves as the first relay
station for pro cessin g olfactory Infermalion in the brain; the bulb conn ects
the nose wi th the olfac tory co rtex,
which then projects to highe r sensory
centers in the cerebral cortex , the area
of the brain that controls though ts an d
behavi ors .
A Family of Receptors
omewhere in this arrangement lies
S
an intricate logic that the brain uses
to ident ify the odor detected in the
nose, dis tinguish it from others and trigger an emotional or behavi oral respo nse.
To pro be the organization of the brain,
my co-workers and I began where an
odor is first physically perceived-e-at
the odor recep tor proteins,
Inst ead of examlnlng odo r recepto rs
directly, Li nda Buck, then a postdoctor al fellow in my labora tory and now a
professo r at Harva rd University, and I
se t out to find the genes encod ing odo r
receptors. Genes provide the template
for pro teins, the molecu les that carry
out the function s o f cells , Once we isolat e the genes that e ncode a protein,
\",' C can usc them as tool s to study the
structu re and function of the odor reo
cep tors them selves.
Furthermore, using gene s to invest igate pro teins is mu ch simpler and
fast er than studying the recept ors di·
rectly. By arttttdalty manipulating genes,
we can easily alter odo r receptors in
ways that help us un derstand how the
molecules enable the nose and brain to
perceive smell. Afte r we understand
how the recep tors work, we can then
study how olfacto ry information is
transmitted to the brain and processed
to permi t the discrimination of smells.
Using the technique of gene clonin g,
we were able to isolate the genes encoding the odo r recept ors. This
family of receptor ge nes exhibited several properties
that su ited it to its role in
odor recognition. First , the
genes encoded prot eins that
SCENT OF A FLOWER is
translated from a sniff to a
smile by the olfactory se nsory system. An odor is
first detected in the upper
region of the nose , at the ol-
factory epithelium. Wilhln
this area. odor molecules
bind to receptors on hairlike
projections, or cilia . The receptors are part of neuron s
lhat can extend three to four
centimeters from the inside
of the nose to the brain.
Structures known as axo ns
run from the neuronal cell
bod y to the olfactory hulb
In the brain. In the bulb, ax'
ons converge at sites called
glomeruli; from there signals are relayed to other regions of the brain, including
the olfactory cortex . The
v omerona sal organ is part
of a separa te se nsory system that gov erns innate responses tn some mammals.
lis role in human behavior
Is not well Imown.
gions of the vi sibl e spectrum, so the brain can
compa re Input from aU
three types of detectors
" to identify a color. OUr
j
data sugges t that a small
number of odo r recepI tors would not be able to
i recogni ze and di scrimi~
Ii
nate the full arr ay of
J sce
nts that can be perceived by mamm als .
i
i
Mamm als can detect at
lea st 10,000 od ors; con~ sequen tlv, each of the
1,000 different receptors
I mu s t respond to several
i. odor molecules, and each
.I odo r must bind to sever-
i
!
l al
~
b
f
receptors . Scie ntis ts
b eli eve that various recep tors respond to dis-
cre te parts of an odor's
and that an
odo r ~onsists o f se veral
chemical gro ups that
8 structure
SENSORY NEURON in the human olfactory epithelium (Iell) is surround.
ed by s uppo rt cells and sits over a
layer of neuronal s tem cell s, wh ich
generate new olfactory neurons duro
ea~h .actlvate a charactensn c rece ptor. For e.xam ple, the mo lecu les reo
Ing an organism's life. Hairlike cilia
sp on sible for the scents
protrude from the tip of an individual neuron (above), shown magnifled 17,500 times; receptors located on cilia bind to odor mole cule s.
These images were taken by R. M.
C~~tanzo and Eo Eo Morr~ son ?f Vir'
guua Commonwealth Unlversitv.
faU squar ely within a pr evi ou sly described group of receptors that pass
th rough the cell membran e of the neuron seven times; these receptors acttvate signaling proteins knO\\TI as G protein s. Early studies by D oron Lancet of
the wetzm ann Institute of Science and
Rand all R. Reed of the John s IIupkins
Sd lOOI of Medicine have established that
complementary stran d In the chro moso me \...·c can loca te a gen e and determin e both the number and position o f
a gene or gene family.
Specifically, by exa mining DNA from
a variety of mamm als, including humans, we determin ed that around 1,000
o f jasmine and freshly
baked bread are made
up o f different structural
gro ups, and each group
activates a dis tinct set of
recep tors; to distinguish
the s mell, the brain must
the n determin e the precise combinatio n of receptors activated
by a particular odo r.
Ilow does the brain identify which of
the 1,000 typ es of receptors have been
turned on? Severa l scenarios are possi ble, If every neuron carries all 1,000
types , every neuron would sen d a sig-
gen es encode 1,000 differcn t o do r re-
nal to the brain eve ry time an odor was
sensed. All the engaged recepto rs would
in the transmission of an electrical im-
ceptors. (Each type of recept or is expr essed in thousand s of ncuron s.) Gtven that mammalian DNA probably contams around 100,000 genes, this find ing
pulse along the o lfacto ry se nsory axon.
indicates that I percen t of all our genes
Second, the genes encoding the odo r
are devot ed to the detection of odors,
making this the largest gene family thu s
signals to deciph er the identit y o f the
smel l. Ahe rna tively, if eac h neu ron fea-
odor receptors, too, usc G prote ins to
initiate the casca de of events resulting
receptor prot ein s arc active onl y in olfactory neurons. Allhough nearly every
cell of the body carries a copy of every
gene, many genesarc expressed only in
s pecia lize d cells.
Finally. a broad range o f odo r recep tor genes app ears to mirror the striking
range of odors. The technique known
as mol ecular hvbrtdi zati on allowed us
10 det ermin e th e numb er of these
ge nes in the chro mosome . This proce-
dure works because genes consist of
two s trands of DNA that come together
to create a doubl e helix. We can label
o ne s trand so that wh en it finds its
156
far identifi ed in mamm als . The enormou s amo unt of ge ne tic informa tion
devot ed to smell perh ap s reflects the
s ignificance of this se nsory sys tem fo r
the survival and repro duction o f mos t
mammalian species.
The large fam ily of od or recep tor s
then need t o co ntribute some distin cttve compone nt to the neuron' s s ignal;
the brain could then compa re these
tures only one type of receptor, the
prob lem of distinguis hing which reccp tor was activated by a particular odor
reduces to the probl em of Identifyi ng
which neuro ns fired. Such a model
woul d grea tly simplify the tas k of the
brain in so rting out which of the numerous receptors have been activated.
contrasts sharply with the far more res tricted rep ertoire of recept ors in the
eye . Humans, for exa mple, can di scri rntnatc among severa l hundred hues using o nly three kinds o f receptors o n
the retina , These photoreccptors detect
light in different but overlapping rc-
Scrrxmrtc AMr.RICAN Ocrober / 995
One Neuron, One Receptor
o in vesti gate which of these two
sc he mes occ urs in the detectio n of
smells. we again loo ked at gene exp ressian in the olfactory neurons. Using the
T
procedure of molecular hybridization.
Andrew Chess, John Ngai an d Robert
Vassar, the n all at Columbia Umvers lty,
and I observe d that in mammals, each
of the 1,000 receptors is exp ressed in
about 0.1 perce nt of the neuro ns. In fish,
which have 100 odo r recep tor s, each
recep tor can be found in about 1 percent of th e neurons. These resu lts suggest that , in both cases, eac h neu ron
may express only one receptor gene.
Furthermore, in recent experime nts,
Catherine Dulac, also a t Colum bia, an d
I have used th e polymerase chai n reaction, which a mplifies small pa rts of
DNA. to clone th e odor recep tor genes
th at arc expressed In Individual olfactoryneurons. when such receptor genes
arc isolat ed fro m a single neuron , the y
all appear 10 be ide nt ical. When th e
same procedure is applied to a collection of neurons, however, hundreds of
diffe rent receptor genes are ob tained.
Taken togeth er, these observations indicate that each se nsory neuron expresses only one receptor and is therefor e fun ctionally disti nct.
This simp le co rrelation between recepto rs and neurons docs not exp lain
the much morc co mplex processing
tha t the brain mus t em ploy 10 discrim·
inate an odo r. For example. how does
the brain determine which olfa ctory
neurons have fired? In all othe r sensory systems. th e brain relies on defined
spatial pa tt erns of neurons as well as
th e position of th e neurons' ultimate
targets to defin e th e qu ah ry of a sensation. Perhaps the brain applies a simi·
lar logic to the se nse of smell.
There ar e a number of poten tial scenarios for arranging neurons and axons In the nose an d brain [see tIIusrra ·
tion below ). In one mod el, neuro ns tha t
bear a given type of rece ptor wou ld be
locali zed in the olfac tory epithelium.
Activation of neurons at specific sites
would th en define the quality of a n
odo r. Altematively, neurons carrying
one type of recep tor could be randomly
positi on ed in th e epithe lium, bu t th eir
axons would co nverge on discret e areas in the brain. In this case, exposure
10 a particular odor would result In defined patt erns of acttvity in the brain.
In a th ird model, bo th the neurons and
their projecti on s to the brain could be
arranged randomly. To int erpre t the
scent, the brain would have to use a sophisticated algorit hm to decod e the
random signals.
Some neurons In th e nose are spatially segregated according 10 th e sce nts
Ihey d etect. Most manunals, including
humans, possess a "sexual nose," or
vomero nasal organ, Ihal is physically
se pa rate from th e main olfactory epithelium. The vomeronasal organ detects
the ph eromones that govern reproductive and social beh avi ors. Sexual acnvtty in male rodents, for example, is an
inn ate res ponse , prompted by the detection at the vome ronasal organ o f
ph eromones secreted by fema les. If the
neuron s in the vomeronasal system in
virgin mice are destroyed, the mice can
still smell with their main olfac tory
system, but the damage to their vomer'
ona sal organ prevents the m from ever
mating.
Addit ionally, as Dulac an d I have
shown by studying the genes enco ding
pheromone recept ors, the sequence of
amino acids (th e building block s of
proteins) in the recep tors of the vorneronasal organ is completely different from
that in th e receptors of the main olfactory epit helium. Th ese differences sug ges t that the two systems may have
evolved independently of eac h ot he r.
Finally , neu rons in the main olfa ctory epitheli um project Ihei r axo ns to an
a rea of th e brain th at is distinct fro m
the region where neurons in the \ '0meronasal organ se nd nerve impulses,
Consequently, signals from these 1\\'0
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PATITRNS OF NEURONScan help the brain Interpret a smell.
Seve ral arrangements are possible. In one scenario ( a ), neurons that contain a particular type of receptor (I ndicated here
by color) would be locali zed In the olfactory epilbeIlwn; In
thi s way, th e brain could identify an odor by determining
what area of the olfactory epith elium was activated by the
smell. Alternatively (b), neurons may be arranged randomly
throughout the epithelium. but lheir axons may conve rge on
localized regions of the olfactory bulh known a s glomeru lL
An odor would therefore be identified by a characteristic pattern of acti vlrv In the glomeruli. Finally (e), both the neurons
and their axons may be arranged randomly.
scrorrmc AMI:JUCAN
Oerober 1995
I S7
BLUE NEURONS rev eal the pathway of sensory infonnation
from the olfactory ep ithe lium in th e nose 10 th e olfa ctory
regions of the nose prod uce \"(.'. r)' dif-
bulb in the brain (a) . By ge netica lly modifying th e od or rece p tor gen es in mice. th e au th or and his collea gues dyed
ga nization, th e most impo rta nt fea tur e
of this arra ngemen t is th e ran do m distri bution of recept ors wi thin each region . Recause we were unable to de tect
a more precise spa tial pa ttern o f neuron s In the epithelium, we searched for
a pa tt ern in th e proj ection s o f' axons
into th e brain.
If such a patt ern is ind eed employed,
neurons expres sing a given receptor,
thoug h randomly dist ribut ed throughout a region of the epitheliu m, must
project th eir axons to a small number
of glomeruli. Several pieces of evi dence
suppo rt this model.
First , th e number of glomeruli is
roug hly the same as the nu mb er of
typ es of recepto rs; becau se each neu-
ron exp resses only one recep tor, each
type of neu ron may connec t to a characteristic glomerulus. Second, physiological experiment s have revealed that
d ifferent odors elicit distinct pattern s
of ac tivi ty in the brain. For example,
Gordo n M . She phe rd and his colleagues
at Yale Univers ity es tablished that exposure of newbo rn rod ents 10 their
mother' s milk led to ac tivi ty in res tri cted regions of the olfactory bulb. SimiOrgani zed Axons
larly, John s. Kauer of Tuft s Univers ity
u sed voltagc-scns inve dyes to show
he ana to mic segrega tion of th ese
that the pattern of activity in th e olfacto ry bulb is d istinct for various odo rs.
two functionally distinct olfartory
systems imme dia tely prompte d us to
Furthermore, clcct rcphysiologtcal studexam ine wheth er neurons wit hin the
ies by Kcn saku Mori of the Osaka Biomain olfacto ry system itse lf also exscience Institute dire ctly demonstrated
that distinct glomeruli arc
ploit spa tial seg rega tion to
define th e qu ality of an
activated by differe nt odors.
odo r. Some of this spatial
My colleagues and 1 have
orga nization is well known :
d evised two mo lecul ar apeach neu ron proj ects a sinpr oach es to stu dy the spagle, unb ra nched a xon totial segrega tion of neurons
ward th e brain. As the co land axo ns. First, Vassa r,
lection of axons eme rges
Steve K. Chao a nd Leslie B.
fro m th e olfactory epitheVosshall , working in my
lium, abo ut 10 million axlab orat ory, mo dified the
on s come toget her to form
technique of molecular hyth e olfa ctory nerve, which
bridiza tion used in p revithen ente rs th e brain. Once 3
ou s wor k so tha t we could
inside the brain, groups of i
exa mine receptor RNA in
10,00 0 axo ns converge at §
th e lips of the axcns, where
site s called glom erul i in th e .
the y co nverge in the olfacolfac tory bulb. In the glo- ~>'
lory bulb. These exp ertrncru li the axons comm uniments, as well as indepencate with neu ron s tha t pro- ...
dent work by Buck, ind ica tjeer to higher centers in the c~
ed that neu ron s express ing
brain.
~
a given recept or project to
Expcnments done by Vas- ~
one or, at most , a few glosar in my labo ra tory at Col- '"
meruli am ong the thouumbia, as well as ind ep en- ~
sa nds wi thin the olfactory
dent research carrie d out :;;
bu lb. Moreover, the posi by Buck, showed that the
OI.FACTORY BULB of a ra t is see n in cross section in this mi- tions of the glomeru li arc
olfactory epithelium is di - cro grap h. The two white s po ts indi cat e wher e axons that be ar fixed, as suring thai a given
vided int o four broad re- a speci fic rece ptor gene conve rge. Becau se eac h axon projects odor will elicit the same patgions according 10 the types 10 a charac teristlc location in th e olfactory bulb, th e bulb p ro- tern of activi ty in th e brains
of receptors foun d in each vides a two-dimensional map of odor quality, which the olfac- of all animals in a species.
zone. Despite this coarse or- lory corte x employs to decipher a n odor.
In ano the r approac h, l'cferent behavioral resp onses. The neurons of the vomeronasa l orga n bypass
the cognitive cente rs of the brain and
send signa ls directly to those areas that
control innate beh avioral and emotional respons es. In contrast, th e main epithelium sends signal s to higher centers
in the olfacto ry co rte." th at elicit mo re
measured responses.
T
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15 8
SCIINTlrI C AMERICAN Octoher 1995
deep blue the neurons thaI bear a partlcular type of receplor-and are therefore sensitive to a limited number of odors.
ler Mombaerts and Fan Wang, also at
Columbia, and I have genetically altered
mice, breeding experimental animals in
which neurons that activate a sp ecific
recepto r were dyed blue. Our procedure
involves isola ting a gene for one of the
odor receptors and th en attaching to it
a second. marker ge ne. This marker
gene, which will become active whenever th e odor rece p tor gene Is exp ressed ,
triggers a che mica l rea ction th at turns
th e neuron and Its axon blue. Th e modIfied gene is inserted into cells th at are
th en int roduced into a mou se embryo.
In th e resulting mice that develop, neuro ns that make thi s particular recept or
a ppear blu e, allowing us to see where
th e cells are located .
We examined th e olfac to ry epithelia
an d brains of the mice and observed
that about one In 1,000 neuron s were
blu e, Most impo rtant, tndlvidual axons
stretching fro m th e neurons coul d be
Identified an d followed loto the brain.
Th e blue axons projected to only two
of the 2,000 glomeruli lo th e olfactory
bulb. The se experim ent s provide convincing visu al evidence that neuro ns
that activate one type of receptor -and
th ere for e res po nd to a limited number
of odors- project their axons to a small
number of glo meruli in the brain. Because the glom eruli In the olfactory
bulb are differentially sensitive to
specific odors, and th e po sitions of th e
individua l glomeruli are topologically
defined, the olfac tory bulb provide s a
two-dimen sional map th at ide ntifies
which of the num erous receptors have
been activa ted lo th e nose. We believe a
given odor wi ll activate a characteristic
combination of glomeruli In the olfactory bulb; signals from the glomeruli
are th en transmitted to the olfactory
cortex , where they mu st be processed
to allow odo r discrtmlnation.
Decoding th e Signal
A ccording to thi s mod el of sme ll,
J-\. mammals sbould, In theo ry, be
able to detect an ext rao rdina rily large
number of od ors. Becau se odors int eract with multiple recep tors ra ther than
with individual ones, th e possible cornbinations exceed by severa l orders of
magnitude th e nu mber of odors a nimals can actu ally de tec t, Consequently,
just as with ot her senses, th e olfactory
system offers a meager represen ta tion
of the environment . Presumably, animals dis crtmlnate only th ose odors that
are biologically Important to their survival and reproduction.
TItis vi ew of olfactory perception
shares several bas ic features with perception In other sensory systems, For
example, In vi ston the brai n analyzes
The Author
RICHARD AXEl Is professor of
Randomly positioned neurons on the olfactory ep ithelium (bl
converge at one location In the olfactory bulb ( c~
an Image by lot erpretlng the Individual
compo nents: form, location, movement,
color. The unity of an Image is accomplished by reconstructing the signals In
th e visual centers of the higher cortex.
in compari son, th e brain analyzes an
odo r by dissectin g th e struc tu ra l features of th e scent. The odor is the n recons tructed by the olfactory cortex .
But how does th e olfac to ry co rtex ,
which receives signals fro m the olfactory bulb, decod e th e map provided by
the olfactory bulb? TItis qu esti on is one
of the central and m ost elu sive problem s in neurobiology. It see ms likely
that some form of spatial seg regation,
similar to that se en in the olfactory
bulb but undoubtedly far m ore co mplex , "ill be malntained as th e signals
project Into th e cortex . This arran gemen t, however, merely places th e problem of Interpreting spatlallnforrnatlon
one level beyond the olfactory bulb, In
the cortex. How does the cortex prompt
the range of emotional or behavioral
res ponses that smells often provoke?
To wha t extent is th e recogni tion of
od ors in humans consdous or nonconsdous, an d how much of behavior or
mood is governed by th e perception of
odors lo our environment? We have
only begun to exp lore the logic of smell
and how It can evoke the "vas t structu re of reco llection.'
Further Reading
CO!'lfffiIBU1l0NS
or T OPOGRAI'HY AN U PARA1lH. PROCESSING TO OOOR CODING
IN TI lE VI:R.TI:BRATE
biochemistryandmolecular biophvs- OUACTORY PATIfWAY. J. S. Kauer In TrendsIn Neurosciences; Vol. 14 , No.2, pages 79 -85; February
ics at Columbia Unfverslty, where he
1991.
is also an Investigator \..1th the How- A Nova MULTIGENE FN-ULY MAY ENCODE ODORANT RECIPTORS: A MOILCUl AR BASIS fOR OOOR Rrard Hughes Medlcal lnstttute, Axel Is CIPTION. Unda Buck and RIchard Axel In Cell , Vol. 65, No. I , pages 175-t 87; April 5, 1991.
a molecular biol ogist who now ap- SIGNAllNG PATIfWAYS IN OOORANT nrrrcnos. R. R. Reed in Neuron, Vol. 8. No.2, pages 20 5- 209 ;
plles the tcchntqu cs of reco mbinan t
February t992.
DNA and m olecul ar genetics to prob-
lems In ncuroblology. Most recentl y
he has focu sed o n the molecular biology of perception.
D ISCRIMINATION OFMOl..ICULAR SIGNALS BY TIlE OlfACTORY RrcFnoR NruRON. G. M. Shepherd tn
Neuron. Vol. 13, No. 4, pages 771-790: OCtober 1994.
MOlLCUlAR RICOGNTI10 N AND OUA(.I ORY PROCESSING IN
nu: MAMMA.1JAN
OlfACTORY SVSlT.M.
K. Mort and Y. Yoshthara In Progress in Neurobiology, Vol. 45, No.6, pages 585-619: April 1995.
SCIENTIFICAMi:RJCAN Ocrober 1995
159