OLFACTION: FROM ODORANT MOLECULE TO SMELL

Ceocccececccccscococcccoccce COCCCOOOOOEEOOOOOEEOT OOOO OOOOH OSOS OHO HOSE O SE OOOO OOOO OSOOOLOOOHS HOLL EEOHHOSHOHLOSESOOOHOEOOOTOOOOOOOOOEOEOS

he sense of smell is

critical for many

organisms’ survival
and plays an important role
in our daily lives. The
process by which individ-
ual odorant molecules give
tise to the perception of a
particular scent has been a
topic of research by scien-
tists for many years. New
techniques in molecular
biology and neurophysiolo-
gy have given rise to many
exciting discoveries in the
field of olfaction. The
process by which molecules
bind to individual recep-
tors, utilize second messen-
gers which open ion chan-
nels and eventually leads to
desensitization has become
much clearer in the last
five years.

Just as there are recep-
tors for neurotransmitters
and hormones, there are
receptors on olfactory neu-
rons which bind odorant
molecules. The first evi-
dence for olfactory recep-
tors came from optical iso-
mers of odorant molecules.
For instance, L—carvone is
perceived as smelling like
carraway while D-carvone
is perceived as smelling like
spearmint (Dodd &
Castellucci, 1991). The
fact that optical isomers
could produce different
smells suggested that the
separate isomers bound to
different receptors. Prior to
the cloning of the recep-
tors, researchers noted a
rise in cAMP levels in rat
olfactory cilia in vitro
(Breer et al., 1993B). A rise
in cAMP upon exposure to
a ligand suggested that the
olfactory receptors proba-
bly belonged to the super
family of G linked recep-
tors which span the mem-
brane seven times. Indeed,
by screening a cDNA
library made from rat olfac-
tory cells using other Ga