The olfactory GPCRs are in the membranes of the olfactory receptor
cells in the nasal epithelium. Activation of an olfactory GPCR initiates
an intracellular cascade leading to the synthesis of cAMP, which then
interacts with a type of cation channel that is gated by the binding of
cyclic nucleotides. The result is an influx of Ca** and Na‘, depolariz-
ing the cell and contributing to signal generation.
Olfactory receptor cells send axons into the olfactory bulb of the
brain, in humans located immediately above and adjacent to the
nasal cavity. These nerve fibers between the nose and the olfactory
bulb constitute cranial nerve 1. In the olfactory bulb, the axons form
synapses with dendrites of mitral cells, so called because their trian-
gular shape is reminiscent of a mitre, the ceremonial headgear worn
by some religious officials. Mitral cells send axons to the pyriform
cortex—buried deep in the interior of the brain—and to the amygdala,
in the limbic system. The pyriform cortex sends axons to the thala-
mus, and from there connections are made to the orbitofrontal cortex
of the frontal lobe. Interconnections with the hippocampus and the
hypothalamus are also present. Somehow, out of all this comes the
conscious perception of aroma, including various associated emo-
tional signatures: wonderful and alluring, or yucky and aversive.
The durian fruit (genus Durio) from Southeast Asia is a food known
for its powerful aromatic qualities (Fig. 12.4). Many who are familiar
with this fruit absolutely adore it, looking forward to the times of the
year when it is freshly available. Often revered as the “king of fruits”
in Southeast Asia, it is at the same time not served by most restau-
rants, and often banned from hotels, trains, and buses, because of its
very strong aroma. The aroma of durian can evoke robust attractive
or repulsive emotional responses; perhaps it is best characterized as a