I find the following press release from Columbia University to be very
interesting.  Indirectly it gives some credence to the practice of
instinctive eating where the use of smell (and  taste) is used to select
foods to be eaten.

Best, Peter
[log in to unmask]

=============================================
Contact: Bob Nelson, Office of Public Affairs
[log in to unmask]
(212) 854-6580
Columbia University

Columbia Biologists Match Odor Receptor to Odor

Research Uncovers Details of How Sense of Smell Works; First
Aroma Scientists Detect Is That of Meat

Molecular biologists at Columbia University for the first time have linked a
particular odor with the proteins in the human nose that detect it. They
made their first match with the smell of meat.

The research, by a team of biologists led by Stuart Firestein, associate
professor of biological sciences at Columbia, is reported in the Jan. 9 issue of the
journal Science. It builds on work conducted at Columbia that discovered the
receptors' proteins that stick out from nerve cells in the nasal cavity and connect to
molecules floating in the air, setting in motion a cascade of reactions that create a
perception of odor in the brain.

"I believe this experiment will prove to be a Rosetta stone for olfaction,
in that we can now begin to match odorants to receptors and decode this elusive
sense," said Darcy Kelley, professor of biological sciences at Columbia, in an interview.

Researchers sprayed 74 individual scents, one at a time, over rat nerve
cells that contained a particular odor receptor they had inserted in the cells. The
first odor they matched to a receptor was that of octanal, which to humans smells like
meat.

Linda Buck, a neuroscientist at Harvard Medical School, and Richard Axel,
Higgins Professor of Biochemistry and Molecular Biophysics at Columbia's
College of Physicians & Surgeons, in 1991 discovered both the family of
transmembrane proteins that they believed to be odor receptors and some of the
genes that code for those proteins. They found nearly 1,000 receptors,
which in the human body number second only to the receptors in the immune system. Yet
researchers had been unable to pair any single receptor or group of
receptors with any particular odor until Professor Firestein's team reported their results.

If humans can make 1,000 odor receptors, they must have 1,000 genes to do so,
which would account for between 1 and 2 percent of the 50,000 to 100,000
genes thought to reside in the human genome. "That's an enormous number
devoted to a single sensory activity," Professor Firestein said. "We'd like
to know why olfaction is so important that a hundredth of the entire genome is
devoted to it."

Nerve cells in the epithelium, sensitive tissue lining the nasal cavity,
are capable of recognizing and responding to an extraordinarily large repertoire of
stimuli -- some 10,000 chemical odors. They accomplish this feat, at least in part, with
numerous mucus-coated fibers, which contain the receptor proteins. Those receptors
recognize different chemicals and transmit that information to the brain,
which perceives the chemicals as an odor.

Professor Firestein developed a powerful approach to understanding the
coding of smell. The idea is a simple one: if a large enough population of olfactory
neurons were forced to produce one particular receptor, then the odor that
activated that receptor would cause a much larger response than normal, one that could be
easily measured.

The Columbia team inserted two linked genes, one that codes for a rat
olfactory receptor, called rat I7, and a gene for green fluorescent protein (GFP), a
substance found normally in fluorescent jellyfish but now used by molecular
biologists to mark genetically altered cells, into a disabled adenovirus --
the same virus that causes colds. The modified adenovirus was in turn introduced
into rat olfactory neurons. The genes carried by the adenovirus were taken up by
about 2 percent of the olfactory neurons exposed to them. Cells that carried the
rat I7 gene also carried the GFP gene, and could be discerned because they glowed bright
green when exposed to blue light.

Professor Firestein's graduate student, Haiqing Zhao, now at Johns Hopkins
Medical School, treated rats with the modified adenovirus and then exposed
their olfactory neurons to various odorants. He monitored the electrical activity
in the neurons, producing a chart called an electro-olfactogram. Electrical
activity was highest when the nerve cells were exposed to octanal, an aldehyde that smells
meaty to humans. Related aldehydes that smell grassy or fruity to humans
produced no effect in the modified rat nerve cells. Single olfactory
neurons also showed specific responses to octanal, confirming that rat I7 protein
responds to the chemical.

The discovery will help answer many questions about smell, the least
understood of the human senses. Do receptors that are coded by similar genes detect
odors of  the same chemical class, or is genetic sequence unrelated to odor
chemistry? Do individual receptors recognize multiple odorants, or do single neurons have
multiple receptors? And how does the brain use this vast genetic resource to form and
remember olfactory perceptions?

Professor Firestein, who holds a Ph.D. in biology from the University of
California, Berkeley, and joined the Columbia faculty in 1993, is widely acknowledged as a
leader in the field of olfaction. Despite a relatively brief scientific
career, he has already received a number of awards, the most recent being the Nakanishi Award
for Excellence in Olfaction Research. Most recently, he has demonstrated that
olfactory neurons are capable of detecting and responding to single odor
molecules, placing them alongside photoreceptors in the eye as biological
detectors evolved to the physical limits of perception.

The work was supported by the McKnight Foundation, the Whitehall Foundation
and the National Institutes of Health.