A rose is a rose is a rapture of the nose Scientists explore taste, touch, smell

November 10, 1992|By Douglas Birch | Douglas Birch,Staff Writer

You don't have to be a scientist to know that a whiff of bourbon and cigars can evoke memories of a favorite uncle or that a rose's scent can return you to your mother's garden, even in the dead of winter.

But how does this happen? How are the molecules that float off that stogie detected with such precision, and how is a rose's scent stamped into the circuitry of the brain?

A team of researchers at Johns Hopkins School of Medicine are tackling these and related questions as part of their work at the new Keck Center for the Study of Special Senses, established recently with a $2 million grant from the W. M. Keck Foundation of Los Angeles.

While some of the researchers are investigating aspects of sight and hearing, others are exploring areas that have been studied less extensively -- taste, touch and "olfaction," or smell. Dr. Solomon Snyder, director of neuroscience at the medical school and head of the Keck Center, is an internationally recognized expert on smell. And he says that nosing around doesn't get the respect that it's due.

"A lot is known about how the brain processes seeing, a reasonable amount for hearing and hardly anything about smell and taste," said Dr. Snyder.

But odors have more influence than we might think in how we perceive the world, Dr. Snyder said, pointing out that smell is associated with a part of the brain called the hippocampus, which also seems to play a major role in memory, emotion and sex.

"There's a very close link between memory and smell," said Dr. Snyder, an enthusiastic researcher who talks in a kind of stage whisper and peppers his sentences with sound effects -- snapping his fingers or making popping sounds to simulate the firing of a nerve cell.

"In earlier, more primitive mammals, the sense of smell is enormously important. And a very large part of the brain is devoted to smell," he said.

There are about 5,000 known smells, Dr. Snyder said, and all of them could probably be detected through various combinations of perhaps a dozen receptors inside the nose. Receptors are the individual cells that recognize when an odor is present, and alert the brain.

A researcher at Columbia University has found that a person has about 1,000 genes that contain the blueprints for an equal number of odorant receptors in the nose. "Which is surprising seeing as how common sense tells you you don't really need 1,000 receptors to identify 5,000 odorants, but maybe God decided to do it that way," Dr. Snyder said.

These 1,000 genes make up 1 percent -- a significant fraction -- of each person's complete library of about 100,000 genes.

"It's not altogether surprising," Dr. Snyder said, "because in lower mammals smell is crucial for the survival of the self."

"Most animals remember a friend or an enemy based on their smell, not on what they look like," he said. "And in the survival of the species, olfaction is critical for courtship and mating." By making small cuts or lesions in a certain part of a rat's brain, he said, researchers can restrict the processing of smells that help trigger courtship.

"They can still smell. But if the cuts eliminate the responsiveness to pheromone, Dr. Snyder said, "Whap! They don't mate, no more sex!"

Dr. Snyder is looking at how the molecules that float into the nose trigger odor receptors, starting the cascade of events that leads to the sensation, and memory, of smelling a rose.

It works this way: Objects give off molecules of gas that contain the odor. Each molecule, or odorant, fits like a key in the lock of a tiny, specialized receptor in the nose. Inserting this "key" triggers the formation of a substance inside the receptor, typically a molecule called cyclic AMP.

"The most crucial part . . . is that the second messenger, cyclic AMP, triggers the adding of phosphate groups to proteins in the cell," Dr. Snyder said. "And that turns out to be about the most universal of all communication within the cell."

The chemical change, called phosphorylation, is amplified a million times in a kind of chain reaction inside the cell.

Ultimately, the intensity of the reaction causes the receptor to fire, sending an electrochemical message through nerve cells to the brain. Researchers think the receptor simply turns on and off, signaling the brain that a particular odor is present.

Once the smell signal reaches the brain cells, or neurons, in an area of the brain called the hippocampus, it registers as a specific sensation in ways that scientists are still debating. But some progress is being made on how the sensation is recorded.

"Think about learning and memory," Dr. Snyder said.

"All we know is that something is changed more or less permanently. The brain works by neurons firing. This neuron 'A' fires and makes neuron 'B' fire and neuron 'C' fires. If they stop firing . . . then you've forgotten. How are you going to remember?

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