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Get a Whiff of This

An issue devoted to the latest research on how smells lead to actions

By | October 1, 2013

ANDRZEJ KRAUZEFor Proust, it was madeleines; for my kids, pizza. The Brooklyn neighborhood in which they grew up was dotted with pizzerias from which wafted the mouthwatering smells of newly baked pies. Eating pizza was a definite pleasure for them, but evoking the experience was another thing altogether. One of their favorite books was a scratch-and-sniff book that let the reader smell his way through the construction of a pizza—tomato sauce, cheese, mushrooms, oregano, the finished pie—odors remarkably like the real ones. Even scratching the pages now, some 30 years later, you can still inhale faint traces. Or maybe that’s just the scent of memories . . .

Once a year, TS devotes the better part of an issue to one of the senses. In 2011 it was taste; 2012, touch; this year, smell. As we were pulling content together, the staff realized that there were many, many ways to cover this particular sense. Olfaction is complex. It’s less acute in humans than in most animals, however, and is processed somewhat differently by other species—insects, fish, rodents—commonly used as models for studying smell. We decided to concentrate mostly on the reports of researchers using the latest tools and techniques to probe into the nitty-gritty of how the olfactory system is wired to deliver an odor message to the brain, and how that delivery translates into behavior.

Humans don’t use antennae to parse odors, as insects do, nor do humans seem to have special organs that respond to a class of compounds called pheromones. While there is little debate about the role pheromones play in eliciting insect reproductive behaviors, and general agreement about their role in certain mammals, there’s plenty of contention when it comes to humans. In “A Pheromone by Any Other Name,” C. Ron Yu lays out what is currently known about the vertebrate vomeronasal organ (found in vestigial form in human infants), the molecular machinery it uses to translate the perception of pheromones into action, and the developing realization that these elusive molecules are involved in more than just sexual behavior.

Behavioral changes are hallmarks of a number of neurodegenerative diseases. And so are disturbances in the ability to smell. In “Smell and the Degenerating Brain,” Richard Doty describes how olfactory loss often accompanies Alzheimer’s and Parkinson’s diseases, and how diagnosing smell dysfunction early could help differentiate between these and other disorders. Doty marshals evidence to support the hypothesis that “cholinergic dysfunction plays a significant role in the olfactory loss seen in a number of neurological diseases.”

This month’s profile, “An Olfaction Odyssey,” describes John Hildebrand’s contributions to the study of smell. He pioneered using the giant sphinx moth as a model to study the development of the insect antennal olfactory system, including a gender-bending experiment that he describes as “one of the greatest ‘wow, gee-whiz’ discoveries” to come out of his lab. And this past January, he and colleagues published in Science what he considers the “culmination” of his work on olfaction: the discovery that action potentials are sparked deep in the brain when a sphinx moth smells an odor that is “behaviorally significant,” and that learned odor preferences can be added to the moths’ innate repertoire.

You can read about optogenetics in many places, but Amber Dance (“Scents in a Flash”) covers the technique’s use in sorting out different aspects of olfaction’s complexity.

Finally, the entire Literature section is devoted to recent scientific publications that relate to olfaction. Two studies use calcium imaging: one to follow scent selectivity by individual neurons in olfactory glomeruli; another to show that a network of as few as 25 nerve cells activated in a Drosophila mushroom body encode enough odor information to explain a fly’s behavior. The third paper reports that odors evoke a neural afterimage that may play a role in memory formation. Madeleines? Pizza?

Next year: the sense of sight.


Mary-Beth Aberlin  Editor-in-Chief  eic@the-scientist.com

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Avatar of: James V. Kohl

James V. Kohl

Posts: 162

October 1, 2013

TS mentioned the 1995 publication of The Scent of Eros: Mysteries of Odor in Human Sexuality here: see Scent of a Book Deal. Human pheromone-deniers can now be approached in the context of what's been scientifically supported by experiments in different model organisms since 1995.

For example, although moths perceive diverse floral species similarly, nutrient-dependent reproduction is pheromone-controlled in species from microbes to man via the conserved molecular mechanisms of ecological, social, neurogenic and socio-cognitive niche construction. Olfactory / pheromonal input epigenetically effects the experience-dependent alternative splicings that link the de novo creation of olfactory receptor genes in moths to the hormone-organized and hormone-activated behavior of invertebrates and vertebrates, like us, as we detailed in our 1996 Hormones and Behavior review article.  

There are now examples of nutrient-dependent pheromone-controlled epigenetic cause and effect in many different model organisms, including yeasts, nematodes, insects, other mammals, and a human population that adaptively evolved in what is now central China during the past ~30,000 years. I included these examples in a more recent review of nutrient-dependent amino acid substitutions, although some of the model organisms were also mentioned in our section on molecular epigenetics in our 1996 review.

The most telling example in the now-refuted context of mutation-driven evolution remains a species of moth in which a single nutrient-dependent substitution of a critical amino acid creates a new pheromone blend. Although the substitution might be considered by some to be the result of a mutation, we now know there is no evidence that mutations are fixed in the organized genome of any species. (Sickle-cell anemia, for example, is not the result of a gene that is fixed in all human populations.)

What is obvious, for comparison, is that new alleles are fixed via nutrient uptake (e.g., glucose and amino acids) and the thermodynamics of pheromone-controlled reproduction associated with organism-level thermoregulation. Thus, fiixation can be directly attibuted to the epigenetic effects of olfactory/pheromonal input that are associated with other sensory input during the development of food preferences and mate preferences in species from microbes to man.  Therefore, we have reached the point the where nutrient-dependent pheromone-controlled  physiology of reproduction must be the first consideration when any attempt is made to link sensory input to the Physiology [that] is rocking the foundations of evolutionary biology.

Clearly, the conserved molecular mechanisms of nutrient-acquisition and pheromone-controlled reproduction are the foundations of evolutionary biology and the Mysteries of Odor in Human Sexuality. This was predicted more that 30 years ago.

"I should think we might fairly guage the future of biological science, centuries ahead by estimating the time it will take to reach a complete comprehensive understanding of odor. It may not seem a profound enough problem to dominate all the life sciences, but it contains, piece by piece, all the mysteries." -- Lewis Thomas (1980) as cited in The Scent of Eros: Mysteries of Odor in Human Sexuality
 

Avatar of: Roy Niles

Roy Niles

Posts: 58

October 17, 2013

Here's Kohl again, trying to hawk his perfumery products by quoting pseudo-scientfic baloney from his own publictions, but never showing evidence of any experimental work to justify this baloney that any reputable scientific journal would have seen fit to publish.  He did write an article abut his "model" that was published, but no experimental evidence was detailed for review.

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