The Chemosensory World of Pets

Transcription

1 The Monell Connection Fall 2000 The Monell Connection... from the Monell Chemical Senses Center, a nonprofit scientific institute devoted to research on taste, smell, and chemosensory irritation. The Chemosensory World of Pets TSM/Rick Gayle, 2000 Increasing knowledge of the chemical senses will help open the doors into the world inhabited by our pets I ve tried every brand of food in the grocery store, but Princess won t eat any of them! Cats and dogs are a source of comfort and companionship to many of us; there are currently over 72.6 million cats and 58.5 million dogs living in US households. Pets often are regarded as part of the family, and owners go to great extent to keep their pets happy. Yet, even though humans are dedicated to our pets, we are also often perplexed by their behavior. Finicky eating habits, odor production, and anti-social behavior are just a few of the problems that can frustrate even the most well-intentioned pet owner. Every animal lives in its own sensory world. Dr. M.R. Kare, founder of Monell. As is often the case, pet behavior can be better understood by considering the world from their point of view. The chemosensory world of pets, in particular, is in many ways different from ours, and reflects their differing evolutionary origins. Both dogs and cats are disposed to detect odors and tastes associated with meat. Cats are among the most committed meat eaters of all the carnivores, and their taste buds are particularly responsive to amino acids. On the other hand, research conducted at Monell in the 1970 s by Dr. Gary Beauchamp demonstrated that cats can t even taste sweet sugars. And, because meat provides a plentiful source of sodium, cats have a poorly-developed sodium appetite and are relatively insensitive to salt. While dogs are also carnivorous, they ingest a wider range of foods and appear to have receptors sensitive to sweet carbohydrates. Dog taste buds are also responsive to amino acids, but the distribution of sensitivity differs from that of the cat. When compared to humans, olfaction is particularly important to dogs and cats. Cats are solitary creatures. They use olfactory signals to mark territories and avoid surprise meetings, relying primarily on visual signals (the domestic cat has at least 9 facial expressions and 16 different tail and body postures) to communicate at close range. In contrast, dogs are social animals, and olfactory communication is important to their social nature. Dogs use olfactory information for individual recognition, to maintain affiliations, and to reduce competition. Advanced technology is helping chemists to identify and synthesize some of the specific compounds pets use for marking behavior and for individual recognition purposes. This information could be useful in developing deterrents to urine marking behavior and specific antagonists or blockers for urine odor. It may even be possible to directly influence pet social behavior, including reducing antagonistic behavior in cohabiting pets, decreasing aggressive behavior in dogs or continued on page 5

2 A Perspective... Paola Tagliamonte Gary K. Beauchamp Director Almost 25 years ago I was co-author of an article* that concluded by recommending that the term pheromone not be used when referring to mammalian chemical communication. Pheromones originally were defined based on results of insect research, and in our article we argued that the increased complexity of mammals made use of the term misleading. For example, insect pheromones were thought to be single compounds that elicited rote, genetically programmed, behavioral or physiological responses: a moth will attempt to copulate with a piece of pheromone-impregnated paper. In contrast, communication in mammals involves complex odor bouquets, and learning and context play central roles in determining how an animal will respond to any given signal. Although most of the arguments we made then are valid today indeed, subsequent research has strengthened some points the naiveté of our recommendation is exceeded only by the extent to which it has been ignored. For years I studiously avoided using the Defining Pheromones p word in print. This was a mistake for several reasons. First, merely because there may be gray areas where a term fails (e.g., are odors that convey signals of individual identity considered pheromones?) does not necessarily mean the term is not useful. Second, there is a growing realization that in many mammals (although probably not in humans), an anatomically separate structure the vomeronasal organ (VNO)/accessory olfactory bulb exists that is specialized to detect some classes of pheromones. This is not to say that all pheromones are detected by the VNO; indeed there are prominent examples where they are detected by the main olfactory system. Nor does it mean that the VNO only detects pheromones. But there is little doubt that it is an organ at least partially specialized for pheromone detection in mammals. Third, and perhaps most important, the term provides a useful and powerful marketing tool for good as well as bad science. It is much more sexy to study human (or even non-human) pheromones rather than to study chemosignals or chemical communication in humans. This is true whether the marketing target is the mass audience of newspaper, television or the internet, or a more restricted group such as grant reviewers. But if the term pheromones can be useful, then ought it have a generally agreed-upon definition? It clearly does not. For some, the definition necessarily involves the VNO. A problem here is that humans apparently do not possess a functioning VNO, thereby eliminating the possibility of human pheromones by fiat. For others, a pheromone has to be sensed unconsciously. Why this must be so is unclear, as is exactly what this stipulation means for non-humans. And for others, a pheromone is anything that can be put into a bottle and so labeled. This definitional problem recalls an issue described by a member of our International Advisory Council, Dr. Solomon Snyder, in a lecture at the Center last May. His lecture focused on neurotransmitters (the chemical substances by which nerves communicate). Sol pointed out that by designating a very clear and specific set of criteria as necessary to classify a substance as a neurotransmitter, some neuroscientists became unwilling or unable to place novel compounds that did not fulfill these criteria into their proper biological context. In other words, definitions that are too specific can become straightjackets. So now, when I am asked whether there are human pheromones I say: of course. But that is only the start of the conversation and of the research. What we really want to know is what is the chemistry involved, how is physiology effected, and how this is translated into a behavioral response. These are the same questions we had 25 years ago. Now, with more powerful tools both in chemistry and biology, we are in the process of getting much better answers. Will there ever be a pheromone someone can put into a perfume (or the water supply) to make us all love one another? I strongly doubt it. Does a pheromone exist that can enhance attraction, modify mood, or alter hormonal activity? Most likely but right now, buyer beware! Only good science and time will reveal the answer. *Beauchamp, G.K., Doty, R.L., Moulton, D.G., and Mugford, R.A. The pheromone concept in mammalian communication: A critique. In: R.L. Doty (ed), Mammalian Olfaction, Reproductive Processes and Behavior. New York: Academic Press, 1976, Pp

3 Paola Tagliamonte Danielle Reed, Ph.D. Working on the Taste Genome Project Peanut butter... brussels sprouts... cotton candy... Do differences in our DNA predict what we like to eat? H ow do our genes determine what we eat and how much we weigh? These questions are at the heart of the research conducted at Monell by Dr. Danielle Reed, the Center s newest faculty member. A behavioral geneticist, Reed is riding the crest of interest and advances in efforts to map the human genome. She has narrowed her search to concentrate on those genes that govern food selection and body weight regulation, work that is painstaking, but also promising and ultimately rewarding. Reed is currently taking a three-pronged approach to finding her way around the genetic maze, each project focusing on a different aspect of the role genetics plays in determining food preferences. She notes that genes probably influence the selection of macronutrients in the diet more strongly than preference for most individual food items. One project zeroes in on a particular bitter compound that presents a striking study of genetic variance. Throughout all cultures and human populations, there are some people who can taste the bitter compound phenylthiocarbamide (PTC) and others who cannot taste it at all. Inability to taste PTC may be associated with food preferences and with 3 several diseases, including disorders of thyroid metabolism. Dr. Reed has mapped the gene for this taste polymorphism, and has narrowed down its location on the genome. She is now exploring whether sequence differences can explain people s varying degrees of ability to taste PTC. In an extension of this work, she is looking at the genetics of other types of bitter blindness in collaboration with Dr. Paul Breslin. Reed hypothesizes that understanding the PTC and other bitter taste genes may provide a powerful tool to help understand the genetic basis for food preferences and also the relationship between taste status and health outcomes. There are some Native American tribes where virtually everyone can taste PTC, but about thirty percent of the U.S. Caucasian population can t taste it at all, Reed notes. Because there are such prevalent differences in the global frequency, there has to be some evolutionary importance to this. C an You Inherit a Sweet Tooth? In another project, Reed is working with Dr. Alexander Bachmanov and Dr. Gary Beauchamp to search for a gene that encodes a sweet receptor. Once they identify the gene in itself a monumental effort they will look at variations in the gene to see if everyone with a particular variant has the same behavior. Reed points out that studies with mice show a strong genetic determination

4 Dr. Reed supervises high school student Sarah Obenrader as she transfers samples of human DNA onto gel plates that allow researchers to visualize the DNA. of preferences for saccharin and sugar. When we locate this sweet gene in humans, we ll be able to see whether differences in the functioning of the receptor are related to differences in their DNA, she says. This may help explain why some people are more or less interested in sugary foods. Or... a Fat Tooth? Similarly, Reed is attempting to identify genes that contribute to the liking for dietary fat. We know that there are genes that contribute to body weight and obesity, and also that eating too much fat plays a role in the development of obesity, Reed explains. But, not all obesity in humans is associated with increased preference for dietary fat. There may be separate genes that increase the likelihood of eating fat. The Benefits of Genetic Research Reed believes there are clear applications to some genetic discoveries. For example, she anticipates that once genes that contribute to liking for dietary fat are identified and cloned, a test could determine whether people have a normal gene, or if there are mutations that might predict whether someone has a predisposition toward a diet laden with fat. Doctors or nutritionists could then help devise a plan of action for keeping fat intake under control. But in some cases, Reed speculates there will not be a clear path from the genetic information itself to what to do about it. There will be situations where it s unclear how to intervene or where we ll be unable to provide any further help, she says. Reed has found that just having specific knowledge about the origins of a physical problem is important for many people. For a lot of the obese people I ve worked with, it helps to know why, she says. They ve suffered trying to have a normal life and control their weight with diet and exercise. They have known there s something different about them, and it s a relief to finally get concrete answers about what those differences are. A Match Made at Monell Raised in a small farming community in Washington State, Reed did not uncover her interest in science until she was a student at the University of Washington in Seattle. I was an economics major and doing fine, but then I took a class in physiology and behavior, and everything just clicked, she recalls. Reed came to Philadelphia in 1988 while enrolled as a psychology graduate student at Yale University to conduct her dissertation research on the determinants of food selection under the supervision of Monell Assistant Director Mark Friedman. After receiving her Ph.D. from Yale, she continued her association with Monell as an Affiliated Scientist while serving on the faculty at the University of Pennsylvania. She joined the Monell faculty earlier this year. Among the supportive colleagues Reed worked with when she first arrived at Monell was Dr. Michael Tordoff, then a post-doctoral student in Friedman s lab, and now a Member. He kept wanting to collaborate on experiments and it took me a while to understand why, Reed says. Eventually I figured it out. The two married in 1988, and now have two children, ages 7 and 8. Reed and Tordoff also share a passion for bicycle riding. They bike to and from Monell everyday, a 25-mile round trip from their home in suburban Philadelphia. With the demands of work and home, Reed says it is helpful to have a spouse who understands the nature of her work. Michael understands the stress of having a grant due, which helps to cope with the pressure, she says. Reed has also learned to discipline herself to focus on a select number of projects at one time. There is so much interesting work to be done, but I tell myself I have a whole lifetime to do it. 4 Illustrations: DOE Human Genome Program

5 The Chemosensory World of Pets continued from page 1 Rover needs oral medication twice a day, but he always manages to spit it out! Toodles and the new cat are always fighting I wish they could just get along! even increasing affection from aloof cats! As in many other vertebrate species, pheromones are most likely involved in the reproductive behavior and territory marking of both cats and dogs. In certain species, some pheromones are detected with the vomeronasal organ. Cat owners are probably familiar with the facial response known as flehmen, consisting of an elevation of the upper lip and a slight opening of the mouth. This behavior, often seen in response to urine, is thought to facilitate transport of chemical stimuli, including, perhaps, pheromones from other cats, into the vomeronasal organ. Currently, almost nothing is known of the stimuli, physiology or function of the vomeronasal system in either cats or dogs, including whether this chemical sensing organ detects pheromones. Olfaction plays an important role in food selection by both cats and dogs, but specific knowledge of this sensory system is still incomplete. Even less is known about the canine and feline taste systems. Pet owners spent $9.9 billion on dog and cat food in 1998, an expenditure that is likely to grow as more detailed information on pet sensory systems leads to the development of specialized dietary products. For example, studies in humans, many conducted at Monell, have revealed profound effects of aging on olfactory function, mediated in part by degenerative changes. Such changes are often accompanied by functional declines of dietary intake and nutritional status. Detailed information about olfactory capabilities of aging pets, and how the aging olfactory system interacts with other sensory systems, may permit new strategies to encourage good eating habits and maintain nutritional status in aging animals. Similar information on the effects of illness, trauma, and various medications on pet chemosensory function will lead to the availability of specific approaches to encourage and maintain healthy eating habits. Identification of specific blockers to unpleasant tastes and identification of attractive flavors will facilitate administration of medication a difficult and unpleasant task for both pet and owner and may help to encourage eating behavior or nutritional supplementation in ill and recovering animals. Advances in molecular biology are increasing knowledge of pet chemosensation in many ways. Olfactory receptor genes have been characterized from the dog, and are very similar to human olfactory receptor genes. Dogs, however, appear to have many more functional receptors than do humans. Interestingly, the number of olfactory receptor genes appears to be stable across breeds, regardless of whether the breed is specifically known for its olfactory acuity (for example, scent hounds) or not (sight hounds or toy breeds). Once genes for taste receptors are identified, this information may open the doors to developing in vitro (test tube) assays to test new flavors. Information on individual differences in taste and olfactory sensitivity can explain differing effects of odors and tastes on pet behavior. Studies using rodents at Monell and elsewhere have revealed prominent genetically-based strain differences in taste sensitivity, and it is likely that such differences also exist in dogs and cats. Increasing knowledge of differences in taste sensitivity and the effects of taste mixtures on perception may help manufacturers come up with palatable foods that can be readily accepted by particular breeds. Genetically-determined differences in susceptibility to diet-induced obesity have been described in laboratory species and are most likely present in pet animals. In fact, obesity is currently the most prevalent nutritional disease affecting dogs (and cats), with some breeds, such as Labrador retriever, terrier, spaniel, dachshund, basset hound, and beagle, among the most commonly affected. Do species and strain differences or aging influence how chemosensory-mediated cephalic reflexes affect food metabolism and digestive function of pets? Food and flavor preferences in humans are determined in large part by experience, and experience also influences the food preferences of pets. Our ability to understand our companion animals and the unique worlds they live in will increase as scientists continue to explore and decipher the chemical senses. 5

6 M onell Publications Here are brief summaries of selected papers describing ongoing work of Monell scientists. For information about these studies or other research at Monell, call When Taste and Smell Get Together Flavor is the combination of taste, smell, and other sensory stimuli that we perceive when a substance such as food is taken into the mouth. Dr. Dalton and Dr. Breslin used a novel psychophysical method to examine how stimuli from the distinct sensory systems of olfaction and taste work together. Subjects were tested to determine individual nasal detection thresholds for benzaldehyde, a tasteless chemical that has a cherry/almond odor. Oral detection thresholds were determined for saccharin, which tastes sweet and has no odor. Then, detection thresholds for benzaldehyde were assessed again while subjects were holding a subthreshold concentration of saccharin in their mouths. Sensitivity to the odor of benzaldehyde was increased by 28% to previously undetectable levels by the presence of a imperceptible concentration of saccharin in the mouth. When the experiment was repeated using a savory taste stimulus (monosodium glutamate; MSG), threshold for benzaldehyde was unaffected. This suggests that the increased sensitivity seen for the benzaldehyde-saccharin pairing was related to a learned congruency between these two stimuli: in American culture, cherry/almond odor is more likely to be associated with sweet rather than savory taste. The data provide strong evidence for the existence of a site within the brain where olfactory and taste information are integrated to produce what is known as flavor. Further, it appears that information from previous experience can influence whether integration occurs. Future experiments will seek to determine the role of experience in taste/olfaction integration and where in the brain this processing occurs. The merging of the senses: integration of subthreshold taste and smell. P. Dalton, N. Doolittle, H. Nagata, and P.A.S. Breslin. Nature Neuroscience, 2000, 3, The Monell Connection 3500 Market Street Philadelphia, PA Leslie J. Stein, Ph.D. Editor & Writer Karen Ivory Writer Mary M. Chatterton Resource Development Sandra Gelak Design How Does the Nose Know? How the olfactory system is able to detect and discriminate among thousands of different odorants is a mystery that continues to elude scientists. Odorant molecules interact with olfactory receptors located on olfactory receptor neurons (ORNs) inside the nose. The binding of odorant with receptor initiates a series of intracellular changes in the ORN that sends a message to the brain. It is not known whether individual ORNs contain receptors that respond to more than one odorant quality. The answer to this question has important implications for unraveling how odorant quality is encoded by the olfactory system. Dr. Rawson and her colleagues sought to address this important question using molecular biological techniques to determine how many different receptor types a single ORN has the potential to produce. Over the past decade, advances in molecular genetics have enabled researchers to identify olfactory receptor genes, made of sequences of nucleotides. The present study looked at specific gene sequences and found that Telephone info@monell.org Web Site while most ORNs examined contained only one olfactory receptor sequence, some ORNs contained two distinct receptor sequences. The two sequences represented different receptor gene families, suggesting that the receptors are probably activated by odorants with distinctly different qualities. These findings indicate that at least some ORNs express more than one receptor type and imply that some coding of odorant quality may be occurring at the level of the olfactory receptor cell. Expression of mrnas encoding for two different olfactory receptors in a subset of olfactory receptor neurons. N. E. Rawson, J. Eberwine (University of Pennsylvania), R. Dotson (University of Colorado), J. Jackson*, P. Ulrich, and D. Restrepo (University of Colorado). Journal of Neurochemistry, 2000, 75, *Jennifer Jackson worked in Dr. Rawson s laboratory for two summers as a participant in Monell s Student Apprentice Program Address correction requested Nonprofit Org. U.S. POSTAGE PAID Philadelphia, PA Permit No. 1994