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NEUROBIOLOGY: ON MAMMALIAN PHEROMONES

The following points are made by P.A. Brennan and E.B. Keverne (Current Biology 2004 14:R81):

1) Chemical signals that convey information between members of the same species are commonly termed "pheromones". This term was first used by Karlson and Loescher [1] and defined as "substances secreted to the outside of an individual and received by a second individual of the same species in which they release a specific reaction, for example, a definite behavior or developmental process" [1]. However, there are problems with defining mammalian pheromones in this way, as it is difficult to define what constitutes a "specific reaction" [2]. Mammals are subject to more complex influences on their behavior than insects. For instance, odors that convey information about individuality may bias the behavior of other individuals of the same species [3], but because they do not elicit a specific behavior they would not qualify as pheromones according to the original definition.

2) The meaning of the term is further muddled by many investigators adding their own criteria for what constitutes a pheromone. For example, some investigators have stated that a substance should be airborne and not be consciously perceived in order for it to be defined as a pheromone [4]. An easy way out would be to redefine the term pheromone to encompass all chemical substances that convey information among individuals of the same species. Nevertheless, there are many mammalian examples of single molecules or cocktails of a few molecules that elicit dramatic behavioral effects. Such substances are often referred to as "releaser pheromones", whereas chemosignals that cause longer term changes in neuroendocrine or developmental states are usually referred to as "primer pheromones".

3) One of the best characterized mammalian pheromone is the rabbit nipple "search pheromone". Sensed by rabbit pups via their main olfactory system, it elicits a characteristic nipple search behavior that quickly results in the location of a nipple [5]. This guidance cue is particularly important for rabbits, as a doe only nurses her pups for around four minutes once a day and the quick location of a nipple in the face of sibling competition is vital for survival. This pheromone has recently been shown to be a single molecule, 2-methylbut-2-enal, which is produced in rabbit milk and is sufficient to elicit full nipple search and grasping behavior when presented on its own. Whereas the rabbit nipple search pheromone is a single molecule, many pheromones consist of blends of two or more molecules, which can elicit a greater response than any individual component. For example, the androgen derivatives 5a-androst-16-en-3-one and 5a-androst-16-en-3-ol are present at high concentrations in boar saliva. Each component elicits pheromonal effects to attract estrus sows and cause them to adopt a characteristic mating stance known as standing.

4) In summary: Olfaction is the dominant sensory modality for most animals and chemosensory communication is particularly well developed in many mammals. Our understanding of this form of communication has grown rapidly over the last 10 years since the identification of the first olfactory receptor genes. The subsequent cloning of genes for rodent vomeronasal receptors, which are important in pheromone detection, has revealed an unexpected diversity of around 250 receptors belonging to two structurally different classes. Recent studies using genetically modified mice and electrophysiological recordings have highlighted the complexities of chemosensory communication via the vomeronasal system and the role of this system in handling information about sex and genetic identity. Although the vomeronasal organ is often regarded as only a pheromone detector, evidence is emerging that suggests it might respond to a much broader variety of chemosignals.

References (abridged):

1. Karlson, P. and Loescher, M. (1959). Pheromones: a new term for a class of biologically active substances. Nature 183, 55-56

2. Doty, R.L. (2003). Mammalian pheromones: fact or fantasy?. In Handbook of Olfaction and Gustation. Doty, R.L. ed. (: Marcel Dekker Inc)

3. Beauchamp, G.K. and Yamazaki, K. (2003). Chemical signalling in mice. Biochem. Soc. Trans. 31, 147-151

4. Stern, K. and McClintock, M.K. (1998). Regulation of ovulation by human pheromones. Nature 392, 177-179

5. Distel, H. and Hudson, R. (1985). The contribution of the olfactory and tactile modalities to the performance of nipple-search behavior in newborn rabbits. J. Comp. Physiol. [A] 157, 599-605

Current Biology http://www.current-biology.com

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BEHAVIORAL BIOLOGY: ON MAMMARY PHEROMONES

The following points are made by B. Schaal et al (Nature 2003 424:68):

1) Mammals owe part of their evolutionary success to the harmonious exchanges of information, energy and immunity between females and their offspring. This functional reciprocity is vital for the survival and normal development of infants, and for the inclusive fitness of parents(1,2). It is best seen in the intense exchanges taking place around the mother's offering of, and the infant's quest for, milk. All mammalian females have evolved behavioral and sensory methods of stimulating and guiding their inexperienced newborns to their mammae, whereas newborns have coevolved means to respond to them efficiently(3). Among these cues, maternal odors have repeatedly been shown to be involved(4,5), but the chemical identity and pheromonal nature of these cues have not been definitively characterized until now.

2) The extremely parsimonious strategy of maternal care evolved by the rabbit, Oryctolagus cuniculus, offers a unique opportunity to understand the cues that regulate neonatal behavior in a mammal. The female nurtures her litter for 4 to 5 minutes once a day during the 2 weeks after birth: this is therefore less than 0.35% of her time. Because the survival of the pups is conditional on milk intake on the first 2 days of life, the pups need a reliable sensory tether for the rapid location of nipples, and competent behavior to obtain milk successfully in a context of harsh competition between littermates. Rabbit pups are endowed with keen chemosensory and tactile abilities linked with a typical head-searching pattern that ends in the grasping of a nipple within 3 to 5 seconds. The release of this behavior is under the main control of chemical cues emitted by lactating females on the nipples and in milk. As these skin and milk signals are functionally equivalent, the authors focused their analytical effort on rabbit milk.

3) The volatiles in milk were extracted, trapped and then desorbed into a gas chromatograph (GC) equipped with a sniffing device, permitting concurrent detection by neonatal rabbits and the flame ionization detector (FID). The pups subjected to such gas chromatography-olfaction (GCO) tests responded either by short-range searching motions of the head directed to the sniff-port, or by attempts to seize it orally. The cumulative frequency of both responses allowed the authors to determine behaviorally active regions on the chromatographic profiles. Although some compounds could not be identified because of uninterpretable mass spectra caused by concentration or co-elution problems, most of the GC peaks corresponding to the behavioral peaks were identified by GC-mass spectrometry (GC-MS). The 21 compounds identified were screened for behavioral activity with a bioassay presenting them (aqueous dilution of 1 microg/ml) on a glass rod. One volatile, 2-methylbut-2-enal (2MB2), released both responses to a considerably greater extent than any of the other volatiles and than the solvent alone.

4) The generality of 2MB2 activity was assessed in pups from other rabbit strains and breeds including angora, castor, chinchilla, laghmere, butterfly and Belgian hare. 2MB2 assays elicited searching-grasping in more than 80% of those pups, showing that genotype has no effect on its releasing power. 2MB2 reception specificity was also examined in newborns belonging to taxa having more or less phylogenetic distance from Oryctolagus. Newborns of a closely related lagomorph, Lepus europaeus, and of murine rodents (Rattus rattus, Mus musculus) and a carnivore (Felis cattus) were tested: the outcome was negative, making clear the species specificity of 2MB2.

5) The authors suggest this finding has several broader implications for future research. First, such key compounds releasing clear behavioral responses are pressingly needed to unravel further the general principles of olfactory functioning, plasticity and development in vertebrate models. At the systems level, the pheromone-behavior coupling will permit the explanation of the chemosensory subsystem mediating its detection. It should then help in deciphering the specificity of the activity pattern elicited in these chemosensory pathways from peripheral to higher-order neural levels.

References (abridged):

1. Clutton-Brock, T. H. The Evolution of Parental Care (Princeton Univ. Press, 1991)

2. Rosenblatt, J. S. & Snowdon, T. C. Parental Care, Evolution, Mechanisms and Adaptive Significance (Academic, Orlando, 1996)

3. Rosenblatt, J. S. Olfaction mediates developmental transition in the altricial newborn of selected species of mammals. Dev. Psychobiol. 16, 347-375 (1983)

4. Leon, M. & Moltz, H. Maternal pheromone: Discrimination by pre-weanling albino rats. Physiol. Behav. 7, 265-267 (1971)

5. Blass, E. M. & Teicher, M. H. Suckling. Science 210, 15-22 (1980)

Nature http://www.nature.com/nature

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MECHANISMS OF OLFACTION

The following points are made by Stuart Firestein (Nature 2001 413:211):

1) The sensitivity and range of olfactory systems is remarkable, enabling organisms to detect and discriminate between thousands of low molecular mass, mostly organic compounds, which we commonly call "odors". Represented in the olfactory repertoire are aliphatic and aromatic molecules with varied carbon backbones and diverse functional groups, including aldehydes, esters, ketones, alcohols, alkenes, carboxylic acids, amines, imines, thiols, halides, nitriles, sulfides and ethers. This remarkable chemical-detecting system, developed over eons of evolutionary time, has received considerable attention in the past decade, revealing sensing and signaling mechanisms common to other areas of the brain, but developed here to unusual sophistication.

2) How does the olfactory system manage this sophisticated discriminatory task? Beginning with the identification of a large family of G-protein-coupled receptors (GPCRs) in the nose, the foundations of a comprehensive understanding have emerged in surprisingly short order. The advent of advanced molecular and physiological techniques, as well as the publication of eukaryotic genomes from Caenorhabditis elegans to Homo sapiens, has provided the critical tools for unveiling some of the secrets. We now possess a detailed description of the transduction mechanism responsible for generating the stimulus-induced signal in primary sensory neurons, and also an explicit picture of the neural wiring, at least in the early parts of the system. From this body of work a view of molecular coding in the olfactory system has arisen that is surely incomplete, but nonetheless compelling in its simplicity and power.

3) Among higher eukaryotes, from flies through to mammals, there is a striking evolutionary convergence towards a conserved organization of signaling pathways in olfactory systems(1). Two olfactory systems have developed in most animals. The common or main olfactory system is the sensor of the environment, the primary sense used by animals to find food, detect predators and prey, and mark territory. It is noteworthy for its breadth and discriminatory power. Like the immune complex, it is an open system built on the condition that it is not possible to predict, a priori, what molecules it (that is, you) might run into. Therefore, it is necessary to maintain an indeterminate but nonetheless precise sensory array.

4) A second, or accessory, olfactory system has developed for the specific task of finding a receptive mate -- a task of sufficient complexity that evolution has recognized the need for an independent and dedicated system. Known as the vomeronasal system, it specializes in recognizing species-specific olfactory signals produced by one sex and perceived by the other, and which contain information not only about location but also reproductive state and availability. In addition to its role in sexual behaviors, it is important in influencing other social behaviors such as territoriality, aggression and suckling.

5) In summary: The human nose is often considered something of a luxury, but in the rest of the animal world, from bacteria to mammals, detecting chemicals in the environment has been critical to the successful organism. An indication of the importance of olfactory systems is the significant proportion -- as much as 4% -- of the genomes of many higher eukaryotes that is devoted to encoding the proteins of smell. Growing interest in the detection of diverse compounds at single-molecule levels has made the olfactory system an important system for biological modelling.(2-5)

References (abridged):

1. Hildebrand, J. G. & Shepherd, G. M. Mechanisms of olfactory discrimination: converging evidence for common principles across phyla. Annu. Rev. Neurosci. 20, 595-631 (1997)

2. Buck, L. B. The molecular architecture of odor and pheromone sensing in mammals. Cell 100, 611-618 (2000)

3. Mombaerts, P. Seven-transmembrane proteins as odorant and chemosensory receptors. Science 286, 707-711 (1999)

4. Mombaerts, P. et al. Visualizing an olfactory sensory map. Cell 87, 675-686 (1996)

5. Buck, L. & Axel, R. A novel multigene family may encode odorant receptors: a molecular basis for odor recognition. Cell 65, 175-187 (1991)

Nature http://www.nature.com/nature

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