The nose knows who's who: chemosensory individuality and mate recognition in mice
Section snippets
Chemosensory signals of individuality
Olfaction is the dominant sensory modality for many animals, and chemosensory communication plays a vital role in conveying information about individuality. It has long been known that mice can be trained to discriminate between the urine odors of congenic mice that differ genetically only at the H2 locus of their MHC (Yamazaki et al., 1979). Different individuals carry different combinations of the 50 or so genes that make up the highly polymorphic MHC family and confer individuality at a
Urine individuality and the pregnancy block effect
In the pregnancy block effect (Bruce effect), exposure of a recently mated female mouse to an unfamiliar male, for 2 days following mating, results in a high incidence of pregnancy failure (Bruce, 1959). This is caused by testosterone-dependent, pregnancy-blocking substances in male mouse urine (Bruce, 1965). Although the mating male also produces pregnancy-blocking chemosignals in his urine, they do not block his mate's pregnancy (Parkes and Bruce, 1961). This is because the female learns the
The chemical nature of pregnancy-blocking chemosignals
It has been suggested that testosterone derivatives present in male urine, such as 17β-estradiol, might block pregnancy directly by interfering with female reproductive endocrinology (deCatanzaro et al., 2001). However, there is substantial evidence that this response is mediated via unidentified urinary chemosignals, which are sensed by the vomeronasal organ (VNO) and activate an excitatory pathway to the hypothalamus, via the accessory olfactory bulb (AOB) and corticomedial amygdala Li et
The vomeronasal organ
In addition to the main olfactory epithelium, rodents possess a well-developed vomeronasal organ (VNO) that is commonly regarded as being specialized for the detection of pheromonal information (Dulac and Torello, 2003). This is a blind-ended, mucus-filled tube situated in the nasal septum and connected anteriorly to the nasal cavity via the narrow vomeronasal duct (Døving and Trotier, 1998). The vomeronasal sensory neurons (VSNs) are located in the crescent-shaped sensory epithelium on the
Vomeronasal receptors and the MHC
The ability of the vomeronasal system to convey information about individuality is not surprising in light of the diversity of vomeronasal receptor types. There are two classes of these seven transmembrane domain G-protein-coupled receptors. These share little sequence homology with each other, suggesting that they may respond to different types of ligand Herrada and Dulac, 1997, Matsunami and Buck, 1997, Ryba and Tirindelli, 1997. The V1R class consists of at least 137 functional receptors
Neural processing of vomeronasal information
There are important differences in the morphology of the projection neurons between the MOB and the AOB that may affect the coding and integration of information (Mori, 1987). In contrast to the highly specific projections of OSNs of the main olfactory epithelium, in which MOB mitral cells project a single primary dendrite to a single glomerulus (Mombaerts et al., 1996), VSNs expressing a particular receptor send a branched primary dendritic tree that collects information from several AOB
Neural mechanisms mediating individual recognition
Learning plays a major role in individual discrimination. The Y-maze training of mice to discriminate between MHC-congenic urine odors is likely to be mediated by the same main olfactory learning mechanisms that deal with associative learning of general odors. Rodents are also able to distinguish between individuals from their previous encounters in a social habituation test, directing more investigation and interest to novel individuals than familiar ones. This discrimination depends on the
Future prospects
There are currently more questions than answers about the nature of individuality cues mediated by the vomeronasal system. However, many answers may be waiting to be discovered in genetically manipulated mice. Mouse strains have been developed that have general vomeronasal dysfunction such as the trp-2 knockout mice, which have reduced vomeronasal sensitivity Leypold et al., 2002, Stowers et al., 2002. Other strains have more specific dysfunctions targeted at either the V1R or V2R classes of
Acknowledgements
The author's work is supported by a Medical Research Council Career Development Award.
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