Odor and Pheromone Sensing in Mammals
The mammalian olfactory system detects a multitude of structurally diverse chemicals in the external environment. Most are perceived as odors, but some, such as pheromones and predator odors, stimulate hormonal changes or instinctive behaviors. To explore the mechanisms underlying odor and pheromone sensing, we are using the mouse as a model organism.
Our initial studies showed that odor detection is mediated by a family of ~1,000 different odorant receptors (ORs). Each olfactory sensory neuron in the nasal olfactory epithelium expresses only one OR gene. We found that ORs are used in a combinatorial fashion to detect odorants, with different odorants detected and thereby encoded, by different combinations of ORs. Even a slight change in an odorant’s structure can alter its combinatorial receptor code, explaining how we can distinguish closely related odorants and perceive nearly identical odorants as having odors as different as sweaty versus orange.
We subsequently asked how signals from 1,000 different ORs are organized in the olfactory system. In the nose, thousands of neurons with the same OR are randomly dispersed within one OR expression zone, producing a mosaic of interspersed neurons expressing different ORs. However, in the olfactory bulb, the axons of those neurons all converge in a few glomeruli at characteristic sites. The result is a nearly stereotyped map of OR inputs in which each glomerulus and each relay neuron that transmits signals to the olfactory cortex is dedicated to one OR. Studies from several groups indicate that OR inputs return to being highly distributed in the olfactory cortex, but how olfactory signals are organized in higher cortical areas and ultimately generate diverse odor perceptions are still unsolved mysteries.
The vomeronasal organ (VNO) is an olfactory structure found in many mammals that detects pheromones and other social cues that elicit innate responses. Efforts in our lab and others identified two families of VNO receptors with over 100 members, called V1Rs and V2Rs. We found that single pheromones activate a minute fraction of VNO neurons, suggesting that individual pheromones may be detected by only one or a few dedicated receptors rather than by a complex combination of receptors, as seen for odorants. More recently, we and others uncovered another VNO receptor family, comprising five of seven members of the formyl peptide receptor (FPR) family. The other two FPRs are expressed by immune system cells and appear to stimulate chemotaxis to sites of bacterial infection or tissue damage, raising interesting questions about the functions of VNO FPRs.
To investigate the neural circuits responsible for pheromone effects on reproduction, we expressed a lectin transneuronal tracer in GnRH neurons in the hypothalamus. GnRH neurons control reproductive hormones and are also linked to sexual behaviors. Remarkably, these studies suggested that the 800 GnRH neurons in mice directly communicate with ~50,000 other neurons in dozens of functionally diverse brain areas, including several associated with sexual behaviors. Surprisingly, these and several other studies also suggested that pheromones may be detected not only in the VNO but also in the nose.
These observations raised a question as to how pheromones are detected in the nose. To explore whether the nose might have other types of chemosensory receptors beyond ORs, we conducted a high-throughput search for additional G protein–coupled receptors. This effort identified a second family of 14 receptors in the nose, called trace amine-associated receptors (TAARs). By testing those receptors with over 200 odorants, we identified ligands for four TAARs, each of which recognized a distinct set of volatile amines. One TAAR detects a compound present in urine from stressed animals, while two others detect compounds enriched in male versus female mouse urine. TAAR genes are also found in fish and humans. We identified multiple TAAR sequences among fish olfactory epithelium expressed sequence tags, suggesting that TAARs might function as olfactory receptors in numerous organisms. The evolutionary conservation of TAARs suggests that they may serve a function distinct from ORs. One potential function suggested by these studies is the detection of social cues. We are now investigating the functions of TAARs and their associated neural circuits. We are also using viral transsynaptic tracers to explore the mechanisms and neural circuits by which olfactory stimuli elicit innate responses and impact basic drives.
In addition to our major focus on olfaction, our laboratory has been interested in aging. One long term goal of aging research is to identify drugs that would delay age-associated disease. Previous studies of the short-lived nematode Caenorhabditis elegans have uncovered numerous genes that influence the lifespan of this organism. At least some aging mechanisms identified in C. elegans appear to be evolutionarily conserved in mice. Given the presence of many homologous proteins with related functions in nematodes and mammals, we reasoned that the identification of drugs that increase nematode longevity might point to drugs that could be tested for beneficial effects on mammalian aging.
As a first step in this direction, we screened 88,000 chemicals for the ability to increase the lifespan of C. elegans. We found over 100 that did so. Further investigation of one chemical led to the finding that nematode lifespan can be increased about 30% by mianserin, a drug used as an antidepressant in humans. This effect required SER-4, a specific serotonin receptor, as well as SER-3, a receptor for octopamine, another neurotransmitter. Similar to its inhibition of human serotonin receptors, mianserin inhibited both receptors but was a more potent antagonist of SER-4 than of SER-3. Testing mianserin on aging mutants or dietary-restricted nematodes suggested that the drug increases lifespan via mechanisms linked to dietary restriction but without reducing food intake. In C. elegans, serotonin appears to signal food availability and octopamine signals starvation. One possible explanation for the effect of mianserin on longevity is that its greater inhibition of SER-4 than of SER-3 mimics a reduction in food intake, thereby triggering aging mechanisms associated with dietary restriction.
More recently, we have been investigating the effects of drugs with known or suspected mammalian targets for the ability to extend the lifespan of C. elegans. Identification of such drugs could facilitate determination of their targets in nematodes and provide candidates for testing in mice.