For many animals, making sense of odors is a matter of survival. Researchers studying mice have now discovered that a hormone called prolactin enhances the sense of smell by stimulating the development of new neurons. The upgrade is kick-started by mating and pregnancy, which cause prolactin levels to surge.
Odors from putrid to pleasant are detected by specialized neurons that line the nose. These neurons alert the brain's smell center, the olfactory bulb. From there, information is sent around the brain and processed, eventually confirming, for example, that yep, that milk has gone sour, and perhaps conjuring up the memory of the time you took a big gulp without checking. New olfactory neurons are created throughout life, but until now, researchers haven't understood what regulates this neurogenesis.
By inserting a marker molecule into dividing cells in the brain, Samuel Weiss of the University of Calgary in Alberta, Canada, and Tetsuro Shingo of the Okayama University Medical School in Japan monitored production of new cells in virgin and pregnant mice. During early pregnancy and just after birth, pregnant mice produced 50% to 100% more stem cells in their brains than did the virgins, the team reports in the 3 January issue of Science. These newly born stem cells migrated to the olfactory bulb and differentiated into olfactory neurons in significantly higher numbers in the pregnant mice. To the researchers' surprise, mating was enough to spur stem cell production. Females mated with vasectomized males generated 42% more stem cells than virgin females did.
Prompted by earlier findings that the hormone prolactin surges after sex and during pregnancy and breastfeeding in humans, Weiss and Shingo tested that hormone's influence on neurogenesis. Female mice injected with prolactin produced 56% more stem cells than did nontreated mice and had more stem cells in the olfactory bulb. This enhanced olfactory prowess is adaptive for these rodents, Weiss says: A heightened sense of smell during pregnancy may help alert moms to danger. Plus, the scent of their young is known to stimulate protective, nurturing behavior in mouse mothers.
These results make a "strong argument for the functional role of prolactin in neurogenesis," says neuroscientist Larry Squire of the University of California, San Diego. Even though the work is in mice, it "confirms the myth that pregnant women have a heightened sense of smell."
Longer Science news article on neurogenesis
Samuel Weiss's site
Larry Squire's site