January 2, 2003. Copyright 2002. Graphic News. All rights reserved.

Sexy scents help bond lovers

LONDON, January 2, Graphic News:  Holiday advertising urges guys and gals to buy their lovers romantic perfumes and aftershaves, and Canadian researchers may now have uncovered the physiological underpinnings of this sales technique. 

A new study in mice shows that prolactin -- a hormone known to surge after orgasm in men and women -- can double the number of new neurons in the mouse brainÕs olfactory bulb, according to a report in the January 3 issue of the journal Science.

A team at the University of Calgary in Alberta found the birth of new neurons -- a process called neurogenesis -- occurs in the olfactory system of the adult brain throughout life, in both invertebrates and vertebrates.

Until recently, precious brain neurons were thought to be irreplaceable. ÒAdult brains, we were taught, are unable to make new neurons, so those with which we are endowed at birth must last a lifetime,Ó said ScienceÕs Marcia Barinaga. 

ÒMajor cracks in that dogma began to appear in the 1980s with the discovery that some songbirds -- warm-blooded vertebrates like ourselves -- give birth to waves of new brain neurons seasonally. Then researchers observed the birth of new neurons in the brains of adult mammals, and the old view came crashing down.Ó 

Interest in newborn neurons surged, and researchers quickly discovered conditions that turn the birthrate of new neurons up or down. Stress and depression, for example, suppress neurogenesis, whereas exercise, stimulating environments, and antidepressant drugs all give it a boost. Now the Calgary reseach has added pregnancy to the list of neurogenesis-enhancing conditions.

The sense of smell is essential for behaviours more fundamental than distinguishing cinnamon from paprika, including a mother rodentÕs ability to recognize and nurture her young, explained Calgary neurobiologist Dr. Samuel Weiss.

A team from WeissÕs lab, led by Dr. Tetsuro Shingo of JapanÕs Okayama University Medical School, injected pregnant rats with a chemical to label new neurons. Neurogenesis rates were found to jump during pregnancy by 65%, peaking around the seventh day of a 21-day gestation and again after delivery.

They also saw a subsequent increase in new neurons integrated into the olfactory bulb. The team identified prolactin, a hormone that increases during pregnancy, as the neurogenesis trigger and discovered that mice genetically altered to be deficient in prolactin receptors have only half the surge in neurogenesis during pregnancy. Weiss notes that other groups have shown that the receptor-deficient mice tend to ignore their young, a behaviour that could be due to an olfactory deficit.

But, he cautions, Òyou have to take that with a big grain of salt, because transgenic animals may display unusual behaviours.Ó Pregnancy isnÕt the only condition that increases prolactin production, Weiss adds: ÒIn males and females one hormone is increased immediately after sexual activity, and that is prolactin.Ó That fact, coupled with his new report, has led some people to joke that sex must be good for the brain. ThatÕs Ògoing off the deep end,Ó Weiss laughs. ÒWe donÕt know the relevance that has to human neurogenesis at all.Ó

The authors suggest that if increased Òscents-itivityÓ helps mice recognize their mates and pups then a similar pattern may help humans cement relationships during the long period between mating and birth. So if you missed your chance to choose just the right fragrance at Christmas, ValentineÕs Day is right around the corner!

/ENDS
Source: Science