Pregnant mice block out unwelcome admirers to protect ...( Mouse mothers-to-be have a remarkabl...)

Mouse mothers-to-be have a remarkable way to protect their unborn pups. Because the smell of a strange male's urine can cause miscarriage and reactivate the ovulatory cycle, pregnant mice prevent the action of such olfactory stimuli by blocking their smell. Researchers from the European Molecular Biology Laboratory (EMBL) in Monterotondo, Italy, have now revealed the nature of this ability. A surge of the chemical signal dopamine in the main olfactory bulb - one of the key brain areas for olfactory perception creates a barrier for male odours, they report in the current issue of Nature Neuroscience.

Social odours, such as pheromones, influence many aspects of human and animal behaviour perhaps most widely known reproductive behaviour. For example, exposing a newly pregnant mouse to the smell of an alien male's urine prevents the implantation of her embryos into the uterus and brings her back into the ovulatory cycle. The scent affects pregnancy by inhibiting the release of the pregnancy hormone prolactin. This phenomenon is often called the Bruce effect and creates a mating opportunity for the alien male. It is also beneficial for the female because it avoids infanticide by the strange male after birth. After day 3 of pregnancy, however, the smell of an alien male's urine no longer affects pregnancy. At this stage the embryos have already been implanted into the uterus and loosing them would bear a high cost for the female.

Liliana Minichiello and her team at the EMBL Mouse Biology Unit now discovered the molecular mechanism that underpins this change in sensitivity to male odours.

"At day 3 of the pregnancy a chemical change occurs in the brain of the expectant mother that makes her unable to perceive male odours. This seems to mark a point of no return for the pregnancy," explains Minichiello.

Following coitus, a progressive surge of the chemical signal dopamine takes place in the main olfactory bulb, the most
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