October 5, 2012
Newborn Mice Rely On Mother’s Odor For Breastfeeding
[Watch Video: Newborn Mice Depend On Mom's Signature Scent]
April Flowers for redOrbit.com - Your Universe OnlineSometimes, not finding what you are looking for can lead to even more interesting avenues of research. That is what a recent international research team found out when they were searching for a pheromone trigger for newborn infant suckling responses in mice. The results of their study have been published in the journal, Current Biology.
Long-held wisdom has been that pheromones — chemicals that trigger innate behaviors — drive the suckling response of newborn mammals. Previous research on rabbits showed that the pheromones were present, leading researchers to assume that all mammals were likely to use the same mechanism. The researchers in this current study, from Wellcome Trust Sanger Institute and the Scripps Research Institute, were keen to find the pheromone in other mammals and chose mice because they have a parenting style similar to that of humans.
Suckling is the defining behavior in mammals, and most are born knowing how to find the breast and drink. They would quickly perish if this were not the case, which makes suckling an instinctive behavior hard wired into the brain.
"So far, little is known about how innate behaviors are coded in the brain, what triggers them and what represses them," said Lisa Stowers of the Scripps Institute. Many things may seem to be triggers for an instinctive behavior, but only because the animal has learned to associate them with the behavior. This study focused on finding the essential, or first — trigger, which is a painstaking process of elimination requiring an array of sophisticated laboratory methods.
"We were expecting to find a pheromone controlling suckling in mice, but we found a completely different mechanism at work," says Dr Darren Logan, from the Wellcome Trust Sanger Institute. "We have shown for the first time that it is not a pheromone response in mice, but a learned response, founded on a mix of odors: the unique signature smell of the mother."
Newborn mice, delivered by C-section — were introduced to breasts that had been washed clean and then soaked in one of the fluids that a baby would first inhale at birth. These fluids included amniotic fluid, the mother's saliva, breast milk and urine. Only the smell of amniotic fluid initiated suckling behaviors in the young.
The team looked for a pheromone in the amniotic fluid. To change the mother's scent, they fed pregnant mice strong smelling foods such as garlic. If there was a pheromone involved in the suckling response, the garlic would have no effect. What they found, however, was that only those mice that had previous experience with the amniotic fluid with the strong smell from their mother were able to feed successfully. This proves the signature smell of the mother must be learned.
The team even fractionated the amniotic fluid, in an attempt to find the pheromone they were sure must be there, breaking down the fluid into molecular weight ranges. No single fractionated part of the fluid triggered suckling on its own.
"Our work shows us that there is no species-wide pheromone that makes newborn mice feed, but that the mouse pups are actually learning their mother's unique and variable mix of smells at birth," explains Associate Professor Lisa Stowers, "So, although the suckling response may look like a pheromone-mediated behavior, it is actually initiated through a fundamentally different process."
Genetic research gave supporting evidence for this conclusion. The team found that mice who lack a critical gene in the pheromone-detecting region of the nose, called the vomeronasal organ (VNO), were able to locate the nipple and suckle. Mice babies who lacked the ability to smell regular smells detected by the main olfactory epithelium (MOE) struggled with feeding.
"This is a neat study which shows the value of studying the development underlying an apparently 'innate' behavior," said Dr Tristram Wyatt of the University of Oxford. "The surprising result is that mouse pups use the individual odors of the mother to find their first feed. It is a reminder of the way that evolution uses whatever works: there is more than one way to find the first milk meal. The rabbit has a pheromone in the milk, humans may have one around the nipple, and mice learn the individual odor of their mother. All three enable the vital task of getting the newborn to suckle."
Learned recognition of signature odors may be a critical component of other innate mammalian behaviors. Humans also form intensive, nurturing bonds with their babies, suggesting that genetic manipulation of the ability to smell in mice will be a useful way to research the neural pathways involved in human instinctive behavior.