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New mothers protected from stress, have heightened reward, smell

12.11.2003


New studies find much to recommend in pregnancy and motherhood. Findings include that: pregnancy produces heightened smell sensitivity; suckling one’s young puts brain reward systems into high gear; lactation increases the rate of wound healing; and motherhood protects against stress.

Morning sickness, food cravings, and food aversions are not the only side effects of pregnancy. Many women also say that they perceived changes in their chemical senses during pregnancy; perhaps a certain dish had a different flavor than they were used to or they were especially sensitive to an odor in their environment.

Indeed, a new study finds that almost 70 percent of 126 pregnant women questioned claimed to have abnormal smell sensitivity early in pregnancy. A majority of these women also reported that they perceived some odors to be stronger than normal, says Daniel Broman, a PhD student at Umea University in Umea, Sweden. Broman and his colleagues found that, for example, the smells of cooking odors, cigarette smoke, perfume, and coffee were reported to be experienced as stronger among more than 30 percent early in pregnancy. Furthermore, almost 60 percent reported food aversions early in pregnancy and, interestingly, about 70 percent of those who reported abnormal smell sensitivity early in pregnancy also reported food aversions.



The study was conducted with a questionnaire, including questions about smell, taste, food cravings, aversions, etc., that was sent to a group of pregnant women twice during pregnancy and once after delivery and to a control group three times.

Next, Broman and his colleagues investigated different aspects of the sense of smell in pregnant women in a laboratory setting in order to better understand these perceived olfactory changes. They studied detection sensitivity, intensity, and unpleasantness for a specific odor in pregnant and nonpregnant women. A set of 32 common, everyday-life odors, such as coffee, vanilla, and lemon were also tested. For these odors, the participants made different judgments, for example, about the pleasantness and edibility of the odorant.

From this part of the study, which looked at 36 pregnant and 36 nonpregnant women, those who were pregnant had better absolute odor sensitivity, which means that they could detect and smell a specific odor at a weaker concentration than the nonpregnant women. Furthermore, pregnant women were even more sensitive with respect to odor unpleasantness. The judgments of the 32 common odors showed that pregnant women reported odors to be more unpleasant and more likely to evoke nausea compared to nonpregnant women.

Broman’s study gives a broad description of the chemical senses and their relation to some aspects of food behavior during pregnancy. Furthermore, the laboratory part shows that differences actually exist in olfactory functions at a perceptual level, and not only in cognitive evaluations or interpretations of odors, between pregnant and nonpregnant women. This finding, to the author’s knowledge, has not clearly been shown before.

An interesting question is whether changes in the chemical senses can have an important function. It has previously been suggested that, for example, a change in taste experience during pregnancy can have a biological meaning and support a more functional diet.

Broman and his colleagues now hope to put all these pieces of information together in order to better understand the role the olfactory system plays during pregnancy. "Hopefully, such knowledge can help us understand problems like severe nausea during pregnancy, unhealthy food aversions and cravings, and perhaps in a longer perspective also help us to make the life easier for the pregnant women that have these complaints," he says.

Another new study using magnetic resonance imaging to measure the brain response of female rats when they are suckling their offspring finds that brain areas that mediate ’reward’ are stimulated by pup suckling.

“These are the same brain areas that are stimulated when a rat is given cocaine,” says study author Craig Ferris, PhD, of the University of Massachusetts Medical Center in Worcester. “Many people consider reward as roughly analogous to ‘pleasure’ in humans. Behavior that activates the reward system is likely to be repeated since reward is a positive reinforcer. So activation of a pleasure sensation in the mother may be part of the reason that rat mothers bond with their pups, and this may also occur in other species,” Ferris adds.

The study supports recent work by Alison Fleming of the University of Toronto and Anna Lee of Toronto’s Hospital for Sick Children showing that rat mothers will ’work’ (press a lever) to obtain their pups when the pups are very young; indeed mother rats will prefer to press a lever to obtain a pup than to press a lever that delivers cocaine. However, the data also show that the mothers prefer to work for cocaine rather than pups once pups are older.

Ferris and his colleagues also found that when older pups suckle, less activation occurs in these ’pleasure’ pathways. “Cocaine is an effective activator of the reward systems, so it’s not surprising that in the lever pressing studies the pups compete with cocaine when young, but don’t when older,” Ferris says.

In the study, the brains of a small group of rat mothers were scanned while they were conscious in a high-powered MRI scanner. Initially, on first scanning, the mothers had young pups; as the experiment progressed, the mothers were scanned again, as the pups got older and older. In each case, the mother was restrained in a specially designed holder, to ensure that the rat was comfortable and yet unable to move its head, to allow for detailed images of the brain activity. The pups were suspended in a small box beneath their mothers’ abdomen under the teats and a sliding divider was placed between mother and pups. The scientists imaged the mothers’ brains when the pups could not access the teats, then removed the sliding divider such that the pups could access the milk and feed. The divider was slid in and out such that the researchers could compare and contrast brain activity with and without suckling in each mom in each experiment.

Ferris and his colleagues also found that several other brain areas seemed to be activated when the pups suckle. They are planning to investigate these areas further to describe the precise changes in activation of brain areas as the pups age. The technology used to hold the animals during the imaging sessions is commercially available through Insight Neuroimaging Systems, a company owned by Ferris.

In a study of mice, scientists found that lactation increased the rate of wound healing and was associated with lower levels of the stress hormone corticosterone when compared with controls.

“Our study was conducted in mice but may have clinical implications,” say study authors A. Courtney DeVries, PhD, and Tara Craft of Ohio State University. “Whether giving birth naturally or via cesarean section, women sustain substantial tissue trauma. Because many of the hormonal changes that occur following birth and during lactation are similar in mice and women, it is possible that lactation (i.e., breastfeeding) may improve healing in women also.”

In the study, adult female mice were mated and allowed to give birth. The pups either remained with the mother (postpartum lactation group) or were removed immediately after birth (postpartum non-lactating group). Twenty-four hours after birth, the mothers were given a small superficial skin wound on the scruff of the neck. Wound size was measured daily to determine healing rate. In a subset of mice, the tissue surrounding the wound was analyzed to provide a profile of immune changes that were occurring in the skin during healing. A blood sample was also taken to determine hormone concentrations.

DeVries and her colleagues found that lactation suppressed “stress hormone” concentrations, altered immune function, and improved wound healing compared to unmated females.

The next step in this research is to examine healing of wounds in women who choose to breastfeed versus bottle feed following birth. The goal would be to determine if breastfeeding improves healing of tissue damage that occurs during natural or cesarean birth.

Previous clinical studies have suggested that stress can influence the rate of wound healing in humans, says DeVries. “Wound healing studies in mice provide strong evidence that an increase in "stress hormones" is the mechanism through which stress delays wound healing. Breastfeeding (lactation) can suppress the release of some stress hormones. Thus, it is possible that in our study, lactation improved wound healing by suppressing stress-hormone concentrations following birth.”

Another new study finds that females can respond very differently to stressful experiences following giving birth.

In previous studies, exposure to an acute stressful event was found to severely impair new learning and enhance anxiety in adult virgin females. In the new study, scientists examined how stress affects these behaviors in the postpartum female.

Unlike virgin females, exposure to the stressful event did not affect their ability to learn or enhance anxiety when they were examined during the postpartum period. These data indicate that some cognitive and emotional responses to stressful experience are suppressed during the postpartum period. “These unique responses may be important in helping new mothers care for their young during stressful times,” says Tracey Shors, PhD, of Rutgers University. “More generally, these results show that the female response to stressful experience is dynamic and can change dramatically during different stages of reproductive life.”

Postpartum is often associated with depression and other emotional problems. But for most women, it is one of the most wonderful and exciting times in their lives. How they respond to life events and handle their new responsibilities directly affects the survival and development of their young.

These findings are the first to show that new mothers may be protected from some of the negative consequences of stressful life events, at least in terms of their ability to form some types of new memories.

These findings also illustrate that new mothers are remarkably resistant to stressful life events. This occurs in spite of the fact that females are most susceptible to mental disorders associated with stress and stressful life events. Compared to men, women have a much higher incidence of depression, generalized anxiety, and posttraumatic stress disorder. By examining the impact of stress on cognitive and emotional function during stages of reproductive life such as the postpartum period, scientists may begin to understand the reasons for the high incidence of mental illness in females, especially postpartum depression and psychosis.

At a basic science level, the researchers believe that these unique cognitive responses to stress during the postpartum period may be a useful animal model for investigating how the brain prepares the new mother for the responsibilities of motherhood.

In the study, Shors and her graduate student Benedetta Leuner examined whether stress impairs learning and enhances anxiety in postpartum female rats as it does in adult virgin females. To accomplish this, groups of adult virgin and postpartum females were exposed to an acute stressful experience of restraint and tail stimulations or left unstressed. Twenty-four hours later, all rats were trained on a classical eye blink conditioning task. In this task, a tone signals the occurrence of an eyelid stimulation, which causes an eye blink. As the animal learns that the tone predicts the eyelid stimulation, it blinks in response to the tone. The scientists used the version of this task known as trace conditioning in which the tone and eyelid stimulation are separated in time and which is dependent on the hippocampus, a part of the brain involved in learning and memory. After training, anxiety-related behavior was measured.

“Our data show that postpartum females show very different and unique responses as compared to females without reproductive and maternal experience.” Shors says. Exposure to the stressful event severely impaired new learning and increased anxiety in virgin females. In contrast, exposure to the stressful event did not affect learning or anxiety behavior during the postpartum period.

In their next studies, the researchers are addressing the age-old question: Is it nurture or nature? In other words, is the stress response suppressed because of behaviors that occur during motherhood such as those directly involving nurture and care for their young or is it mediated by the very different hormonal environment within the postpartum female?

In other experiments, they are examining the effects of stressful experience on the postpartum brain. Researchers have previously found that exposure to stressful experiences alters the anatomy of cells in the female hippocampus, a brain region necessary for memory. They are currently examining whether the brain of the postpartum female is protected from the adverse effects of stress, as is her ability to learn.

“Motherhood is a one of the most exciting but also the most demanding times in a woman’s life. Her ability to learn and respond appropriately to new environmental events is vital to the survival and well-being of her children,” Shors says.

“Our data suggest that even in rats, postpartum females’ response to stressful life events is very different than in a female during her normal cycle. Under normal cycling conditions females do not learn as well after stressful life events. However, during postpartum, their ability to learn is not adversely affected by stress,” Shors adds. “It may be this unique response system that prevents the new mother from over-responding to stressors in the environment and thus allows her to deal more effectively and wisely with the lives of her offspring.”

Dawn McCoy | Society for Neuroscience
Further information:
http://www.sfn.org/content/AboutSFN1/NewsReleases/am2003_pregnancy.html

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