This Is Your Brain on Sex (21 page)

Lo and behold, when lactating dams were exposed to cocaine and then scanned in the magnet, there was a
suppression
of brain activity in the reward system. “Not only did the cocaine not activate the motivational system in the moms—it shut it down,” said Ferris.

Ferris argues that this period in a dam’s life is essential to the survival of the species. A mother rat has to nurse those babies or they aren’t going to live. It is a critical behavior—so critical that it has to be more than just pleasurable to the mom. It has to trump any other pleasant distractions that may come her way. After childbirth and lactation the brain is organized in such a way that it will put a negative valence on anything that isn’t in the best interest of the pups,
even if it is enjoyable, including cocaine. The simple experience of being around the pups is a reinforcer, so to speak, one that helps to change the way the brain’s reward circuits operate in these moms.

Those changes result in babies coming first, and everything that might get in the way of caring for those babies second. It reminds me of what my friend, a mom of three, told me when I confessed my marriage was changing for the worse after the birth of my son. She may have had a point when she told me that my intense feelings for my husband might shift to my son. If nothing else, motherhood changed my brain in ways I couldn’t have imagined before giving birth. It is hypothesized that the experience of childbirth alters the reward and motivation centers of the brain so mothers not only have an overwhelming love for their children, but also have a strong focus on their children’s well-being and proper rearing. What might facilitate these kinds of changes? Wouldn’t you know it: animal studies involving both rats, sheep, and prairie voles have demonstrated that our old friends oxytocin and dopamine are involved.

Motherhood Changes Everything

Innately we all, whether we are parents or not, seem to know that there is something unique about the relationship between a mother and her child. Like romantic love, this bond is the stuff of inspirational novels and movies and even gets a fair amount of play in religious lore. Social psychologists have been studying maternal bonds for quite some time, as well as their effect on the physical and mental health of offspring, but neurobiological inquiries into the phenomenon are quite new. The system underlying it, however, is as old as our reptilian brains.

“Motherhood is the biological prototype, the neural and endocrine prototype for all aspects of mammalian sociality,” said Sue Carter, one of the pioneers of oxytocin and pair-bond research. “The neurobiological mechanisms involved in the mammalian mother-infant relationship are highly conserved. And I argue that pair-bonds, romantic love, friendship, any and all social relationships take root from the circuits and processes we see in maternal bonds.”

Long before researchers like Carter were studying the role of oxytocin in pair-bonds, it was understood
that this neuropeptide was involved with motherhood. Physiologically oxytocin has been linked to uterine contractions during birth as well as the milk letdown reflex in lactation, discussed in chapter 3. For a long time scientists believed that was the extent of its role. But soon it became clear that this neuropeptide was also making some kind of cognitive connection between the mind and the body. Consider a nursing mother. She may be resting in the living room, watching a little television or talking with a friend on the telephone, as her infant naps upstairs. If that baby wakes and cries for her, it’s not uncommon for milk letdown to occur within a few seconds of Mom hearing the sound. That letdown can occur without any direct touch or interaction with the infant; the baby’s cry alone is enough to cause the reaction. This kind of phenomenon made it clear that oxytocin was doing more than just facilitating birth and nursing. It was also playing some kind of psychogenic role, connecting the perception of a cry with an actual bodily response.

In addition, when researchers gave rats, hamsters, and prairie voles infusions of oxytocin, the animals were able to elicit maternal behaviors even if they had not yet given birth. The sum of these results suggests that this neuropeptide has a strong role in maternal behaviors—and, by extension, is related to that special attachment between mother and child.

Receptors and Good Mothering

We already know that lack of oxytocin in rodent models leads to impaired social recognition, problems with spatial memory, and issues with pair-bonds.
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Unsurprisingly, it works in similar ways with mothers and their children. After all, in order to have a special bond with your offspring, you need to be able to recognize and locate the baby before you even have a chance to care for it. It appears, however, that it is not a particular level of oxytocin in the blood that determines whether rats will have a loving bond with their offspring, but rather the number of oxytocin receptors in different parts of the brain.

As outlined in chapter 4, maternal behavior in rats influences how different genes are expressed later in life, ultimately influencing neural development and the
offspring’s own behavior down the line. There is a lot of naturally occurring variation in maternal behavior in these species: you’ll see high licking and grooming (LG) dams that dote on their babes as well as low-LG dams that fall more into the negligent category. Michael Meaney and Frances Champagne, those epigenetics researchers extraordinaire, wondered what brain changes underlie those differences in maternal behaviors.
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Meaney, Champagne, and their colleagues took a close look at the brains of high- and low-LG dams. High-LG animals showed significantly higher oxytocin receptor density all over the brain, especially in a few areas implicated in love, like the medial preoptic area (mPOA), the lateral septum, the central nucleus of the amygdala, the paraventricular nucleus (PVN) of the hypothalamus, and the bed nucleus of the stria terminalis. There was a strong correlation between those receptor levels and the quality of maternal behaviors. To try to better link those oxytocin receptor levels to the behaviors, the group then injected some of the high-LG moms with a compound that blocked the oxytocin receptors. Doing so removed any differences in licking and grooming behaviors between the high- and the low-LG groups. Basically, when oxytocin couldn’t be gobbled up by those receptors, every female rat was a low-quality licker.

From the epigenetic standpoint, it would seem that being a high-LG mom helps oxytocin receptor genes create a lot of receptors in certain brain areas. That in turn helps facilitate high-LG behaviors once a mom starts lactating. What about dopamine? Ferris’s work demonstrated that lactating lit up dopaminergic reward processing areas in the brain. Mouse studies that knock out dopamine seem to deprive the moms of the ability to properly care for their young. Might dopamine signaling also have something to say about variations in maternal care? In a follow-up study Champagne, Meaney, and their colleagues took a look at the role of dopamine in high- and low-LG behavior.
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Using an in vivo technique called voltammetry, a way to measure dopamine activity in live rat mommies, the group found that dopamine neurons fired like crazy in the nucleus accumbens shell, not only as moms licked and groomed their babies, but in anticipation of it. The rate of the activity, in terms of both magnitude and duration, was positively correlated with whether the rat was a high-LG or
a low-LG dam. That is, high-LG moms showed significantly higher magnitude and duration of dopamine signaling in the nucleus accumbens shell than their low-LG peers. The researchers argued that this difference might account for the discrepancies observed in the quality of parental care. That higher magnitude and duration of dopamine release in the reward system simply made taking care of pups more pleasurable; hence high-LG behaviors.

In a recent study Meaney’s lab connected oxytocin and dopamine in the natural variations observed in maternal care. The ventral tegmental area (VTA) has been implicated in romantic love in a number of studies; it also happens to receive input from the mPOA and PVN, areas that show a higher concentration of oxytocin receptors in high-LG moms. The VTA is connected to the nucleus accumbens, which shows more dopamine activity in the high-LG dams. It’s like the brain version of the old song “Dem Bones,” but instead of the knee bone connecting to the thighbone, you have the mPOA connected to the VTA, the VTA connected to the nucleus accumbens, and so on. You get the idea. When Meaney and his colleagues directly infused some oxytocin into the VTA, they saw increased dopamine signaling in the nucleus accumbens and higher LG behaviors. When they blocked the oxytocin receptors, they saw lower dopamine activity as well as lower LG behaviors. This study is the first to show that oxytocin directly acts on dopamine release in the brain. So oxytocin release during nursing leads not only to more pleasure but to higher-quality maternal care in high-LG rats.
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It’s easy to read about these studies and say, “Aha! Good and more loving moms have more oxytocin receptors and more dopamine signaling. We just need to up both of those in all women!” Unfortunately, that line of thinking is an oversimplification.

“I don’t really enjoy the good mother–bad mother distinction,” Champagne told me. “It doesn’t make sense from an evolutionary standpoint. It’s not necessarily good or bad to be reared by a low-LG mother. It just prepares you for reproduction and the environment in a different way.” As Moshe Szyf, another leading epigeneticist, said, if the genome is the hardware and the epigenome is the software, the mother is the programmer. Her behavior tells the offspring what kind of environment it should expect. Though some of the same systems seem to be at work, both in terms of neural substrate and neurochemicals, licking and grooming during a weeklong
nursing period seem hardly akin to what we humans consider maternal love. Might we see the same kind of maternal neurobiology in an animal model that is a little similar to humans?

The Neurobiology of Alloparental Behavior

Prairie voles have a very different parenting model than rats. This species is alloparental; that is, everyone plays a part in raising litters. Seeming to intrinsically know that it takes a village to raise a vole pup, the fathers, brothers, and sisters get in on the act. It’s not uncommon for a burrow to house a mother and father as well as several litters of pups of various ages. Young females who have not yet reached sexual maturity may be naturally maternal, grooming babies and retrieving them if they get out of the nest, but this kind of spontaneous parental care occurs in only about half of the girls. It would seem that environment dictates whether these girls will be natural mothers; staying put in the nest after weaning and having Dad around are highly correlated with a prairie vole female’s being naturally affectionate.

Oxytocin is also important. A single treatment of oxytocin twenty-four hours after birth can make prairie vole mommies more attentive and loving to their offspring. Given that the density of oxytocin receptors in the nucleus accumbens is profoundly variable across animals, as observed in rats, Larry Young and his colleagues hypothesized that more receptors would be linked to higher alloparental behavior in young females. To test that hypothesis, Young, a neuroscientist at Emory University who studies social bonding, used a viral vector to enhance the production of oxytocin receptors in this brain area. Surprisingly, he found no difference between these animals and controls. It’s never quite that simple, is it? Clearly oxytocin is mediating some effects on maternal behavior, but it’s unclear how it’s doing so in the prairie vole.
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Looking to Humans

Rats and prairie voles offer
neuroscientists a model, a way to look at how neurochemicals may mediate behavior. But it’s not exactly the same behavior we see in humans. People do not have litters of offspring; typically we stick to one child a time. As such, it’s not a stretch to imagine that people will naturally have different types of biological parenting strategies, even if we share some neural substrate with rodents. There has been some work done with both sheep and rhesus macaques, animals who show a preferential bond with their young, which demonstrates that oxytocin plays an important role in the recognition of the offspring. Those studies have also found natural variations in maternal care. How oxytocin may be regulated by the environment or other factors, making changes to the brain or influencing other neurochemicals downstream, is still under examination.

Given those limitations, what can we extrapolate from animal models of maternal behavior? It depends on who you ask. When I asked Champagne about how easily the findings translate across species, she was cautiously optimistic. “I think there are certainly clear indicators that, at least at its most basic level, the same processes apply in humans,” she said. “Of course, the devil will be in the details when it comes to understanding how it all works.”

And there are a lot of details that still need to be discovered. Instead of relying on animal models, many neuroscientists are now trying to study the effects of oxytocin and dopamine directly in human subjects. Ruth Feldman, a psychologist at Bar-Ilan University in Israel, has looked at correlations between oxytocin levels and behavior in women both during pregnancy and after birth. She and her colleagues measured the plasma oxytocin levels of more than sixty pregnant women during their first trimester and third trimester and within one month of giving birth. Then they observed the mothers interacting with their babies, paying careful attention to eye gaze, emotional response, touch, and vocalizations. They also had the mothers fill out an extensive questionnaire about their emotions and behaviors. When Feldman and her colleagues analyzed their data, they found something surprising. A mother’s oxytocin level was stable across the pregnancy and after birth. As expected, high oxytocin levels were most predictive
of a strong mother-child bond, complete with lots of cooing, attention, and affection, after the child was born, but that level was established long before a mom ever set eyes on her newborn.