This Is Your Brain on Sex (10 page)

The resulting pups from methyl-rich-fed parents were smaller, fitter, and a more typical mousy brown. They were also much less vulnerable to conditions like obesity and cancer than their parents. The differences from mom to baby were fairly dramatic, yet the agouti gene hadn’t changed. The pups still had exactly the same variation as their fatter, yellower moms. Those methyl groups present in the diet simply attached to the agouti gene and suppressed its expression, allowing the pups to be healthier and brown. “When you see an effect like this, it changes everything,” said Jirtle. “You will never think of genetics the same way again.”

It certainly brings a whole new meaning to “You are what you eat” (and makes me want to make sure my kid gets his recommended daily intake of methyl donors). This is where the cheeseburgers come in. A lot may come down to my mother’s preference for that particular fast food while she was pregnant. Who knows what kind of epigenetic markers that hankering left on my genome while I was still in the womb, changing my body and my health? I am not sure I want to know. And since I doubt that particular fast-food restaurant will be funding any studies to find out, I am not likely to either. The important take-away is that even something like nutrition, in the womb and beyond, has the power to alter gene expression—and consequently brain development and behavior. Cool (and somewhat scary), no?

Early Life Experience

Epigenetics goes beyond imprinting or food.
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Your early life experience also plays a role. For example, a mother’s level of care and affection during the early years has the power to make serious changes to the epigenome. Certainly there is a lot of evidence suggesting that appropriate care in early life is important to behavior. In the 1950s Harry Harlow, a psychologist at the University of Wisconsin, separated newborn monkeys from their mothers and put artificial wire or cloth “surrogates” into the baby monkeys’ cages. The researchers
soon learned that sustenance was not enough; in order to thrive, those babies needed a soft touch. Without regular cuddling, the babies demonstrated almost autistic-like behavior: rocking themselves, making odd noises, and avoiding novel stimuli.
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This phenomenon is not unique to monkeys. Children who were the unlucky residents of Romanian orphanages, neglected and left isolated in cribs, during Nicolae Ceausescu’s rule also showed a host of mental, physical, and emotional disabilities later in life. At the time these differences were chalked up to strict nurture effects. But the work of a Canadian psychobiologist at McGill University named Michael Meaney suggests that epigenetic effects are at play too. In a landmark study Meaney demonstrated that differences in early maternal care could change the way a certain stress-related gene was expressed in offspring.

Rat mommies do not show their love with expensive toys, extra hugs, or regular trips to the park. They focus instead on a lot of licking and grooming until their pups are weaned. This nurturing behavior does not just keep the baby clean; it also enhances growth and development by facilitating hormone systems. If you threw together a group of rat mommies (often called dams), you would see a wide variety of different licking behaviors. Meaney and his colleagues compared two groups of dams, those that spent a lot of time licking and grooming their pups (high LG) and those that did not (low LG).
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When the researchers observed the offspring of these rats, they noticed a few interesting things. First, once female pups grew up and had their own litters, they tended to show the same kind of licking strategy as their mother did. If they were licked a lot as babies, they licked their own babies a lot too; if they did not receive a lot of licking attention when they were younger, they showed the same kind of laissez-faire behavior toward their young’uns. It was a very stable, predictable effect. Grandbaby rats also showed the same licking strategy. What’s more, this happened even when rats were cross-fostered with a different type of mom. So even if a rat’s biological mom was a big licker, if she was raised by a low licker, she would end up as a low licker herself. Behavior was somehow trumping biology.

Meaney’s group also discovered that low-LG pups could not handle stress as well as their high-LG peers. Though all the rats would startle at an unexpected loud noise, the pups
that had attentive moms could handle it, quickly going back to whatever rat-type activity they had been up to before the stressor sounded. Pups whose moms did not lick them as much, however, weren’t as resilient, cowering in fear for some time after the noise had abated. High-LG pups also showed increased learning over the low-LG pups. Meaney wondered if these differences, with significant transgenerational staying power, could be the product of a gene-environment interaction. He partnered with Moshe Szyf, a scientist studying epigenetic effects in cancer, to find out.

Sure enough, when the group looked at the rat genomes, they found that differences in observed behaviors were linked to the amount of circulating stress hormones, particularly in one type of stress chemical called glucocorticoid. Looking deeper, Meaney and Szyf discovered that a mother’s licking and grooming removed methyl groups from the offspring’s glucocorticoid receptor gene. A mother’s extra licking attention allowed the production of those receptors to flourish, taking up all that extra glucocorticoid and making for a more mellow, easygoing rat.

Are these results applicable to humans? It is unethical to study humans in the same way we do rats. No one is going to ask moms to neglect their kids so scientists can try to measure their stress hormones in a controlled setting. But given that a history of childhood abuse has been strongly linked to stress and depression, it makes sense that scientists would see the same kind of changes in humans. To test the idea, Meaney’s group looked posthumously at the glucocorticoid receptor gene in suicide victims. Looking at DNA from hippocampal cells, they discovered that suicide victims with a history of childhood abuse showed methylation at this gene site—even though they were long into adulthood. Those with happier childhoods did not show the same kind of epigenetic marks.

When I spoke with Szyf, he told me the same thing I learned from Jirtle regarding epigenetics: the genome is the hardware and the epigenome is the software. But he added, “The programmer is the mother. She may not know that she is, but she is.” Szyf argues that a mother’s behavior is a signal to the child about what kind of environment to expect. “If she gives her child a lot of fatty foods, that is one signal. If she gives him vegetables, that’s a different signal. These different signals result in programming,
programmed changes in the epigenome to prepare that child for the world around him.”

It might be easy to suggest that stress-related programming is permanent—if not “hard-wired,” then certainly very stable. This is the case with methylation in genomic imprinting and other types of epigenetic programming. Yet it is not irreversible. When Meaney and his colleagues paired the low-LG rats with high-LG mothers after they reached puberty or put the animals into an enriched environment (a cage with running wheels, toys, and other stimulating artifacts), they saw a disrupted inheritance of the low-LG behavior: the rats behaved more like the high-LG moms when they had their own babies. It would seem there is quite a bit of plasticity, or malleability, in these systems; environment and biology tightly couple to result in learning and behavior. “This is not deterministic either,” said Frances Champagne, a former student of Meaney’s who now has her own epigenetics laboratory at Columbia University. “There are layers of information around a gene that affect how that gene will be expressed. And these confer plasticity. That’s the key.”

The brain is always changing. Every experience, interaction, and relationship has the power to change the relative connections between neurons and, by extension, change the circuitry of the brain itself. Your brain at birth is not the same as it is in adolescence, nor the same as it will be in adulthood. In the past decade neuroscientists have been astounded to learn just how plastic the brain is. Previously it was believed that once the brain was done developing—sometime in the teen years—its structure was set in stone. But new research shows that experience has the power to transform the brain at any age, particularly at the molecular level. And those small changes can add up over time. Epigenetic changes are just one way those transformations can occur.

Meaney and his colleagues discovered that high licking and grooming behaviors in mothers show another interesting effect. Not only can they help predict the parenting style of the offspring, but they can also play a role in that baby rat’s future mating behaviors.

“High levels of maternal care lead to high levels of maternal care in daughters. It also leads to reduced sexual receptivity. Whereas the offspring who have low levels of care have heightened sexual receptivity,” Champagne told me. “So there seems to be a trade-off in different aspects of reproduction as a function of maternal care.”

Attentive moms produce pups that will grow up to be not only prolific lickers themselves but also somewhat prudish in demeanor. It is going to take more than just a “How do you do” for a male to mate with this girl. She will take longer to reach sexual maturity, and once she gets there, she will be less receptive to sex. She is picky and takes her time. She will make the boys work for it. In contrast, low-LG moms produce female offspring who are hot to trot. They are not only more receptive to sex, they actually are more likely to solicit it. Nicole Cameron, a student of Meaney’s who spearheaded this work, soon discovered that maternal licking and grooming behaviors do not just epigenetically alter stress gene receptor expression; they also make changes to a promoter for a type of estrogen receptor. To the group’s surprise, they found that low-LG mothers showed an increase in this promoter, which is probably responsible for these differences in observed sexual behaviors. Again, when animals were cross-fostered, the group saw that the effects were determined by the mother’s behavior, not her genes. It was not being the biological offspring of a high-LG or low-LG dam that made the difference; it all came down to experiencing a particular level of licking and grooming in early life.
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Since Meaney and Szyf started this line of behavioral epigenetic research, more scientists have gotten on board. I could list a dozen more studies that talk about the different ways epigenetics may affect behavior, not just in early life but across the life span. It is likely that every significant experience and interaction we have with others has the power to change our biology. Some of the most pertinent studies will be discussed in the following chapters. But I introduce the concept here to illustrate how important it is. As much as people want to dispute all manner of individual traits and behaviors in terms of nature
or
nurture, there is no real way to tease the two apart. Our biology impacts the way we are built, the way we perceive what is out there, and the way we interact with the world around us. Our environment provides the sensory information to help the body adapt and respond, ultimately changing the underlying biology. And then the circle goes back around again. How much of nature or nurture has certain
effects on our behavior is incredibly context-dependent—and likely impossible to determine in a quantitative fashion. Szyf jokes that it is much easier to explain the concept of epigenetics to your average mom than to a guy who has been studying the finer points of molecular biology for ten years. Moms immediately understand it, especially if they have a few kids who all grow up to be unique individuals despite similar biology and home environment. But it has not been as easy for those invested in genetic determinism to get behind the theory.

“These effects have been completely dismissed by geneticists in the past. Now we realize they are powerful forces that can be modeled both mathematically and experimentally. That changes the whole picture,” said Szyf. “You can’t just study the cell anymore. There isn’t just a cell. The cell acts in a body, that body has a brain, and that body acts in an environment. You can’t disconnect them. Our family, our community, our city, our country, our world—all of these are important to understanding biology.” And, as it so happens, they are important to understanding behaviors related to sex, love, and parenting too.

I have a confession to make. As much as I may like to jokingly blame my mother and her penchant for greasy burgers for my insatiable love of carbs or my affection for dark-eyed men who will ultimately break my heart, it is only one factor in thousands, millions really, that have shaped me into the person I am, the way I behave—and, by extension, the way I love. There is no separating my biology—my brain—from my environment. And that offers no easy explanations, no general guidelines or rules, when it comes to my relationships.

Chapter 5