A recent Michigan State experiment shed light on which parts of the male brain sprout new neurons during puberty. In the 2013 study, researchers injected adolescent male hamsters with a special chemical marker to detect the growth of new cells. When the hamsters matured into adults, they were allowed to mingle and even mate with the females. Immediately after these interactions, scientists examined the brains and discovered the new cells that formed during puberty had been integrated into the amygdala, an almond-shaped region deep inside the brain that is thought to play a role in such social behaviors as mating. The new research suggests this nerve growth is important for adult reproduction because it may have created neural pathways that enabled the males to interact with females.
“We know that experience is at least as powerful a regulator and shaper of brain structure and function as hormones, and boys and girls have very different experiences,” says Sisk, who was involved in the study. “The brain metamorphosis of puberty … is not just about the fine tuning of synapses or making more of a particular neurotransmitter. It’s really a complete makeover that includes the addition of brand-new cells in places we never considered before to give us the tools we need to navigate our way through the human social fabric as adults.” The turmoil of the teenage years also can drive hormonal changes that permanently alter neural pathways for emotional regulation. How each sex handles these stresses provides clues into the biological roots of gender differences in incidences of mental illnesses, and illuminates why women have higher levels of anxiety and depression. In 1989, University of Wisconsin researchers launched a longitudinal study, called the Wisconsin Study of Families and Work, which collected medical and demographic data on several hundred children from birth to early adulthood. In a 2002 study that followed 174 of these kids, researchers reported that 4-year-olds living in stressful environments — their mothers were depressed, their parents fought, or there were financial difficulties — had high levels of the stress hormone cortisol in their saliva. When the children were observed two years later, those with more cortisol exhibited greater behavioral problems, such as aggression and impulsivity.
The researchers checked back in with the study subjects when they turned 18 to find out how the increased cortisol affected their brain function. Researchers scanned the brain connections of 57 participants — 28 females and 29 males — using fMRI. Brains of teenage girls exposed to high levels of family stress when they were toddlers showed reduced connections between the amygdala, which is also known for processing fear and emotions, and the ventromedial prefrontal cortex, an outer region responsible for emotional regulation. This correlated with anxiety in adolescence: Girls with higher scores on anxiety tests have weaker synchrony between these two regions. Yet the young men in the study didn’t exhibit any of these neural patterns, suggesting that this may be a developmental pathway that makes females more prone to becoming anxious. “Males are better at avoiding depression,” says Georgia State’s de Vries, “and experiments like these may illuminate their protective factors.”
Parenting Rewires the Brain
As we move into adulthood, parenting also generates brain changes along sex-related lines. Expectant mothers spend nine months marinating in a flood of hormones that alter their brain circuitry. Once they give birth, hormones are released to stimulate lactation and to cement an emotional bond with their newborns. Preparing for parenting rewires fathers’ brains as well, but in a different way. For mothers, that hormone surge is part of an exquisitely choreographed internal program that nurtures developing fetuses throughout pregnancy. For fathers, the social interaction with their offspring spawns binding neural ties.
One study found that when paternal mice snuggled with their newborn pups in the nest, it prompted the formation of new brain cells that created a lasting connection with their offspring. Samuel Weiss, director of the Hotchkiss Brain Institute at the University of Calgary, and his colleagues reported that nerve cells sprouted in the olfactory bulb, the seat of the sense of smell, and in the hippocampus, the brain’s memory bank. These particular brain cells are also regulated by prolactin, a hormone that orchestrates the milk production in the breasts of new mothers. In the fathers, a surge of prolactin helped the neurons form a permanent circuit in the brain, which integrated a pup’s scent into the father’s long-term memory. As a consequence, even when the fathers were separated from their babies for a few weeks — normally enough time to forget cage mates — they easily recognized their pups when they were reunited. But new neurons formed only if the father had physical contact in the nest with the pups.
“The nuzzling stimulates the production of the hormone prolactin,” says Weiss. “If you block prolactin, it stops brain cell production, and memories aren’t formed because no nerve cells are produced. But this has long-term implications for mental health because these social interactions yield the release of hormones that change the brain, which, in turn, forms social memories. And these memories reinforce positive social interactions, creating positive feedback loops.”
On the epigenetic side of the equation, research into different parenting behaviors indicates that positive experiences may become embedded in our DNA — and in a way that also breaks down along gender lines. While Yehuda’s research on the children of Holocaust survivors suggests we can’t escape the legacy of trauma experienced by our parents, the opposite may be true, too: Healthy parenting can have a salutary effect on not only their offspring but also future generations.
Weiss’ group looked at how different parenting models affected new nerve growth in the brain, and the behavioral impact of the neurological changes. They used 8-week-old mice and placed them into three distinct environments. In the first group, mothers raised their litters alone until their pups were weaned; in the second, the impregnated females were put in cages with virgin females who helped them rear the young mice; and the third group consisted of pups reared by both parents. When the young animals were successfully weaned, researchers gave them a series of tests to gauge their fear response, along with their cognitive, memory and social skills. The mice were also injected with a dye that could illuminate the footprints of new nerve cell growth in the brain.
Perhaps not surprisingly, two parents were better than just one, although it didn’t matter whether it was a combination of mom and dad or the two females. The extra attention the offspring received in the enriched environments — nursing, licking and grooming — translated to denser nerve growth in the dentate gyrus, which is in the hippocampus, the brain’s memory warehouse believed responsible for learning and storing short-term memories.
But while male pups raised by two parents produced more gray matter in the memory-processing regions, dual-parented females sprouted twice the number of nerve cells in the corpus callosum, a thick bundle of nerve fibers that enhances communications between both sides of the brain and facilitates spatial coordination and sociability.
In fact, female mice raised by two parents were more proficient at negotiating a ladder with uneven rungs than females with just one parent — and all females were far more adept at this task than the males, even those reared by two parents. These effects endured not only throughout the animals’ lives but were carried on to the next generation and along the same gender lines: The offspring of dual-parented pups turned in superior performances on tests of cognitive ability and social skills than mice raised by single parents.
“We already know that in humans, positive early experiences lead to stronger adults that have less problems coping and managing life’s challenges, but the generational results are mind-numbing — who would imagine that if you have a positive early experience that your offspring would benefit?” says Weiss. “We’re not that far away from the point where we will be able to explore similar things in humans.”