It’s a study in mice, but results from an intriguing experiment suggests that having one or two parents can affect new nerve growth in the brain, and that male and females respond differently to these influences.
Extensive studies involving mice and rats showed that early life experiences could have profound effects on intellectual, emotional and social development. Negative parenting environments can affect the stress response in young pups, and make them hypervigilant to threats, while nurturing and supporting upbringing can instill resilience and novelty-seeking behaviors. Studies of how new nerves developed in the brains of young pups also hinted at the importance of parental care; positive parental environments tended to promote the growth of neurons in the dentate gyrus, a region of the brain responsible for learning, storing memories and spatial coordination. So researchers from the University of Calgary’s Hotchkiss Brain Institute (HBI) decided to take a closer look at different parenting models to figure out how they affected nerve growth and the behavioral consequences of that neural development.
They started with eight-week old mice and placed them in three separate rearing environments. In the first group, impregnated females were left to birth their litters and raise their pups alone until the offspring were weaned; in the second group, impregnated females were placed in cages with a virgin female who helped the mother raise the pups until they were weaned; and in the third group, females were placed with the male fathers of their litters. Once the young animals were weaned, the researchers put them through a series of tests to measure their cognitive, memory and social skills, as well as their fear response. They also injected the animals with a dye that could track the growth of new neurons wherever they sprouted in the brain.
To their surprise, they discovered that being raised in either of the two-parent situations boosted nerve growth in the dentate gyrus, but especially for the male mice. Female mice showed the same amount of neural growth regardless of whether they were raised by one or two parents, but they still developed more new nerves in the memory-processing area of the brain than male mice raised just by their mothers.
Because the dentate gyrus involves learning, the male mice raised by two parents were also more likely to freeze when conditioned with contextual cues to recognize a threat. Such a heightened sensitive could reflect their denser population of nerves in the part of the brain responsible for such learning and recall.
Female mice, on the other hand, tended to show differences in another type of nerve development when raised by two parents compared to one. Females with two parents developed twice the number of neurons in the corpus callosum, a thick band of nerve fibers that facilitates communication between the two sides of the brain and coordinates balance, attention and arousal. Previous studies showed that nurturing or enriched environments can promote connectivity in this region, and enhance social behavior and spatial coordination. Indeed, in the study, female mice raised in two-parent cages were more adept at navigating a ladder made of uneven rungs than female mice raised by their mothers only. All females, however, were better at the task than males, since the rearing environment appeared to affect corpus callosum development in the females more than the males.
By observing how the parents cared for their offspring in the different situations, the scientists say that pups raised by two parents experienced more licking and grooming overall compared to those being cared for by just their mothers; the sex differences could have risen from the fact that mothers tend to preferentially lick male pups more than female offspring, and in two-parent environments, the other parent may simply follow the mother’s lead, leading to differences in the nerve growth between the genders.
So how are these results important for understanding how parental environments affect brain development in young children? Understanding the potential influence that different rearing situations can have on nerve growth could help researchers gain better insight into the cognitive, emotional and behavioral abnormalities that affect people, the study authors say.
For example, the study also showed that the effects of rearing appeared to last throughout the animals’ lifetime, and were passed on to the next generation. The pups raised in two-parent cages also had offspring that fared better on cognitive and social skills tests than the pups of animals raised in the single-parent cages. “We found that parental attention can directly impact long term brain health into adulthood. It shows there is a mechanism that increases brain cells as adults. The fact that they carried on to the next generation is astounding, which means it is almost imprinted or encoded in the offspring,” says senior author Samuel Weiss, the director of HBI.
Weiss says anecdotal evidence suggests similar mechanisms may occur in humans. “We are not at a point yet–although we will be shortly–where we can actually measure brain cell turn over in the adult human brain. [But] there will be an opportunity down the line to see if the same mechanism will apply in the human population where there is increased parental attention and long term brain health,” he says.
There is some preliminary evidence that Weiss’ findings in mice may have some validity in the human setting; previous studies in humans have found that children raised in a positive, nurturing environment tend to have a much healthier trajectory and are less likely to encounter problems with violent and aggressive behavior, for example, or engage in drug or alcohol abuse. Taken together, that data adds more weight to the importance of early childhood experiences in shaping the brain and, ultimately, who we are.
The study is published in the journal PLOS ONE.