Perinatal environmental enrichment affects murine neonates' brain structure before their active engagement with environment

Every living creature carries the imprint of its earliest days. The world a newborn encounters, even before it can walk or speak, leaves a mark on how its body and mind develop. Scientists have long known that a complex, stimulating world can reshape the brain, making it more capable and resilient. In adult animals, researchers have seen this clearly: when rodents are placed in cages filled with toys, tunnels, and social partners, their brains physically change, growing new connections and structures, particularly in the area responsible for memory. But a crucial question has remained unanswered: does this happen the same way when the animal is just a baby? Does the environment shape the brain of a newborn that is still too young to explore, or is the effect reserved for those old enough to interact with the world?

A team of researchers set out to answer this by looking at mice during the perinatal period, the time just before and after birth. They wanted to see if the environment the mother lived in could change the brain structure of her offspring before the babies had any direct contact with the outside world. To do this, they raised some mouse mothers in standard, quiet cages and others in enriched environments filled with novel objects and social complexity. They waited until the newborn mice were seven days old, a time when the pups are still blind, deaf, and entirely dependent on their mothers. At this stage, the babies cannot climb, play, or solve problems; they simply exist in the nest. The researchers then used high-resolution magnetic resonance imaging, a powerful scanning technique that allows scientists to see the inside of the brain without cutting it open, to measure the physical structure of these seven-day-old brains.

The scans revealed that the brains of the newborns were indeed different depending on the environment their mothers had lived in. However, the changes were not what one might expect based on studies of adult mice. In adult rodents, the most dramatic changes usually appear in the hippocampus, the region linked to learning and memory. In these newborns, the hippocampus showed very little difference. Instead, the most significant structural changes appeared in other parts of the brain: the hindbrain, which controls basic life functions; the dorsal striatum, involved in movement and habit formation; and the medial habenula, a small structure deep in the brain. This suggests that the brain responds to environmental complexity in a different way during early development than it does in adulthood.

Since the newborn mice were too young to interact with the enriched toys or the complex social setting, the researchers asked how the environment could possibly reach the baby's brain. They hypothesized that the mother was the bridge. They observed that mothers in the enriched environments behaved differently than those in the standard cages, providing a different style of care to their pups. The data showed a clear link: the specific ways the mothers cared for their young correlated directly with the structural changes seen in the babies' brains. Furthermore, the effects of the enriched environment and the effects of the maternal care were similar, suggesting that the mother's behavior is at least partly responsible for passing on the benefits of a complex world to her offspring. The study concludes that the brain of a developing mouse is shaped by its environment, but for a newborn, this happens not through its own exploration, but through the altered care it receives from its mother.