Giraffes’ long necks are perfectly suited to harvesting tender leaves beyond the reach of other herbivores. Pondering the genesis of this phenomenon, two giants of modern biology, Jean-Baptiste Lamarck and Charles Darwin, arrived at remarkably different hypotheses. Lamarck believed that constant stretching of the neck somehow stimulated its growth. The giraffe would then pass along this new trait to its offspring. In effect, this newer, longer neck was a direct result of a giraffe’s interaction with its environment. By contrast, Darwin’s theory posited that traits evolve as part of a random, gradual process. The giraffes that happened to have been born with longer necks thanks to a random genetic mutation were better fed and thus healthier than their shorter-necked counterparts, making them more likely to live long enough to breed and pass on this trait. Because this mutation conferred a specific advantage to long-necked giraffes that aided their survival, the trait was preserved in future generations.
Lamarckian theories about the influence of the environment were largely abandoned after scientists discovered that heritable traits are carried on the genes encoded by our DNA. A recent study, however, published by neuroscientists Junko A. Arai, Shaomin Li and colleagues at Tufts University, shows that not only does the environment an animal is reared in have marked effects on its ability to learn and remember, but also that these effects are inherited. The study suggests that we are not the mere sum of our genes: what we do can make a difference.
The neurobiological investigation of environmental effects on learning and memory began in the late Sixties and early Seventies, when Mark Rosenzweig and colleagues examined how manipulating levels of sensory stimulation, exercise and social interaction affected rats’ behavior. Laboratory rats typically live in a cage with bedding, food and water but little else. In the enriched environments (EE) that Rosenzweig’s group created, animals got access to a changing roster of toys, and increased opportunities for socialization and exercise. The brains of EE rats were larger and they outperformed controls (which were housed in typical cages) in learning and memory tasks. Subsequent work by researchers looking at the cellular level has shown that EE triggers changes in neural morphology (shape), resistance to neurodegenerative disease and learning-related neural activity.
Recently, Arai, Li and colleagues extended this line of inquiry, examining the role that EE plays in long-term potentiation (LTP), a form of synaptic strengthening that supports learning and memory. The physiological signature of LTP is an increase in the baseline level of a neuron’s electrical activity. Arai and Li showed that LTP in the hippocampus, a key brain structure involved in learning and memory processes, is greater in mice reared in EE.
What’s more surprising, however, is that EE is also sufficient to “rescue” a memory defect present in genetically altered mice. Parent mice born with the defect that were then exposed to EE as juveniles did not pass the same memory defects to their offspring. Their enriched surroundings corrected their genetic deficit.
How does this correction occur? Specific molecular pathways are required to generate LTP. When scientists silence the parts of the DNA code involved in the function of one of these pathways using what geneticists call “knock out” technology, as was the case in the mutant mice with a memory defect, both LTP and memory functioning are impaired. Arai and Li showed that EE increased LTP volume in wild-type (non-mutant) mice. Interestingly, mice that have had a standard molecular pathway required to induce LTP knocked out can still induce LTP. The researchers found that this EE-related LTP is induced via a novel molecular pathway that arises as a direct result of EE exposure. Moreover, they found that the enhanced LTP capacity of wild-type mice, and the rescued capacity for LTP in knock-out mice, can be transmitted epigenetically (that is, without any changes in their genetic code) from mother to offspring. Surprisingly, this transmission was true even when their offspring were raised in a conventional environment.
Is It Really the Environment?