A study of young adult rats by researchers provides evidence that explains exactly how exposure to lead during brain development produces learning deficits.
The study shows that exposure to levels of lead that are similar to those measured in lead-intoxicated children reduce the birth and survival of new neurons (neurogenesis) in the brain. Lead also alters the normal development of newly born neurons in a part of the brain (hippocampus) known to be important for learning and memory.
"There was a dogma in neuroscience that you were born with all the neurons you would ever have, but that thinking has changed dramatically in the last 20 years," said Tomás R. Guilarte, PhD, senior author of the study and professor of environmental health sciences at the Johns Hopkins Bloomberg School of Public Health. "The exciting idea is that scientists have discovered ways to increase the number of new neurons, and this may facilitate learning in the hippocampus portion of the brain."
The researchers studied young adult rats, using a group of lead-treated and non-treated (control) rats. When they examined the brains of lead-exposed rats, they found that fewer neurons were born and those neurons that were born survived for a shorter amount of time and had abnormal development.
Guilarte explained that newly born neurons extend processes to form new connections with other neurons in the brain—like branches growing off of a tree limb—which allow learning and memory to take place. The length of the neuronal processes, called dendrites, in lead-exposed rats were shorter and twisted when compared to the long and relatively straight dendrites in neurons found in control rats.
"The fewer connections between neurons found in lead-exposed rats decrease their ability to communicate, which has a major impact on the rat's ability to learn. Previously, we knew that lead impaired cognitive function, but we didn't know exactly how," said Guilarte. "Now that we know that lead decreases neurogenesis in the hippocampus and alters the ability of these new neurons to communicate, in future studies we hope to determine if environmental enrichment can reverse these detrimental effects of lead."
Guilarte has studied lead's effects at the molecular level on rat brain development for more than a decade. In November 2002, Guilarte and colleagues reported that environmental enrichment that stimulates brain activity can reverse the long-term learning deficits caused by lead poisoning.