Too much information? New research suggests brain edits memories over time.

Developmental neurons in the hippocampus.

Image: Neurons born in infancy (green). Credit: Shaina Cahill.

Why, in an otherwise healthy brain, do some neurons just die? And is apoptosis – controlled, intentional cell death – part of the natural life cycle of a normal neuron? Many of the neurons in the dentate gyrus, part of the hippocampus and responsible for memory, are formed in the perinatal stage of brain development. However, due to intrigue surrounding neuron birth in the adult brain, research has focused less on the neurons born in infancy. As a result, developmental neurons are poorly understood.

New research from graduate student Shaina Cahill and Dr. Jason Snyder, published this week in the journal Hippocampus, finds that neurons formed during brain development in rats mature faster and age differently than neurons formed in older brains, a discovery that has implications for understanding memory formation and disorders including depression.

"We expect neurons to die in degenerative diseases such as Alzheimer’s," explains Dr. Snyder. In Alzheimer's disease, the hippocampus is affected by cell death causing memory loss and trouble forming new memories. "What we found is that some neuronal death seems to be a normal part of the aging process, possibly involved in 'de-cluttering' of the brain over time."

The researchers found that developmental and adult-born neurons behave differently, with different patterns of survival. Developmental neurons were stable in early development, but once they reach two months of age they began to show signs of intentional cell death. In contrast, neurons born in the adult brain do not die off once they have reached a mature state.

"We think that developmental neurons play an important role in the plasticity of the brain," says Dr. Snyder. "The brain is constantly changing, with new neurons added each day throughout the lifespan, but brain growth is finite. We speculate that if you’re gaining new neurons, you’re losing older ones for a reason. Memories are only useful sometimes," Dr. Snyder explains. "Children soak up an enormous amount of information, so it’s possible that over time we’re simply editing that existing information for relevance."

Memory plays an important role in connecting a person’s experience with emotional responses. People with depression can experience impairments in memory as the hippocampus shrinks, suggesting that the brain is not adding as many neurons or preserving essential developmentally born neurons at a sustainable rate.

For Snyder and Cahill, the next challenge is determining whether it’s possible to manipulate the death of developmentally born neurons by adding more new neurons in adulthood.

(How does one "add more new neurons?" Aerobic exercise and antidepressant drugs stimulate new cell growth, especially in the hippocampus.)

"If we can add more new neurons in adulthood, will this be balanced by increased death of the neurons that were born in infancy?" asks Dr. Snyder. "We’re hoping we can uncover what effect this might have on memories that were acquired in early life."