Replenishing the brain with new neurons

Humans do, after all, replenish the neurons they lose over time and the belief that the ones we are born with are all we will ever have has been shown not to be the case.

In what has been described as a landmark study, Swedish biologists have found that humans, like mice and rats, in fact make new neurons throughout life, many in a region of the brain called the hippocampus. This is an area that plays an essential role in learning and in forming new memories.

Not only that, but the new neurons are able to form connections or synapses with pre-existing cells, although the strength of the immature synapses depends on the level of electrical activity within the newly made neuronal networks.

The biologists say this means that the new circuits are highly adaptable, changing in response to the body’s experiences. These changes are believed to be especially important when laying down new memories in adult brains.

In a report on the research, published in the journal Cell, the biologists from the Karolinska Institute in Stockholm provide the first proof that new neurons are made in a critical part of the human hippocampus. They say this changes the way scientists think about the brain.

The breakthrough was made when the cell biologists invented a complex technique to measure the amount of carbon isotope 14 (C14) in the DNA of human cells obtained from a large number of people of different ages who had died.

C14 levels in the atmosphere peaked during the period of above-ground atomic bomb tests in the 1950s and early 1960s, and have declined ever since.

When cells divide, they incorporate carbon into the newly formed DNA, so measurement of how much C14 is in the DNA gives an indication of when the cells were born. The Swedish measurements revealed surprisingly high levels of neurogenesis (formation of new neurons) in the hippocampus that continued into old age.

The new cells are born in a part of the hippocampus called the dentate gyrus, a region essential for forming and consolidating memories. Using complex modelling, the researchers calculated that there was a sub-population of hippocampal neurons – about a third probably comprising most of the dentate gyrus – that was continually being renewed.

The researchers found that the proportion of ‘exchangeable’ neurons was much higher in humans than in rodents and that the generation of new cells was similar in males and females. Although there was a slight decline with age, this was nowhere near as much as has been found with mice and rats.

Neurons in the non-renewing parts of the brain are gradually lost throughout life while new neurons appear not to survive for more than about 10 years. The effect is that, overall, humans do end up with fewer hippocampal neurons at age 80 than at 20.

The researchers suggest reduced neurogenesis in the human brain may be a factor in psychiatric disease and could lead to depression and other mood disorders, even Alzheimer’s disease. This raises the possibility that scientists could manipulate neuron formation as a type of therapy for neurological problems.

The finding also provides clues as to how our brains can be nurtured as we age. Exercise in mice has been found to boost the number of new neurons while people who exercise have been found to have a bigger hippocampus.

The lesson here is that physical activity may increase the factors that aid neurogenesis in our brains and slow down or prevent the onset of dementia.