Brain Regeneration in Humans: Can We Manipulate This Process?
Multiple evidence collected over the past 30 years supports the notion of brain neurogenesis in animals. Until recently, however, regeneration of neurons in humans seemed impossible. Proof that humans are not unique came from the laboratory of Fred Gage of the Salk Institute for Biological Studies in California in November 1998.
The brains of five cancer patients, who were injected with bromodeoxyuridine (BrdU) to tract their tumor cells, were analyzed post mortem. BrdU is a non-specific marker of DNA replication and, thus, tracts any dividing cell. The researchers found ample evidence of cell division in neurons of the dentate gyrus of hippocampus, a region involved in learning and memory.
Dentate gyrus is the first relay station for sensory information entering the hippocampus. It gets hit with a lot of glutamate, an excitatory neurotransmitter that damages brain cells. Thus, neurogenesis in dentate gyrus may be part of brain repair. The new neurons are short-lived (only a few weeks) and could play a role in new memory formation. These memories may then be stored elsewhere for the long term.
If brain neurons can regenerate, what are the factors potentiating or retarding this process? Studies of mice that suffered a stroke showed that exercise speeds up the recovery. Interestingly, exercising had to be absolutely voluntary or else it did not induce neuronal proliferation. Also, living in spacious well-equipped cages resulted in the doubling of the number of new brain cells. Surprisingly, learning did not induce neuronal formation in mice, unlike in rats.
Neurogenesis can be chemically manipulated. For example, increasing serotonin raises the number of neurons in rodents. Importantly, this happens even if serotonin is increased by an anti-depressant such as Prozac. Estrogen also increases neurogenesis. This is expected since for a long time it has been theorized that hormone replacement therapy protects older women against mental decline. In contrast, corticosteroids (stress hormones) stunt neuronal regeneration and survival. Stress is well known to impair memory. Also, the levels of corticosteroids are three times higher in the elderly than in young people. Interestingly, when corticosteroid levels are lowered in older rats, cell division of neurons is increased.
Thus, neuronal regeneration appears to be a reality. On going studies are looking into various ways of manipulating and controlling this process. Hopefully, this will enable us to induce neuronal recovery in cases where brain damage has occured.
- Motluk A. Grow your own. New Scientist, 12 February 2000, p. 24.
- Zigova T, Sanberg PR. Neural stem cells for brain repair. Science & Medicine, Sep/Oct 1999, p.18.