Some of the oldest monkeys in the world have helped scientists to solve one of the greatest puzzles of ageing – why do oldies slow down?
Like people, monkeys suffer from a gradual and irreversible loss in the ability to handle sensory information from the eyes, ears and other sense organs. Now scientists have established that, in addition to the sense organs themselves deteriorating, deterioration also occurs in the information-processing centre of the brain which handles incoming sensory data, according to a study published in the journal Science.
A team led by Audie Leventhal of the University of Utah School of Medicine in Salt Lake City discovered a key explanation for this mental decline by studying very old macaque monkeys in a captive colony set up in China by scientists in the 1950s. Some members of the colony are now 32 years old – equivalent to a human age of 96, which makes them twice the age of the oldest macaques found in the wild.
The study investigated a particular region of the cerebral cortex – the brain's "higher" information-processing centre – involved in handling visual information from the eyes. It found that the very old monkeys' nerves in this region lose their ability to discriminate between one signal and another and that this loss was directly related to the presence of a chemical called gamma-aminobutyric acid (Gaba), a neurotransmitter that appears to dwindle in old age.
Dr Leventhal said the loss of Gaba seems responsible for the indiscriminate "firing" of electrical impulses by old nerves which suggests that drugs aimed at restoring Gaba could also revive mental agility. Dr Leventhal said: "The good news is that there are a lot of drugs that can facilitate Gaba function and maybe some of them will help. If it's going on in the visual cortex, it's probably going on in other parts of the cortex." The experiment involved showing images to both old and young monkeys and observing the electrical reactions of the visual cortex nerves.
As expected many of the nerve cells of the older animals reacted indiscriminately by responding to a wide range of orientations while a smaller proportion of the nerves belonging to young animals showed such a lack of discrimination.
However, when the scientists added Gaba or Gaba-like drugs to the older nerves, they behaved just like younger ones with a doubling or tripling of their discriminatory powers.
Being able to boost sensory discrimination within the information-processing centre of the brain could in effect make it possible for older people to react just as quickly as younger people to fast-moving changes.
"Many sensory problems suffered by the elderly stem not from deterioration of the eyes and ears, but from declines in the brain regions that process sensory information," Science said adding that, "[Gaba] could be bigger than Viagra and drug companies are already drooling: a treatment that turns back time on the ageing brain and makes old neurons act young again."
Dr Leventhal said it was remarkable that his was the only lab in the world studying higher brain functions in ageing monkeys. "Old monkeys are rare, but the world is full of old human primates," he said. Monkeys "really do look like grandpa, they have thinning hair and wrinkles".