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Old Neurons Revisit Their Youth
1 May 2003 (All day)
It could be bigger than Viagra: A treatment that turns back time on the aging brain and makes old neurons act young again. Such a potion is still a long way off, but a team of neuroscientists reports an advance in this direction in the 2 May issue of Science. The researchers applied tiny amounts of the neurotransmitter aminobutyric acid (GABA) to neurons in the brains of old monkeys. The treatment restored the neurons' ability to distinguish the orientation of lines and the direction of moving objects, renewing long-lost powers of discrimination.
For most of a macaque monkey's life, neurons in the primary visual cortex (V1) respond selectively to lines oriented at a particular angle and to bars moving in a particular direction. But in 2000, Audie Leventhal of the University of Utah School of Medicine in Salt Lake City and co-workers reported that V1 neurons in very old monkeys lose much of this selectivity.
In the new study, Leventhal and his colleagues tried to reverse this decline in monkeys who were 26 to 32 years old, equivalent to 78 to 96 human years. As expected, most cells in older animals responded indiscriminately to a range of orientations and directions. But when the researchers spritzed on GABA, the brain's major inhibitory neurotransmitter, the proportion of orientation-selective cells nearly doubled and that of direction-selective cells roughly tripled, approximating the proportions found in monkeys just 7 to 9 years old.
Leventhal suggests that GABA-dependent neural communication declines with age, and that this decline is to blame for old animals' indiscriminate neurons. Indeed, when the team blocked GABA in younger animals, their neurons lost their orientation and direction selectivity, in effect aging 20 years in an instant.
"Anything that can show a reversal in the effects of aging is really exciting and potentially beneficial," says Julie Mendelson, a neuroscientist at the University of Toronto. Many sensory problems suffered by the elderly--trouble following a conversation at a loud party or navigating through traffic, for instance--stem not from deterioration of the eyes and ears, but from declines in the brain regions that process sensory information, she says. Leventhal has "really got something," agrees Donald Caspary, a neuropharmacologist at Southern Illinois University Medical School in Springfield whose group has found a similar decline in GABA-supported networks in the aging auditory system.