The research published in the Journal of Neuroscience provides fresh evidence that the decline in the capacity of brain cells to change - called 'plasticity' - rather than a decline in total cell number, may underlie some of the sensory and cognitive declines associated with normal brain ageing.
Scientists at Massachusetts Institute of Technology (MIT) in the US showed that they could restore a significant degree of lost plasticity to the cells by treating mice with the commonly used antidepressant medication fluoxetine, also known as Prozac.
"Despite common belief, loss of neurons due to cell death is quite limited during normal ageing and unlikely to account for age-related functional impairments," researchers said.
"Rather it seems that structural alterations in neuronal morphology and synaptic connections are features most consistently correlated with brain age, and may be considered as the potential physical basis for the age-related decline," they said.
Researchers focused on the ageing of inhibitory interneurons which is less well-understood than that of excitatory neurons, but potentially more crucial to plasticity.
Plasticity, in turn, is key to enabling learning and memory and in maintaining sensory acuity. In the study, while they focused on the visual cortex, the plasticity they measured is believed to be important elsewhere in the brain as well.
The team counted and chronically tracked the structure of inhibitory interneurons in dozens of mice aged to 3, 6, 9, 12 and 18 months.
The team tracked the growth of dendrites, which are the tree-like structures on which a neuron receives input from other neurons.
At three months of age mice showed a balance of growth and retraction, consistent with dynamic remodelling. However, between three and 18 months they saw that dendrites progressively simplified, exhibiting fewer branches, suggesting that new growth was rare while retraction was common.
Researchers put the antidepressant in the drinking water of mice at various ages for various amounts of time.
About 25 per cent of cells in 6-month-old mice treated for three months showed significant new growth. Among 3-month-old mice treated for six months, 67 per cent of cells showed new growth by the age of 9 months, showing that treatment starting early and lasting for six months had the strongest effect.
"Our finding that fluoxetine treatment in aging mice can attenuate the concurrent age-related declines in interneuron structural and visual cortex functional plasticity suggests it could provide an important therapeutic approach towards mitigation of sensory and cognitive deficits associated with aging, provided it is initiated before severe network deterioration," researchers said.
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