A new study in primates has shown that Hormone replacement therapy (HRT) may protect the ageing brain cells of post-menopausal women if it's given early enough.

The study suggests that oestrogen protects the neurons in the post-menopausal female brain from unavoidable, age-related weakening if it is given around the time of perimenopause - the period before menopause when production of the sex hormones oestrogen and progesterone begins to slow down.

John Morrison at the Mount Sinai School of Medicine in New York, US, and colleagues injected hormone oestrogen into rhesus macaques, whose menstrual cycle and menopause is similar to humans. The monkeys were divided into two groups of older and younger animals, and, within these groups, half were given hormone shots, and the remaining acted as controls.

On short-term memory tests, the younger animals performed equally well, irrespective of whether they got oestrogen or not, but it was a totally different story in the older animals.

The older rhesus monkeys that got the oestrogen boosters performed as well as the younger animals, whereas the older untreated monkeys displayed dramatic cognitive declines, the paper said.

Autopsies demonstrated that the oestrogen had affected the neurons in the pre-frontal cortex - a region of the brain linked to cognitive tasks. The treated monkeys had an elevated density of synaptic spines, neural structures that connect brain cells, than the untreated animals.

These spines link brain cells to one another, assisting brain cell communication, and are vitally important for learning and memory, but characteristically reduce with age.

The researchers believe the oestrogen treatment encourages growth of new, more active spines, which partly compensate for the effect of ageing.

"We found that this increase in synaptic spines in the prefrontal cortex in the older oestrogen-treated monkeys appears to have prevented age-related cognitive decline," New Scientist.com quoted Morrison, lead author on the paper, as saying.

Morrison said that the higher density of spines seen in the brains of the animals treated with hormones indicates that oestrogen permits better neuroplasticity.

"The younger animals retain neural plasticity in the absence of oestrogen, but what's happening with the older animals is this double hit of both age and oestrogen decline," he said.

The findings of this study propose that the timing of any hormonal interference may be significant, and that there is hope to protect brain function before it has weakened too far.

"If the brain is too old, then age-related decline may be difficult to reverse. However, our study suggests that if we jump before it's too late, we may possibly prevent memory loss," Morrison said.