Healthy mind, healthy sexuality...

Indeed, experiments on mice with Alzheimer's disease (AD) (the most common form of dementia) suggest that treatment with testosterone, the main male hormone, might slow its progression.

AD affects 4.5 million Americans.

Now, researchers at the University of Southern California (USC) and the University of California, Irvine, found - for the first time - that loss of testosterone speeds the development of Alzheimer's disease in mice, while hormone replacement prevents it.

In this case, testosterone-based hormone therapy may be useful in the treatment and prevention of Alzheimer's disease in aging men. The research team found a correlation between low testosterone and elevated beta-amyloid (Aâ), a protein that abnormally accumulates in AD patients and the study suggests that testosterone may lower Aâ levels.

"We've known that low testosterone is a risk factor for Alzheimer's disease but now we know why," said Christian Pike, associate professor at the Leonard Davis School of Gerontology at USC. "Because testosterone is rapidly converted to estrogen after entry into neurons, the new data are logical, and they dovetail well with historical data."

Using mice with three mutations that triggered AD, researchers checked how experimental manipulation of sex hormones affected the progress of the disease.

Young adult male mice were castrated and over several months, some received testosterone treatment and others a placebo. The mice with lowered testosterone registered high levels of beta-amyloid. Mice that were given testosterone showed reduced accumulation of beta-amyloid and less behavioral impairment associated with AD.

The castrated models that received the placebo showed poor working memory and high brain levels of Aâ. "These results are exciting because they tell us that we are on to something that is worth pursuing," said Pike. "The next step is to look at what the long term effects of testosterone therapy are in aging men."

"Although the results of the study predict that androgen therapy has the potential to reduce the risk of AD in at least some men," Pike said, "clinical studies will be required to determine when and how to use androgen therapy."

Recent evidence has suggested that testosterone may be useful in other neurological conditions. Chien-Ping Ko, professor of biological sciences at USC, found that testosterone therapy improved muscle coordination in mice with a form of Amyotrophic Lateral Sclerosis, Lou Gehrigs Disease.