A milder type of mental decline that often precedes Alzheimer's disease is alarmingly more common than has been believed, and in men more than women, doctors reported Monday.

Nearly a million older Americans slide from normal memory into mild impairment each year, researchers estimate, based on a Mayo Clinic study of Minnesota residents.

That's on top of the half million Americans who develop full-blown Alzheimer's or other forms of dementia - a problem sure to grow as baby boomers age. The oldest boomers turn 62 this year.

But that may be changing. Researchers on Monday reported early, somewhat encouraging results from an experimental nose spray that seemed to improve certain memory measures in a study of mildly impaired people.

The drug, for now just called AL-108, needs testing in a longer, larger study. It is being developed by Allon Therapeutics Inc., based in Vancouver, B.C.

Doctors said it shows the potential for new types of medicines that target the protein tangles that kill nerve cells, instead of targeting the sticky brain deposits that have gotten most of the attention up to now.

Astrocytes produce fat-like lipid neurons used to create their outer membranes and they also produce glutamate, the most abundant neurotransmitter in the nervous system and one of the most important chemical messengers in the brain.

Recent experiments, however, revealed that astrocytes form connections with blood vessels and control the flow of nutrients, including oxygen, to neurons. When brain activity increases, neurons trigger astrocytes to release calcium, which in turn affects other chemical messengers that can cause blood vessels to either dilate or contract.

The classical symptoms of memory loss and dementia associated with Alzheimer's are the result of neurons dying over a period of years. Brain scans of Alzheimer's patients show decreased blood flow to critical parts of the brain, and doctors have always assumed that this was because there was less of a demand for blood because there were fewer neurons to feed.

The new result could mean that Alzheimer's affects mainly astrocytes and not neurons and that blood flow to the brain is not decreased because the neurons are dying, but that the neurons are dying because there is decreased blood flow.

"It may be that for whatever reason, astrocytes are not doing their job properly, and then blood flow decreases," Nedergaard said. "This could lead to the death of the neurons, which would starve from a lack of nutrients, since the neurons depend on the astrocytes for their survival."