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Cholesterol in your brain is key to the cell connections needed for memory and learning. The researchers zeroed in on cholesterol through experiments with cells in which the lipid triggered the formation of synapses -- the connections through which nerve cells communicate.

Cholesterol levels in the blood do not determine the brain's supply, as blood cholesterol molecules are too large to cross the blood-brain barrier. The blood-brain barrier is a mechanism that strictly controls the type of molecule allowed to enter into the brain from blood vessels.

The availability of cholesterol also appears to limit synapse development. In addition, the investigators found that, when cultured alone, neurons produced some cholesterol. But only when glial cells were present was there a cholesterol supply abundant enough for "massive" synapse formation.

According to the researchers, these findings suggest that any "genetic or age-related defects" in the brain's cholesterol use may impair the circuitry behind mental functioning.

Past research has suggested that brain "support cells" known as glial cells produce a substance that allows the brain's nerve cells, or neurons, to communicate, according to Science (November 9, 2001;294:1354-1357).