The findings could explain earlier behavioral studies in which choline improved learning and memory in animals, say the researchers from Duke University Medical Center in the US. It also supports the supplementation of pregnant women's diets, as choline could affect their children's lifelong learning and memory.
"Previous studies at Duke have shown that choline-supplemented animals are smarter and have a greater learning capacity, but we hadn't known until now whether the cells that make up memory-relevant brain circuits are changed by choline," said study leader Qiang Li. "Choline didn't just change the general environment of the brain, it changed the fundamental building blocks of brain circuits -- the cells themselves."
In theory, choline could boost cognitive function, diminish age-related memory decline, and reduce the brain's vulnerability to toxic insults.
Choline is found naturally in egg yolks, milk, nuts, fish, liver and other meats as well as in human breast milk. It is the essential building block for a memory-forming brain chemical called acetylcholine, and it plays a vital role in the formation of cell membranes throughout the body.
The US Food & Drug Administration established a nutrient content claim for products containing qualifying amounts of choline in 2001 and the Institute of Medicine recently raised choline to the status of an essential nutrient for humans - particularly pregnant and nursing women.
In the current study, the researchers explored the effects of choline on neurons in the hippocampus, a brain region that is critical for learning and memory. They fed pregnant rats extra amounts of choline during a brief but critical window of pregnancy, then studied how their hippocampal neurons differed from those of control rats.
The researchers found that hippocampal neurons were larger, and they possessed more tentacle-like dendrites that reach out and receive signals from neighboring neurons.
"Having more dendrites means that a neuron has more surface area to receive incoming signals," said Scott Swartzwelder, senior author of the study. "This could make it easier to push the neuron to the threshold for firing its signal to another neuron."
When a neuron fires a signal, it releases brain chemicals called 'neurotransmitters' that trigger neighbouring neurons to react. As neurons successively fire, one to the next, they create a neural circuit that can process new information, he said.
Not only were neurons structured with more dendrites, they also 'fired' electrical signals more rapidly and sustained their firing for longer periods of time, the study showed. The neurons also rebounded more easily from their resting phase in between firing signals. These findings complement a previous study by this group showing that neurons from supplemented animals were less susceptible to insults from toxic drugs that are known to kill neurons.
Collectively, these behaviours should heighten the neurons' capacity to accept, transmit and integrate incoming information, said Swartzwelder. "We've seen before that the brains of choline-supplemented rats have a greater plasticity -- or an ability to change and react to stimuli more readily than normal rats -- and now we are beginning to understand why," he said.
Previous studies support the brain effects of choline. Steven Zeisel at the University of North Carolina at Chapel Hill has demonstrated that choline alters a gene called CDKN-3 by adding a 'methyl group' of atoms to the gene. The methyl group switches off the gene and, in doing so, uninhibits the cell division process in the memory centres of the brain.
Also, Tiffany Mellott and Jan Krzysztof Blusztajn at Boston University recently found that two hippocampal proteins known to participate in learning and memory, called MAPK and CREB, are activated to a greater extent in the animals prenatally supplemented with choline.
Results of the new study will be published in the April issue of the Journal of Neurophysiology.
Also this week, the US Agricultural Research Service has set up a new online database that allows researchers to estimate daily choline intake from consumption of the more than 400 foods listed. Scant analytical data on the choline content in foods existed beforethe two-year ARS project began. Stephen H. Zeisel also worked on the project.
Experts suggest that an adequate choline intake is 425mg daily for women and 550mg a day for men.