Want to improve your memory? Take choline. Well, not exactly - it's probably too late.
Since coming to Duke from Columbia University, Dr. Christina Williams, associate professor in the Department of Experimental Psychology, has made the startling finding that pre- and early post-natal dietary supplementation with choline leads to long-lasting changes in the brain and major improvements in memory capacity in rats.
In the past, many researchers have investigated the cholinergic hypothesis for memory. They know, for example, that damage to the cholinergic system, which consists of acetylcholine-producing neurons, results in memory loss. Alzheimer's-type dementia is partly the result of loss of cholinergic neurons in the brain such that memory loss occurs.
To date, many researchers have given choline supplements to various test populations, ranging from normal college students to Alzheimer's patients. Yet, in most studies, memory has not been enhanced in normal individuals and the people who had deteriorated cholinergic systems demonstrated only some transient improvement.
Working to improve memory, Dr. Williams decided to study alterations in the cholinergic systems of the brain induced by choline supplementation. Previous research has already demonstrated that increased availability of choline can increase activity in the cholinergic system and, thus, memory function (Bartus et al., 1985). Also, in mature rats, increased choline concentration in blood plasma will improve cholinergic function if the cholinergic neurons have an increased need for choline (Jope, 1982).
Most studies done so far have assumed that choline aids in memory just because it is the precursor of the neurotransmitter acetylcholine. Choline, however, is multi-functional. When present in high concentrations, it can act like acetylcholine (Krynjevic et al., 1982). It is an integral part of neuronal membranes (Shinizky, 1986), and is required for neuron growth.
Dr. Williams, in collaboration with Dr. Warren H. Meck, associate professor in the Department of Experimental Psychology, opted for a different approach to the problem. They decided to build an improved cholinergic system by adding choline to the diet when the cholinergic cells are being formed and making the synaptic connections in the brain.
Cholinergic cells are special because they need choline to make acetylcholine but cholinergic cells, like all cells, also require choline to maintain their cell membranes. "Thus, cholinergic cells doubly require choline," said Dr. Williams. So, Dr. Williams supplemented pregnant rats with choline in their drinking water. Choline was injected postnatally into the maturing rats.
The rats had their spatial memory performance tested in a 12-arm radial maze, in which the equidistant arms radiate out from a central platform. In each trial, the same pattern of eight arms would have food at their ends. The rats' job is to find those baited eight arms without repeating the same arm (since food is not replenished in a given trial) and avoiding those arms that never have food.
This task taps into working and reference memories. Working memory stores information specific to that day's task, while reference memory is for rules like avoiding those arms that never have any food. "Rats, being born foragers, are great at this task," said Dr. Williams.
"Amazingly enough, the rats which had pre-natal or post-natal or both pre- and post-natal supplementation of choline made fewer mistakes on the first day of training and the choline animals continually perform better than control rats even as adults," said Dr. Williams. In fact, rats which had both pre- and post-natal supplementation of choline demonstrated the greatest amount of permanent improvement in their memory capacity and precision.
"Since those experiments were completed, the sensitive periods for choline administration have been determined to be prior to birth on days 12 to 17 in development and also days 15 and 30 after birth," said Dr. Williams.
The former period occurs when all the cholinergic neurons in the basal forebrain form. The latter period also seems to be highly significant because it is when these developing rats are being weaned and synaptic connections are being made in the hippocampus and cortex that are critical in visuospatial learning and memory.
Interestingly enough, the effects of choline are sexually dimorphic. Females do not have the second sensitive period. Dr. Williams will further investigate this line of research by examining if and how steroid hormones secreted by the male around the time of birth may influence the effectiveness of choline on memory. Brains of rats treated with choline and either exposed to androgen or estrogen will be inspected to see if these hormone and choline interactions affect hippocampal circuitry.
The importance of this research lies in the fact that the memory improvement lasts through old age. Normally, rats' memory shows deterioration as they reach old ageÑabout 2-2.5 yearsÑwhile rats treated perinatally with choline still perform like young adult rats even around 26 months of age. Thus, early supplementation of choline appears to protect against age-related declines in memory capacity and precision.
In collaboration with a number of other researchers, Drs. Williams and Meck have been trying to determine the changes in the brain that provide the substrate for the improved memory. They have found two- to three-fold increases in nerve growth factor (NGF) in the hippocampi of rats supplemented perinatally with choline, compared with untreated controls. NGF permits greater cell plasticity so that the cells can alter and change synapses throughout life. NGF may thus lead to an altered circuitry in the hippocampus, which may allow for more accurate memory.
Neuroanatomical studies have also demonstrated changes in the circuitry of the hippocampus in rats treated perinatally with choline. The alterations include 10-15% increases in the size of the acetylcholine-producing cells in the basal forebrain.
Also collaborating with Drs. Williams and Meck is Dr. Steven Zeisel, Chair of the Department of Nutrition at the University of North Carolina, Chapel Hill, who uses biochemical techniques to understand how the metabolism of rats and humans deals with the added choline. For instance, he discovered that human milk contains huge amounts of choline, much more than in the mother's blood. Thus, the mammary glands must concentrate choline into milk for the infant. The newborn organism may need choline during the first week of life, but more research is required to determine why the choline is needed.
Dr. Zeisel also discovered that a rat mother eating a standard rat chow goes into choline deficiency during pregnancy and lactation. He observed that as the rats' gestation progresses, the fat content of the liver increases, which is a sign of choline depletion. In post-natal rats, blood choline concentration is extraordinarily high at about 65 mM, compared with adult rat levels of about 10 mM (Zeisel and Wurtman, 1981). Human milk choline concentration is also extremely high at about 650 mM at birth, but then declines to 150 mM in mature milk. A hypothesis for why choline concentration in the mother's milk rapidly declines is that the mother is simply running out of choline.
Along this line of research, Dr. Zeisel has been investigating different populations of humans which were post-natally fed breast milk or various formulas. Infant formulas based on cow milk have about 107 nmoles/ml of choline, while soybean-based formulas have about 648 nmoles/ml of choline. By tracking down different individuals, Dr. Zeisel may be able to see a correlation between the amount of choline ingested postnatally and the individual's memory capacities and accuracy.
Another line of Dr. Williams' choline research involves Dr. Scott Swartzwelder, associate professor in the Department of Psychology.
"By collaborating with Dr. Swartzwelder, who is examining Long Term Potentiation [LTP] in hippocampal slices from choline-treated and control subjects, we hope to relate the changes in LTP to behavioral changes," said Dr. Williams. LTP is a physiological process involving the modification of synapses that results in the process of learning. Thus, the LTP could be used as a functional intermediate to bridge the gap between the biochemical/neuroanatomical and behavorial findings.
Dr. Williams is currently trying to determine other behavioral effects of choline treatment. She has also just begun a long-term aging study to examine systematically the benefits of choline during aging. This study is designed to unravel the methods by which memory is saved into old age after perinatal choline treatment.
Quite possibly, a choline pill administered during early development for memory improvement may be accessible in the coming years. For the next generation, memorizing Shakespeare quotations and organic chemistry equations may be less of a headache.
At the time this article was written, Holly Weng was a Trinity College sophomore majoring in biology.
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