Sci. Aging Knowl. Environ., 17 April 2002
This Is Your Brain ... And This Is Your Brain on Calcium
Misplaced your wallet? Forgotten your niece's birthday again? You might be able to blame your forgetfulness on calcium
Abstract: Most older brains don't learn or remember as well as younger ones do. One possible reason is that rising calcium concentrations within neurons hamper memory and learning. Researchers have accumulated a small pile of supporting evidence for the calcium hypothesis, but key questions remain about calcium's interaction with other mechanisms of aging and its effects on neuron activity.
They're known as senior moments: those embarrassing and frustrating mental stumbles that happen more often as we get older. We misplace our checkbook, or forget where we parked our car at the mall, or can't recall who won the Oscar for best actor the day after watching the entire awards extravaganza. Lapses like these don't necessarily herald the onset of Alzheimer's disease, but they do demonstrate that--for many people, at least--age corrodes the ability to learn and remember.
No one can say for sure what sabotages the aging brain, but some neuroscientists blame elderly forgetfulness on increasing calcium concentrations within the organ's neurons. Over the last 20 years, researchers have gleaned intriguing circumstantial evidence backing the so-called calcium hypothesis of brain aging. They've shown that some parts of the cell's calcium-regulating machinery become sloppy as we age, and they've identified calcium-driven changes in how older neurons respond to stimulation that could lead to mental sluggishness. The findings, based on measurements from isolated tissues or cells, aren't conclusive. "It's still a hypothesis," says neuroscientist Thomas Foster of the University of Kentucky, Lexington. According to supporters, recent work bolsters the idea, but at least one long-term backer of the hypothesis says he's changed his mind.
Fingering a Shady Character
Calcium has a reputation as one of the good guys in aging. Eating plenty of it is supposed to help keep our bones solid and our posture upright. But according to the calcium hypothesis, the intricate mechanism that regulates the mineral's concentration inside brain cells might malfunction with age, regardless of how much calcium we consume in our diets.
Suspicions that calcium might be messing with our heads sprouted in the early 1980s, says neuroscientist Philip Landfield, also at the University of Kentucky. Several labs, including his own, turned up hints that neurons from older animals no longer scrupulously control their internal calcium concentrations, allowing more of the stuff to accumulate. The rising tide of calcium might spark subtle functional changes that could impair the brain's ability to store knowledge--or so the hypothesis ran. Some researchers went further, arguing that elevated amounts of calcium might even slay neurons: High concentrations of intracellular calcium are toxic to cells. Calcium overload might even explain the mass slaughter of neurons seen in Alzheimer's disease.
Although supporting data were meager, the idea made sense because calcium manages so many brain functions. For one, it enables us to think and move, thanks to its role in the release of neurotransmitters, which carry messages from neuron to neuron. A neuron fires when it receives enough of its favored stimulation. As part of the process, channels in its membrane snap open and allow calcium to pour in. This influx, along with the movements of other ions, upends the electric charge in the cell and triggers the release of neurotransmitters that pass the signal to the next nerve cell.
Calcium also helps regulate two mechanisms that might underlie learning and memory: long-term potentiation and long-term depression. (Although they sound like titles of self-help books, they have nothing to do with mental health.) Long-term potentiation refers to the process in which a neuron becomes more excitable after repeated stimulation. It fires more easily the next time it's goosed--an ability that could provide the basis for memory. Long-term depression, by contrast, inhibits cell firing. "It's a mechanism for forgetting," speculates Foster. Both long-term depression and long-term potentiation change with age, according to several studies: Researchers find it easier to provoke long-term depression in old rats than in young ones, whereas the opposite is true for long-term potentiation.
In 1986 Landfield's team dredged up the first evidence that elevated calcium concentrations might hamper the brain. Measuring the electrical activity of rat neurons in culture, they determined the length of a phase called afterhyperpolarization (see figure). Stimulated by the entry of calcium, afterhyperpolarization is a recovery period that allows the cell to restore its ion balance after firing repeatedly. During this adjustment, the cell is less excitable. Landfield found that the afterhyperpolarization was longer in cells from older animals, a sign that their neurons might be harder to rouse than neurons from youthful rats.
Building on that result, neuroscientist John Disterhoft of Northwestern University School of Medicine in Chicago and colleagues linked calcium influx to defects in learning. They used a Pavlovian test called trace eye-blink conditioning to gauge the learning ability of rabbits. In the test, they play a tone at the same time that a puff of air hits the rabbit's eye. The animal associates the sound with the irritating stimulus and learns to blink at the tone alone--just as Pavlov's dogs eventually would slobber when he clanged a bell, even if no food was around.
Compared with young animals, old rabbits required more than twice as many trials to make the connection--and human senior citizens show impairments when performing the same test. However, treating the graying rabbits with nimodipine, a drug that plugs one kind of channel through which calcium enters cells, banished the age difference. In further experiments, Disterhoft and colleagues connected Landfield's afterhyperpolarization to mental languor. The team separated the old rabbits by how quickly they learned the eye-blink test and measured the activity of their neurons. The slow learners showed longer and deeper afterhyperpolarizations.
Staying Alive in the Hippocampus
Until the early 1990s, many neuroscientists believed that forgetfulness and impaired learning reflected the gradual death of neurons in the hippocampus, an area of the brain where memories are made. The calcium hypothesis got entangled with this idea. Because high calcium concentrations kill cells, some researchers reasoned that rising calcium concentrations might cause this winnowing of neurons.
With a few twirls of his microscope dials, Mark West of Aarhus University in Denmark brought all of those neurons back to life. Previously, researchers had gauged the density of cells in samples from the hippocampus. Because cell density seemed to decline with age, they reasoned that neurons were perishing. But West's counts of the neurons in many slices from the hippocampus turned up no signs of cellular attrition in the memory regions--a result confirmed by other researchers. West used a new technique to ensure that he didn't count any neurons twice and took samples from the hippocampus at regular intervals, allowing him to estimate the number of cells rather than their density.
"Our understanding has changed," says Disterhoft. "Except in a neurodegenerative disease, we don't think that cells are lost." Calcium poisoning might still slaughter neurons in Alzheimer's disease, says Landfield: amyloid, one of the proteins that proliferates in the brains of Alzheimer's patients, can trigger calcium influx into cells. But subtle changes in cell function are what's important for normal aging, he says.
Increasing a cell's calcium content over the long term requires a malfunction in the complex system that normally keeps the quantities under control (see figure). Protein channels in the cell membrane are doors for calcium. It can enter a neuron through two kinds of channels, one controlled by electrical currents, the other by neurotransmitter molecules such as glutamate. Another type of channel, known as Ca2+-ATPase, behaves like a bouncer, 86-ing the mineral. Calcium can also be absorbed or released by the endoplasmic reticulum, a system of sinuous tubes within the cell, and by mitochondria. Furthermore, molecules such as calmodulin and calbindin act as buffers and abduct free-floating calcium.
Early studies suggest that the buffering system and the calcium channels go awry as organisms grow old. Landfield and colleagues showed that aging cells install more of one type of channel, which could allow increased inflow of calcium when the neuron discharges. Calmodulin and Ca2+-ATPase also work less efficiently in older animals, according to research published in 1996 by Mary Michaelis, a neurobiologist at the University of Kansas, Lawrence, and colleagues.
One of the crucial questions scientists are keen to answer is how calcium interacts with other possible mechanisms of brain aging. Something has to instigate the defects in calcium regulation, after all. "Calcium can't be a primary cause, but it can be a critical part of a cascade that leads to age-related abnormalities," says neuroscientist Gary Gibson of Cornell Medical College in New York City.
Triggering these cascades, many neuroscientists say, are reactive oxygen species, electron-stealing molecules such as peroxide that attack proteins, DNA, and lipids and sap a cell's energy (see "The Two Faces of Oxygen"). Neuroscientists are just beginning to probe the relation between calcium and oxidative stress. Do oxidants attack calcium-handling proteins such as calmodulin, leading to lax calcium control, for instance? Michaelis and colleagues found evidence supporting this possibility when they soaked calmodulin in solutions of reactive oxygen species. The proteins developed faults similar to those seen in molecules from aged animals. And oxidants might mangle the calcium-control system in other ways that haven't been discovered.
The Case of the Missing Measurements
Another avenue of research is leading scientists back to the basics. That's because support for the calcium hypothesis remains circumstantial. Researchers haven't accomplished two key experiments that would seal its credibility. First, no one has published convincing measurements of brain calcium concentrations in a live animal, Landfield says. Neuroscientists have had to rely on studies of isolated cells or slices of tissue from the hippocampus removed from recently killed animals. Of these surrogates, brain slices come closest to replicating physiological conditions, preserving some of the connections between cells--but they don't substitute for an intact organ.
Even using brain slices, scientists haven't determined whether calcium concentrations within neurons really do climb as we age. Although the question seems elementary, several studies have come up with conflicting answers. There's good reason for the discrepancies. "It's enormously difficult to measure calcium in cells," says Landfield, who is considered a virtuoso at the techniques. Even getting healthy neurons to study can be a challenge, because teasing apart cells or separating them with enzymes can cause injury. The patch-clamp method of measuring calcium inflow--in which electrodes are glued to the cell's membrane--is very sensitive but is also hard to pull off. And any pieces of cellular debris that get between electrode and membrane can skew the readings.
According to Gibson, Landfield's most recent work--published in December 2001--comes closest to solving the mystery. When young neurons are stimulated repeatedly over a short period of time, the strength of their response increases--a phenomenon that Landfield says could lead to long-term potentiation. Using confocal microscopy, which allows researchers to monitor calcium concentrations using a fluorescent compound, Landfield and colleagues showed that calcium quantities were the same in young and old neurons--when the cells were at rest. When stimulated, however, old cells allowed in about 50% more calcium than did younger neurons. Are these brief surges enough to bollix a neuron? Landfield says they are, noting that the same cells were less responsive to stimulation and that the calcium imbalance shows up while the cell is in action: "We can finally say, 'Yes, there are changes in calcium influx, and yes, they do affect neuron function.' "
But neuroscientist Alexej Verkhratsky of the University of Manchester, U.K., is skeptical. The measurements undercut the hypothesis, he says, because such transient changes couldn't drive a major alteration in brain function. The results confirm his suspicion that calcium is only a sideshow, a minor manifestation of a larger change in brain chemistry. "I spent 15 years on the calcium hypothesis and was a firm believer," he says. But now "the more I see, the less I believe." The real memory thieves, he suggests, are oxidants, which deprive neurons or their supporting cells of the energy they need.
Only further research will decide who's right. Such studies could also help us hold onto our faculties into old age, Michaelis points out. The pharmaceutical company Bayer spent 15 years testing nimodipine, which blocks calcium channels, as a treatment for Alzheimer's disease. It failed, but something like nimodipine might work as a memory enhancer, helping tune up the minds of old people who show "normal" brain aging. And if oxidants are driving damage to the calcium-regulating machinery, people have many more options for fighting memory loss. So remember to eat your antioxidant-rich broccoli, and maybe you'll remember where you left your keys.
April 17, 2002
Mitch Leslie is a, er, um, ah, yes, a science writer in Albuquerque, New Mexico. He's forgotten the witty aphorism he intended to put here.
Suggested ReadingBack to Top
Science of Aging Knowledge Environment. ISSN 1539-6150