Suicide is all in the head

三月 13, 1998

Next Wednesday is European Brain Day. The THES gets cerebral

In response to some changes, the adult brain may reconfigure and, in the process, kills some of it cells. Changing socks is not sufficient to cause brain cells to commit suicide, but having a baby or changing one's environment might be, scientists say.

Neurobiologists at Birmingham University are trying to discover what scale of change starts the reconfiguration.

The human body is made of millions of cells in a state of flux, dying and being replaced. Cells die as a result either of an event that causes physical trauma and local cell death or of suicide, when some cells in effect turn on themselves, a process called apoptosis.

In most parts of the body, a cell that dies is replaced by a new one. But brain cells, or neurons, cannot regenerate. Lose one of the 100,000 million brain cells and you will have to do without it for ever.

Scientists want to understand brain cell suicides better, especially as they have thought for some time that degenerative diseases such as Parkinson's occur when neurons commit suicide inappropriately.

There must be positive advantages to the brain's maintaining its suicide system. "The temptation for neurons to undergo this death process would otherwise seem bizarre," says Ian Mitchell of Birmingham University's psychology department.

Dr Mitchell explains that scientists have at least one hypothesis for the cells' suicidal streak in early life and are on the trail of an explanation for its occurrence in adults.

They believe the brain contains so many neurons that have to be wired in such a complex way that DNA cannot possibly contain such detail. Instead, they suggest, DNA carries only rough information on brain structure, and there is another process that helps neurons to wire properly.

They think the body makes an enormous excess of neurons, each of which has the ability to commit suicide. Very early in the brain's development, each neuron tries to connect with others to form circuits. To alert their neighbours to their presence, neurons produce certain chemicals, which also serve to prevent cells from turning on themselves.

If this chemical is limited in supply, only the best connected neurons receive enough, Dr Mitchell says. As a result of this process, only the best-placed neurons with the most connections and a good supply of the chemicals survive .

The team at Birmingham is trying to model this process on computerised neural networks to see whether the circuits their computer suggests match those of the brain.

These artificial neural networks are made of hundreds of connections. But if their environment alters, connections in the network also need to change.

"This is what seems to happen in animal adults, too," says Dr Mitchell. He suggests that a new situation, such as pregnancy, may trigger some rewiring of the brain to cope with the changed environment. Establishing new connections means some neurons become isolated, starve and die.

"There seem to be waves of this sort of cell death," Dr Mitchell says. "It appears that a selective method of killing individual neurons is maintained so the adult brain can be sufficiently plastic to cope with change.

"If the brain were to reconnect every time you changed your socks, and it involved apoptosis, you would end up with a hole in the head. What we are trying to find out is how significant a situation has to be for the brain to rewire and cells to commit suicide."



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