Brain explosion

How nuclear testing made all people participants in the experiment

"Nerve cells do not recover!" - says popular wisdom. True, almost more often than the sacramental maxim itself, you can now hear the words that it is just a myth that science has long refuted. Recently, an article was published in the prestigious journal Neuron , from which it follows that the myth, apparently, turned out to be a little more true than we would all like.

The most amazing thing in this story is that the experiment on the basis of which the researchers made their conclusion began in the middle of the last century and all of humanity took part in it, without knowing it.

Canaries and the father of neuroscience

Santiago Ramon y Cajal in his laboratory. Photo from the site molbiol.ru

The impossibility of dividing neurons was established in science simultaneously with the very concept of a neuron as a working unit of the brain - that is, in fact, from the very beginning. Santiago Ramon y Cajal, the generally recognized founder of neurobiology, who shared the Nobel Prize in Medicine with Golgi in 1906, wrote about the development of nerve cells: “At the end of [embryonic] development, the sources of growth dry up forever. The neural pathways in adults are unchanged. Anything can happen.” be destroyed, but nothing can be restored."

These words of the great physiologist, of course, greatly influenced not only popular culture, but also his colleagues in the workshop. Even more than half a century after Cajal’s work, the scientific community still considered the impossibility of the formation of new neurons to be a proven fact.

Perhaps because of this, or perhaps because of the lack of convincingness of his experiments, the scientific community completely ignored the work of the first researcher who doubted this doctrine, Joseph Altmann from the Massachusetts Institute of Technology. As early as the 1960s, he provided evidence for the existence of neurogenesis in adult rats, guinea pigs and cats. However, then they did not convince anyone; they laughed at Altman. The real revolution happened much later and required much more “tangible” evidence.

Pathway of neuroblades from their site of formation in the lateral ventricles (LV) to the olfactory bulb (OB) in rodents. Image by Jessica B Lennington, Zhengang Yang and Joanne C Conover

In the mid-80s, Rockefeller University employee Fernando Nottebohm turned to an unusual model organism - canaries. He noticed that in adult male canaries, the brain size changes depending on the time of year. During the breeding season, when males burst into mating songs, their brains literally “swell” to cope with the complex intellectual task of learning melodies. At the end of the season, the musically developed brain becomes unnecessary and decreases until the following spring. This was the first conclusive evidence of neurogenesis in adult vertebrates.

work Nottebohm's spurred new research that generally confirmed his observations. Almost immediately, scientists switched to their favorite model objects: mice and rats. It turned out that in adult rodents, neurons are formed in significant numbers, and this process is very complexly coordinated. Neural stem cells formed on the surface of the ventricles of the brain migrate to the olfactory bulbs, where they turn into either auxiliary glial cells (mainly astrocytes) or neurons. This migration occurs in only one direction: from the site of neuroblast formation to the site of cell maturation.

Similar results were obtained in monkeys. True, in their case, the level of formation of new cells was significantly lower, and the migration and maturation of cells in macaques took six long months.

It immediately turned out that the formation of new cells in adult animals is incomparable in scale with neurogenesis in embryonic and early individual development.

Hundreds of studies have been devoted to how to increase the rate of neurogenesis. A more diverse environment that stimulates brain activity has been shown to stimulate the formation of new neurons. Running and physical activity also contributed to this process. Moreover, an unexpected stimulant effect was discovered in the antidepressant Prozac, which sharply boosted both the interest of pharmaceutical companies and the level of public expectations. There was only one annoying nuisance: all these experiments were carried out on model animals, and not a single one of the conclusions was confirmed in humans - no one simply understood how this could be done at all.

Pulse chase

Winemakers and criminologists

The analysis of the “bomb carbon” content, which Spalding was able to use to determine the age of DNA, was first carried out by criminologists Wald and Kucera from the University of Vienna. They used isotope analysis of tooth enamel to help investigators solve the strange deaths of two sisters found dead in their home in a Vienna suburb. The women had died years before police discovered their mummified corpses, and insurance companies were trying to figure out which relative died first so they could collect their pension savings. Wild and Kucera's analysis showed that one of the women died in 1988, and the other only in 1989. All this time she lived next to her dead sister.
After Spalding's work, the method became widely known and began to be used in a wide variety of fields. Several years ago, Australian scientists, commissioned by a local wine producers' association, developed a technology for determining vintage based on carbon-14 analysis. Later, whiskey producers followed the winemakers.

Model animals exist for the purpose of conducting experiments that cannot be repeated in humans. The first experiments on neurogenesis, which were carried out on mice and monkeys, involved feeding the animals radioactive substances, which could then be seen on thin sections of the brain.

This type of radioactive labeling, which in the early days of molecular biology helped reveal the secrets of how DNA works, is what scientists call “pulse chase.” It involves adding a radioactive isotope to the environment, food or blood of an experimental organism for a very short time, and then replacing it with a large amount of a normal isotope. As a result, the radioactive label is incorporated only into those substances that were synthesized precisely during the “pulse”.

For example, if you introduce radioactive nucleotides, which are the building blocks of DNA, into the body, they will mark only the nucleic acid that was synthesized when the isotope was added. In brain tissue, the tag will be embedded only in those cells in which DNA was synthesized during the pulse, which means it will be possible to directly see where and how dividing neuronal precursors are formed. Generally speaking, the label does not have to be radioactive, it just has to be different from other substances in some way to be noticeable on the drugs.

In 1998, Fred Gage of the Salk Institute realized that a drug given to some cancer patients, bromodeoxyuridine (BrdU), could be just such a marker. This modified nucleotide was supposed to reduce the growth of tumors, but, being a nucleotide, it was also integrated into the synthesized DNA of brain cells. Judging by the specimens from five patients that Gage analyzed, neurogenesis did indeed occur in humans. But there were many problems with BrdU: firstly, it was very toxic, and secondly, it could stimulate DNA synthesis itself, which made it difficult to understand whether synthesis actually occurs or whether it is an artifact of the method.

Forced experiment

But how can one find out whether cell division and DNA synthesis are occurring in the human brain if a label cannot be introduced into the molecule? And how to introduce a tag if it is not possible to use volunteers?

Paradoxically, it turned out that in fact volunteers are not needed: all of humanity, since the middle of the last century, has already been participating in a large-scale experiment, without even knowing it.

Kirsty Spalding in her laboratory at Karolinska Institutet, pioneer of the bomb chase method. Photo Science/Stefan Zimmerman

Everyone remembers the impressive photographs of nuclear explosions and is aware of the consequences of the release of radioactive isotopes. Not everyone knows what benefits can be derived from this. It turns out that from the Manhattan Project until the ban on open testing in 1963, huge amounts of the isotope carbon-14 were released into the atmosphere. It is a harmless isotope that is usually formed when atmospheric nitrogen is exposed to cosmic radiation. Its concentration was constant in the environment before the start of nuclear testing. It is its content that is the basis of the radiocarbon dating method - when organisms die, they stop absorbing atmospheric carbon-14, which gradually disintegrates in the remains. Knowing the degree of decay of the isotope, it is possible to determine, with an accuracy of up to a couple of hundred years, the time when the organism under study last breathed.

It turns out that the release of large amounts of carbon-14 into the atmosphere as a result of nuclear testing makes dating orders of magnitude more accurate. The concentration of carbon-14 in the atmosphere has been steadily declining since 1963, almost uniformly across the planet. By knowing the isotope concentration in any organic molecule, you can determine its age, and knowing the age of DNA, you can determine the age of any cell.

The young Australian Kirsty Spalding, who worked at the Karolinska Institute in Stockholm in the early 2000s, was able to come up with and carry out this experiment. She determined the age of horses from their brain samples. She managed to achieve amazing accuracy - using the developed method, you can determine the age of any tissue with an accuracy of up to a year.

Since Spalding developed this unique method, she has systematically worked to determine the regeneration time of different tissues. Along the way, Kirsty greatly enriched the arsenal of criminologists with her method and even founded a small company that determines a person’s age based on the isotopic composition of tooth enamel.

But the most important thing that Spalding was able to establish is that in the adult brain, at least in the areas she studied, there is absolutely no cell division at all. All neurons in the brain are the same age as the person himself.

Determination of DNA synthesis time (horizontal axis) by carbon-14 content (vertical). The graph shows a peak corresponding to nuclear tests. Image from Bergmann et al.

In the latest work on this topic in the journal Neuron , which has already been discussed, the authors studied DNA synthesis in neurons of the olfactory bulbs. This is the very region of the brain where neuronal stem cells migrate most actively in rodents. Scientists have shown that almost all neurons of the human olfactory bulbs are old, they are formed only during embryonic development.

The absence of young neurons where scientists hoped to find them becomes a very strong argument against the possibility of regeneration.

It turns out that Ramon y Cajal was right and brain degradation is inevitable with age? Not certainly in that way.

Yes, there is no hope that neurons damaged and lost during a stroke or as a result of Alzheimer's disease will be restored over time as a result of training and physical activity. At least, there is practically no scientific evidence of this possibility at the moment.

On the other hand, humanity does not have to rely solely on natural regeneration mechanisms. Work on growing stem cells in vitro suggests that they may well mature, specialize and become full-fledged neurons. So far, these studies are still very far from practical application, and very little is known about the side effects. There is only one practical conclusion from this: research must be continued.

Alexander Ershov

Brain explosion

Canaries and the father of neuroscience

Pulse chase

Forced experiment

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Lenta.ru: Progress: Brain Explosion