Do neurons regenerate? It has always been thought not. It seems that most of our neurons are born when we are still in the womb of our mother, and with the passage of time, they do not reproduce, but they are dying little by little.
However, this has not been a cause for concern in normal situations. It is common for a generous amount of neurons to be lost every day, what begins to be pathological is an excessive loss like that in the dementia .
But, the loss of neurons that is considered normal does not affect our cognitive abilities. In fact, neurons reorganize their connections continuously, to always secure the most useful in each moment and to discard the useless ones.
But what if I told you that evidence has been found that the neurons regenerate? Do you know that there are certain areas of our brain in which these cells reproduce, even if we are adults?
Regeneration of neurons in the hippocampus
It appears that, in most mammals, neurons are regenerated in the hippocampus And in the Olfactory bulb . The hippocampus is essential for learning, memory And spatial orientation, while the olfactory bulb gives meaning to the information that captures our sense of smell.
This makes sense, because the explanation given to our brain to produce new neurons is that it needs to maintain a set of cells with specific properties, but these last for a limited time. In addition, they are essential because they are specialized to carry out a very specific neural processing.
Numerous studies appear to indicate that neurons are born in a portion of the lateral ventricle and then migrate to the olfactory bulb. There they will integrate with existing cells and participate in the olfactory memory and in the conditioning of fear through smell.
They can also migrate to the dentate gyrus of the hippocampus, acquiring an important role in spatial learning and memory of contextual cues.
Humans are distinguished from other mammals in that they do not possess regeneration in the olfactory bulb. However, this regeneration has been shown to occur in the hippocampus. It seems that explains why we are not as dependent on smell as other animals, while we have a greater degree of cognitive adaptation.
Before 1998, it was already known that Neurogenesis (Birth of new neurons) in rodents and adult monkeys. But what about humans?
In that year, Eriksson and his team were the first to demonstrate that neuronal regeneration occurs in the human hippocampus. They used human brain tissue postmortem, proving that during the whole life the neurons reproduce in the gated cogwheel.
Thus, the hippocampus cells have an annual renewal rate of 1.75%.
However, human neurogenesis in the cerebral cortex only occurs in our early development and is not maintained in adulthood.
Regeneration of neurons in the striatum
In 2014, a group of scientists from the Karolinska Institute discovered that neurogenesis exists in the brains of adult humans.
These researchers found Neuroblasts On the wall of our lateral ventricle. We can say that neuroblasts are primitive cells that have not yet evolved, and that, in the future, will differentiate into neurons or glial cells.
But that is not all, they also found that these neuroblasts grow and integrate in a nearby area: the nucleus striatum. This part of our brain Is fundamental to control our movements, and a damage in this place would produce motor alterations like tremors and tics.
In fact, the same authors discovered that in the Huntington's disease , Where motor deficits are present, neurons are hardly regenerated in the striatum. In addition, in advanced stages of the disease, regeneration stops completely.
Regeneration in other areas of the brain
There are authors who have found adult neuronal regeneration in other non-conventional areas, such as neocortex, piriform cortex and Limbic structures Such as the amygdala, Hypothalamus The preoptical area. The latter have an essential role in social behavior.
However, there are researchers who have obtained conflicting results or have used inaccurate methods that have been able to alter the results. Therefore, it is necessary to continue research to confirm these findings.
On the other hand, it is necessary to mention that it is complicated to study in humans the regeneration of neurons by the existing ethical limits. For that reason, there are more advances in the animal field.
However, a noninvasive technique called magnetic resonance spectroscopy has been developed that can explore the existence of progenitor cells in the living human brain.
It is hoped that in the future these techniques can be perfected to know more about neurogenesis in adult humans.
Factors that enhance neuronal regeneration in adults
- Enriched environment and physical activity
It seems that a more complex environment increases the opportunity to live experiences, and produces sensory, cognitive, social and motor stimulation.
This fact does not seem to increase neurogenesis, but it does increase the survival of hippocampal cells in rodents and their level of specialization.
However, only the physical activity Has shown to increase neurogenesis in addition to the survival of these cells in adult mice.
If we consider the enriched environment as greater opportunities for learning, it has been confirmed that learning itself is decisive in hippocampal neurogenesis.
- Learning tasks
In a 1999 study by Gould et al., It was shown that learning Potentiates neurogenesis in the hippocampus. They marked the new cells in rats and observed where they were going while performing different learning tasks.
Thus, they verified that the number of regenerated neurons was doubled in the dentate gyrus when the rats carried out learning tasks that involved the hippocampus. While, in activities in which the hippocampus did not participate, this increase did not occur.
This is confirmed in other studies, such as Shors et al. In 2000, or as Van Praag et al. (2002), although they add that new cells evolve and become functional mature cells similar to those already existing in the dentate gyrus.
As for the learning activities in which the hippocampus is involved we find: blink conditioning, food preference, or space navigation learning.
- Social Interactions
In an interesting study by Lieberwirth & Wang (2012) it was found that positive social interactions (such as mating) increase adult neurogenesis in the limbic system, while negative interactions (such as isolation) decrease it.
However, these results should be compared with new studies to be confirmed.
- Neurotrophic factors
Nerve growth promoting substances, such as BDNF (brain derived neurotrophic factor), CNTF (ciliary neurotrophic factor), IGF-1 (Type I insulin-like growth factor), or VEGF (endothelial growth factor vascular).
- Neurotransmitters
There are certain types of neurotransmitters that regulate cell proliferation.
For example, GABA, which is inhibitory, regulates hippocampal neurogenesis. More specifically, it reduces it, but at the same time it increases the integration of the new neurons with the previous ones.
Another neurotransmitter, glutamate, decreases neuronal regeneration. As if injecting a substance with an opposite (antagonistic) effect, the regeneration is increased again.
On the other hand, Serotonin Increases neurogenesis in the hippocampus, while its absence reduces it.
- Antidepressants
In a study by Malberg et al. (2000) it has been shown that prolonged exposure to Antidepressants Increases cell proliferation in the hippocampus. However, this has only been proven in rats.
Factors that inhibit neuronal regeneration in adults
- Stress
Numerous studies show that an increase in stress Produces a significant decrease in neuronal regeneration of the hippocampus.
In addition, if the Stress is chronic , Reduces both neurogenesis and the survival of these cells.
- Steroids
The Corticosteroids , As glucocorticoids, which are released during the stress response, produce a decrease in hippocampal neurogenesis. The opposite occurs if the levels of this substance are reduced.
Something similar happens with gonadal steroids. In fact, in females the neuronal proliferation varies according to the levels of steroids existing in each phase of the hormonal cycle.
If estrogen is given to females for less than 4 hours, neuronal proliferation increases. However, if administration continues for up to 48 hours, such proliferation is suppressed.
- Social isolation
It seems that social failure, like isolation, decreases neuronal regeneration and survival in animals such as monkeys, mice, rats and shrews.
- Drugs abuse
A reduction in neurogenesis and cell survival due to chronic use of alcohol, cocaine, ecstasy, nicotine, and opioids has been shown.
References
- Eriksson, P.T., Ekaterina P., Björk-Eriksson, T., Alborn, A.M., Nordborg, C., Peterson, D.A. & Gage, F.H. (1998). Neurogenesis in the adult human hippocampus. Nature Medicine, 4, 1313-1317.
- Ernst, A.A., Alkass, K.A., Bernard, S.A., Salehpour, M.A., Perl, S.A., Tisdale, J.A., &... Uppsala universitet, T. O. (2014). Neurogenesis in the Striatum of the Adult Human Brain. Cell, 1072.
- G Ould, E., Beylin, A., Tanapat, P., Reeves, A. & Shors, T.J. (1999). Learning enhances adult neurogenesis in the hippocampal formation. Nature Neuroscience, 2, 260-265.
- Lieberwirth, C. & Wang, Z. (2012). The social environment and neurogenesis in the adult mammalian brain. Front Hum. Neurosci., 6, pp. 1-19.
- Lieberwirth, C., Pan, Y., Liu, Y., Zhang, Z., & Wang, Z. (2016). Hippocampal adult neurogenesis: Its regulation and potential role in spatial learning and memory. Brain Research, 1644: 127-140.
- Malberg J.E., Eisch A.J., Nestler E.J., Duman R.S. (2000). Chronic antidepressant treatment increases neurogenesis in adult rat hippocampus. J. Neurosci., 20, pp. 9104-9110.
- Shors, T.J., Miesegaes, G., Beylin, A., Zhao, M., Rydel, T., & Gould, E. (2001). Neurogenesis in the adult is involved in the formation of trace memories. Nature, 410 (6826), 372.
- Van Praagh., Schneider A.F., Christie B.R., Toni N., Palmer T.D., Gage F.H. (2002). Functional neurogenesis in the adult hippocampus. Nature; 415 (6875): 1030-4.
- Yuan, T., Li, J., Ding, F., & Arias-Carrion, O. (2014). Evidence of adult neurogenesis in non-human primates and human. Cell And Tissue Research, (1), 17.