Dr Thanh-Tam Pham - 22/11/2020
Neurogenesis is the generation and incorporation of new nerve cells into the brain circuits.
Previously, neuroanatomist such as Santiago Ramon Cajal believed the nervous system was not capable of regeneration and that neurogenesis in humans no longer continued after the embryonic development phase.
The conventional thinking was that the brain was made of a fixed network of neurons that formed a circuit and once we were grown-up, all we could do was watch our brain cells die. People assumed that the incorporation of new cells into this circuit would disrupt the brain’s ability to function.
In 1962, the first evidence of adult neurogenesis was demonstrated by Joseph Altman and more substantial evidence was provided by Fernanndo Nottebohm in 1980.
It is now widely accepted that adult neurogenesis occurs in the hippocampus, a region involved in regulating learning, memory and emotions. Recently neurogenesis has been found in the amygdala. Each amygdala is located close to the hippocampus in the frontal portion of the temporal lobe. It is a processing centre, receiving incoming messages from our senses and internal organs and is associated with emotional processes, emotional behaviour and motivation. It has an important role in anxiety and post traumatic stress disorder by putting emotions to memories. The newly formed cells in the amygdala are interneuron cells that dampen down activity in neural circuits such as helping to keep emotions like fear in check.
Alzheimer’s disease: some studies suggest that decreased hippocampal neurogenesis can lead to the development of Alzheimer’s.
Schizophrenia: studies suggest that people with schizophrenia have a reduced hippocampal volume which is believed to be caused by a reduction of adult neurogenesis.
Major depressive disorder: hippocampal neurogenesis is also reduced.
Modulation of adult neurogenesis
1- Factors that increase neurogenesis:
a- Exercise: Aerobic exercise such as running, cycling, swimming, dancing and even sex are effective ways to boost neurogenesis. The aim is to get the heart pumping over 20 min on a regular basis. In contrast, aerobic exercise combined with low or high intensity strength exercise was found to decrease these benefits in a study.
Yoga and meditation also increase neurogenesis.
b- An enriched environment: activities such as learning a new skill, a new language, travelling, exposure to sunlight ( or vitamin D supplement) or other intellectual challenges stimulate neurogenesis. It is good to get out of the comfort zone, experience something new and being mentally challenged on a regular basis. These activities not only trigger new neurons growth but those cells are more likely to survive in the long term.
- Omega 3 fatty acids
- Flavonoids: such as dark chocolate, blueberries, red wine…
Moderate alcohol over a long period of time can enhance the formation of new nerve cells may be by enhancing GABA (gamma-Aminobutyric acid). GABA excites the new neurons and prepares them for integration into the adult brain.
- Calorie restriction diet or extending the time between meals (intermittent fasting) increase neurogenesis.
- Low dose Cucurmin.
Long term meditation can support neurogenesis by increasing grey matter density, hippocampal volume and melatonin levels (study of MRI brain- Department of neurology UCLA). Meditation is also known to be effective in reducing stress, anxiety and depression.
e- Psychoactive substances: cannabinoids and antidepressant such as SSRI (Selective Seretonin receptor inhibitor) may increase neurogenesis. In a normal brain Serotonin causes the suppression of Corticotropin Releasing Hormone (CRH) through the connection to the hippocampus to reduce Cortisol. SSRI efficacy to treat depression may be explained by neurogenesis. It normally takes 3 to 6 weeks for new neurons to mature which is the same amount of time it takes for SSRI to take effect. This correlation strengthens the hypothesis that SSRI act through neurogenesis to decrease depression.
2- Factors that reduce neurogenesis:
a- Stress and depression: many now believe stress is the most significant factor for the onset of depression. Hippocampal cells are sensitive to stress which can lead to a reduction of neurogenesis. The adrenal gland is producing Cortisol in response to a stressor. Chronic elevation of cortisol causes down regulation of Serotonin receptors and suppresses the birth of new neurons.
b- Sleep deprivation: one study has linked lack of sleep to a reduction in rodent hippocampal neurogenesis. Normal levels of neurogenesis after chronic sleep deprivation return after 2 weeks with a temporary increase in neurogenesis.
c- Inactivity like binge watching televisions for hours on the couch can actually reduce cell growth in the brain.
d- Aging: there is a well known reduction in the rate of neuronal generation in the ageing brain. But there was a study demonstrating that brain can produce new cells even at the age of 90. Alzheimer’s disease accelerates the rate of nerve cells loss and also limits the creation of new ones. Fortunately, a study in mice found that chronic exercise such as running can produce new nerve cells and more efficient integration of new neurons in a running mouse comparing to in a sedentary mouse.
e- Excess alcohol has a negative effect on neurogenesis while moderate alcohol consumption improves it. Most drugs of abuse such as nicotine, heroin, cocaine have a negative effect except for cannabinoids.
- high saturated fat diet
- Thiamine deficiency
- Vitamin A deficiency
- Folate deficiency
- Zinc deficiency
- Increased homocysteine: homocysteine is mostly from meat and is often associated with low levels vit B6, B12, folate and renal disease
- Soft diet: soft diet in rats is linked to impairment in learning and memory. Chewing plays a role in adult neurogenesis. This could be particularly relevant to the ageing population with cognitive decline where dental problems might limit the chewing ability.
In summary, modulating adult neurogenesis by diet and exercise could be a good choice to prevent cognitive decline during ageing as well as reducing the effect of stress and prevent depression. It is not all about growing new neurons but the more we use our mind, the better it will be as it was shown that the dendrites of a nerve cell can show dramatic growth with usage.