Molecular Biology of Depression
Dysregulation of the brain in early life can have lasting effects, and the effects of stress and depression can also accumulate. At the 2015 meeting of the Society of Biological Psychiatry, researcher Huda Akil explained that behavioral pathology can “take on a life of its own, leading to deteriorating course of illness and treatment resistance.” She illustrated how preclinical work in animals can help clarify the molecular biology of depression and develop new targets for therapeutics.
Early Life Experiences are Key
Akil discuss studies of rodents in which she used new molecular genetic techniques to increase the number of glucocorticoid receptors in the hippocampus early in life (prior to weaning). Glucocorticoid receptors mediate the effects of the stress hormone cortisol in people and corticosterone in rodents. More receptors help shut off cortisol secretion after a stressful event. People with post-traumatic stress disorder (PTSD) have high levels of glucocorticoid receptors while people with depression have low levels, leading to over-secretion of cortisol in depression.
The increased glucocorticoid receptors led to a long-term increase in anxiety behaviors and response to stimulants. When Akil carried out the same manipulation on rats that had already been weaned, it had no long-lasting effects, showing that there is a vulnerability window for some long-lasting effects on behavior.
CLOCK Genes and Circadian Rhythms
Akil also studied CLOCK genes in rodents. These genes, including BMAL-1, Per 1, Per 2, and Per3, play a role in circadian rhythms, and their transcription induces these 24-hour cycles. In rodents who were induced into a depression-like state, the CLOCK genes were dysregulated and did not correspond to normal circadian rhythms. These data show that depressive states can induce changes in CLOCK genes and circadian rhythms. Others have shown the converse, that abnormal CLOCK genes can induce behavioral abnormalities including mania-like behaviors.
Fibroblast Growth Factor
Levels of fibroblast growth factor 2 (FGF2) in the hippocampus are low in people with depression. In rodents, FGF2 inhibits anxiety. Decreases in FGF2 are seen in the hippocampus of animals in a depression-like state following repeated defeat by a larger animal. It appears that FGF2 is an endogenous antidepressant (i.e. one that is produced by the brain). When the rodent brain is manipulated to eliminate FGF2, the animals become anxious.
In addition, animals bred to have high stress, low social responsivity, and resistance to new learning also have low FGF2. Treatment with FGF2 reversed these behavioral abnormalities and also increased the production of new neurons. For the stressed rats, receiving FGF2 on their second day of life increased new neuron production, decreased anxiety, decreased proneness to social defeat stress and increased the bonding hormone oxytocin in the amygdala into adulthood.
FGF2 had no effect on rats bred for low stress and high social responsivity, indicating that it only worked for the rats that needed it. Akil compared FGF2 to “personalized medicine for rats.”
Defeat stress affects the way genes are transcribed, and FGF2 was able to reverse one of these specific transcriptional effects, suggesting it could potentially ameliorate some of the long-lasting effects of stress and depression.
The Human Brain
Akil also studied the brains of people who had died of depression, bipolar disorder, or schizophrenia. In bipolar disorder, the nucleus accumbens, the reward center of the brain, was enlarged.
In contrast, Akil described the brains of those people who had died with depression as being “low on fertilizer.” That is, they showed less cell growth, less production of new neurons, more abnormalities in cell shape, and more cell death. Akil said that by the time someone is severely ill, the pathology is all over the brain. The changes Akil saw in the brains of people who were depressed are also consistent with data indicating that several neuroprotective factors, including BDNF and VEG-F, are low in the frontal cortex and the hippocampus of depressed people (while BDNF is high in the nucleus accumbens).
