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Some mother rats spend a lot of time licking, grooming and nursing their pups. Others seem to ignore their pups. Highly nurtured rat pups tend to grow up to be calm adults, while rat pups who receive little nurturing tend to grow up to be anxious.

It turns out that the difference between a calm and an anxious rat is not genetic – it’s epigenetic. The nurturing behavior of a mother rat during the first week of life shapes her pups’ epigenomes. And the epigenetic pattern that mom establishes tends to stay put, even after the pups become adults.

High-nurturing mothers raise high-nurturing offspring, and low-nurturing mothers raise low-nurturing offspring. This may look like a genetic pattern, but it's not. Whether a pup grows up to be anxious or relaxed depends on the mother that raises it - not the mother that gives birth to it.

The Glucocorticoid Receptor (GR) Helps Shut Down the Stress Response

When we’re confronted with danger, the body turns on stress circuitry in the brain. Stress circuitry activates the adrenaline-driven Fight or Flight response and causes the hormone cortisol to be released into the bloodstream. Cortisol is important for freeing stored energy, which helps with both fighting and fleeing. But too much cortisol can be a bad thing. High levels can lead to heart disease, depression, and increased susceptibility to infection.

Cortisol also travels to an area of the brain called the hippocampus, where it binds to GRs. When enough cortisol is bound, the hippocampus sends out signals that turn off the stress circuit, shutting down both the Fight or Flight response and cortisol production.

Rats (and people) with higher levels of GR are better at detecting cortisol, and they recover from stress more quickly.

The Stress Circuit, also called the HPA Axis (for Hypothalamus-Pituitary-Adrenal).

Stress signals travel from the hypothalamus to the pituitary gland and then to the adrenal glands. The adrenal glands release the hormone cortisol (and adrenaline, not shown).

When cells in the hippocampus detect cortisol, which binds to the GR receptor, they send a signal to the hypothalamus that shuts down the stress circuit.

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