Tell the story about the licking rat pup experiment. What did it tell us about epigenetics, and how did early life experiences affect the rat pup?
What did you personally learn about the process of gene expression? Provide an example of how a neuron’s pattern of gene expression can be influenced by something in the local cellular environment (i.e., the inside fluid constituents of the cell), and how that fluid environment of the neuron may be impacted by the broader local environment the person is actively behaving in?
The experiment showed that the difference in maternal care wasn't just behavioral; it had a direct, lasting biological effect on the pups' brains. Pups raised by High-LG mothers grew up to be calmer and less fearful than those raised by Low-LG mothers.
To prove that the effect was due to the nurture and not the genes, researchers performed cross-fostering: pups born to High-LG mothers were given to Low-LG mothers to raise, and vice versa. The results confirmed that the adult stress response of the pups followed the behavior of their adoptive mother, not their biological one.
Epigenetics and Early Life Effects
The experiment provided a clear example of how epigenetic mechanisms mediate the effect of the early environment on development.
Gene Target: The key gene involved is the one that codes for the glucocorticoid receptor (GR) in the hippocampus (a brain region involved in stress regulation).
The Mechanism (Epigenetics):
Low-LG Pups: Minimal licking/grooming meant the pups grew up with a methylated GR gene promoter. Methylation is an epigenetic tag that acts like a "dimmer switch," turning the gene's expression down. Consequently, these rats produced fewer GR receptors, making their stress response system less sensitive to shutting off circulating stress hormones (like cortisol/corticosterone). This resulted in a heightened, prolonged stress response in adulthood—they were anxious and fearful.
High-LG Pups: Frequent licking/grooming signals in early life removed the methylation tags from the GR gene promoter (a process called demethylation). This allowed the gene's expression to be turned up. These rats produced more GR receptors, making their stress response system more efficient at detecting and clearing stress hormones. This resulted in a calm, low-stress response in adulthood.
In summary, the experiment told us that environmental factors (maternal care) can physically change the chemical structure around the DNA, altering the pattern of gene expression without changing the underlying DNA sequence itself. This is the definition of epigenetics.
Sample Answer
The "licking rat pup" experiment is a classic study in epigenetics that demonstrated how maternal behavior can change the expression of genes in her offspring, influencing their stress response later in life.
1. The Licking Rat Pup Experiment
The Story
The experiment, led by Michael Meaney and Moshe Szyf, focused on the behavior of mother rats (dams) and their pups. Researchers observed significant natural variations in maternal care:
High-Licking/Grooming (LG) Dams: These mothers frequently licked, groomed, and nursed their pups in an arched-back posture.
Low-Licking/Grooming (LG) Dams: These mothers exhibited minimal licking and grooming behavior.