Combat-like stress in fathers alters brain chemistry in offspring

Fathers who experience severe trauma before having children can pass biological and behavioral changes on to their offspring. A new animal study published in Neurobiology of Stress shows that combat-like stress in male rats alters the pain perception, social behavior, and brain chemistry of the offspring they conceive later. These inherited biological shifts offer clues into how trauma might influence the physical and mental health of future generations.

Military veterans experience high rates of post-traumatic stress and physical brain injuries like concussions. These experiences often lead to lasting conditions such as chronic pain, severe anxiety, and depression. The biological toll of this extreme stress affects the hypothalamic-pituitary-adrenal axis, the body’s central hormone response system.

When a person or animal encounters a threat, this system floods the body with stress hormones. In individuals with chronic trauma, the system remains activated, leading to a state of biological wear and tear. This extreme stress can also leave chemical markers on a person’s DNA, a process known as epigenetic modification.

Epigenetic markers do not alter the genetic code itself, but they dictate how genes are turned on or off. Because sperm production continues throughout a male’s life, stress-induced epigenetic changes in sperm can be passed down to offspring. Researchers Glenn R. Yamakawa, Richelle Mychasiuk, and their colleagues conducted the study to see how a father’s severe stress might manifest in the next generation.

The researchers used an experimental animal model to simulate the sensory and physical overload of combat. They divided male rats into a control group and a combat trauma group. Over three weeks, the combat trauma group experienced a series of random, intense stressors.

These stressors included the visual threat of a model hawk, the scent of fox urine, sudden loud noises, and physical restraint. The animals also received a mild traumatic brain injury simulating a concussion. This combination was designed to overload the animals’ stress response systems in a way that mimics warfare.

Following the three-week period, the traumatized male rats exhibited higher levels of the stress hormone corticosterone. They also showed increased anxiety in behavioral maze tests compared to the control group. The researchers observed elevated reward-seeking behaviors in the traumatized rats, such as increased consumption of sugar water and heightened interaction with unfamiliar rats.

Tissue analysis of the traumatized fathers revealed altered gene expression across multiple brain regions, including the prefrontal cortex, hippocampus, and hypothalamus. They observed distinct changes in specific genes that help regulate the body’s response to a class of steroid hormones. Altered expression of these genes suggests that the combat trauma protocol profoundly disrupted the animals’ natural stress response mechanisms.

After the stress period concluded, the male rats were mated with control females. The fathers were removed from the environment entirely, ensuring they had no contact with the mothers or the resulting offspring. Removing the fathers allowed the researchers to isolate the effects of biological inheritance from the effects of parental behavior.

When the offspring reached adolescence, the researchers ran them through a battery of behavioral tests. Half of the offspring were also exposed to a secondary stressor, which was the visual threat of a model hawk. This secondary test was used to see if the paternal trauma made the offspring more sensitive to stress in their own lives.

The offspring of the traumatized fathers displayed pronounced behavioral differences compared to the offspring of the control fathers. Early in life, they showed elevated anxiety and increased unprompted freezing behavior during startle testing. These offspring also engaged in more aggressive social play.

Testing revealed physical changes as well, specifically regarding nociception, which is the nervous system’s ability to process pain. Female offspring of traumatized fathers showed reduced sensitivity to noxious heat and cold, while male offspring became more sensitive to cold. In tests using mechanical pressure on the paws, female offspring became more sensitive, while male offspring exhibited reduced sensitivity.

The researchers then analyzed the brain and adrenal gland tissues of the offspring. They found shortened telomeres, which are the protective caps at the ends of chromosomes. Shortened telomeres are typically associated with cellular aging and a higher risk for chronic disease.

Gene expression in the offspring’s brains mirrored many of the chemical changes seen in their fathers. The researchers found changes in a specific dopamine receptor gene within the adrenal glands of both generations. This particular gene is deeply involved in the secretion of adrenaline and the body’s fight-or-flight response.

The offspring also had altered expression of genes that regulate dopamine and serotonin across various brain regions. These neurotransmitters are chemical messengers that dictate mood, sociability, and reward processing. They also found changes in genes that manage the body’s stress hormone receptors.

The secondary stressor in adolescence did not dramatically compound these effects, though it did interact with the paternal trauma to alter certain behaviors like sugar consumption. Animal models of trauma provide a controlled environment to study biology, but they do not perfectly replicate the human experience of war. Because the fathers in this study were removed immediately after mating, the results strictly reflect biological inheritance.

In human families, a parent living with trauma will interact with their child, which introduces learned behaviors and environmental stressors that shape a child’s development. The researchers note that the exact timing of mating could influence the results. The rats were mated just days after the trauma protocol ended, reflecting an acute phase of biological stress.

Future studies could explore whether mating the animals months after the trauma produces the same inherited effects. Understanding these inherited biological markers opens the door to future treatments. Because epigenetic changes are chemical modifications rather than permanent DNA mutations, they are potentially reversible. Researchers might one day develop targeted therapies or specific diets to help offset the inherited risks associated with paternal trauma.

The study, “Intergenerational Influences of Paternal Combat-Related Trauma on Offspring Behavioral and Brain Function,” was authored by Glenn R. Yamakawa, James Freeman, Sydney Harris, Marissa Sgro, Elaina Vlassopoulos, Crystal N. Li, Josep Roman-Juan, Melanie Noel, Sabrina Salberg, and Richelle Mychasiuk.