Animal research

Studying stress in pregnancy

Stress during pregnancy can have lasting effects on the unborn child in later life. Animal studies can help us investigate the impact of stress on early development in the womb, and whether these effects can be prevented or reversed.

Stress is the body's natural reaction to feeling threatened or under pressure. Most people feel stressed sometimes and some people find it can be helpful or even motivating. Experiencing stress over long periods of time, or extreme stress however, can lead to long term mental and physical health problems, including anxiety and depression, raised blood pressure and risk of heart attack.

What do we know already?

We know from studies of natural or man-made disasters that babies born to mothers who were stressed during pregnancy are more likely to develop learning, behavioural, and mood disorders in childhood, as well as complex diseases such as obesity and cardiovascular conditions in later life.

Brain scanning studies have shown that exposure to stress in early life, before and after birth, can affect brain development in the child. However, we still know relatively little about the biological mechanisms that link maternal health with the onset of disease in the child.

Why do you need to use animals?

Studying the biology of stress in people is challenging because it is unethical to expose pregnant women to varying levels of stress, given what we know about its potential impact on their unborn child. Stress affects the whole body and its effects cannot be modelled in cell cultures. Additionally, it is not possible to replicate the complex mechanisms involved in mammalian development in a laboratory dish.

At Edinburgh, we use rats to investigate the biological mechanisms linking maternal stress and offspring health. Rats are genetically very similar to humans and many of the biological pathways that are involved in stress in people are also present in rats. In addition, rats have complex social interactions and display reproducible behaviours that are well characterised. We can use infra-red video tracking tools and specialist computer software to easily monitor changes in their behaviour. 

What have you found?

Our research has demonstrated that maternal exposure to social stress during pregnancy is linked with low birth weight, anxious behaviour, insulin resistance (a warning sign of diabetes), cognitive deficits and abnormal social behaviours in the offspring. We investigate the mechanisms in the brain that underpin these changes and whether the impact of stress exposure during development can be prevented or reversed.

Recently, in studies with rats, we identified molecules released by the placenta that transmit the effects of stress to the unborn foetus. These molecules are linked to a chemical pathway called oxidative stress, and we found we could block their effects by treating with an antioxidant called mitoquinone.

This is a really promising finding because mitoquinone has already been used in a number of human trials, for example it has been found to lower blood pressure in older people. However, the molecule has not been tested during pregnancy and a lot more research is required before mitoquinone could be used as a treatment for women.

How do you induce stress in the animals?

In our studies, we expose pregnant rats to a stressful situation by placing them in a cage with an unfamiliar female rat for a short period of time. This form of social stress is relevant to the natural behaviour of rats and involves very little pain, aside from the occasional nip and pregnancy proceeds normally.

How do you test for the impact of stress?

Monitoring the effects of stress in an animal is complex. Rats can’t tell us how they’re feeling so we need to use a range of tests to look for behaviours that can tell us something about their overall psychological state. We regularly review the scientific literature to make sure we are using the most refined and appropriate tests that give us the most robust scientific answers with the least possible impact on the animal’s welfare.

To investigate behaviours related to anxiety, we place the rats in a maze with a mixture of open and closed arms. Rats prefer to be in a closed environment, but equally have an innate curiosity to explore new environments. Anxious or fearful rats will spend less time in the open parts of the maze. We can monitor the rats’ movements with cameras and check how long they spend in each part of the maze.

An image of a rat exploring an elevated plus maze, which is used to measure behaviours linked to anxiety
Rats exposed to prenatal stress spend less time on the open arms of the elevated plus maze than control animals, indicating greater anxiety-like behaviour.

Another related test involves placing the rats into a light-dark box, which comprises a transparent box connected to a lidded opaque box. The rats are placed into the dark chamber and allowed to explore freely for 5 minutes. We use cameras to monitor movements and the amount of time they spend in the light, transparent box is recorded. We also time how long it takes the rats to venture into the light box, and how far they move around within the boxes. Anxious rats are less likely to venture into the light box and will spend more time in the preferred, dark environment.

What other tests do you use to study stress?

To assess how the rats born to stressed mothers cope with stress themselves, we use a test called the Porsolt Swim Test, sometimes called the Forced Swim Test. This involves placing the rat in an enclosed tank of water (warmed to around 23oC) and monitoring its swimming behaviour. Swimming is a natural behaviour for rats but when they are forced to do it, it is stressful for them. The test lasts for a maximum of 5 minutes. Healthy rats don’t have any problems staying afloat but if at any point the rat were to show signs that they were struggling, they would be removed from the water. All animals are towel dried after swimming.

During the test, some rats take an active approach to stress and continue to swim, dive and seek an escape route. Others take a passive approach – they give up trying to escape and float in the water instead. Active stress coping strategies are associated with greater resiliency to stress, whereas passive coping is associated with increased vulnerability to stress.

How do you look for signs of depression in a rat?

Losing interest in things that were previously pleasurable is a warning sign of depression. We can check for this behaviour in rats by giving them free access to a tasty sugar solution and monitoring how much of it they drink over a fixed period. Healthy rats will naturally choose the sugar solution over plain water, but a rat showing depressive-like behaviour will be less selective.

Why is this work important?

Understanding the causes of mental ill health and the factors that affect our ability to cope with stress has never been more important. A report from the Office for National Statistics revealed that in 2020, as many as one in five adults in the UK were likely to be experiencing some form of depression, which is almost double rates from before the pandemic. One in eight adults developed moderate to severe depressive symptoms during the pandemic.

In addition to the personal cost of mental illness to those affected and their families, there is also a huge economic burden through costs to the health service and reduced productivity at work. In total, mental illness is estimated to cost the UK economy £77 billion per year.