Bruce McEwen - Beating stress with joined-up thinking on brain function

Published on Friday 1 August 2008 20:52

STRESS contributes to the onset of cardiovascular disease and depression, among other illnesses.

It is not only major stressful life events that exact a toll on our bodies – the many conflicts and demands of daily life elevate and sometimes disrupt the workings of our response systems for stress, causing wear and tear on the body and brain. This burden of chronic stress, called "allostatic overload", reflects not only the impact of life experiences but also our genetic constitution. Individual habits such as diet, exercise, the quality and quantity of sleep, and substance abuse also play a major role, as do early life experiences that set lifelong patterns of behaviour and physiological reactivity.

There are three categories of stress:

&#149 Positive stress, for which a person feels rewarded by surmounting a challenge.

&#149 Tolerable stress, which results from serious life events – for example, divorce, death of a loved one, loss of a job – but where the affected person has good support systems.

&149 Toxic stress, which involves the same types of serious events, as well as the accumulation of daily struggles, but without good support systems.

The difference between tolerable and toxic stress depends on the perceived degree of control that a person experiences. Moreover, low self-esteem exacerbates a feeling of helplessness and lack of control. Social support by friends and family is vital to ameliorating the effects of tolerable stress and keeping it from turning toxic.

These are all functions of the brain – the key organ in our response to stress. The brain interprets what is threatening and therefore stressful; regulates behavioural and physiological stress responses – the latter through the autonomic, immune and neuroendocrine systems; and is a target of stress, undergoing structural and functional remodelling of its circuits that affects its performance.

The recognition of the brain's vulnerability and plasticity under stress began with investigations of the hippocampus, and it now includes the amygdala, a brain region involved in fear, anxiety and mood, and the prefrontal cortex, which is important in decision-making, memory and top-down control of impulsive behaviour, as well as regulation of the autonomic nervous system and stress hormone axis.

Because the remodelling of neurons by stress is reversible, researchers now believe that chronic anxiety disorders and depression represent a lack of resilience, or spontaneous recovery, in susceptible individuals. Such a lack of recovery then requires medication, behavioural interventions, or both.

Hormones associated with stress protect the body and brain in the short run and promote adaptation, but the chronic activity of these same hormones brings about changes in the body that cause allostatic overload, along with its potential follow-on diseases. For example, the immune system is enhanced by acute stress but suppressed by chronic stress.

Developmental influences involving the quality of parenting and acquisition of attachment have a powerful influence on subsequent stress vulnerability during the rest of our lives – for example, abuse and neglect in childhood increase our vulnerability to physical and mental disorders, including obesity, cardiovascular disease, depression, post-traumatic stress disorder, substance abuse and antisocial behaviour.

Among the most potent causes of stress in adult life are those arising from competitive interactions between animals of the same species, leading to the formation of dominance hierarchies. Psychosocial stress of this type not only alters cognitive function in lower-ranking animals, but it can also promote disease (for example, atherosclerosis) among those vying for the dominant position.

Public and private-sector policies can have a positive impact on health, providing a top-down benefit to brain and body function. This may occur through policies that improve education, provide better housing, improve commuting, regulate working conditions, increase availability of health foods and provide tax relief for those in the lower and middle classes. Such policies might well prevent disease, thereby saving money, reducing human suffering and promoting healthier and more meaningful lives.

&#149 Bruce S McEwen is a researcher in the Laboratory of Neuroendocrinology at Rockefeller University, New York.