Blog by Dr. Nora Volkow, Director, National Institute on Drug Abuse (NIDA)
The brain adapts and responds to the environments and conditions in which a person lives. When we speak of addiction as a chronic disorder of the brain, it thus includes an understanding that some individuals are more susceptible to drug use and addiction than others, not only because of genetic factors but also because of stress and a host of other environmental and social factors in their lives that have made them more vulnerable.
Opioid addiction is often described as an “equal opportunity” problem that can afflict people from all races and walks of life, but while true enough, this obscures the fact that the opioid crisis has particularly affected some of the poorest regions of the country, such as Appalachia, and that people living in poverty are especially at risk for addiction and its consequences like overdose or spread of HIV. The Centers for Disease Control and Prevention (CDC) considers people on Medicaid and other people with low-income to be at high risk for prescription drug overdose.
Some of the reasons have to do with access and quality of health care received by people in economically disadvantaged regions. According to the US Department of Health and Human Services, people on Medicaid are more likely to be prescribed opioids, at higher doses, and for longer durations—increasing their risk for addiction and its associated consequences. They are also less likely to have access to evidence-based addiction treatment. But psychological factors also play a role.
Last year, economists Anne Case and Angus Deaton attributed much of the increased mortality among middle-aged white Americans to direct and indirect health effects of substance use especially among those with less education, who have faced increasing economic challenges and increased psychological stress as a result.
Environmental and social stresses are an important predictor of many mental disorders, and decades of research using animal models have told us a great deal about how such stresses increase risk for substance use and even make the brain more prone to addiction. Among the best-known animal models of environmental stress and addiction risk are those involving social exclusion and isolation: Solitary animals show greater opioid self-administration than animals housed together, for example—a finding originally made famous by the “Rat Park” experiment of Bruce K. Alexander in the 1970s and replicated by other researchers.
Even more pertinent to the question of how low social status might affect addiction risk is research by Michael Nader, who showed that male monkeys who are dominant in their social group demonstrate less cocaine self-administration than lower-ranked (subordinate) animals or solitary ones. Some evidence point to brain circuitry in the insula (a region important in processing social emotions) that may link feelings of social exclusion to increased drug craving, as well as possibly altered dopamine-receptor availability in the striatum (part of the reward circuit) depending on social status.
The relationship may be bidirectional—exclusion not only increases risk for using drugs but increased drug use can increase social isolation further, in a vicious cycle. By the same token, when people have strong family or community relationships, these can act as protective factors against the risk of becoming addicted, and can facilitate recovery among those striving to achieve it.
Another animal model of environmental stress is an environment without opportunities for play, exploration, and exercise. Rodents housed in unenriched environments have been shown to be more sensitive to the rewarding effects of heroin compared to those in more enriched environments. A team of researchers at the University of Texas Medical Branch in Galveston recently explored the molecular mechanisms that mediate the protective effects of enriched environments. They analyzed the transcriptome—or the parts of the genome that are expressed—in the nucleus accumbens (part of the reward circuit) following cocaine exposure in animals raised in either enriched or dull environments. They identified a number of molecules and signaling pathways, including a pathway involving retinoic acid (a product of Vitamin A metabolism), that may underlie the effects of an enriched environment on the brain’s processing of reward.
The researchers suggest that the mild stressors and surmountable challenges presented by an enriched environment act to “inoculate” against stress, making individuals in those environments more resilient.
Although highly simplified, animal models of social and environmental stress can tell us a great deal about how stressful human environments may act as risk factors for substance use and other adverse outcomes and, conversely, how socially supportive and rewarding environments may offer protection. Prevention efforts targeting some of the environmental determinants of substance use, especially in young people, have already shown great success by applying the principles of boosting social support and creating the human equivalent of “enriched environments.” For example, a primary prevention model implemented in Iceland drastically reduced teen substance use in that country by increasing parental involvement and youth participation in team sports.
Blame for the opioid crisis now claiming 91 lives every day is often placed on the supply side: overprescription of opioid pain relievers and the influx of cheap, high-quality heroin and powerful synthetics like fentanyl, which undoubtedly have played a major role. But we cannot hope to abate the evolving crisis without also addressing the lost hope and opportunities that have intensified the demand for drugs among those who have faced loss of jobs and homes due to economic downturns. Reversing the opioid crisis and preventing future drug crises of this scope will require addressing the economic disparities, housing instability, poor education quality, and lack of access to quality health care (including evidence-based treatment) that currently plague many of America’s disadvantaged individuals, families, and communities.