Brain Changes in Youth Who Use Substances: Cause or Effect?

A widely accepted assumption in the addiction field is that neuroanatomical changes observed in young people who use alcohol or other substances are largely the consequence of exposure to these substances.

But a new study suggests neuroanatomical features in children, including greater whole brain and cortical volumes, are evident before exposure to any substances.

Investigators, led by Alex P. Miller, PhD, assistant professor, Department of Psychiatry, Indiana University, Indianapolis, noted that the findings add to a growing body of work that suggests individual brain structure, along with environmental exposure and genetic risk, may influence risk for substance use disorder.

The findings were published online on December 30, 2024, in JAMA Network Open.

Neuroanatomy a Predisposing Risk Factor?

Earlier research showed that substance use is associated with lower gray matter volume, thinner cortex, and less white matter integrity. While it has been widely thought that these changes were induced by the use of alcohol or illicit drugs, recent longitudinal and genetic studies suggest that the neuroanatomical changes may also be predisposing risk factors for substance use.

To better understand the issue, investigators analyzed data on 9804 children (mean baseline age, 9.9 years; 53% men; 76% White) at 22 US sites enrolled in the Adolescent Brain Cognitive Development (ABCD) Study that’s examining brain and behavioral development from middle childhood to young adulthood.

Researchers collected information on the use of alcohol, nicotine, cannabis, and other illicit substances from in-person interviews at baseline and years 1, 2, and 3, as well as interim phone interviews at 6, 18, and 30 months. MRI scans provided extensive brain structural data, including global and regional cortical volume, thickness, surface area, sulcal depth, and subcortical volume.

Of the total, 3460 participants (35%) initiated substance use before age 15, with 90% reporting alcohol use initiation. There was considerable overlap between initiation of alcohol, nicotine, and cannabis.

Researchers tested whether baseline neuroanatomical variability was associated with any substance use initiation before or up to 3 years following initial neuroimaging scans. Study covariates included baseline age, sex, pubertal status, familial relationship (eg, sibling or twin), and prenatal substance exposures. Researchers didn’t control for sociodemographic characteristics as these could influence associations.

Significant Brain Differences

Compared with no substance use initiation, any substance use initiation was associated with larger global neuroanatomical indices, including whole brain (β = 0.05; P = 2.80 × 10⁻⁸), total intracranial (β = 0.04; P = 3.49 × 10⁻⁶), cortical (β = 0.05; P = 4.31 × 10⁻⁸), and subcortical volumes (β = 0.05; P = 4.39 × 10⁻⁸), as well as greater total cortical surface area (β = 0.04; P = 6.05 × 10⁻⁷).

The direction of associations between cortical thickness and substance use initiation was regionally specific; any substance use initiation was characterized by thinner cortex in all frontal regions (eg, rostral middle frontal gyrus, β = −0.03; P = 6.99 × 10⁻⁶), but thicker cortex in all other lobes. It was also associated with larger regional brain volumes, deeper regional sulci, and differences in regional cortical surface area.

The authors noted total cortical thickness peaks at age 1.7 years and steadily declines throughout life. By contrast, subcortical volumes peak at 14.4 years of age and generally remain stable before steep later life declines.

Secondary analyses compared initiation of the three most commonly used substances in early adolescence (alcohol, nicotine, and cannabis) with no substance use.

Findings for alcohol largely mirrored those for any substance use. However, the study uncovered additional significant associations, including greater left lateral occipital volume and bilateral para-hippocampal gyri cortical thickness and less bilateral superior frontal gyri cortical thickness.

Nicotine use was associated with lower right superior frontal gyrus volume and deeper left lateral orbitofrontal cortex sulci. And cannabis use was associated with thinner left precentral gyrus and lower right inferior parietal gyrus and right caudate volumes.

The authors noted results for nicotine and cannabis may not have had adequate statistical power, and small effects suggest these findings aren’t clinically informative for individuals. However, they wrote, “They do inform and challenge current theoretical models of addiction.”

Associations Precede Substance Use

A post hoc analysis further challenges current models of addiction. When researchers looked only at the 1203 youth who initiated substance use after the baseline neuroimaging session, they found most associations preceded substance use.

“That regional associations may precede substance use initiation, including less cortical thickness in the right rostral middle frontal gyrus, challenges predominant interpretations that these associations arise largely due to neurotoxic consequences of exposure and increases the plausibility that these features may, at least partially, reflect markers of predispositional risk,” wrote the authors.

A study limitation was that unmeasured confounders and undetected systemic differences in missing data may have influenced associations. Sociodemographic, environmental, and genetic variables that were not included as covariates are likely associated with both neuroanatomical variability and substance use initiation and may moderate associations between them, said the authors.

The ABCD Study provides “a robust and large database of longitudinal data” that goes beyond previous neuroimaging research “to understand the bidirectional relationship between brain structure and substance use,” Miller said in a press release.

“The hope is that these types of studies, in conjunction with other data on environmental exposures and genetic risk, could help change how we think about the development of substance use disorders and inform more accurate models of addiction moving forward,” Miller said.

Reevaluating Causal Assumptions

In an accompanying editorial, Felix Pichardo, MA, and Sylia Wilson, PhD, from the Institute of Child Development, University of Minnesota Twin Cities, Minneapolis, suggested that it may be time to “reevaluate the causal assumptions that underlie brain disease models of addiction” and the mechanisms by which it develops, persists, and becomes harmful.

Neurotoxic effects of substances are central to current brain disease models of addiction, wrote Pichardo and Wilson. “Substance exposure is thought to affect cortical and subcortical regions that support interrelated systems, resulting in desensitization of reward-related processing, increased stress that prompts cravings, negative emotions when cravings are unsated, and weakening of cognitive control abilities that leads to repeated returns to use.”

The editorial writers praised the ABCD Study for its large sample size for providing a level of precision, statistical accuracy, and ability to identify both larger and smaller effects, which are critical for addiction research.

Unlike most addiction research that relies on cross-sectional designs, the current study used longitudinal assessments, which is another of its strengths, they noted.

“Longitudinal study designs like in the ABCD Study are fundamental for establishing temporal ordering across constructs, which is important because establishing temporal precedence is a key step in determining causal links and underlying mechanisms.”

The inclusion of several genetically informative components, such as the family study design, nested twin subsamples, and DNA collection, “allows researchers to extend beyond temporal precedence toward increased causal inference and identification of mechanisms,” they added.

The study received support from the National Institutes of Health. Study authors and editorial writers had no relevant conflicts of interest.