Experiencing social isolation early in development may increase later anxiety and preference for alcohol. A new study in rats shows that these early stressors physically change the brain’s response to alcohol, changing how it regulates the chemical dopamine, particularly in areas associated with reward processing. The results of this study were published in the journal Addiction Neuroscience.
As children and teens reach critical periods of brain development, social contact helps form neural circuits. Environmental stressors during this sensitive period can disrupt normal developmental trajectories. Experiencing isolation and neglect in early life can increase the risk of developing mood disorders and substance use problems in adulthood.
Researchers from Binghamton University and Brigham Young University wanted to understand the biological mechanisms behind this vulnerability. Lead author Gavin J. Vaughan and senior author Anushree N. Karkanis, both of Binghamton University, focused on a brain structure called the ventral pallidum.
The ventral globus pallidus is a collection of small cells located deep in the brain. It serves as a central hub for evaluating the value of different experiences. This area helps individuals weigh whether the stimulus is rewarding and worth pursuing, or aversive and worth avoiding.
Cells in the ventral pallidum receive constant chemical signals from other parts of the brain. One of the main messenger chemicals they receive is dopamine. Dopamine is a neurotransmitter that helps the brain recognize and learn from rewarding events.
The researchers designed an experiment to see how a lack of peer interaction at a young age affects the functioning of dopamine signals. They specifically wanted to see how early isolation changed the interaction between alcohol and brain tissue.
The research team used a laboratory rat model to closely study the effects of early childhood stress on development. They divided young Long-Evans rats into two different living situations. One group grew up alongside their peers in a standard communal housing arrangement.
The second group was kept completely alone for a period corresponding to human adolescence. After a six-week developmental period, the researchers tested the fully grown rats’ anxiety levels. They exposed both animals to a series of behavioral disturbances.
One such test was an open field assay in which animals were placed in a large enclosed arena. Rats are natural predators and tend to cling to the walls of their new environment and avoid exposed centers. Lonely rats were even more reluctant to leave their safe corners and step into the center of the arena.
Another test involved a high platform shaped like a cross. Two of the platform arms had high walls, while the other two were completely open and exposed to the bright light of the room. Isolated rats spent much less time exploring within the open arms and had a strong preference for enclosed spaces.
The third evaluation involved a long graded track divided into four different zones. The first section had a high protective wall, but the last section was completely unscreened. Isolated rats spent more time hiding in the first enclosed section than group-housed rats.
Across a variety of tests, socially isolated rats consistently showed heightened anxiety-related behaviors. Female rats in both isolated and group-housed categories tended to exhibit elevated overall anxious behavior compared to male rats. Researchers have confirmed that the stress of isolation in adolescents causes lasting psychological changes.
After establishing the behavioral differences, the researchers tested the animals’ natural preference for alcohol. Over an 8-week period, rats had ad libitum access to two beverage bottles in their cages. One bottle contained regular tap water and the other bottle contained a 20 percent alcohol solution.
The researchers used an intermittent access structure for the sobriety test to simulate binge-like consumption patterns. Animals were given alcohol solution for 24 hours at a time, followed by water only for 1 day. Socially isolated animals consistently showed higher relative preference for alcohol solutions than group-housed animals.
To see if this preference was forced, the team modified the experiment. They introduced quinine, a bitter-tasting compound, into the alcoholic solution in varying amounts. The goal was to measure aversive tolerant drinking behavior.
Aversive resistant consumption mimics the behavior seen in severe alcohol use disorder in humans, where a person continues to consume alcohol despite significant negative emotional or physical consequences. The researchers found that by adding bitterness, they were able to deter the behavior in both animals as well.
The isolated rats did not ingest as much bitter alcohol as the other rats. In all groups, male rats had a slightly higher tolerance for bitter tastes than female rats, but the overall reduction in alcohol consumption was consistent. Although early isolation induced a general preference for alcohol, it did not make the rats more tolerant of bitter tastes.
To see what was happening on a biological level, the researchers examined delicate slices of the subjects’ abdominal pallidum tissue. They used a laboratory technique called fast-scan cyclic voltammetry. This process utilizes small carbon fiber electrodes to measure rapid chemical changes.
The researchers suspended the brain slices in a bath of oxygenated liquid that mimics the natural environment of the skull. A weak electric current passed through tissue stimulates local neurons, prompting the release of dopamine. The probe read dopamine levels in real time as the chemicals flooded the tissue and were removed.
Initially, the baseline release of dopamine in the ventral pallidum was identical in both groups of rats. Growing alone did not change the natural dormant state of this particular chemical pathway. The rate of dopamine release and absorption was unchanged.
The functional differences first became apparent when the researchers applied a liquid alcohol solution directly to the brain slices. In typical group-housed rats, exposure to alcohol clearly decreased the total amount of dopamine released in the ventral pallidum.
The brain tissue of socially isolated rats responded quite differently. In these animals, alcohol was much less effective at suppressing dopamine release. The developmental stress of growing up alone had fundamentally suppressed the brain’s typical chemical response to drugs.
The researchers also varied the electrical stimulation to mimic the rapid bursts of dopamine release that occur naturally when neurons fire in quick succession. They found that the effects of alcohol on these rapid bursts varied significantly depending on both the sex of the rat and its residential history.
In group-housed male rats, alcohol was successful in suppressing these rapid dopamine bursts. The depressant effect completely disappeared in isolated men. In contrast, alcohol had no effect on dopamine bursts in group-housed females, but actively suppressed dopamine bursts in isolated females.
These gender-based physical differences highlight the extreme difficulty of modeling human mental states in animals. Animal behavior can vary widely depending on the specific testing environment. The researchers note that laboratory animals are very sensitive to small changes, such as the brightness of the lighting in the evaluation room.
Measuring psychological concepts like anxiety and addiction requires relying on multiple, overlapping tests to get an accurate picture. Although this finding points to clear anatomical changes in the ventral pallidum, the precise molecular changes that cause these sex differences remain unclear. It is not entirely clear which cellular receptors are altered in response to early life stress.
Alcohol interacts with several signaling proteins in the brain, including receptors that respond to a chemical called acetylcholine. Researchers suspect that early stress may change the physical shape and amount of these specific receptors on the surface of dopamine cells.
The researchers now hope to identify the precise proteins that mediate this altered dopamine response. Precisely identifying these microscopic cellular targets is an early step toward developing targeted medicine. The ultimate goal is to find targeted therapies that can reduce alcohol use disorders caused by childhood adversity.
The study, titled “Adolescent social isolation is associated with changes in ethanol-induced dopamine regulation in the ventral pallidum,” was authored by Gavin J. Vaughan, Makenzie R. Lehr, Gina M. Magardino, Abigail M. Kelley, Michelle A. Chan, Madison C. Heitkamp, Jordan T. Yorgason, and Anushree N. Karkhanis.
