Denise Cummins Ph.D.

Good Thinking


What Horses And Genes Are Teaching Us About Addiction

Genes have been identified that increase risk of addiction to marijuana.

Posted Apr 08, 2016

In two previous blog posts, I discussed what we currently understand about addiction. You can read them here and here.

More recently, a study reported in the March 30, 2016 issue of the Journal of the American Medical Association-Psychiatry  has identified three genes that increase risk of addiction to cannabis (marijuana). These genes also put a person at greater risk for major depressive disorder and schizophrenia.

The research was led by Dr. Joel Gelertner, a professor of psychiatry and genetics at Yale University. Gelernter and his team analyzed the genes of nearly 15,000 people from three independent studies. Between 18 and 36 percent of the sample were addicted to cannabis according to criteria specified by the DSM-IV. The three genes identified were rs143244591, rs146091982, and rs77378271. They impact central nervous system development, and are crucial in maintaining stable calcium levels in neurons.

Notice that the researchers state these genes put one at risk for marijuana addiction, not that they doom a person to marijuana addiction. The question is what other factors must be present in order for these genetic predispositions to manifest.

The prevailing belief among Americans is that addiction is a moral failing. For example, the Johns Hopkins Medical School recently surveyed attitudes toward mental illness and addiction among 709 participants from all points in the American demographic spectrum. The researchers summarized the results this way:  

While drug addiction and mental illness are both chronic, treatable health conditions, the American public is more likely to think of addiction as a moral failing than a medical condition. In recent years, it has become more socially acceptable to talk publicly about one's struggles with mental illness. But with addiction, the feeling is that the addict is a bad or weak person, especially because much drug use is illegal.

The medical community, on the other hand, takes a very different view of addiction, namely, the brain disease model of addiction. According to this model, addiction is characterized by altered brain structure and functioning. These alterations occur when people with particular genetic make-ups are exposed to potentially addictive substances in the presence of environmental triggers. This model is strongly supported by scientific evidence. In a Lancet commentary, NIAAA Director Dr. George Koob stated 

Addiction is a complex disease of a complex brain; ignoring this fact will only hamper our efforts to find effective solutions through a comprehensive and systematic understanding of underlying phenomena.

Despite the scientific evidence and treatment outcomes supporting this model, it continues to be questioned. According to a recent review article published in the ​New England Journal of Medicine, people resist this model because it

challenges deeply ingrained values about self-determination and personal responsibility that frame drug use as a voluntary, hedonistic act. In this view, addiction results from the repetition of voluntary behaviors. How, then, can it be the result of a disease process? The concept of addiction as a brain disease has even more disconcerting implications for public attitudes and policies toward the addict. This concept of addiction appears to some to excuse personal irresponsibility and criminal acts instead of punishing harmful and often illegal behaviors.

So let's take a different approach to the question by looking at addiction of a different sort in a different species: Cribbing in horses.

Cribbing is a behavior in which the horse grabs a solid object (the stall door or fence rail) with its front teeth, then arches his neck, pulls against the object, and sucks in air. Cribbers will do this for hours on end, sometimes even ignoring opportunities to eat in order to crib. This is what it looks like.

Why do they do it? Cribbing slows the horse's heart rate, lowers cortisol, a hormone associated with stress, and most importantly, releases endorphins in the horse's brain, which may produce a feeling of pleasure (like a runner's high). Cribbing also stimulates a release of dopamine, the "feel good hormone", to levels three times higher than normal, thus rewarding the horse for the behavior.  Horses become addicted to cribbing because it alters the structure and functioning of their brain's reward circuitry.

I think most people would agree that it is ridiculous to attribute this kind of equine addiction to a horse's "bad moral character" and a refusal to "exhibit voluntary control." Instead, the most accepted explanation—and the one with the most scientific support—is that two factors must be present in order for a horse to become a cribber:

The horse must have a genetic predisposition to cribbing AND certain environmental triggers must be present.

The first clue comes from the fact that cribbing is not seen in free-ranging horses, which suggests that the way domesticated horses are kept and cared for may cause the behavior. The second is that, contrary to common wisdom, horses don't learn to crib simply by watching other horses do it. There is definitely a genetic component: A study involving 396 horses in Finland estimated the heritability of cribbing to be 0.68 (on a range of 0 to 1.0), which means that cribbing is likely to be passed onto offspring. And cribbing tends to manifest early in a horse's development, often within the first year of life.

The third is that while there is no simple cure for the addiction, the best treatment for reducing the behavior is to improve the afflicted horse's living conditions. In the wild, horses live in small herds and spend most of their time grazing in each other's company, grooming each other, and migrating from place to place. In contrast, domesticated horses often spend most of their time in 10'X12' box stalls, have minimal contact with other horses, are fed diets high in grain-based carbohydrates, and (too often) allowed virtually no time grazing or moving freely in open pastures. Research has repeatedly shown that the best ways to prevent and treat cribbing is re-create the living environment in which horses typically thrive. This means making forage (hay or pasture) the mainstay of a horse's diet, allowing daily freedom of movement, and most importantly, and to allow daily interaction with other horses. This type of horse keeping not only reduces the risk of cribbing, it also yields calmer behavior. Implementing these changes lowers cortisol levels and restores balance to the horse's brain reward circuitry.

Does this apply to humans?

There are four reasons to believe so. The first is that the brain's reward circuitry is virtually identical across mammalian species. The second is that genetic predispositions have indeed been implicated in the expression of addictive behavior. The third is that addiction risk is highest in the developing brain. Studies have shown that children and adolescents with evidence of structural or functional changes in frontal cortical regions at greater risk for substance-use disorders.

The fourth and most important reason is the most effective behavioral interventions to addiction are those that help restore balance in brain circuitry that has been affected by drugs. These include strategies that enhance the salience of natural, healthy rewards such as social contact and exercise, those that mitigate a person’s stress reactivity, and strategies that help patients recovering from addiction to avoid social environments that include drug-associated cues and triggers.

In contrast, centuries of efforts to curb addictive behavior by punishing it has proven no more affective than curbing cribbing behavior by punishing the horses who do it. When a living being has come under this kind of stimulus control, the most effective strategy is to change the behavioral choices available to them by changing their social environments, and these interventions are particularly important during childhood and adolescence.

Copyright Dr. Denise Cummins April 8, 2016

Dr. Cummins is a research psychologist, a Fellow of the Association for Psychological Science, and the author of Good Thinking: Seven Powerful Ideas That Influence the Way We Think.

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