CU Boulder scientists estimate the heritability of opioid use disorder with a rodent study

Opioid use disorder is an ongoing global health crisis. In the United States alone, almost 108,000 people died from drug overdose in 2022, and about 75% of those deaths involved opioids.

Although many factors contribute to this crisis—and there are many approaches to addressing it as a result—one important line of research is into the genetic factors that increase people’s risk for developing an opioid use disorder (OUD). Once these risk factors are known, doctors may be able to prescribe opioids more strategically to people at higher risk of OUD, and such individuals could make more informed choices.

In recently published research, scientists from the University of Colorado Boulder—including Eamonn Duffy, Jack Ward, Luanne Hale, Kyle Brown and Ryan Bachtell of the Bachtell Laboratory, and Andrew Kwilasz, Erika Mehrhoff, Laura Saba and Marissa Ehringer—tested the influence of genetics on opioid-related behaviors, which include OUD. Specifically, they looked at its heritability by conducting an experiment in which rats were given the ability to self-administer oxycodone, a semi-synthetic opioid that is used medically to treat pain.

 

CU Boulder researchers tested the influence of genetics on opioid-related behaviors, specifically looking at its heritability by conducting an experiment in which rats were given the ability to self-administer oxycodone, a semi-synthetic opioid that is used medically to treat pain. (Photo: Jon Anders Wiken/Dreamstime.com)

Experimental design

More than 260 inbred rats from 15 strains were used for the study. In this case, an inbred strain is defined as a population produced by 20 or more generations of brother-sister mating. This was important for the study because the rats within inbred strain are isogenic: “They’re almost like clones; their genomes are identical, except for the X and Y chromosomes between males and females,” Duffy explains.

Like the use of identical-twin research involving humans, this makes the results more reliable. In a twin study, most differences between twins are caused by their environment, so researchers can determine the genetic influence on a trait by how much it varies. Similarly, within an inbred strain, most individual differences are caused by sex differences, and this provides insight into the importance of biological sex to a given trait. Between inbred strains, differences are attributable to either the strains’ different genes, sex differences, or a combination of the two.

The animals in the study could self-administer the oxycodone using levers, so their behaviors could be measured. There were two retractable levers in the testing chamber: one active, which would give the rats a dose of oxycodone after being pulled, and one inactive, which would do nothing.

After the active lever was pulled, there was a cooldown period of 20 seconds, during which time pulling the lever would not dispense another dose. Regardless of whether pulling a lever had an effect, it would be recorded. This allowed researchers to measure two substance-use behaviors in addition to the total amount of oxycodone consumed. These variables were referred to as “timeout responding” and “lever discrimination.”

Timeout responses were pulls on the active lever that happened during the cooldown period. Lever discrimination was a measure of how often rats pulled the inactive lever. Both essentially tracked the rats’ ability to self-administer substances in a regulated manner, although lever discrimination could have other associations. Attempting to get more oxycodone very quickly (timeout responding) and attempting to get it in an illogical way (low lever discrimination, especially once the animals had time to learn how the levers worked) are signs of dysregulated drug use.

These measures are important in addition to total dosage because the rats naturally consumed more oxycodone as they developed a tolerance to the drug, making it difficult to characterize their drug use on that basis alone. “With addiction,” Duffy says, “it’s a complicated story. They’re developing tolerance, and they’re showing dysregulated use.”

Push the lever, get the oxycodone

The tests were split into two phases: acquisition and escalation. Although the number of daily doses the rats received generally increased over time, especially between the two phases, their self-administration behaviors varied significantly by strain.

For example, in the escalation phase, the females of one strain pushed the lever for a total oxycodone dose of less than 100 mg/kg, whereas rats of another strain took a total of about 300. There was also variation between males and females within a strain, though not always: In some strains, males and females consumed a similar amount of oxycodone, while in others, consumption was notably divergent, with males consuming around 200 mg/kg more oxycodone overall.

 

Once the genetic factors that increase people's risk for developing an opioid use disorder (OUD) are known, doctors may be able to prescribe opioids more strategically to people at higher risk of OUD, and such individuals could make more informed choices. (Photo illustration: iStock)

This is evidence for a strain-sex interaction, meaning that the rats’ substance-use behaviors were determined by a combination of genetic background and biological sex, not either alone, according to the researchers. Although the obvious explanation for this would be different genes encoded on the sex chromosomes of the various strains, this isn’t necessarily the case.

“Some of our collaborators in San Diego have performed several genetic mapping studies,” Duffy says, “and they found that the Y chromosome didn’t appear to play much of a role in regulating behavioral traits.”

It is possible that X-chromosome genes are a greater factor. However, the biggest influence would probably be sex hormones or related differences, Duffy adds. For example, according a separate study, the sex hormone estradiol can increase oxycodone metabolism indirectly by raising the concentration of a protein in the brain.

Moreover, Duffy says, “there could be developmental aspects to the sex difference, so seeing if they’re exposed to testosterone versus estrogen as they’re growing up, that may affect how their brain is wired.”

Several other strains showed notably divergent behaviors. Some strains were fairly stable in their use, while others increased their oxycodone intake rapidly during the acquisition phase. Lever discrimination also varied by strain, with one strain increasing its lever discrimination quickly, for example, while another failed to increase its lever discrimination much over time.

The biggest discovery that emerged from the research was the discovery of how heritable several behaviors related to opioid use are.

The influence of genetics

Heritability is a measure of what part of the variation in a group is due to genetic or heritable characteristics.

“With heritability,” Duffy explains, “when you’re looking at everything that goes into some kind of trait, like opioid use disorder, the average genetic component will be your heritability. You also have environmental influences, which could be things such as diet.”

Taking OUC as an example, variation might be understood qualitatively in terms of how destructive the effects of drug use are on individuals, from having minimal effect on people’s lives to potentially causing overdoses and death, Duffy adds.

If the heritability of OUD were 0, the fact that some people use the drug safely and others die because of it would be explained entirely by non-genetic factors. If the heritability of OUD were 1, this fact would be explained entirely by genetics. However, as with most traits, OUD appears to be caused by a combination of genetic and environmental factors.

According to the study, measures of oxycodone intake ranged between 0.26 and 0.54 heritability. The high end of this range is total oxycodone intake over the course of the experiment, while the low end is change in intake (increase in intake over the acquisition phase). The other behavioral phenotypes had heritability scores of 0.25 to 0.42, with timeout responding being more heritable than lever discrimination.

“About half of that variability is due to genetic background,” Duffy says, referring to total intake. “That’s really strong heritability.” However, because these data come from rats, the heritability of these behavioral phenotypes may be different in humans. “We’re not going to capture everything about OUD in a rat model, but we can capture specific aspects and use that to put together a bigger picture.

“OUD is hard to study in humans because there aren’t as many people using opioids as alcohol or nicotine, and of that smaller population, we also have people using several types of drugs, so it’s harder to calculate these heritability values, but I believe ours do fall within the range for opioid dependence and opioid use disorder in humans.”

It's also important to recognize that heritability is a population-level statistic. This means that it does not represent the chance for any individual to develop a trait, even if that trait could be inherited from the individual’s parents. However, a higher heritability of some trait would correspond to a greater resemblance between parents and offspring in that respect throughout the population, Duffy says.

What genes contribute to OUD?

While it is useful to know how heritable opioid use disorder is, meaningfully assessing the risk for individuals requires knowing what genes contribute to it. This study doesn’t identify these genes, but progress has already been made to this end.

“There’ve been a number of studies in humans that have found that these SNPs, or single nucleotide polymorphisms, are associated with your risk of developing conditions like opioid dependence or opioid use disorder,” Duffy says. “There’s another group that is performing some genetic mapping in outbred rats, and that’s going to be the next stage of this project for us as well.”

One potential gene influencing OUD in mice is an SNP in the Oprm1 gene, which is explained in the study to affect the brain’s response to reward-related behavior generally and analgesics like oxycodone specifically. Common Oprm1 SNPs have also been associated with dysregulated use of an opioid in humans, specifically heroin.

Once relevant SNPs are identified, however, the situation remains complex. “It’s not going to be a simple answer,” Duffy says. “Like, you have this one SNP in Oprm1 and that’s going to increase or influence your risk for OUD. It’s probably going to be a multitude of SNPs, and those additive effects are going to influence the risk for this disorder.”

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