Like stressed-out humans, sometimes rats go looking for a little relief too — in the form of cannabis, a new study has found.

And the experiment could shed light on why some humans gravitate toward the popular recreational drug more than others, and how to prevent harm and excessive cannabis use, a Canadian co-author said.

The study, published in the scientific journal Neuropsychopharmacology, found that rats with a high level of corticosterone (the rat-equivalent of the human stress hormone, cortisol) were more likely to take a hit of vapourized cannabis than rats with lower levels. 

Matthew Hill, a neuroscientist at the University of Calgary and co-author of the study, called it the first of its kind to use a special vapour chamber apparatus (which allows rats to self-administer cannabis) to better understand predictive factors of cannabis use.

Led by Washington State University, the study tested the rats under various conditions to see what factors prompted individual rats to seek cannabis, Hill explained. Those traits included social behaviours, sex and cognition, among others.

When an individual rat wanted a hit, it would poke its nose into a hole in the chamber which would trigger a three-second puff of cannabis vapour.

This vapour chamber, developed in previous research, was a big advancement in the field, Hill said, because finding a way to model the “rewarding, addictive aspects of cannabis” in rodents had been a challenge for years.

“It’s allowed us to finally, actually start to break down factors like this that might relate to cannabis use patterns,” Hill said.

Using this method, the researchers observed the rats for an hour a day over the course of three weeks as they were given access to cannabis. The study found that the rats’ natural resting stress levels were one of the biggest predictors of whether or not they would access the drug.

Another predictor, the study found, was a weaker ability to handle tasks that tested their cognitive flexibility — in other words, their ability to adapt to a change in routine.

“Let’s say you always park your car … in the same parking lot, and every day you’re going to that parking lot over and over again. And then all of a sudden that parking lot shuts down … and now you have to move to a new parking lot.” Hill explained.

“So the process of shifting your behaviour requires you to both unlearn the original rule as well as learn the new rule.”

When researchers modelled this type of scenario on the rats, those that performed worse in flexibility tasks were also more likely to self-administer cannabis.

Margaret Haney, a professor of neurobiology at Columbia University and the director of the Cannabis Research Laboratory, said this study “significantly advances the field” by testing which individual behavioural and biological factors can predict whether individuals will seek out cannabis.

“Cannabis has long posed a challenge, as it is widely used by humans yet historically difficult to model reliably in animals,” said Haney, who was not involved in this particular study.

It also offers a roadmap for future studies that could ultimately reduce the risk of problematic cannabis use, she said.

Hill said he’s motivated to advance what he and his co-authors have learned from this research. After testing how cannabis use rises with higher levels of resting stress, he’ll aim next to investigate what happens when the animals are exposed to external stressors.