Pregnancy and weed probably shouldn’t go together, new research shows.

Baby rats! Awww!
Image credits Karsten Paulick.

Researchers from the Washington State University (WSU) found that heavy cannabis exposure during pregnancy can lead to cognitive changes in the offspring of rats. Cannabis is the most commonly used illicit substance among pregnant women and may have similar effects when used by human mothers.

Pass on the puff, puff

“Prenatal exposure to cannabis may cause meaningful changes in brain development that can negatively impact cognitive functioning into adulthood,” the authors wrote in a summary of the work presented yesterday at the Society for Neuroscience’s annual meeting in San Diego.

For the study, McLaughlin — an assistant professor of Integrative Physiology and Neuroscience at the WSU — and his team exposed pregnant rats (dams) to various concentrations of cannabis vapor. This method was selected as it better recreates how people most often use the drug. The team then documented how these rats’ offspring performed in a lab test that required learning, and later adjusting, a strategy to get sugar rewards.

The vapor was administered in atmospherically controlled cages during two hour-long sessions each day. The treatment started from before pregnancy and carried until they gave birth. Rats in the control group received cannabis-free vapor, while other groups received vapor with low to high levels of cannabis. The treatment was designed to raise the rats’ blood THC levels to that of a person who has had a few puffs of the drug, the team notes.

Roughly 60 offspring of these rats were then submitted to a task similar to the Wisconsin Card Sorting Test (WCST). The WCST is a method used to test a person’s flexibility when the stimulus for positive reinforcement changes. The rats were trained to press one of two levers. These levers were tied to lights, and the rats learned that they’d receive a treat when pressing the lever close to the shining light. After this, however, the team shook things up: the reward was assigned to one of the levers permanently, regardless of which light was shining.

Cognitive changes

The rats who were exposed to cannabis in utero (in the womb) had no difficulties learning the first rule, the team reports. However, those who were exposed to higher concentrations “showed marked deficits in their ability to shift strategies when the new rule was implemented,” the researchers add.

This doesn’t mean they were unable to learn the new strategy, mind you — these rats (from dams exposed to high levels of cannabis) appeared to understand the change, as they pressed the correct lever several times in a row. But they simply wouldn’t hold to it. They would give up before striking the right lever ten times — which is what the offspring of dams exposed to less or no cannabis did.

“The general take-home message is that we see deficits, particularly in the domain of cognitive flexibility, in rats prenatally exposed to high doses of cannabis vapor,” McLaughlin said. “The impairment is not a general learning deficit, as they can learn the initial rule just fine.”

“The deficit only emerges when the learned strategy is no longer resulting in reward delivery. They cannot seem to adapt properly and tend to commit more regressive errors as a result, which suggests impairment in maintaining the new optimal strategy.”

McLaughlin cautions against jumping to conclusions, however. He says that high-exposure rats aren’t necessarily less intelligent, just less motivated. They might not be very interested in the task itself, the sugar reward, or they’d simply rather explore other activities during the test.

“They don’t have these opinions about how they need to perform because they don’t want to be perceived as ‘the stupid rat,'” he said. “Clearly that’s not what’s motivating their behavior. They’re just going to try to get as many sugar pellets as they can.”

“But at some point, do sugar pellets continue to motivate your behavior after you’ve eaten 100? Do you still care as much about them?”

The findings are still preliminary, and the team has a lot of work ahead of them. Among others, they plan to look for differences in gene expression and protein levels in the brain to determine why the rats’ behavior changed.

The findings have been presented at the Society for Neuroscience’s annual meeting Neuroscience 2018 in San Diego.