Researchers Find Cognitive Changes in Rats Exposed to Cannabis in Utero

Researchers at Washington State University have observed cognitive changes in the offspring of rats exposed to heavy amounts of cannabis. In a summary of the research presented at the annual meeting of the Society for Neuroscience on Sunday, WSU assistant professor of Integrative Physiology and Neuroscience Ryan McLaughlin and his colleagues wrote that prenatal cannabis exposure can have long-lasting effects.

“Prenatal exposure to cannabis may cause meaningful changes in brain development that can negatively impact cognitive functioning into adulthood,” the researchers wrote.

Two-Hour Vape Sesh

To conduct the study, researchers exposed pregnant rats, known as dams, to cannabis vapor in various concentrations for two hours per day in special cages. Some rats received no vapor to serve as a control, some received vapor with no cannabis, and some with low or high amounts of cannabis. The rats were exposed to the cannabis prior to getting pregnant and throughout the gestation period.

About 60 offspring of the dams in the study were then trained to press one of two levers to receive a reward. At first, the rats received a sugar pellet for pressing the lever located near a light. The following day, the rats received the reward when they pressed either the left or right lever, without regard to the light.

The researchers found that the rats exposed to cannabis learned the first rule without difficulty. But the group that had received high levels of cannabis in the vapor “showed marked deficits in their ability to shift strategies when the new rule was implemented,” they wrote.

It appeared that the rats from mothers that had been exposed to high levels of cannabis were not able to learn the new reward strategy. They would hit the correct lever several times in a row, but would not persist with the new strategy to hit the proper level ten consecutive times like the rats exposed to less or no cannabis.

“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 noted, however, that the rats exposed to high levels of cannabis were not necessarily less intelligent than the others. Instead, they could be less motivated to complete the task because of disinterest or a lower desire for sugar.

“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?”

Cannabis Use by Pregnant Women

McLaughlin noted that cannabis is the most commonly used illicit substance by pregnant women. He told High Times in an email that studies of cannabis-use by pregnant women are made difficult by other factors out of the control of researchers.

“There are certainly many studies that have examined effects of maternal cannabis-use in human users, although many of the cross-sectional studies are often confounded by other extraneous variables, such as the use of alcohol, nicotine, and/or other drugs, maternal nutrition, socioeconomic status, etc., which make it difficult to disentangle the effects of prenatal cannabis exposure from these other associated factors,” McLaughlin said.

McLaughlin added that research into other aspects of the long-term cognitive effects of prenatal cannabis exposure is continuing.

“There have been a couple of ongoing longitudinal studies that have followed cannabis-exposed offspring for many years, doing periodic assessment of other cognitive factors, but none to my knowledge have specifically looked at cognitive flexibility,” he said.