When newborn female rats are given a substance mimicking cannabis, their brains become more masculine – as does their behaviour.

Margaret McCarthy from the University of Maryland in Baltimore and colleagues found that newborn female rats usually make more new cells than males in a part of their brain called the amygdala, an area that governs social and emotional behaviour.

They also found that females had a smaller endocannabinoid system, involving brain receptors that react to cannabis. That correlation made them wonder whether injecting substances that mimicked cannabis would alter the rate of cell proliferation in the amygdala. To find out, the team injected newborn rats with a compound that triggers cannabinoid receptors in the brain. They also injected a chemical that allowed them to see cell division in brain tissue. To find out how these changes affected rats’ behaviour, the team also studied the playing habits of the pups after four weeks.

Without treatment, female rats produced between 30 and 50 per cent more glial cells – which help maintain homeostasis and protect neurons – in the amygdala than males. They also played 30 to 40 per cent less than males. But females that were given cannabinoid compounds had cell proliferation rates and play behaviour similar to those of males.


“Play behaviour is similarly sex-specific in humans,” says McCarthy. “The ultimate goal is now to find out whether the neurological underpinnings of this behaviour, which we are beginning to understand in this study, are similar in humans”.

Javier Fernández Ruiz from the Complutense University in Madrid, Spain, says the study is well-designed and convincing. But he emphasises that what it describes is a physiological process in the brains of rats, and that since the study did not use a plant-derived cannabinoid, or higher doses to reflect the fact that mouse metabolism is quicker than humans, no conclusions can be made about the effects of conventional cannabis on human babies.

Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1005003107