Molecules produced by our brain which are similar to the active ingredient of cannabis play a key role in the brain's development, according to new research. The findings offer an explanation as to how smoking cannabis during pregnancy could affect the developing brain. The study, which was partly funded by the EU through the Sixth Framework Programme (FP6) and brought together scientists from around the world, is published in the latest edition of the journal Science. Our ability to think, move, remember and show emotions depends on our hundreds of billions of brain cells being connected to each other correctly. These connections are formed during a relatively short period of time while we are still in the womb. In this latest study, the scientists looked at the role in brain development of endogenous cannabinoids, 'the brain's own marijuana'. These molecules, which are produced naturally in the brain, attach themselves to the CB1 cannabinoid receptor on nerve cells, just like tetrahydrocannabinol (THC), the active ingredient in cannabis. Now the researchers have shown that endogenous cannabinoids play a significant role in ensuring the correct formation of connections between nerve cells during foetal development. The do this by guiding the long 'arms' (axons) of the nerve cells to establish the right connections. The researchers note that marijuana smoked during pregnancy could disrupt this process. 'Besides identifying a fundamental mechanism in brain development, our findings may provide new perspectives to identifying the molecular changes in the brains of individuals prenatally affected by maternal cannabis use,' said Dr Tibor Harkany of the Karolinska Institute in Sweden, who lead the research. 'This is of social impact given the continuous growing use of marijuana, the most common illicit drug, in our society.' Previous studies have shown that the children of mothers who smoked marijuana during pregnancy are at an increased risk of suffering from cognitive deficits, concentration disorders, hyperactivity and problems with social interactions.