Marijuana hampers short-term memory by activating a signalling pathway between neurons and non-neuronal cells called astrocytes, work in mice suggests. This is surprising because astrocytes have mostly been considered to be just support cells.

Giovanni Marsicano at the French National Institute of Health and Medical Research in Bordeaux, Xia Zhang of the University of Ottawa in Canada and their colleagues found that marijuana's psychoactive ingredient, tetrahydrocannabinol (THC), weakens the connections, or synapses, between neurons in the hippocampus — a brain region crucial for memory formation. They tested the effects of THC on mice engineered so that either their neurons or their astrocytes lacked the CB1 receptor that THC acts on. In mice missing the astrocyte receptors, THC had no effect on hippocampal synapses or working memory, as shown in a behavioural test.

The authors propose that the activation of astrocyte CB1 receptors by THC causes receptors for a compound called AMPA to be removed from the membranes of neurons, which is known to weaken synapses.

Cell 148, 1039–1050 (2012)