Researchers have identified the mechanism by which marijuana may affect learning and memory

Marijuana use has long been known to cause problems with learning and memory. Now, researchers at the Albert Einstein College of Medicine of Yeshiva University have identified the mechanism by which marijuana may affect activity in the hippocampus, the portion of the brain responsible for learning and memory. They reported their findings in the September 16 issue of the journal Neuron.

The active chemical in marijuana, THC, is also present in marijuana-like molecules called endocannabinoids that occur naturally in our brains, explains Dr. Pablo Castillo, associate professor of neuroscience at Einstein. These naturally occurring endocannabinoids are neurotransmitters that send signals between neurons in the brain, permitting the intake and storage of information that is critical for effective learning and memory. This communication process between neurons is referred to as synaptic plasticity. (Synapses are the gaps between nerve cells into which one nerve cell secretes a neurotransmitter that stimulates the adjacent nerve cell; and synaptic plasticity refers to the ability of these impulses to pass between neurons.)

But Dr. Castillo notes that when cannabinoids are introduced from outside the body through marijuana use, they serve to short circuit the internal system and impair synaptic plasticity in particular, thereby interfering with the way neurons signal one another, gather information, and store it.

“When a person smokes marijuana, an outside signal triggered by the THC is received in the brain,” he says. “But at the same time, the brain’s naturally occurring cannabinoids are also sending messages via neuronal synapses. These conflicting cannabinoid-triggered signals create confusion, interfering with normal brain function, synaptic plasticity and, ultimately, with the ability to learn.”

The researchers were surprised to find that endocannabinoid signaling could be confined to a local region of neurons. "Edocannabinoids did not operate in as diffuse a manner as we had imagined,” says Dr. Castillo. “Instead, they're released into specific synapses in the hippocampus, where endocannabinoid signaling plays a complex and important role in the storage of memories.

Through further study of the internal system, Dr. Castillo and his colleagues hope to gain understanding of how activation of the endogenous system occurs. “By learning how the naturally occurring cannabinoid system is activated,” says Dr. Castillo, “we can begin to develop a strategy for understanding the effects that the cannabinoids from marijuana smoking have on it. Only then can we think about developing agents for blocking their adverse effects.”