02 May How Marijuana Impairs Memory
Memory is the general term used to describe the ability to store information about people, facts and events, and then recall that information at a later point in time. In order to make memories, the brain relies on lasting connections between nerve cells called neurons. The active ingredient in marijuana, called tetrahydrocannabinol (THC), can disrupt the neuron connections required to form short-term memories. However, much of this disruptive ability is unrelated to THC’s direct effects on neurons; instead, it comes from THC’s effects on other cells in the brain called astrocytes, which play a supporting role in several key aspects of brain health.
Memory storage and recall inside the brain are the result of an interactive process between neurons that doctors and researchers refer to as “synaptic plasticity.” Generally speaking, when notable events occur, synaptic plasticity allows neighboring neurons to change their relationships with each other and form unique electrical and chemical connections that represent those occurrences as memories. Some neurons form long-lasting relationships with each other, and therefore form long-term memories that remain stable over extended periods of time. Other neurons form relatively short-term relationships with each other, and therefore form short-term memories that get discarded (or, more accurately, disconnected) when they no longer serve a significant purpose. The portion of the brain with chief responsibility for memory formation, organization and storage is called the hippocampus.
Astrocytes get their name because they have shapes that resemble crude depictions of stars. They belong to a larger group of brain cells called glial cells, and are also known as astroglial cells. Glial cells outnumber neurons in the brain by anywhere from 10 to 50 to 1, and provide the support services required to make the neurons work properly. Known services provided by astrocytes include physical support of the neurons, transportation of the nutrients required for ongoing neuron health, digestion of the leftover material from dead neurons, and disposal of loose cellular debris inside the brain. Astrocytes also help reinforce the integrity of the blood-brain barrier, a grouping of tiny blood vessels (capillaries) that separates the brain from the body’s main blood flow and helps the brain maintain a stable internal environment.
In order to produce the flexibility required for synaptic plasticity and the formation of new memories, the brain relies heavily on a neurotransmitting chemical called glutamate. Neurotransmitters get their name because they transmit chemical messages between neighboring neurons. In the case of glutamate, the message passed between neurons is a signal to increase the rate of internal neuron activity. Glutamate passes on this message by attaching itself at specific sites on the targeted neurons, called receptor sites, which allow the neurons to “hear” the incoming chemical broadcast.
When the THC from marijuana enters the brain, it alters normal function inside the hippocampus by essentially erasing some of the receptors required to receive glutamate messages, according to a study published in 2012 in the journal Cell. In turn, this receptor erasure triggers a process that leads to a change in synaptic plasticity and a degradation of the ability to properly store memories. In particular, the memories affected by this degradation are the short-term memories (also known as working memories) that keep a person anchored in his or her immediate reality and provide the basis for ongoing participation in everyday routines and events.
THC molecules achieve their damaging effects on glutamate receptors by attaching themselves to other receptor sites in the brain, called cannabinoid receptors. These receptors are found on both neurons and astrocytes. Previously, most researchers had assumed that THC’s impact comes from its attachment to cannabinoid receptors on the neurons, not cannabinoid receptors on the astrocytes. However, according to the authors of the study published in Cell, THC molecules that attach themselves to the brain’s neurons have essentially no effect on the stability of short-term memory. On the other hand, THC molecules that attach themselves to the brain’s astrocytes produce clear deficits in short-term memory formation and stability.
Other neurotransmitters inside the brain that may cause memory problems when adversely influenced by the THC content in marijuana include substances called dopamine and acetylcholine. Apart from the hippocampus, parts of the brain where THC-related memory disruption can occur include the prefrontal cortex-which plays a vital role in the formation of short-term memories-and the amygdala, which helps the brain store and recall emotion-based memories.
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