Marijuana’s Effects on the Brain: A Scientific Look At Cannabis and the Brain

 

Marijuana is becoming a more socially acceptable psychoactive substance, and coincidentally, a more abused substance. The other most popular abused psychoactive substances are caffeine, alcohol, and nicotine. Marijuana, also known as cannabis, has been climbing in popularity, but what are the benefits and consequences that can arise from recreational usage? Are they similar to medical implications?

Marijuana's effects on the brain

Marijuana’s effects on the brain

Marijuana’s effects on the brain

Your brain already naturally produces molecules that are chemically similar to marijuana, called endocannabinoids, or just cannabinoids. These substances work on the endogenous endocannabinoid system, which is a rather unique system of pathways that allows for the presynaptic neurons to be stimulated. This system governs over many crucial biological functions such as sleep/wake cycle, appetite, immune response, and pain regulation.

Normally, when discussing how neurons in the central nervous system interact with each other, we break down communication into a synapse consisting of a pre and a postsynaptic neuron. The presynaptic neuron releases neurotransmitters into the synaptic cleft, the space between the two neurons. The postsynaptic neuron has receptor sites for these neurotransmitters to bind. This in turn enacts changes in the postsynaptic neuron that allows communication to continue downstream to the targeted areas.

Types of neurotransmitters

The endocannabinoid system does not strictly stick to this model of functioning. Instead, when the postsynaptic neuron is stimulated it releases endocannabinoids which travel backwards towards the presynaptic neuron. It is there that the endocannabinoids bind.

Why Does it Matter Where the Cannabinoids Bind?

Since the endocannabinoids bind on the presynaptic neuron, this enables the neurotransmitter to impact what happens to the neuron when it is next stimulated.

The presynaptic neuron has specific receptor sites for endocannabinoids, and when bound, these neurotransmitters change how many calcium channels are open on the presynaptic neuron. Calcium channels, found in the presynaptic membrane, are what control neurotransmitter release from the presynaptic neuron. By decreasing the number of open calcium channels, endocannabinoids decrease the amount of other neurotransmitters that can be expelled from the the presynaptic neuron.

This has an inhibitory effect on the other neurons downstream of the endocannabinoid bound neuron. Since less neurotransmitter is being released, this decreases the frequency of the firing potential of the postsynaptic neuron.

Understanding how the system works is important in understanding how the different chemical compounds of marijuana impact the brain.

What Makes Up Marijuana?

There are three main cannabinoids that are found in marijuana: THC, CBN, and CBD. It is important to note that when these cannabinoids enter the body, they are referred to as exogenous cannabinoids because they originated outside of the body. Endocannabinoids, as described earlier, pertain to the cannabinoids that your body naturally produces.

THC is most likely the compound that you have heard of before. Delta9-tetrahydrocannabinol (THC) is the psychoactive ingredient in marijuana and has a chemical compound very closely related to anandamide, an endocannabinoid. When THC enters the body it binds to anandamide receptors (CB1), found through out the brain, and in turn propagates a heightened level of effects as compared to anandamide.

In addition to interacting with the endocannabinoid system, THC also acts on the mesolimbic dopamine pathway. The mesolimbic pathway is integral to reward processing. Under normal conditions, with the absence of THC, dopaminergic neurons are phasically active. In other words, these neurons fire action potentials that result in dopamine release from the ventral trigeminal area in the midbrain and act on the nucleus accumbens in the basal ganglia, in response to normal rewarding behaviors. The nucleus accumbens then projects excitatory signals to the amygdala and prefrontal cortex, which creates positive feelings and in turn motivates the behaviors. The lack of dopamine release would subsequently deter the engagement in non-rewarding behaviors.

See more about the parts of the brain

When this pathway is active in the presence THC, levels of dopamine are increased by a large dump of the neurotransmitter from a presynaptic neuron onto a post synaptic neuron. Dopamine released from the ventral trigeminal area act on the nucleus accumbens, the heavy saturation caused by stimulation from THC allows neurons in the nucleus accumbens to continue to be excited.

The second important compound in marijuana is CBN, also known as cannabinol. It is also psychoactive, but much less so than THC. In fact, CBN is oxidized version of THC. CBN binds to two different types of cannabinoid receptors, CB1 and CB2. CB1 is most prominent in the brain, and CB2 is found in the peripheral nervous system.

The third compound is CBD, or cannabidiol. CBD is a non-psychoactive component of marijuana and does little to interfere with normal brain functioning. CBD does not actually bind to any receptor in the brain, instead it inhibits enzyme breakdown of anandamide.

Marijuana’s Effects on The Brain: Recreational vs. Medical Usage

Marijuana's effects on the brain

Marijuana’s effects on the brain

One of the main differences between recreational usage and medical usage of marijuana is the various concentrations of the three main components highlighted previously.

Most recreational users want a high concentration of THC, so that they can experienced the heightened psychoactive effects. However, the larger amount of THC present in marijuana, the more likely dependence and addiction will occur. This is due to THC’s relationship with dopamine. As mentioned previously, THC enacts on the mesolimbic dopamine system. Prolonged use to achieve the euphoric satisfaction that dopamine enables eventually alters the brain’s biological functions. Subsequently, neural circuits change in order to compensate and maintain normal functioning when exposed to dopamine. This change is what causes dependency on the drug.

What are the functions of dopamine?

When observing marijuana’s medical usages, it is not a surprise that it is implemented to treat a variety of ailments. This is due to the extensive reach of the endocannabinoid system over biological functions. However, a higher quantity of CBD is common among most medical strands of cannibis. By increasing CBD, common unwanted side effects of medical marijuana are reduced. CBD and THC work together in these cases.

Marijuana’s Effects on the Brain: Examples of Medical Usage

Marijuana's effects on the brain

Marijuana’s effects on the brain

Neurobiological diseases related to aging encapsulate one area where medical marijuana has shown improvements in functionality. In many of these diseases, such as Alzheimer’s and Parkinson’s, a lack of dopamine in the brain is seen as a cause.

Parkinson’s disease in particular is caused by a the brain’s inability to produce as much dopamine as it needs. This is often concentrated in a particular area of the brain called the substantia nigra. This area of the brain is responsible for both reward and movement, and is an integral part of the dopaminergic system. The common pharmaceutical treatment for this disease is L-DOPA, which is a precursor to dopamine, that the body can then convert into dopamine. By doing this, the symptomatic tremors associated with the disease are greatly reduced. However, this drug loses it’s effectiveness overtime.

Medical marijuana may be an alternative to L-DOPA and other treatments for Parkinson’s disease. Instead of converting L-DOPA to dopamine, marijuana acts on the dopaminergic system and essentially dumps dopamine into synaptic clefts to saturate postsynaptic neurons. When this activity is seen in the substantia nigra, tremors and other forms of dyskinesia, usually subside. However, muscular movement abnormalities are only one symptom of Parkinson’s disease, and often patients experience executive functioning impairments as well. For an individual that experiences these symptoms, it is not wise to medicate with a substance that will further impair mental functioning.

Taking advantage of marijuana’s ability to have a wide cast effect on the body can be beneficial when trying to treat over-arching generalized symptoms, or more specific symptoms. Although, its ability to have this wide effect due to the engagement with the endogenous endocannabinoid system also can increase likelihood for multiple areas of dependence, whether used medically or recreationally. More research should be done to be able to get a better picture of what long term effects marijuana has on the brain.

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Jacquelyn is currently an undergraduate student at the University of Pittsburgh. She is studying both Neuroscience and Psychology, and earning a minor in Chemistry. Jacquelyn is particularly interested in neuromuscular research and neurobiological diseases related to aging, and hopes to apply her passions to future functional neuroscience research.