Marijuana has always been a popular narcotic throughout the years, increasingly so as legalization occurs more and more in several locations around the world. Marijuana is used for both medicinal and recreational purposes, and therefore a lot of research and trials have been produced surrounding neurobiology. Although we mostly have an understanding on how it affects our brain and its functions, the International Institute of Information Technology in India conducted an experiment that specifically studied how long-term cannabis usage affected particular regions in the hippocampus.

The hippocampus is a tiny organ in the brain primarily responsible for memory storage and managing, as well as regulating emotions. For further clarification, think of this process as a shipping center in which the hippocampus receives information (as a shipping center would receive a package), registers and processes the information (a shipping center would identify where the package is supposed to go) and eventually transferring information to store in long-term memory (shipping the package to the costumer). The hippocampus also possesses the highest amount of cannabinoid receptors (what the THC in cannabis binds to in order to alter your state of mind, causing that “high” feeling) throughout the entire brain, and therefore extremely vulnerable to cannabis.

The experiment consisted of 17 regular cannabis users and 17 healthy, non-users both undergoing magnetic resonance imaging (MRI) scans, once to form a baseline at the beginning of the study and one three years later as a follow up to observe the subsequent changes. The results indicated that cannabis users had increased volumes, but only in specific regions of the hippocampus. These regions include both CA3 and CA4 in the right side of the brain, and maintaining our shipping center analogy, would be sections within the center where specific packages are stored and eventually transferred. The subiculum in both hemispheres also showed a decrease in volume, which outputs information from CA regions- the shipping truck that carries the packages out of the facility.

The non-cannabis users experienced lower growth rates in those regions, leading to a hypothesis in which regular cannabis usage may delay volume reduction in certain regions of the hippocampus.

Reference: Garimella, A., Rajguru, S., Singla, U. K., & Alluri, V. (2020). Marijuana and the hippocampus: A longitudinal study on the effects of marijuana on hippocampal subfields. Progress in Neuro-Psychopharmacology and Biological Psychiatry, 101, 109897.