Health and Medicine Neuroscience

Cannabis Scientifically Proven to Have Adverse Effects


The function of important mechanisms involved in the formation of neural circuits in the brain have recently been uncovered in a new study. It was also found that disruption of neural circuits within the cortex is caused by a psychoactive substance found in cannabis – delta-9-tetrahydrocannabinol (THC). Researchers believe the results may find application in the functional recovery of cases of dementia and brain injury. It also clarifies why cannabis may be harmful.

It is known that neural activity plays a significant role in the creation of neural circuits, yet little is known about the types of neural activities that are involved in this process. The only thing we for example know about projections from the thalamus to the cortex is that, as they develop, redundant projections are eliminated and only correct projections are left behind. This process is one of the most complex neural activities known.

Fumitaka Kimura, an associate professor at the Department of Molecular Neuroscience, Graduate School of Medicine, Osaka University, recently led a group of researchers that explained the contribution of several mechanisms used in the development of this neural circuit. At the same time, the study revealed scientific evidence that shows how cannabis intake causes trimming of neural connections, ultimately leading to a breakdown of neural circuits.

cannabis proof
Suppression of thalamocortical projection by chronic administration of Δ9-THC (cannabinoid, active ingredient of marijuana). Photomicrograph of cerebral cortex from transgenic mice expressing GFP in thalamocortical axons at postnatal day 7 (P7). (left) : Normal thalamocortical projections. In the middle layer (layer 4), blobs of GFP showing dense termination of thalamocortical axons can be seen (under number 1~5). (right): Thalamocortical projection at P7 from a mouse received chronic administration of Δ9-THC (P2~7). Massive retraction of thalamocortical projections including middle layer (layer 4) can be observed. Image credit: Osaka University

Synaptic strength is a functional measure of connections between neurons and Spike Timing-Dependent Plasticity (STDP) is a rule that is used to determine synaptic strength. The research team discovered that in a different section of the cortex, the STDP suddenly changed at a certain point in development. Using this finding as a basis, the group examined whether the same occurred in the projection from the thalamus to the cortex. Initially, the synapses were reinforced due to the coordinated activities of the pre- (thalamic) and post- (cortical) synaptic neurons. After the projections had spread widely however, the coordinated activities weakened in most synapses, thereby removing redundant projections to enable the ones that are more systematic. As the synapses weaken, neural cells release endogenous cannabinoid through these synchronized activities. This leads to a deterioration of neuron projections that are not required. The study also found that when cannabinoid was taken externally, the same deterioration takes place.

The findings may help to decrease abuse of marijuana as scientific evidence is now available that show the adverse effects of cannabis consumption on brain development. These findings are likely to have an impact on further research aimed at improving our understanding of the mechanisms involved in the formation of neural circuits. It also has the potential to aid in the development of new therapies to improve recovery from dementia and brain damage.

This research was published online in the Journal of Neuroscience.