Education

The Dopaminergic System

The reinforcement and reward models of drug addiction have suggested that dopamine neurotransmission plays a key role in the motivation to use drugs. Dopamine appears to be at least partially responsible for the stimulus-reward association that is involved in addiction, and abnormal dopamine levels have even been correlated with schizophrenic and psychotic symptoms as well. 

Similar to cannabinoids and the endocannabinoid system, dopamine binds to dopaminergic receptors D2 and D3. Studies conducted in 2001 and 2011 indicated that chronic cannabis use may dull these receptors, but better designed, peer-reviewed studies conducted in 2008, 2011, 2012, 2013 and 2014 have failed to replicate these findings. There are no differences in D2 and D3 dopamine receptors in those who do and do not consume cannabis.

Studies

In numerous preclinical studies, dopamine and cannabinoids were found to have inconsistently reported interactions with one another. Five studies from 1991 and 1997 indicated an increase in dopamine levels following the administration of THC, but 17 further studies conducted in between 1998 and 2015 seemed to either find difficulty replicating the same results, or producing data that contradicts those initial studies, with Jentsch 1998’s work demonstrating decreased dopamine levels in chronic THC administration. It should be noted that many of these studies did not test THC itself, and instead used synthetic cannabinoids WIN55212-2 and HU-210, both of which have been found in the dangerous designer drug Spice, and CP55940, which was developed for the treatment of brain cancer. In human trials, no consistent relationship has been found between cannabinoids and dopamine. Studies in 2013 and 2015 conducted positron-emission tomography (PET) scans on volunteers who inhaled 8 mg of THC or a placebo, and both found no effect of THC on dopamine levels.