Nicotine and cannabis have counteracting effects on resting cerebral connectivity
A recent study funded by the NIDA points out that the combination of cannabis with nicotine can produce effects that differ from the sum of their individual effects. More specifically, researchers have shown that nicotine or cannabis users have reduced connectivity in many brain networks but that consumers of both substances combined have similar connectivity to non-users.
The study demonstrates that:
- Nicotine and cannabis have interactive effects on the structure and function of the brain.
- Suggests that specialized treatment may be appropriate for those who use both substances.
Dr. Francesca Filbey of the Center for Brain Health at the University of Texas in Dallas and Dr. Bharat Biswal of the New Jersey Institute of Technology performed resting functional magnetic resonance imaging (MRIRRI) to assess brain network connectivity at 28 nicotine smokers, 53 cannabis, 26 users of both substances and 30 non-users.
In fMRI, subjects are asked to relax and let their mind wander during imaging. Network connectivity while at rest provides a basic indicator of how the regions within it can coordinate when responding to environmental stimuli or deficiencies.
The Filbey Lab (from this study) focuses on the combination of neuroimaging techniques and genetic techniques to characterize the neural mechanisms associated with the dysfunction of the reward system (eg, addictive disorders). More specifically, how environmental factors (for example, early adolescent use, early-life stress) play a role in the neural mechanisms associated with changes in the reward system and the extent to which genetic risk mitigates these effects. Current projects involve the determination of these effects using neuroimaging tools (fMRI, IRT, fMRI during signal exposure tasks, reward and punishment tasks, response inhibition tasks). and stress tasks, fMRI at rest) as well as genetic studies of drug addicts, compulsive consumers and individuals at risk.
- Non-consumer users of both substances (CTRL) had greater connectivity than users of the mediating medicine IC06 and the IC11 network. in the posterior cingulate gyrus.
- Non-users also had greater connectivity than nicotine users in most other networks observed.
Figure 1 illustrates the differences in functional connectivity at rest between nicotine-only users, cannabis-only users and non-users of nine brain networks. Each brain network is represented by a panel presenting four illustrations of the brain. The two illustrations at the top of each panel represent a longitudinal exterior view of the left and right sides of the brain. The two illustrations at the bottom of each panel represent longitudinal cross sections on the left and right sides of the brain. The areas highlighted in red indicate that functional non-users (CTRL) at rest are higher than nicotine users, and the areas highlighted in green indicate that the functional connectivity at rest is higher. among non-users than among cannabis users.
FMRI images revealed that nicotine and cannabis users each had reduced connectivity compared to non-users in two networks, one of which supports saliency (assigning importance to environmental stimuli) (see Figure 1). Nicotine users also had reduced connectivity in seven additional networks supporting functions such as cognition, vision, and body awareness. Users of both drugs had comparable connectivity to non-users in all networks; greater than nicotine or cannabis users in six networks; and greater connectivity than nicotine users only but not cannabis users only in four additional networks (see Figure 2).
- Users of both had greater connectivity than nicotine or cannabis users in six networks (IC03, IC04, IC07, IC08, IC10, and IC15) and greater connectivity than nicotine users in only four additional networks ( IC01, IC05, IC06 and IC09).
Figure 2 illustrates the differences in functional connectivity between nicotine-only users, cannabis-only users, and concurrent users of nicotine and cannabis in 12 brain networks. Each brain network is represented by a panel presenting four illustrations of the brain. The two illustrations at the top of each panel represent a longitudinal exterior view of the left and right sides of the brain. The two illustrations at the bottom of each panel represent longitudinal cross sections on the left and right sides of the brain. The areas highlighted in red indicate that functional connectivity at rest is higher among concurrent users of nicotine and cannabis than among cannabis-only users; areas highlighted in green indicate that functional connectivity at rest is higher for concurrent users of nicotine and cannabis than for nicotine users alone,
Dr. Filbey summarizes: "The functional connectivity between several regions of the brain is affected by the use of nicotine or cannabis but the connectivity of those who use the two substances together is similar to that of non-users." According to her, new findings are consistent with suggestions that cognitive enhancing effects of nicotine may facilitate cannabis use by counteracting the negative impact of cannabis on cognition-supporting networks.
The new study adds to others that demonstrate multiple individual and complex interactive effects of nicotine and cannabis on the structure and function of the brain. In previous studies, Dr. Filbey and his colleagues have shown that regular marijuana use reduced the amount of gray matter in the orbitofrontal cortex and that exposure to nicotine and cannabis reversed the normal association between larger hippocampal volume and higher memory performance.
Dr. Filbey hopes that elucidation of the interactions between nicotine and cannabis will eventually lead to more effective treatment interventions for those who use both substances, which represents about 39% of cannabis users.
Dr. Filbey said, "These results show that there are complex interactions between nicotine and cannabis. Although we know that these two substances have overlapping effects, we do not know much about these interactions. It is clear, however, that understanding how the brain reacts to nicotine and cannabis has important implications for treatment. "