Researchers at the University of Massachusetts Medical School and The Scripps Research Institute say they have identified the circuitry in the brain responsible for the increased anxiety commonly experienced during withdrawal from nicotine addiction.
“We identified a novel circuit in the brain that becomes active during nicotine withdrawal, specifically increasing anxiety,” said principal investigator Andrew Tapper, Ph.D., associate professor of psychiatry. “Increased anxiety is a prominent nicotine withdrawal symptom that contributes to relapse in smokers attempting to quit.”
The study yielded several discoveries about interconnected brain mechanisms that induce anxiety during nicotine withdrawal, as well as possible ways to derail these mechanisms in order to treat, or even prevent the symptom.
The main finding of the study (“Increased CRF signalling in a ventral tegmental area-interpeduncular nucleus-medial habenula circuit induces anxiety during nicotine withdrawal”), published online in Nature Communications, is that the interpeduncular nucleus in the brain is activated and appears to cause anxiety during nicotine withdrawal. Scientists were intrigued to learn that the sub region of the interpeduncular nucleus, which is activated and linked to anxiety during withdrawal, is distinct from another sub-region, previously identified by Dr. Tapper, where physical nicotine withdrawal symptoms such as headaches, nausea, and insomnia originate.
Anxiety is an affective symptom often likely to thwart smokers' attempts to quit. The newly discovered sub region offers a distinct target for dampening the affective symptoms of nicotine withdrawal.
Also newly identified is the fact that input from neurons in two other brain regions converge onto the interpeduncular nucleus to stimulate anxiety-provoking neurons. Surprisingly, the ventral tegmental area, which is traditionally associated with the rewarding or pleasurable effects of abused drugs, activates neuron receptors through corticotropin releasing factor (CRF), a protein neurotransmitter released in response to stress.
Also surprising, neurons in the medial habenula stimulate interpeduncular nucleus neurons by releasing glutamate, the major excitatory neurotransmitter in the brain, an effect that is increased by corticotropin releasing factor receptor activation.
“These data indicate that increased CRF receptor signalling within a VTA–IPN–MHb circuit triggers anxiety during nicotine withdrawal,” wrote the investigators.
“Both of these inputs are important. We could alleviate anxiety during nicotine withdrawal by either preventing corticotropin releasing factor synthesis in the ventral tegmental area, or by silencing the medial habenula inputs into the interpeduncular nucleus,” said Dr. Tapper.
Investigators were able to alleviate anxiety in mice by quieting the activity of those activated neurons, suggesting the same might be possible for humans.
“There are already drugs that block the CRF receptor that contributes to activation of these anxiety-inducing neurons,” noted Dr. Tapper. “These receptors have previously been linked to anxiety and depression, so our findings may also have implications for anxiety disorders in general.”
Next steps for this research will be expanding the scope of scientists' understanding of the interactions between anxiety, stress, reward, and withdrawal from addictive substances.
“We're now exploring whether the circuitry that we identified is involved in stress-induced anxiety in general, or specific to nicotine withdrawal-induced anxiety,” Tapper said. “We're also exploring if this circuitry is engaged with other drugs of abuse.”