By Austėja Čiulkinytė
Nicotine is an addictive chemical compound and the main psychoactive ingredient in tobacco. It acts on nicotinic acetylcholine receptors (nAChRs) in the mesolimbic reward pathway of the brain. Neurones of this pathway release dopamine, which helps establish nicotine addiction over time. Nicotine also activates the habenulo-interpeduncular pathway, which suppresses the centres responsible for withdrawal symptom development. Different types of nAChRs, which vary in their sensitivity to nicotine and ability to desensitise, are present in these pathways. This allows nAChRs to adapt to prolonged nicotine exposure in a way that discourages quitting. Various pharmaceutical, biotechnological and legislative efforts are being made to overcome the addiction associated adaptations in the brain.
Nicotine addiction has a huge impact on healthcare, economy, and social systems. The importance of preventing and treating nicotine addiction cannot be underestimated, but to do so, we need to understand why and how it develops.
Our brain communicates within itself via electrical and chemical signals that pass information from one neuronal cell to the other. These signals are in part carried by small organic molecules: neurotransmitters. One of the most important neurotransmitters is acetylcholine, which allows neurons involved in regulating mood, attention, learning and memory to communicate with each other.
This is made possible by the presence of nicotinic acetylcholine receptors (nAChRs) on neuronal cells. These receptors are found on all parts of neuronal cells throughout the brain. However, in addition to acetylcholine, they can also be recognised and stimulated by nicotine. By smoking, chewing, and picking nicotine-containing plants, nicotine can quickly enter the brain and alter its activity. This causes addiction: an intense drive to consume nicotine over and over again. This article describes the principal mechanisms of nAChR contribution to nicotine addiction, and how it is addressed.
Overview of nAChRs
The nicotinic acetylcholine receptor is a protein complex, embedded in the membrane of a neuronal cell. It consists of five protein subunits arranged in a circle, with a water-filled pore in the middle. The pore is a channel for positively charged ions to flow through, but is only open when agonists such as acetylcholine or nicotine are bound to the receptor. In neurons, influx of Na+ and Ca2+ ions launches a cascade of signalling molecules that triggers the neuron to produce an impulse. The signal travels down the length of the neuron until it is released as an electric or chemical signal to trigger another neuron, forming a neuronal pathway (Dani, 2015).
Each of the five protein subunits can be either type α or type β. In addition, each type is further divided into subtypes. In humans, there are nine α subtypes and three β subtypes, termed α2-α10 and β2-β4, respectively (Wittenberg et al., 2020). nAChRs can be divided into two major structural groups, depending on whether all subunits are of the same (homodimeric), or of different (heterodimeric) types (Figure 1).
Figure 1. Nicotinic acetylcholine receptor (nAChR) structural groups. A: Homodimeric nAChRs are composed of five identical α subunits. They contain five acetylcholine/nicotine binding sites located at each α-α subunit interface. B: Heterodimeric nAChRs contain at least one α-subunit and one β-subunit. A typical heterodimeric nAChR is composed of two α-subunits, two β-subunits, and one accessory subunit which can be of either type. They typically contain two acetylcholine/nicotine binding sites located at α-β subunit interfaces. All nAChRs also contain a water-filled pore in the middle which allows cations to flow through. ACh – acetylcholine. (Dani, 2015; figure created in Biorender.com)
Subunit composition determines properties of the entire receptor, such as affinity for acetylcholine and nicotine, as well as the time it takes for the channel to open and close, permeability to cations, and ability to become insensitive to stimuli (Wittenberg et al., 2020). Even small changes in receptor composition, such as genetic mutations within individual subunits, can have a large impact on nicotine response and likelihood of addiction (Table 1).
|SNP||Mutant gene||Nucleotide mutation||Amino acid substitution||Effect on cell||Effect on organism|
|rs16969968||CHRNA5(α5 subunit)||G>A||Asp398Asn||Decreased response to nicotineReduced Ca2+ permeabilityFaster desensitization||High susceptibility to nicotine addictionHeavy smokingIntense nicotine cravings|
|rs1051730||CHRNA3(α3 subunit)||G>A||None||Lower expression of CHRNA5||Same as rs16969968|
|rs12914008||CHRNB4(β4 subunit)||G>A||Thr90Ile||Unknown||High susceptibility to nicotine addiction|
Table 1. Common genetic mutations associated with increased risk of nicotine addiction. rs1051730 is a mutation in an untranslated region of DNA. Therefore, it has no effects at protein or cellular level, but is very often found together with rs16969968. SNP – single nucleotide polymorphism (Sherry et al., 2001; Wittenberg et al., 2020).
Addiction to nicotine is a learnt reward-based process. It takes place in the mesolimbic reward pathway, which is a group of neurons that are stimulated in the midbrain, and respond by releasing dopamine in the forebrain. Dopamine stimulates the reward centre, which regulates positive emotional response, motivation, and learning (Salgado and Kaplitt, 2015).
The mesolimbic reward pathway receives signals from several regulatory neuronal groups within the midbrain. Some regulatory neurons release glutamate, which activates the mesolimbic pathway. Others release gamma-aminobutyric acid (GABA), which inhibits it. Both types of input neurons, as well as mesolimbic pathway neurons themselves, contain nAChRs. However, some types of nAChRs are more likely to be desensitised to nicotine than others. When nicotine is present for prolonged periods of time, inhibitory GABA neurons and dopamine neurons of the reward pathway become unresponsive to nicotine. Meanwhile, excitatory glutamate neurons remain active for longer. This results in a net positive activation of the mesolimbic reward pathway and causes a dopamine rush (Figure 2; Wittenberg et al., 2020).
Figure 2. Changes in the mesolimbic reward pathway in nicotine addiction. A. Normal state: dopamine neurons of the pathway receive balanced inputs from excitatory glutamate neurons and inhibitory GABA neurons in the midbrain. nAChRs on all neuron types can be activated by nicotine. B: Addicted state. Heteropentameric α4/β2 nAChRs on GABA and dopamine neurons become desensitised to nicotine and no longer respond to stimulation. Homopentameric α7 nAChRs on glutamine neurons are much less likely to desensitise, therefore the mesolimbic reward pathway only receives excitatory signals. GABA – gamma-aminobutyric acid. (Wittenberg et al., 2020; figure created in Biorender.com)
When nAChRs are desensitised by prolonged nicotine exposure, they are unresponsive to acetylcholine. Neurons sense a drop in acetylcholine signalling and respond by speeding up the creation of new nAChRs. As a result, the total number of nAChRs increases (Wittenberg et al., 2020). If nicotine is not consumed, endogenous acetylcholine can no longer activate the positive emotional response centres in the brain to the same extent. This causes a drop in mood and withdrawal symptoms such as: irritability, anxiety and impatience.
While nicotine addiction initially develops via the mesolimbic reward pathway, it is maintained via the habenulo-interpeduncular pathway, located almost entirely in the midbrain (Antolin-Fontes et al., 2015; Wittenberg et al., 2020). Habenular neurons release acetylcholine and glutamate, which stimulates the interpeduncular neurons. These release GABA, which suppresses the brain centres that produce pain, fear and anxiety and are involved in depression and insomnia, and deactivate the mesolimbic reward pathway (Figure 3a; Antolin-Fontes et al., 2015).
Both habenular and interpeduncular neurons are rich in nearly all subtypes of nAChRs, especially those that contain α5, α3 and β4 subunits, known to have a significant role in nicotine addiction (Table 1). When nicotine stimulates these nAChRs, it activates the entire pathway and ultimately inhibits the pain and fear processing centres (Figure 3b; Wittenberg et al., 2020).
Just like with the mesolimbic reward pathway, prolonged exposure to nicotine promotes creation of new nAChRs in the habenulo-interpeduncular pathway (Antolin-Fontes et al., 2015). If nicotine is suddenly withdrawn, normal acetylcholine levels cannot stimulate the pathway to the same level that nicotine would (Figure 3c). As a result, suppression of the pain and fear centres is lifted, and withdrawal symptoms develop.
Interestingly, habenular neurons also have a unique ability to generate impulses on their own, similar to the pacemaker cells of the heart (Antolin-Fontes et al., 2015). Initially, nicotine does not affect this. However, if nicotine is withdrawn and then reintroduced, the frequency of impulses suddenly increases. This inhibits the pain and fear processing centres even more so than before. It is not yet known why or how this happens, but is important to understand to prevent relapses in quitters (Görlich et al., 2013).
Figure 3. Changes in the habenulo-interpeduncular pathway in nicotine addiction and withdrawal. A: Normal state. Habenular neurons activate interpeduncular neurons. Interpeduncular neurons inhibit pain and fear centres at a baseline level. B: Addicted state. α3, β4 and α5 nAChRs are stimulated and increase activity of all neurons involved in the pathway. In addition, β4 nAChRs are now expressed on interpeduncular neurons, increasing nicotine stimulation potential even more. C: Nicotine withdrawal. nAChRs are no longer stimulated. Presence of additional nAChRs contributes to overall lower levels of pathway stimulation compared to previous state. Pain and fear centre inhibition is lifted, which allows withdrawal symptoms to develop (Lee et al., 2019; figure created in Biorender.com).
So, can the cycle be broken? The unpleasant withdrawal symptoms make it very difficult to reverse nicotine addiction. However, it is known that their severity is linked with sudden changes in nicotine concentration in the brain. Various nicotine replacement devices, such as patches, gum, or lozenges, can help to gradually decrease nicotine levels in the brain and alleviate some of these symptoms (Molyneux, 2004).
The NHS also offers two prescription medications: varenicline and bupropion (NHS, 2019). Varenicline is a small molecule that binds to nAChRs, but does not activate them as strongly as nicotine or acetylcholine would. This has two effects. First, varenicline stimulates the habenulo-interpeduncular pathway just enough to suppress withdrawal effects. Second, varenicline takes up nAChRs and ensures that if nicotine is consumed again, it can no longer speed habenular signalling back up, or trigger a dopamine release in the mesolimbic reward pathway (Tonstad et al., 2020).
Bupropion is a repurposed antidepressant medication. It reduces total dopamine content within the mesolimbic reward pathway, and can block certain nAChRs altogether. While the mechanism is not completely understood, it is proposed that bupropion dampens the effects of nicotine on the brain, particularly by restricting the dopamine rush (Roddy, 2004). Over time, both varenicline and bupropion will prevent the brain processing nicotine as rewarding.
Treatment would not be necessary if addiction could be prevented. Recently, researchers in Germany have created a nicotine-free tobacco plant by disrupting its nicotine synthesis genes via CRISPR gene editing techniques (Schachtsiek and Stehle, 2019). In addition to biotechnology development, there is legislative effort focused at reducing nicotine content in cigarettes. For example, the EU has imposed a maximum nicotine limit of 1 mg per cigarette (The European Parliament and Council of the European Union, 2014), while the FDA had planned to shift from current average of 15 mg nicotine per cigarette to about 0.4 mg per cigarette (Food and Drug Administration, 2018; Denlinger-Apte et al., 2019).
nAChRs relay nicotine stimulation to the neuronal pathways within the brain. These pathways just so happen to allow the brain to associate nicotine consumption with reward and pleasure, and nicotine absence with anxiety and upset. Differences in nAChR structure, types, affinity for nicotine and ability to desensitise determines how active these brain pathways are in relation to nicotine levels in the body.
Since the neuropharmacological pathways feed into each other and promote repeated nicotine consumption, quitting is very difficult without outside help. The pharmaceutical industry has already offered products to help established smokers, while biotechnological advances, combined with legislation, aim to make it harder to become addicted in the first place. Despite this, the neuropharmacology of nicotine is not understood enough to confidently assess which addiction reduction measures would work best and what other courses of action are possible.
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