Nicotine Addiction

Nicotine Addiction

To understand how nicotine affects the brain, we must first understand the role of addiction in substance use and abuse.

Nicotine addiction refers to the dependency on nicotine, the toxic active agent in tobacco, which works as a stimulant drug.

Signs and Symptoms of Nicotine Addiction

Nicotine addiction generally has the same effects on people across the board. Signs and symptoms of nicotine addiction include:

• Urges to smoke.
• Unable to stop smoking.
• Withdrawal symptoms when quitting smoking (anxiety, restlessness, cravings, sadness and depression, irritability, frustration, and difficulty concentrating).
• Continued smoking when physical health declines.
• Inability to continue social activities to a full extent, as smoking takes precedence.

Nicotine Addiction Treatment

Treating nicotine addiction typically involves various psychological and biological therapies. Treatment for nicotine addiction includes:

• Nicotine replacement therapy (NRT), i.e., patches, chewing gum, oral strips.
• Bupropion (Zyban).
• E-Cigarette usage (reduced nicotine), although addiction issues are becoming increasingly problematic if e-cigarettes merely replace cigarettes rather than treat nicotine addiction.
• Cognitive behavioural therapy (CBT)
• Motivational interviewing (MI).

Multiple theories of why nicotine addiction develops have been proposed, including learning theory and the biological approach.

Learning Explanations for Nicotine Addiction

Learning theory has been proposed to explain nicotine addiction. It states that people who start smoking do so because they see people around them - such as family or friends - engaging in such behaviours (observation and modelling) and understand the consequences.

Based on operant conditioning, this can be positively or negatively reinforced or vicariously reinforced (witnessing the positive consequences of behaviours in others).

Once an individual has started smoking (initiation phase), the individual is likely to continue smoking for a certain period (maintenance phase). During the maintenance phase, the individual develops an addiction. Finally, the last step, according to the learning theory, is the relapse phase, in which an individual may try to stop smoking but would not be able to do so.

In addition to the above, learning theory states that addiction occurs when an association is made between the consumption of a substance and being in certain environments, which prompts the need to smoke when placed in similar environments. This is known as cue reactivity (classical conditioning), and is associated with relapse rates.

Learning theory explains nicotine addiction. However, it is not exempt from criticism.

• The learning theory may predict smoking behaviours but does not offer a model based on which all human smoking behaviour is explained. It does not account for the negative consequences of smoking, such as health issues.
• Learning theory tends to ignore individual differences and free will.

Effects of Nicotine on the Brain and Nervous system

Several hypotheses and models have been suggested that explain the effect of nicotine on the brain. Theories and models typically explain nicotine addiction in terms of the dopaminergic system, which is associated with the reward system.

Nicotine Receptors: Desensitisation

The desensitisation hypothesis explains nicotine addiction at the synaptic level. When it comes to nicotine, the neurotransmitter (NT) that comes into play is acetylcholine (ACh). ACh binds to ACh receptors. However, one type of ACh receptor is nicotinic acetylcholine receptors (nAChRs).

Nicotine seems to have a specific effect on these receptors. It stimulates the nAChRs, and shortly after that, the receptors are less sensitive due to ion channel inactivation (Ochoa et al., 1989). The phenomenon by which a response is diminished or absent is called desensitisation.

• Nicotine addicts (typical daily smokers) continue to smoke throughout the day, which results in a near-constant state of saturation of the nAChRs (Brody et al., 2006). When sleeping or attempting to stop smoking, smokers experience cravings and withdrawal symptoms because nAChRs return to a sensitised state. To reduce cravings and withdrawal symptoms, smokers will take in nicotine once more, which will desensitise the receptors again.

Nicotine also stimulates dopamine release, especially in the nucleus accumbens and ventral tegmental area, all of the dopaminergic reward pathways.

• Dopamine provides feelings of reward and relaxation.

Once smoking stops, dopamine goes through the reuptake process and the reward-like feeling stops, so smokers may want to increase their nicotine intake to regain these feelings.

Fig. 2: The reward structures in the brain.

In general, nicotinic receptors can be found throughout the brain. Areas of the brain include the hippocampus, the thalamus, the basal ganglia, and the cerebellum, to name a few.

Evidence from Studies using Brain-Imaging techniques

Evidence of this model comes from studies using imaging techniques which compare blood flow in certain brain areas. In 2005, Zubieta and colleagues conducted a PET study investigating brain activation patterns when participants consumed tobacco.

• The study presented a double-blind design, meaning neither participants nor researchers knew which participants consumed nicotine or a placebo. Brain scans were taken after the day's first and second cigarette.
• The VTA and NAcc (and other brain areas) were activated in the scans.

Brain Regions Affected by Nicotine

Olds and Milners (1954), two scientists working in Canada, conducted a study using rats.

• Researchers implanted electrodes in the NAcc of rats.
• The electrode would stimulate the NAcc once a lever would be pressed.
• Interestingly, once rats learned the association between pressing the lever and receiving the stimulation, they would keep pressing the lever to receive the stimulation (up to 2000 times in one hour).

The NAcc activation would trigger the activation of the VTA, which both are part of the reward system, which aligns with the desensitisation hypothesis. The consumption of nicotine triggers the reward system.

Nicotine and the Blood-Brain Barrier

The blood-brain barrier (BBB) is the brain's protective system, which keeps neurotoxins away from the brain. It was in the 1800s when Ehrlich noticed that staining when rats were injected with a blue stain, the stain would spread across all cells except those in the central nervous system.

• In this way, the brain and spinal cord would remain the same colour. When the stain was injected between the brain and the skull, the whole brain would change colour, while the rest of the body would remain the same colour.

Ehrlich discovered that the BBB was a barrier formed of cells strategically placed very close to one another. This allowed some materials, such as water and glucose, to cross the barrier and enter the brain, while at the same time, the barrier prevented other materials, such as neurotoxins, from entering the brain.

Interestingly, nicotine is a substance which can cross the BBB. This, in turn, has the effect of nicotine reaching the brain in around 15 seconds ². The fact that nicotine can enter the brain in such a short amount of time contributes to nicotine addiction.

Impulsivity, Serotonin and Smoking

Research has pointed attention to the role serotonin has in smoking initiation and relapse. It is well established that serotonin plays a role in the inhibition of impulses ⁴.

Low levels of serotonin have been associated with higher impulsive behaviours. At the same time, nicotine consumption has been shown to increase serotonin levels in the brain, while nicotine withdrawal decreases serotonin levels.

Thus, it has been hypothesised that nicotine withdrawal increases individuals' impulsivity, making them more prone to maintaining their nicotine consumption.

Opioid System and Nicotine Addiction

There is preliminary evidence of the role that the opioid system may be playing in nicotine addiction. Nicotine consumption results in the increase of levels of opioids in the brain, which have an effect on the brain's reward system.

• Krishnan-Sarin et al. (1999) conducted a study to investigate whether the opioid system was different in smokers compared to non-smokers. Twenty participants were tested in four experimental conditions in which they received different amounts of naloxone.

• The comparisons between smokers and non-smokers suggested that those who consumed nicotine regularly had a different response to naloxone. This, in turn, was taken as evidence suggesting that nicotine consumption can alter the opioid system.

Evaluation of the Biological Explanation of Nicotine Addiction

The biological approach to the research on Nicotine Addiction has offered great insights into the topic. We need to consider the strengths and weaknesses of the explanations for nicotine addiction.

Among the strengths:

• A significant amount of research has supported the link between nicotine and the dopaminergic system.
• Understanding the brain neurochemistry behind the consumption of nicotine has practical implications in the programmes supporting individuals quit the consumption of nicotine.

And of course, there are limitations to this approach:

• The biological explanation of addiction is criticised for being reductionist. Explaining addiction in terms of brain neurochemistry does not account for other components, such as social learning and environmental influences, that may also be playing a role in nicotine dependency.
• Further, the approach is criticised for being deterministic. One of the biological approach's assumptions is that biological foundations determine behaviour. This, in turn, cancels out individuals' rational ability to freely choose their actions.