Explanations for Nicotine Addiction

Explanations for Nicotine Addiction

1. Acetylcholine Receptors

   • Dopamine producing neurons have on their surfaces receptors that bind with another neurotransmitter - ACH
   • Binding allows communication across synapses to take place
   • In neurotransmission, when enough ACh molecules bind with receptors and electrical impulse is able to continue from one neuron to another
   • One substitute of ACh receptor: nicotinic acetylcholine receptor (nAChR
     • Can bind with both ACh and nicotine
     • When nicotine molecules bind with nAChRs the neuron becomes stimulated and transmits dopamine. But immediately after this, the nicotine receptor shuts down and temporarily cannot respond to neurotransmitters.
       • The neuron is said to be desensitised and this leads to downregulation, a reduction in the number of active neurons because fewer of them are available

2. Dopamine transmission

   • nAChRs are concentrated in the ventral tegmental area (VTA) of the brain
     • When nAChRs in the VTA are stimulated by nicotine, the neurotransmitter dopamine is transmitted along the mesolimbic pathway to the nucleus accumbens (NA)
       • This triggers the release of dopamine from the NA into the frontal cortex
         • Dopamine is also transmitted along the mesocortical pathway to be released in the frontal cortex
           • The mesolimbic and mesocortical pathways are part of the brain's reward and please centre
             • So nicotine powerfully activates this system and results in pleasurable effects
               • This mechanism is thought to be very similar to the involved in addictions to classic drugs of abuse
                 • cocaine
                 • heroin
               • mild euphoria
               • increased alertness
               • reduction in anxiety

3. The Nicotine regulation model

   • As long as the individual is smoking, nicotine receptors are desensitised and cravings and withdrawal symptoms held off
     • When smokers go without smoking for a prolonged period, nicotine disappears from the body
       • This allows more nicotine receptors to become functional
         • So dopamine neurons resensitise and more become available
   • Withdrawal
     • More nAChRs are available but not stimulated, so the smoker experiences ac acute withdrawal syndrome with symptoms including anxiety and agitation
       • At this point, after a period of abstinence, nAChRs are at their most sensitive
         • This is why smokers often describe the first cigarette of the day as the most enjoyable - it re-activates the dopamine reward system
   • Denedance and Tolerance
     • According to the model, this explains how nicotine dependance is maintained
     • The smoker is motivated to avoid unpleasurable physiological and psychological withdrawal states by having another cigarette
     • But repetition of the cycle of daytime downregulation and nigh-time regulation creates chronic desensitisation of nAChRs.
     • So continuous exposure of nAChRs to nicotine causes permanent changes to brain neurochemistry - a decrease in the number of active receptors
     • Tolerance develops as the smoker has to smoke more to get the same effects