Why do Alcohol and Tobacco usually go together?

The danger of smoking and drinking is already spread to all the society. Advertising campaigns try to alert people of the evils involved in smoking and drinking, showing how this kind of behavior can affect not only your own healthy, but others around you.
The neural mechanisms involved in the onset and maintenance of addiction are subject of many scientific researches, that try to solve the puzzle of addiction, in order to help people who are in this condition.
Nicotine is a stimulant substance present in Tobacco and it can bind to a type of acetylcholine receptor, called nicotinic receptor. The nicotinic receptors are present within the brain and muscular junctions. When nicotine binds to the nicotinic receptors, the neurons end up releasing different kinds of neurotransmitters in the brain, such as norepinephrine, epinephrine, serotonin, and dopamine, causing different feelings of adrenaline and a later relaxation.
Nicotinic receptors are present in different parts of the brain, including the reward pathway, which together with other parts responsible for memory and decision making, form the circuit of addiction. The use of drugs of abuse can alter this circuit, building up the physical and chemical dependence.
Nicotine can alter the availability of receptors in the brain, increasing the number of the receptors. With more receptors, the sensitivity to nicotine is higher, which is correlated with nicotine addiction. Thus, nicotine can alter not just the function, but also the structure of neurons, and that may explain why is so hard to smokers to quit this habit.
Recent studies that focused in nicotine and alcohol addiction revealed important connections between the brain receptors involved in the maintenance of smoking and alcoholism. Ethanol does not bind directly to nicotinic receptors, or any kind of acetylcholine receptors. However, alcohol can increase the response of acetylcholine receptors, including nicotine receptors. That is, alcohol can potentiate the response of nicotinic receptors to nicotine, what could explain the high number of alcoholics that are also heavy smokers.
It is important that scientists continue to investigate the neural circuits involved in addiction, and how different drugs of abuse can relate to each other, so it will be possible to help people and to regulate these kind of drugs.