Smoking has the dual experiential effects: stimulant + tranquilizer, that puts one in a good place for dealing with stress; creating relaxation (from worry, and so hightened concentration), without becoming sedated.

Both are important, but of the two, I'd say that the 'mild tranquilizer' effect is the key. Even without a stimulant, lessening negative thoughts and feeling calmer allows more focussed attention and increased productivity.

Nicotine is able to produce these effects via enhacing production of dopamine (boosted and widened by MAOIs). As noted before, there are tie-ins with opioid receptors and their reward system.

"Marina Picciotto, a professor of psychiatry at Yale University, says that the findings suggest that nicotine's action ultimately relies on mu-opioid receptors, much as the actions of many other addictive drugs do. "There are common circuits involved in mediating the effects of these drugs ..."



Smoking's reward: nicotine triggers opiate-pleasure response. - Free Online Library

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GABA (Gamma Amino Butyric Acid - the chief inhibitory neurotransmitter) has been shown to be able to completely substitute for nicotine in animal studies.