Nicotine is a stimulant which causes adrenaline to be released which results in Serotonin being released.  Thus, in a sense, nicotine is a way to self medicate as an anti-depressant. 

Unfortunately, unless one sleeps and eats in ways that allows the body to regenerate Serotonin, these self medicating ways ultimately contribute to further depletion of Serotonin. 

This is part of the reason why anti depressants (SSRI's--Selective Serotonin Reuptake Inhibitors) ultimately commonly fail in the long run.

As any addict knows, there comes a time when the stimulant of choice does not produce the desired effect.   That is because the storage supply of serotonin in your brain has been used up.  No matter how many cigarettes you smoke, cups of coffee you drink, or carbohydrates you eat, you cannot get the rush you so desire.  You end up serotonin depleted, extremely depressed and possibly even suicidal.  That is why all stimulants eventually become depressants.

There are many common factors of neurotransmitter and hormonal imbalances with schizophrenia and bipolar disorder that are not unlike biochemical addictive conditions such as alcoholism and drug addiction. 

According to Gary Craig EFT founder an addiction is something that you use to help reduce your anxiety level.  EFT can help these people get over their smoking addictions.

Mary

This is a sad but very true piece of information. Nicotine actually does provide some temporary relief for those who suffer the effects of the likes of depression and schizophrenia. ...  The flip side (apart from the fact that smoking kills the body), . is the fact that smoking also promotes depression.
 
It is pathetic also to discover the fact that in the states,.. cigarette smoking for thousands of those suffering from mental illness was started while they were patients in state psychiatric hospitals. ... The hospitals actually encrouraged smoking and used cigarettes as a tool to manipulate patients into cooperative behavior. ....... Now those "patients" are dying from emphasema  and cancer etc.