Thanks for your question. The biggest difference between these two medications is based on their mechanism of action and the chemicals that they effect in the brain. There are 3 major chemicals ( aka neurotransmitters) that are responsible for our mood. These three chemicals are seritonin,norepinephrine, and dopamine. The biggest difference between all of the antidepressants on the market are the chemical or chemicals that they effect in the brain.
For example Celexa which belongs to a larger group of medications known as SSRI's decrease the symptoms of depression by increasing the amount of seritonin that is in the brain. It does this by decreasing the chemical's breakdown and removal. Seritonin is considered to have the largest impact on an individual's mood. It is the same neurotransmitter that is released in large amount when an individual eats a particularly good meal and is responsible for that feeling of satisfaction we all get from certain activities. Depressed individuals often have low levels of seritonin in the brain and therefore find it hard to find satisfaction in the things that they normally would. This proves the point that depression really is disease and not something that can be cured by "Cheering Up".
It has been determined that seritonin is not the only chemical that alters an individuals mood. The other two chemicals also are considered to effect mood, but the exact relationship is not completely understood. This is where Wellbutrin comes into play and the reason that it can be sucessfully added to Celexa treatment.
Wellbutrin is a unique medication that is in a class of its own. It works on the other two chemicals (dopamine and norepinephrine) that are responsible for mood. How this drug works is not completely understood yet, but it works to increase levels of these other two chemicals.
So basically this combination works to improve levels of all the chemicals that are responsible for a persons mood. The differences are not based on the part of the brain they effect,rather the chemicals.
Thank you for question and hopefully this helped,