Dopamine and norepinephrine are, along with serotonin, related because they are three very important neurotransmitters that are the main constituents of the monoamine neurotransmitter group. All three are indicated in being potentially affected when diseases like depression occur, and where levels of any of the three might be lower than normal. It might seem like each neurotransmitter is produced on its own, but actually these substances have a special relationship to each other.
As scientists continue to learn more about dopamine and norepinephrine one thing is quite clear. In absence of dopamine, norepinephrine is unlikely to be produced. Dopamine specifically creates norepinephrine with use of other chemicals like beta-hydroxylase. Low levels of dopamine tend to suggest low levels of norepinephrine, and the absence or near absence of these chemicals in concert could result in very serious depression or anxiety.
With this in mind, pharmaceutical companies have worked to hard to develop new types of antidepressants that act on norepinephrine and dopamine receptors. One of these is the successful Wellbutrin® or Zyban® (bupropion), which is called an NDRI, or norepinephrine dopamine reuptake inhibitor. For some people this NDRI has tamed depression well, and Zyban® has been used as a stop-smoking aid.
Given the relationship between dopamine and norepinephrine other people have advocated for a strictly dopamine reuptake inhibitor since that might satisfy norepinephrine production too. There are actually a few of these. The best known is the popular attention deficit disorder (ADD) drug Ritalin® (methylphenidate). Some people with depression do respond to a solely dopamine reuptake inhibitor, but others seem to respond more to an NDRI or to other medications that stimulate or inhibit the reuptake of serotonin.
When in good supply, norepinephrine and dopamine can act on the mind in positive ways. Norepinephrine may help regulate stress, help people feel pleasure, keep people attentive and aid in learning and normal emotional developmental. Dopamine also helps people perceive pleasure, concentrate, think more clearly, maintain an even mood, remember things, and learn new behaviors. Together the two may interfere with rewards caused by dangerous substances like cocaine or nicotine, which explains why Zyban® has been marketed as a quit smoking aid, though it is not entirely successful.
There can be too much of a good thing, and having overly high amounts of these two neurotransmitters may be disastrous. If dopamine increases in supply it might stimulate greater production of norepinephrine, and this could result in shaking, anxiety, mania, paranoia or other extremely undesirable features. The balance has to be just right, or dopamine and norepinephrine can wreak havoc on the brain and body. Nowhere is this better demonstrated than when people with bipolar disorder are medicated with an NDRI; most of them quickly progress to manic states.