Dopamine functions as both an inhibitory and excitatory neurotransmitter depending upon where in the brain and at which particular receptor site it binds to. Adequate dopamine levels are needed to allow us to focus our attention in the moment and attend to matters at hand (remember that attention deficit is at least in part due to low dopamine). Dopamine is the main player in regulating our reward circuitry and pleasure centers (hence dopamine's role in addictions). This all important brain chemical is also critical for memory and motor skills.
Problems can ensue if dopamine is too high or too low. For example, dramatically elevated levels, the so-called "dopamine storm," can be associated with hallucinations, delusions, agitation, mania, and frank psychosis. Such a state, fortunately rare, is clinically obvious and constitutes a medical emergency. Low dopamine states, on the other hand are quite common and sometimes go years without being identified let alone treated.
Dopamine is responsible for motivation, interest, and drive. It is associated with positive stress states such as being in love, exercising, listening to music, and sex . When we don't have enough of it we don't feel alive, we have difficulty initiating or completing tasks, poor concentration, no energy, and lack of motivation. Dopamine also is involved in muscle control and function. Low Dopamine levels can drive us to use drugs (self medicate), alcohol, smoke cigarettes, gamble, and/or overeat. Low dopamine states, on the other hand are quite common and sometimes go years without being identified let alone treated. Such states can cause memory, concentration, and attention problems. Stimulants such as prescription medications for ADD/ADHD, caffeine, and some street drugs temporarily address symptoms of low dopamine by pushing your existing (but dwindling) supply into the space between two neurons (synpase). This kind of approach can in the short run improve symptoms but if continued for any lenght of time inhibit natural transmission and actually cause/hasten dopamine depletion.