The dopamine agonists pramipexole and ropinirole are drugs used in the treatment of Parkinson Disease.
These drugs are now well-established in treating restless legs syndrome (RLS) and periodic limb movement disorder of sleep (PLMS), which are frequent problems afflicting about 10% of the population, and which can negatively impact quality of life & mood symptoms.
There is a small body of evidence showing possible benefits of dopamine agonists in the treatment of depression.
Unfortunately, dopamine agonists can exacerbate addictive/compulsive behaviour: http://www.ncbi.nlm.nih.gov/pubmed/20484726
Here are some references about the role of dopamine agonists in RLS and PLMS:
Here's a good review article on the use of ropinirole to treat these conditions: http://www.ncbi.nlm.nih.gov/pubmed/20421915
Here's a recent review of dopamine agonists in general to treat RLS: http://www.ncbi.nlm.nih.gov/pubmed/20206780
Here's a 2008 meta-analysis comparing ropinirole with pramipexole for treating RLS. Pramipexole is shown to be slightly superior:
In this 2010 study, gabapentin was compared to ropinirole for treating RLS. While ropinirole was superior in reducing objective measures of periodic limb movements, subjects taking gabapentin had a higher subjective benefit:
Here's a case study showing remission of depressive symptoms with ropinirole used as an augmenting agent:
Here's a 2005 study looking at ropinirole augmentation in treating depression:
This 2010 review in Lancet showed a direct antidepressant effect of pramipexole in Parkinson Disease patients: http://www.ncbi.nlm.nih.gov/pubmed/20452823
Here's a rather weak but positive 2010 paper describing a group of patients with bipolar depression who appeared to benefit from longer-term pramipexole treatment; doses averaged about 1 mg/d: http://www.ncbi.nlm.nih.gov/pubmed/20425143
The side effects from these drugs include frequent nausea and dizziness, possibly some daytime sleepiness. Psychiatric adverse effects can include hallucinations, and increased compulsive or impulse-control problems.
In summary, I think dopamine agonists have a role in selected psychiatric conditions, particularly if there are restless-legs symptoms contributing to insomnia or nocturnal discomfort. They may help treat refractory depression, but there is a risk of causing impulse control problems or hallucinations in predisposed individuals.