Dopamine is a neurotransmitter (or chemical in the brain) that either increases or reduces the activity of neurons (nerve cells).
Dopamine has a variety of influences on brain function, including playing a role in regulating attention, cognition, movement, pleasure, and hormonal processes. Parkinsons disease, attention deficit disorder, and schizophrenia all involve abnormalities in the dopamine system.
Some researchers believe that dopamine dysfunction may be also involved in the development of borderline personality disorder (BPD) because of evidence that dopamine is involved in cognitive and emotional processing and the regulation of impulsive behaviors (which are related to BPD symptoms). In addition, antipsychotic medications that target dopamine receptors in the brain seem to reduce BPD symptoms.
Source:
Friedel RO. Dopamine Dysfunction in Borderline Personality Disorder: A Hypothesis. Neuropsychopharmacology. 29:1029-1039, 2004.

