Dopamine receptors play vital roles in processes such as the control of learning, motivation, fine motor control and modulation of neuroendocrine signaling (Giralt JA and Greengard P, 2004). Abnormalities in dopamine receptor signaling may lead to neuropsychiatric disorders such as Parkinson's disease and schizophrenia. Dopamine receptors are prominent in the CNS and the neurotransmitter dopamine is the primary endogenous ligand for these receptors. In humans, there are five distinct types of dopamine receptor, D1-D5. They are subdivided into two families; D1-like family (D1 and D5) which couple with the G protein alpha-s and are excitatory and D2-like family (D2,D3 and D4) which couple with the G protein alpha-i and are inhibitory (Kebabian JW and Calne DB, 1979).