Dopamine, a neurotransmitter widely distributed throughout the central nervous system (CNS), is responsible for regulating movement, cognition, and pleasure by activating specific dopamine receptors [46]. There are two different types of dopamine receptors: the D1 and the D2 family. D1 and D5 receptors of the D1 family are located primarily in the amygdala, nucleus accumbens (NAc), substantia nigra, hypothalamus, and thalamus. D2, D3, and D4 receptors of the D2 family are more sparingly expressed in the hypothalamus and the nuclei of the thalamus [58]. A profile of the function and involvement of receptors is provided in Table 1. Evidence suggests that dopamine release and dopaminergic receptor expression in the mesolimbic dopamine system is associated with traits of addictive behaviour such as impulsivity, novelty-seeking, and reward deficiency syndrome [33, 46]. As a result, the dopamine receptor genes have become prime candidates for the study of genetic polymorphisms and their effects on opioid dependence vulnerability [46, 47, 59-61].
