The DRD2 TaqI A is a SNP with two variants: A1, the less frequent allele, and A2. The A1 allele is associated with a reduction in the density of D2 receptors at the striatum [8-10], although one study failed to replicate this finding [11]. The hypofunction of D2 receptors associated with A1 allele could be a risk factor for SUD [12]. Healthy men who feel psychostimulant effects to be pleasant have lower D2 receptor availability than those who feel them to be unpleasant [13]. This result supports a model that links the A1 allele with SUD through the following sequence of facts: low D2 receptor density, moderate substance stimulation of dopamine system, and rewarding feelings after substance use. The addictive process could get started and continue more easily in men than in women, because D2 receptor density is lower in healthy men [9,14] as well as in heroin-dependent men [15].
