Figure 6.

Normal developmental trajectories of dopamine and cannabinoid expressions in the human prefrontal cortex. Expression of dopamine receptor, DRD1 suggests an inverted U-shape with increasing expression of receptors until the mid-twenties after which there is a slow decline (Rothmond et al., 2012). The DRD2 isoforms and DRD5 show an inverse pattern, peaking prenatally with a continuous decline in expression throughout life, excepting a small increase in the early twenties (Rothmond et al., 2012). The expression of DRD4 as determined by in situ hybridization is stable across the lifespan (Weickert et al., 2007). Measurement of this receptor by quantitative PCR has been quite variable and no statistical differences in expression across age groups have been detected (Rothmond et al., 2012). Cannabinoid receptor mRNA has its highest measured expression at the time of birth and shows a stepwise decline with age, reaching 30% of maximal expression in middle age (Long et al., 2012).