More recently, investigators have turned their attention to OPRD1 and OPRK1 (Levran et al. 2008; Zhang et al. 2008). Opioid drugs bind delta opioid receptors (DORs), but with a much lower affinity than MORs. DORs play an important role in the development of opioid tolerance (Daniels et al. 2005) and are involved in the rewarding and analgesic effects of opioids (Le Merrer et al. 2009). Overall, kappa opioid receptor (KOR) actions are commonly in opposition to those of MORs. Administration of KOR agonists results in conditioned place aversion rather than conditioned place preference (CPP) (Le Merrer et al. 2009). KORs may play a major role in the dysphoria experienced with opioid withdrawal (Le Merrer et al. 2009).
