string: Several previous studies have indicated that κ and δ opioid receptors can exist not only as monomers but also as multi-protein structures including homodimers, heterodimers, and larger oligomer complexes that may potentially correspond to several pharmacologically defined opioid receptor subtypes that have so far eluded clear understanding (Bhushan et al. 2004; Daniels et al. 2005; Gomes et al. 2002; Jordan and Devi 1999; Portoghese and Lunzer 2003; Waldhoer et al. 2005; Wang et al. 2005; Xie et al. 2005; Ansonoff et al. 2010).