Opioid receptor family includes three classic receptors, μ, δ, and κ, also known as OP1, OP2 and OP3, respectively. The receptors are Gi/o-coupled GPCRs which will reduce intracellular cAMP levels after activation. δ-opioid receptor modulates many kinase cascades including ERKs, Akts, JNKs, STAT3, P38 involving Src, Ras, Rac, Raf-1, Cdc42, RTKs. In addition, δ-opioid receptor has also been proposed to interact with μ receptors. The observed pharmacological cross-talk may partially arise from agonist cross-reactivity.