Fig. 2. Summary of opioid receptor signaling. Figure depicts opioid receptor signal transduction and trafficking. In general, all four opioid receptor subtypes (mu [μ], delta [δ], kappa [κ], and opioid receptor like-1 [ORL1]) share these common pathways. New research indicates that selective ligands at each opioid receptor can direct opioid receptors to favor one or more of these signaling events (biased agonism or ligand-directed signaling). Arrows  refer to activation steps; T lines  refer to blockade or inhibition of function. βγ = G protein β-γ subunit; cAMP = cyclic adenosine monophosphate; ERK = extracellular signal-regulated kinase; JNK = c-jun N-terminal kinase; MAPK = mitogen-activated protein kinases; P = phosphorylation.