Kappa but not delta or mu opioid receptors form homodimers at low membrane densities.

Opioid receptors (ORs) have been observed as homo- and heterodimers, but it is unclear if the dimers are stable under physiological conditions, and whether monomers or dimers comprise the predominant fraction in a cell. Here, we use three live-cell imaging approaches to assess dimerization of ORs at expression levels that are 10-100 × smaller than in classical biochemical assays. At membrane densities around 25/µm, a split-GFP assay reveals that κOR dimerizes, while µOR and δOR stay monomeric.