G-protein-coupled receptors are generally thought to be organized as dimers; whether they form higher order oligomers is a topic of much controversy. We combined bioluminescence/fluorescence complementation and energy transfer to demonstrate that at least four dopamine D2 receptors are located in close molecular proximity in living mammalian cells, consistent with their organization as higher order oligomers at the plasma membrane. This implies the existence of multiple receptor interfaces. In addition to the symmetrical interface in the fourth transmembrane segment (TM4) we identified previously by cysteine (Cys) crosslinking, we now show that a patch of residues at the extracellular end of TM1 forms a second symmetrical interface. Crosslinking of D2 receptor with Cys substituted simultaneously into both TM1 and TM4 led to higher order species, consistent with our novel biophysical results. Remarkably, the rate and extent of crosslinking at both interfaces were unaltered over a 100-fold range of receptor expression. Thus, at physiological levels of expression, the receptor is organized in the plasma membrane into a higher order oligomeric structure.

G protein–coupled receptor (GPCR) oligomers have been proposed to play critical roles in cell signaling, but confirmation of their existence in a native context remains elusive, as no direct interactions between receptors have been reported. To demonstrate their presence in native tissues, we developed a time-resolved FRET strategy that is based on receptor labeling with selective fluorescent ligands. Specific FRET signals were observed with four different receptors expressed in cell lines, consistent with their dimeric or oligomeric nature in these transfected cells. More notably, the comparison between FRET signals measured with sets of fluorescent agonists and antagonists was consistent with an asymmetric relationship of the two protomers in an activated GPCR dimer. Finally, we applied the strategy to native tissues and succeeded in demonstrating the presence of oxytocin receptor dimers and/or oligomers in mammary gland.

Synaptic dopamine levels appear to affect the in vivo binding of many D2 receptor radioligands. Thus, release of endogenous dopamine induced by administration of amphetamine decreases ligand binding, whereas subchronic dopamine depletion increases binding. This is generally thought to be due to binding competition between endogenous dopamine and the radioligands. However, the temporal dissociation between amphetamine-induced increases in dopamine, which last on the order of 2 hours as measured by microdialysis, and the prolonged decrease in ligand binding, which lasts on the order of a day, has suggested that agonist-induced D2 receptor internalization may contribute to the sustained decrease in D2 receptor binding potential seen following a dopamine surge. To test this hypothesis, we developed an in vitro system showing robust agonist-induced D2 receptor internalization following treatment with the agonist quinpirole. HEK293 cells were stably co-transfected with human D2 receptor, G-protein-coupled-receptor kinase 2 and arrestin 3. Agonist-induced D2 receptor internalization was demonstrated by fluorescence microscopy, flow cytometry and radioligand competition binding. The binding of seven antagonists and four agonists to surface and internalized receptors was measured in intact cells. All imaging ligands bound with high affinity to both surface and internalized D2 receptors. Affinity to internalized receptors was modestly lower, supporting the hypothesis that internalization would reduce binding potential measured in imaging studies performed with these ligands. However, between-ligand differences in the magnitude of the internalization-associated affinity shift only partly accounted for the data obtained in neuroimaging experiments, suggesting that mechanisms beyond competition and internalization are involved.

GABA B receptors are the G-protein coupled receptors for the main inhibitory neurotransmitter in the brain, GABA. GABA B receptors were shown to associate with homo-oligomers of auxiliary KCTD8, KCTD12, KCTD12b, and KCTD16 subunits (named after their T1 K ϩ -channel tetramerization domain) that regulate G-protein signaling of the receptor. Here we provide evidence that GABA B receptors also associate with hetero-oligomers of KCTD subunits. Coimmunoprecipitation experiments indicate that two-thirds of the KCTD16 proteins in the hippocampus of adult mice associate with KCTD12. We show that the KCTD proteins hetero-oligomerize through selfinteracting T1 and H1 homology domains. Bioluminescence resonance energy transfer measurements in live cells reveal that KCTD12/ KCTD16 hetero-oligomers associate with both the receptor and the G-protein. Electrophysiological experiments demonstrate that KCTD12/KCTD16 hetero-oligomers impart unique kinetic properties on G-protein-activated Kir3 currents. During prolonged receptor activation (one min) KCTD12/KCTD16 hetero-oligomers produce moderately desensitizing fast deactivating K ϩ currents, whereas KCTD12 and KCTD16 homo-oligomers produce strongly desensitizing fast deactivating currents and nondesensitizing slowly deactivating currents, respectively. During short activation (2 s) KCTD12/KCTD16 hetero-oligomers produce nondesensitizing slowly deactivating currents. Electrophysiological recordings from hippocampal neurons of KCTD knock-out mice are consistent with these findings and indicate that KCTD12/KCTD16 hetero-oligomers increase the duration of slow IPSCs. In summary, our data demonstrate that simultaneous assembly of distinct KCTDs at the receptor increases the molecular and functional repertoire of native GABA B receptors and modulates physiologically induced K ϩ current responses in the hippocampus.

The properties of glycine receptors (GlyRs) depend upon their subunit composition. While the prevalent adult forms of GlyRs are heteromers, previous reports suggested functional α homomeric receptors in mature nervous tissues. Here we show two functionally different GlyRs populations in the rat medial nucleus of trapezoid body (MNTB). Postsynaptic receptors formed α1/β containing clusters on somatodendritic domains of MNTB principal neurons, co-localizing with glycinergic nerve endings to mediate fast, phasic inhibitory postsynaptic currents (IPSCs). By contrast, presynaptic receptors on glutamatergic calyx of Held terminals were composed of dispersed, homomeric α1 receptors. Interestingly, the parent cell bodies of the calyces of Held, the globular bushy cells (GBCs) of the cochlear nucleus, expressed somatodendritic receptors (α1/β heteromers) and showed similar clustering and pharmacological profile as GlyRs on MNTB principal cells. These results suggest that specific targeting of glycine receptor β subunit produces segregation of GlyR subtypes involved in two different mechanisms of modulation of synaptic strength.