SUMMARY Why do opiates make human beings itch ? Spinal opioid-induced itch, a prevalent side effect of pain management, has been considered to occur as a result of pain inhibition. We report that morphine-induced scratching (MIS) is abolished in mice lacking either gastrin-releasing peptide receptor (GRPR) or the μ opioid receptor (MOR). Using exon-specific knockdown, we identified the MOR1D isoform as essential for MIS, whereas MOR1 is important for morphine-induced analgesia (MIA) with no cross activity present. MOR1D and GRPR form constitutive heterodimers in the spinal cord and relay itch information upon morphine activation. Morphine induces internalization of both GRPR and MOR1D, whereas GRP induces that of GRPR but not MOR1D, when co-expressed. Moreover, GRP-induced scratching (GIS) is independent of MOR activation. These results suggest a unidirectional cross-activation of GRPR signaling by MOR1D via heterodimerization, and that opioid-induced itch is an active process concomitant with but independent of opioid analgesia.

BackgroundPlants have evolved a unique epigenetic process to target DNA cytosine methylation: RNA-directed DNA methylation (RdDM). During RdDM, small RNAs (smRNAs) guide methylation of homologous DNA loci. In Arabidopsis thaliana, the de novo DNA methyltransferase that ultimately methylates cytosines guided by smRNAs in all sequence contexts is DOMAINS REARRANGED METHYLTRANSFERASE 2 (DRM2). Recent reports have shown that DRM2 requires the catalytic mutated paralog DRM3 to exert its function through a still largely unknown process. To shed light on how DRM3 affects RdDM, we have further characterized its role at the molecular and cytological levels.FindingsAlthough DRM3 is not required for RdDM loci transcriptional silencing, it specifically affects loci’s DNA methylation. Interestingly, DRM3 and DRM2 regulate the DNA methylation in a subset of loci differently.Fluorescence In Situ Hybridization and immunolocalization analyses showed that DRM3 is not required for the large-scale nuclear organization of heterochromatin during interphase, with the notable exception of the 45S ribosomal RNA loci. DRM3 localizes exclusively to the nucleus and is enriched in a round-shaped domain located in the nucleolar periphery, in which it colocalizes with components of the RdDM pathway.ConclusionsOur analyses reinforce the previously proposed chaperone role of DRM3 in RdDM. Overall, our work further demonstrates that DRM3 most likely functions exclusively with DRM2 in RdDM and not with other A. thaliana DNA methyltransferases. However, DRM3’s regulation of DNA methylation is likely target- or chromatin context-dependent. DRM3 hypothetically acts in RdDM either upstream of DRM2, or in a parallel step.Electronic supplementary materialThe online version of this article (doi:10.1186/1756-0500-7-721) contains supplementary material, which is available to authorized users.

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