The role of regulator of G protein signaling 4 in delta-opioid receptor-mediated behaviors

Abstract

Rationale Regulator of G protein signaling (RGS) proteins act as negative modulators of G protein signaling. RGS4 has been shown to negatively modulate G protein signaling mediated by the delta opioid receptor (DOPr) in vitro. However, the role of RGS4 in modulating DOPr-mediated behaviors in vivo has not been elucidated. Objective The aim of this study was to compare the ability of the DOPr agonist SNC80 to induce DOPr-mediated antinociception, antihyperalgesia, antidepressant-like effects, and convulsions in wildtype and RGS4 knockout mice. Methods Antinociception was assessed in the acetic acid stretch assay. Antihyperalgesia was measured in a nitroglycerin-induced thermal hyperalgesia assay. Antidepressant-like effects were evaluated in the forced swim and tail suspension tests. Mice were also observed for convulsive activity post-SNC80 treatment. SNC80-induced phosphorylation of MAP kinase in striatal tissue from RGS4 wild-type and knockout mice was quantified by Western blot. DOPr number from forebrain tissue was measured using [3H]DPDPE saturation binding. Results Elimination of RGS4 potentiated SNC80-induced antinociception and antihyperalgesia. SNC80-induced antidepressant-like effects were potentiated in RGS4 knockout mice in the forced swim test but not the tail suspension test. Additionally, RGS4 knockout did not alter SNC80-induced convulsions. SNC80-induced phosphorylation of MAP kinase was potentiated in striatum from RGS4 knockout mice. Loss of RGS4 did not affect total DOPr number. Conclusions Overall, these findings demonstrate that reduction of RGS4 functionally increases the therapeutic index of SNC80. These results provide the first evidence of differential regulation of DOPr-mediated behaviors by RGS proteins and G protein signaling pathways.

Role of signalling molecules in behaviours mediated by the δ opioid receptor agonist SNC80

“…The potency of SNC80 to produce antihyperalgesia and antidepressant‐like effects was significantly increased in the Gα o RGS‐insensitive heterozygous mice. These data indicate that these δ‐receptor‐mediated behaviours signal through Gα o and are negatively regulated by RGS proteins, consistent with our previous finding that RGS4 negatively regulates these behaviours (Dripps et al, ). Furthermore, these enhanced effects of SNC80 were observed in mice with only one mutant copy of Gα o , demonstrating that δ‐receptor‐mediated signalling in vivo is highly sensitive to the effects of RGS proteins.…”

“…We found that Gα o and arrestins differentially regulate the antihyperalgesia, antidepressant‐like effects and convulsions produced by the δ‐receptor agonist SNC80. In the NTG‐induced thermal hyperalgesia assay, SNC80 produced antihyperalgesia in wild‐type mice, consistent with previous studies (Pradhan et al, ; Dripps et al, ). SNC80 also decreased immobility in the FST, consistent with the well‐established antidepressant‐like effects of δ‐receptor agonists (Broom et al, ; Naidu et al, ; Saitoh et al, ).…”

“…In the present study, δ‐receptor‐mediated convulsions were not altered in Gα o RGSi and Gα o knockout mice. In addition, we previously observed that SNC80‐induced convulsions were unaltered in RGS4 knockout mice (Dripps et al, ). Overall, these data may suggest that signalling mechanisms mediating δ‐receptor agonist‐induced convulsions are distinct from those mediating antihyperalgesia and antidepressant‐like effects.…”

“…Targeted knockdown of specific G protein subunits using antisense nucleotides inhibited δ‐receptor‐mediated spinal and supraspinal antinociception in mice, implicating multiple Gα i/o subtypes in the regulation of these effects (Standifer et al, ; Sánchez‐Blázquez and Gárzon, ). Loss of regulator of G protein signalling 4 (RGS4) potentiated the antinociceptive, antihyperalgesic and antidepressant‐like effects of the δ‐receptor agonist SNC80 suggesting that these behaviours are generated through G protein signalling (Dripps et al, ). However, this study also found that the frequency of SNC80‐induced convulsions was not altered in RGS4 knockout mice suggesting that δ‐receptor‐mediated convulsions may signal through a G protein‐independent mechanism.…”

A highly conserved δ‐opioid receptor region determines RGS4 interaction

“…In contrast, when similar experiments were performed using the truncated DΝ 17 RGS4 preincubated with increasing concentrations of i4 peptide, no shift in protein migration was observed, suggesting that the DΝ 17 RGS4 charge and its hydrodynamic size remained unchanged (Fig. 4B, lanes [14][15][16]. Similarly, no alterations in protein charge were detected using the control peptide MELVPSAR (Fig.…”