Role of β-arrestin-2 in short-and long-term opioid tolerance in the dorsal root ganglia
{beta}-arrestin-2 has been implicated in the mechanism of opioid-induced antinociceptive tolerance. G-protein-biased agonists with reduced {beta}-arrestin-2 activation are being investigated as safer alternatives to clinically-used opioids. Opioid-induced analgesic tolerance is classically considered as centrally-mediated, but recent reports implicate nociceptive dorsal root ganglia (DRG) neurons as critical mediators in this process. Here, we investigated the role of {beta}-arrestin-2 in the mechanism of opioid tolerance in DRG nociceptive neurons using {beta}-arrestin-2 knockout mice and the G-protein-biased -opioid receptor agonist, TRV130. Whole-cell current-clamp electrophysiology experiments revealed that 15-18-hour overnight exposure to 10 M morphine in vitro induced acute tolerance in {beta}-arrestin-2 wild-type but not knockout DRG neurons. Furthermore, in wild-type DRG neurons circumventing {beta}-arrestin-2 activation by overnight treatment with 200 nM TRV130 attenuated tolerance. Similarly, in {beta}-arrestin-2 knockout male mice acute antinociceptive tolerance induced by 100 mg/kg morphine s.c. was prevented in the warm-water tail-withdrawal assay. Treatment with 30 mg/kg TRV130 s.c. also inhibited antinociceptive tolerance in wild-type mice. Alternately, in {beta}-arrestin-2 knockout DRG neurons tolerance induced by 7-day in vivo exposure to 50 mg morphine pellet was conserved. Likewise, {beta}-arrestin-2 deletion did not mitigate in vivo antinociceptive tolerance induced by 7-day exposure to 25 mg or 50 mg morphine pellet in both female or male mice, respectively. Consequently, these results indicated that {beta}-arrestin-2 mediates acute but not chronic opioid tolerance in DRG neurons and to antinociception. This suggests that opioid-induced antinociceptive tolerance may develop even in the absence of {beta}-arrestin-2 activation, and thus significantly affect the clinical utility of biased agonists.