RGSz1 actions in the periaqueductal gray modulate opioid reward and analgesic tolerance
Authors: Sakloth Farhana , Singh Mandakini , Serafini Randal , Gomes Ivone , Devi Lakshmi , Zachariou Venetia 
Published: April 2020

Regulator of G protein signaling z1 (RGSz1), is a member of the RGS family of proteins, and present in several networks expressing mu opioid receptors. Previous studies in our lab have shown that RGSz1 knockout increases the analgesic efficacy of synthetic opioids, and delays the development of tolerance. On the other hand, knockout of RGSz1 decreases the rewarding efficacy of morphine in the place conditioning paradigm and prevents its locomotor sensitizing actions (Gaspari, et al, PNAS 2018). We further demonstrated the impact of RGSz1 in tolerance to synthetic opioids using a three‐week tolerance paradigm. These opposing roles of RGSz1 in reward versus analgesic efficacy make it an interesting target for the development of therapeutic interventions for chronic pain management. The goal of this work is to identify the RGSz1 circuits involved in the modulation of morphine’s rewarding and analgesic actions.

To gain insight on the regional role of RGSz1 in the rewarding effects of morphine, we conditionally downregulated RGSz1 in the nucleus accumbens and in the ventrolateral periaqueductal gray (vlPAG). For these experiments, RGSz1fl\fl mice were infected in the vlPAG with adeno‐associated vectors expressing Cre recombinase. Downregulation of RGSz1 in the vlPAG of adult mice decreased sensitivity to the rewarding effects of morphine in the conditioned place preference (CPP) assay. Next, we hypothesized that RGSz1 actions in vlPAG projections to the ventral tegmental area (VTA) modulate the rewarding actions of opioids. We used a chemogenetic approach to test if activation of vlPAG‐VTA circuitry promotes morphine place preference. For these studies, the VTA of male C57BL/6 mice was infected with pAAV‐pkg‐Cre vectors and the vlPAG with pAAV‐hSyn‐DIO‐hM3D(Gq)‐mCherry (AAV8), and mice were tested in the CPP assay five weeks later. Activation of vlPAG‐VTA projections increased morphine reward sensitivity as pAAV‐hSyn‐DIO‐hM3D(Gq)‐mCherry expressing mice showed significant place preference to 3mg/kg, whereas control animals developed CPP to 5 mg/kg of morphine. Notably, this intervention did not affect the analgesic actions of morphine. Circuit‐specific gene down regulation approaches are currently used to test the hypothesis that RGSz1 actions in vlPAG‐VTA projections affect the rewarding efficacy of morphine. For these experiments, the VTA of RGSz1fl\fl mice was infected with pAAV‐EF1a‐fDIO‐Cre and the vlPAG with pAAV‐Ef1a‐Flpo vectors, and 5 weeks later mice are tested in the CPP assay.

Overall, our studies have identified that RGSz1 actions in the vlPAG control the rewarding efficacy of opioid analgesics. Furthermore, we provide insight on the mechanism of RGSz1 action in vlPAG‐VTA circuit as an important pathway for mediating morphine’s rewarding actions. Therefore, targeting RGSz1 serves as potential avenue that optimizing the actions of prescription opioids.

Support or Funding Information

Supported by R01 NS086444, R56 NS111351 NINDS, and P01 DA008227, NIDA