Role of Regulator of G-Protein Signaling Proteins in Serotonin and Opioid Mediated Behaviors

Abstract

Multiple classes of drugs, including antidepressants as well as opioid analgesics, exert their therapeutic effects at least in part by direct or indirect actions at G-protein-coupled receptors (GPCR’s). This class of receptor propagates a signal inside the cell by activating heterotrimeric G-proteins (comprised of Gα and Gβγ subunits). Following receptor activation GDP bound to the Gα subunit of the heterotrimer is exchanged for GTP, followed by separation of the Gα and Gβγ subunits. Both Gα and Gβγ then activate downstream signaling pathways inside the cell. The regulator of G-protein signaling (RGS) proteins are a class of intracellular regulatory proteins that serve as negative modulators of GPCR signaling. They exert their actions by binding to active Gα-GTP subunits and accelerating the hydrolysis of GTP to GDP with subsequent reformation of the Gα/βγ heterotrimer, thus terminating signaling.

In this work, I describe the use of mice expressing Gα subunits of one of two types (either Gαi2 or Gαo) that do not bind to RGS proteins and so are RGS insensitive (RGSi) to gain a better understanding of how RGS proteins regulate both antidepressant-like and antinociceptive behaviors. Site-specific microinjections of a serotonin 1A receptor (5HT1AR) antagonist and agonist are used to show that mice expressing RGSi Gαi2 have a robust antidepressant-like phenotype dependent on hippocampal 5-HT1AR activity. Ex vivo recording from hippocampal tissue confirms that a 5-HT1AR agonist inhibits cellular activity more effectively in mice expressing RGSi Gαi2. Furthermore, I demonstrate that hippocampal administration of an RGS4/19 inhibitor (CCG203769) produces antidepressant-like effects. I show that mice expressing RGSi Gαo have a complex phenotype including increased and decreased sensitivity to noxious stimuli consistent with alterations in both nociceptin receptor (NOPR) and mu opioid receptor (MOPR) activity. The balance between the antinociceptive MOPR and pronociceptive NOPR systems is disturbed in the RGSi Gαo mice and in wild type mice during inflammatory pain. This work expands upon previous findings showing profound differences between mice with enhanced signaling downstream of different Gαi/o proteins due to the loss of RGS control, and provides novel information regarding the receptor systems and brain regions involved, with implications for both the treatment of depressive disorders and pain. This work is significant because it provides a greater understanding of the G-proteins involved in the perception of pain and depression, as well as the receptor systems and RGS proteins that control their activity.