Gβγ that interacts with adenylyl cyclase in opioid tolerance originates from a Gs protein

Abstract

We previously demonstrated that chronic morphine induces a change in G protein coupling by the mu opioid receptor (MOR) from Gi/o to Gs, concurrent with the instatement of an interaction between Gbetagamma and adenylyl cyclase types II and IV. These two signaling changes confer excitatory effects on the cell in place of the typical inhibition by opioids and are associated with morphine tolerance and dependence. Both signaling changes and these behavioral manifestations of chronic morphine are attenuated by cotreatment with ultra-low-dose naloxone. In the present work, using striatum from chronic morphine-treated rats, we isotyped the Gbeta within Gs and Go heterotrimers that coupled to MOR and compared these to the Gbeta isotype of the Gbetagamma that interacted with adenylyl cyclase II or IV after chronic morphine treatment. Isotyping results show that chronic morphine causes a Gs heterotrimer associated with MOR to release its Gbetagamma to interact with adenylyl cyclase. These data suggest that the switch to Gs coupling by MOR in response to chronic morphine, which is attenuated by ultra-low-dose opioid antagonist cotreatment, leads to a two-pronged stimulation of adenylyl cyclase utilizing both Galpha and Gbetagamma subunits of the Gs protein novel to this receptor.

Functional Selectivity at the μ-Opioid Receptor: Implications for Understanding Opioid Analgesia and Tolerance

“…After longterm treatment with morphine (twice-daily injections of 10 mg/kg morphine for 7 days), there is a switch from the inhibitory G␣ i/o coupling to the stimulatory G␣ s coupling in all three brain regions. Moreover, the switch in the G␣ subunit coupling is associated with a stimulation of adenylyl cyclases (Wang et al, 2005;Wang and Burns, 2006). These studies not only provide evidence for a switch in G␣ coupling but also illustrate how morphine-mediated OR coupling can differ depending upon the tissue in which it is expressed.…”

Tolerance to Repeated Morphine Administration Is Associated with Increased Potency of Opioid Agonists

“…Increased agonist potency may simply increase the number of bg subunits after activation of the G i -protein. In addition, several studies have also provided evidence that the m-opioid receptor couples to the G as subtype after chronic morphine pretreatment (Chakrabarti et al, 1998(Chakrabarti et al, , 2005Wang and Burns, 2006). GIRK channels are directly activated by bg subunits (Huang et al, 1995), so an increase in bg release after increased excitatory G as signaling mediated by m-opioid receptors could also increase activation of GIRK channels.…”

Regulation by Ca2+-Signaling Pathways of Adenylyl Cyclases

“…There are 23 studies in isolated membranes or whole cells overexpressing AC2 that document this activation, and some of these are very frequently cited Federman et al1992). An additional four studies show direct Gbg binding to AC2, and this has also been reported in endogenous expression systems (Wang et al 2005;Wang and Burns 2006). In stark contrast, there is much less evidence for the activation of AC4 by Gbg subunits, with only two studies conducted in overexpression systems (Gao and Gilman 1991;Marjamaki et al 1997).…”