The G‐protein‐biased agents PZM21 and TRV130 are partial agonists of μ‐opioid receptor‐mediated signalling to ion channels

Abstract

Background and Purpose Opioids remain the most efficient medications against severe pain; they act on receptors that couple to heterotrimeric G‐proteins in the Gαi/o family. Opioids exert many of their acute effects through modulating ion channels via Gβγ subunits. Many of their side effects are attributed to β‐arrestin recruitment. Several biased agonists that do not recruit β‐arrestins, but activate G‐protein‐dependent pathways, have recently been developed. While these compounds have been proposed to be full agonists of G‐protein signalling in several high throughput pharmacological assays, their effects were not studied on ion channel targets. Experimental Approach Here, we used patch‐clamp electrophysiology and Ca2+ imaging to test the effects of TRV130, PZM21, and herkinorin, three G‐protein‐biased agonists of μ‐opioid receptors, on ion channel targets of Gαi/o/Gβγ signalling. We also studied G‐protein dissociation using a FRET‐based assay. Key Results All three biased agonists induced smaller activation of G‐protein‐coupled inwardly rectifying K+ channels (Kir3.2) and smaller inhibition of transient receptor potential melastatin (TRPM3) channels than the full μ receptor agonist DAMGO. Co‐application of TRV130 or PZM21, but not herkinorin, alleviated the effects of DAMGO on both channels. PZM21 and TRV130 also decreased the effect of morphine on Kir3.2 channels. The CaV2.2 channel was also inhibited less by PZM21 and TRV130 than by DAMGO. We also found that TRV130, PZM21, and herkinorin were less effective than DAMGO at inducing dissociation of the Gαi/Gβγ complex. Conclusion and Implications TRV130, PZM21, and potentially herkinorin are partial agonists of μ receptors.

Functional expression and pharmacological modulation of TRPM3 in human sensory neurons

“…We observed that PS responses were rapidly and reversibly inhibited by DAMGO in about 50% of the hSCDS neurons. Such a variable inhibition of TRPM3 by μ‐opioid receptor activation was also reported in isolated mouse sensory neurons, where PS responses were insensitive to DAMGO or morphine in between 5% and 30% of the neurons (Dembla et al, ; Quallo et al, ; Yudin & Rohacs, ). The variable inhibition of the TRPM3 responses may reflect the unequal expression of the μ‐opioid receptor or its downstream signalling pathway in sensory neurons.…”

“…In rodents, it has been previously shown that TRPM3 activity is suppressed following activation of a variety of GPCRs, including the μ‐opioid receptor, via a mechanism involving the G βγ subunit of trimeric G proteins (Badheka et al, ; Dembla et al, ; Quallo et al, ; Yudin & Rohacs, ). Considering the role of TRPM3 in nociception, it has been proposed that this pathway could contribute to the analgesic effects of peripheral opioids.…”

“…Interestingly, it was recently shown that activation of the μ‐opioid receptor inhibits TRPM3‐mediated responses in mouse sensory neurons, via a mechanism that involved direct interaction of G βγ with the channel. Accordingly, peripherally applied opioids such as morphine or DAMGO were shown to inhibit TRPM3‐dependent pain responses in mice (Dembla et al, ; Quallo et al, ; Yudin & Rohacs, ) and it has been suggested that this mechanism may contribute to the peripheral analgesic effect of opioids (Dembla et al, ; Quallo et al, ). We observed that PS responses were rapidly and reversibly inhibited by DAMGO in about 50% of the hSCDS neurons.…”

“…Strikingly, TRPM3 −/− mice do not develop inflammatory hyperalgesia in response to injection of complete Freud's adjuvant (Vriens et al, ) and systemic TRPM3 antagonists were shown to alleviate mechanical and thermal hyperalgesia in mouse and rat models of inflammatory and neuropathic pain (Chen, Chen, Qian, Fang, & Zhu, ; Jia, Zhang, & Yu, ; Krugel, Straub, Beckmann, & Schaefer, ). Moreover, TRPM3 activity in sensory neurons is strongly suppressed by μ‐opioid receptor activation in sensory neurons, suggesting that TRPM3 may contribute to peripheral analgesic effects of opioids (Dembla et al, ; Quallo, Alkhatib, Gentry, Andersson, & Bevan, ; Yudin & Rohacs, ).…”

Biased versus Partial Agonism in the Search for Safer Opioid Analgesics

“…In particular the following rank order of maximal effects was determined: DAMGO = fentanyl = methadone > morphine = oxycodone > oliceridine = PZM21 ≥ SR-17018 ≥ buprenorphine which was highly conserved in all the assays (r 2 always ≥ 0.79) including ligand-induced C-terminal phosphorylation of the mu receptor. Of note is that the partial agonist behavior of PZM21 and oliceridine at the mu receptor has already been reported for ion channel signaling using electrophysiological and Ca 2+ imaging techniques [ 65 ] and in biochemical assays (Azzam et al personal communication). Surprisingly, this same rank order of maximal effects was measured in receptor regulatory pathway assays (GRK2 and βarr2 and receptor internalization) for all compounds; thus, suggesting that they have similar activity in both G protein and receptor regulatory pathways.…”