Non-Peptide Opioids Differ in Effects on Mu-Opioid (MOP) and Serotonin 1A (5-HT1A) Receptors Heterodimerization and Cellular Effectors (Ca2+, ERK1/2 and p38) Activation

Abstract

The importance of the dynamic interplay between the opioid and the serotonin neuromodulatory systems in chronic pain is well recognized. In this study, we investigated whether these two signalling pathways can be integrated at the single-cell level via direct interactions between the mu-opioid (MOP) and the serotonin 1A (5-HT1A) receptors. Using fluorescence cross-correlation spectroscopy (FCCS), a quantitative method with single-molecule sensitivity, we characterized in live cells MOP and 5-HT1A interactions and the effects of prolonged (18 h) exposure to selected non-peptide opioids: morphine, codeine, oxycodone and fentanyl, on the extent of these interactions. The results indicate that in the plasma membrane, MOP and 5-HT1A receptors form heterodimers that are characterized with an apparent dissociation constant = (440 ± 70) nM). Prolonged exposure to all non-peptide opioids tested facilitated MOP and 5-HT1A heterodimerization and stabilized the heterodimer complexes, albeit to a different extent: = (80 ± 70) nM), = (200 ± 70) nM, = (100 ± 70) nM and = (200 ± 70) nM. The non-peptide opioids differed also in the extent to which they affected the mitogen-activated protein kinases (MAPKs) p38 and the extracellular signal-regulated kinase (Erk1/2), with morphine, codeine and fentanyl activating both pathways, whereas oxycodone activated p38 but not ERK1/2. Acute stimulation with different non-peptide opioids differently affected the intracellular Ca2+ levels and signalling dynamics. Hypothetically, targeting MOP–5-HT1A heterodimer formation could become a new strategy to counteract opioid induced hyperalgesia and help to preserve the analgesic effects of opioids in chronic pain.

Why Search for Alternative GPCR Agonists?

“…However, we do not know if this is a current active area of research. Nevertheless, this interplay between these monoaminergic neurotransmitters and the receptors raises many questions, including on the cross-regulation due to heterodimer formations, such as those described for MT 2 and 5-HT 2C [24], between MT 1 and the orphan receptor GPR50 [25] or µ-opioid receptor and 5-HT 1A [26]. The last-mentioned considerations suggest that it is certainly possible, in some cases, to have better than, or at least as good as, the endogenous agonist.…”

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“…Recent in vitro evidence also suggests that fentanyl may act on the a1 adrenoceptor subtypes, dopamine D1 and D4 receptors, and serotonin 5-HT1A and 5-HT2A receptors, in addition to blocking catecholamine uptake through actions on the vesicular monoamine transporter 1 5,[8][9][10] . Interestingly, with serotonergic signaling, fentanyl has been shown to modulate activity of a recently identified μopioid/5-HT1A receptor heterodimer at the plasma membrane in vitro, in which fentanyl increased the downstream intracellular 5-HT1A signaling pathways for MAPK p38 and Erk1/2 11 . Moreover, long-term treatment with fentanyl appears to induce persistent changes in receptor and related circuit function, for instance as evidenced at the μ-opioid receptor level with tolerance and increased desensitization 12 .…”

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“…Three articles in this Special Issue explore dimerization and intracellular interactions and positive or negative cooperativity between the µOR and angiotensin (AT2) receptors [ 30 ], serotonin (5HT 1A ) receptor [ 31 ], and free fatty acid (FFA) receptors [ 32 ]. Kiraly et al reviewed positive cooperativity between µOR analgesics and angiotensin receptor inhibition [ 30 ].…”

“…The FFAR4 antagonist was also tested in the presence of the δOR agonist DPDPE but also without a strong effect. Finally, Radoi et al utilized fluorescence cross-correlation spectroscopy to examine whether the opioids morphine, codeine, oxycodone, and fentanyl promoted heterodimerization between the serotonin 5HT 1A receptor and µOR [ 31 ]. The authors further assessed the ability of the four opioids to stimulate ERK1/2 and p38 phosphorylation in cells co-expressing µOR and 5HT 1A receptors.…”