Modulation of μ opioid receptor signalling
Opioids are most widely used for treating moderate to severe pain despite their association with some serious adverse effects. The clinically used opioids act at the μ receptor, a prototypical G protein-coupled receptor. Some novel strategies that aim to enhance the analgesic efficacy of opioids and reduce their clinically limiting adverse effects focus on modulating μ receptor signalling. The studies presented in this thesis are undertaken to understand the molecular basis through which drugs act at the μ receptor and its common structural variants.
Tapentadol is a commonly prescribed opioid which combines the mechanism of μ receptor activation with nor-adrenaline reuptake inhibition in a single molecule. In an assay that quantitatively determined efficacy using operational model of pharmacological agonism, tapentadol demonstrated low intrinsic efficacy to activate μ receptor as compared to oxycodone and morphine. Its effect was further compared in the common variants of the receptor-N40D and A6V, which have been associated with the variation of μ receptor signalling and function. We observed no effect mediated by these SNPs on the efficacy and potency of opioids to activate GIRK channel in AtT20 cells. As these SNPs are shown to have cell and pathway-specific responses, we further challenged their effect in HEK cells on two signalling endpoints of μ receptor (cAMP inhibition and GIRK activation) and yet observed no difference.
Besides their primary role in analgesia, opioids are commonly misused for recreation either alone or mixed with other compounds. Co-ingestion of opioids with gabapentinoids (pregabalin and gabapentin) and cannabinoids, cannabidiol (CBD) and Δ9-tetrahydrocannabinol (THC) is a common occurrence among unregulated opioid users. We utilised the concept of allosteric modulation to understand the molecular mechanism underlying the mixing of these compounds with opioids. Results revealed no allosteric effect at therapeutic concentrations of gabapentinoids and cannabinoids on morphine-induced μ receptor signalling, however, higher concentrations of CBD and THC negatively regulated the opioid response. Future work on these modulatory compounds should consider their effect on the intrinsic efficacy of opioids at other signalling pathways and the possible effect of functional selectivity.
The findings from this thesis provide valuable insight to μ receptor modulation and supports future investigations into developing compounds that may positively modulate the therapeutic properties of opioids.
