Background The most common functional single nucleotide polymorphism of the human OPRM1 gene, A118G, has been shown to be associated with inter-individual differences in opioid analgesic requirements, particularly with morphine, in patients with acute postoperative pain. The purpose of the present study was to examine whether this polymorphism would modulate the morphine and fentanyl pharmacological profile of sensory neurons isolated from a humanized mouse model homozygous for either the 118A or 118G allele. Methods The coupling of wild-type and mutant mu opioid receptors to voltage-gated Ca2+ channels after exposure to either ligand was examined by employing the whole-cell variant of the patch-clamp technique in acutely dissociated trigeminal ganglion neurons. Morphine-mediated antinociception was measured in mice carrying either the 118AA or 118GG allele. Results The biophysical parameters (cell size, current density, and peak current amplitude potential) measured from both groups of sensory neurons were not significantly different. In 118GG neurons, morphine was approximately 5-fold less potent and 26% less efficacious than that observed in 118AA neurons. On the other hand, the potency and efficacy of fentanyl were similar for both groups of neurons. Morphine-mediated analgesia in 118GG mice was significantly reduced compared to the 118AA mice. Conclusions This study provides evidence to suggest that the diminished clinical effect observed with morphine in 118G carriers results from an alteration of the receptor’s pharmacology in sensory neurons. Additionally, the impaired analgesic response with morphine may explain why carriers of this receptor variant have an increased susceptibility to become addicted to opioids.

The pharmacology of G protein-coupled receptors can be influenced by factors such as constitutive receptor activation and Na ϩ ions. In this study, we examined the coupling of natively and heterologously expressed nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptors with voltage-dependent

Key point• Sympathetic stellate ganglion (SG) neurons that innervate cardiac muscle play an important role in regulating heart rate and contractility.• In this study, we examined which Gβ protein subunit couples Ca 2+ channels to opioid and adrenergic receptors expressed in SG neurons.• We show that Gβ2 and Gβ4 are important signalling elements that maintain the transduction pathway of both receptor subtypes and Ca 2+ channels, and how their expression levels are differentially compensated.• Our results also indicate that Gβ1 subunits do not appear to play a role in either signalling pathway.• The elucidation of specific G proteins that couple Ca 2+ channels with opioid and adrenergic receptors will help us better understand how these receptors regulate synaptic transmission of SG neurons that innervate cardiac muscle.Abstract Cardiac function is regulated in part by the sympathetic branch of the autonomic nervous system via the stellate ganglion (SG) neurons. Neurotransmitters, such as noradrenaline (NA), and neuropeptides, including nociceptin (Noc), influence the excitability of SG neurons by modulating Ca 2+ channel function following activation of the adrenergic and nociceptin/orphanin FQ peptide (NOP) opioid receptors, respectively. The regulation of Ca 2+ channels is mediated by Gβγ, but the specific Gβ subunit that modulates the channels is not known. In the present study, small interference RNA (siRNA) was employed to silence the natively expressed Gβ proteins in rat SG tissue and to examine the coupling specificity of adrenergic and NOP opioid receptors to Ca 2+ channels employing the whole-cell variant of the patch-clamp technique. Western blotting analysis showed that Gβ1, Gβ2 and Gβ4 are natively expressed. The knockdown of Gβ2 or Gβ4 led to a significant decrease of the NA-and Noc-mediated Ca 2+ current inhibition, while Gβ1 silencing was without effect. However, sustaining low levels of Gβ2 resulted in an increased expression of Gβ4 and a concomitant compensation of both adrenergic and opioid signalling pathways modulating Ca 2+ channels. Conversely, Gβ4-directed siRNA was not accompanied with a compensation of the signalling pathway. Finally, the combined silencing of Gβ2 and Gβ4 prevented any additional compensatory mechanisms. Overall, our studies suggest that in SG neurons, Gβ2 and Gβ4 normally maintain the coupling of Ca 2+ channels with the receptors, with the latter subtype responsible for maintaining the integrity of both pathways.

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Several studies have shown that human carriers of the single nucleotide polymorphism of the µ-opioid receptor, OPRM1 A118G, exhibit greater drug and alcohol use, increased sensitivity to pain, and reduced sensitivity to the antinociceptive effects of opiates. In the present study, we employed a ‘humanized’ mouse model containing the wild-type (118AA) or variant (118GG) allele to examine behavior in our model of drug-induced suppression of a natural reward cue and to compare the morphine pharmacological profile in acutely isolated sensory neurons. Compared with 118AA mice, our results demonstrate that homozygous 118GG mice exhibit greater avoidance of the cocaine-paired saccharin cue, a behavior linked to an aversive withdrawal-like state. Electrophysiological recordings confirmed the reduced modulation of Ca2+ channels by morphine in trigeminal ganglion (TG) neurons from 118GG mice compared to the 118AA control cells. However, repeated cocaine exposure in 118GG mice led to a leftward shift of the morphine concentration-response relationship when compared with 118GG control mice, while a rightward shift was observed in 118AA mice. These results suggest that cocaine exposure of mice carrying the 118G allele leads to a heightened sensitivity of the reward system and a blunted modulation of Ca2+ channels by morphine in sensory neurons.