The biased OTR ligands -atosiban and carbetocin- differentially inhibit early or late formalin-induced nociception in rats.

Males are more sensitive to intrathecal oxytocin-induced antinociception than females. This antinociception has been linked to oxytocin receptor (OTR) activation. Canonically, OTR is coupled to Gq but can also activate Gi/o proteins. In males, the formalin test showed that oxytocin prevented early nociception (flinches) via the Gq pathway, whereas long-lasting hypersensitivity was halted by Gi/o activation. Here, we tested the effects of biased OTR ligands carbetocin (Gq) and atosiban (Gi/o) on formalin-induced nociception in male and female Wistar rats. Specifically, we assessed the effects of intrathecal carbetocin and atosiban on early (flinches) and late (paw withdrawal threshold) formalin-induced nociception. Pretreatment with L-368,899 (OTR antagonist), U-73122 (phospholipase C inhibitor), L-NAME (nitric oxide synthase inhibitor), or pertussis toxin (a Gi/o inhibitor) was used to dissect the pathways involved. Furthermore, late activation of Akt, ERK1/2, and S6 ribosomal (S6) protein was tracked in spinal tissue by immunoblotting. Carbetocin prevented early nociception in males, whereas atosiban precluded late nociception in both sexes. The antinociception induced by carbetocin and atosiban was abolished by L-368,899, pointing out the role of OTR. Pretreatment with U-73122 or L-NAME blocked the carbetocin effect, whereas pertussis toxin prevented the atosiban effect. Late hypersensitivity correlated with increased levels of phosphorylated S6 protein in the spinal tissue, an effect partly blocked by atosiban. These data suggest that carbetocin prevents early nociception in males via OTR-Gq, and atosiban blocks late hypersensitivity in both sexes via OTR-Gi/o, implying that OTR-biased activation underlies the sexual dimorphism observed in oxytocin-induced antinociception.