Preliminary results from a randomized, controlled, cross-over trial of intrathecal oxytocin for neuropathic pain

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.

Subcutaneous Oxytocin Injection Reduces Heat Pain: A Randomized-Controlled Trial

Oxytocin (OT) is a neuropeptide broadly implicated in social relationships and behavior. OT also exerts antinociceptive and pain-reducing effects in both humans and rodents. Recent research in rodents demonstrates that these effects can be peripheral and local. In human studies, intravenous OT has reduced visceral pain, and subcutaneous injection of OT has reduced postsurgical pain. However, the local effects of subcutaneous OT on experimental pain have not been studied. We conducted a 2-session crossover study during which healthy adults received a subcutaneous injection of synthetic OT (4 mcg/2 mL) or saline placebo (isotonic saline 2 mL), in a randomized and double-blinded manner. Eighteen participants completed full study procedures. We hypothesized that 10 minutes after injection, OT would reduce measures of acute mechanical pain, pressure pain, and heat pain perception. Subcutaneous OT significantly reduced ratings of heat pain intensity and unpleasantness (both P < .01), but did not alter mechanical pain, pressure pain, or heat pain threshold (all P > .05). Changes in heat pain were observed only on the injected arm and not on the contralateral arm, confirming a localized mechanism. These findings confirm the ability of OT in or near the skin to modulate nociceptive processes in cutaneous tissues in human adults, opening exciting avenues for further mechanistic research as well as potential clinical applications for acute pain. PERSPECTIVE: This randomized-controlled trial showed that a subcutaneous injection of OT could reduce perception of heat pain tested with a thermode. OT did not alter mechanical or pressure pain or thresholds for perceiving heat pain. These findings are relevant to scientists and clinicians seeking nonaddictive local drug treatments for pain.