Successful Pain Management with Epidural Oxytocin

Evaluating the efficacy of oxytocin for pain management: An updated systematic review and meta-analysis of randomized clinical trials and observational studies

Background

There is a need for novel analgesics with favorable risk to benefit profiles. Oxytocin has recently gained attention for its potential analgesic properties.

Aim

The aim of this study was to perform an updated systematic review and meta-analysis evaluating the effect of oxytocin for pain management.

Method

Ovid MEDLINE, Embase, PsycINFO, CINAHL, and Clinicaltrials.gov were searched for articles reporting on associations between oxytocin and chronic pain management from January 2012 to February 2022. Studies published before 2012 that were identified in our previous systematic review were also eligible. Risk of bias of included studies was assessed. Synthesis of results was performed using meta-analysis and narrative synthesis.

Results

Searches returned 2087 unique citations. In total, 14 articles were included that reported on 1504 people living with pain. Results from meta-analysis and narrative review were mixed. Meta-analysis of three studies indicated that exogenous oxytocin administration did not result in a significant reduction in pain intensity relative to placebo (N = 3; n = 95; g = 0.31; 95% confidence interval [CI] -0.10, 0.73). Narrative review provided encouraging evidence that exogenous oxytocin administration reduced pain sensitivity among individuals with back pain, abdominal pain, and migraines. Results suggested that individual difference factors (e.g., sex and chronic pain condition) may influence oxytocin-induced nociception, but the heterogeneity and limited number of studies identified precluded further investigation.

Discussion

There is equipoise for the benefit of oxytocin for pain management. Future studies are imperative and should undertake more precise exploration of potential confounds and mechanisms of analgesic action to clarify inconsistency in the literature.

The role of the endocannabinoid 2-arachidonoylglycerol in the in vivo spinal oxytocin-induced antinociception in male rats

Oxytocin receptor (OTR) activation at the spinal level produces antinociception. Some data suggest that central OTR activation enhances social interaction via an increase of endocannabinoids (eCB), but we do not know if this could occur at the spinal level, modulating pain transmission. Considering that oxytocin via OTR stimulates diacylglycerol formation, a key intermediate in synthesizing 2-arachidonylglycerol (2-AG), an eCB molecule, we sought to test the role of the eCB system on the spinal oxytocin-induced antinociception. Behavioral and electrophysiological experiments were conducted in naïve and formalin-treated (to induce long-term mechanical hypersensitivity) male Wistar rats. Intrathecal RHC 80267 injections, an inhibitor of the enzyme diacylglycerol lipase (thus, decreasing 2-AG formation), produces transient mechanical hypersensitivity, an effect unaltered by oxytocin but reversed by gabapentin. Similarly, in in vivo extracellular recordings of naïve spinal wide dynamic range cells, juxtacellular picoinjection of RHC 80267 increases the firing of nociceptive Aδ-, C-fibers, and post-discharge, an effect unaltered by oxytocin. Interestingly, in sensitized rats, oxytocin picoinjection reverses the RHC 80627-induced hyperactivity of Aδ-fibers (but not C- or post-discharge activity). In contrast, a sub-effective dose of JZL184 (a monoacylglycerol lipase inhibitor, thus favoring 2-AG levels), which does not have per se an antinociceptive effect in the formalin-induced hypernociception, the oxytocin-induced antinociception is boosted. Similarly, electrophysiological experiments suggest that juxtacellular JZL184 diminishes the neuronal firing of nociceptive fibers, and co-injection with oxytocin prolongs and enhances the antinociceptive effect. These data may imply that 2-AG formation may play a role in the spinal antinociception induced by oxytocin.

Estrogen can abolish oxytocin-induced spinal anti-hyperalgesia

Our previous study verified a sex difference of anti-hyperalgesia in rats and anti-allodynia in mice induced by intrathecal oxytocin (OT). In the model of intraplantar carrageenan-induced inflammatory hyperalgesia, intrathecal OT injection induced a substantial anti-hyperalgesia in male rats even at a low dose (0.125 nmol). In contrast, female rats only responded to an extremely high dose (1.25 nmol). This sex difference concurs with a lower expression of OT receptors and higher expression of insulin-regulated aminopeptidase (IRAP; OT degrading enzyme) in the spinal cords of female rats. In this study, we further determined the role of female hormones in this sex difference by using ovariectomized rats. Our results show that a low dose of intrathecal OT caused a significant anti-hyperalgesia effect in ovariectomized female rats, similar to that seen in male rats. Ovariectomy did not cause any change of paw edema except at the late stage of convalescence when compared with the sham-operated group. Ovariectomy-induced faster recovery from edema but did not affect the severity of hyperalgesia. Moreover, there was a similar amount of IRAP expression in ovariectomized and sham rats. When estradiol (E2) was given together with OT, OT-induced anti-hyperalgesia was abolished at the developmental stage of hyperalgesia in ovariectomized rats. These results show an inhibitory role of female hormones generated from ovaries (mainly estrogen) in the sex difference of anti-hyperalgesia induced by OT. This study suggests the feasibility of a novel OT-based remedy to treat hyperalgesia in men and in menopausal women no receiving hormonal supplements.

Ultrastructural Evidence for Oxytocin and Oxytocin Receptor at the Spinal Dorsal Horn: Mechanism of Nociception Modulation

Oxytocin is a hypothalamic neuropeptide involved in the inhibition of nociception transmission at spinal dorsal horn (SDH) level (the first station where the incoming peripheral signals is modulated). Electrophysiological, behavioral, and pharmacological data strongly support the role of this neuropeptide and its receptor (the oxytocin receptor, OTR) as a key endogenous molecule with analgesic properties. Briefly, current data showed that oxytocin release from the hypothalamus induces OTR activation at the SDH, inducing selective inhibition of the nociceptive Aδ- and C-fibers (probably peptidergic) activity, but not the activity of proprioceptive fibers (i.e. Aβ-fibers). The above inhibition could be a direct presynaptic mechanism, or a mechanism mediated by GABAergic interneurons. However, the exact anatomical localization of oxytocin and OTR remains unclear. In this context, the present study set out to analyze the role of OTRs, GABAergic cells and CGRP fibers in the SDH in rats by using electron microscopy. Ultrastructural analyses of the SDH tissue show that: (i) oxytocin and OTR are found in asymmetrical synapsis; (ii) OTR is found in GABAergic interneurons (near unmyelinated fibers), CGRPergic fibers and glial cells; (iii) whereas oxytocin is present in supraspinal descending projection fibers. These anatomical data strongly support the notion that oxytocin released at the SDH could presynaptically inhibit the nociceptive input from the peripheral primary afferent fibers. This inhibitory action could be direct or use a GABA interneuron. Furthermore, our findings that OTR is exhibited in glial tissue at the SDH requires further exploration in nociception assays.

Anti-nociceptive effects of oxytocin receptor modulation in healthy volunteers-A randomized, double-blinded, placebo-controlled study

BACKGROUND

There is increasing evidence for oxytocin as a neurotransmitter in spinal nociceptive processes. Hypothalamic oxytocinergic neurons project to the spinal dorsal horn, where they activate GABA-ergic inhibitory interneurons. The present study tested whether the long-acting oxytocin-analogue carbetocin has anti-nociceptive effects in multi-modal experimental pain in humans.

METHODS

Twenty-five male volunteers received carbetocin 100 mcg and placebo (0.9% NaCl) on two different sessions in a randomized, double-blinded, cross-over design. Multi-modal quantitative sensory testing (QST) including a model of capsaicin-induced hyperalgesia and allodynia were performed at baseline and at 10, 60 and 120 min after drug administration. QST data were analysed using mixed linear and logistic regression models. Carbetocin plasma concentrations and oxytocin receptor genotypes were quantified and assessed in an exploratory fashion.

RESULTS

An anti-nociceptive effect of carbetocin was observed on intramuscular electrical temporal summation (estimated difference: 1.26 mA, 95% CI 1.01 to 1.56 mA, p = .04) and single-stimulus electrical pain thresholds (estimated difference: 1.21 mA, 95% CI 1.0 to 1.47 mA, p = .05). Furthermore, the area of capsaicin-induced allodynia was reduced after carbetocin compared to placebo (estimated difference: -6.5 cm² , 95% CI -9.8 to -3.2 cm² , p < .001).

CONCLUSIONS

This study provides evidence of an anti-nociceptive effect of carbetocin on experimental pain in humans.

SIGNIFICANCE

This study provides evidence of the anti-nociceptive effect of intravenous administration of the oxytocin agonist carbetocin in healthy male volunteers.

Intrathecal Oxytocin Improves Spontaneous Behavior and Reduces Mechanical Hypersensitivity in a Rat Model of Postoperative Pain

The first few days post-surgery, patients experience intense pain, hypersensitivity and consequently tend to have minor locomotor activity to avoid pain. Certainly, injury to peripheral tissues produces pain and increases sensitivity to painful (hyperalgesia) and non-painful (allodynia) stimuli. In this regard, preemptive pharmacological treatments to avoid or diminish pain after surgery are relevant. Recent data suggest that the neuropeptide oxytocin when given at spinal cord level could be a molecule with potential preemptive analgesic effects, but this hypothesis has not been properly tested. Using a validated postoperative pain model (i.e. plantar incision), we evaluated in male Wistar rats the potential preemptive antinociceptive effects of intrathecal oxytocin administration measuring tactile hypersensitivity (across 8 days) and spontaneous motor activity (across 3 days). Hypersensitivity was evaluated using von Frey filaments, whereas spontaneous activity (total distance, vertical activity episodes, and time spent in the center of the box) was assessed in real time using a semiautomated open-field system. Under these conditions, we found that animals pretreated with spinal oxytocin before plantar incision showed a diminution of hypersensitivity and an improvement of spontaneous behavior (particularly total distance and vertical activity episodes). This report provides a basis for addressing the therapeutic relevance of oxytocin as a potential preemptive analgesic molecule.

Oxytocin prevents neuronal network pain-related changes on spinal cord dorsal horn in vitro

Recently, oxytocin (OT) has been studied as a potential modulator of endogenous analgesia by acting upon pain circuits at the spinal cord and supraspinal levels. Yet the detailed action mechanisms of OT are still undetermined. The present study aimed to evaluate the action of OT in the spinal cord dorsal horn network under nociceptive-like conditions induced by the activation of the N-methyl-d-aspartate (NMDA) receptor and formalin injection, using calcium imaging techniques. Results demonstrate that the spontaneous Ca²⁺-dependent activity of the dorsal horn cells was scarce, and the coactivity of cells was mainly absent. When NMDA was applied, high rates of activity and coactivity occurred in the dorsal horn cells; these rates of high activity mimicked the activity dynamics evoked by a neuropathic pain condition. In addition, although OT treatment increased activity rates, it was also capable of disrupting the conformation of coordinated activity previously consolidated by NMDA treatment, without showing any effect by itself. Altogether, our results suggest that OT globally prevents the formation of coordinated patterns previously generated by nociceptive-like conditions on dorsal horn cells by NMDA application, which supports previous evidence showing that OT represents a potential therapeutic alternative for the treatment of chronic neuropathic pain.

Sex Difference of Angiotensin IV-, LVV-Hemorphin 7-, and Oxytocin-Induced Antiallodynia at the Spinal Level in Mice With Neuropathic Pain

BACKGROUND

We demonstrated previously that angiotensin IV (Ang IV) and LVV-hemorphin 7 (LVV-H7) act through the blockade of insulin-regulated aminopeptidase to decrease oxytocin degradation, thereby causing antihyperalgesia at the spinal level in rats. We determined that intrathecal oxytocin can induce significant antihyperalgesia in male rats with inflammation but not in female rats. Thus, we speculate that Ang IV, LVV-H7, and oxytocin can induce antiallodynia, which could be of great therapeutic potential. Because the antihyperalgesia by using these peptides was with sex difference, their possible antiallodynia was examined in male and female mice for comparison. We investigated whether Ang IV, LVV-H7, and oxytocin produce antiallodynia at the spinal level in mice and whether this antiallodynia differs between the sexes.

METHODS

Partial sciatic nerve ligation surgery was performed on adult male and female C57BL/6 mice from the same litter (25-30 g). The effects of intrathecal injections of Ang IV (25.8 nmol), LVV-H7 (27.2 nmol), and oxytocin (0.125 or 1.25 nmol) were assessed through the von Frey test 3 days after partial sciatic nerve ligation.

RESULTS

Intrathecal injection of Ang IV, LVV-H7, and oxytocin all produced a potent antiallodynia in male mice. However, these antiallodynia effects were either extremely weak or absent in female mice at the same dose.

CONCLUSIONS

Intrathecal Ang IV, LVV-H7, and oxytocin can all cause significant antiallodynia in male mice. The Ang IV-, LVV-H7-, and oxytocin-induced antiallodynia effects differed between the sexes at the spinal level in mice.

Neuropathic Pain Up-Regulates Hypothalamo-Neurohypophysial and Hypothalamo-Spinal Oxytocinergic Pathways in Oxytocin-Monomeric Red Fluorescent Protein 1 Transgenic Rat

Despite the high incidence of neuropathic pain, its mechanism remains unclear. Oxytocin (OXT) is an established endogenous polypeptide produced in the supraoptic nucleus (SON) and paraventricular nucleus (PVN) of the hypothalamus. OXT, which is synthesized by OXT neurons in the SON and the magnocellular part of the PVN (mPVN), is delivered into the posterior pituitary (PP), then released into the systemic blood circulation. Meanwhile, OXT-containing neurosecretory cells in the parvocellular part of the PVN (pPVN) are directly projected to the spinal cord and are associated with sensory modulation. In this study, the OXT system in the hypothalamo-neurohypophysial and hypothalamo-spinal pathway was surveyed using a rat neuropathic pain model induced by partial sciatic nerve ligation (PSL). In the present study, we used transgenic rats expressing an OXT-monomeric red fluorescent protein 1 (mRFP1) fusion gene. In a neuropathic pain model, mechanical allodynia was observed, and glial cell activation was also confirmed via immunohistochemistry. In this neuropathic pain model, a significant increase in the OXT-mRFP1 expression was observed in the PP, the SON, mPVN, and pPVN. Furthermore, OXT-mRFP1 granules with positive fluorescent reaction were remarkably increased in laminae I and II of the ipsilateral dorsal horn. Although the plasma concentrations of OXT did not significantly change, a significant increase of the mRNA levels of OXT and mRFP1 in the SON, mPVN, and pPVN were observed. These results suggest that neuropathic pain induced by PSL upregulates hypothalamic OXT synthesis and transportation to the OXTergic axon terminals in the PP and spinal cord.

Role of central oxytocin and dopamine systems in nociception and their possible interactions: suggested hypotheses

Central oxytocin and dopamine have an important role in the process of nociception at the spinal level as well as supraspinal structures, e.g. anterior cingulate cortex, insular cortex, amygdala, nucleus accumbens, and hypothalamus. Many studies have pointed out the importance of both systems in the pain descending modulatory system and in pain-related symptoms in some chronic disorders, e.g. Parkinson disease and fibromyalgia. The interaction between oxytocin and dopamine systems has been addressed in some motivational behaviors, e.g. maternal and sexual behaviors, pair bonding, and salience. In this aspect, we propose that an oxytocin-dopamine interaction could be present in nociception, and we also explain the possible hypotheses of such an interaction between these systems.

Oxytocin inhibits the rat medullary dorsal horn Sp5c/C1 nociceptive transmission through OT but not V1A receptors

The medullary dorsal horn (MDH or Sp5c/C1 region) plays a key role modulating the nociceptive input arriving from craniofacial structures. Some reports suggest that oxytocin could play a role modulating the nociceptive input at the MDH level, but no study has properly tested this hypothesis. Using an electrophysiological and pharmacological approach, the present study aimed to determine the effect of oxytocin on the nociceptive signaling in the MDH and the receptor involved. In sevoflurane, anesthetized rats, we performed electrophysiological unitary recordings of second order neurons at the MDH region responding to peripheral nociceptive-evoked responses of the first branch (V1; ophthalmic) of the trigeminal nerve. Under this condition, we constructed dose-response curves analyzing the effect of local spinal oxytocin (0.2-20 nmol) on MDH nociceptive neuronal firing. Furthermore, we tested the role of oxytocin receptors (OTR) or vasopressin V_1A receptors (V_1AR) involved in the oxytocin effects. Oxytocin dose-dependently inhibits the peripheral-evoked activity in nociceptive MDH neurotransmission. This inhibition is associated with a blockade of neuronal activity of Aδ- and C-fibers. Since this antinociception was abolished by pretreatment (in the MDH) with the potent and selective OTR antagonist (L-368,899; 20 nmol) and remained unaffected after the V_1AR antagonist (SR49059; 20 nmol or 200 nmol), the role of OTR is implied. This electrophysiological study demonstrates that oxytocin inhibits the peripheral-evoked neuronal activity at MDH, through OTR activation. Thus, OTR may represent a new potential drug target to treat craniofacial nociceptive dysfunction in the MDH.

Oxytocin Signaling in Pain: Cellular, Circuit, System, and Behavioral Levels

Originally confined to the initiation of parturition and milk ejection after birth, the hypothalamic nonapeptide oxytocin (OT) is now recognized as a critical determinant of social behavior and emotional processing. It accounts for the modulation of sensory processing and pain perception as OT displays a potent analgesic effect mediated by OT receptors (OTRs) expressed in the peripheral and central nervous systems. In our chapter, we will first systemically analyze known efferent and afferent OT neuron projections, which form the anatomical basis for OT modulation of somatosensory and pain processing. Next, we will focus on the synergy of distinct types of OT neurons (e.g., magno- and parvocellular OT neurons) which efficiently control acute inflammatory pain perception. Finally, we will describe how OT signaling mechanisms in the spinal cord control nociception, as well as how OT is able to modulate emotional pain processing within the central amygdala. In the conclusions at the end of the chapter, we will formulate perspectives in the study of OT effects on pain anticipation and pain memory, as well as propose some reasons for the application of exogenous OT for the treatment of certain types of pain in human patients.

The potential role of serotonergic mechanisms in the spinal oxytocin-induced antinociception

The role of oxytocin (OXT) in pain modulation has been suggested. Indeed, hypothalamic paraventricular nuclei (PVN) electrical stimuli reduce the nociceptive neuronal activity (i.e., neuronal discharge associated with activation of Aδ- and C-fibers) of the spinal dorsal horn wide dynamic range (WDR) cells and nociceptive behavior. Furthermore, raphe magnus nuclei lesion reduces the PVN-induced antinociception, suggesting a functional interaction between the OXT and the serotoninergic system. The present study investigated in Wistar rats the potential role of spinal serotonergic mechanisms in the OXT- and PVN-induced antinociception. In long-term secondary mechanical allodynia and hyperalgesia induced by formalin or extracellular unitary recordings of the WDR cells we evaluated the role of 5-hydroxytryptamine (5-HT) effect on the OXT-induced antinociception. All drugs were given intrathecally (i.t.). OXT (1×10^-5-1×10^-4nmol) or 5-HT (1×10^-3-1×10^-1nmol) prevented the formalin-induced sensitization, an effect mimicked by PVN stimulation. Moreover, administration of OXT (1×10^-5nmol) plus 5-HT (1×10^-3nmol) at ineffective doses, produced antinociception. This effect was antagonized by: (i) d(CH₂)₅[Tyr(Me)²,Thr⁴,Tyr-NH₂⁹]OVT (oxytocin receptor antagonist; 2×10^-2nmol); or (ii) methiothepin (a non-specific 5-HT_1/2/5/6/7 receptor antagonist; 80nmol). Similar results were obtained with PVN stimulation plus 5-HT (5×10^-5nmol). In WDR cell recordings, the PVN-induced antinociception was enhanced by i.t. 5-HT and partly blocked when the spinal cord was pre-treated with methiothepin (80nmol). Taken together, these results suggest that serotonergic mechanisms at the spinal cord level are partly involved in the OXT-induced antinociception.