Extremely low doses of oxytocin reduce pain sensitivity in men

Prolactin and oxytocin: potential targets for migraine treatment

Migraine is a severe neurovascular disorder of which the pathophysiology is not yet fully understood. Besides the role of inflammatory mediators that interact with the trigeminovascular system, cyclic fluctuations in sex steroid hormones are involved in the sex dimorphism of migraine attacks. In addition, the pituitary-derived hormone prolactin and the hypothalamic neuropeptide oxytocin have been reported to play a modulating role in migraine and contribute to its sex-dependent differences. The current narrative review explores the relationship between these two hormones and the pathophysiology of migraine. We describe the physiological role of prolactin and oxytocin, its relationship to migraine and pain, and potential therapies targeting these hormones or their receptors.In summary, oxytocin and prolactin are involved in nociception in opposite ways. Both operate at peripheral and central levels, however, prolactin has a pronociceptive effect, while oxytocin appears to have an antinociceptive effect. Therefore, migraine treatment targeting prolactin should aim to block its effects using prolactin receptor antagonists or monoclonal antibodies specifically acting at migraine-pain related structures. This action should be local in order to avoid a decrease in prolactin levels throughout the body and associated adverse effects. In contrast, treatment targeting oxytocin should enhance its signalling and antinociceptive effects, for example using intranasal administration of oxytocin, or possibly other oxytocin receptor agonists. Interestingly, the prolactin receptor and oxytocin receptor are co-localized with estrogen receptors as well as calcitonin gene-related peptide and its receptor, providing a positive perspective on the possibilities for an adequate pharmacological treatment of these nociceptive pathways. Nevertheless, many questions remain to be answered. More particularly, there is insufficient data on the role of sex hormones in men and the correct dosing according to sex differences, hormonal changes and comorbidities. The above remains a major challenge for future development.

Evaluating the efficacy of intranasal oxytocin on pain and function among individuals who experience chronic pain: a protocol for a multisite, placebo-controlled, blinded, sequential, within-subjects crossover trial

INTRODUCTION

Current treatments for chronic pain (eg, opioids) can have adverse side effects and rarely result in resolution of pain. As such, there is a need for adjuvant analgesics that are non-addictive, have few adverse side effects and are effective for pain management across several chronic pain conditions. Oxytocin is a naturally occurring hormone that has gained attention for its potential analgesic properties. The objective of this trial is to evaluate the efficacy of intranasal oxytocin on pain and function among adults with chronic pain.

METHODS AND ANALYSIS

This is a placebo-controlled, triple-blind, sequential, within-subject crossover trial. Adults with chronic neuropathic, pelvic and musculoskeletal pain will be recruited from three Canadian provinces (British Columbia, Alberta and Newfoundland and Labrador, respectively). Enrolled patients will provide one saliva sample pretreatment to evaluate basal oxytocin levels and polymorphisms of the oxytocin receptor gene before being randomised to one of two trial arms. Patients will self-administer three different oxytocin nasal sprays twice daily for a period of 2 weeks (ie, 24 IU, 48 IU and placebo). Patients will complete daily diaries, including standardised measures on day 1, day 7 and day 14. Primary outcomes include pain and pain-related interference. Secondary outcomes include emotional function, sleep disturbance and global impression of change. Intention-to-treat analyses will be performed to evaluate whether improvement in pain and physical function will be observed posttreatment.

ETHICS AND DISSEMINATION

Trial protocols were approved by the Newfoundland and Labrador Health Research Ethics Board (HREB #20227), University of British Columbia Clinical Research Ethics Board (CREB #H20-00729), University of Calgary Conjoint Health Research Ethics Board (REB20 #0359) and Health Canada (Control # 252780). Results will be disseminated through publication in peer-reviewed journals and presentations at scientific conferences.

TRIAL REGISTRATION NUMBER

NCT04903002; Pre-results.

Intranasal Oxytocin Attenuates Reactive and Ongoing, Chronic Pain in a Model of Mild Traumatic Brain Injury

BACKGROUND

Approximately 1.7 million Americans sustain a traumatic brain injury (TBI) each year and chronic pain is a common complication.

OBJECTIVE

We studied the effects of intranasally administered oxytocin as a potential treatment for chronic pain in an animal model of mild TBI.

METHODS

The lateral fluid percussion model of mild TBI was chosen for this purpose and after exposure to mild TBI the rats (n = 12) developed hind paw and facial allodynia compared to sham animals (n = 6). Oxytocin or a vehicle was afterwards administered intranasally and reactive pain was assessed by hind paw and facial von Frey testing. Some animals received the oxytocin receptor antagonist, atosiban, in addition to oxytocin/vehicle treatment (n = 12). The effect of oxytocin on ongoing and spontaneous pain was examined through conditioned place preference testing. To determine whether the effects of intranasal oxytocin could be attributed to delivery via the peripheral blood stream, some TBI animals received an intravenous injection of the same oxytocin dose that was given intranasally. ELISA immunoassays were carried out (n = 6) to measure concentrations of oxytocin in the trigeminal ganglia, pons, spinal cord, and olfactory bulb after intranasal administration and evaluate the most likely route of entry.

RESULTS

These studies confirmed that the fluid percussion model can be used to study post-TBI facial allodynia. Oxytocin attenuated both reactive and spontaneous, ongoing non-reactive pain following mild TBI for at least 3-4 hours after intranasal administration by binding to OT or VA1-receptors most likely by a peri-trigeminal nerve mediated uptake.

CONCLUSIONS

Intranasal oxytocin attenuates measures of reactive and non-reactive pain in a model of mild TBI and may represent a novel treatment for chronic pain in TBI patients.

The Association Between Progesterone, Estradiol, and Oxytocin and Heat Pain Measures in Pregnancy: An Observational Cohort Study

BACKGROUND

Hormonal action has been implicated as a possible mechanism for pregnancy-induced analgesia. Previous investigators have reported an increase in heat pain tolerance during labor compared with nonpregnant controls and postulated it was because of the hormonal changes during pregnancy. However, these previous reports did not include measurement of hormonal values. The purpose of our study was to quantitatively test if changes in pregnancy hormone concentrations correlated with changes in temperature ratings.

METHODS

This was a prospective cohort study consisting of 32 women scheduled for elective cesarean delivery at term between July 2010 and January 2013. Heat pain threshold and tolerance, estrogen, progesterone, and oxytocin levels were measured twice in each patient at term and again 4 to 8 weeks postpartum.

RESULTS

All hormone levels decreased significantly between term pregnancy and the postpartum visit (all P values < 0.029). However, there were no statistically significant differences between term and postpartum heat pain measurements. The mean baseline heat pain threshold was 40.9°C at term compared with 40.3°C °postpartum (P = 0.47; mean change, -0.6°C; 95% confidence interval of change, -1.8°C to +0.7°C). The mean baseline heat pain tolerance was 46.1°C at term and 46.0°C postpartum (P = 0.59; mean change, -0.1°C; 95% confidence interval of change, -0.8°C° to +0.6°C).

CONCLUSIONS

Our findings show that amounts of estradiol and progesterone changed significantly between the term and the postpartum visit; however, the thermal pain tolerance did not significantly change. In summary, we did not observe an association between hormonal changes and changes in pain threshold measures. This finding argues against the concept of simple progesterone- or estrogen-induced analgesia in humans.

Experimenter Effects on Pain Reporting in Women Vary across the Menstrual Cycle

Background. Separate lines of research have shown that menstrual cycling and contextual factors such as the gender of research personnel influence experimental pain reporting. Objectives. This study examines how brief, procedural interactions with female and male experimenters can affect experimentally reported pain (cold pressor task, CPT) across the menstrual cycle. Methods. Based on the menstrual calendars 94 naturally cycling women and 38 women using hormonal contraceptives (M age = 19.83,  SD = 3.09) were assigned to low and high fertility groups. This assignment was based on estimates of their probability of conception given their current cycle day. Experimenters (12 males, 7 females) engaged in minimal procedural interactions with participants before the CPT was performed in solitude. Results. Naturally cycling women in the high fertility group showed significantly higher pain tolerance (81 sec, d = .79) following interactions with a male but not a female experimenter. Differences were not found for women in the low fertility or contraceptive groups. Discussion. The findings illustrate that menstrual functioning moderates the effect that experimenter gender has on pain reporting in women. Conclusion. These findings have implications for standardizing pain measurement protocols and understanding how basic biopsychosocial mechanisms (e.g., person-perception systems) can modulate pain experiences.

The effect of intranasal oxytocin administration on acute cold pressor pain: a placebo-controlled, double-blind, within-participants crossover investigation

BACKGROUND

This study examined the effect of synthetic oxytocin delivered intranasally on acute pain sensitivity using a placebo-controlled, double-blind, within-participant crossover design.

METHODS

Thirty-seven (18 were male) pain-free young adults underwent two laboratory sessions separated by 1 week. Each session consisted of baseline, administration, second baseline, pain, and recovery phases, completed in a fixed order. Participants were given an intransal administration of 40 IU oxytocin or placebo. Blood pressure and heart rate (HR) were measured at 1-minute intervals throughout each phase. Pain was induced by submersing the nondominant hand in cold (2°C) water. Pain threshold, intensity, unpleasantness, and Short-Form McGill Pain Questionnaire-2 pain descriptors were rated immediately after pain testing. Mood was assessed using visual analog scales after baseline, second baseline, and pain phases. The second laboratory session was identical to the first, with the exception that a different nasal spray was administered.

RESULTS

Participants reported lower pain intensity (50.57 [20.94] versus 56.73 [20.12], p = .047), pain unpleasantness (47.00 [27.24] versus 55.78 [22.46], p = .033), and Short-Form McGill Pain Questionnaire-2 pain descriptors (53.38 [31.18] versus 60.92 [31.17], p = .031) and higher pain threshold (45.70 [59.55] versus 38.35 [59.12], p = .040) after oxytocin administration relative to placebo. There was a nasal spray by phase interaction on HR (p = .006). Pain-related increase in HR was attenuated by oxytocin nasal spray. Systolic and diastolic blood pressure increased during pain testing but were unaffected by nasal spray.

CONCLUSIONS

These results suggest that oxytocin can lead to decreased acute pain sensitivity.

Fluctuating experimental pain sensitivities across the menstrual cycle are contingent on women's romantic relationship status

We explored the social-signaling hypothesis that variability in exogenous pain sensitivities across the menstrual cycle is moderated by women's current romantic relationship status and hence the availability of a solicitous social partner for expressing pain behaviors in regular, isochronal ways. In two studies, we used the menstrual calendars of healthy women to provide a detailed approximation of the women's probability of conception based on their current cycle-day, along with relationship status, and cold pressor pain and ischemic pain sensitivities, respectively. In the first study (n = 135; 18-46 yrs., Mage = 23 yrs., 50% natural cycling), we found that naturally-cycling, pair-bonded women showed a positive correlation between the probability of conception and ischemic pain intensity (r = .45), associations not found for single women or hormonal contraceptive-users. A second study (n = 107; 19-29 yrs., Mage = 20 yrs., 56% natural cycling) showed a similar association between greater conception risk and higher cold-pressor pain intensity in naturally-cycling, pair-bonded women only (r = .63). The findings show that variability in exogenous pain sensitivities across different fertility phases of the menstrual cycle is contingent on basic elements of women's social environment and inversely correspond to variability in naturally occurring, perimenstrual symptoms. These findings have wide-ranging implications for: a) standardizing pain measurement protocols; b) understanding basic biopsychosocial pain-related processes; c) addressing clinical pain experiences in women; and d) understanding how pain influences, and is influenced by, social relationships.