No pain, no social gains: A social-signaling perspective of human pain behaviors

Neuroimmune Pain and Its Manipulation by Pathogens

Recent studies highlight extensive crosstalk that exists between sensory neurons responsible for pain and the immune system. Cutaneous pain neurons detect harmful microbes, recruit immune cells, and produce anticipatory immunity in nearby tissues. These complementary systems generally protect hosts from infections. At the same time, neuroimmune pain is vulnerable to manipulation. Some pathogens evade immunity activated by nociceptors by producing opioid analogs and by interfering with sensory nerve function. Other organisms manipulate neuroimmune pain by increasing it. Hosts may gain protection from interference by adjusting pain sensitivity. Nociceptive sensitization follows expectations of signal detection theory and the smoke detector principle, allowing pain to be more easily triggered in response to microbial threats and damage. However, pain sensitization at the spinal level and cortical responses to pain are themselves the target of manipulation by parasites and other organisms. Here we review examples of parasites, bacteria, and other medically important organisms that interfere with pain signaling and describe their implications for public health, infectious disease, and the treatment of pain.

Understanding feeling "high" and its role in medical cannabis patient outcomes

Introduction: We measure for the first time the associations between subjective patient experiences of feeling "high" and treatment outcomes during real-time Cannabis flower consumption sessions. Methods: Our study uses data from the mobile health app, Releaf App™, through which 1,882 people tracked the effects of Cannabis flower on a multitude of health conditions during 16,480 medical cannabis self-administration sessions recorded between 6/5/2016 and 3/11/2021. Session-level reported information included plant phenotypes, modes of administration, potencies, baseline and post-administration symptom intensity levels, total dose used, and real-time side effect experiences. Results: Patients reported feeling high in 49% of cannabis treatment sessions. Using individual patient-level fixed effects regression models and controlling for plant phenotype, consumption mode, tetrahydrocannabinol (THC) and cannabidiol (CBD) potencies, dose, and starting symptom level, our results show that, as compared to sessions in which individuals did not report feeling high, reporting feeling high was associated with a 7.7% decrease in symptom severity from a mean reduction of -3.82 on a 0 to 10 analog scale (coefficient = -0.295, p < 0.001) with evidence of a 14.4 percentage point increase (p < 0.001) in negative side effect reporting and a 4.4 percentage point (p < 0.01) increase in positive side effect reporting. Tetrahydrocannabinol (THC) levels and dose were the strongest statistical predictors of reporting feeling high, while the use of a vaporizer was the strongest inhibitor of feeling high. In symptom-specific models, the association between feeling high and symptom relief remained for people treating pain (p < 0.001), anxiety (p < 0.001), depression (p < 0.01) and fatigue (p < 0.01), but was insignificant, though still negative, for people treating insomnia. Although gender and pre-app cannabis experience did not appear to affect the relationship between high and symptom relief, the relationship was larger in magnitude and more statistically significant among patients aged 40 or less. Discussion: The study results suggest clinicians and policymakers should be aware that feeling high is associated with improved symptom relief but increased negative side effects, and factors such as mode of consumption, product potency, and dose can be used to adjust treatment outcomes for the individual patient.

Alleviative effects of Cannabis flower on migraine and headache

OBJECTIVE

Few studies to date have measured the real-time effects of consumption of common and commercially available Cannabis products for the treatment of headache and migraine under naturalistic conditions. This study examines, for the first time, the effectiveness of using dried Cannabis flower, the most widely used type of Cannabis product in the United States, in actual time for treatment of headache- and migraine-related pain and the associations between different product characteristics and changes in symptom intensity following Cannabis use.

METHODS

Between 06/10/2016 and 02/12/2019, 699 people used the Releaf Application to record real-time details of their Cannabis use, including product characteristics and symptom intensity levels prior to and following self-administration; data included 1910 session-level attempts to treat headache- (1328 sessions) or migraine-related pain (582 sessions). Changes in headache- or migraine-related pain intensity were measured on a 0-10 scale prior to, and immediately, following Cannabis consumption.

RESULTS

Ninety-four percent of users experienced symptom relief within a two-hour observation window. The average symptom intensity reduction was 3.3 points on a 0-10 scale (standard deviation = 2.28, Cohen's d = 1.58), with males experiencing greater relief than females (P < 0.001) and a trend that younger users (< 35 years) experience greater relief than older users (P = 0.08). Mixed effects regression models showed that, among the known (i.e., labeled) product characteristics, tetrahydrocannabinol levels 10% and higher are the strongest independent predictors of symptom relief, and this effect is particularly prominent in headache rather than migraine sufferers (P < 0.05), females (P < 0.05) and younger users (P < 0.001). Females and younger users also appear to gain greater symptom relief from flower labeled as "C. indica" rather than "C. sativa" or other hybrid strains.

CONCLUSION

These results suggest that whole dried Cannabis flower may be an effective medication for treatment of migraine- and headache-related pain, but the effectiveness differs according to characteristics of the Cannabis plant, the combustion methods, and the age and gender of the patient.

The Therapeutic Effectiveness of Full Spectrum Hemp Oil Using a Chronic Neuropathic Pain Model

BACKGROUND

Few models exist that can control for placebo and expectancy effects commonly observed in clinical trials measuring 'Cannabis' pharmacodynamics. We used the Foramen Rotundum Inflammatory Constriction Trigeminal Infraorbital Nerve injury (FRICT-ION) model to measure the effect of "full-spectrum" whole plant extracted hemp oil on chronic neuropathic pain sensitivity in mice.

METHODS

Male BALBc mice were submitted to the FRICT-ION chronic neuropathic pain model with oral insertion through an incision in the buccal/cheek crease of 3 mm of chromic gut suture (4-0). The suture, wedged along the V2 trigeminal nerve branch, creates a continuous irritation that develops into secondary mechanical hypersensitivity on the snout. Von Frey filament stimuli on the mouse whisker pad was used to assess the mechanical pain threshold from 0-6 h following dosing among animals (n = 6) exposed to 5 μL of whole plant extracted hemp oil combined with a peanut butter vehicle (0.138 mg/kg), the vehicle alone (n = 3) 7 weeks post-surgery, or a naïve control condition (n = 3).

RESULTS

Mechanical allodynia was alleviated within 1 h (d = 2.50, p < 0.001) with a peak reversal effect at 4 h (d = 7.21, p < 0.001) and remained significant throughout the 6 h observation window. There was no threshold change on contralateral whisker pad after hemp oil administration, demonstrating the localization of anesthetic response to affected areas.

CONCLUSION

Future research should focus on how whole plant extracted hemp oil affects multi-sensory and cognitive-attentional systems that process pain.

The Effects of Partner Presence and Sexual Stimulation on the Appraisal of Vaginal Pressure and Sexual Arousal

BACKGROUND

Sex research lacks experimental studies in which both partners participate in a laboratory procedure. This is relevant in the context of genital pain because painful vaginal sensations often occur in the presence of the partner.

AIM

To examine the effects of partner presence, sexual stimulation, and vaginal pressure on the appraisal of vaginal sensations and sexual arousal, ultimately aiming to increase the ecologic validity of laboratory designs.

METHODS

A community sample of 42 women and their male partners watched sexual and neutral films while separated or together. We induced gradually increasing vaginal pressure in the women using an intravaginal inflatable rubber balloon.

OUTCOMES

Women reported on pleasant and painful vaginal pressure and perceived genital arousal. Men and women reported on subjective sexual arousal. We also examined whether these appraisals were moderated by relationship satisfaction.

RESULTS

The appraisal of vaginal pressure varied as a function of relationship satisfaction. Less satisfied women reported more painful pressure than women who were highly satisfied and highly satisfied women appraised the pressure as more pleasant in the context of a sex film and in the presence (vs absence) of their partner. In men and women, although partner presence had a negative effect on subjective sexual arousal, the presence of the partner did increase women's perception of genital arousal when vaginal pressure was induced during a sex film, particularly when women felt highly satisfied with their relationship. Also, the effects on subjective sexual arousal were moderated by relationship satisfaction. For couples in which the woman was less satisfied, the induction of vaginal pressure resulted in higher subjective sexual arousal when the partner was absent compared with when he was present, whereas when the man felt less satisfied, partner presence had a positive effect on sexual arousal.

CLINICAL IMPLICATIONS

Interventions need to focus on the importance of sexual arousal during vaginal pressure stimulation and the way this is shaped by partner and relationship variables. Our results indicate that enhancing the relationship climate is an important target of intervention.

STRENGTHS AND LIMITATIONS

We did not include physical indices of genital arousal and did not use a clinical sample of women with genital pain.

CONCLUSIONS

The appraisal of vaginal sensations and sexual arousal are context-dependent responses that vary as a function of partner presence and sexual stimulation. Including both partners in the laboratory setting is important to create more valid models on sexual responding. Dewitte M, Schepers J, Melles R. The Effects of Partner Presence and Sexual Stimulation on the Appraisal of Vaginal Pressure and Sexual Arousal. J Sex Med 2018;15:539-549.

How nurse gender influences patient priority assignments in US emergency departments

The goals of this study were to compare whether emergency department (ED) patients' pain intensity (PI) is measured differently by male and female nurses and to determine whether PI, heart rate (HR), and respiratory rate (RR) were used to prioritize patient urgency differently by male and female nurses. The associations between patients' PI|HR|RR and the Emergency Severity Index (ESI) scores they were assigned by attending nurses were analyzed using a national database of electronic medical records of US Veterans Affairs ED patients from 2008 to 2012. A total of 129,991 patients presenting for emergency care (Mage = 59.5, 92% males) and their triage nurses (n = 774, Mage = 47.5, 18% males) were analyzed, resulting in a total of 359,642 patient-provider interactions. Patients' PI did not differ by the nurse's gender; however a cross-classified mixed-effects model showed that nurse gender influenced how PI and RR measurements informed the ESI levels that male patients received. Higher PI levels were associated with more urgent (higher priority) ESI levels by female nurses, yet less urgent ESI levels by male nurses. In contrast, male patients with high RR received more urgent ESI levels by male nurses, whereas the nurse gender did not influence ESI assignments for female patients. These findings show that ED patients receive disparate treatment based on inherent characteristics of their triage nurses, and more standardized (eg, automated) protocols that can account for implicit social factors on health care practice for reliably assessing and prioritizing ED patients may be currently warranted.

Multi-level selection, social signaling, and the evolution of human suffering gestures: The example of pain behaviors

Pain suffering has been naturally selected to be experienced and expressed within a wider social system. The communication of pain improves group coordination and decision-making about engaging in resource dependent and potentially risky endeavors. Recent findings warrant the development of a cohesive framework for understanding the reciprocal nature of pain expression and individual and group-level outcomes that can generate novel predictions on the heuristical expression of human suffering in naturalistic and clinical settings.

The Curse of Curves: Sex Differences in the Associations Between Body Shape and Pain Expression

This study examines the associations between objective and subjective measurements and impressions of body shape and cold pressor pain reporting in healthy adults. On the basis of sexual selection theory (SST), we hypothesized that body characteristics that are universally preferred by the opposite sex-specifically, lower waist-to-hip ratios (WHR) in women and higher shoulder-to-hip ratios (SHR) in men-and characteristics (e.g., proportion of body fat in women) that infer attractiveness differently across cultures will correspond to higher experimental pain reporting in women and lower pain reporting in males. A convenience sample of young adults (n = 96, 58 females, 18-24 years; mean age = 19.4) was measured for body mass index (BMI), WHR, SHR, and subjective body impressions (SBI), along with cold pressor pain reporting. The findings showed that BMI was positively associated with WHR and less-positive SBI in both sexes. Consistent with SST, however, only BMI and WHR predicted variability in pain expression in women, whereas only SHR predicted variability in men. Subjective body impressions were positively associated with SHR among males and unrelated to WHR among females, yet only females showed a positive association between SBI and higher pain reporting. The findings suggest that sexually selected physical characteristics (WHR and SHR) and culturally influenced somatic (BMI) and psychological (SBI) indicators of attractiveness correspond with variability in pain reporting, potentially reflecting the general tendency for people to express clusters of sexually selected and culturally influenced traits that may include differential pain perception.

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.

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.