Involvement of Opioid Receptors and α2-Adrenoceptors in Inhibitory Pain Modulation Processes: A Double-Blind Placebo-Controlled Crossover Study

Prazosin as an Adjuvant to Increase Effectiveness of Duloxetine in a Rat Model of Oxaliplatin-Induced Peripheral Neuropathy

OBJECTIVES

Duloxetine, the only American Society of Clinical Oncology (ASCO) treatment recommended for chemotherapy-induced peripheral neuropathy (CIPN) in cancer survivors, is not effective for 40% of survivors. This study examined the ability of a duloxetine-prazosin combination to prevent the development of allodynia and hyperalgesia in a rat model of oxaliplatin-induced peripheral neuropathy (OPIN).

METHODS

Female (n = 24) and male (n = 41) rats were started on duloxetine (15 mg), prazosin (2 mg), or a duloxetine-prazosin combination one week prior to administration of the chemotherapy drug, oxaliplatin, and continued the duloxetine-prazosin combination for 32 days. Behavioral testing for mechanical allodynia and mechanical hyperalgesia was done with selected von Frey filaments over the course of the study.

RESULTS

Overall percent paw withdrawal for rats that received the duloxetine-prazosin combination was significantly lower in female (p < .001 for both conditions) and male (p = .029 for allodynia; p < .001 for hyperalgesia) than those that received water. No significant posttreatment differences were found for allodynia or hyperalgesia between rats treated with duloxetine and rats that received the duloxetine-prazosin combination in either sex.

CONCLUSIONS

These finding provide preliminary evidence that a duloxetine-prazosin combination can prevent the posttreatment development of allodynia and hyperalgesia in both male and female rats; however, the results suggest that the duloxetine-prazosin combination is no more efficacious than duloxetine alone in preventing chronic OIPN.

IMPLICATIONS FOR NURSING PRACTICE

The profession of nursing is built on clinical practice supported by scientific research. The current study addressed the clinical practice problem of prevention and management of painful OIPN, which is a priority area in oncology nursing.

The Effect of Observing High or Low Pain on the Development of Central Sensitization

It is unknown whether watching other people in high pain increases mechanical hypersensitivity induced by pain. We applied high-frequency electrical stimulation (HFS) on the skin of healthy volunteers to induce pinprick mechanical hypersensitivity. Before HFS participants were randomly allocated to 2 groups: in the low pain group, which was the control condition, they watched a model expressing and reporting lower pain scores, in the high pain group the model expressed and reported higher scores. The 2 videos were selected on the basis of a pilot/observational study that had been conducted before. We tested the differences in perceived intensity of the HFS procedure, in the development of hypersensitivity and the role of fear and empathy. The high pain group reported on average higher pain ratings during HFS. The perceived intensity of hypersensitivity, but not the unpleasantness or the length of the area was higher in the high pain group. Our results suggest that watching a person expressing more pain during HFS increases one's own pain ratings during HFS and may weakly facilitate the development of secondary mechanical hypersensitivity, although this latter result needs replication. PERSPECTIVE: Observing a person in high pain can influence the perceived pain intensity of a procedure leading to secondary mechanical hypersensitivity, and has a weak effect on hypersensitivity itself. The role of fear remains to be elucidated.

Caffeine Attenuates Electroacupuncture Effect on Pressure Pain Threshold and Tolerance in Healthy Individuals: A Randomized Controlled Trial

Introduction

The effect of caffeine on acupuncture analgesia in humans is unclear. This study aimed to investigate whether caffeine-containing beverage intake influences the effect of electroacupuncture (EA) on static quantitative sensory testing (QST) and dynamic QST in healthy subjects.

Methods

A total of 40 healthy subjects were enrolled and randomly assigned to receive coffee containing moderate doses of caffeine (coffee group) or non-caffeinated juice (juice group) for 4 weeks. The primary outcome measures were the pressure pain threshold (PPT), pressure pain tolerance (PPTo), and heat pain threshold (HPT) as static QST parameters. Numerical rating scales (NRS) of heat stimulus and nociceptive flexor reflex (RIII reflex), as parameters of dynamic QST, were also examined. EA stimulation with tolerance intensity was performed at ST36 (Zusanli)-GB34 (Yanglingquan) points at weeks 0, 2, and 4. PPT, PPTo, and HPT were detected pre- and post- EA. The NRS scores were examined pre-, during, and post-EA, and 1 min after EA was completed. The RIII reflex was examined pre- and 1–5 min post-EA.

Results

At week 0, both groups showed increased PPT and PPTo and decreased NRS scores of heat stimuli and RIII reflex after EA, but HPT was not affected. After 4 weeks, the effects of EA on PPT and PPTo were attenuated in the coffee group compared to the juice group, whereas the effect of EA on the NRS scores and RIII reflex were not influenced. There was no significant difference found at week 2 for these indications. EA also did not affect the HPT in both groups at week 4.

Conclusion

Moderate caffeine intake reduced the effects of EA on PPT and PPTo in healthy subjects.

Safety Assessment and Pain Relief Properties of Saffron from Taliouine Region (Morocco)

Saffron is the most expensive spice in the world. In addition to its culinary utilization, this spice is used for medicinal purposes such as in pain management. In this study, the analgesic activity of Crocus sativus stigma extract (CSSE) was evaluated in rodents and its possible physiological mechanism was elucidated. The anti-nociceptive effect of CSSE was evaluated using three animal models (hot plate, writhing, and formalin tests). The analgesic pathways involved were assessed using various analgesia-mediating receptors antagonists. The oral administration of CSSE, up to 2000 mg/kg, caused no death or changes in the behavior or in the hematological and biochemical blood parameters of treated animals nor in the histological architecture of the animals’ livers and kidneys. CSSE showed a central, dose-dependent, anti-nociceptive effect in response to thermal stimuli; and a peripheral analgesic effect in the test of contortions induced by acetic acid. The dual (central and peripheral) analgesic effect was confirmed by the formalin test. The anti-nociceptive activity of CSSE was totally or partially reversed by the co-administration of receptor antagonists, naloxone, atropine, haloperidol, yohimbine, and glibenclamide. CSSE influenced signal processing, by the modulation of the opioidergic, adrenergic, and muscarinic systems at the peripheral and central levels; and by regulation of the dopaminergic system and control of the opening of the ATP-sensitive K⁺ channels at the spinal level. The obtained data point to a multimodal mechanism of action for CSSE: An anti-inflammatory effect and a modulation, through different physiological pathways, of the electrical signal generated by the nociceptors. Further clinical trials are required to endorse the potential utilization of Moroccan saffron as a natural painkiller.

Biased, Bitopic, Opioid–Adrenergic Tethered Compounds May Improve Specificity, Lower Dosage and Enhance Agonist or Antagonist Function with Reduced Risk of Tolerance and Addiction

This paper proposes the design of combination opioid–adrenergic tethered compounds to enhance efficacy and specificity, lower dosage, increase duration of activity, decrease side effects, and reduce risk of developing tolerance and/or addiction. Combinations of adrenergic and opioid drugs are sometimes used to improve analgesia, decrease opioid doses required to achieve analgesia, and to prolong the duration of analgesia. Recent mechanistic research suggests that these enhanced functions result from an allosteric adrenergic binding site on opioid receptors and, conversely, an allosteric opioid binding site on adrenergic receptors. Dual occupancy of the receptors maintains the receptors in their high affinity, most active states; drops the concentration of ligand required for full activity; and prevents downregulation and internalization of the receptors, thus inhibiting tolerance to the drugs. Activation of both opioid and adrenergic receptors also enhances heterodimerization of the receptors, additionally improving each drug’s efficacy. Tethering adrenergic drugs to opioids could produce new drug candidates with highly desirable features. Constraints—such as the locations of the opioid binding sites on adrenergic receptors and adrenergic binding sites on opioid receptors, length of tethers that must govern the design of such novel compounds, and types of tethers—are described and examples of possible structures provided.

"Big girls don't cry": the effect of the experimenter's sex and pain catastrophising on pain

OBJECTIVES

The expression of pain in males and females involves complex socio-psychological mechanisms. Males may report lower pain to a female experimenter to appear strong, whereas females may report higher pain to a male experimenter to appear weak and to seek protection. However, evidence to support these stereotypes is inconclusive. Individuals who catastrophise about pain rate higher pain than those who do not. How pain catastrophising interacts with the effect of the experimenter's sex on pain reports is yet to be explored. Thus, the aim of this study was to determine whether pain catastrophising moderated the effect of the experimenter's sex on pain reports in healthy males and females.

METHODS

Participants (n=60, 30 males) were assigned to one of four experimental conditions: males tested by male experimenters, males tested by female experimenters, females tested by male experimenters, and females tested by female experimenters. Participants completed the Pain Catastrophising Scale, and then sensitivity to heat and to blunt (pressure-pain threshold) and sharp stimuli was assessed on both forearms, and to high frequency electrical stimulation (HFS) administered to one forearm.

RESULTS

Females reported lower pressure-pain thresholds than males irrespective of the experimenters' sex. Females reported lower sharpness ratings to male than female experimenters only when the test stimuli were moderately or intensely sharp. Higher pain catastrophising scores were associated with higher sharpness ratings in females but not males. Additionally, higher pain catastrophising scores were associated with greater temporal summation of pain to HFS, and with lower pressure-pain thresholds in females who were tested by male experimenters.

CONCLUSIONS

These findings indicate that the experimenters' sex and the participant's pain catastrophising score influence pain reports, particularly in females. Awareness of these psychosocial factors is important in order to interpret pain responses in a meaningful way, especially when females are tested by male experimenters. A greater awareness of sex/gender role biases and their potential interaction with pain catastrophising may help researchers and clinicians to interpret pain reports in meaningful ways. In turn, this may help to improve delivery of treatments for patients with chronic pain.

Perceptual correlates of homosynaptic long-term potentiation in human nociceptive pathways: a replication study

Animal studies have shown that high-frequency stimulation (HFS) of peripheral C-fibres induces long-term potentiation (LTP) within spinal nociceptive pathways. The aim of this replication study was to assess if a perceptual correlate of LTP can be observed in humans. In 20 healthy volunteers, we applied HFS to the left or right volar forearm. Before and after applying HFS, we delivered single electrical test stimuli through the HFS electrode while a second electrode at the contra-lateral arm served as a control condition. Moreover, to test the efficacy of the HFS protocol, we quantified changes in mechanical pinprick sensitivity before and after HFS of the skin surrounding both electrodes. The perceived intensity was collected for both electrical and mechanical stimuli. After HFS, the perceived pain intensity elicited by the mechanical pinprick stimuli applied on the skin surrounding the HFS-treated site was significantly higher compared to control site (heterotopic effect). Furthermore, we found a higher perceived pain intensity for single electrical stimuli delivered to the HFS-treated site compared to the control site (homotopic effect). Whether the homotopic effect reflects a perceptual correlate of homosynaptic LTP remains to be elucidated.

Pharmacology of Herbal Sexual Enhancers: A Review of Psychiatric and Neurological Adverse Effects

Sexual enhancers increase sexual potency, sexual pleasure, or libido. Substances increasing libido alter the concentrations of specific neurotransmitters or sex hormones in the central nervous system. Interestingly, the same pathways are involved in the mechanisms underlying many psychiatric and neurological disorders, and adverse reactions associated with the use of aphrodisiacs are strongly expected. However, sexual enhancers of plant origin have gained popularity over recent years, as natural substances are often regarded as a safer alternative to modern medications and are easily acquired without prescription. We reviewed the psychiatric and neurological adverse effects associated with the consumption of herbal aphrodisiacs Areca catechu L., Argemone Mexicana L., Citrus aurantium L., Eurycoma longifolia Jack., Lepidium meyenii Walp., Mitragyna speciosa Korth., Panax ginseng C. A. Mey, Panax quinquefolius L., Pausinystalia johimbe (K. Schum.) Pierre ex Beille, Piper methysticum G. Forst., Ptychopetalum olacoides Benth., Sceletium tortuosum (L.) N. E. Brown, Turnera diffusa Willd. ex. Schult., Voacanga africana Stapf ex Scott-Elliot, and Withania somnifera (L.) Dunal. A literature search was conducted on the PubMed, Scopus, and Web of Science databases with the aim of identifying all the relevant articles published on the issue up to June 2020. Most of the selected sexual enhancers appeared to be safe at therapeutic doses, although mild to severe adverse effects may occur in cases of overdosing or self-medication with unstandardized products. Drug interactions are more concerning, considering that herbal aphrodisiacs are likely used together with other plant extracts and/or pharmaceuticals. However, few data are available on the side effects of several plants included in this review, and more clinical studies with controlled administrations should be conducted to address this issue.

Opioid-Induced Hyperalgesia Is Associated with Dysregulation of Circadian Rhythm and Adaptive Immune Pathways in the Mouse Trigeminal Ganglia and Nucleus Accumbens

The benefits of opioid-based treatments to mitigate chronic pain can be hindered by the side effects of opioid-induced hyperalgesia (OIH) that can lead to higher consumption and risk of addiction. The present study advances the understanding of the molecular mechanisms associated with OIH by comparing mice presenting OIH symptoms in response to chronic morphine exposure (OIH treatment) relative to control mice (CON treatment). Using RNA-Seq profiles, gene networks were inferred in the trigeminal ganglia (TG), a central nervous system region associated with pain signaling, and in the nucleus accumbens (NAc), a region associated with reward dependency. The biological process of nucleic acid processing was over-represented among the 122 genes that exhibited a region-dependent treatment effect. Within the 187 genes that exhibited a region-independent treatment effect, circadian rhythm processes were enriched among the genes over-expressed in OIH relative to CON mice. This enrichment was supported by the differential expression of the period circadian clock 2 and 3 genes (Per2 and Per3). Transcriptional regulators in the PAR bZip family that are influenced by the circadian clock and that modulate neurotransmission associated with pain and drug addiction were also over-expressed in OIH relative to CON mice. Also notable was the under-expression in OIH relative to CON mice of the Toll-like receptor, nuclear factor-kappa beta, and interferon gamma genes and enrichment of the adaptive immune processes. The results from the present study offer insights to advance the effective use of opioids for pain management while minimizing hyperalgesia.

Painful stimulation of a sensitized site in the forearm inhibits ipsilateral trigeminal nociceptive blink reflexes

Exposure to moderate levels of ultraviolet B radiation (UVB) is painless but nevertheless induces an inflammatory response that sensitizes primary afferent nociceptors. Subsequently, heating the UVB-treated site can sensitize spinal nociceptors. We used a repeated-measures design to determine whether heating the UVB-treated site also triggers ipsilateral inhibitory controls. Specifically, a 2-cm diameter site on the forearm of 20 participants was exposed to UVB at twice the minimum erythema dose. 48 h later mechanical and thermal sensitivity had increased at the UVB-treated site, indicating primary hyperalgesia. In addition, sensitivity to blunt pressure had increased in the ipsilateral forehead, implying activation of an ipsilateral supra-spinal pro-nociceptive mechanism. Despite this, the area under the curve of the ipsilateral nociceptive blink reflex decreased when the UVB-treated site was heated to induce moderate pain. Together, these findings suggest that the UVB treatment sensitized primary nociceptive afferents and generated an ipsilateral supra-spinal pro-nociceptive mechanism. In addition, sensitization to heat induced by the UVB treatment strengthened an ipsilateral anti-nociceptive process elicited by heat-pain. Infrequent but enduring discharge of sensitized primary nociceptive afferents, driven by inflammation after UVB exposure, might initiate a lateralized supra-spinal pro-nociceptive influence that heightens awareness of impending harm on the sensitized side of the body. In addition, a lateralized anti-nociceptive response triggered by an intense barrage of nociceptive signals may help to differentiate stronger from weaker sources of pain.

Endogenous Opiates and Behavior: 2016

This paper is the thirty-ninth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2016 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

Efficacy and Safety of Tamsulosin in Medical Expulsive Therapy for Distal Ureteral Stones with Renal Colic: A Multicenter, Randomized, Double-blind, Placebo-controlled Trial

BACKGROUND

Recent large high-quality trials have questioned the clinical effectiveness of medical expulsive therapy using tamsulosin for ureteral stones.

OBJECTIVE

To evaluate the efficacy and safety of tamsulosin for distal ureteral stones compared with placebo.

DESIGN, SETTING, AND PARTICIPANTS

We conducted a double-blind, placebo-controlled study of 3296 patients with distal ureteral stones, across 30 centers, to evaluate the efficacy and safety of tamsulosin.

INTERVENTION

Participants were randomly assigned (1:1) into tamsulosin (0.4mg) or placebo groups for 4 wk.

OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS

The primary end point of analysis was the overall stone expulsion rate, defined as stone expulsion, confirmed by negative findings on computed tomography, over a 28-d surveillance period. Secondary end points included time to stone expulsion, use of analgesics, and incidence of adverse events.

RESULTS AND LIMITATIONS

Among 3450 patients randomized between September 1, 2011, and August 31, 2013, 3296 (96%) were included in the primary analysis. Tamsulosin benefits from a higher stone expulsion rate than the placebo (86% vs 79%; p<0.001) for distal ureteral stones. Subgroup analysis identified a specific benefit of tamsulosin for the treatment of large distal ureteral stones (>5mm). Considering the secondary end points, tamsulosin-treated patients reported a shorter time to expulsion (p<0.001), required lower use of analgesics compared with placebo (p<0.001), and significantly relieved renal colic (p<0.001). No differences in the incidence of adverse events were identified between the two groups.

CONCLUSIONS

Our data suggest that tamsulosin use benefits distal ureteral stones in facilitating stone passage and relieving renal colic. Subgroup analyses find that tamsulosin provides a superior expulsion rate for stones >5mm, but no effect for stones ≤5mm.

PATIENT SUMMARY

In this report, we looked at the efficacy and safety of tamsulosin for the treatment of distal ureteral stones. We find that tamsulosin significantly facilitates the passage of distal ureteral stones and relieves renal colic.

Effect of combined opioid receptor and α2-adrenoceptor blockade on anxiety and electrically evoked startle responses

The R3 component of the electrically evoked blink reflex may form part of a startle reaction. Acoustic startle responses are augmented by yohimbine, an α₂-adrenoceptor antagonist that blocks α₂-autoreceptors, and are potentiated by opioid receptor blockade. To investigate these influences on electrically evoked startle responses, 16 mg yohimbine, with (16 participants) or without 50 mg naltrexone (23 participants), was administered in separate double-blind placebo-controlled cross-over experiments. In each experiment, R3 (a probable component of the startle response) was examined before and after high-frequency electrical stimulation of the forearm, a procedure that initiates inhibitory pain controls. Anxiety and somatic symptoms were greater after yohimbine than placebo, and were potentiated by naltrexone. Pain ratings for the electrically evoked startle stimuli decreased after high-frequency electrical stimulation in the placebo session but remained stable after drug administration. Yohimbine with naltrexone, but not yohimbine alone, also blocked an inhibitory effect of high-frequency electrical stimulation on electrically evoked sharp sensations and R3. Together, the findings suggest that adding naltrexone to yohimbine potentiated anxiety and blocked inhibitory influences of high-frequency electrical stimulation on electrically evoked sensations and startle responses. Thus, opioid peptides could reduce activity in nociceptive and startle-reflex pathways, or inhibit crosstalk between these pathways. Failure of this inhibitory opioid influence might be important in chronically painful conditions that are aggravated by startle stimuli.