Beta-endorphin response to an acute pain stimulus

Biomarkers for assessing pain and pain relief in the neonatal intensive care unit

Newborns admitted to the neonatal intensive care unit (NICU) regularly undergo painful procedures and may face various painful conditions such as postoperative pain. Optimal management of pain in these vulnerable preterm and term born neonates is crucial to ensure their comfort and prevent negative consequences of neonatal pain. This entails accurate and timely identification of pain, non-pharmacological pain treatment and if needed administration of analgesic therapy, evaluation of treatment effectiveness, and monitoring of adverse effects. Despite the widely recognized importance of pain management, pain assessment in neonates has thus far proven to be a challenge. As self-report, the gold standard for pain assessment, is not possible in neonates, other methods are needed. Several observational pain scales have been developed, but these often rely on snapshot and largely subjective observations and may fail to capture pain in certain conditions. Incorporation of biomarkers alongside observational pain scores holds promise in enhancing pain assessment and, by extension, optimizing pain treatment and neonatal outcomes. This review explores the possibilities of integrating biomarkers in pain assessment in the NICU.

Pain, Fear, Anxiety, and Stress: Relations to the Endogenous Opioid System

Pain, fear, stress, and anxiety are separate yet interrelated phenomena. Each of these concepts has an extensive individual body of research, with some more recent work focusing on points of conceptual overlap. The role of the endogenous opioid system in each of these phenomena is only beginning to be examined and understood. Research examining the ways in which endogenous opioids (e.g., beta-endorphin; βE) may mediate the relations among pain, fear, stress, and anxiety is even more nascent. This chapter explores the extant evidence for endogenous opioid activity as an underpinning mechanism of these related constructs, with an emphasis on research examining βE.

Oxytocin Is a Positive Allosteric Modulator of κ-Opioid Receptors but Not δ-Opioid Receptors in the G Protein Signaling Pathway

Oxytocin (OT) influences various physiological functions such as uterine contractions, maternal/social behavior, and analgesia. Opioid signaling pathways are involved in one of the analgesic mechanisms of OT. We previously showed that OT acts as a positive allosteric modulator (PAM) and enhances μ-opioid receptor (MOR) activity. In this study, which focused on other opioid receptor (OR) subtypes, we investigated whether OT influences opioid signaling pathways as a PAM for δ-OR (DOR) or κ-OR (KOR) using human embryonic kidney-293 cells expressing human DOR or KOR, respectively. The CellKey^TM results showed that OT enhanced impedance induced by endogenous/exogenous KOR agonists on KOR-expressing cells. OT did not affect DOR activity induced by endogenous/exogenous DOR agonists. OT potentiated the KOR agonist-induced Gi/o protein-mediated decrease in intracellular cAMP, but did not affect the increase in KOR internalization caused by the KOR agonists dynorphin A and (-)-U-50488 hydrochloride (U50488). OT did not bind to KOR orthosteric binding sites and did not affect the binding affinities of dynorphin A and U50488 for KOR. These results suggest that OT is a PAM of KOR and MOR and enhances G protein signaling without affecting β-arrestin signaling. Thus, OT has potential as a specific signaling-biased PAM of KOR.

Efficacy of Acupuncture on Pain Mechanisms, Inflammatory Responses, and Wound Healing in the Acute Phase of Major Burns: An Experimental Study on Rats

OBJECTIVES

We investigated acupuncture, a potential contributor for burn-care, on physiological and pathological pain mechanisms and systemic and local inflammatory responses in a rat experimental burn model.

METHODS

Forty male Sprague-Dawley rats were divided into 2 groups. One-hour groups(5 rats/group) were observed for 1 hour and included Sh1(sham/observation), ShA1(sham+acupuncture/observation), Brn1(burn/observation), and BrnA1(burn+acupuncture/observation). Seven-day groups(5 rats/group) were observed for 7 days and included Sh7(sham/observation), ShA7(sham+acupuncture/observation), Brn7(burn/observation), and BrnA7(burn+acupuncture/observation). "Pain-distress scores" were noted daily, acupuncture was repeated within every wound-dressing change on alternate days. After observation periods, blood samples for interleukin-6 and beta-endorphin and skin biopsies for inflammatory-changes and immunohistochemical-staining of interleukin-6 were collected for analysis( P< .05 ).

RESULTS

In 1-hour groups, interleukin-6 accumulation in burn wounds of BrnA1 was less than Brn1, with Brn1 having the highest mean blood level(P< .05). Mean beta-endorphin levels were higher in ShA1, Brn1, and BrnA1 than in Sh1(P< .05). In all 7-day groups, the agonizing period was 48 to72 hours after burn, with Brn7 most affected(P< .05). Microvessels were multiplied in Brn7group, with significantly higher numbers in burn wounds of BrnA7(P˂ .05). Burn wounds of BrnA7 had less accumulation of interleukin-6 than Brn7 with Brn7-group having the highest mean blood level and Sh7, ShA7, and BrnA7 having similarly low levels(P˃ .05). Beta-endorphin levels in ShA7, Brn7, and BrnA7 were lower than in Sh7(P< .05).

CONCLUSIONS

Acupuncture contributed to management of physiological and pathological pain, modulation of inflammatory responses, and associated enhancement of angiogenesis in acute phase of burn injury in rats.

From Pharmacology to Physiology: Endocrine Functions of μ-Opioid Receptor Networks

The steady rise in opioid users and abusers has uncovered multiple detrimental health consequences of perturbed opioid receptor signaling, thereby creating the need to better understand the biology of these systems. Among endogenous opioid networks, μ-receptors have received special attention due to their unprecedented biological complexity and broad implications in homeostatic functions. Here, we review the origin, molecular biology, and physiology of endogenous opioids with a special focus on μ-opioid receptor networks within the endocrine system. Moreover, we summarize the current evidence supporting an involvement of the latter in regulating distinct endocrine functions. Finally, we combine these insights to present an integrated perspective on μ-opioid receptor biology and provide an outlook on future studies and unresolved questions in this field.

Crosstalk between Opioid and Anti-Opioid Systems: An Overview and Its Possible Therapeutic Significance

Opioid peptides and receptors are broadly expressed throughout peripheral and central nervous systems and have been the subject of intense long-term investigations. Such studies indicate that some endogenous neuropeptides, called anti-opioids, participate in a homeostatic system that tends to reduce the effects of endogenous and exogenous opioids. Anti-opioid properties have been attributed to various peptides, including melanocyte inhibiting factor (MIF)-related peptides, cholecystokinin (CCK), nociceptin/orphanin FQ (N/OFQ), and neuropeptide FF (NPFF). These peptides counteract some of the acute effects of opioids, and therefore, they are involved in the development of opioid tolerance and addiction. In this work, the anti-opioid profile of endogenous peptides was described, mainly taking into account their inhibitory influence on opioid-induced effects. However, the anti-opioid peptides demonstrated complex properties and could show opioid-like as well as anti-opioid effects. The aim of this review is to detail the phenomenon of crosstalk taking place between opioid and anti-opioid systems at the in vivo pharmacological level and to propose a cellular and molecular basis for these interactions. A better knowledge of these mechanisms has potential therapeutic interest for the control of opioid functions, notably for alleviating pain and/or for the treatment of opioid abuse.

Neuropeptide oxytocin enhances μ opioid receptor signaling as a positive allosteric modulator

Oxytocin (OT) is a 9-amine neuropeptide that plays an essential role in mammalian labor, lactation, maternal bonding, and social affiliation. OT has been reported to exert an analgesic effect in both humans and animals, and the results of certain animal experiments have shown that the analgesic effect of OT is partially blocked by opioid receptor antagonists. To investigate the relationship between OT and μ opioid receptor (MOR), we evaluated how OT affects MOR in vitro by performing an electrical impedance-based receptor biosensor assay (CellKey™ assay), an intracellular cAMP assay, and a competitive receptor-binding analysis by using cells stably expressing human MOR and OT receptor. In both the CellKey™ assay and the intracellular cAMP assay, OT alone exerted no direct agonistic effect on human MOR, but treatment with 10^-6 M OT markedly enhanced the MOR signaling induced by 10^-6 M endomorphin-1, β-endorphin, morphine, fentanyl, and DAMGO. Moreover, in the competitive receptor-binding assay, 10^-6 M OT did not alter the affinity of endomorphin-1 or morphine for MOR. These results suggest that OT could function as a positive allosteric modulator that regulates the efficacy of MOR signaling, and thus OT might represent a previously unrecognized candidate analgesic agent.

Acute pain and peripheral sensitization following cautery disbudding in 1- and 4-week-old calves

Acute pain and peripheral sensitization development after cautery disbudding was investigated in 33 calves administered preventive multimodal analgesia. The animals were assigned randomly to three groups: 1) Group SH (Control), undergoing sham disbudding at 1 and 4weeks of age; 2) Group ED (Early Disbudding), undergoing disbudding at 1week of age and sham disbudding at 4weeks of age; 3) Group LD (Late Disbudding), undergoing sham disbudding at 1week of age and disbudding at 4weeks of age. Physiological parameters (heart rate, respiratory rate, rectal temperature, invasive blood pressure, cortisol, β-endorphin, interleukin-1β, interleukin-6, tumor necrosis factor-α and haptoglobin plasmatic concentration), local variables (tactile sensitivity score, pressure pain thresholds and horn temperature), behavior and pain scores [multidimensional pain scale and visual analogue scale (VAS)] were assessed at baseline and at several pre-determined time points until 24h after disbudding. Tactile sensitivity score significantly and equally increased in both groups ED and LD and pressure pain thresholds significantly decreased in group LD until 24h after disbudding compared to group SH. Pain and VAS scores significantly and equally increased in both groups ED and LD until 24h after disbudding compared to group SH. No significant differences in physiological parameters, behavior and horn temperature were detected among groups. The present study suggests that acute pain and peripheral sensitization develop and do not differ in calves disbudded at 1week and 4weeks of age. Moreover, the use of physiological and behavioral parameters as sole indicators of acute pain might lead to improper conclusions and should be reassessed.

Imaging Mass Spectrometric Analysis of Neurotransmitters: A Review

Imaging mass spectrometry (IMS) is a toolbox of versatile techniques that enable us to investigate analytes in samples at molecular level. In recent years, IMS, and especially matrix-assisted laser desorption/ionisation (MALDI), has been used to visualise a wide range of metabolites in biological samples. Simultaneous visualisation of the spatial distribution of metabolites in a single sample with little tissue disruption can be considered as one important advantage of MALDI over other techniques. However, several technical hurdles including low concentrations and rapid degradation rates of small molecule metabolites, matrix interference of signals and poor ionisation, need to be addressed before MALDI can be considered as a reliable tool for the analysis of metabolites such as neurotransmitters in brain tissues from different sources including humans. In the present review we will briefly describe current MALDI IMS techniques used to study neurotransmitters and discuss their current status, challenges, as well as future prospects.

Subcutaneous administration of tramadol after elective surgery is as effective as intravenous administration in relieving acute pain and inflammation in dogs

Subcutaneous (SC) administration of tramadol was compared with intravenous (IV) administration to evaluate analgesia following canine ovariohysterectomy (OHE). Healthy female dogs (n = 12) between 1 and 3 years of age (1.95 ± 0.65 years), weighing between 10.5 and 17.1 kg (13.12 ± 1.95 kg), were used. Pain was assessed at baseline before surgery and then hourly for 8 hr after surgery. Tramadol was administered both SC and IV at a dose of 3 mg/kg and provided significant postoperative analgesia, as indicated by analgesiometry, β-endorphin levels, and interleukin 6 (IL-6) levels. The respiratory rates and rectal temperatures remained normal and were not significantly different between or within the groups. A significant increase in heart rate was observed at 4 hr for dogs in both groups relative to the baseline, but there was no significant difference in heart rates between the groups at any time point. A significant decrease in mechanical pain threshold was observed within each group after surgery, but both groups responded similarly, suggesting that SC administration of tramadol is as effective as IV administration. Increased serum levels of both IL-6 and β-endorphin 3 hr postoperatively further indicate that both routes of administration achieve similar pain control. Thus, the relative analgesic efficacy of SC tramadol is comparable to that of IV administration and can be used to achieve similar effects for postsurgical pain management in dogs undergoing OHE.

Possible involvement of β-endorphin in docosahexaenoic acid-induced antinociception

We have previously demonstrated that the n-3 polyunsaturated fatty acid docosahexaenoic acid (DHA) has an antinociceptive effect on various pain stimuli in a naloxone-reversible manner. In the present study, the role of the endogenous opioid peptide β-endorphin in DHA-induced antinociception was examined. DHA-induced antinociception was abolished when mice were pretreated with the μ-opioid receptor antagonist β-funaltrexamine (β-FNA) and the δ-opioid receptor antagonist naltrindole, but not by the κ-opioid receptor antagonist nor-binaltorphimine (nor-BNI) in the acetic acid-induced writhing test. In the radioligand binding assay, DHA itself did not have affinity for μ- , δ- or κ-opioid receptors. On the other hand, the pretreatment of anti-β-endorphin antiserum inhibited DHA-induced antinociception. Furthermore, the intracerebroventricular injection of DHA dose-dependently reduced writhing behavior, and this effect was inhibited by d-Phe-Cys-Tyr-Orn-Thr-Pen-Thr-NH(2) (CTOP) and naltrindole, but not nor-BNI. β-endorphin-induced antinociception was inhibited by the pretreatment of β-FNA, but not naltrindole or nor-BNI, and its levels in plasma were increased by DHA treatment. These findings suggest that the induction of antinociception by DHA may partially involve the μ-opioid receptor via the release of β-endorphin.

Neurobiology of aversive states

Hoebel and colleagues are often known as students of reward and how it is coded in the CNS. This article, however, attempts to focus on the significant advances by Hoebel and others in dissecting out behavioral components of distinct aversive states and in understanding the neurobiology of aversion and the link between aversive states and addictive behaviors. Reward and aversion are not necessarily dichotomous and may reflect an affective continuum contingent upon environmental conditions. Descriptive and mechanistic studies pioneered by Bart Hoebel have demonstrated that the shift in the reward-aversion spectrum may be, in part, a result of changes in central dopamine/acetylcholine ratio, particularly in the nucleus accumbens. The path to aversion appears to include a specific neurochemical signature: reduced dopamine release and increased acetylcholine release in "reward centers" of the brain. Opioid receptors may have a neuromodulatory role on both of these neurotransmitters.

Endogenous opiates and behavior: 2009

This paper is the 32nd consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2009 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 (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Neurobiological mechanisms involved in nicotine dependence and reward: participation of the endogenous opioid system

Nicotine is the primary component of tobacco that maintains the smoking habit and develops addiction. The adaptive changes of nicotinic acetylcholine receptors produced by repeated exposure to nicotine play a crucial role in the establishment of dependence. However, other neurochemical systems also participate in the addictive effects of nicotine including glutamate, cannabinoids, GABA and opioids. This review will cover the involvement of these neurotransmitters in nicotine addictive properties, with a special emphasis on the endogenous opioid system. Thus, endogenous enkephalins and beta-endorphins acting on mu-opioid receptors are involved in nicotine-rewarding effects, whereas opioid peptides derived from prodynorphin participate in nicotine aversive responses. An up-regulation of mu-opioid receptors has been reported after chronic nicotine treatment that could counteract the development of nicotine tolerance, whereas the down-regulation induced on kappa-opioid receptors seems to facilitate nicotine tolerance. Endogenous enkephalins acting on mu-opioid receptors also play a role in the development of physical dependence to nicotine. In agreement with these actions of the endogenous opioid system, the opioid antagonist naltrexone has shown to be effective for smoking cessation in certain sub-populations of smokers.

The endogenous opioid system: a common substrate in drug addiction

Drug addiction is a chronic brain disorder leading to complex adaptive changes within the brain reward circuits that involve several neurotransmitters. One of the neurochemical systems that plays a pivotal role in different aspects of addiction is the endogenous opioid system (EOS). Opioid receptors and endogenous opioid peptides are largely distributed in the mesolimbic system and modulate dopaminergic activity within these reward circuits. Chronic exposure to the different prototypical drugs of abuse, including opioids, alcohol, nicotine, psychostimulants and cannabinoids has been reported to produce significant alterations within the EOS, which seem to play an important role in the development of the addictive process. In this review, we will describe the adaptive changes produced by different drugs of abuse on the EOS, and the current knowledge about the contribution of each component of this neurobiological system to their addictive properties.