Prior exposure to repeated morphine potentiates mechanical allodynia induced by peripheral inflammation and neuropathy

Sex Differences in the Serum Proteomic Profile During Acute Low Back Pain-A Preliminary Study of the Relationship to Future Low Back Pain

The molecular processes driving the transition from acute to chronic low back pain (LBP) remain poorly understood and are likely to be sexually dimorphic. This study aimed to explore sex differences in the serum proteomic profile of people experiencing an acute LBP episode and determine if serum protein concentrations were associated with three-month outcome. Serum samples were collected through venepuncture from 30 female and 29 male participants experiencing an acute LBP episode. Serum samples underwent trypsin digestion and fractionation using hydrophobic interaction chromatography and were then analysed using mass-spectrometry. Mass-spectrometry spectra were searched in the Swissprot database for protein identification. Sex differences in protein abundance changes were evident upon inspection of fold changes. Multivariable data analysis identified 21 serum proteins during the acute episode that correctly classified 93% of males and 23 serum proteins that correctly classified 90% of females with ongoing LBP at 3 months. Pathway analysis suggested the differentially expressed proteins during acute LBP were frequently involved in immune, inflammatory, complement, or coagulation responses. This data provides preliminary evidence that biological processes during an acute LBP episode may contribute to the resolution, or persistence, of LBP symptoms at 3 months, however, these processes differ between males and females. PERSPECTIVE: Differential expression of serum proteins was observed between male and female participants during an acute LBP episode. This preliminary work provides a foundation for future research targeting distinct immune system processes in males and females that may interfere with the transition from acute to chronic LBP.

Morphine aggravates inflammatory, behavioral, and hippocampal structural deficits in septic rats

Although pain and sepsis are comorbidities of intensive care units, reported data on whether pain control by opioid analgesics could alter inflammatory and end-organ damage caused by sepsis remain inconclusive. Here, we tested the hypothesis that morphine, the gold standard narcotic analgesic, modifies behavioral and hippocampal structural defects induced by sepsis in male rats. Sepsis was induced with cecal ligation and puncture (CLP) and behavioral studies were undertaken 24 h later in septic and/or morphine-treated animals. The induction of sepsis or exposure to morphine (7 mg/kg) elicited similar: (i) falls in systolic blood pressure, (ii) alterations in spatial memory and learning tested by the Morris water maze, and (iii) depression of exploratory behavior measured by the new object recognition test. These hemodynamic and cognitive defects were significantly exaggerated in septic rats treated with morphine compared with individual interventions. Similar patterns of amplified inflammatory (IL-1β) and histopathological signs of hippocampal damage were noted in morphine-treated septic rats. Additionally, the presence of intact opioid receptors is mandatory for the induction of behavioral and hemodynamic effects of morphine because no such effects were observed when the receptors were blocked by naloxone. That said, our findings suggest that morphine provokes sepsis manifestations of inflammation and interrelated hemodynamic, behavioral, and hippocampal deficits.

Fractalkine/CX3CR1-Dependent Modulation of Synaptic and Network Plasticity in Health and Disease

CX3CR1 is a G protein-coupled receptor that is expressed exclusively by microglia within the brain parenchyma. The only known physiological CX3CR1 ligand is the chemokine fractalkine (FKN), which is constitutively expressed in neuronal cell membranes and tonically released by them. Through its key role in microglia-neuron communication, the FKN/CX3CR1 axis regulates microglial state, neuronal survival, synaptic plasticity, and a variety of synaptic functions, as well as neuronal excitability via cytokine release modulation, chemotaxis, and phagocytosis. Thus, the absence of CX3CR1 or any failure in the FKN/CX3CR1 axis has been linked to alterations in different brain functions, including changes in synaptic and network plasticity in structures such as the hippocampus, cortex, brainstem, and spinal cord. Since synaptic plasticity is a basic phenomenon in neural circuit integration and adjustment, here, we will review its modulation by the FKN/CX3CR1 axis in diverse brain circuits and its impact on brain function and adaptation in health and disease.

The potential interplay between Opioid and the Toll-Like Receptor 4 (TLR-4)

CONTEXT

Opioids are available for the management of severe and chronic pain. However, long-term use of high-dose opioids could lead to physiologic tolerance, hyperalgesia, gastrointestinal immobility, addiction, respiratory depression, tumor progression, and inhibition of the immune system. It seems some of these adverse effects of opioids might be induced by TLR-4 signaling.

OBJECTIVE

The review aims to investigate the potential interplay between opioids and TLR-4 in CNS, gastrointestinal, cancer, and immune system.

METHODS

The search of PubMed, Embase, Scopus, web of sciences, and Google scholar was performed for all relevant studies published. From a total of 513 papers obtained at the initial database search, publications including in silico, in vitro, and in vivo studies were selected for the review.

RESULTS

A comprehensive review of studies indicated that using opioids for the reduction of pain might induce adverse effects such as analgesic tolerance, hyperalgesia, cancer progression, and suppression of the immune system. Some studies have indicated these effects may be due to a change in the level of expression and signaling pathway of TLR-4. The generalizability of the results was limited due to the inconsistency of findings.

CONCLUSIONS

More studies are needed to clarify TLR-4-mediated opioid effects on the biology or stages of the disease as well as the role of different types of opioids, appropriate dosage, and exposure in various contexts. Designing the drug candidate and doing many formulation studies for different diseases and various stages of disease could be associated with effective treatment and pain management.

Repeated blast mild traumatic brain injury and oxycodone self-administration produce interactive effects on neuroimaging outcomes

Traumatic brain injury (TBI) and drug addiction are common comorbidities, but it is unknown if the neurological sequelae of TBI contribute to this relationship. We have previously reported elevated oxycodone seeking after drug self-administration in rats that received repeated blast TBI (rbTBI). TBI and exposure to drugs of abuse can each change structural and functional neuroimaging outcomes, but it is unknown if there are interactive effects of injury and drug exposure. To determine the effects of TBI and oxycodone exposure, we subjected rats to rbTBI and oxycodone self-administration and measured drug seeking and several neuroimaging measures. We found interactive effects of rbTBI and oxycodone on fractional anisotropy (FA) in the nucleus accumbens (NAc) and that FA in the medial prefrontal cortex (mPFC) was correlated with drug seeking. We also found an interactive effect of injury and drug on widespread functional connectivity and regional homogeneity of the blood oxygen level dependent (BOLD) response, and that intra-hemispheric functional connectivity in the infralimbic medial prefrontal cortex positively correlated with drug seeking. In conclusion, rbTBI and oxycodone self-administration had interactive effects on structural and functional magnetic resonance imaging (MRI) measures, and correlational effects were found between some of these measures and drug seeking. These data support the hypothesis that TBI and opioid exposure produce neuroadaptations that contribute to addiction liability.

Repeated Endomorphin Analogue MEL-0614 Reduces Tolerance and Improves Chronic Postoperative Pain without Modulating the P2X7R Signaling Pathway

The clinical treatment of chronic postoperative pain (CPSP) remains challenging. The side effects of chronic morphine treatment limit its clinical application. MEL-0614, a novel endomorphin analogue that is highly selective and agonistic for μ opioid receptor (MOR), produces a more powerful analgesic effect than that of morphine. In this study, we explored the difference in antinociceptive tolerance and related mechanisms between MEL-0614 and morphine in CPSP induced in a skin/muscle incision and retraction (SMIR) mice model. We found that acute administration of MEL-0614 (1, 3, 5, and 10 nmol, i.t.) produced a dose-dependent analgesic effect that was superior to that of morphine in the SMIR mice model. Long-term MEL-0614 treatment (10 nmol, i.t.) did not induce tolerance compared with morphine. Notably, tolerance induced by morphine could be greatly prevented and/or inhibited via cross-administration or coadministration between MEL-0614 and morphine. In addition, MEL-0614 accelerated the recovery of postoperative pain, whereas morphine aggravated postoperative pain and prolonged its recovery time regardless of preoperative or postoperative treatment. In addition, MEL-0614 did not activate microglia and the P2X7R signaling pathway and showed reduced expression iba1 and P2X7R compared with that observed after morphine administration. Release of inflammatory factors was induced by continued administration of morphine during SMIR surgery, but MEL-0614 did not promote the activation of inflammatory factors. Our results showed that MEL-0614 has superior analgesic effects in CPSP and leads to tolerance to a lesser degree than morphine. Further, MEL-0614 may be used as a promising treatment option for the long-term treatment in CPSP.

Enhanced H3K4 Trimethylation in TNF-α Promoter Gene Locus with Cell Apoptosis in the Ventral-Medial Striatum following Opioid Withdrawal of Neonatal Rat Offspring from Morphine-Addicted Mothers

Prenatal opioid exposure might disturb epigenetic programming in the brain of neonatal offspring with various consequences for gene expressions and behaviors. This study determined whether altered trimethylation of histone 3 at lysine 4 (H3K4me3) in the promoter of the tumor necrosis factor-α (tnf-α) gene with neural cell apoptosis was involved in the ventral-medial striatum, an important brain region for withdrawal symptoms, of neonatal rat offspring from morphine-addicted mothers. Female adult rats were injected with morphine before gestation and until 14 days after giving birth. On postnatal day 14 (P14), rat offspring from morphine-addicted mothers were subjected to an opioid-withdrawal protocol and were analyzed 2 or 8 h after administration of that protocol. Expressions of the TNF-α protein, H3K4me3 in the tnf-α promoter gene, and neural cell apoptosis within the ventral-medial striatum of neonatal rat offspring were evaluated. In the absence of significant opioid withdrawal (2 h after initiation of the opioid-withdrawal protocol on P14), prenatal morphine exposure led to increased levels of H3K4me3 in the tnf-α promoter gene, of the TNF-α protein, and of neural cell apoptosis within the ventral-medial striatum of neonatal rat offspring. Following opioid withdrawal (8 h after initiation of the opioid-withdrawal protocol on P14), differential expression of H3K4me3 in the tnf-α promoter gene locus and upregulation of the level of TNF-α protein expression were further enhanced in these offspring. In addition, increased levels of caspase-3 and neural cell apoptosis were also observed. Taken together, this study revealed that prenatal opioid exposure can activate an epigenetic histone mechanism which regulates proinflammatory factor generation, which hence, led to cell apoptotic damage within the ventral-medial striatum of neonatal rat offspring from morphine-addicted mothers. More importantly, the opioid-withdrawal episode may provide augmented effects for the abovementioned alterations and could lead to deleterious effects in the neonatal brain of such offspring.

Pain management in the surgical ICU patient

PURPOSE OF REVIEW

Acute pain management in the surgical ICU is imperative. Effective acute pain management hastens a patient's return to normal function and avoid the negative sequelae of untreated acute pain. Traditionally, opioids have been the mainstay of acute pain management strategies in the surgical ICU, but alternative medications and management strategies are increasingly being utilized.

RECENT FINDINGS

Extrapolating from lessons learned from enhanced recovery after surgery protocols, surgical intensivists are increasingly utilizing multimodal pain regimens (MMPRs) in critically ill surgical patients recovering from major surgical procedures and injuries. MMPRs incorporate both oral medications from several drug classes and regional blocks when feasible. In addition, although MMPRs may include opioids as needed, they are able to achieve effective pain control while minimizing opioid exposure.

SUMMARY

Even after major elective surgery or significant injury, opioid-minimizing MMPRs can effectively treat acute pain.

Chronic cancer and non-cancer pain and opioid-induced hyperalgesia share common mechanisms: neuroinflammation and central sensitization

Neuroinflammation, a peculiar form of inflammation that occurs in response to noxious stimuli in peripheral and central nervous system (CNS), consists in altered vascular permeability followed by leukocyte recruitment and activation in the inflamed tissue, release of inflammatory mediators including cytokines and chemokines, and finally in the activation of microglia and astrocytes in the spinal cord and CNS. This phenomenon mediates and even worsen the inflammatory pain in many painful states and is responsible for central sensitization leading to pain chronicity. We describe the major neuroinflammatory mechanisms shared by cancer and non-cancer pain. Particular attention is given to two different chronic inflammatory painful diseases such as the complex regional pain syndrome and the rheumatoid arthritis as prototypes of neuroinflammatory diseases (gliopathies). In addition, we describe the complexity of tumor microenvironment, their main cellular components (tumor cells, tumor infiltrating leukocytes and sensory neurons) and their reciprocal interactions that characterize different forms and intensity of cancer pain. We also hypothesize that one type of cancer pain, the breakthrough pain, can be attributable to receptor-mediated interaction of opioids with tumor cells and intratumoral leukocytes. Surprisingly, long-term opioid treatment shares the same neuroinflammatory potential responsible for the chronicity of both cancer and non-cancer pain; thus, resulting in paradoxical worsening rather than relieving pain. This paradox has upset the world of pain therapy, with neuroinflammation now being a main target of emerging therapies.

Sex differences in neuro(auto)immunity and chronic sciatic nerve pain

Chronic pain occurs with greater frequency in women, with a parallel sexually dimorphic trend reported in sufferers of many autoimmune diseases. There is a need to continue examining neuro-immune-endocrine crosstalk in the context of sexual dimorphisms in chronic pain. Several phenomena in particular need to be further explored. In patients, autoantibodies to neural antigens have been associated with sensory pathway hyper-excitability, and the role of self-antigens released by damaged nerves remains to be defined. In addition, specific immune cells release pro-nociceptive cytokines that directly influence neural firing, while T lymphocytes activated by specific antigens secrete factors that either support nerve repair or exacerbate the damage. Modulating specific immune cell populations could therefore be a means to promote nerve recovery, with sex-specific outcomes. Understanding biological sex differences that maintain, or fail to maintain, neuroimmune homeostasis may inform the selection of sex-specific treatment regimens, improving chronic pain management by rebalancing neuroimmune feedback. Given the significance of interactions between nerves and immune cells in the generation and maintenance of neuropathic pain, this review focuses on sex differences and possible links with persistent autoimmune activity using sciatica as an example.

Preoperative Opioids Associated With Increased Postoperative Opioid Use in Pediatric Appendicitis

BACKGROUND

In light of current opioid-minimization efforts, we aimed to identify factors that predict postoperative opioid requirement in pediatric appendicitis patients.

METHODS

A single-center retrospective cohort study was conducted of children (<18 y) who underwent laparoscopic appendectomy for acute appendicitis between January 1, 2018 and April 30, 2019. Patients who underwent open or interval appendectomies were excluded. The primary outcome was morphine milliequivalents (MMEs) per kilogram administered between 2 and 24 h after surgery. Multivariable analyses were performed to evaluate predictors of postoperative opioid use. Clinically sound covariates were chosen a priori: age, weight, simple versus complicated appendicitis, preoperative opioid administration, and receipt of regional or local anesthesia.

RESULTS

Of 546 patients, 153 (28%) received postoperative opioids. Patients who received postoperative opioids had a longer median preadmission symptom duration (48 versus 24 h, P < 0.001) and were more likely to have complicated appendicitis (55% versus 21%, P < 0.001). Patients who received postoperative opioids were more likely to have received preoperative opioids (54% versus 31%, P < 0.001). Regional and local anesthesia use was similar between groups. Nearly all patients (99%) received intraoperative opioids. Each preoperative MME per kilogram that a patient received was associated with receipt of 0.29 additional MMEs per kilogram postoperatively (95% confidence interval, 0.19-0.40).

CONCLUSIONS

Preoperative opioid administration was independently associated with increased postoperative opioid use in pediatric appendicitis. These findings suggest that preoperative opioids may potentiate increased postoperative pain. Limiting preoperative opioid exposure, through strategies such as multimodal analgesia, may be an important facet of efforts to reduce postoperative opioid use.

Oxycodone, fentanyl, and morphine amplify established neuropathic pain in male rats

Opioids are widely prescribed for chronic pain, including neuropathic pain, despite growing evidence of long-term harm. Previous preclinical studies have documented exacerbation of nociceptive hypersensitivity, including that induced by peripheral nerve injury, by morphine. The present series of behavioral studies sought to replicate and extend our prior research, which demonstrated a multimonth exacerbation of nociceptive hypersensitivity by a 5-day course of morphine initiated 10 days after nerve injury. The current studies demonstrate that enduring exacerbation of nociceptive hypersensitivity is not restricted to morphine, but rather is also created by the clinically relevant opioids fentanyl and oxycodone when these are likewise administered for 5 days beginning 10 days after nerve injury. Furthermore, enduring exacerbation of nociceptive hypersensitivity is also observed when the same dosing regimen for either morphine, fentanyl, or oxycodone begins 1 month after nerve injury. Finally, a striking result from these studies is that no such exacerbation of nociceptive hypersensitivity occurs when either morphine, fentanyl, or oxycodone dosing begins at the time of nerve injury. These results extend our previous findings that morphine exacerbates nociceptive hypersensitivity to the clinically relevant opioids fentanyl and oxycodone when administered after the development of nociceptive hypersensitivity, while also providing possible clinically relevant insight into when these opioids can be safely administered and not exacerbate neuropathic pain.

Morphine Exacerbates Postfracture Nociceptive Sensitization, Functional Impairment, and Microglial Activation in Mice

BACKGROUND

Emerging evidence suggests that opioid use immediately after surgery and trauma may worsen outcomes. In these studies, the authors aimed to determine whether morphine administered for a clinically relevant time period (7 days) in a tibia fracture orthopedic surgery model had adverse effects on postoperative recovery.

METHODS

Mice were given morphine twice daily for 7 days after unilateral tibial fracture and intramedullary pin fixation to model orthopedic surgery and limb trauma. Mechanical allodynia, limb-specific weight bearing, gait changes, memory, and anxiety were measured after injury. In addition, spinal cord gene expression changes as well as glial activation were measured. Finally, the authors assessed the effects of a selective Toll-like receptor 4 antagonist, TAK-242, on nociceptive and functional changes after injury.

RESULTS

Tibial fracture caused several weeks of mechanical nociceptive sensitization (F(1, 216) = 573.38, P < 0.001, fracture + vehicle vs. sham + vehicle, n = 10 per group), and this change was exacerbated by the perioperative administration of morphine (F(1, 216) = 71.61, P < 0.001, fracture + morphine vs. fracture + vehicle, n = 10 per group). In additional testing, injured limb weight bearing, gait, and object location memory were worse in morphine-treated fracture mice than in untreated fracture mice. Postfracture expression levels of several genes previously associated with opioid-induced hyperalgesia, including brain-derived neurotrophic factor and prodynorphin, were unchanged, but neuroinflammation involving Toll-like receptor 4 receptor-expressing microglia was observed (6.8 ± 1.5 [mean ± SD] cells per high-power field for fracture + vehicle vs. 12 ± 2.8 fracture + morphine, P < 0.001, n = 8 per /group). Treatment with a Toll-like receptor 4 antagonist TAK242 improved nociceptive sensitization for about 2 weeks in morphine-treated fracture mice (F(1, 198) = 73.36, P < 0.001, fracture + morphine + TAK242 vs. fracture + morphine, n = 10 per group).

CONCLUSIONS

Morphine treatment beginning at the time of injury impairs nociceptive recovery and other outcomes. Measures preventing glial activation through Toll-like receptor 4 signaling may reduce the adverse consequences of postoperative opioid administration.

Repeated Morphine Prolongs Postoperative Pain in Male Rats

BACKGROUND

Opioids are effective postoperative analgesics. Disturbingly, we have previously reported that opioids such as morphine can worsen inflammatory pain and peripheral and central neuropathic pain. These deleterious effects are mediated by immune mediators that promote neuronal hyperexcitability in the spinal dorsal horn. Herein, we tested whether perioperative morphine could similarly prolong postoperative pain in male rats.

METHODS

Rats were treated with morphine for 7 days, beginning immediately after laparotomy, while the morphine was tapered in a second group. Expression of genes for inflammatory mediators was quantified in the spinal dorsal horn. In the final experiment, morphine was administered before laparotomy for 7 days.

RESULTS

We found that morphine treatment after laparotomy extended postoperative pain by more than 3 weeks (time × treatment: P < .001; time: P < .001; treatment: P < .05). Extension of postoperative pain was not related to morphine withdrawal, as it was not prevented by dose tapering (time × treatment: P = .8; time: P < .001; treatment: P = .9). Prolonged postsurgical pain was associated with increased expression of inflammatory genes, including those encoding Toll-like receptor 4, NOD like receptor protein 3 (NLRP3), nuclear factor kappa B (NFκB), caspase-1, interleukin-1β, and tumor necrosis factor (P < .05). Finally, we showed that of preoperative morphine, concluding immediately before laparotomy, similarly prolonged postoperative pain (time × treatment: P < .001; time: P < .001; treatment: P < .001). There is a critical window for morphine potentiation of pain, as a 7-day course of morphine that concluded 1 week before laparotomy did not prolong postsurgical pain.

CONCLUSIONS

These studies indicate the morphine can have a deleterious effect on postoperative pain. These studies further suggest that longitudinal studies could be performed to test whether opioids similarly prolong postoperative pain in the clinic.

Morphine immunomodulation prolongs inflammatory and postoperative pain while the novel analgesic ZH853 accelerates recovery and protects against latent sensitization

Background

Numerous studies have identified the proinflammatory, pronociceptive effects of morphine which ultimately exacerbate pain. Our novel endomorphin analog ZH853 does not produce proinflammatory effects on its own and gives potent, long-lasting analgesia. This study investigates whether ZH853’s lack of interaction with the neuroimmune system reduces the risk of prolonged pain.

Methods

Adult male Sprague-Dawley rats were subjected to one of two treatment paradigms. Either (1) chronic pain followed by chronic treatment with morphine, ZH853 or vehicle, or (2) chronic drug administered prior to pain induction. Complete Freund’s adjuvant (CFA) was injected or paw incision surgery was performed on the left hind plantar foot pad. Drugs were administered through Alzet osmotic minipumps at a rate of 1 μl/h for 5 days at appropriate doses based on prior experiments. Animals were tested for mechanical allodynia and thermal hyperalgesia using von Frey filaments and the Hargreaves apparatus, respectively. Additionally, several gait parameters were measured using the CatWalk XT. When all animals had recovered from pain, 1 mg/kg of naltrexone was administered to test for development of latent sensitization (LS). A second set of animals was used to investigate dorsal horn inflammation following CFA and drug treatment. ANOVAs were used to assess differences between drug treatment groups.

Results

As expected, morphine increased and prolonged pain in all experiments compared to vehicle treatment. However, ZH853 treatment reduced the overall time spent in pain and the severity of pain scores compared to morphine. ZH853 not only reduced inflammation versus morphine treatment but also, in some instances, acted as an anti-inflammatory drug compared to vehicle treatment. Finally, ZH853 prevented the development of LS while vehicle- and morphine-treated animals showed robust relapse to pain.

Conclusions

ZH853 has a favorable side effect profile versus morphine and provides superior analgesia in a number of pain states. We now know that chronic use of this compound reduces time spent in a chronic pain state, the opposite of common opioids like morphine, and reduces the risk of LS, making ZH853 an excellent candidate for clinical development in humans for inflammatory and postoperative pain.

Protraction of neuropathic pain by morphine is mediated by spinal damage associated molecular patterns (DAMPs) in male rats

We have recently reported that a short course of morphine, starting 10days after sciatic chronic constriction injury (CCI), prolonged the duration of mechanical allodynia for months after morphine ceased. Maintenance of this morphine-induced persistent sensitization was dependent on spinal NOD-like receptor protein 3 (NLRP3) inflammasomes-protein complexes that proteolytically activate interleukin-1β (IL-1β) via caspase-1. However, it is still unclear how NLRP3 inflammasome signaling is maintained long after morphine is cleared. Here, we demonstrate that spinal levels of the damage associated molecular patterns (DAMPs) high mobility group box 1 (HMGB1) and biglycan are elevated during morphine-induced persistent sensitization in male rats; that is, 5weeks after cessation of morphine dosing. We also show that HMGB1 and biglycan levels are at least partly dependent on the initial activation of caspase-1, as well as Toll like receptor 4 (TLR4) and the purinergic receptor P2X7R-receptors responsible for priming and activation of NLRP3 inflammasomes. Finally, pharmacological attenuation of the DAMPs HMGB1, biglycan, heat shock protein 90 and fibronectin persistently reversed morphine-prolonged allodynia. We conclude that after peripheral nerve injury, morphine treatment results in persistent DAMP release via TLR4, P2X7R and caspase-1, which are involved in formation/activation of NLRP3 inflammasomes. These DAMPs are responsible for maintaining persistent allodynia, which may be due to engagement of a positive feedback loop, in which NLRP3 inflammasomes are persistently activated by DAMPs signaling at TLR4 and P2X7R.

MicroRNA-124 and microRNA-146a both attenuate persistent neuropathic pain induced by morphine in male rats

We have recently reported that a short course of morphine, starting 10 days after sciatic chronic constriction injury (CCI), prolonged the duration of mechanical allodynia for months after morphine ceased. Maintenance of this morphine-induced persistent sensitization was dependent on microglial reactivity and Toll-like receptor 4 signaling. Given that microRNAs (miRNAs) such as miR-124 and miR-146a possess the ability to modulate such signaling, we directly compared their function in this model. We found that both miRNAs reversed established allodynia in our model of morphine-induced persistent sensitization. The efficacy of miR-124 and miR-146a were comparable, and in both cases allodynia returned within hours to days of miRNA dosing conclusion. Our findings demonstrate that miRNAs targeting Toll-like receptor signaling are effective in reversing neuropathic pain, which underscores the clinical potential of these non-coding RNAs.

Astrocytic hypertrophy in the rat ventral tegmental area following chronic morphine differs with age

The ventral tegmental area (VTA) is the origin of the mesolimbic dopaminergic system known to play an integral role in mediating reward and development of drug addiction. Although the differences in neuronal plasticity of VTA at various ages remain to be understood, age is known to influence the effects of chronic opioids. In addition, adaptations associated with exposure to opioids within glial populations located in the VTA are poorly understood. The objective of the study was to determine if there are changes in astrocytic immunofluorescent labeling in the VTA following chronic morphine administration in a rat model at different ages: newborn at postnatal day (PD)7 and adult (estimated PD57). We hypothesized that increased immunohistochemical labeling of an astrocytic marker, glial fibrillary acidic protein (GFAP) in the VTA following chronic administration of morphine will not differ with age. Two groups of rats were analyzed: chronic morphine and saline control treatment groups. Either morphine (10 mg/kg) or equal volume of saline was given subcutaneously twice daily for 6½ days. On the 7^th day of treatment, animals were anesthetized and perfused at one hour after the final drug injection. Coronal sections of the midbrain were processed for immunofluorescent identification of GFAP that was noted at both ages. We report an increase in both (1) GFAP labeling intensity, as well as (2) the percent area occupied by astrocytes that are immunoreactive for GFAP following chronic morphine when compared to saline treatment in the VTA only for the adults (n=6/group) but not infant rats at PD7 (n=5/group). Our findings suggest that adaptations in the mesolimbic dopaminergic system produced by repeated exposure to opioids may be associated with changes in glial function that differ with age.

Heroin-piracetam mixture: Suggested mechanisms of action and risks of misinterpretation for drug users

Piracetam is a positive allosteric modulator of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor that has been frequently used in the treatment of cognitive disorders. Press and internet reports indicate that the use of piracetam, as a heroin adulterant, has spread rapidly in some countries, especially in Asia and Europe. Its use, as adulterant, is believed to produce more profound desirable effects, while decreasing hangover. Recent surveys demonstrated that piracetam protects neurons from heroin-induced apoptosis. The protective role of this adulterating substance may be related to restoration of beta-endorphin levels and to its neuroprotective effects. The aim of this paper is to review the relevant literature and suggest the main hypothetical mechanisms that justify its use as a heroin adulterant, try to understand if its use could help people who want to come off heroin by reducing withdrawal symptoms and, finally, give useful information that permit us to understand why drug trafficking organisations started to use piracetam as heroin adulterant.

Glial and neuroinflammatory targets for treating substance use disorders

BACKGROUND

The plenary session at the 2016 Behavior, Biology and Chemistry: Translational Research in Addiction Conference focused on glia as potential players in the development, persistence and treatment of substance use disorders. Glia partake in various functions that are important for healthy brain activity. Drugs of abuse alter glial cell activity producing several perturbations in brain function that are thought to contribute to behavioral changes associated with substance use disorders. Consequently, drug-induced changes in glia-driven processes in the brain represent potential targets for pharmacotherapeutics treating substance use disorders.

METHODS

Four speakers presented preclinical and clinical research illustrating the effects that glial modulators have on abuse-related behavioral effects of psychostimulants and opioids. This review highlights some of these findings and expands its focus to include other research focused on drug-induced glia abnormalities and glia-focused treatment approaches in substance use disorders.

RESULTS

Preclinical findings show that drugs of abuse induce neuroinflammatory signals and disrupt glutamate homeostasis through their interaction with microglia and astrocytes. Preclinical and clinical studies testing the effects of glial modulators show general effectiveness in reducing behaviors associated with substance use disorders.

CONCLUSIONS

The contribution of drug-induced glial activity continues to emerge as an intriguing target for substance use disorder treatments. Clinical investigations of glial modulators have yielded promising results on substance use measures and indicate that they are generally safe and well-tolerated. However, results have not been entirely positive and more questions remain for continued exploration in the development and testing of glial-directed treatments for substance use disorders.

Exploring acute-to-chronic neuropathic pain in rats after contusion spinal cord injury

Spinal cord injury (SCI) causes chronic pain in 65% of individuals. Unfortunately, current pain management is inadequate for many SCI patients. Rodent models could help identify how SCI pain develops, explore new treatment strategies, and reveal whether acute post-SCI morphine worsens chronic pain. However, few studies explore or compare SCI-elicited neuropathic pain in rats. Here, we sought to determine how different clinically relevant contusion SCIs in male and female rats affect neuropathic pain, and whether acute morphine worsens later chronic SCI pain. First, female rats received sham surgery, or 150kDyn or 200kDyn midline T9 contusion SCI. These rats displayed modest mechanical allodynia and long-lasting thermal hyperalgesia. Next, a 150kDyn (1s dwell) midline contusion SCI was performed in male and female rats. Interestingly, males, but not females showed SCI-elicited mechanical allodynia; rats of both sexes had thermal hyperalgesia. In this model, acute morphine treatment had no significant effect on chronic neuropathic pain symptoms. Unilateral SCIs can also elicit neuropathic pain that could be exacerbated by morphine, so male rats received unilateral T13 contusion SCI (100kDyn). These rats exhibited significant, transient mechanical allodynia, but not thermal hyperalgesia. Acute morphine did not exacerbate chronic pain. Our data show that specific rat contusion SCI models cause neuropathic pain. Further, chronic neuropathic pain elicited by these contusion SCIs was not amplified by our course of early post-trauma morphine. Using clinically relevant rat models of SCI could help identify novel pain management strategies.

Biphalin, a Dimeric Enkephalin, Alleviates LPS-Induced Activation in Rat Primary Microglial Cultures in Opioid Receptor-Dependent and Receptor-Independent Manners

Neuropathic pain is relatively less responsive to opioids than other types of pain, which is possibly due to a disrupted opioid system partially caused by the profound microglial cell activation that underlines neuroinflammation. We demonstrated that intrathecally injected biphalin, a dimeric enkephalin analog, diminished symptoms of neuropathy in a preclinical model of neuropathic pain in rats (CCI, chronic constriction injury of the sciatic nerve) at day 12 postinjury. Using primary microglial cell cultures, we revealed that biphalin did not influence cell viability but diminished NO production and expression of Iba1 in LPS-stimulated cells. Biphalin also diminished MOP receptor level, as well as pronociceptive mediators (iNOS, IL-1β, and IL-18) in an opioid receptor-dependent manner, and it was correlated with diminished p-NF-κB, p-IκB, p-p38MAPK, and TRIF levels. Biphalin reduced IL-6, IL-10, TNFα, p-STAT3, and p-ERK1/2 and upregulated SOCS3, TLR4, and MyD88; however, this effect was not reversed by naloxone pretreatment. Our study provides evidence that biphalin diminishes neuropathy symptoms, which might be partially related to reduced pronociceptive mediators released by activated microglia. Biphalin may be a putative drug for future pain therapy, especially for the treatment of neuropathic pain, when the lower analgesic effects of morphine are correlated with profound microglial cell activation.

Evaluation of TSPO PET imaging, a marker of glial activation, to study the neuroimmune footprints of morphine exposure and withdrawal

INTRODUCTION

A growing area of research suggests that neuroimmunity may impact the pharmacology of opioids. Microglia is a key component of the brain immunity. Preclinical and clinical studies have demonstrated that microglial modulators may improve morphine-induced analgesia and prevent the development of tolerance and dependence. Positron emission tomography (PET) using translocator protein 18kDa (TSPO) radioligand is a clinically validated strategy for the non-invasive detection of microglial activation. We hypothesized that TSPO PET imaging may be used to study the neuroimmune component of opioid tolerance and withdrawal.

METHODS

Healthy rats (n=6 in each group) received either saline or escalating doses of morphine (10-40mg/kg) on five days to achieve tolerance and a withdrawal syndrome after morphine discontinuation. MicroPET imaging with [¹⁸F]DPA-714 was performed 60h after morphine withdrawal. Kinetic modeling was performed to estimate [¹⁸F]DPA-714 volume of distribution (V_T) in several brain regions using dynamic PET images and corresponding metabolite-corrected input functions. Immunohistochemistry (IHC) experiments on striatal brain slices were performed to assess the expression of glial markers (Iba1, GFAP and CD68) during 14days after morphine discontinuation.

RESULTS

The baseline binding of [¹⁸F]DPA-714 to the brain (V_T=0.086±0.009mLcm^-3) was not increased by morphine exposure and withdrawal (V_T=0.079±0.010mLcm^-3) indicating the absence of TSPO overexpression, even at the regional level. Accordingly, expression of glial markers did not increase after morphine discontinuation.

CONCLUSIONS

Morphine tolerance and withdrawal did not detectably activate microglia and had no impact on [¹⁸F]DPA-714 brain kinetics in vivo.

Morphine amplifies mechanical allodynia via TLR4 in a rat model of spinal cord injury

Central neuropathic pain (CNP) is a pervasive, debilitating problem that impacts thousands of people living with central nervous system disorders, including spinal cord injury (SCI). Current therapies for treating this type of pain are ineffective and often have dose-limiting side effects. Although opioids are one of the most commonly used CNP treatments, recent animal literature has indicated that administering opioids shortly after a traumatic injury can actually have deleterious effects on long-term health and recovery. In order to study the deleterious effects of administering morphine shortly after trauma, we employed our low thoracic (T13) dorsal root avulsion model (Spinal Neuropathic Avulsion Pain, SNAP). Administering a weeklong course of 10mg/kg/day morphine beginning 24h after SNAP resulted in amplified mechanical allodynia. Co-administering the non-opioid toll-like receptor 4 (TLR4) antagonist (+)-naltrexone throughout the morphine regimen prevented morphine-induced amplification of SNAP. Exploration of changes induced by early post-trauma morphine revealed that this elevated gene expression of TLR4, TNF, IL-1β, and NLRP3, as well as IL-1β protein at the site of spinal cord injury. These data suggest that a short course of morphine administered early after spinal trauma can exacerbate CNP in the long term. TLR4 initiates this phenomenon and, as such, may be potential therapeutic targets for preventing the deleterious effects of administering opioids after traumatic injury.

The dark side of opioids in pain management: basic science explains clinical observation

Although there is no doubt about opioids' ability to relieve pain in the short term, it is not always clear why longer-term analgesic efficacy seems to be impaired. Tolerance and hyperalgesia have been suggested as mechanisms for opioid analgesic deterioration. But could there also be an effect of opioids on pain itself?

Introduction:

In the past 2 decades, opioids have been used increasingly for the treatment of persistent pain, and doses have tended to creep up. As basic science elucidates mechanisms of pain and analgesia, the cross talk between central pain and opioid actions becomes clearer.

Objectives:

We aimed to examine the published literature on basic science explaining pronociceptive opioid actions, and apply this knowledge to clinical observation.

Methods:

We reviewed the existing literature on the pronociceptive actions of opioids, both preclinical and clinical studies.

Results:

Basic science provides a rationale for the clinical observation that opioids sometimes increase rather than decrease pain. Central sensitization (hyperalgesia) underlies pain chronification, but can also be produced by high dose and high potency opioids. Many of the same mechanisms account for both central pain and opioid hyperalgesia.

Conclusion:

Newly revealed basic mechanisms suggest possible avenues for drug development and new drug therapies that could alter pain sensitization through endogenous and exogenous opioid mechanisms. Recent changes in practice such as the introduction of titration-to-effect for opioids have resulted in higher doses used in the clinic setting than ever seen previously. New basic science knowledge hints that these newer dosing practices may need to be reexamined. When pain worsens in a patient taking opioids, can we be assured that this is not because of the opioids, and can we alter this negative effect of opioids through different dosing strategies or new drug intervention?

Morphine paradoxically prolongs neuropathic pain in rats by amplifying spinal NLRP3 inflammasome activation

Opioid use for pain management has dramatically increased, with little assessment of potential pathophysiological consequences for the primary pain condition. Here, a short course of morphine, starting 10 d after injury in male rats, paradoxically and remarkably doubled the duration of chronic constriction injury (CCI)-allodynia, months after morphine ceased. No such effect of opioids on neuropathic pain has previously been reported. Using pharmacologic and genetic approaches, we discovered that the initiation and maintenance of this multimonth prolongation of neuropathic pain was mediated by a previously unidentified mechanism for spinal cord and pain-namely, morphine-induced spinal NOD-like receptor protein 3 (NLRP3) inflammasomes and associated release of interleukin-1β (IL-1β). As spinal dorsal horn microglia expressed this signaling platform, these cells were selectively inhibited in vivo after transfection with a novel Designer Receptor Exclusively Activated by Designer Drugs (DREADD). Multiday treatment with the DREADD-specific ligand clozapine-N-oxide prevented and enduringly reversed morphine-induced persistent sensitization for weeks to months after cessation of clozapine-N-oxide. These data demonstrate both the critical importance of microglia and that maintenance of chronic pain created by early exposure to opioids can be disrupted, resetting pain to normal. These data also provide strong support for the recent "two-hit hypothesis" of microglial priming, leading to exaggerated reactivity after the second challenge, documented here in the context of nerve injury followed by morphine. This study predicts that prolonged pain is an unrealized and clinically concerning consequence of the abundant use of opioids in chronic pain.

Endomorphin analog analgesics with reduced abuse liability, respiratory depression, motor impairment, tolerance, and glial activation relative to morphine

Opioids acting at the mu opioid receptor (MOR) are the most effective analgesics, however adverse side effects severely limit their use. Of particular importance, abuse liability results in major medical, societal, and economic problems, respiratory depression is the cause of fatal overdoses, and tolerance complicates treatment and increases the risk of side effects. Motor and cognitive impairment are especially problematic for older adults. Despite the host of negative side effects, opioids such as morphine are commonly used for acute and chronic pain conditions. Separation of analgesia from unwanted effects has long been an unmet goal of opioid research. Novel MOR agonist structures may prove critical for greater success. Here we tested metabolically stable analogs of the endomorphins, endogenous opioids highly selective for the MOR. Compared to morphine, the analogs showed dramatically improved analgesia-to-side-effect ratios. At doses providing equal or greater antinociception than morphine in the rat, the analogs showed reduced a) respiratory depression, b) impairment of motor coordination, c) tolerance and hyperalgesia, d) glial p38/CGRP/P2X7 receptor signaling, and e) reward/abuse potential in both conditioned place preference and self-administration tests. Differential effects on glial activation indicate a mechanism for the relative lack of side effects by the analogs compared to morphine. The results suggest that endomorphin analogs described here could provide gold standard pain relief mediated by selective MOR activation, but with remarkably safer side effect profiles compared to opioids like morphine.

Opioid-induced central immune signaling: implications for opioid analgesia

Despite being the mainstay of pain management, opioids are limited in their clinical utility by adverse effects, such as tolerance and paradoxical hyperalgesia. Research of the past 15 years has extended beyond neurons, to implicate central nervous system immune signaling in these adverse effects. This article will provide an overview of these central immune mechanisms in opioid tolerance and paradoxical hyperalgesia, including those mediated by Toll-like receptor 4, purinergic, ceramide, and chemokine signaling. Challenges for the future, as well as new lines of investigation will be highlighted.

Opioid and chemokine receptor crosstalk: a promising target for pain therapy?

Chemokines and opioids are important regulators of immune, inflammatory and neuronal responses in peripheral and central pain pathways. Recent studies have provided insights into the functional interactions between chemokine receptors and opioid receptors, and their role in pain modulation. In this Progress article, we discuss how crosstalk between these two systems might provide a molecular and cellular framework for the development of novel analgesic therapies for the management of acute and/or chronic pain.

Endogenous opiates and behavior: 2013

This paper is the thirty-sixth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2013 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; alcohol and drugs of abuse; 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.

Interactions between glia, the immune system and pain processes during early development

Pain is a serious problem for infants and children and treatment options are limited. Moreover, infants born prematurely or hospitalized for illness likely have concurrent infection that activates the immune system. It is now recognized that the immune system in general and glia in particular influence neurotransmission and that the neural bases of pain are intimately connected to immune function. We know that injuries that induce pain activate immune function and suppressing the immune system alleviates pain. Despite this advance in our understanding, virtually nothing is known of the role that the immune system plays in pain processing in infants and children, even though pain is a serious clinical issue in pediatric medicine. This brief review summarizes the existing data on immune-neural interactions in infants, providing evidence for the immaturity of these interactions.

Pathological pain and the neuroimmune interface

Reciprocal signalling between immunocompetent cells in the central nervous system (CNS) has emerged as a key phenomenon underpinning pathological and chronic pain mechanisms. Neuronal excitability can be powerfully enhanced both by classical neurotransmitters derived from neurons, and by immune mediators released from CNS-resident microglia and astrocytes, and from infiltrating cells such as T cells. In this Review, we discuss the current understanding of the contribution of central immune mechanisms to pathological pain, and how the heterogeneous immune functions of different cells in the CNS could be harnessed to develop new therapeutics for pain control. Given the prevalence of chronic pain and the incomplete efficacy of current drugs--which focus on suppressing aberrant neuronal activity--new strategies to manipulate neuroimmune pain transmission hold considerable promise.

Suppression of voluntary wheel running in rats is dependent on the site of inflammation: evidence for voluntary running as a measure of hind paw-evoked pain

Decreased voluntary wheel running has recently been proposed as a preclinical pain measure for inflammatory pain, but whether this reflects pain evoked by use of the affected limbs is unknown. To assess the role of inflammation site as a determinant of this measure, complete Freund's adjuvant (CFA), formalin, or equivolume vehicle was subcutaneously injected into the plantar surface of the hind paws (bilateral) or L1 dorsum dermatome (leaving paws unaffected) of male Sprague Dawley rats. CFA-induced hind paw mechanical allodynia (P < .001) did not correlate with reduced voluntary wheel running. Intraplantar formalin did not attenuate voluntary running, despite eliciting robust licking/writhing/flinching behavior and hind paw mechanical allodynia (P < .001). Subcutaneous L1 dorsum dermatome formalin, but not CFA, induced licking/writhing/flinching behavior (P < .001), but neither induced hind paw mechanical allodynia or attenuated voluntary running. That voluntary running is decreased by hind paw CFA, but not by L1 dorsum CFA, implies that the behavior is a measure of CFA-induced pain evoked by use of the affected limbs rather than supraspinal pain processing that is independent of inflammation site. Furthermore, the results suggest that interpretation of voluntary wheel running data cannot simply be explained by correlation with mechanical allodynia.

PERSPECTIVE

Whether decreased voluntary running is dependent on inflammation site is unknown. We show that intraplantar, but not L1 dorsum, CFA suppressed voluntary running and formalin-induced licking/writhing/flinching behavior but had no effect on voluntary running. These data suggest that suppressed voluntary running by CFA likely reflects pain evoked by use of the affected limbs.

Cytokines, chaperones and neuroinflammatory responses in heroin-related death: what can we learn from different patterns of cellular expression?

Heroin (3,6-diacetylmorphine) has various effects on the central nervous system with several neuropathological alterations including hypoxic-ischemic brain damage from respiratory depressing effects and neuroinflammatory response. Both of these mechanisms induce the release of cytokines, chemokines and other inflammatory mediators by the activation of many cell types such as leucocytes and endothelial and glial cells, especially microglia, the predominant immunocompetent cell type within the central nervous system. The aim of this study is to clarify the correlation between intravenous heroin administration in heroin related death and the neuroinflammatory response. We selected 45 cases among autopsies executed for heroin-related death (358 total cases); immunohistochemical studies and Western blotting analyses were used to investigate the expression of brain markers such as tumor necrosis factor-α, oxygen-regulated protein 150, (interleukins) IL-1β, IL-6, IL-8, IL-10, IL-15, cyclooxygenase-2, heat shock protein 70, and CD68 (MAC387). Findings demonstrated that morphine induces inflammatory response and cytokine release. In particular, oxygen-regulated protein 150, cyclooxygenase-2, heat shock protein 70, IL-6 and IL-15 cytokines were over-expressed with different patterns of cellular expression.

Novel fentanyl-based dual μ/δ-opioid agonists for the treatment of acute and chronic pain

Approximately one third of the adult U.S. population suffers from some type of on-going, chronic pain annually, and many more will have some type of acute pain associated with trauma or surgery. First-line therapies for moderate to severe pain include prescriptions for common mu opioid receptor agonists such as morphine and its various derivatives. The epidemic use, misuse and diversion of prescription opioids have highlighted just one of the adverse effects of mu opioid analgesics. Alternative approaches include novel opioids that target delta or kappa opioid receptors, or compounds that interact with two or more of the opioid receptors.

AIMS

Here we report the pharmacology of a newly synthesized bifunctional opioid agonist (RV-Jim-C3) derived from combined structures of fentanyl and enkephalin in rodents. RV-Jim-C3 has high affinity binding to both mu and delta opioid receptors.

MAIN METHODS

Mice and rats were used to test RV-Jim-C3 in a tailflick test with and without opioid selective antagonist for antinociception. RV-Jim-C3 was tested for anti-inflammatory and antihypersensitivity effects in a model of formalin-induced flinching and spinal nerve ligation. To rule out motor impairment, rotarod was tested in rats.

KEY FINDINGS

RV-Jim-C3 demonstrates potent-efficacious activity in several in vivo pain models including inflammatory pain, antihyperalgesia and antiallodynic with no significant motor impairment.

SIGNIFICANCE

This is the first report of a fentanyl-based structure with delta and mu opioid receptor activity that exhibits outstanding antinociceptive efficacy in neuropathic pain, reducing the propensity of unwanted side effects driven by current therapies that are unifunctional mu opioid agonists.

Mast cell activation contributes to sickle cell pathobiology and pain in mice

Sickle cell anemia (SCA) is an inherited disorder associated with severe lifelong pain and significant morbidity. The mechanisms of pain in SCA remain poorly understood. We show that mast cell activation/degranulation contributes to sickle pain pathophysiology by promoting neurogenic inflammation and nociceptor activation via the release of substance P in the skin and dorsal root ganglion. Mast cell inhibition with imatinib ameliorated cytokine release from skin biopsies and led to a correlative decrease in granulocyte-macrophage colony-stimulating factor and white blood cells in transgenic sickle mice. Targeting mast cells by genetic mutation or pharmacologic inhibition with imatinib ameliorates tonic hyperalgesia and prevents hypoxia/reoxygenation-induced hyperalgesia in sickle mice. Pretreatment with the mast cell stabilizer cromolyn sodium improved analgesia following low doses of morphine that were otherwise ineffective. Mast cell activation therefore underlies sickle pathophysiology leading to inflammation, vascular dysfunction, pain, and requirement for high doses of morphine. Pharmacological targeting of mast cells with imatinib may be a suitable approach to address pain and perhaps treat SCA.

The role of the angiotensin AT2 receptor on the diurnal variations of nociception and motor coordination in rats

Phasic pain demonstrates significant diurnal variation in rats. Angiotensin II modulates pain transmission and the diurnal variation in nociception in several rodent pain models. The participation of AT2 receptors in the diurnal regulation of nociception is not yet elucidated. In the present study we investigated the effects of selective peptide AT2 agonist CGP 42112A and the nonpeptide AT2 receptor antagonist PD 123319 on the nociception, motor coordination and arterial blood pressure. Male Wistar 12 weeks old rats were used. CGP 42112A was injected at single doses of 1 and 5 μg/rat intracerebroventricularly (ICV) and infused chronically ICV at a dose of 12 μg/rat/day during 14 days by osmotic minipumps. PD123319 was injected at single doses of 1 and 5 μg/rat, ICV and chronically subcutaneously at a dose of 10 mg/kg/day/14 days. Nociception was assessed by an analgesimeter, arterial blood pressure (ABP) was measured by tail cuff method, and motor coordination by Rota-rod method. Single doses of CGP 42112A (1 and 5 μg/rat) provoked a short lasting antinociception. Unlike acute injection, chronic CGP 42112A infusion increased nociception at the beginning and the end of light phase thus attenuating the diurnal variations observed in the controls. Moreover, it produced an increase of ABP and improved motor coordination. Both acute (1 μg/rat) and chronic PD 123319 treatment resulted in a decrease of pain threshold and chronic treatment attenuated its diurnal fluctuation. Our data support a role for Ang II type 2 receptors in the control of diurnal variations of nociception in rats.