Development of antinociceptive tolerance and changes of opioid receptor ligand binding in central nervous system of the mouse forced to single and repeated swimming in the cold water

The changes of nociception and the signal molecules expression in the dorsal root ganglia and the spinal cord after cold water swimming stress in mice

Several studies have previously reported that exposure to stress provokes behavioral changes, including antinociception, in rodents. In the present study, we studied the effect of acute cold-water (4°C) swimming stress (CWSS) on nociception and the possible changes in several signal molecules in male ICR mice. Here, we show that 3 min of CWSS was sufficient to produce antinociception in tail-flick, hot-plate, von-Frey, writhing, and formalin-induced pain models. Significantly, CWSS strongly reduced nociceptive behavior in the first phase, but not in the second phase, of the formalin-induced pain model. We further examined some signal molecules' expressions in the dorsal root ganglia (DRG) and spinal cord to delineate the possible molecular mechanism involved in the antinociceptive effect under CWSS. CWSS reduced p-ERK, p-AMPKα1, p-AMPKα2, p-Tyk2, and p-STAT3 expression both in the spinal cord and DRG. However, the phosphorylation of mTOR was activated after CWSS in the spinal cord and DRG. Moreover, p-JNK and p-CREB activation were significantly increased by CWSS in the spinal cord, whereas CWSS alleviated JNK and CREB phosphorylation levels in DRG. Our results suggest that the antinociception induced by CWSS may be mediated by several molecules, such as ERK, JNK, CREB, AMPKα1, AMPKα2, mTOR, Tyk2, and STAT3 located in the spinal cord and DRG.

Exploring the anti-stress effects of imatinib and tetrabenazine in cold-water immersion-induced acute stress in mice

The aim of the present study was to explore the ameliorative role of imatinib and tetrabenazine in acute stress-induced behavioural and biochemical changes in mice. Cold-water immersion (5 min duration) was employed to induce acute stress and the resulting changes in the locomotor activity, exploratory behaviour, motor activity and social behaviour were assessed using the actophotometer, the hole board, the open field and the social interaction tests. The biochemical alterations were assessed by measuring the plasma corticosterone levels using ELISA kit. Cold-water immersion-induced acute stress diminished the locomotor activity, exploratory behaviour, motor activity and social behaviour along with increase in the plasma corticosterone levels. Administration of imatinib (50 and 100 mg/kg, i.p.), a tyrosine kinase inhibitor, significantly attenuated the cold-water immersion-induced behavioural alterations with normalization of the plasma corticosterone levels in a dose-dependent manner. Moreover, administration of tetrabenazine (1 and 2 mg/kg, i.p.), a vesicular monoamine transporter 2 (VMAT2) inhibitor, also abolished the acute stress-induced behavioural and biochemical changes in a dose-dependent manner. The beneficial effects of imatinib and tetrabenazine in normalizing acute stress-induced biochemical and behavioural changes make them promising therapeutic agents in the treatment of acute stress-related problems.

The Molecular Signatures of Acute-immobilization-induced Antinociception and Chronic-immobilization-induced Antinociceptive Tolerance

In the present study, the productions of antinociception induced by acute and chronic immobilization stress were compared in several animal pain models. In the acute immobilization stress model (up to 1 hr immobilization), the antinociception was produced in writhing, tail-flick, and formalin-induced pain models. In chronic immobilization stress experiment, the mouse was enforced into immobilization for 1 hr/day for 3, 7, or 14 days, then analgesic tests were performed. The antinociceptive effect was gradually reduced after 3, 7 and 14 days of immobilization stress. To delineate the molecular mechanism involved in the antinociceptive tolerance development in the chronic stress model, the expressions of some signal molecules in dorsal root ganglia (DRG), spinal cord, hippocampus, and the hypothalamus were observed in acute and chronic immobilization models. The COX-2 in DRG, p-JNK, p-AMPKα1, and p-mTOR in the spinal cord, p-P38 in the hippocampus, and p-AMPKα1 in the hypothalamus were elevated in acute immobilization stress, but were reduced gradually after 3, 7 and 14 days of immobilization stress. Our results suggest that the chronic immobilization stress causes development of tolerance to the antinociception induced by acute immobilization stress. In addition, the COX-2 in DRG, p-JNK, p-AMPKα1, and p-mTOR in the spinal cord, p-P38 in the hippocampus, and p-AMPKα1 in the hypothalamus may play important roles in the regulation of antinociception induced by acute immobilization stress and the tolerance development induced by chronic immobilization stress.

Chronic stress impacts the cardiovascular system: animal models and clinical outcomes

Psychological stresses are associated with cardiovascular diseases to the extent that cardiovascular diseases are among the most important group of psychosomatic diseases. The longstanding association between stress and cardiovascular disease exists despite a large ambiguity about the underlying mechanisms. An array of possibilities have been proposed including overactivity of the autonomic nervous system and humoral changes, which then converge on endothelial dysfunction that initiates unwanted cardiovascular consequences. We review some of the features of the two most important stress-activated systems, i.e., the humoral and nervous systems, and focus on alterations in endothelial function that could ensue as a result of these changes. Cardiac and hematologic consequences of stress are also addressed briefly. It is likely that activation of the inflammatory cascade in association with oxidative imbalance represents key pathophysiological components of stress-induced cardiovascular changes. We also review some of the commonly used animal models of stress and discuss the cardiovascular outcomes reported in these models of stress. The unique ability of animals for adaptation under stressful conditions lessens the extrapolation of laboratory findings to conditions of human stress. An animal model of unpredictable chronic stress, which applies various stress modules in a random fashion, might be a useful solution to this predicament. The use of stress markers as indicators of stress intensity is also discussed in various models of animal stress and in clinical studies.

Pharmacological traits of delta opioid receptors: pitfalls or opportunities?

RATIONALE

Delta opioid receptors (DORs) have been considered as a potential target to relieve pain as well as treat depression and anxiety disorders and are known to modulate other physiological responses, including ethanol and food consumption. A small number of DOR-selective drugs are in clinical trials, but no DOR-selective drugs have been approved by the Federal Drug Administration and some candidates have failed in phase II clinical trials, highlighting current difficulties producing effective delta opioid-based therapies. Recent studies have provided new insights into the pharmacology of the DOR, which is often complex and at times paradoxical.

OBJECTIVE

This review will discuss the existing literature focusing on four aspects: (1) Two DOR subtypes have been postulated based on differences in pharmacological effects of existing DOR-selective ligands. (2) DORs are expressed ubiquitously throughout the body and central nervous system and are, thus, positioned to play a role in a multitude of diseases. (3) DOR expression is often dynamic, with many reports of increased expression during exposure to chronic stimuli, such as stress, inflammation, neuropathy, morphine, or changes in endogenous opioid tone. (4) A large structural variety in DOR ligands implies potential different mechanisms of activating the receptor.

CONCLUSION

The reviewed features of DOR pharmacology illustrate the potential benefit of designing tailored or biased DOR ligands.

A review on animal models for screening potential anti-stress agents

Stress is a state of threatened homeostasis that produces different physiological as well as pathological changes depending on severity, type and duration of stress. The animal models are pivotal for understanding the pathophysiology of stress-induced behavioral alterations and development of effective therapy for its optimal management. A battery of models has been developed to simulate the clinical pain conditions with diverse etiology. An ideal animal model should be able to reproduce each of the aspects of stress response and should be able to mimic the natural progression of the disease. The present review describes the different types of acute and chronic stress models including immersion in cold water with no escape, cold environment isolation, immobilization/restraint-induced stress, cold-water restraint stress, electric foot shock-induced stress, forced swimming-induced stress, food-deprived activity stress, neonatal isolation-induced stress, predatory stress, day-night light change-induced stress, noise-induced stress, model of post-traumatic stress disorder and chronic unpredictable stress models.

Role of peroxisome proliferator-activated receptor-alpha in ileum tight junction alteration in mouse model of restraint stress

Restraint stress induces permeability changes in the small intestine, but little is known about the role of endogenous peroxisome proliferator-activated receptor-alpha (PPAR-alpha) ligand in the defects of the tight junction function. In the present study, we used PPAR-alpha knockout mice to understand the roles of endogenous PPAR-alpha on ileum altered permeability function in models of immobilization stress. The absence of a functional PPAR-alpha gene in PPAR-alpha knockout mice resulted in a significant augmentation of the degree of 1) TNF-alpha production in ileum tissues; 2) the alteration of zonula occludens-1, occludin, and beta-catenin (immunohistochemistry); and 3) apoptosis (terminal deoxynucleotidyltransferase-mediated dUTP nick-end labeling staining, Bax, Bcl-2 expression). Taken together, our results demonstrate that endogenous PPAR-alpha ligands reduce the degree of tight junction permeability in the ileum tissues associated with immobilization stress, suggesting a possible role of endogenous PPAR-alpha ligands on ileum barrier dysfunction.

Role of TNF-alpha in ileum tight junction alteration in mouse model of restraint stress

Restraint stress induces permeability changes in the small intestine, but little is known about the role of tumor necrosis factor (TNF)-alpha in the defects of the TJ function. In the present study, we used tumor necrosis factor-R1 knockout mice (TNF-alpha-R1KO) to understand the roles of TNF-alpha on ileum altered permeability function in models of immobilization stress. The genetic TNF-alpha inhibition significantly reduced the degree of 1) TNF-alpha production in ileum tissues; 2) the alteration of zonula occludens-1 (ZO-1), claudin-2, claudin-4, claudin-5, and beta-catenin (immunohistochemistry); and 3) apoptosis (TUNEL staining, Bax, Bcl-2 expression). Taken together, our results demonstrate that inhibition of TNF-alpha reduces the tight junction permeability in the ileum tissues associated with immobilization stress, suggesting a possible role of TNF-alpha on ileum barrier dysfunction.

Changes in pain behavior induced by formalin, substance P, glutamate and pro-inflammatory cytokines in immobilization-induced stress mouse model

In the present study, we examined the change of pain behaviors induced by formalin injected subcutaneously (s.c.) into the hind paw, or substance P (SP), glutamate, and pro-inflammatory cytokines (TNF-alpha, IL-1beta, and IFN-gamma) injected intrathecally (i.t.) in the mouse immobilization stress model. The mouse was restrained either once for 1h or five times for 5 days (once/day). In the formalin test, a single immobilization stress attenuated pain behaviors (licking, biting or scratching) in the second phase, while it had no effect on the pain behaviors revealed during the first phase. In addition, repeated immobilization stress attenuated pain behaviors revealed during the second phase but not in the first phase. A single as well as repeated immobilization stress decreased pain behaviors induced by substance P i.t. injection, but there were no significant changes in the glutamate test. In the pro-inflammatory cytokine pain model, a single immobilization stress decreased the pain behaviors induced by TNF-alpha, IL-1beta administered i.t. but not by IFN-gamma administered i.t. Moreover, a mouse applied with repeated immobilization stress did not show any changes in pain behaviors elicited by pro-inflammatory cytokines (TNF-alpha, IL-1beta and IFN-gamma) compared to the control group. These results suggest that a single and repeated immobilization stress differentially affects such nociceptive processing induced by formalin, SP, glutamate and pro-inflammatory cytokines in different manners.

Dopamine receptor mechanism(s) and antinociception and tolerance induced by swim stress in formalin test

In the present study, involvement of D1 and D2 dopamine receptors in the antinociception and tolerance induced by water swim stress in the formalin test has been investigated. Water swim stress at 20 degrees C temperature induced antinociception in both phases of the formalin test. Intraperitoneal administration of the D2 dopamine receptor antagonist, sulpiride (25 and 50 mg/kg) reduced swim stress-induced antinociception in the second phase of the formalin test. A higher dose of the D1 dopamine receptor antagonist, SCH23390 (0.1 mg/kg, intraperitoneal) also reduced swim stress-induced antinociception in both phases of the test. Exposure to 3 min water swimming stress, once daily for 3 days, induced tolerance to swim stress-induced antinociception in the second phase of the formalin test. Administration of sulpiride (12.5, 25 and 50 mg/kg), during exposure to water swimming stress (once daily for 3 days), decreased tolerance in the second phase, whereas the antagonist (12.5 and 50 mg/kg) increased pain scores in the first phase of the formalin test. Sulpiride (25 mg/kg) treatment however, once daily for 3 days with no water swimming stress, did not alter swim stress-induced antinociception (0.5, 1 and 3 min tests). Similarly, repeated treatment with SCH23390 (0.05 mg/kg) and water swimming stress did not alter tolerance induced by water swimming stress. Repeated administration of the antagonist in the absence of water swimming stress also did not change swim stress-induced antinociception. The results may indicate a possible involvement of both dopamine D1 and D2 receptors in the antinociception induced by swim stress and D2 receptor mechanism in the tolerance induced by repeated swim stress.

Cross-tolerance between antinociception induced by swim-stress and morphine in formalin test

The present study investigated cross-tolerance between antinociception induced by water swim-stress and morphine in the formalin test. Intraperitoneal administration of morphine (3, 6 and 9 mg/kg) induced dose-dependent antinociception in both phases of the formalin test. Mice treated with a lower dose of morphine (25 mg/kg), once daily for 3 days, showed tolerance to antinociception induced by a lower test dose of morphine (3 mg/kg). Similar repeated treatments with a higher dose of morphine (50 mg/kg) produced tolerance to antinociception induced by different test doses of morphine (3, 6 and 9 mg/kg). Exposure to water swim-stress, once daily for 2 or 3 days in order to induce tolerance, also decreased morphine-induced antinociception. Swim-stress exposure for 2 or 3 days also tends to potentiate tolerance induced by a lower dose of morphine. Acute swim-stress of different durations (0.5, 1 and 3 min) induced antinociception in both phases of the formalin test, which was not reduced by naloxone, but showed even more antinociception in the second phase. The response to swim stress was decreased in mice treated with higher doses of morphine, but not those animals that received swimming stress (3 min) once daily for 2-3 days, in order to induce habituation to swim-stress-induced antinociception. The results may indicate a possible cross-tolerance between antinociception induced by morphine and by swim stress.

Inhibition of morphine analgesia by LPS: role of opioid and NMDA receptors and spinal glia

Intraperitoneal (i.p.) injection of toxins, such as the bacterial endotoxin lipopolysaccharide (LPS), is associated with a well-characterized increase in sensitivity to painful stimuli (hyperalgesia) [Watkins LR, Maier SF, Goehler LE. Immune activation: the role of pro-inflammatory cytokines in inflammation, illness responses and pathological pain states. Pain 1995;63:289-302. [53]] and a longer-lasting reduction in opioid analgesia (anti-analgesia) when pain sensitivity returns to basal levels [Johnston IN, Westbrook RF. Acute and conditioned sickness reduces morphine analgesia. Behav Brain Res 2003;142:89-97]. Here we show that this inhibition of morphine analgesia 24 h after a single i.p. injection of LPS involves mechanisms that contribute to illness-induced hyperalgesia and the development of analgesic tolerance to morphine. Specifically, morphine analgesia was restored if LPS was preceded by systemic administration of a non-competitive NMDA receptor antagonist (MK-801), spinal infusion of a glial metabolic inhibitor (fluorocitrate), or intracerebroventricular microinjection of an opioid receptor antagonist (naloxone). Morphine analgesia was also restored if MK-801 was administered after LPS. These results demonstrate that LPS recruits similar, if not the same mechanisms that reduce morphine tolerance following opiate administration: namely, stimulation of opioid and NMDA receptors and recruitment of spinal glia.

Endogenous opiates and behavior: 2003

This paper is the 26th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2003 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).