In silico characterization of ligand-receptor interactions for U-47700, N,N-didesmethyl-U-47700, U-50488 at mu- and kappa-opioid receptors

The increasing misuse of novel synthetic opioids (NSOs) represents a serious public health concern. In this regard, U-47700 (trans-3,4-dichloro-N-[2-(dimethylamino)cyclohexyl]-N-methylbenzamide) and related "U-compounds" emerged on recreational drug markets as synthetic substitutes for illicit heroin and constituents of counterfeit pain medications. While the pharmacology of U-compounds has been investigated using in vitro and in vivo methods, there is still a lack of understanding about the details of ligand-receptor interactions at the molecular level. To this end, we have developed a molecular modeling protocol based on docking and molecular dynamics simulations to assess the nature of ligand-receptor interactions for U-47700, N,N-didesmethyl U-47700, and U-50488 at the mu-opioid receptor (MOR) and kappa-opioid receptor (KOR). The evaluation of ligand-receptor and ligand-receptor-membrane interaction energies enabled the identification of subtle conformational shifts in the receptors induced by ligand binding. Interestingly, the removal of two key methyl groups from U-47700, to form N,N-didesmethyl U-47700, caused a loss of hydrogen bond contact with tryptophan (Trp)229, which may underlie the lower interaction energy and reduced MOR affinity for the compound. Taken together, our results are consistent with the reported biological findings for U-compounds and provide a molecular basis for the MOR selectivity of U-47700 and KOR selectivity of U-50488.

κ-Opioid Receptors Improve Vascular Endothelial Dysfunction in Salt-Sensitive Hypertension via PI3K/Akt/eNOS Signaling Pathway

κ-Opioid receptors (κ-OR) are widely used to regulate the activity of the cardiovascular system. To explore the effect and mechanism of κ-OR on salt-sensitive hypertensive endothelial dysfunction, we used Dah1 rats to construct a rat model of salt-sensitive hypertension on a high-salt (HS) diet. Then, the rats were treated with κ-OR activators U50,488H (1.25 mg/kg) and inhibitor nor-BNI (2.0 mg/kg) for 4 weeks, respectively. The rat aortas were collected to detect the contents of NO, ET-1, AngII, NOS, T-AOC, SO, and NT. Protein expression was determined for NOS, Akt, and Caveolin-1. In addition, the vascular endothelial cells were extracted, and the levels of NO, TNF-α, IL-1, IL-6, IL-8, IL-10, p-Akt, and p-eNOS in cell supernatants were detected. In vivo results showed that compared with the HS group, treated with U50,488H promoted rats' vasodilation by increasing the NO content and decreasing ET-1 and AngII contents. U50,488H reduced endothelial cell apoptosis and attenuated vascular, smooth muscle cell and endothelial cell injury. U50,488H also enhanced the rats' response to oxidative stress by increasing the NOS and T-AOC contents. Moreover, U50,488H increased the eNOS, p-eNOS, Akt, and p-AKT expression and decreased the iNOS and Caveolin-1 expression. In vitro results showed that U50,488H promoted NO, IL-10, p-Akt, and p-eNOS levels in endothelial cell supernatants versus the HS group. And U50,488H reduced the adhesion of peripheral blood mononuclear cells and polymorphonuclear neutrophils to endothelial cells and the migration function of polymorphonuclear neutrophils. Our study suggested that κ-OR activation may improve vascular endothelial dysfunction in salt-sensitive hypertensive rats through the PI3K/Akt/eNOS signaling pathway. This may be a potential therapeutic approach in the treatment of hypertension.

Inhibition of P2X7 receptors by Lu AF27139 diminishes colonic hypersensitivity and CNS prostanoid levels in a rat model of visceral pain

Visceral pain is a prominent feature of various gastrointestinal diseases. The P2X7 receptor is expressed by multiple cell types including dorsal root ganglion satellite glial cells, macrophages, and spinal microglia, all of which have been implicated in nociceptive sensitization. We have used the selective and CNS penetrant P2X7 receptor antagonist Lu AF27139 to explore this receptor's role in distinct rat models of inflammatory and visceral hypersensitivity. Rats injected with CFA in the hindpaw displayed a marked reduction in hindpaw mechanical threshold, which was dose-dependently reversed by Lu AF27139 (3-30 mg/kg, p.o.). In rats injected with TNBS in the proximal colon, the colorectal distension threshold measured distally was significantly lower than sham treated rats at 7 days post-injection (P < 0.001), indicative of a marked central sensitization. Colonic hypersensitivity was also reversed by Lu AF27139 (10-100 mg/kg) and by the κ-opioid receptor agonist U-50,488H (3 mg/kg, s.c.). Moreover, both Lu AF27139 and U-50,488H prevented a TNBS-induced increase in spinal and brain levels of PGE2 and LTB4, as well as an increase in brain levels of PGF2α and TXB2. Lu AF27139 was well tolerated as revealed by a lack of significant effect on rotarod motor function and coordination at all doses tested up to 300 mg/kg. Thus, P2X7 receptor antagonism is efficacious in a rat model of visceral pain, via a mechanism which potentially involves attenuation of microglial function within spinal and/or supraspinal pain circuits, albeit a peripheral site of action cannot be excluded.

Pharmacological activation of kappa opioid receptors in the nucleus accumbens core and ventral tegmental area increases the aversive effects of nicotine

Aversive effects of nicotine play an important role in the development of nicotine dependence. However, neural substrates and/or brain regions that play a role in the aversive effects of nicotine have not been fully identified. Previous work done in our laboratory showed that systemic administration of kappa opioid receptors (KORs) agonist ±U50488 increased the aversive effects of nicotine. In this study, we assessed the effects of KOR activation in specific brain regions, namely, the nucleus accumbens (NAcc) core and ventral tegmental area (VTA) on the aversive effects of nicotine using the conditioned taste aversion model. Separate groups of Wistar rats were implanted with cannulae above either the NAcc core or the VTA. KOR agonist (±U50488) was bilaterally infused in the NAcc core (0, 0.3, and 3 ug/0.5 ul/side) or VTA (0, 0.3, 1.5, and 3 ug/0.5 ul/side) prior to receiving nicotine (0.4 mg/kg, base; s.c.) during conditioning. Bilateral infusion of the KOR agonist (3 ug/0.5 ul/side) in the NAcc core or the VTA increased the aversive effects of nicotine compared with respective saline controls. Together, these results suggest that pharmacological activation of the KORs in the NAcc core and VTA dose dependently modulate the aversive effects of nicotine. Because aversive effects of nicotine determine susceptibility to development of nicotine dependence, we can conclude that KOR activity in the NAcc and VTA after administration of nicotine may determine susceptibility to the development of nicotine dependence.

Antidepressant-like effects of dezocine in mice: involvement of 5-HT1A and κ opioid receptors

Dezocine is an opioid with low efficacy at μ-opioid and κ-opioid receptors. It also inhibits the reuptake of norepinephrine and serotonin. Dezocine is an effective analgesic against various clinical painful conditions and is widely used in many Asian countries. Given the unique pharmacology of dezocine, the drug may also have antidepressant-like properties. However, no published preclinical study has explored this possibility. This study examined the potential antidepressant-like activity of dezocine in mice. Male ICR mice were used in the forced swimming test, the tail suspension test, the warm water tail withdrawal test and locomotor activity test to test the effects of dezocine (0.3-3.0 mg/kg). The 5-HT1A receptor antagonist WAY-100635 (1 mg/kg), the μ-opioid receptor antagonist β-funaltrexamine (2 mg/kg) and the κ-opioid receptor agonist U50488 (1 mg/kg) were also studied in combination with dezocine. Dezocine produced a dose-dependent decrease in the immobility time in the forced swimming test and tail suspension test at doses that did not alter the motoric activity as determined in the locomotion test. WAY-100635 and U50488 but not β-funaltrexamine pretreatment significantly blocked the effects of dezocine. Dezocine dose-dependently increased the latency in the tail withdrawal test which was blocked by WAY-100635 and β-funaltrexamine. Combined, these results suggest that dezocine may have antidepressant-like effects. Considering the well-documented analgesic property of dezocine, it may be useful to treat pain and depression comorbidity.

κ Opioid Receptor Agonist Inhibits Myocardial Injury in Heart Failure Rats through Activating Nrf2/HO-1 Pathway and Regulating Ca2+-SERCA2a

OBJECTIVES

We aimed to observe the protective effect of κ opioid receptor (κ-OR) agonist on myocardial injury in heart failure (HF) rats and its effect on Ca2+-SERCA2a and to explore the regulatory mechanism with the Nrf2/HO-1 signaling pathway.

METHODS

50 Sprague-Dawley rats were randomly divided into the following groups: the sham operation group (sham group), HF model group (HF group), HF+κ-OR agonist U50488 group (HU group), HF+U50488H+novel calmodulin-dependent protein kinase II (CaMKII) agonist (oleic acid) (HUO group), and HF+U50488H+Nrf2 inhibitor (HUM group). The HF rat's model was established through surgical ligation of the left anterior descending coronary artery and the exhausting swimming exercise. After that, rat's cardiac function was monitored by echocardiography. HE and MASSON staining was used to detect the myocardial injury, and TUNEL staining was used to detect the myocardial apoptosis. ELISA was performed to detect the biomarkers of oxidative stress. Moreover, the distribution of reactive oxygen species (ROS) and Nrf2 was detected under immunofluorescence. The expression of sarco/endoplasmic reticulum calcium (Ca2+) ATPase (SERCA) 2a, calmodulin, endoplasmic reticulum stress- (ERS-) related proteins, and Nrf2/HO-1 signaling pathway-related proteins were detected by Western Blotting.

RESULTS

κ-OR agonist U50488H can significantly enhance rat's cardiac function, reduce the injury and apoptosis of myocardial cells, and alleviate endoplasmic reticulum stress injury in HF rats via upregulating the SERCA2a expression and inhibiting the Ca2+ influx. Furthermore, U50488H could also inhibit the phosphorylation of CaMKII and cAMP-response element binding protein (CREB). Additionally, administration of CaMKII-specific agonist could partially block the therapeutic effect of κ-OR agonist on the myocardium of HF rats. Interestingly, the antagonist of Nrf2 could also significantly reverse the therapeutic effect of κ-OR agonist. Therefore, these results suggested that the effect of U50488H on HF rats is dependent on regulating CaMKII phosphorylation and activating the Nrf2/HO-1 pathway.

CONCLUSION

κ-OR agonists U50488H can improve ERS in cardiomyocytes and relieve myocardial injury in HF rats through activating the Nrf2/HO-1 pathway and regulating Ca2+-SERCA2a to inhibit Ca2+ influx.

Activation of κ-opioid receptor inhibits inflammatory response induced by sodium palmitate in human umbilical vein endothelial cells

OBJECTIVES

The current study aims to investigate the effect of κ-opioid receptor (κ-OR) activation on sodium palmitate (SP)-induced human umbilical vein endothelial cells (HUVECs) inflammatory response and elucidate the underlying mechanisms.

METHODS

A hyperlipidemic cell model was established and treated with κ-OR agonist (U50,488H), and antagonist (norbinaltorphimine, nor-BNI), or inhibitors targeting PI3K, Akt or eNOS (LY294002, MK2206-2HCl or L-NAME, respectively). Furthermore, the expression levels of NLRP3, caspase-1, p-Akt, Akt, p-eNOS, and total eNOS were evaluated. Additionally, the production of reactive oxygen species, and levels of inflammatory factors, such as TNF-α, IL-1β, IL-6, IL-1 and adhesion molecules, such as ICAM-1, VCAM-1, P-selectin, and E-selectin were determined. The adherence rates of the neutrophils and monocytes were assessed as well.

RESULTS

The SP-induced hyperlipidemic cell model demonstrated increased expression of NLRP3 and caspase-1 proteins (P < 0.05) and elevated ROS levels (P < 0.01), and decreased phosphorylated-Akt and phosphorylated-eNOS expression (P < 0.05). In addition, SP significantly increased TNF-α, IL-1β, IL-6, ICAM-1, VCAM-1, P-selectin, and E-selectin levels (P < 0.01), decreased IL-10 levels (P < 0.01), and increased the adhesion rates of monocytes and neutrophils (P < 0.01). The SP-induced inflammatory response in HUVECs was ameliorated by κ-OR agonist, U50,488H. However, the protective effect of U50,488H was abolished by κ-OR antagonist, nor-BNI, and inhibitors of PI3K, Akt and eNOS.

CONCLUSION

Our findings suggest that κ-OR activation inhibits SP-induced inflammation by activating the PI3K/Akt/eNOS signaling pathway.

κ‑opioid receptor agonist U50488H attenuates postoperative cognitive dysfunction of cardiopulmonary bypass rats through the PI3K/AKT/Nrf2/HO‑1 pathway

Postoperative cognitive dysfunction (POCD) is a common complication following cardiopulmonary bypass (CPB). U50488H, a κ‑opioid receptor (KOR) agonist, can specifically activate KORs on hippocampal nerve cells, resulting in neuroprotective effects. The present study established a CPB rat model, observed the protective effect of U50488H on CPB‑induced POCD and brain damage and explored the regulatory mechanism of the PI3K/AKT/nuclear factor erythroid 2‑related factor 2 (Nrf2)/heme oxygenase (HO)‑1 pathway. Sprague‑Dawley rats were divided into the following groups: Sham operation (Sham group), CPB (CPB group), KOR agonist (U50488H) + CPB (U50488H group), CPB + U50488H + HO‑1 antagonist (ZnPP‑IX; ZnPP group) and CPB + U50488H + PI3K antagonist (LY294002; LY294002 group), with 10 rats in each group. Neurological scores and the Morris water maze test were used to evaluate cognitive function; hematoxylin and eosin and terminal deoxynucleotidyl transferase dUTP nick end labeling assays were performed to observe hippocampal neuron damage in rats. Immunofluorescence was used to detect reactive oxygen species, glial fibrillary acidic protein and Nrf2 expression in the hippocampus. Enzyme‑linked immunosorbent assays were used to detect inflammatory and oxidative stress factors. Western blotting was used to examine the expression of PI3K/AKT/Nrf2/HO‑1‑related proteins. It was demonstrated that U50488H significantly reduced the neural function score of rats with POCD induced by CPB, relieved cognitive dysfunction, reduced hippocampal neuron damage, inhibited the rate of apoptosis, repaired oxidative stress injury and protected against brain damage caused by CPB. In addition, U50488H could promote Nrf2 entry into the nucleus and upregulate HO‑1 and thioredoxin 1 (Trx1) expression. In CPB rats treated with PI3K inhibitors, less Nrf2 was detected in the nucleus and HO‑1 and Trx‑1 expression levels were reduced in the nucleus. Therefore, U50488H, a KOR agonist, can activate Nrf2/HO‑1 via the PI3K/AKT pathway to improve cognitive function and reduce brain damage in CPB rats.

Quantification of observable behaviors induced by typical and atypical kappa-opioid receptor agonists in male rhesus monkeys

RATIONALE

Kappa-opioid receptor (KOR) agonists are antinociceptive but have side effects that limit their therapeutic utility. New KOR agonists have been developed that are fully efficacious at the KOR but may produce fewer or reduced side effects that are typical of KOR agonists.

OBJECTIVES

We determined behavioral profiles for typical and atypical KOR agonists purported to differ in intracellular-signaling profiles as well as a mu-opioid receptor (MOR) agonist, oxycodone, using a behavioral scoring system based on Novak et al. (Am J Primatol 28:124-138, 1992, Am J Primatol 46:213-227, 1998) and modified to quantify drug-induced effects (e.g., Duke et al. J Pharmacol Exp Ther 366:145-157, 2018).

METHODS

Six adult male rhesus monkeys were administered a range of doses of the typical KOR agonists, U50-488H (0.0032-0.1 mg/kg) and salvinorin A (0.00032-0.01 mg/kg); the atypical KOR agonists, nalfurafine (0.0001-0.001 mg/kg) and triazole 1.1 (0.01-0.32 mg/kg); the MOR agonist, oxycodone (0.0032-0.32 mg/kg); and as controls, cocaine (0.032-0.32 mg/kg) and ketamine (0.32-10 mg/kg). For time-course determinations, the largest dose of each KOR agonist or MOR agonist was administered across timepoints (10-320 min). In mixture conditions, oxycodone (0.1 mg/kg) was followed by KOR-agonist administration.

RESULTS

Typical KOR agonists produced sedative-like and motor-impairing effects. Nalfurafine was similar to typical KOR agonists on most outcomes, and triazole 1.1 produced no effects on its own except for reducing scratch during time-course determinations. In the mixture, all KOR agonists reduced oxycodone-induced scratching, U50-488H and nalfurafine reduced species-typical activity, and U50-488H increased rest/sleep posture.

CONCLUSIONS

Atypical "biased" KOR agonists produce side-effect profiles that are relatively benign (triazole 1.1) or reduced (nalfurafine) compared to typical KOR agonists.

Role of RGS12 in the differential regulation of kappa opioid receptor-dependent signaling and behavior

Kappa opioid receptor (KOR) agonists show promise in ameliorating disorders, such as addiction and chronic pain, but are limited by dysphoric and aversive side effects. Clinically beneficial effects of KOR agonists (e.g., analgesia) are predominantly mediated by heterotrimeric G protein signaling, whereas β-arrestin signaling is considered central to their detrimental side effects (e.g., dysphoria/aversion). Here we show that Regulator of G protein Signaling-12 (RGS12), via independent signaling mechanisms, simultaneously attenuates G protein signaling and augments β-arrestin signaling downstream of KOR, exhibiting considerable selectivity in its actions for KOR over other opioid receptors. We previously reported that RGS12-null mice exhibit increased dopamine transporter-mediated dopamine (DA) uptake in the ventral (vSTR), but not dorsal striatum (dSTR), as well as reduced psychostimulant-induced hyperlocomotion; in the current study, we found that these phenotypes are reversed following KOR antagonism. Fast-scan cyclic voltammetry studies of dopamine (DA) release and reuptake suggest that striatal disruptions to KOR-dependent DAergic neurotransmission in RGS12-null mice are restricted to the nucleus accumbens. In both ventral striatal tissue and transfected cells, RGS12 and KOR are seen to interact within a protein complex. Ventral striatal-specific increases in KOR levels and KOR-induced G protein activation are seen in RGS12-null mice, as well as enhanced sensitivity to KOR agonist-induced hypolocomotion and analgesia-G protein signaling-dependent behaviors; a ventral striatal-specific increase in KOR levels was also observed in β-arrestin-2-deficient mice, highlighting the importance of β-arrestin signaling to establishing steady-state KOR levels in this particular brain region. Conversely, RGS12-null mice exhibited attenuated KOR-induced conditioned place aversion (considered a β-arrestin signaling-dependent behavior), consistent with the augmented KOR-mediated β-arrestin signaling seen upon RGS12 over-expression. Collectively, our findings highlight a role for RGS12 as a novel, differential regulator of both G protein-dependent and -independent signaling downstream of KOR activation.

Activation of kappa opioid receptors in the dorsal raphe have sex dependent effects on social behavior in California mice

Kappa opioid receptor activation has been linked to stress and anxiety behavior, thus leading to kappa antagonists being popularized in research as potential anxiolytics. However, while these findings may hold true in standard models, the neuromodulatory effects of social defeat may change the behavioral outcome of kappa opioid receptor activation. Previous research has shown that social defeat can lead to hyperactivity of serotonergic neurons in the dorsal raphe nucleus, and that inhibition of this increase blocks the social deficits caused by defeat. Kappa opioid receptor activation in the dorsal raphe nucleus works to decrease serotonergic activity. We injected the kappa opioid receptor U50,488 directly into the dorsal raphe nucleus of male and female, defeat and control adult California mice. Here we show evidence that U50,488 induces anxiety behavior in control male California mice, but helps relieve it in defeated males. Consistent with previous literature, we find little effect in females adding evidence that there are marked and important sex differences in the kappa opioid system.

κ-opioid receptor activation protects against myocardial ischemia-reperfusion injury via AMPK/Akt/eNOS signaling activation

This study aims to investigate the effect of κ-opioid receptor activation on myocardial ischemia and reperfusion(I/R) injury and elucidate the underlying mechanisms. Myocardial I/R rat model and simulated I/R cardiomyocytes model were established. In vivo study showed that U50,488 H improved cardiac function, reduced myocardial infarct size and serum cTnT significantly. The effect of U50,488 H was abolished by nor-BNI(a κ-opioid receptor antagonist), Compound C(an AMPK inhibitor), Akt inhibitor and L-NAME(an eNOS inhibitor). AICAR, an AMPK activator, mimicked the effect of U50,488 H. U50,488 H up-regulated p-AMPK, p-Akt, and p-eNOS, which were abolished by nor-BNI. AICAR increased p-Akt and p-eNOS, which was abolished by Compound C. In vitro study showed that U50,488 H increased p-AMPK, p-Akt, and p-eNOS via κ-OR activation. The effect of U50,488 H on p-AMPK was abolished by compound C, but not Akt inhibitor and L-NAME. The effect of U50,488 H on p-Akt was abolished by compound C and Akt inhibitor, but not L-NAME. AICAR increased p-Akt and p-eNOS, which was abolished by Akt inhibitor, but not L-NAME. U50,488 H and AICAR also increased the viability of cardiomyocytes subjected to simulated I/R, the effects of U50,488 H and AICAR were blocked by nor-BNI, Compound C, Akt inhibitor, and L-NAME, respectively. In conclusion, κ-OR activation confers cardioprotection via AMPK/Akt/eNOS signaling.

Responsiveness of rat fetuses to sibling motor activity: Communication in utero?

Previous research has revealed that fetuses detect and respond to extrauterine stimuli such as maternal movement and speech, but little attention has been cast on how fetuses may directly influence and respond to each other in the womb. This study investigated whether motor activity of E20 rat fetuses influenced the behavior of siblings in utero. Three experiments showed that; (a) contiguous siblings expressed a higher frequency of synchronized movement than noncontiguous siblings; (b) fetuses that lay between two siblings immobilized with curare showed less movement relative to fetuses between saline or uninjected controls; and (c) fetuses between two siblings behaviorally activated by the opioid agonist U50,488 also showed less activity and specific behavioral changes compared to controls. Our findings suggest that rat fetuses are directly impacted by sibling motor activity, and thus that a rudimentary form of communication between siblings may influence the development of fetuses in utero.

The effects of kratom on restraint-stress-induced analgesia and its mechanisms of action

ETHNOPHARMACOLOGICAL RELEVANCE

Mitragyna speciosa and its extracts are called kratom (dried leaves, extract). They contain several alkaloids with an affinity for different opioid receptors. They are used in traditional medicine for the treatment of different diseases, as a substitute by opiate addicts, and to mitigate opioid withdrawal symptoms. Apart from their medical properties, they are used to enhance physical endurance and as a means of overcoming stress.

PURPOSE

The aim of this study was to determine the mechanisms underlying the effects of kratom on restraint-stress-induced analgesia which occurs during or following exposure to a stressful or fearful stimulus.

METHODS

To gain further insights into the action of kratom on stress, we conducted experiments using restraint stress as a test system and stress-induced analgesia as a test parameter. Using transgenic mu opioid-receptor (MOR) deficient mice, we studied the involvement of this receptor type. We used nor-binaltorphimine (BNT), an antagonist at kappa opioid receptors (KOR), to study functions of this type of receptor. Membrane potential assay was also employed to measure the intrinsic activity of kratom in comparison to U50,488, a highly selective kappa agonist.

RESULTS

Treatment with kratom diminished stress-induced analgesia in wildtype and MOR knockout animals. Pretreatment of MOR deficient mice with BNT resulted in similar effects. In comparison to U50,488, kratom exhibited negligible intrinsic activity at KOR alone.

CONCLUSIONS

The results suggest that the use of kratom as a pharmacological tool to mitigate withdrawal symptoms is related to its action on KOR.

Car8 dorsal root ganglion expression and genetic regulation of analgesic responses are associated with a cis-eQTL in mice

Carbonic anhydrase-8 (Car8 mouse gene symbol) is devoid of enzymatic activity, but instead functions as an allosteric inhibitor of inositol trisphosphate receptor-1 (ITPR1) to regulate this intracellular calcium release channel important in synaptic functions and neuronal excitability. Causative mutations in ITPR1 and carbonic anhydrase-8 in mice and humans are associated with certain subtypes of spinal cerebellar ataxia (SCA). SCA mice are genetically deficient in dorsal root ganglia (DRG) Car8 expression and display mechanical and thermal hypersensitivity and susceptibility to subacute and chronic inflammatory pain behaviors. In this report, we show that DRG Car8 expression is variable across 25 naïve-inbred strains of mice, and this cis-regulated eQTL (association between rs27660559, rs27706398, and rs27688767 and DRG Car8 expression; P < 1 × 10^-11) is correlated with nociceptive responses in mice. Next, we hypothesized that increasing DRG Car8 gene expression would inhibit intracellular calcium release required for morphine antinociception and might correlate with antinociceptive sensitivity of morphine and perhaps other analgesic agents. We show that mean DRG Car8 gene expression is directly related to the dose of morphine or clonidine needed to provide a half-maximal analgesic response (r = 0.93, P < 0.00002; r = 0.83, P < 0.0008, respectively), suggesting that greater DRG Car8 expression increases analgesic requirements. Finally, we show that morphine induces intracellular free calcium release using Fura 2 calcium imaging in a dose-dependent manner; V5-Car8 ^WT overexpression in NBL cells inhibits morphine-induced calcium increase. These findings highlight the 'morphine paradox' whereby morphine provides antinociception by increasing intracellular free calcium, while Car8 and other antinociceptive agents work by decreasing intracellular free calcium. This is the first study demonstrating that biologic variability associated with this cis-eQTL may contribute to differing analgesic responses through altered regulation of ITPR1-dependent calcium release in mice.

Effects of the psychotomimetic benzomorphan N-allylnormetazocine (SKF 10,047) on prepulse inhibition of startle in mice

N-allylnormetazocine (NANM; SKF 10,047) is a benzomorphan opioid that produces psychotomimetic effects. (+)-NANM is the prototypical agonist for the sigma-1 (σ1) receptor, and there is a widespread belief that the hallucinogenic effects of NANM and other benzomorphan derivatives are mediated by interactions with σ1 sites. However, NANM is also an agonist at the κ opioid receptor (KOR) and binds to the PCP site located within the channel pore of the NMDA receptor, interactions that could potentially contribute to the effects of NANM. NMDA receptor antagonists such as phencyclidine (PCP) and ketamine are known to disrupt prepulse inhibition (PPI) of acoustic startle, a measure of sensorimotor gating, in rodents. We recently found that racemic NANM disrupts PPI in rats, but it is not clear whether the effect is mediated by blockade of the NMDA receptor, or alternatively whether interactions with KOR and σ1 receptors are involved. The present studies examined whether NANM and its stereoisomers alter PPI in C57BL/6J mice, and tested whether the effects on PPI are mediated by KOR or σ1 receptors. Racemic NANM produced a dose-dependent disruption of PPI (3-30mg/kg SC). (+)-NANM also disrupted PPI, whereas (-)-NANM was ineffective. Pretreatment with the selective KOR antagonist nor-binaltorphimine (10mg/kg SC) or the selective σ1 antagonist NE-100 (1mg/kg IP) failed to attenuate the reduction in PPI produced by racemic NANM. We also found that the selective KOR agonist (-)-U-50,488H (10-40mg/kg SC) had no effect on PPI. These findings confirm that NANM reduces sensorimotor gating in rodents, and indicate that the effect is mediated by interactions with the PCP receptor and not by activation of KOR or σ1 receptors. This observation is consistent with evidence indicating that the σ1 receptor is not linked to hallucinogenic or psychotomimetic effects.

Activation of κ-opioid receptor by U50,488H improves vascular dysfunction in streptozotocin-induced diabetic rats

BACKGROUND

Evidence suggests that activation of κ-opioid receptor (KOR) by U50,488H exhibits potential cardiovascular protective properties. However, the effects of U50,488H on vascular dysfunction in diabetes mellitus (DM) are still not clear. The present study was designed to investigate the effects of U50,488H on vascular dysfunction in diabetic rats and explore the underlying mechanisms involved.

METHODS

Rats were randomly divided into control, DM, DM + vehicle, DM + U50,488H and DM + nor-binaltorphimine (nor-BNI) groups. Streptozotocin injection was used to induce DM. Weight, blood glucose, blood pressure and plasma insulin for each group were measured. Arterial functions were assessed with isolated vessels mounted for isometric tension recordings. Angiotensin II (ANG II), soluble intercellular adhesion molecule-1 (sICAM-1), interleukin (IL)-6 and IL-8 levels were measured by ELISA, and endothelial nitric oxide synthase (eNOS) phosphorylation and NF-κB p65 translocation were measured by Western blot.

RESULTS

Activation of KOR by U50,488H reduced the enhanced contractility of aortas to KCl and noradrenaline and increased acetylcholine-induced vascular relaxation, which could also protect the aortal ultrastructure in DM. U50,488H treatment resulted in reduction in ANG II, sICAM-1, IL-6 and IL-8 levels and elevation in NO levels, while these effects were abolished by nor-BNI treatment. Further more, eNOS phosphorylation was increased, and NF-κB p65 translocation was decreased after U50,488H treatment.

CONCLUSIONS

Our study demonstrated that U50,488H may have therapeutic effects on diabetic vascular dysfunction by improving endothelial dysfunction and attenuating chronic inflammation, which may be dependent on phosphorylation of eNOS and downstream inhibition of NF-кB.

Suppression of sex behavior by kappa opiates and stress steroids occurs via independent neuroendocrine pathways

Endocannabinoids and their receptors are found throughout the brain of all vertebrates. By virtue of their wide distribution, endocannabinoids have the potential to affect many behaviors. Prior research has shown that cannabinoids inhibit courtship-clasping and mediate behavioral responses to stress in male rough-skinned newts, Taricha granulosa, and cannabinoid signaling is initiated by rapid actions of the steroid corticosterone (CORT) at its specific membrane receptor (mCR). This same mCR also recognizes κ-opioid receptor agonists and antagonists. Prior behavioral studies show that κ-opioid agonists suppress clasping behavior in a dose dependent manner. Combined, these studies suggest that κ-opioid agonists might suppress clasping behavior via the same pathway initiated by CORT: up-regulation of endocannabinoid signaling. We examined whether pretreatment with a CB1 antagonist, AM281, would block κ-opioid-mediated suppression of clasping. We found that the CB1 antagonist did not reverse κ-opioid-induced suppression of clasping, revealing that while endocannabinoids mediate CORT-induced suppression of clasping, endocannabinoids do not mediate the κ-opioid-induced suppression of clasping.

[κ-opioid receptor agonist U50, 488H attenuates myocardial ischemia-reperfusionvia modulating Toll-like receptor 4/nuclear factor-κB signaling in rat]

OBJECTIVE

To observe the effects of κ-opioid receptor agonist U50, 488H on myocardial ischemia and reperfusion injury and related mechanism.

METHODS

Rats were randomly divided into sham operation, myocardial ischemia and reperfusion(I/R, 30 min ischemia followed by 120 min reperfusion), and MI/R+U50, 488H (1.5 mg/kg) and I/R+U50, 488H+ selective κ-opioid receptor antagonist Nor-BNI (2 mg/kg, n = 8 each). The infarction size and the incidence of ventricular arrhythmias were observed.Real-time PCR and DAB staining were used to define the myocardium Toll-like receptor 4(TLR4) expression. Myeloperoxidase level, TNF-α induction and the expression of NF-κB were also examined in rats.

RESULTS

After I/R, the expressions of myocardial TLR4 and NF-κB increased significantly both in ischemia area and area at risk. Compared with I/R, κ-opioid receptor stimulation with U50, 488H significantly attenuated the expressions of TLR4 and NF-κB and reduced myeloperoxidase (MPO) levels, myocardial TNF-α production, myocardial infarct sizes and the incidence of ventricular arrhythmias and arrhythmia score (2.9 ± 0.7 vs. 4.4 ± 0.9, P < 0.05) , above effects of U50, 488H were partly abolished by co-treatment with Nor-BNI.

CONCLUSION

These data provide evidence for the first time that κ-opioid receptor stimulation could attenuate myocardial I/R injury via downregulating TLR4/NF-κB signaling in rats.

A G protein-coupled receptor (GPCR) in red: live cell imaging of the kappa opioid receptor-tdTomato fusion protein (KOPR-tdT) in neuronal cells

INTRODUCTION

In contrast to green fluorescent protein and variants (GFPs), red fluorescent proteins (RFPs) have rarely been employed for the generation of GPCR fusion proteins, likely because of formation of aggregates and cell toxicity of some RFPs. Among all the RFPs, tdTomato (tdT), one of the non-aggregating RFP, has the highest brightness score (about 3 times that of eGFP) and unsurpassed photostability.

METHODS

We fused tdT to the KOPR C-terminus. The KOPR-tdT cDNA construct was transfected into a Neuro2A mouse neuroblastoma cell line (Neuro2A cells) and rat cortical primary neurons for characterization of pharmacological properties and imaging studies on KOPR trafficking.

RESULTS

KOPR-tdT retained KOPR properties (cell surface expression, ligand binding, agonist-induced signaling and internalization) when expressed in Neuro2A cells and rat primary cortical neurons. Live cell imaging of KOPR-tdT enables visualization of the time course of agonist-induced internalization of KOPR in real time for 60 min, without photobleaching and apparent cell toxicity. U50,488H-induced KOPR internalization occurred as early as 4min and plateaued at about 30 min. A unique pattern of internalized KOPR in processes of primary neurons was induced by U50,488H.

DISCUSSION

tdT is an alternative to, or even a better tool than, GFPs for fusion to GPCR for trafficking studies, because tdT has higher brightness and thus better resolution and less photobleaching problems due to the reduced laser power used. It also has advantages associated with its longer-wavelength emission including spectral separation from autofluorescence and GFPs, reduced cell toxicity that the laser may impose, and greater tissue penetration. These advantages of tdT over GPFs may be critical for live cell imaging studies of GPCRs in vitro and for studying GPCRs in vivo because of their low abundance.

Neurokinin B activates arcuate kisspeptin neurons through multiple tachykinin receptors in the male mouse

Kisspeptin neurons located in the arcuate nucleus (ARN) coexpress dynorphin and neurokinin B (NKB) and may interact to influence gonadotropin secretion. Using a kisspeptin-green fluorescent protein mouse model, the present study examined whether the neuropeptides kisspeptin, dynorphin, and NKB modulate the electrical activity of ARN kisspeptin neurons in the adult male mouse. Cell-attached recordings showed that kisspeptin itself had no effect on kisspeptin neuron firing. Dynorphin and the κ-opioid receptor agonist U50-488 evoked a potent suppression of all ARN kisspeptin neuron firing that was blocked completely by the κ-opioid receptor antagonist nor-Binaltorphimine. Both NKB and Senktide, a neurokinin 3 receptor agonist, exerted a potent stimulatory action on ∼95% of ARN kisspeptin neurons. Although the selective neurokinin 3 receptor antagonists SB222200 and SR142801 blocked the effects of Senktide on kisspeptin neurons, they surprisingly had no effect on NKB activation of firing. Studies with selective neurokinin 1 receptor (SDZ-NKT343) and neurokinin 2 receptor (GR94800) antagonists revealed that the activation of kisspeptin neurons by NKB was only blocked completely by a cocktail of antagonists against all 3 tachykinin receptors. Whole-cell recordings revealed that individual kisspeptin neurons were activated directly by all 3 tachykinins substance, P, neurokinin A, and NKB. These experiments show that dynorphin and NKB have opposing actions on the electrical activity of kisspeptin neurons supporting the existence of an interconnected network of kisspeptin neurons in the ARN. However, the effects of NKB result from an unexpected activation of multiple tachykinin receptors.

Role of dopamine D2 and D3 receptors in mediating the U-50,488H discriminative cue: comparison with methamphetamine and cocaine

Substitutions of the dopamine D(2) or D(3) receptor agonists for the discriminative stimulus effect induced by U-50,488H, methamphetamine (METH) and cocaine in rats were examined. The D(2) receptor agonist R-propylnorapomorphine [(-)-NPA] failed to substitute for U-50,488H cue, while the D(3) receptor-preferred agonist (+/-)-7-hydroxy-dipropylaminotetralin hydrobromide (7-OH-DPAT) produced dose-related increases in drug-appropriate responding up to 0.03 mg/kg, which fully substituted. At doses greater than 0.03 mg/kg of 7-OH-DPAT, there was a dose-dependent decrease in the percentage of responses on the U-50,488H-appropriate lever. Furthermore (-)-NPA and 7-OH-DPAT at high doses substituted for the discriminative stimulus effect induced by both METH and cocaine, indicating that 7-OH-DPAT at high doses may interact with D(2) receptors. These results suggest that the stimulation of D(2) receptor may be critical for the production of the discriminative stimulus effect induced by METH and cocaine, whereas the stimulation of D(3) receptor may contribute to the production of the U-50,488H cue.

Dysregulation of kappa-opioid receptor systems by chronic nicotine modulate the nicotine withdrawal syndrome in an age-dependent manner

RATIONALE

Mechanisms that mediate age differences during nicotine withdrawal are unclear.

OBJECTIVE

This study compared kappa-opioid receptor (KOR) activation in naïve and nicotine-treated adolescent and adult rats using behavioral and neurochemical approaches to study withdrawal.

METHODS

The behavioral models used to assess withdrawal included conditioned place and elevated plus maze procedures. Deficits in dopamine transmission in the nucleus accumbens (NAcc) were examined using microdialysis procedures. Lastly, the effects of KOR stimulation and blockade on physical signs produced upon removal of nicotine were examined in adults.

RESULTS

Nicotine-treated adults displayed a robust aversion to an environment paired with a KOR agonist versus naïve adults. Neither of the adolescent groups displayed a place aversion. KOR activation produced an increase in anxiety-like behavior that was highest in nicotine-treated adults versus all other groups. KOR activation produced a decrease in NAcc dopamine that was largest in nicotine-treated adults versus all other groups. Lastly, KOR activation facilitated physical signs of withdrawal upon removal of nicotine and KOR blockade reduced this effect.

CONCLUSION

Chronic nicotine enhanced the affective, anxiogenic, and neurochemical effects produced by KOR activation in adult rats. Our data suggest that chronic nicotine elicits an increase in KOR function, and this may contribute to nicotine withdrawal since KOR activation facilitated and KOR blockade prevented withdrawal signs upon removal of nicotine. Given that chronic nicotine facilitated the neurochemical effects of KOR agonists in adults but not in adolescents, it is suggested that KOR regulation of mesolimbic dopamine may contribute to age differences in nicotine withdrawal.

GPR54-dependent stimulation of luteinizing hormone secretion by neurokinin B in prepubertal rats

Kisspeptin, neurokinin B (NKB) and dynorphin A (Dyn) are coexpressed within KNDy neurons that project from the hypothalamic arcuate nucleus (ARC) to GnRH neurons and numerous other hypothalamic targets. Each of the KNDy neuropeptides has been implicated in regulating pulsatile GnRH/LH secretion. In isolation, kisspeptin is generally known to stimulate, and Dyn to inhibit LH secretion. However, the NKB analog, senktide, has variously been reported to inhibit, stimulate or have no effect on LH secretion. In prepubertal mice, rats and monkeys, senktide stimulates LH secretion. Furthermore, in the monkey this effect is dependent on kisspeptin signaling through its receptor, GPR54. The present study tested the hypotheses that the stimulatory effects of NKB on LH secretion in intact rats are mediated by kisspeptin/GPR54 signaling and are independent of a Dyn tone. To test this, ovarian-intact prepubertal rats were subjected to frequent automated blood sampling before and after intracerebroventricular injections of KNDy neuropeptide analogs. Senktide robustly induced single LH pulses, while neither the GPR54 antagonist, Kp-234, nor the Dyn agonist and antagonist (U50488 and nor-BNI, respectively) had an effect on basal LH levels. However, Kp-234 potently blocked the senktide-induced LH pulses. Modulation of the Dyn tone by U50488 or nor-BNI did not affect the senktide-induced LH pulses. These data demonstrate that the stimulatory effect of NKB on LH secretion in intact female rats is dependent upon kisspeptin/GPR54 signaling, but not on Dyn signaling.

Sigma-1 receptor function is critical for both the discriminative stimulus and aversive effects of the kappa-opioid receptor agonist U-50488H

The present study was undertaken to identify possible similarities between the effects of kappa-opioid receptor agonist, N-methyl-D-aspartate-receptor antagonist, and sigma receptor agonist on the discriminative stimulus effects of U-50488H, and the possible involvement of sigma receptors in the discriminative stimulus and aversive effects of U-50488H. The kappa-opioid receptor agonist U-50488H produced significant place aversion as measured by the conditioned place preference procedure, and this effect was completely abolished by treatment with the putative sigma-1 receptor antagonist NE-100. In addition, phencyclidine (+)-SKF-10047 and (+)-pentazocine, which are sigma receptor agonists, generalized to the discriminative stimulus effects of U-50488H in rats that had been trained to discriminate between U-50488H (3.0 mg/kg) and saline. Furthermore, NE-100 significantly attenuated the discriminative stimulus effects of U-50488H and the U-50488H-like discriminative stimulus effects of phencyclidine. These results suggest that the sigma-1 receptor is responsible for both the discriminative stimulus effects and aversive effects of U-50488H.

Stress-induced activation of the dynorphin/κ-opioid receptor system in the amygdala potentiates nicotine conditioned place preference

Many smokers describe the anxiolytic and stress-reducing effects of nicotine, the primary addictive component of tobacco, as a principal motivation for continued drug use. Recent evidence suggests that activation of the stress circuits, including the dynorphin/κ-opioid receptor system, modulates the rewarding effects of addictive drugs. In the present study, we find that nicotine produced dose-dependent conditioned place preference (CPP) in mice. κ-receptor activation, either by repeated forced swim stress or U50,488 (5 or 10 mg/kg, i.p.) administration, significantly potentiated the magnitude of nicotine CPP. The increase in nicotine CPP was blocked by the κ-receptor antagonist norbinaltorphimine (norBNI) either systemically (10 mg/kg, i.p.) or by local injection in the amygdala (2.5 μg) without affecting nicotine reward in the absence of stress. U50,488 (5 mg/kg, i.p.) produced anxiety-like behaviors in the elevated-plus maze and novel object exploration assays, and the anxiety-like behaviors were attenuated both by systemic nicotine (0.5 mg/kg, s.c.) and local injection of norBNI into the amygdala. Local norBNI injection in the ventral posterior thalamic nucleus (an adjacent brain region) did not block the potentiation of nicotine CPP or the anxiogenic-like effects of κ-receptor activation. These results suggest that the rewarding effects of nicotine may include a reduction in the stress-induced anxiety responses caused by activation of the dynorphin/κ-opioid system. Together, these data implicate the amygdala as a key region modulating the appetitive properties of nicotine, and suggest that κ-opioid antagonists may be useful therapeutic tools to reduce stress-induced nicotine craving.

Effect of ΔFosB overexpression on opioid and cannabinoid receptor-mediated signaling in the nucleus accumbens

The stable transcription factor ΔFosB is induced in the nucleus accumbens (NAc) by chronic exposure to several drugs of abuse, and transgenic expression of ΔFosB in the striatum enhances the rewarding properties of morphine and cocaine. However, the mechanistic basis for these observations is incompletely understood. We used a bitransgenic mouse model with inducible expression of ΔFosB in dopamine D(1) receptor/dynorphin-containing striatal neurons to determine the effect of ΔFosB expression on opioid and cannabinoid receptor signaling in the NAc. Results showed that mu opioid-mediated G-protein activity and inhibition of adenylyl cyclase were enhanced in the NAc of mice that expressed ΔFosB. Similarly, kappa opioid inhibition of adenylyl cyclase was enhanced in the ΔFosB expressing mice. In contrast, cannabinoid receptor-mediated signaling did not differ between mice overexpressing ΔFosB and control mice. These findings suggest that opioid and cannabinoid receptor signaling are differentially modulated by expression of ΔFosB, and indicate that ΔFosB expression might produce some of its effects via enhanced mu and kappa opioid receptor signaling in the NAc.

The expression of genes encoding opioid precursors and the influence of opioid receptor agonists on steroidogenesis in porcine adrenocortical cells in vitro

Endogenous opioid peptides are involved in the regulation of the HPA-axis function and stress response mechanism. However, there is a scarcity of data on opioid involvement in the regulation of the adrenocortical endocrine function. This study was performed to: 1) establish the expression of proenkephalin, POMC and prodynorphin genes in the porcine adrenal cortex and test in vitro the influence of ACTH, angiotensin II, CRH and epinephrine on this expression, and 2) determine the effects of opioid receptor agonists on basal and ACTH- or angiotensin II-affected secretion of cortisol, aldosterone and progesterone by the cultured adrenocortical cells. Our experiment has demonstrated the presence of mRNAs for opioid precursors in cells isolated from the adrenal cortex and the significant effects of ACTH and angiotensin II, but not CRH or epinephrine, on adrenocortical transcription of the analyzed genes. Angiotensin II reduced the expression of the POMC gene but stimulated that of prodynorphin. In turn, ACTH decreased the transcription of prodynorphin. The study has also demonstrated the effects of selective opioid receptor agonists - DPLPE (delta), FK33-824 (mu) and U50,488 (kappa) - on adrenal steroidogenesis in pigs. Basal secretion of cortisol was enhanced after the activation of mu or kappa receptors, whereas ACTH-stimulated cortisol output was increased only by the mu receptor agonist. Angiotensin II-treated cells significantly decreased aldosterone secretion in the presence of the kappa receptor agonist. The present results suggest that opioid peptides are synthesized in the porcine adrenal cortex, indicating their involvement in the regulation of adrenal steroidogenesis through autocrine and/or paracrine interactions.

Role of Intracellular Calcium and Cyclic Nucleotides in Realization of Cardioprotective Effects of δ(1)- and κ(1)-Opioid Receptor Agonists

The role of cyclic nucleotides (cAMP, cGMP) and Ca(2+)-ATPase of the sarcoplasmic reticulum in the mechanism of cardioprotective effects of selective δ(1)- and κ(1)-opioid receptor agonists DPDPE and U-50488 was studied under conditions of global ischemia and reperfusion of isolated and perfused rat heart. Activation of both types of opioid receptors 2-fold reduced the reperfusion release of creatine phosphokinase. The cardioprotective effect of U-50488 was paralleled by a 2-fold decrease in cAMP content in the myocardium, while DPDPE did not modify the content of cAMP throughout the experiment. None of these substances changed the content of cGMP in the myocardium. The cardioprotective effect of DPDPE was not observed after inhibition of sarcoplasmic reticulum Ca(2+)-ATPase with cyclopiazonic acid. The cardioprotective effect of U-50488 was associated with reduction of cAMP level in the myocardium, while the cytoprotective effect of DPDPE was mediated by opioidergic modulation of Ca(2+) transport at the level of the sarcoplasmic reticulum.

[Model of platelet activation as experimental test system for screening kappa-opioid receptor ligands]

A new model is proposed for in vitro testing the efficiency of new chemical substances with kappa-opioid receptor agonist ligand properties.

[Effects of kappa-opioid receptor stimulation on high glucose induced myocardial hypertrophy of neonatal rats]

OBJECTIVE

To observe the effects of kappa-opioid receptor stimulation on high glucose induced myocardial hypertrophy of neonatal rats.

METHODS

Using cultured myocardial cells as a model, the protein content was assayed with Lowrys method. The cardiomyocytes volumes were measured by computer photograph analysis system. The level of p-ERK44/42 was determined by Western blot.

RESULTS

Compared with the control, U50488H significantly inhibited the protein content and volumes of cultured hypertrophic myocardial cells induced by high glucose. Meanwhile the role of ERK was important.

CONCLUSION

The stimulation of kappa-opioid can inhibit myocardial hypertrophy induced by high glucose, which is possibly via attenuating p-ERK.

[The study of effects and mechanism of U50, 488H on electrical coupling during ischemia in the perfused isolated rat heart]

OBJECTIVE

To determine the effect of activation of lambda-opioid receptor with U50, 488H, a selective kappa-opioid receptor agonist, on the changes in electrical coupling during prolonged ischemia and to explore the possible mechanism.

METHODS

The isolated rat heart was perfused in a Langendorff apparatus. The effect of U50, 488H on electrical coupling parameters including onset of uncoupling, plateau time, slope and fold increase in r(t) was observed in isolated perfused rat heart subjected to global no-flow ischemia. The effect of U50, 488H on connexin 43 (Cx43) expression of ventricular muscle during ischemia was determined by immunohistochemistry.

RESULTS

In the prolonged ischemia model, U50, 488H concentration dependently delayed the onset of uncoupling, increased time to plateau, and decreased the maximal rate of uncoupling during ischemia. The effect of U50, 488H on electrical uncoupling parameters during ischemia was abolished by a selective kappa-opioid receptor antagonist nor-BNI or a PKC inhibitor chelerythrine. The amount of Cx43 immunoreactive signal in ventricular muscle was greatly reduced after ischemia. U50, 488H markedly increased Cx43 expression during ischemia and its effect was also attenuated by nor-BNI or chelerythrine.

CONCLUSION

These results demonstrated that U50, 488H delayed the onset of uncoupling and plateau time, decreased the maximal rate of uncoupling and increased Cx43 expression of ventricular muscle during ischemia, and these effects of U50, 488H were mediated by kappa-opioid receptor, in which activation of PKC was involved. The effect of U50, 488H on electrical coupling during ischemia was probably correlated with preservation of Cx43 in cardiac muscle.

The effects of kappa-opioid receptor ligands on prepulse inhibition and CRF-induced prepulse inhibition deficits in the rat

RATIONALE

Kappa-opioid receptor (KOR) agonists produce dysphoria and psychotomimesis in humans. KORs are enriched in the prefrontal cortex and other brain regions that regulate mood and cognitive function. Dysregulation of the dynorphin/KOR system has been implicated in the pathogenesis of schizophrenia, depression, and bipolar disorder. Prepulse inhibition of the acoustic startle reflex (PPI), a sensorimotor gating process, is disrupted in many psychiatric disorders.

OBJECTIVES

The present study determined whether KOR ligands alter PPI in rats.

RESULTS

Utilizing a range of doses of the synthetic KOR agonists (+/-) U50,488, (-) U50,488, and U69,593 and the naturally occurring KOR agonist, Salvinorin A, we demonstrate that KOR activation does not alter PPI or startle reactivity in rats. Similarly, selective KOR blockade using the long-acting antagonist nor-binaltorphimine (nor-BNI) was without effect. In contrast to KOR ligands, MK-801 and quinpirole produced deficits in PPI. Stress and corticotropin-releasing factor (CRF) decrease PPI levels. The dynorphin/KOR system has been suggested to be a key mediator of various behavioral effects produced by stress and CRF. We therefore examined the contribution of KORs to CRF-induced alterations in PPI. Intracerebroventricular infusion of CRF decreased PPI. Administration of nor-BNI failed to affect the CRF-evoked disruption in PPI.

CONCLUSIONS

Together, these results provide no evidence of a link between the dynorphin/KOR system and deficits in sensory gating processes. Additional studies, however, examining whether dysregulation of this opioid system contributes to cognitive deficits and other behavioral abnormalities associated with psychiatric disorders are warranted.

One-trial cocaine-induced behavioral sensitization in preweanling rats: role of contextual stimuli

Using a one-trial procedure, preweanling rats exhibit robust sensitization regardless of whether drug pretreatment and testing occur in the same or different environments. The purpose of the present study was to determine whether one-trial context-specific and context-independent sensitization of preweanling rats could be dissociated by varying the pretreatment dose of cocaine, by varying the pretreatment drug, or by minimizing interoceptive cues. In Experiments 1a and 1b, rats were pretreated with a broad dose range of cocaine (0-40 mg/kg) before placement in a novel activity chamber or the home cage. In Experiment 2, rats were pretreated with a locomotor-enhancing drug (e.g., methylphenidate, U50,488, or MK-801) before placement in a novel activity or anesthesia chamber. In Experiment 3, rats were anesthetized with isoflurane before cocaine administration to minimize the effects of interoceptive and injection cues. In all experiments, rats were challenged with cocaine on the test day (24 hr later), with locomotion being measured in activity chambers. Results showed that (a) the pretreatment dose of cocaine (10-40 mg/kg) did not differentially affect context-specific and context-independent sensitization; (b) cross-sensitization between methylphenidate and cocaine was observed in the context-specific condition, but not when using a context-independent procedure; and (c) sensitization was evident if injection and interoceptive cues were minimized. One possibility is that associative processes do not modulate the one-trial sensitization of preweanling rats. Alternatively, "unitization" may cause preweanling rats to treat the different environments as equivalent, thus permitting robust sensitization even when drug pretreatment and testing occur in different environments.

Regulation of cocaine-reinstated drug-seeking behavior by kappa-opioid receptors in the ventral tegmental area of rats

RATIONALE

Relapse is one of the main challenges facing the current treatment of cocaine addiction. Understanding its neurobiological mechanism is a critical step toward developing effective anti-relapse therapies.

OBJECTIVES

Emerging evidence indicates that glutamate-mediated activation of dopamine (DA) neurons in the ventral tegmental area (VTA) may be critically involved in cocaine-induced relapse to drug-seeking behavior. Activity of VTA DA neurons is modulated by multiple neurotransmitter systems including opioids, serotonin, dopamine, and acetylcholine. Recent studies demonstrated that activation of kappa-opioid receptors (kappaORs) in the rat VTA directly inhibits the activity of a subpopulation of DA neurons projecting to the prefrontal cortex (PFC) and amygdala. Because we previously showed that blockade of DA receptors in the dorsal PFC inhibits cocaine-induced reinstatement of extinguished cocaine-seeking behavior suggesting a critical role of the VTA-PFC DA circuit in this process, we tested the hypothesis that activation of kappaORs in the VTA will block cocaine-induced reinstatement in rats.

METHODS

Rats were trained to self-administer intravenous cocaine (0.125 mg/infusion) under a modified fixed-ratio five schedule. After extinction of the learned behavior, the effects of activation of VTA kappaORs on cocaine-induced reinstatement were studied.

RESULTS

The kappaOR agonist U50 488 (0-5.6 microg/side) microinjected into the VTA dose-dependently decreased cocaine-induced reinstatement. The effects could not be explained by either a disruption of operant behavior or diffusion of the drug to the areas surrounding the VTA. Moreover, the effect was reversed by norbinaltorphimine.

CONCLUSIONS

The VTA DA neurons expressing functional kappaORs are critically involved in cocaine-induced reinstatement in rats.

[Comparing effects of U50488H, prazosin and/or propranolol on cardiac hypertrophy induced by NE in rat]

OBJECTIVE

To demonstrate the inhibitory effect of kappa-opioid receptor activation by U50488H on hypertrophy induced by NE in cultured neonatal rat cardiac myocytes and compare its effect with that of prazosin and propranolol.

METHODS

The cellular proliferation was determined with crystal violet staining. The protein content was assayed with Lowry's method. The cardiomyocytes volumes were measured by computer photograph analysis system. The protein synthesis was assayed with [3H]-lencine incorporation method.

RESULTS

(1) NE significantly induced the increase of protein content, [3H]-leucine incorporation and cell size without a concomitant increase in cell number in low serum medium. OThese responses were partially suppressed by prazosin or propranolol alone and completely abolished by both in combination. U50488H significantly inhibited the NE-induced increase of protein content, [3H]-leucine incorporation and cell size. The inhibitory effects of U50488H on NE-induced cardiac hypertrophy were greater than either prazosin or propranolol, but comparable to combination of both.

CONCLUSION

NE, acting via both alpha1- and beta-adrenergic pathway, stimulates myocyte hypertrophy. Stimulating kappa-opioid receptor significantly inhibits NE-induced cardiac hypertrophy, which may be related with alpha1- and beta1-adrenergic pathway.

Blockade of ethanol reward by the kappa opioid receptor agonist U50,488H

Alcoholism is a pervasive social problem, and thus understanding factors that regulate alcohol (ethanol) reward is important for designing effective therapies. One putative regulatory system includes the kappa opioid receptor (KOR) and its endogenous ligand, dynorphin. Previously, we demonstrated that acute ethanol increased preprodynorphin expression via brain-derived neurotrophic factor (BDNF) in striatal neurons, and that blockade of the KOR attenuated decreases in ethanol intake observed following increased expression of BDNF. As high doses of KOR agonists can generate an aversive state, we hypothesized that endogenous dynorphin may regulate ethanol intake by interfering with the rewarding properties of ethanol. We found that low, nonaversive doses of the KOR agonist U50,488H blocked the rewarding properties of ethanol during conditioning, thus impairing the acquisition of conditioned place preference. Importantly, we demonstrate that U50,488H also inhibited the conditioned increase in locomotor activation normally observed in the ethanol-paired chamber on test day. Taken together, these data indicate that the KOR/dynorphin system may acutely regulate ethanol intake via inhibition of the rewarding properties of ethanol.

Delayed uncoupling is related to cardioprotection induced by κ-agonist U-50,488H in rat heart

OBJECTIVES

To determine whether the kappa-opioid receptor agonist U50,488H affects electrical uncoupling during prolonged ischemia and, if so, whether the changes are associated with its cardioprotective action.

DESIGN

The isolated rat heart was perfused in a Langendorff apparatus. Formazan content, lactate dehydrogenase (LDH) and hemodynamic parameters were measured to confirm the cardioprotective effect of U50,488H. The effects of U50,488H on electrical coupling during prolonged ischemia were also measured.

RESULTS

U50,488H concentration-dependently increased formazan content and reduced LDH release, and the ameliorating effect of 10(-5) mol/L U50,488H was abolished by 5 x 10(-6) mol/L nor-binaltorphimine (nor-BNI), a selective kappa-opioid receptor antagonist, or 10(-4) mol/L 5-hydroxydecanoate (5-HD), a selective mitochondrial ATP-sensitive K(+) (K(ATP)) channel blocker. The onset of electrical uncoupling during prolonged ischemia was delayed by U50,488H, and the delay was not only abolished, but also advanced by nor-BNI or 5-HD relative to the control group.

CONCLUSIONS

These results demonstrate that delayed uncoupling during prolonged ischemia is associated with the cardioprotection of U50,488H, and these effects of U50,488H are mediated by mitochondrial K(ATP) channels.

Kappa and delta opioid receptor signaling is augmented in the failing heart

The opioidergic system, an endogenous stress pathway, modulates cardiac function. Furthermore, opioid peptide and receptor expression is altered in a number of cardiac pathologies. However, whether the response of myocardial opioid receptor signaling is altered in heart failure progression is currently unknown. Elucidating possible alterations in and effects of opioidergic signaling in the failing myocardium is of critical importance as opioids are commonly used for pain management, including in patients at risk for cardiovascular disease. A hamster model of cardiomyopathy and heart failure (Bio14.6) was used to investigate cardiac opioidergic signaling in heart failure development. This study found an augmented negative inotropic and lusitropic response to administration of agonists selective for the kappa opioid receptor and delta opioid receptor in the failing heart that was mediated by a pertussis toxin-sensitive G-protein. The augmented decrease in cardiac function was manifested by increased inhibition of cAMP accumulation and the amplitude of the systolic Ca(2+) transient. Furthermore, increased depression of cardiac function and of two important second messengers, cAMP and intracellular Ca(2+), were independent of changes in cardiac opioid peptide or receptor expression. Thus, the cardiomyopathy-induced failing heart experiences increased cardiac depressant effects following opioid receptor stimulation which could exacerbate diminished cardiac function in end-stage heart failure. As cardiac function is already depressed in heart failure patients, administration of opioids could exacerbate the degree of cardiac dysfunction and worsen disease progression.

[Intracellular mechanisms of opioidergic regulation of the myocardial function during normoxia and postischemic reperfusion]

Cardioprotective and inotropic effects of selective agonists of delta1- and k1-opioid receptors (OR): DPDPE (0.1 microM/L) and U-50.488 (0.1 microM/L) were studied during 45 min global ischemia and 30 min reperfusion of the rat isolated perfused heart. Activation of both OR types promoted a 2-fold reduction in reperfusion leakage of creatine kinase. Cardioprotective effect of U-50.488 was accompanied by a 2-fold decrease in cAMP levels in myocardium. The selective delta1-agonist DPDPE had no effect on the cAMP content. Cardioprotective effect of DPDPE was not demonstrated after inhibition of Ca2+-ATPase in sarcoplasmic reticulum (SR) by cyclopiazonic acid. Stimulation of myocardial delta1- and K1-OR decreased the heart rate and force of contraction both before ischemia and during reperfusion. In summary, cardioprotective effect of U-50.488 depends on the reduction in myocardial cAMP levels whereas cytoprotective effect of DPDPE is mediated via opioidergic alteration in Ca2+-transport at SR level. Decrease in pump function of heart in response to OR activation does not depend upon alteration in cAMP levels in the myocardium.

Nociceptin produces antinociception after spinal administration in amphibians

Nociceptin, also known as orphanin FQ, is a opioid-like neuropeptide that mediates its effects at the nociceptin receptor, a member of the G protein-coupled receptor superfamily. In mammals, nociceptin produces analgesia after spinal administration, however the role of nociceptin and nociceptin receptors in the modulation of noxious stimuli in non-mammalian species has not been examined. In an amphibian pain model using the acetic acid test with Rana pipiens, nociceptin and nociceptin1-13 amide produced dose-dependent antinociception (1-100 nmol), blocked by the nociceptin antagonist, [Nphe1]-nociceptin1-13 amide (30 nmol), but not the opioid antagonist, naltrexone (100 nmol/g, s.c.). Conversely, the antinociceptive effects of micro, delta, and kappa opioid receptor agonists were not blocked by the nociceptin antagonist. Nociceptin and nociceptin1-13 amide were the least potent of the opioid agonists tested. These studies demonstrate that spinal nociceptin receptors and not opioid receptors mediate the antinociceptive effect of nociceptin. Considered with previous findings, these behavioral data supports a role for nociceptin inhibition of spinal nociception in amphibians and perhaps all vertebrates.

[Role of kappa1 opioid receptors and cAMP in regulation of cardiac tolerance to ischemia and reperfusion]

Cardioprotective, inotropic, and antiarrhythmic effects of the selective agonist of k1-opioid receptors (k1-ORs) U-50.488H have been studied after 45-min global ischemia and 30-min reperfusion of isolated perfused rat hearts. The heart k1-ORs were stimulated by adding 0.1 or 1 mmol/l U-50.488H to the perfusion solution. The opioid did not affect the frequency of reperfusion arrhythmias. At a concentration of 0.1 mmol/l, it induced a twofold decrease in the reperfusion release of creatine phosphokinase (CPK), which positively correlated with a decrease in the myocardial cAMP level (r = 0.89, p < 0.01). Application of U-50.488H at a final concentration of 1 mmol/l did not change the cAMP level and CPK release. These results suggest that the cardioprotective effect of U-50.488H is due to a decrease in the level of cAMP in cardiomyocytes. Activation of k1-ORs decreased the frequency and force of myocardial contractions. It has been shown that the negative inotropic and chronotropic effects of U-50.488H are independent of changes in the myocardial cAMP level. A hypothesis is proposed that the absence of cardioprotective effect of 1 mM U-50.488H is a result of activation of nonopioid receptors in cardiomyocytes.

Cysteine protease inhibitors suppress the development of tolerance to morphine antinociception

The effects of various protease inhibitors on the development of antinociceptive tolerance to morphine were examined in mice. Intrathecal (i.t.) administration of morphine (0.01-1 nmol) produced a dose-dependent and significant antinociceptive effect in the 0.5% formalin test. When the doses of morphine (mg/kg, s.c. per injection) were given as pretreatment twice daily for two days [first day (30) and second day (60)], i.t. administration of morphine (0.1 nmol) was inactive due to antinociceptive tolerance on the third day. Tolerance to i.t. morphine was significantly suppressed by the i.t. injection of N-ethylmaleimide or Boc-Tyr-Gly-NHO-Bz, inhibitors of cysteine proteases involved in dynorphin degradation, as well as by dynorphin A, dynorphin B and (-) U-50,488, a selective kappa-opioid receptor agonist. On the other hand, amastatin, an aminopeptidase inhibitor, phosphoramidon, an endopeptidase 24.11 inhibitor, lisinopril, an angiotensin-converting enzyme inhibitor, and phenylmethanesulfonyl fluoride, a serine protease inhibitor, were inactive. These results suggest that cysteine protease inhibitors suppress the development of morphine tolerance presumably through the inhibition of dynorphin degradation.

Pharmacologic evaluation of pressor and visceromotor reflex responses to bladder distension

AIMS

Several mechanisms that are involved in acute rat bladder nociception were examined. The nociceptive response was measured by analyzing both cardiovascular and visceromotor reflex responses to urinary bladder distension. The contributions of micro-opioid receptor, kappa-opioid receptor, sodium channels, muscarinic receptors, and cyclooxygenase, were explored with morphine, U50,488, mexiletine, oxybutynin, and naproxen, respectively.

METHODS

Female Sprague-Dawley rats were acutely instrumented with jugular venous, carotid arterial, and bladder cannulas. Needle electrodes were placed directly into the abdominal musculature to measure myoelectrical activity subsequent to repeated phasic urinary bladder distension (60 mmHg for 20 sec in 3 min intervals) under 1% isoflurane. Drugs were administered by i.v. bolus injection 2 min prior to distension.

RESULTS

The analgesics morphine (ID50 0.69 mg/kg), U50,488 (1.34 mg/kg), and mexiletine (2.60 mg/kg) significantly inhibited the visceromotor reflex response to noxious urinary bladder distension. Oxybutynin also attenuated reflex responses to noxious urinary bladder distension to 41% of the maximal pressor response and 32% of the control visceromotor reflex response (3.01 and 5.05 mg/kg), respectively, indicating a role of muscarinic receptors in bladder nociception. Naproxen did not attenuate the pressor response, but moderately inhibited visceromotor reflex to 45% of control at 30 mg/kg (P < 0.05).

CONCLUSIONS

Current results using the rat urinary bladder distension model are consistent with previous research demonstrating a role of the analgesics (morphine, U50,488, and mexiletine) in the inhibition of visceral nociceptive transmission. The utility of the reflex responses to urinary bladder distension may provide a method useful to examine mechanisms which target the bladder sensory pathway.

The comparison of effects of processed Aconiti tuber, U50488H and MK-801 on the antinociceptive tolerance to morphine

In the previous studies, we demonstrated that an oriental herbal medicine, processed Aconiti tuber (PAT), at subanalgesic doses could inhibit or reverse the antinociceptive tolerance to morphine. In the present study, we compared the effect of PAT, trans-(+/-)-3,4-dichloro-N-methyl-N-(2-(1-pyrrolidin)cyclohexyl)-benzeneacetamide methane sulfonate hydrate (U50488H), a selective kappa opioid receptor (KOR) agonist, and (-)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine-maleate (MK-801), a N-methyl-D-aspartate (NMDA) receptor antagonist, on the antinociceptive tolerance to morphine in the same experimental condition. Mice received subcutaneous morphine (10 mg/kg), and oral PAT at a subanalgesic dose (0.3 g/kg for mechanical or 1.0 g/kg for thermal test), or intraperitoneal U50488H at a subanalgesic dose (3 mg/kg), or MK-801 at a subanalgesic dose (0.1 mg/kg) once daily for 14 days. The mechanical nociceptive threshold was measured before, and at 60 min by tail pressure testing, and thermal nociceptive latency was measured before, and at 30 min by hot plate testing, after daily morphine injections. PAT and U50488H could not only inhibit the development of morphine tolerance but also reverse the already-developed morphine tolerance, while MK-801 could only inhibit the development of morphine tolerance but not reverse the already-developed morphine tolerance, in both mechanical and thermal nociceptive tests. These data suggested that PAT, an indirect-acting KOR agonist, share the common pharmacological property of KOR agonists on morphine tolerance, and that PAT may be superior to some NMDA receptor antagonists which do not reverse already-developed morphine tolerance.

[Role of kappa1 opioid receptors and cAMP in regulation of cardiac tolerance to ischemia and reperfusion]

The cardioprotective, inotropic, and antiarrhythmic effects of U-50.488, a selective agonist of kappa1 opioid receptors (kappa1 ORs), was studied using the model of 45-min total ischemia and 30-min reperfusion of isolated rat heart. Cardiac kappa1 ORs were stimulated by adding U-50.488 to the perfusing solution up to the final concentration of 0.1 or 1 micromol/l. The opioid had no influence on the incidence of reperfusion arrhythmias. The addition of 0.1 micromol/l U-50.488 reduced the reperfusion release of creatine phosphokinase (CPK) by half, which positively correlated with the decrease in the myocardial cAMP content (r = 0.89, p < 0.01). At the same time, the addition of U-50.488 in the higher concentration (1 micromol/l) had no effect on either cAMP level or CPK release. These results indicate that the cardioprotective effect of U-50.488 may be connected with the reduction of myocardial cAMP content. Activation of kappa1 ORs caused a decrease in both frequency and amplitude of myocardial contractions. The negative inotropic and chronotropic effect of U-50.488 was shown to be independent of changes in the myocardial cAMP content. A hypothesis is proposed that the absence of any cardioprotective effect of U-50.488 at the higher concentration (1 micromol/l) is accounted for by its interaction with unknown nonopioid receptors of cardiac myocytes.

Involvement of kappa opioid receptors in formalin-induced inhibition of analgesic tolerance to morphine in mice

This study examined the role of kappa opioid receptors (KOR) in the mechanism underlying tolerance to the analgesic effects of morphine induced by chronic pain. The analgesic effect of morphine (10 mg kg(-1)), estimated by the tail-flick test in mice, gradually decreased during repeated daily morphine treatment. A significant decrease in the analgesic effect of morphine was seen on the fifth day of repeated morphine treatment compared with the first day. Chronic pain was induced by subcutaneous administration of 2% formalin into the dorsal part of the left hind paw, which significantly inhibited development of tolerance to morphine analgesia. The effect of formalin-induced pain on inhibition of morphine tolerance was reversed by the KOR antagonist nor-binaltorphimine. Furthermore, an antisense oligodeoxynucleotide, but not a missense oligodeoxynucleotide, against KOR completely suppressed the inhibitory effect of formalin-induced pain on morphine tolerance. Naltrindole, an antagonist of delta opioid receptor, did not affect chronic-pain-induced tolerance to morphine. Our findings show that the inhibitory effect of chronic pain on analgesic tolerance to morphine is mediated by KOR rather than delta opioid receptors.

Long-acting kappa opioid antagonists disrupt receptor signaling and produce noncompetitive effects by activating c-Jun N-terminal kinase

Norbinaltorphimine (NorBNI), guanidinonaltrindole, and atrans-(3R,4R)-dimethyl-4-(3-hydroxyphenyl) piperidine (JDTic) are selective kappa opioid receptor (KOR) antagonists having very long durations of action in vivo despite binding non-covalently in vitro and having only moderately high affinities. Consistent with this, we found that antagonist treatment significantly reduced the subsequent analgesic response of mice to the KOR agonist U50,488 in the tail-withdrawal assay for 14-21 days. Receptor protection assays were designed to distinguish between possible explanations for this anomalous effect, and we found that mice pretreated with the readily reversible opioid antagonists naloxone or buprenorphine before norBNI responded strongly in the tail-flick analgesia assay to a subsequent challenge with U50,488 1 week later. Protection by a rapidly cleared reagent indicates that norBNI did not persist at the site of action. In vitro binding of [(3)H]U69,593 to KOR showed that K(d) and Bmax values were not significantly affected by prior in vivo norBNI exposure, indicating that the agonist binding site was intact. Consistent with the concept that the long-lasting effects might be caused by a functional disruption of KOR signaling, both norBNI and JDTic were found to stimulate c-Jun N-terminal kinase (JNK) phosphorylation in HEK293 cells expressing KOR-GFP but not in untransfected cells. Similarly, norBNI increased phospho-JNK in both the striatum and spinal cord in wild type mice but not in KOR knock-out mice. Pretreatment of mice with the JNK inhibitor SP600125 before norBNI attenuated the long acting antagonism. Together, these results suggest that the long duration KOR antagonists disrupt KOR signaling by activating JNK.

Mechanisms Involved in the Hypotensive Effect of a κ-Opioid Receptor Agonist in Hypertensive Rats

BACKGROUND

It remains unclear whether the activation of kappa-opioid receptors has strong hypotensive effects under hypertensive condition, and the underlying mechanisms have not yet been investigated. Therefore, the present study is designed to use spontaneously hypertensive rats (SHR) to investigate the effects of a kappa-opioid receptor agonist on the regulation of urinary formation in hypertensive conditions and to identify its underlying mechanism.

METHODS

The hemodynamics, urine flow rate, vasodilatation of isolated renal artery, and plasma hormones were determined by physiological in vivo experimental technique, isolated artery perfusion technique and radioimmunoassay.

RESULTS

Intravenous administration of U50, 448H significantly decreased mean arterial blood pressure in both Wistar-Kyoto (WKY) rats and SHR. However, the blood pressure vasodepressor effect of U50, 448H was much more profound in SHR than in WKY rats. Administration of U50, 448H in SHR not only caused significantly greater effects in increasing urine volume and decreasing plasma anti-diuretic hormone than in WKY rats, but also caused significant reduction in plasma angiotensin. Moreover, vasodilatory effect of U50, 488H was significantly exhibited in the renal artery segments isolated from SHR. All effects described above were abolished by nor-binaltorphimine.

CONCLUSIONS

These data indicate that the depressor effect of U50, 488H in SHR is significantly stronger than that in WKY rats, and the effect is mediated or modulated by a kappa-opioid receptor sensitive mechanism. The sensitized hypotensive effect of U50, 488H in SHR may be attributed, in part, to its vasodilatory effect, enhanced beneficial effect on plasma humoral factors, and stronger diuretic effect in these hypertensive animals.

Cardioprotective and antiarrhythmic effect of U50,488H in ischemia/reperfusion rat heart

The objective of this study was to investigate the protective effect of U50,488H, a selective kappa-opioid receptor agonist, in the ischemia/reperfusion (I/R) rat and to delineate the underlying mechanism. Rat heart I/R injury was induced by occluding the left anterior descending coronary artery for 45 min and restoring perfusion for 120 min. U50,488H or vehicle was intravenously injected before ischemia. Electrocardiogram, heart rate (HR), arterial blood pressure (ABP), left ventricular pressure (LVP), systolic function (+dp/dtmax), and diastolic function (-dp/dtmax) were monitored in the course of the experiment. Myocardial infarction size was evaluated. Plasma concentrations of cardiac troponin T (cTnT), creatine kinase (CK), and lactate dehydrogenase (LDH) were measured. Single rat ventricular myocyte was obtained by enzymatic dissociation method. The potassium currents (IK) of isolated ventricular myocytes were recorded with the whole-cell configuration of the patch-clamp technique. Compared with the sham control group, no significant change was found in HR, while ABP, LVP and +/-dp/dtmax were significantly reduced in the I/R group. Administration of U50,488H significantly lowered HR in both control and I/R groups. Compared with the vehicle-treated I/R group, administration of U50,488H had no significant effect on I/R-induced reduction in ABP, LVP, and +/-dp/dtmax. However, this treatment significantly reduced the myocardial infarction size, and markedly decreased the contents of plasma cTnT, CK and LDH. During ischemia and reperfusion, the incidence of ventricular arrhythmia in U50,488H-treated rats was significantly reduced. These effects were independent of the bradycardia induced by U50,488H, as the reducing infarct size and antiarrhythmic effect of U50,488H were still observed in animals in which heart rate was kept constant by electrical pacing. U50,488H and BRL-52537 still produced an antiarrhythmic effect when the rat heart was subjected to a shorter ischemic period of 10 min occlusion of coronary artery, which produced no infarction. IK of the myocytes were inhibited by U50,488H in a dose-dependent manner in normal and hypoxic rat ventricular myocytes. However, the effects of U50,488H on IK did not show any significant difference in normal and hypoxic myocytes. The above-described effects of U50,488H were totally blocked by nor-Binaltorphimine, a selective kappa-opioid receptor antagonist. The results suggest that kappa-opioid agonist U50,488H exerts its direct cardioprotective and antiarrhythmic effects against I/R via kappa-opioid receptor, which participates in the regulation of potassium channels in normal and hypoxic ventricular myocytes.