Morphine-Tolerant Mice Exhibit a Profound and Persistent Cardioprotective Phenotype

µ-Opioid Receptors Expressed by Intrinsically Photosensitive Retinal Ganglion Cells Contribute to Morphine-Induced Behavioral Sensitization

Opioid drugs are the most effective tools for treating moderate to severe pain. Despite their analgesic efficacy, long-term opioid use can lead to drug tolerance, addiction, and sleep/wake disturbances. While the link between opioids and sleep/wake problems is well-documented, the mechanism underlying opioid-related sleep/wake problems remains largely unresolved. Importantly, intrinsically photosensitive retinal ganglion cells (ipRGCs), the cells that transmit environmental light/dark information to the brain's sleep/circadian centers to regulate sleep/wake behavior, express μ-opioid receptors (MORs). In this study, we explored the potential contribution of ipRGCs to opioid-related sleep/circadian disruptions. Using implanted telemetry transmitters, we measured changes in horizontal locomotor activity and body temperature in mice over the course of a chronic morphine paradigm. Mice lacking MORs expressed by ipRGCs (McKO) exhibited reduced morphine-induced behavioral activation/sensitization compared with control littermates with normal patterns of MOR expression. Contrastingly, mice lacking MORs globally (MKO) did not acquire morphine-induced locomotor activation/sensitization. Control mice also showed morphine-induced hypothermia in both the light and dark phases, while McKO littermates only exhibited morphine-induced hypothermia in the dark. Interestingly, only control animals appeared to acquire tolerance to morphine's hypothermic effect. Morphine, however, did not acutely decrease the body temperature of MKO mice. These findings support the idea that MORs expressed by ipRGCs could contribute to opioid-related sleep/wake problems and thermoregulatory changes.

Morphine Accumulates in the Retina Following Chronic Systemic Administration

Opioid transport into the central nervous system is crucial for the analgesic efficacy of opioid drugs. Thus, the pharmacokinetics of opioid analgesics such as morphine have been extensively studied in systemic circulation and the brain. While opioid metabolites are routinely detected in the vitreous fluid of the eye during postmortem toxicological analyses, the pharmacokinetics of morphine within the retina of the eye remains largely unexplored. In this study, we measured morphine in mouse retina following systemic exposure. We showed that morphine deposits and persists in the retina long after levels have dropped in the serum. Moreover, we found that morphine concentrations (ng/mg tissue) in the retina exceeded brain morphine concentrations at all time points tested. Perhaps most intriguingly, these data indicate that following chronic systemic exposure, morphine accumulates in the retina, but not in the brain or serum. These results suggest that morphine can accumulate in the retina following chronic use, which could contribute to the deleterious effects of chronic opioid use on both image-forming and non-image-forming visual functions.

Anesthetic preconditioning increases sirtuin 2 gene expression in a renal ischemia reperfusion injury model

BACKGROUND

Renal transplant surgical proceedings are known to elicit periods of hypoxia and consequent blood flow reestablishment triggering ischemia-reperfusion (I-R) injury. Kidney damage induced by I-R injury associates with a higher risk of graft dysfunction and rejection. Anesthetic preconditioning exerts a beneficial effect on I-R injury by reducing oxidative stress, inflammation and apoptosis. However, the degree of renoprotection stimulated by commonly used anesthetics, as well as their mechanisms of action, are largely unknown. Sirtuins are class III histone deacetylases that reduce cellular stress, promote genome stability and regulate senescence. So far, the relationship between sirtuins and anesthetic preconditioning in the context of renal I-R has not been studied. The main objective of the present work was to determine the renal expression of sirtuins after I-R damage in rats under different anesthetic preconditioning treatments.

METHODS

Unilateral ischemia was performed via occlusion of the left renal hilum for 45 min and followed by 24 hours of reperfusion. Anesthetic preconditioning schemes (morphine 0.5 mg/kg, fentanyl 10 µg/kg, propofol 7.5 mg/kg, or dexmedetomidine 25 µg/kg) were administered 1 hour before ischemia. Creatinine levels were determined in serum, and expression of kidney injury molecule 1 and sirtuin 1, 2, 3 and 7 in kidney tissue was quantified by RT-PCR.

RESULTS

Anesthetic preconditioning with morphine, fentanyl, propofol and dexmedetomidine reduced kidney injury markers after I-R and modulated sirtuin gene expression. Opioids or dexmedetomidine administration before ischemia increased sirtuin 2 expression and correlated with improved renal function.

CONCLUSIONS

Anesthetic preconditioning is a promising strategy to prevent I-R injury associated with transplantation. Our results suggest that sirtuin 2 is involved in the protective mechanisms of some commonly used anesthetics against I-R damage.

Post-infarct morphine treatment mitigates left ventricular remodeling and dysfunction in a rat model of ischemia-reperfusion

Following myocardial infarction, the heart undergoes a series of dramatic compensations which may later form a maladaptive picture characterized by ventricular dilation and pump failure. Among several opioid agents, morphine has been shown to confer protection against reperfusion injury and infarct size. Here, we sought to study the cardioprotective effect of post-infarct morphine treatment against left ventricular adverse remodeling. We induced myocardial infarction in male Sprague - Dawley rats by ligating left anterior descending artery and then, treated these animals with three different doses of morphine -0.3, 3 and 10 mg/kg (i.p.). The echocardiographic evaluation depicted improved cardiac performance and lesser chamber dilation in the animals that had received 3 mg/kg of morphine. Next, we studied the effect of 3 mg/kg morphine administration on left ventricular hemodynamics, infarct size, tissue architecture, changes in lung and heart weight, circulating TNF-α level and post-MI mRNA expression of collagen-1, collagen-3, TGF-β, TNF-α, MMP-2 and MMP-9. Five-day morphine administration markedly improved LV function, and also reduced infarct size, myocyte hypertrophy, fibrosis, index of infarct expansion, heart weight and serum TNF-α level. Moreover, morphine alleviated MI-induced increase in wet and dry lung weight. Morphine also altered the mRNA expression of fibrosis-related genes, TNF-α, MMP-2 and MMP-9. In conclusion, post-infarct morphine treatment can mitigate adverse remodeling and cardiac dysfunction after MI. Beside analgesic effect, we may be able to harvest benefits from the antifibrotic and anti-remodeling action of morphine in patients with the acute coronary syndrome.

Delta Opioid Receptors and Cardioprotection

The opioid receptor family, with associated endogenous ligands, has numerous roles throughout the body. Moreover, the delta opioid receptor (DORs) has various integrated roles within the physiological systems, including the cardiovascular system. While DORs are important modulators of cardiovascular autonomic balance, they are well-established contributors to cardioprotective mechanisms. Both endogenous and exogenous opioids acting upon DORs have roles in myocardial hibernation and protection against ischaemia-reperfusion (I-R) injury. Downstream signalling mechanisms governing protective responses alternate, depending on the timing and duration of DOR activation. The following review describes models and mechanisms of DOR-mediated cardioprotection, the impact of co-morbidities and challenges for clinical translation.

Prospects for Creation of Cardioprotective and Antiarrhythmic Drugs Based on Opioid Receptor Agonists

It has now been demonstrated that the μ, δ1 , δ2 , and κ1 opioid receptor (OR) agonists represent the most promising group of opioids for the creation of drugs enhancing cardiac tolerance to the detrimental effects of ischemia/reperfusion (I/R). Opioids are able to prevent necrosis and apoptosis of cardiomyocytes during I/R and improve cardiac contractility in the reperfusion period. The OR agonists exert an infarct-reducing effect with prophylactic administration and prevent reperfusion-induced cardiomyocyte death when ischemic injury of heart has already occurred; that is, opioids can mimic preconditioning and postconditioning phenomena. Furthermore, opioids are also effective in preventing ischemia-induced arrhythmias.

Opioid receptors and cardioprotection - 'opioidergic conditioning' of the heart

Ischaemic heart disease (IHD) remains a major cause of morbidity/mortality globally, firmly established in Westernized or 'developed' countries and rising in prevalence in developing nations. Thus, cardioprotective therapies to limit myocardial damage with associated ischaemia-reperfusion (I-R), during infarction or surgical ischaemia, is a very important, although still elusive, clinical goal. The opioid receptor system, encompassing the δ (vas deferens), κ (ketocyclazocine) and μ (morphine) opioid receptors and their endogenous opioid ligands (endorphins, dynorphins, enkephalins), appears as a logical candidate for such exploitation. This regulatory system may orchestrate organism and organ responses to stress, induces mammalian hibernation and associated metabolic protection, triggers powerful adaptive stress resistance in response to ischaemia/hypoxia (preconditioning), and mediates cardiac benefit stemming from physical activity. In addition to direct myocardial actions, central opioid receptor signalling may also enhance the ability of the heart to withstand I-R injury. The δ- and κ-opioid receptors are strongly implicated in cardioprotection across models and species (including anti-infarct and anti-arrhythmic actions), with mixed evidence for μ opioid receptor-dependent protection in animal and human tissues. A small number of clinical trials have provided evidence of cardiac benefit from morphine or remifentanil in cardiopulmonary bypass or coronary angioplasty patients, although further trials of subtype-specific opioid receptor agonists are needed. The precise roles and utility of this GPCR family in healthy and diseased human myocardium, and in mediating central and peripheral survival responses, warrant further investigation, as do the putative negative influences of ageing, IHD co-morbidities, and relevant drugs on opioid receptor signalling and protective responses.

Sarcolemmal cholesterol and caveolin-3 dependence of cardiac function, ischemic tolerance, and opioidergic cardioprotection

Cholesterol-rich caveolar microdomains and associated caveolins influence sarcolemmal ion channel and receptor function and protective stress signaling. However, the importance of membrane cholesterol content to cardiovascular function and myocardial responses to ischemia-reperfusion (I/R) and cardioprotective stimuli are unclear. We assessed the effects of graded cholesterol depletion with methyl-β-cyclodextrin (MβCD) and lifelong knockout (KO) or overexpression (OE) of caveolin-3 (Cav-3) on cardiac function, I/R tolerance, and opioid receptor (OR)-mediated protection. Langendorff-perfused hearts from young male C57Bl/6 mice were untreated or treated with 0.02-1.0 mM MβCD for 25 min to deplete membrane cholesterol and disrupt caveolae. Hearts were subjected to 25-min ischemia/45-min reperfusion, and the cardioprotective effects of morphine applied either acutely or chronically [sustained ligand-activated preconditioning (SLP)] were assessed. MβCD concentration dependently reduced normoxic contractile function and postischemic outcomes in association with graded (10-30%) reductions in sarcolemmal cholesterol. Cardioprotection with acute morphine was abolished with ≥20 μM MβCD, whereas SLP was more robust and only inhibited with ≥200 μM MβCD. Deletion of Cav-3 also reduced, whereas Cav-3 OE improved, myocardial I/R tolerance. Protection via SLP remained equally effective in Cav-3 KO mice and was additive with innate protection arising with Cav-3 OE. These data reveal the membrane cholesterol dependence of normoxic myocardial and coronary function, I/R tolerance, and OR-mediated cardioprotection in murine hearts (all declining with cholesterol depletion). In contrast, baseline function appears insensitive to Cav-3, whereas cardiac I/R tolerance parallels Cav-3 expression. Novel SLP appears unique, being less sensitive to cholesterol depletion than acute OR protection and arising independently of Cav-3 expression.

Unique transcriptional profile of sustained ligand-activated preconditioning in pre- and post-ischemic myocardium

Background

Opioidergic SLP (sustained ligand-activated preconditioning) induced by 3–5 days of opioid receptor (OR) agonism induces persistent protection against ischemia-reperfusion (I-R) injury in young and aged hearts, and is mechanistically distinct from conventional preconditioning responses. We thus applied unbiased gene-array interrogation to identify molecular effects of SLP in pre- and post-ischemic myocardium.

Methodology/Principal Findings

Male C57Bl/6 mice were implanted with 75 mg morphine or placebo pellets for 5 days. Resultant SLP did not modify cardiac function, and markedly reduced dysfunction and injury in perfused hearts subjected to 25 min ischemia/45 min reperfusion. Microarray analysis identified 14 up- and 86 down-regulated genes in normoxic hearts from SLP mice (≥1.3-fold change, FDR≤5%). Induced genes encoded sarcomeric/contractile proteins (Myh7, Mybpc3,Myom2,Des), natriuretic peptides (Nppa,Nppb) and stress-signaling elements (Csda,Ptgds). Highly repressed genes primarily encoded chemokines (Ccl2,Ccl4,Ccl7,Ccl9,Ccl13,Ccl3l3,Cxcl3), cytokines (Il1b,Il6,Tnf) and other proteins involved in inflammation/immunity (C3,Cd74,Cd83, Cd86,Hla-dbq1,Hla-drb1,Saa1,Selp,Serpina3), together with endoplasmic stress proteins (known: Dnajb1,Herpud1,Socs3; putative: Il6, Gadd45g,Rcan1) and transcriptional controllers (Egr2,Egr3, Fos,Hmox1,Nfkbid). Biological themes modified thus related to inflammation/immunity, together with cellular/cardiovascular movement and development. SLP also modified the transcriptional response to I-R (46 genes uniquely altered post-ischemia), which may influence later infarction/remodeling. This included up-regulated determinants of cellular resistance to oxidant (Mgst3,Gstm1,Gstm2) and other forms of stress (Xirp1,Ankrd1,Clu), and repression of stress-response genes (Hspa1a,Hspd1,Hsp90aa,Hsph1,Serpinh1) and Txnip.

Conclusions

Protection via SLP is associated with transcriptional repression of inflammation/immunity, up-regulation of sarcomeric elements and natriuretic peptides, and modulation of cell stress, growth and development, while conventional protective molecules are unaltered.

Daily ischemic preconditioning provides sustained protection from ischemia-reperfusion induced endothelial dysfunction: a human study

Background

It is well established that acute ischemic preconditioning (IPC) protects against ischemia–reperfusion (IR) injury; however, the effectiveness of repeated IPC exposure has not been extensively investigated. We aimed to determine whether daily IPC episodes provide continued protection from IR injury in a human forearm model, and the role of cyclooxygenase‐2 in these responses.

Methods and Results

Thirty healthy volunteers were randomized to participate in 2 of 3 protocols (IR alone, 1‐day IPC, 7‐day IPC) in an operator‐blinded, crossover design. Subjects in the IR alone protocol underwent flow‐mediated dilation (FMD) measurements pre‐ and post‐IR (15′ upper‐arm ischemia and 15′ reperfusion). The 1‐day IPC protocol involved FMD measurements before and after 1 episode of IPC (3 cycles of 5′ upper‐arm ischemia and 5′ reperfusion) and IR. Day 7 of the 7‐day IPC protocol was identical to the 1‐day IPC protocol but was preceded by single daily episodes of IPC for 6 days prior. During each protocol, subjects received a 7‐day treatment of either the cyclooxygenase‐2 inhibitor celecoxib or placebo. Pre‐IR FMD was similar between groups. IR alone reduced FMD post‐IR (placebo, ΔFMD: −4.4±0.7%; celecoxib, ΔFMD: −5.0±0.5%). One‐day IPC completely prevented this effect (placebo, ΔFMD: −1.1±0.6%; celecoxib, ΔFMD: 0.0±0.7%; P<0.0001). Similarly, 7‐day IPC demonstrated persistent endothelial protection post‐IR (placebo, ΔFMD: −0.9±0.9%; celecoxib, ΔFMD: 0.0±0.8%; P<0.0001, P<0.0001 for ANOVA effect of IPC protocol). Celecoxib did not alter responses to IR in any protocol.

Conclusions

Daily episodes of IPC provide sustained protection from IR‐induced endothelial dysfunction in humans through a mechanism that appears cyclooxygenase‐2‐independent.

Eribis peptide 94 reduces infarct size in rat hearts via activation of centrally located μ opioid receptors

Eribis peptide 94 (EP 94) is a novel enkephalin derivative that binds with high potency to μ and δ opioid receptors with less affinity for the κ opioid receptor. This compound has recently been shown to produce an acute reduction in myocardial infarct size in the anesthetized pig and rat partially via an endothelial nitric oxide synthase and KATP channel-dependent mechanism. EP 94 also was found to produce a chronic reduction in infarct size 24 hours postdrug administration via the upregulation of inducible nitric oxide synthase in rats. Despite these findings, no data have emerged in which the opioid receptor subtype responsible for cardioprotection has been identified and the site of action, heart, other peripheral organs, or the central nervous system, has not been addressed. In the current study, EP 94, was administered in 2 divided doses (0.5 μg/kg, intravenously) at 5 and 10 minutes into the ischemic period, and the opioid antagonists were administered 10 minutes before the onset of the 30-minute ischemic period. The selective antagonists used were the μ receptor antagonist CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2), the δ receptor antagonists naltrindole and BNTX (7-benzylidenenaltrexone), and the κ receptor antagonist nor-BNI (norbinaltorphimine). Surprisingly, only CTOP completely blocked the cardioprotective effect of EP 94, whereas naltrindole, BNTX, and nor-BNI had modest but nonsignificant effects. Because there is controversial evidence suggesting that μ receptors may be absent in the adult rat myocardium, it was hypothesized that the protective effect of EP 94 may be mediated by an action outside the heart, perhaps in the central nervous system. To test this hypothesis, rats were pretreated with the nonselective opioid antagonist, naloxone hydrochloride, which penetrates the blood-brain barrier or naloxone methiodide, the quaternary salt of naloxone hydrochloride, which does not penetrate the blood-brain barrier before EP 94 administration. In support of a central nervous system site of action for EP 94, naloxone hydrochloride completely blocked its cardioprotective effect, whereas naloxone methiodide had no effect. These results suggest that EP 94 reduces infarct size (expressed as a percent of the area at risk) in the rat primarily via activation of central μ opioid receptors.

Pathophysiology of myocardial reperfusion injury: preconditioning, postconditioning, and translational aspects of protective measures

Heart diseases due to myocardial ischemia, such as myocardial infarction or ischemic heart failure, are major causes of death in developed countries, and their number is unfortunately still growing. Preliminary exploration into the pathophysiology of ischemia-reperfusion injury, together with the accumulation of clinical evidence, led to the discovery of ischemic preconditioning, which has been the main hypothesis for over three decades for how ischemia-reperfusion injury can be attenuated. The subcellular pathophysiological mechanism of ischemia-reperfusion injury and preconditioning-induced cardioprotection is not well understood, but extensive research into components, including autacoids, ion channels, receptors, subcellular signaling cascades, and mitochondrial modulators, as well as strategies for modulating these components, has made evolutional progress. Owing to the accumulation of both basic and clinical evidence, the idea of ischemic postconditioning with a cardioprotective potential has been discovered and established, making it possible to apply this knowledge in the clinical setting after ischemia-reperfusion insult. Another a great outcome has been the launch of translational studies that apply basic findings for manipulating ischemia-reperfusion injury into practical clinical treatments against ischemic heart diseases. In this review, we discuss the current findings regarding the fundamental pathophysiological mechanisms of ischemia-reperfusion injury, the associated protective mechanisms of ischemic pre- and postconditioning, and the potential seeds for molecular, pharmacological, or mechanical treatments against ischemia-reperfusion injury, as well as subsequent adverse outcomes by modulation of subcellular signaling mechanisms (especially mitochondrial function). We also review emerging translational clinical trials and the subsistent clinical comorbidities that need to be overcome to make these trials applicable in clinical medicine.

Sustained ligand-activated preconditioning via δ-opioid receptors

We have previously described novel cardioprotection in response to sustained morphine exposure, efficacious in young to aged myocardium and mechanistically distinct from conventional opioid or preconditioning (PC) responses. We further investigate opioid-dependent sustained ligand-activated preconditioning (SLP), assessing duration of protection, opioid receptor involvement, additivity with conventional responses, and signaling underlying preischemic induction of the phenotype. Male C57BL/6 mice were treated with morphine (75-mg subcutaneous pellet) for 5 days followed by morphine-free periods (0, 3, 5, or 7 days) before ex vivo assessment of myocardial tolerance to 25-min ischemia/45-min reperfusion. SLP substantially reduced infarction (by ∼50%) and postischemic contractile dysfunction (eliminating contracture, doubling force development). Cardioprotection persisted for 5 to 7 days after treatment. SLP was induced specifically by δ-receptor and not κ- or μ-opioid receptor agonism, was eliminated by δ-receptor and nonselective antagonism, and was additive with adenosinergic but not acute morphine- or PC-triggered protection. Cotreatment during preischemic morphine exposure with the phosphoinositide-3 kinase (PI3K) inhibitor wortmannin, but not the protein kinase A (PKA) inhibitor myristoylated PKI-(14-22)-amide, prevented induction of SLP. This was consistent with shifts in total and phospho-Akt during the induction period. In summary, data reveal that SLP triggers sustained protection from ischemia for up to 7 days after stimulus, is δ-opioid receptor mediated, is induced in a PI3K-dependent/PKA-independent manner, and augments adenosinergic protection. Mechanisms underlying SLP may be useful targets for manipulation of ischemic tolerance in young or aged myocardium.

Acute methadone treatment reduces myocardial infarct size via the delta-opioid receptor in rats during reperfusion

BACKGROUND

Methadone is an opioid agonist often given to manage acute and chronic pain. We sought to determine whether methadone compared with morphine dose dependently reduces myocardial infarct size (IS) and whether the mechanism is delta-opioid receptor mediated. Furthermore, we examined whether myocardial IS reduction varies with the timing of methadone administration or duration of induced ischemia.

METHODS

After surgical instrumentation, we divided male Sprague-Dawley rats into 3 sets. The first set was divided into groups, which received methadone (0.03-3 mg/kg), morphine (0.03-3 mg/kg), or water (placebo) 30 min before ischemia. Some animals of the first set also received the delta-opioid antagonist naltrindole (5 mg/kg) before methadone (0.3 mg/kg), morphine (0.3 mg/kg), or placebo administration. The second set of animals was divided into groups that received methadone (0.3 mg/kg) 5 min before reperfusion or 10 s after reperfusion. These 2 sets of animals were subjected to 30 min of myocardial ischemia by left anterior descending coronary artery occlusion and then 2 h of reperfusion. The third set of animals received placebo, methadone (0.3 mg/kg), or morphine (0.3 mg/kg) 5 min before reperfusion and were subjected to 45 min of ischemia by left anterior descending coronary artery occlusion with 2 h of reperfusion. Myocardial IS was assessed by staining myocardial tissue with triphenyltetrazolium chloride and expressed as a percentage of the area at risk (mean +/- sem).

RESULTS

Methadone or morphine administered before ischemia reduced myocardial IS. The greatest effect was achieved at a dose of 0.3 mg/kg (methadone, 46% +/- 1%, P < 0.001 and morphine, 47% +/- 1%, P < 0.001 versus placebo, 61% +/- 1%, respectively). Naltrindole (5 mg/kg) blocked methadone-induced (0.3 mg/kg) and morphine-induced (0.3 mg/kg) cardioprotection (naltrindole + methadone, 58% +/- 1%, P < 0.001 versus methadone; and naltrindole + morphine, 58 +/- 1%, P < 0.001 versus morphine). Methadone (0.3 mg/kg) reduced myocardial IS when given 5 min before reperfusion (46% +/- 1%, P < 0.001 versus placebo) but not 10 s after reperfusion (60% +/- 1%, P = 0.675 versus placebo). No significant myocardial IS differences were seen for placebo when comparing the 45-min ischemia group (64% +/- 1%) with the 30-min ischemia group (60% +/- 1%, P = 0.069). The longer ischemia time of 45 min abrogated methadone-induced IS reduction (64% +/- 2%, P = 0.867 versus 45-min ischemia placebo group) and morphine-induced IS reduction (65% +/- 1%, P = 0.836 versus 45-min ischemia placebo group).

CONCLUSIONS

These findings demonstrate that methadone and morphine produce similar myocardial IS-sparing effects that are delta-opioid receptor mediated and that are dependent on the duration of myocardial ischemia.

Pre-medication and renal pre-conditioning: a role for alprazolam, atropine, morphine and promethazine

Four pre-medication drugs are used to relieve pain, allay anxiety, reduce secretion and enhance hypnosis, were evaluated for their effects on ischemia reperfusion (I/R) injury which is one of the major complications of vascular and transplantation surgery. Right kidney was removed from female rats (210-250 g) 3 weeks before surgical procedure. Different doses of morphine (0.5, 2 and 5 mg/kg), promethazine (1, 2 and 5 mg/kg), atropine (0.1, 0.3 and 0.5 mg/kg) and alprazolam (0.08, 0.32 and 0.64 mg/kg) were administered subcutaneously 30 min before left renal artery occlusion and 6 h reperfusion. Left kidneys were processed for histological evaluations. Creatinine and BUN were measured in serum samples. Morphine, promethazine, atropine and alprazolam at all evaluated doses significantly decreased serum creatinine and BUN levels and histopathological scores. The effects of promethazine (1 mg/kg) and all doses of alprazolam were more potent than other pre-medication drugs and doses. This study suggested a protective effect of these pre-medication drugs on I/R injury. Although obvious studies are required, these findings may lead to effective therapies against I/R injury.

Long-acting phosphodiesterase-5 inhibitor, tadalafil, induces sustained cardioprotection against lethal ischemic injury

The ability of pharmacological preconditioning mimetics to confer long-lasting and sustained cardioprotection may be a logical criterion to develop a drug that can be used clinically for cardioprotection. We propose here that the use of long-acting phosphodiesterase-5 inhibitor, tadalafil, may confer sustained cardioprotection against ischemia. Tadalafil (5 mg/kg) was administered orally to male C57B/6J mice (n = 6 in each treatment subgroup at each time point studied). Hearts were isolated and subjected to 40 min of ischemia and 30 min of reperfusion on Langendorff's apparatus at 1, 12, 24, 36, 48, 60, 72, and 108 h after tadalafil administration. In 1- to 48-h subgroups, tadalafil was given once at 0 h only. In 60- and 72-h subgroups, tadalafil was given twice at 0 and 36 h. Similarly, in the 108-h subgroup, tadalafil was administered at 0, 36, and 72 h. In the same subgroups, wortmannin (15 microg/kg ip), an inhibitor of phosphatidylinositol 3-kinase or 5-hydroxydecanoic acid (5 mg/kg ip), an inhibitor of mitochondrial ATP-sensitive K(+) channels, was given together with tadalafil, and the hearts were subjected to ischemia-reperfusion at 36 h to determine whether the effect of tadalafil on ischemia-reperfusion injury was abolished. As a result, tadalafil treatment reduced left ventricular end-diastolic pressure and increased left ventricular developed pressure as well as reduced lactate dehydrogenase release. This protection remained till 36-40 h, and thereafter it vanished. The readministration of tadalafil at 36 and 72 h restored the protection till 108 h. Tadalafil treatment accelerated Akt phosphorylation in cardiac tissue and decreased myocyte apoptosis. The administration of wortmannin abolished the beneficial effects of tadalafil on hemodynamic parameters and myocyte apoptosis, together with significantly reduced Akt phosphorylation. 5-Hydroxydecanoic acid also abolished the antiapoptotic effect of tadalafil. It is concluded that tadalafil treatment induces the long-term protection of ischemic myocardium via phosphatidylinositol 3-kinase/Akt signaling pathway.

HIV-1 Tat and morphine have interactive effects on oligodendrocyte survival and morphology

Human immunodeficiency virus (HIV)-infected individuals who abuse opiates show faster progression to AIDS, and enhanced incidence of HIV-1 encephalitis. Most opiates with abuse liability are preferential agonists for mu-opioid receptors (MORs), and MORs are expressed on both neurons and glia, including oligodendrocytes (OLs). Tat, gp120, and other viral toxins, cause neurotoxicity in vitro and/or when injected into brain, and co-exposure to opiates can augment HIV-1 protein-induced insults to both glial and neuronal populations. We examined the effects of HIV-1 Tat +/- opiate exposure on OL survival and differentiation. In vivo studies utilized transgenic mice expressing Tat(1-86) regulated by an inducible glial fibrillary acidic protein promoter. Although MBP levels were unchanged on immunoblots, certain structural and apoptotic indices were abnormal. After only 2 days of Tat induction, OLs showed an upregulation of active caspase-3 that was enhanced by morphine exposure. Tat also upregulated TUNEL staining, but only in the presence of morphine. Tat significantly reduced the length of processes in Golgi-Kopsch impregnated OLs. A greater proportion of cells exhibited diminished or aberrant cytoplasmic processes, especially when mice expressing Tat were co-exposed to morphine. Collectively, our data show that OLs in situ are extremely sensitive to effects of Tat +/- morphine, although it is not clear if immature OLs as well as differentiated OLs are targeted equally. Significant elevations in caspase-3 activity and TUNEL labeling, and evidence of increased degeneration/regeneration of OLs exposed to Tat +/- morphine suggest that toxicity toward OLs may be accompanied by heightened OL turnover.

Morphine causes rapid increases in glial activation and neuronal injury in the striatum of inducible HIV-1 Tat transgenic mice

HIV encephalitis (HIVE) is accompanied by brain inflammation, leukocyte infiltration, and glial activation, and HIV patients who abuse opiates are more likely to develop HIVE. To better understand how opiates could alter HIV-related brain inflammation, the expression of astrocyte (GFAP immunoreactivity) and macrophage/microglial (F4/80 or Mac1 immunoreactivity) markers in the striatum, and the percentage of 3-nitrotyrosine (3-NT) positive macrophages/microglia, was determined following a 2-day exposure to morphine (5 mg/kg/day via time-release, subcutaneous implant) and doxycycline in GFAP-driven, doxycycline-inducible HIV-1 Tat transgenic mice. Data show that both morphine and Tat induction via doxycycline increased astrocyte activation, with significant additive increases achieved with combined morphine and doxycycline exposure. By contrast, combined Tat induction and morphine exposure, but neither manipulation alone, significantly increased the proportion of macrophages/microglia present in the striatum of transgenic mice, although morphine exposure was necessary to elevate 3-NT co-detection in Mac1-positive macrophages/microglia. Finally, Tat induction increased the percentage of neurons expressing active caspase-3, and this was even more significantly elevated by co-administration of morphine. In spite of elevations in caspase-3, neuronal TUNEL reactivity was unchanged in all groups, even after 10 days of Tat induction. Importantly, co-administration of naltrexone completely antagonized the effects of morphine. These findings indicate that morphine rapidly and significantly increases the activation of astrocytes and macrophages/microglia in the brains of inducible Tat transgenic mice, supporting the theory that early inflammatory changes in glia could underlie the development of HIVE in opiate-abusing AIDS patients.

Sustained cardioprotection: exploring unconventional modalities

Since Murry et al. [Murry, C.E., Jennings, R.B., Reimer, K.A., 1986. Preconditioning with ischemia: a delay of lethal cell injury in ischemic myocardium. Circulation. 74, 1124-36.] initially reported on the powerful protective effects of ischemic preconditioning (PC), a plethora of experimental investigations have identified varied preconditioning protocols or mimetics to achieve cardioprotection. These stimuli predominantly act via archetypal mediators identified in associated signalling studies (including PI3-K, Akt, PKC, mitochondrial K(ATP) channels). Despite an intense research effort over the last 20 years, there remains a paucity of evidence that this protective paradigm is clinically exploitable. This may arise due to a number of drawbacks to conventional protection, including effects of age, disease, and interactions with other pharmacological agents. This encourages investigation of alternate strategies that trigger protection via unconventional signalling (distinct from conventional PC) and/or mediate sustained shifts in ischemic tolerance in hearts of varying age and disease status. This review considers briefly drawbacks to conventional PC, and focuses on alternate strategies for generating prolonged states of cardiac protection.

Morphine dependence protects rat kidney against ischaemia-reperfusion injury

Ischaemic preconditioning (IPC) protects the heart and kidneys against ischaemia-reperfusion (I/R) injury. It has been shown that opioid receptor activation can mimic cardiac IPC. In a kidney model of I/R, a single dose of morphine failed to mimic IPC. The aim of the present study was to determine the role of chronic morphine (dependence) in protection against renal I/R injury. Male Wistar rats were treated with increasing doses of morphine (20-30 mg/kg per day, s.c., for 5 days) to develop morphine dependence (MD). Three weeks before the I/R procedure, the right kidney was removed. Ischaemia-reperfusion injury was induced by clamping the left renal artery for 45 min, followed by 24 h reperfusion. Some MD rats were pretreated with naloxone (5 mg/kg, s.c.). Twenty-four hours later, creatinine and sodium concentrations were measured in serum and urine, then creatinine clearance (CCr) and the fractional excretion of sodium (FE(Na)) were calculated. Blood urea nitrogen (BUN) was measured only in serum samples. Kidneys were also assessed histologically for evidence of tissue injury. In the present study, MD decreased tissue injury (histological score), serum creatinine and BUN levels, increased CCr and decreased FE(Na) after I/R. Pretreatment with naloxone attenuated the protective effects of MD. Morphine dependence did not have any significant effect on urine volume. In conclusion, it seems that morphine dependence protects the kidney against I/R injury via opioid receptor-dependent pathways. Further studies are required to clearly determine the mechanisms involved.

Interaction of cardiovascular risk factors with myocardial ischemia/reperfusion injury, preconditioning, and postconditioning

Therapeutic strategies to protect the ischemic myocardium have been studied extensively. Reperfusion is the definitive treatment for acute coronary syndromes, especially acute myocardial infarction; however, reperfusion has the potential to exacerbate lethal tissue injury, a process termed "reperfusion injury." Ischemia/reperfusion injury may lead to myocardial infarction, cardiac arrhythmias, and contractile dysfunction. Ischemic preconditioning of myocardium is a well described adaptive response in which brief exposure to ischemia/reperfusion before sustained ischemia markedly enhances the ability of the heart to withstand a subsequent ischemic insult. Additionally, the application of brief repetitive episodes of ischemia/reperfusion at the immediate onset of reperfusion, which has been termed "postconditioning," reduces the extent of reperfusion injury. Ischemic pre- and postconditioning share some but not all parts of the proposed signal transduction cascade, including the activation of survival protein kinase pathways. Most experimental studies on cardioprotection have been undertaken in animal models, in which ischemia/reperfusion is imposed in the absence of other disease processes. However, ischemic heart disease in humans is a complex disorder caused by or associated with known cardiovascular risk factors including hypertension, hyperlipidemia, diabetes, insulin resistance, atherosclerosis, and heart failure; additionally, aging is an important modifying condition. In these diseases and aging, the pathological processes are associated with fundamental molecular alterations that can potentially affect the development of ischemia/reperfusion injury per se and responses to cardioprotective interventions. Among many other possible mechanisms, for example, in hyperlipidemia and diabetes, the pathological increase in reactive oxygen and nitrogen species and the use of the ATP-sensitive potassium channel inhibitor insulin secretagogue antidiabetic drugs and, in aging, the reduced expression of connexin-43 and signal transducer and activator of transcription 3 may disrupt major cytoprotective signaling pathways thereby significantly interfering with the cardioprotective effect of pre- and postconditioning. The aim of this review is to show the potential for developing cardioprotective drugs on the basis of endogenous cardioprotection by pre- and postconditioning (i.e., drug applied as trigger or to activate signaling pathways associated with endogenous cardioprotection) and to review the evidence that comorbidities and aging accompanying coronary disease modify responses to ischemia/reperfusion and the cardioprotection conferred by preconditioning and postconditioning. We emphasize the critical need for more detailed and mechanistic preclinical studies that examine car-dioprotection specifically in relation to complicating disease states. These are now essential to maximize the likelihood of successful development of rational approaches to therapeutic protection for the majority of patients with ischemic heart disease who are aged and/or have modifying comorbid conditions.

Myocardial protection in man--from research concept to clinical practice

Myocardial protection aims at preventing myocardial tissue loss: (a) In the acute stage, i.e., during primary angioplasty in acute myocardial infarction. In this setup, the attenuation of reperfusion injury is the main target. As a "mechanical" means, post-conditioning has already been tried in man with encouraging results. Pharmacologic interventions that could be of promise are statins, insulin, peptide hormones, including erythropoietin, fibroblast growth factor, and many others. (b) The patient with chronic coronary artery disease offers another paradigm, with the target of avoidance of further myocyte loss through apoptosis and inflammation. Various pharmacologic agents may prove useful in this context, together with exercise and "mechanical" improvement of cardiac function with attenuation of myocardial stretch, which by itself is a noxious influence. A continuous effort toward acute and chronically preserving myocardial integrity is a concept concerning both the researcher and the clinician.

Mediating delta-opioid-initiated heart protection via the beta2-adrenergic receptor: role of the intrinsic cardiac adrenergic cell

Stimulation of cardiac beta(2)-adrenergic receptor (beta(2)-AR) or delta-opioid receptor (DOR) exerts a similar degree of cardioprotection against myocardial ischemia in experimental models. We hypothesized that delta-opioid-initiated cardioprotection is mediated by the intrinsic cardiac adrenergic (ICA) cell via enhanced epinephrine release. Using immunohistochemical and in situ hybridization methods, we detected in situ tyrosine hydroxylase (TH) mRNA and TH immunoreactivity that was colocalized with DOR immunoreactivity in ICA cells in human and rat hearts. Western blot analysis detected DOR protein in ICA cells isolated from rat ventricular myocytes. The physiology of DOR expression was examined by determining changes of cytosolic Ca(2+) concentration ([Ca(2+)](i)) transients in isolated rat ICA cells using fluorescence spectrophotometry. Exposing the selective delta-opioid agonist D-[Pen(2,5)]enkephalin (DPDPE) to ICA cells increased [Ca(2+)](i) transients in a concentration-dependent manner. Such an effect was abolished by the Ca(2+) channel blocker nifedipine. HPLC-electrochemical detection demonstrated a 2.4-fold increase in epinephrine release from ICA cells following DPDPE application. The significance of the ICA cell and its epinephrine release in delta-opioid-initiated cardioprotection was demonstrated in the rat myocardial infarction model and ICA cell-ventricular myocyte coculture. DPDPE administered before coronary artery occlusion or simulated ischemia-reperfusion reduced left ventricular infarct size by 54 +/- 15% or myocyte death by 26 +/- 4%, respectively. beta(2)-AR blockade markedly attenuated delta-opioid-initiated infarct size-limiting effect and abolished delta-opioid-initiated myocyte survival protection in rat ICA cell-myocyte coculture. Furthermore, delta-opioid agonist exerted no myocyte survival protection in the absence of cocultured ICA cells during ischemia-reperfusion. We conclude that delta-opioid-initiated myocardial infarct size reduction is primarily mediated via endogenous epinephrine/beta(2)-AR signaling pathway as a result of ICA cell activation.

Impaired p38 MAPK/HSP27 signaling underlies aging-related failure in opioid-mediated cardioprotection

Cardioprotection and preconditioning mediated via G-protein-coupled receptors may be lost or impaired with advancing age, limiting ischemic tolerance and the ability to pharmacologically protect older hearts from ischemic injury. Our preliminary findings indicated a loss of delta-opioid receptor-mediated protection in aged vs. young mouse hearts, which may involve alterations in protective kinase signaling. In the present study, we tested the hypothesis that aging-related loss of opioid-triggered cardioprotection involves failure to activate p38 MAPK and its distal signaling targets. Langendorff-perfused hearts from young (10-14 weeks) or aged (24-26 months) C57 mice underwent 25-min ischemia and 45-min reperfusion in the presence or absence of 1 micromol/l DPDPE (delta-opioid agonist) or 1 micromol/l anisomycin (activator of p38 MAPK), and functional recovery and protein activation/phosphorylation were assessed. Contractile recovery was similar in untreated young and aged hearts (50+/-2% and 53+/-5%, respectively), and was enhanced by DPDPE in young hearts only (67+/-3%). Immunoblot analysis revealed that DPDPE comparably activated or phosphorylated GRK2, Akt, ERK1/2 and p70S6 kinase in young and aged hearts, whereas aging abrogated the stimulatory effects of DPDPE on p38 MAPK and HSP27. Treatment with anisomycin elicited comparable activation of p38 MAPK and HSP27 in both young and aged hearts, coupled with a pronounced and equivalent cardioprotection in the two groups (73+/-3% and 77+/-2%, respectively), an effect abolished by the p38 MAPK inhibitor, SB203580. These data indicate that aging-related loss of delta-opioid-mediated cardioprotection involves failure to activate p38 MAPK and HSP27. Direct targeting of this pathway elicits comparable protection in both age groups.

Systematic evaluation of a novel model for cardiac ischemic preconditioning in mice

Cardioprotection by ischemic preconditioning (IP) remains an area of intense investigation. To further elucidate its molecular basis, the use of transgenic mice seems critical. Due to technical difficulty associated with performing cardiac IP in mice, we developed an in situ model for cardiac IP using a hanging-weight system for coronary artery occlusion. This technique has the major advantage of eliminating the necessity of intermittently occluding the coronary artery with a knotted suture. To systematically evaluate this model, we first demonstrated correlation of ischemia times (10–60 min) with infarct sizes [3.5 ± 1.3 to 42 ± 5.2% area at risk (AAR), Evan’s blue/triphenyltetrazolium chloride staining]. IP (4 × 5 min) and cold ischemia (27°C) reduced infarct size by 69 ± 6.7% and 84 ± 4.2%, respectively ( n = 6, P < 0.01). In contrast, lower numbers of IP cycles did not alter infarct size. However, infarct sizes were distinctively different in mice from different genetic backgrounds. In addition to infarct staining, we tested cardiac troponin I (cTnI) as marker of myocardial infarction in this model. In fact, plasma levels of cTnI were significantly lower in IP-treated mice and closely correlated with infarct sizes ( R2 = 0.8). To demonstrate transcriptional consequences of cardiac IP, we isolated total RNA from the AAR and showed repression of the equilibrative nucleoside transporters 1–4 by IP in this model. Taken together, this study demonstrates highly reproducible infarct sizes and cardiac protection by IP, thus minimizing the variability associated with knot-based coronary occlusion models. Further studies on cardiac IP using transgenic mice may consider this technique.

Cardioprotective effects of acute and chronic opioid treatment are mediated via different signaling pathways

A 5-day exposure to morphine exerts a profound cardioprotective phenotype in murine hearts. In the present study, we examined mechanisms by which morphine generates this effect, exploring the roles of Gi and Gs proteins, PKA, PKC, and β-adrenergic receptors (β-AR) in acute and chronic opioid preconditioning. Langendorff-perfused hearts from placebo, acute morphine (AM; 10 μmol/l)-, or chronic morphine (CM)-treated mice (75-mg pellet, 5 days) underwent 25-min ischemia and 45-min reperfusion. After reperfusion, placebo-treated hearts exhibited marked contractile and diastolic dysfunction [rate-pressure product (RPP), 40 ± 4% baseline; end-diastolic pressure (EDP), 33 ± 3 mmHg], whereas AM hearts showed significant improvement in recovery of RPP and EDP (60 ± 3% and 23 ± 4 mmHg, respectively; P < 0.05 vs. placebo). Furthermore, CM hearts demonstrated a complete return of diastolic function and significantly greater recovery of contractile function (83 ± 3%, P < 0.05 vs. both placebo and AM). Pretreatment with Gi protein inhibitor pertussis toxin abolished AM protection while partially attenuating CM recovery ( P < 0.05 vs. placebo). Treatment with Gs inhibitor NF-449 did not affect AM preconditioning yet completely abrogated CM preconditioning. Similarly, PKA inhibition significantly attenuated the ischemia-tolerant state afforded by CM, whereas it was ineffective in AM hearts. PKC inhibition with chelerythrine was ineffective in CM hearts while completely abrogating AM preconditioning. Moreover, whereas β1-AR blockade with CGP-20712A failed to alter recovery in CM hearts, the β2-AR antagonist ICI-118,551 significantly attenuated postischemic recovery. These data describe novel findings whereby CM preconditioning is mediated by a PKC-independent pathway involving PKA, β2-AR, and Gs proteins, whereas AM preconditioning is mediated via Gi proteins and PKC.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Immunomodulatory effect of morphine: therapeutic implications

The immunosuppressive as well as modulatory effects of morphine have been known in clinical medicine for > 100 years. Recent developments in molecular immunology, including experiments in mu (mu) opioid receptor knockout mice has led to a better understanding of central and peripheral mechanisms involved in this process. Though there is a large volume of literature documenting adverse effects of immunosupression following the use of morphine, several reports confirm its potential usefulness as an immunomodulator. In vitro and in vivo animal experiments have demonstrated wide-spectrum effects of morphine, including anti-inflammatory, antifibrotic, antitumour, cardioprotective and renoprotective. Immunomodulation is an important field in modern medicine with rapid advancement in recent years. Though a final statement regarding the clinical relevance of morphine-induced immunomodulation cannot be made at this juncture, nevertheless, it is worthwhile to review current developments. It may encourage further clinical studies to elucidate the influence of morphine treatment on immune regulation in different specialties of medicine.

Opioid-induced preconditioning: recent advances and future perspectives

Opioids, named by Acheson for compounds with morphine-like actions despite chemically distinct structures, have received much research interest, particularly for their central nervous system (CNS) actions involved in pain management, resulting in thousands of scientific papers focusing on their effects on the CNS and other organ systems. A more recent area which may have great clinical importance concerns the role of opioids, either endogenous or exogenous compounds, in limiting the pathogenesis of ischemia-reperfusion injury in heart and brain. The role of endogenous opioids in hibernation provides tantalizing evidence for the protective potential of opioids against ischemia or hypoxia. Mammalian hibernation, a distinct energy-conserving state, is associated with depletion of energy stores, intracellular acidosis and hypoxia, similar to those which occur during ischemia. However, despite the potentially detrimental cellular state induced with hibernation, the myocardium remains resilient for many months. What accounts for the hypoxia-tolerant state is of great interest. During hibernation, circulating levels of opioid peptides are increased dramatically, and indeed, are considered a "trigger" of hibernation. Furthermore, administration of opioid antagonists can effectively reverse hibernation in mammals. Therefore, it is not surprising that activation of opioid receptors has been demonstrated to preserve cellular status following a hypoxic insult, such as ischemia-reperfusion in many model systems including the intestine [Zhang, Y., Wu, Y.X., Hao, Y.B., Dun, Y. Yang, S.P., 2001. Role of endogenous opioid peptides in protection of ischemic preconditioning in rat small intestine. Life Sci. 68, 1013-1019], skeletal muscle [Addison, P.D., Neligan, P.C., Ashrafpour, H., Khan, A., Zhong, A., Moses, M., Forrest, C.R., Pang, C.Y., 2003. Noninvasive remote ischemic preconditioning for global protection of skeletal muscle against infarction. Am. J. Physiol. Heart Circ. Physiol. 285, H1435-H1443], the CNS [Borlongan, C.V., Wang, Y., Su, T.P., 2005. Delta opioid peptide (d-ala 2, d-leu 5) enkephalin: linking hiberation and neuroprotection. Front Biosci. 9, 3392-3398] and the myocardium [Romano, M.A., Seymour, E.M., Berry, J.A., McNish, R.A., Bolling, S.F., 2004. Relative contribution of endogenous opioids to myocardial ischemic tolerance. J Surg Res. 118, 32-37; Peart, J.N., Gross, G.J., 2004a. Exogenous activation of delta- and kappa-opioid receptors affords cardioprotection in isolated murine heart. Basic Res Cardiol. 99(1), 29-37]. For the purpose of this review, we will focus primarily on the protective effects of opioids against post-reperfusion myocardial stunning and infarction.

Chronic exposure to morphine produces a marked cardioprotective phenotype in aged mouse hearts

Aging is often associated with decreased myocardial ischemic tolerance. We recently reported that chronic preconditioning produced by continuous exposure to morphine affords a profound cardioprotective phenotype in young mice. In this study, we determined if chronic exposure to morphine retained its ability to precondition the myocardium in the young or aged heart. Young (10-14 weeks) or aged (24-26 months) C57/BL6 mice were untreated, administered morphine acutely (30 microM), or implanted with a morphine pellet (75 mg) for 5 days prior to heart isolation and perfusion. Following equilibration, perfused hearts were subjected to 25 min ischemia and 45 min reperfusion. Untreated hearts from both young and aged mice displayed marked contractile dysfunction and LDH release following reperfusion. Acute infusion of morphine improved recovery of end-diastolic pressure and developed pressure in young (P < 0.05 vs. untreated) but not senescent hearts. Hearts from mice exposed to morphine for 5 days displayed a further improvement in post-ischemic contractile function (P < 0.05 vs. acute treatment), and a marked reduction in post-ischemic LDH efflux (P < 0.05 vs. untreated) in both young and senescent hearts. These data demonstrate that aged hearts maintain the ability to be preconditioned by chronic exposure to morphine in the absence of acute protection.