Neuroprotection and functional recovery conferred by administration of kappa- and delta1-opioid agonists in a rat model of global ischemia

Effect of U50488, a selective kappa opioid receptor agonist and levetiracetam against lithium-pilocarpine-induced status epilepticus, spontaneous convulsive seizures and related cognitive impairment

PURPOSE

Kappa opioid receptor (KOR) agonists have anticonvulsant effect but their antiepileptogenic effect is unknown. U50488, a selective KOR agonist is used to determine its effect on status epilepticus (SE), spontaneous convulsive seizures (SS) and cognitive impairment in rat lithium-pilocarpine SE model. Effect of an antiepileptic drug levetiracetam is also studied.

METHOD

Male Wistar rats with SE were divided into three groups namely, LiP, LiP + U50488 (10 mg/kg, i.p.) and LiP + levetiracetam (400 mg/kg, i.p.) group. SE was terminated after 90 min of its onset with diazepam (15 mg/kg, i.p.) and phenobarbitone (25 mg/kg, i.p.). Drug treatment was started after 15 min of onset of SE and repeated once after 4 h. Rats were video monitored 12 h daily (9 AM to 9 PM) to determine severity of SE using modified Racine scale and onset and frequency of SS from day 0 to day 21. Morris water maze (MWM) test was done at baseline i.e. day -1 (before lithium administration) and day 22, to assess cognitive impairment.

RESULTS

As compared to LiP, U50488 decreased the severity of SE (1.98 ± 0.13 vs 2.95 ± 0.12; p-value < 0.0001) but not levetiracetam (2.62 ± 0.09; p-value = 0.3112). Survival increased with both U50488 (90%, n = 10) and levetiracetam (81.8%, n = 11) as compared to NS (56.2%, n = 16). No effect on onset and frequency of SS was found in U50488/levetiracetam group. U50488 improved seizures-induced cognitive impairment. Levetiracetam group showed thigmotactic (wall hugging) behaviour in MWM in 8 out of 9 rats.

CONCLUSION

Acute treatment with U50488, a kappa opioid receptor agonist has a beneficial effect on SE, SE-related mortality and memory impairment. The dual protective effect of U50488 on seizures and related cognitive impairment is advantageous over currently used antiseizure drugs which are known to cause cognitive impairment.

The Kappa Opioid Receptor: A Promising Therapeutic Target for Multiple Pathologies

Kappa-opioid receptors (KOR) are widely expressed throughout the central nervous system, where they modulate a range of physiological processes depending on their location, including stress, mood, reward, pain, inflammation, and remyelination. However, clinical use of KOR agonists is limited by adverse effects such as dysphoria, aversion, and sedation. Within the drug-development field KOR agonists have been extensively investigated for the treatment of many centrally mediated nociceptive disorders including pruritis and pain. KOR agonists are potential alternatives to mu-opioid receptor (MOR) agonists for the treatment of pain due to their anti-nociceptive effects, lack of abuse potential, and reduced respiratory depressive effects, however, dysphoric side-effects have limited their widespread clinical use. Other diseases for which KOR agonists hold promising therapeutic potential include pruritis, multiple sclerosis, Alzheimer's disease, inflammatory diseases, gastrointestinal diseases, cancer, and ischemia. This review highlights recent drug-development efforts targeting KOR, including the development of G-protein-biased ligands, mixed opioid agonists, and peripherally restricted ligands to reduce side-effects. We also highlight the current KOR agonists that are in preclinical development or undergoing clinical trials.

Sex differences in the rodent hippocampal opioid system following stress and oxycodone associated learning processes

Over the past two decades, opioid abuse has risen especially among women. In both sexes hippocampal neural circuits involved in associative memory formation and encoding of motivational incentives are critically important in the transition from initial drug use to drug abuse/dependence. Opioid circuits, particularly the mossy fiber pathway, are crucial for associative memory processes important for addiction. Our anatomical studies, especially those utilizing electron microscopic immunocytochemistry, have provided unique insight into sex differences in the distribution of opioid peptides and receptors in specific hippocampal circuits and how these distributions are altered following stress and oxycodone-associative learning processes. Here we review the hippocampal opioid system in rodents with respect to ovarian hormones effects and baseline sex differences then sex differences following acute and chronic stress. Next, we review sex differences in the hippocampal opioid system in unstressed and chronically stressed rats following oxycodone conditioned place preference. We show that opioid peptides and receptors are distributed within hippocampal circuits in females with elevated estrogen states in a manner that would enhance sensitivity to endogenous and exogenous opioids. Moreover, chronic stress primes the opioid system in females in a manner that would promote opioid-associative learning processes. In contrast, chronic stress has limited effects on the opioid system in males and reduces its capacity to support opioid-mediated learning processes. Interestingly, acute stress appears to prime males for opioid associative learning. On a broader scale the findings highlighted in this review have important implications in understanding sex differences in opioid drug use and abuse.

κ-Opioid Receptor Agonist Ameliorates Postoperative Neurocognitive Disorder by Activating the Ca2+/CaMKII/CREB Pathway

Objective

Cardiopulmonary bypass (CPB) is an important cardiac operation and also a high-risk procedure, leading to postoperative neurocognitive disorder. However, there are few effective drugs to treat the aftermath of CPB. Therefore, we observe the effect of kappa opioid receptor (KOR) agonist on cognitive disorders of rats after cardiopulmonary bypass (CPB) and investigate the mechanism of the Ca²⁺/calmodulin-dependent protein kinase (CaMKII)/cAMP responsive element-binding protein (CREB) pathway.

Methods

A total of 40 Sprague Dawley rats were randomly divided into the sham operation group (sham group, n = 10), CPB model group (CPB group, n = 10), CPB + KOR agonist U50488H group (UH group, n = 10), and CPB + specific CaMKII antagonist + U50488H group (CKU group, n = 10). The changes in the rats' cognitive function were evaluated using the Morris water maze, the hippocampal histopathological changes were observed via hematoxylin-eosin (H&E) staining, and the apoptosis rate of neuronal cells was detected through terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) staining. Moreover, enzyme-linked immunosorbent assay (ELISA) was applied to examine the changes in brain injury markers, inflammatory factors, and oxidative stress factors. The hippocampal variations in Ca²⁺ concentration and oxidative stress index (ROS) levels were measured by immunofluorescence staining, and western blotting was performed to determine the expression changes in the Ca²⁺/CaMKII/CREB pathway.

Results

The KOR agonist could shorten latency, increase the swimming distance and residence time in the target quadrant, and ameliorate postoperative neurocognitive disorder (PND). Meanwhile, the KOR agonist relieved CPB-induced hippocampal and oxidative stress injuries, reduced NSE and S-100β expression, decreased the apoptosis rate, and repressed the inflammatory response, which alleviated the brain injury. In addition, U50488H was able to decrease Ca²⁺ influx and glutamate (Glu) level, inhibit N-methyl-D-aspartate receptor (NMDAR) expression, upregulate CaMKII expression, promote CREB phosphorylation, and increase the brain-derived neurotrophic factor (BDNF) level in CPB rats. However, the protective effects of KORs against PND were suppressed following the application of the CaMKII-specific antagonist.

Conclusion

The KOR agonist activates the Ca²⁺/CaMKII/CREB pathway, which improves the brain injury and relieves PND in CPB rats.

Reversing Postcardiopulmonary Bypass Associated Cognitive Dysfunction Using k-Opioid Receptor Agonists to Regulate Microglial Polarization via the NLRP3/Caspase-1 Pathway

Cardiopulmonary bypass (CPB) is mainly used during cardiac surgeries that treat ischemic, valvular, or congenital heart disease and aortic dissections. The disorders of central nervous system (CNS) that occur after cardiopulmonary bypass are attracting considerable interest. Postoperative neurocognitive disorders (PND) have been reported as the leading cause of patients' disability and death following CPB. The k-opioid receptor (KOR) agonists (U50488H) have been suggested to be vital in the treatment of surgically induced CNS neuroinflammatory responses. In this article, the transitions between the M1 and M2 microglial polarization state phenotypes were hypothesized to significantly affect the regulatory mechanisms of KOR agonists on postcardiopulmonary bypass (post-CPB) neuroinflammation. We investigated the effects of U50488H on neuroinflammation and microglia polarization in rats exposed to CPB and explored the method of the NLRP3/caspase-1 pathway. Thirty SD rats were randomly divided into three groups: sham operation group, cardiopulmonary bypass model group, and CPB+ k-opioid receptor agonist (U50488H) group, with ten rats in each group. The Morris water maze was used to evaluate the changes in the cognitive function of CPB rats. Hematoxylin and eosin (HE) staining and TUNEL were performed to assess the rats' hippocampal damage. Enzyme-Linked Immunosorbent Assay (ELISA) was used to detect changes in brain injury markers and inflammatory factors. Furthermore, immunofluorescence was used to observe the expression of microglia polarization and NLRP3 followed by Western blots to detect the expression of the NLRP3/caspase-1 pathway and microglia polarization-related proteins. Rat microglia were cultured in vitro, with LPS stimulation, and treated with U50488H and a caspase-1 antagonist to evaluate the effects and mechanism of action of U50488H. KORs alleviated hippocampal damage caused by CPB and improved PND. CPB activated the NLRP3 inflammasome and upregulated pro-caspase-1 expression which promoted the expression of pro-IL-lβ and pro-IL-18 and resulted in increased inflammation. However, KORs also inhibited NLRP3 and transformed microglia from the M1 to the M2 state. Caspase-1 inhibitor treatment reduced the microglial polarization induced by KORs. The κ-opioid receptor agonists inhibited the inflammation mediated by microglia and improved PND through the NLRP3/caspase-1 signaling pathway.

Global cerebral ischemia in adolescent male Long Evans rats: Effects of vanillic acid supplementation on stress response, emotionality, and visuospatial memory

The developmental period is critical in delineating plastic response to internal and external events. However, neurobehavioural effects of global cerebral ischemia (GCI) in the maturing brain remain largely unknown. This study characterised the effects of GCI experienced at puberty on adulthood (1) hippocampus CA1 neuronal damage, (2) cognitive and emotional impairments, and (3) glucocorticoid receptor (GR) expression. Effects of adolescent exposure to the phenol vanillic acid (VA) on post-ischemic outcomes were also determined. Male Long Evans rats (n = 35) were supplemented for 21 consecutive days (postnatal days 33-53) with VA (91 mg/kg) or nut paste vehicle (control) prior to a 10-min GCI or sham surgery. As adults, rats were tested in the Open Field Test (OFT), Elevated-Plus Maze (EPM), and Barnes Maze (BM). GR expression was determined in the basolateral amygdala (BLA), CA1, and paraventricular nucleus (PVN), and brain injury assessed via CA1 neuronal density. Adolescent GCI exposure induced extensive hippocampal CA1 injury, which was not prevented by VA supplementation. Behaviourally, GCI increased EPM exploration while having no impact on spatial memory. VA intake increased OFT peripheral exploration. Notably, while no delayed changes in CA1 and PVN GR immunoreactivity were noted, both treatments separately increased BLA GR expression when compared with sham-nut paste rats. Age at GCI occurrence plays a critical role on post-ischemic impairments. The observation of minimal functional impairments despite important CA1 neuronal damage supports use of compensatory mechanisms. Our findings also show daily VA supplementation during adolescence to have no protective effects on post-ischemic outcomes, contrasting adult intake.

Whether short peptides are good candidates for future neuroprotective therapeutics?

Neurodegenerative diseases are a broad group of largely debilitating, and ultimately terminal conditions resulting in progressive degeneration of different brain regions. The observed damages are associated with cell death, structural and functional deficits of neurons, or demyelination. The concept of neuroprotection concerns the administration of the agent, which should reverse some of the damage or prevent further adverse changes. A growing body of evidence suggested that among many classes of compounds considered as neuroprotective agents, peptides derived from natural materials or their synthetic analogs are good candidates. They presented a broad spectrum of activities and abilities to act through diverse mechanisms of action. Biologically active peptides have many properties, including antioxidant, antimicrobial, antiinflammatory, and immunomodulatory effects. Peptides with pro-survival and neuroprotective activities, associated with inhibition of oxidative stress, apoptosis, inflammation and are able to improve cell viability or mitochondrial functions, are also promising molecules of particular interest to the pharmaceutical industries. Peptide multiple activities open the way for broad application potential as therapeutic agents or ingredients of health-promoting functional foods. Significantly, synthetic peptides can be remodeled in numerous ways to have desired features, such as increased solubility or biological stability, as well as selectivity towards a specific receptor, and finally better membrane penetration. This review summarized the most common features of major neurodegenerative disorders, their causes, consequences, and reported new neuroprotective drug development approaches. The author focused on the unique perspectives in neuroprotection and provided a concise survey of short peptides proposed as novel therapeutic agents against various neurodegenerative diseases.

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.

δ-Opioid Receptors, microRNAs, and Neuroinflammation in Cerebral Ischemia/Hypoxia

Hypoxia and ischemia are the main underlying pathogenesis of stroke and other neurological disorders. Cerebral hypoxia and/or ischemia (e.g., stroke) can lead to neuronal injury/death and eventually cause serious neurological disorders or even death in the patients. Despite knowing these serious consequences, there are limited neuroprotective strategies against hypoxic and ischemic insults in clinical settings. Recent studies indicate that microRNAs (miRNAs) are of great importance in regulating cerebral responses to hypoxic/ischemic stress in addition to the neuroprotective effect of the δ-opioid receptor (DOR). Moreover, new discovery shows that DOR can regulate miRNA expression and inhibit inflammatory responses to hypoxia/ischemia. We, therefore, summarize available data in current literature regarding the role of DOR and miRNAs in regulating the neuroinflammatory responses in this article. In particular, we focus on microglia activation, cytokine production, and the relevant signaling pathways triggered by cerebral hypoxia/ischemia. The intent of this review article is to provide a novel clue for developing new strategies against neuroinflammatory injury resulting from cerebral hypoxia/ischemia.

Delta Opioids: Neuroprotective Roles in Preclinical Studies

Since ancient times, opioids have been used clinically and abused recreationally. In the early stages (about 1,000 AD) of opium history, an Arab physician, Avicenna, administered opioids to control diarrhea and eye diseases. ¹ Opioids have very strong pain relieving properties and they also regulate numerous cellular responses. Opioid receptors are expressed throughout the body, including the nervous system, heart, lungs, liver, gastrointestinal tract, and retina. ^2-6 Delta opioid receptors (DORs) are a very attractive target from the perspective of both receptor function and their therapeutic potential. Due to a rapid progress in mouse mutagenesis and development of small molecules as DOR agonist, novel functions and roles of DORs have emerged in recent years. This review article focuses on the recent advances in the neuroprotective roles of DOR agonists in general and retina neuroprotection in particular. Rather than being exhaustive, this review highlights the selected studies of DOR function in neuroprotection. We also highlight our preclinical studies using rodent models to demonstrate the potentials of DOR agonists for retinal neuroprotection. Based on existing literature and our recently published data on the eye, DOR agonists possess therapeutic abilities that protect the retina and optic nerve injury against glaucoma and perhaps other retinopathies as well. This review also highlights the signaling events associated with DOR for neuroprotection in the eye. There is a need for translational research on DORs to recognize their potential for clinical application such as in glaucoma.

Effects of D‑Ala2, D‑Leu5‑Enkephalin pre‑ and post‑conditioning in a rabbit model of spinal cord ischemia and reperfusion injury

It has recently been revealed that during the aorta-clamped period, D-Ala², D-Leu⁵-Enkephalin (DADLE) infusion can protect the spinal cord against ischemia and reperfusion (I/R) injury. However, the protective effects of DADLE administration prior to ischemia or at the time of early reperfusion have not yet been investigated. Drug pre- or post-conditioning can serve as a more valuable clinical strategy. Therefore, the present study was designed to investigate the neuroprotective effect of DADLE infusion at different time intervals in order to determine the optimum time point for ischemic spinal cord protection. A total of 40 New Zealand white rabbits were randomly divided into 5 groups: Sham-operated (Sham), normal saline pre-conditioning (NS), DADLE per-conditioning (D_per), DADLE pre-conditioning (D_pre) and DADLE post-conditioning (D_post). All animals were subjected to spinal cord ischemia for 30 min followed by 48 h reperfusion. Hind limb motor functions were assessed according to the Tarlov criterion when the animals regained consciousness, 6, 24 and 48 h after reperfusion. Histological analysis and the number of viable α-motor neurons were also used to assess the extent of spinal cord injury. Compared with the NS group, the Tarlov scores and the number of normal neurons were significantly higher in the D_per group (P<0.05), which were consistent with the results of a previous study. In addition, the paraplegia rate and loss of normal motor neurons were lower in the DADLE per- and post-conditioning groups compared with the DADLE pre-conditioning; however, these were not statistically significant. DADLE 0.05 mg/kg administration at three time points all mitigated normal motor neuron injury in the anterior horn and decreased the paraplegia rates in rabbits. The therapeutic benefits appeared best in the post-conditioning group with DADLE, and worst in the pre-conditioning group.

Sex Differences in the Subcellular Distribution of Corticotropin-Releasing Factor Receptor 1 in the Rat Hippocampus following Chronic Immobilization Stress

Corticotropin-releasing factor receptors (CRFR1) contribute to stress-induced adaptations in hippocampal structure and function that can affect learning and memory processes. Our prior studies showed that female rats with elevated estrogens compared to males have more plasmalemmal CRFR1 in CA1 pyramidal cells, suggesting a greater sensitivity to stress. Here, we examined the distribution of hippocampal CRFR1 following chronic immobilization stress (CIS) in female and male rats using immuno-electron microscopy. Without stress, total CRFR1 dendritic levels were higher in females in CA1 and in males in the hilus; moreover, plasmalemmal CRFR1 was elevated in pyramidal cell dendrites in CA1 in females and in CA3 in males. Following CIS, near-plasmalemmal CRFR1 increased in CA1 pyramidal cell dendrites in males but not to levels of control or CIS females. In CA3 and the hilus, CIS decreased cytoplasmic and total CRFR1 in dendrites in males only. These results suggest that in naive rats, CRF could induce a greater activation of CA1 pyramidal cells in females than males. Moreover, after CIS, which leads to even greater sex differences in CRFR1 by trafficking it to different subcellular compartments, CRF could enhance activation of CA1 pyramidal cells in males but to a lesser extent than either unstressed or CIS females. Additionally, CA3 pyramidal cells and inhibitory interneurons in males have heightened sensitivity to CRF, regardless of stress state. These sex differences in CRFR1 distribution and trafficking in the hippocampus may contribute to reported sex differences in hippocampus-dependent learning processes in baseline conditions and following chronic stress.

Alterations in the corticotropin-releasing hormone (CRH) neurocircuitry: Insights into post stroke functional impairments

Although it is well accepted that changes in the regulation of the hypothalamic-pituitary adrenal (HPA) axis may increase susceptibility to affective disorders in the general population, this link has been less examined in stroke patients. Yet, the bidirectional association between depression and cardiovascular disease is strong, and stress increases vulnerability to stroke. Corticotropin-releasing hormone (CRH) is the central stress hormone of the HPA axis pathway and acts by binding to CRH receptors (CRHR) 1 and 2, which are located in several stress-related brain regions. Evidence from clinical and animal studies suggests a role for CRH in the neurobiological basis of depression and ischemic brain injury. Given its importance in the regulation of the neuroendocrine, autonomic, and behavioral correlates of adaptation and maladaptation to stress, CRH is likely associated in the pathophysiology of post stroke emotional impairments. The goals of this review article are to examine the clinical and experimental data describing (1) that CRH regulates the molecular signaling brain circuit underlying anxiety- and depression-like behaviors, (2) the influence of CRH and other stress markers in the pathophysiology of post stroke emotional and cognitive impairments, and (3) context and site specific interactions of CRH and BDNF as a basis for the development of novel therapeutic targets. This review addresses how the production and release of the neuropeptide CRH within the various regions of the mesocorticolimbic system influences emotional and cognitive behaviors with a look into its role in psychiatric disorders post stroke.

Effects of Remifentanil Preconditioning on Osteoblasts under Hypoxia-Reoxygenation Condition

BACKGROUND

Ischemia-reperfusion of bone occurs in a variety of clinical conditions, such as orthopedic arthroplasty, plastic gnathoplasty, spinal surgery, and amputation. Usually, cellular models of hypoxia-reoxygenation reflect in vivo models of ischemia-reperfusion. With respect to hypoxia-reoxygenation conditions, the effects of remifentanil on osteogenesis have received little attention. Therefore, we investigated the effects of remifentanil on the proliferation and differentiation of osteoblasts during hypoxic-reoxygenation.

METHODS

After remifentanil (0.1, 1 ng/mL) preconditioning for 2 hours, human osteoblasts were cultured under 1% oxygen tension for 24 hours. Thereafter, the cells were reoxygenated for 12 hours at 37 °C. The naloxone groups were treated with naloxone for 30 minutes before remifentanil treatment. We measured cell viability via MTT assay. Osteoblast maturation was determined by assay of bone nodular mineralization. Quantitative PCR and western blot methods were used to determine BMP-2, osteocalcin, Akt, type I collagen, osterix, TGF-β1, HIF-1α, and RUNX2 expression levels.

RESULTS

Osteoblast viability and bone nodular mineralization by osteoblasts is recovered by remifentanil preconditioning from hypoxia-reoxygenation insult. During hypoxic-reoxygenation condition, remifentanil preconditioning induced the expression of BMP-2, osteocalcin, Akt, type I collagen, osterix, TGF-β1, HIF-1α, and RUNX2 in osteoblasts.

CONCLUSIONS

Under hypoxia-reoxygenation conditions, remifentanil preconditioning enhanced the cell viability and maturation of osteoblasts, and stimulated the expression of proteins associated with osteoblast proliferation and differentiation of the osteoblast. Our results suggest that remifentanil may help in the treatment of bone stress injuries.

Delta opioid receptor agonist BW373U86 attenuates post-resuscitation brain injury in a rat model of asphyxial cardiac arrest

OBJECTIVE

The aim of this study was to investigate whether the DOR agonist BW373U86 conferred neuroprotection following ACA when given after resuscitation and to determine the long-term effects of chronic BW373U86 treatment on ACA-elicited brain injury.

METHODS

Animals were divided into acute and chronic treatment groups. Each group consisted of four sub-groups, including Sham, ACA, BW373U86 (BW373U86+ACA), and Naltrindole groups (Naltrindole and BW373U86+ACA). The DOR antagonist Naltrindole was used to confirm the possible receptor-dependent effects of BW373U86. ACA was induced by 8min of asphyxiation followed by resuscitation. All drugs were administered either immediately after the restoration of spontaneous circulation (ROSC) in acute-treatment groups or over 6 consecutive days in chronic-treatment groups. Alterations of cAMP response element-binding protein (CREB) and phosphorylated CREB (pCREB) were analyzed by western blot and immunohistochemistry. Neurological functions were assessed by neurological deficit score (NDS) and Morris Water Maze performance. Neurodegeneration was monitored by immunofluorescence and Nissl staining.

RESULTS

ACA induced massive neuron loss and serious neurological function deficits. BW373U86 significantly reduced both of these negative effects and increased CREB and pCREB expression in the hippocampus; these effects were reversed with acute Naltrindole treatment. The protective effects of BW373U86 persisted until 28d post-ROSC with chronic treatment, but these effects were not reversed by Naltrindole.

CONCLUSIONS

BW373U86 attenuates global cerebral ischemic injury induced by ACA through both DOR-dependent and DOR-independent mechanisms. CREB might be an important molecule in mediating these neuroprotective effects.

Crosstalk between delta opioid receptor and nerve growth factor signaling modulates neuroprotection and differentiation in rodent cell models

Both opioid signaling and neurotrophic factor signaling have played an important role in neuroprotection and differentiation in the nervous system. Little is known about whether the crosstalk between these two signaling pathways will affect neuroprotection and differentiation. Previously, we found that nerve growth factor (NGF) could induce expression of the delta opioid receptor gene (Oprd1, dor), mainly through PI3K/Akt/NF-κB signaling in PC12h cells. In this study, using two NGF-responsive rodent cell model systems, PC12h cells and F11 cells, we found the delta opioid neuropeptide [d-Ala², d-Leu⁵] enkephalin (DADLE)-mediated neuroprotective effect could be blocked by pharmacological reagents: the delta opioid antagonist naltrindole, PI3K inhibitor LY294002, MAPK inhibitor PD98059, and Trk inhibitor K252a, respectively. Western blot analysis revealed that DADLE activated both the PI3K/Akt and MAPK pathways in the two cell lines. siRNA Oprd1 gene knockdown experiment showed that the upregulation of NGF mRNA level was inhibited with concomitant inhibition of the survival effects of DADLE in the both cell models. siRNA Oprd1 gene knockdown also attenuated the DADLE-mediated neurite outgrowth in PC12h cells as well as phosphorylation of MAPK and Akt in PC12h and F11 cells, respectively. These data together strongly suggest that delta opioid peptide DADLE acts through the NGF-induced functional G protein-coupled Oprd1 to provide its neuroprotective and differentiating effects at least in part by regulating survival and differentiating MAPK and PI3K/Akt signaling pathways in NGF-responsive rodent neuronal cells.

Pharmacological traits of delta opioid receptors: pitfalls or opportunities?

RATIONALE

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

OBJECTIVE

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

CONCLUSION

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

Neuroprotection against hypoxia/ischemia: δ-opioid receptor-mediated cellular/molecular events

Hypoxic/ischemic injury remains the most dreaded cause of neurological disability and mortality. Despite the humbling experiences due to lack of promising therapy, our understanding of the complex cascades underlying the neuronal insult has led to advances in basic science research. One of the most noteworthy has been the effect of opioid receptors, especially the delta-opioid receptor (DOR), on hypoxic/ischemic neurons. Our recent studies, and those of others worldwide, present strong evidence that sheds light on DOR-mediated neuroprotection in the brain, especially in the cortex. The mechanisms of DOR neuroprotection are broadly categorized as: (1) stabilization of the ionic homeostasis, (2) inhibition of excitatory transmitter release, (3) attenuation of disrupted neuronal transmission, (4) increase in antioxidant capacity, (5) regulation of intracellular pathways-inhibition of apoptotic signals and activation of pro-survival signaling, (6) regulation of specific gene and protein expression, and (7) up-regulation of endogenous opioid release and/or DOR expression. Depending upon the severity and duration of hypoxic/ischemic insult, the release of endogenous opioids and DOR expression are regulated in response to the stress, and DOR signaling acts at multiple levels to confer neuronal tolerance to harmful insult. The phenomenon of DOR neuroprotection offers a potential clue for a promising target that may have significant clinical implications in our quest for neurotherapeutics.

δ-opioid receptor activation and microRNA expression of the rat cortex in hypoxia

Prolonged hypoxic/ischemic stress may cause cortical injury and clinically manifest as a neurological disability. Activation of the δ-opioid receptor (DOR) may induce cortical protection against hypoxic/ischemic insults. However, the mechanisms underlying DOR protection are not clearly understood. We have recently found that DOR activation modulates the expression of microRNAs (miRNAs) in the kidney exposed to hypoxia, suggesting that DOR protection may involve a miRNA mechanism. To determine if the miRNAs expressed in the cortex mediated DOR neuroprotection, we examined 19 miRNAs that were previously identified as hypoxia- and DOR-regulated miRNAs in the kidney, in the rat cortex treated with UFP-512, a potent and specific DOR agonist under hypoxic condition. Of the 19 miRNAs tested, 17 were significantly altered by hypoxia and/or DOR activation with the direction and amplitude varying depending on hypoxic duration and times of DOR treatment. Expression of several miRNAs such as miR-29b, -101b, -298, 324-3p, -347 and 466b was significantly depressed after 24 hours of hypoxia. Similar changes were seen in normoxic condition 24 hours after DOR activation with one-time treatment of UFP-512. In contrast, some miRNAs were more tolerant to hypoxic stress and showed significant reduction only with 5-day (e.g., miR-31 and -186) or 10-day (e.g., miR-29a, let-7f and -511) exposures. In addition, these miRNAs had differential responses to DOR activation. Other miRNAs like miRs-363* and -370 responded only to the combined exposure to hypoxia and DOR treatment, with a notable reduction of >70% in the 5-day group. These data suggest that cortical miRNAs are highly yet differentially sensitive to hypoxia. DOR activation can modify, enhance or resolve the changes in miRNAs that target HIF, ion transport, axonal guidance, free radical signaling, apoptosis and many other functions.

Current research on opioid receptor function

The use of opioid analgesics has a long history in clinical settings, although the comprehensive action of opioid receptors is still less understood. Nonetheless, recent studies have generated fresh insights into opioid receptor-mediated functions and their underlying mechanisms. Three major opioid receptors (μ-opioid receptor, MOR; δ-opioid receptor, DOR; and κ-opioid receptor, KOR) have been cloned in many species. Each opioid receptor is functionally sub-classified into several pharmacological subtypes, although, specific gene corresponding each of these receptor subtypes is still unidentified as only a single gene has been isolated for each opioid receptor. In addition to pain modulation and addiction, opioid receptors are widely involved in various physiological and pathophysiological activities, including the regulation of membrane ionic homeostasis, cell proliferation, emotional response, epileptic seizures, immune function, feeding, obesity, respiratory and cardiovascular control as well as some neurodegenerative disorders. In some species, they play an essential role in hibernation. One of the most exciting findings of the past decade is the opioid-receptor, especially DOR, mediated neuroprotection and cardioprotection. The upregulation of DOR expression and DOR activation increase the neuronal tolerance to hypoxic/ischemic stress. The DOR signal triggers (depending on stress duration and severity) different mechanisms at multiple levels to preserve neuronal survival, including the stabilization of homeostasis and increased pro-survival signaling (e.g., PKC-ERK-Bcl 2) and antioxidative capacity. In the heart, PKC and KATP channels are involved in the opioid receptor-mediated cardioprotection. The DOR-mediated neuroprotection and cardioprotection have the potential to significantly alter the clinical pharmacology in terms of prevention and treatment of life-threatening conditions like stroke and myocardial infarction. The main purpose of this article is to review the recent work done on opioids and their receptor functions. It shall provide an informative reference for better understanding the opioid system and further elucidation of the opioid receptor function from a physiological and pharmacological point of view.

Salvinorin A administration after global cerebral hypoxia/ischemia preserves cerebrovascular autoregulation via kappa opioid receptor in piglets

Background

Cerebral hypoxia/ischemia (HI) is not uncommon during the perinatal period. If occurring, it can result in severe neurologic disabilities that persist throughout life. Salvinorin A, a non-opioid Kappa opioid receptors (KOR) selective agonist, has the potential to address this devastating situation. We have demonstrated that salvinorin A administration before HI, preserves pial artery autoregulative function through both the KOR and extracellular signal-regulated kinases (ERK) pathways. In the present study, we tested the hypothesis that administration of salvinorin A after HI could preserve cerebral autoregulation via KOR and ERK pathway.

Methodology/Principal Findings

The response of the pial artery to hypercapnia, hypotension and isoproterenol were monitored before and 1 hour after HI in piglets equipped with a cranial window. Four groups of drug administration were performed after HI. The control group had DMSO (1 µl/kg, i.v.) administrated immediately after HI. Two salvinorin A treated groups had salvinorin A (10 µg/kg, i.v.) administrated 0 and 30 min after HI, respectively. The 4^th group had salvinorin A and the KOR antagonist norbinaltorphimine (Nor-BIN, 1 µM topical) co-administrated 0 min after HI (n = 5). The dilation responses of the pial artery to hypercapnia and hypotension were impaired after global HI and were preserved with salvinorin A administration immediately or 30 min after HI. The preservation of autoregulation was abolished when nor-BIN was administered. Levels of phosphor-ERK(pERK)/ERK in the cerebrospinal fluid (CSF) were measured before and 1 hour after HI. After HI, the pERK/ERK levels significantly increased in both DMSO control group and salvinorin A and nor-BIN co-administration group. The elevated levels of pERK/ERK were not observed with salvinorin A only groups.

Conclusions

Salvinorin A administration 0 and 30 min after HI preserves autoregulation of pial artery to hypercapnia and hypotension via kappa opioid receptor and ERK pathway.

Morphine protects against intracellular amyloid toxicity by inducing estradiol release and upregulation of Hsp70

Certain experimental models support morphine can play a beneficial role against damage in the neuronal system. In this study, we find morphine as well as endomorphin-1 and endomorphin-2 can protect against intracellular amyloid β (iAβ) toxicity in human and rat primary neuronal cultures and in rat brains in vivo. Morphine reverses the electrophysiological changes induced by iAβ, including current density, resting membrane potential and capacitance. Also morphine improves the spatial memory performance in rats infected by iAβ packaged virus and in APP/PS1 mice in Morris water maze tests. Morphine protection is mediated through inducing estradiol release in hippocampal neurons measured by ELISA and liquid chromatography-mass spectrometry, possibly by increasing P450 cytochrome aromatase activity. Released estradiol induces upregulation of heat shock protein 70 (Hsp70). Hsp70 protects against intracellular amyloid toxicity by rescuing proteasomal activity which is impaired by iAβ. This is the first time, to our knowledge, that induction of estradiol release in hippocampal neurons by morphine is reported. Our data may contribute to both Alzheimer's disease therapy and pain clinics where morphine is widely used.

Effect of delta-opioid receptor over-expression on cortical expression of GABAA receptor alpha1-subunit in hypoxia

Recent studies show that both delta-opioid receptors (DOR) and GABA receptors play a neuroprotective role in the mature cortex. Since we have observed that DOR over-expression renders the cortex more tolerant to hypoxic stress, we asked whether DOR over-expression affects GABA receptors expression in the cortex under hypoxia. As the first step, we investigated the expression of GABAA receptor alpha1-subunit (GABAA Ralpha1, the most abundant alpha-subunit of GABA receptors in the adult brain) in the mouse cortex with transgenic DOR over-expression after hypoxia. The results showed that GABAA Ralpha1 expression was lower in the transgenic than wild-type cortex, suggesting that DOR overexpression induces an inhibitory effect on GABAA receptor expression. Hypoxia for 1-3 days significantly increased GABAA Ralpha1 expression in the wild-type cortex, which may be an adaptive strategy for protecting the cortex against hypoxic stress. Interestingly, such increase was not found in the transgenic cortex with DOR over-expression. This may represent an interactive regulation in the transgenic cortex to efficiently balance energy production and consumption for better adaptation to hypoxic environment. Since DOR over-expression increases cortical tolerance to hypoxia, an increase in GABA receptors expression (an energy-costing process) may not be necessary in the cortex with DOR over-expression.

Delta-opioid receptor activation prolongs respiratory motor output during oxygen-glucose deprivation in neonatal rat spinal cord in vitro

Delta opioid receptor (DOR) activation protects the adult mammalian brain during oxygen-glucose deprivation (OGD), but it is not known whether neonatal spinal motor circuits are also protected. Also, it is unclear whether the timing of spinal DOR activation relative to spinal OGD is important for neuroprotection. Thus, a split-bath in vitro neonatal rat brainstem/spinal cord preparation was used to record spontaneous respiratory motor output from cervical (C4-C5) and thoracic (T5-T6) ventral spinal roots while exposing only the spinal cord to OGD solution (0 mM glucose, bubbled with 95% N(2)/5% CO(2)) or DOR agonist drugs (DADLE, DPDPE). Spinal OGD solution application caused respiratory motor output frequency and amplitude to decrease until all activity was abolished (i.e. end-point times) after 25.9±1.4 min (cervical) and 25.2±1.4 min (thoracic). Spinal DOR activation via DPDPE (1.0 μM) prior-to and during spinal OGD increased cervical and thoracic end-point times to 35-48 min. Spinal DADLE or DPDPE (1.0 μM) application 15 min following spinal OGD onset increased cervical and thoracic end-point times to 36-45 min. Brief spinal DPDPE (1.0 μM) application for 10 min at 25 min before spinal OGD onset increased cervical and thoracic end-point times to 41-46 min. Overall, the selective DOR agonist, DPDPE, was more effective at increasing end-point times than DADLE. Naltrindole (DOR antagonist; 10 μM) pretreatment blocked DPDPE-dependent increase in end-point times, suggesting that DOR activation was required. Spinal naloxone (1.0 μM) application before and during spinal OGD also increased end-point times to 31-33 min, but end-point times were not altered by Mu opioid receptor (MOR) activation or DOR activation/MOR blockade, indicating that there are complex interactions between OGD and opioid signaling pathways. These data suggest DOR activation before, during, and after spinal OGD protects central motor networks and may provide neuroprotection during unpredictable perinatal ischemic events.

Ovarian hormones influence corticotropin releasing factor receptor colocalization with delta opioid receptors in CA1 pyramidal cell dendrites

Stress interacts with addictive processes to increase drug use, drug seeking, and relapse. The hippocampal formation (HF) is an important site at which stress circuits and endogenous opioid systems intersect and likely plays a critical role in the interaction between stress and drug addiction. Our prior studies demonstrate that the stress-related neuropeptide corticotropin-releasing factor (CRF) and the delta-opioid receptor (DOR) colocalize in interneuron populations in the hilus of the dentate gyrus and stratum oriens of CA1 and CA3. While independent ultrastructural studies of DORs and CRF receptors suggest that each receptor is found in CA1 pyramidal cell dendrites and dendritic spines, whether DORs and CRF receptors colocalize in CA1 neuronal profiles has not been investigated. Here, hippocampal sections of adult male and proestrus female Sprague-Dawley rats were processed for dual label pre-embedding immunoelectron microscopy using well-characterized antisera directed against the DOR for immunoperoxidase and against the CRF receptor for immunogold. DOR-immunoreactivity (-ir) was found presynaptically in axons and axon terminals as well as postsynaptically in somata, dendrites and dendritic spines in stratum radiatum of CA1. In contrast, CRF receptor-ir was predominantly found postsynaptically in CA1 somata, dendrites, and dendritic spines. CRF receptor-ir frequently was observed in DOR-labeled dendritic profiles and primarily was found in the cytoplasm rather than at or near the plasma membrane. Quantitative analysis of CRF receptor-ir colocalization with DOR-ir in pyramidal cell dendrites revealed that proestrus females and males show comparable levels of CRF receptor-ir per dendrite and similar cytoplasmic density of CRF receptor-ir. In contrast, proestrus females display an increased number of dual-labeled dendritic profiles and an increased membrane density of CRF receptor-ir in comparison to males. We further examined the functional consequences of CRF receptor-ir colocalization with DOR-ir in the same neuron using the hormone responsive neuronal cell line NG108-15, which endogenously expresses DORs, and assayed intracellular cAMP production in response to CRF receptor and DOR agonists. Results demonstrated that short-term application of DOR agonist SNC80 inhibited CRF-induced cAMP accumulation in NG108-15 cells transfected with the CRF receptor. These studies provide new insights on opioid-stress system interaction in the hippocampus of both males and females and establish potential mechanisms through which DOR activation may influence CRF receptor activity.

Delta opioid peptide [D-Ala2, D-Leu5] enkephalin (DADLE) triggers postconditioning against transient forebrain ischemia

Preconditioning with selective delta opioid peptide [d-Ala2, d-Leu5] enkephalin (DADLE) provides ischemic tolerance following transient forebrain ischemia in rats. However, whether DADLE postconditioning retains its neuroprotective efficacy and the underlying molecular mechanism in ischemic brain is largely unknown. We investigated DADLE postconditioning protection of hippocampal CA1 neurons against transient forebrain ischemia. 6 days after being implanted with cannula at the right lateral ventricle, rats underwent 10 min of forebrain ischemia by four vessel occlusion. Hippocampal CA1 neuronal survival and degeneration were measured in the hippocampi of rats at 3 days after ischemia. The behavioral and cognitive improvements of DADLE treatment in rats were also evaluated on days 5-9 using open-field and Morris water maze tests. The results showed that DADLE at doses of 0.25 and 2.5 nmol, but not 25 nmol, could significantly protect CA1 neurons against ischemia/reperfusion injury. Co-administration with the delta-opioid receptor antagonist naltrindole or pretreatment with the Akt antagonist LY294002 completely abolished the DADLE postconditioning effect. Furthermore, DADLE postconditioning exhibited cognitive benefits in rats with transient forebrain ischemia. The study thus suggested a therapeutic opportunity of postconditioning neuroprotection by DADLE and also provided important information in understanding the mechanism of DADLE action in the ischemic brain.

Opioid receptor agonist reduces myocardial ischemic injury when administered during early phase of myocardial ischemia

AIM OF THE STUDY

Postresuscitation myocardial dysfunction is one of the leading causes of early death after initial success of resuscitation, the mechanisms of postresuscitation myocardial dysfunction remain controversial. We hypothesize that ischemia injury, rather than reperfusion injury is the major cause of postresuscitation myocardial dysfunction. We proposed to investigate the separate effects of ischemia and reperfusion injury on postresuscitation myocardial dysfunction.

METHODS

Thirty-three Langendorff-perfused isolated rat hearts were subjected to 15 min of global ischemia followed by 120 min of reperfusion. Pentazocine was utilized as a myocardial protective agent, either before ischemia or during reperfusion. All hearts were randomized into 3 groups: (1) "ischemia protection", in which pentazocine was infused 10 min prior to global ischemia, (2) "reperfusion protection", in which pentazocine was infused during 2h of reperfusion and (3) "control", with no pentazocine infusion. Left ventricular (LV) functions were measured by the maximal rate of LV pressure rise (dP/dt(max)) and decline (-dP/dt(max)), the maximal LV diastolic pressure (LVDP). The incidences of postischemic arrhythmias were measured.

RESULTS

When pentazocine was administered before onset of ischemia, the LV systolic and diastolic functions were significantly greater, and the postischemic arrhythmias were significantly less in comparison to those with reperfusion protection (p<0.05) and the control group (p<0.05).

CONCLUSIONS

In this model, the severity of postischemic myocardial dysfunction was less when the heart was protected during ischemia. Ischemia injury may therefore be the major cause of postresuscitation myocardial dysfunction.

Effects of intracerebroventricular application of the delta opioid receptor agonist [D-Ala2, D-Leu5] enkephalin on neurological recovery following asphyxial cardiac arrest in rats

The delta opioid receptor (DOR) agonist [D-Ala2, D-Leu5] enkephalin (DADLE) has been implicated as a novel neuroprotective agent in the CNS. The current study was designed to evaluate the effects of intracerebroventricular (ICV) application of DADLE on neurological outcomes following asphyxial cardiac arrest (CA) in rats. Male Sprague-Dawley rats were randomly assigned to four groups: Sham group, CA group, DADLE group (DADLE+CA), and Naltrindole group (Naltrindole and DADLE+CA). All drugs were administered into the left cerebroventricle 30 min before CA. CA was induced by 8-min asphyxiation and the animals were resuscitated with a standardized method. DOR protein expression in the hippocampus was significantly increased in the CA group at 1 h after restoration of spontaneous circulation (ROSC) compared with the Sham group. As time progressed, expression of DOR proteins decreased gradually in the CA group. Treatment with DADLE alone or co-administration with Naltrindole reversed the down-regulation of DOR proteins in the hippocampus induced by CA at 24 h after ROSC. Compared with the CA group, the DADLE group had persistently better neurological functional recovery, as assessed by neurological deficit score (NDS) and Morris water maze trials. The number of surviving hippocampal CA1 neurons in the DADLE group was significantly higher than those in the CA group. However, administration of Naltrindole abolished most of the neuroprotective effects of DADLE. We conclude that ICV administration of DADLE 30 min before asphyxial CA has significant protective effects in attenuating hippocampal CA1 neuronal damage and neurological impairments, and that DADLE executes its effects mainly through DOR.

Ionic storm in hypoxic/ischemic stress: can opioid receptors subside it?

Neurons in the mammalian central nervous system are extremely vulnerable to oxygen deprivation and blood supply insufficiency. Indeed, hypoxic/ischemic stress triggers multiple pathophysiological changes in the brain, forming the basis of hypoxic/ischemic encephalopathy. One of the initial and crucial events induced by hypoxia/ischemia is the disruption of ionic homeostasis characterized by enhanced K(+) efflux and Na(+)-, Ca(2+)- and Cl(-)-influx, which causes neuronal injury or even death. Recent data from our laboratory and those of others have shown that activation of opioid receptors, particularly delta-opioid receptors (DOR), is neuroprotective against hypoxic/ischemic insult. This protective mechanism may be one of the key factors that determine neuronal survival under hypoxic/ischemic condition. An important aspect of the DOR-mediated neuroprotection is its action against hypoxic/ischemic disruption of ionic homeostasis. Specially, DOR signal inhibits Na(+) influx through the membrane and reduces the increase in intracellular Ca(2+), thus decreasing the excessive leakage of intracellular K(+). Such protection is dependent on a PKC-dependent and PKA-independent signaling pathway. Furthermore, our novel exploration shows that DOR attenuates hypoxic/ischemic disruption of ionic homeostasis through the inhibitory regulation of Na(+) channels. In this review, we will first update current information regarding the process and features of hypoxic/ischemic disruption of ionic homeostasis and then discuss the opioid-mediated regulation of ionic homeostasis, especially in hypoxic/ischemic condition, and the underlying mechanisms.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

delta-Opioid receptors protect from anoxic disruption of Na+ homeostasis via Na+ channel regulation

Hypoxic/ischemic disruption of ionic homeostasis is a critical trigger of neuronal injury/death in the brain. There is, however, no promising strategy against such pathophysiologic change to protect the brain from hypoxic/ischemic injury. Here, we present a novel finding that activation of delta-opioid receptors (DOR) reduced anoxic Na+ influx in the mouse cortex, which was completely blocked by DOR antagonism with naltrindole. Furthermore, we co-expressed DOR and Na+ channels in Xenopus oocytes and showed that DOR expression and activation indeed play an inhibitory role in Na+ channel regulation by decreasing the amplitude of sodium currents and increasing activation threshold of Na+ channels. Our results suggest that DOR protects from anoxic disruption of Na+ homeostasis via Na+ channel regulation. These data may potentially have significant impacts on understanding the intrinsic mechanism of neuronal responses to stress and provide clues for better solutions of hypoxic/ischemic encephalopathy, and for the exploration of acupuncture mechanism since acupuncture activates opioid system.

delta-Opioid receptor activation attenuates oxidative injury in the ischemic rat brain

Background

We have recently shown that δ-opioid receptors (DORs) play an important role in neuroprotection from hypoxic injury via the regulation of extracellular signaling-regulated kinase (ERK) and cytochrome c release. Since ERK and cytochrome c are differentially involved in caspase signaling of oxidative injury that significantly contributes to neuronal damage in ischemia/reperfusion, we considered if DOR activation protects the ischemic brain by attenuating oxidative injury.

Results

We observed that, in a model of cerebral ischemia with middle cerebral artery occlusion, DOR activation increased the activity of major antioxidant enzymes, glutathione peroxidase and superoxide dismutase, and decreased malondialdehyde and nitric oxide levels in the cortex exposed to cerebral ischemia/reperfusion. In addition, DOR activation reduced caspase 3 expression, though it did not significantly affect the increase in interleukin (IL)1β and tumor necrosis factor (TNF)α expression at the same timepoint. PD98059, an inhibitor of mitogen-activated protein kinase (MAPK) extracellular signaling-regulated kinase kinase, accelerated animal death during ischemia/reperfusion.

Conclusion

DOR activation attenuates oxidative injury in the brain exposed to ischemia/reperfusion by enhancing antioxidant ability and inhibiting caspase activity, which provides novel insights into the mechanism of DOR neuroprotection.

Defective neuropeptide processing and ischemic brain injury: a study on proprotein convertase 2 and its substrate neuropeptide in ischemic brains

Using a focal cerebral ischemia model in rats, brain ischemia-induced changes in expression levels of mRNA and protein, and activities of proprotein convertase 2 (PC2) in the cortex were examined. In situ hybridization analyses revealed a transient upregulation of the mRNA level for PC2 at an early reperfusion hour, at which the level of PC2 protein was also high as determined by immunocytochemistry and western blotting. When enzymatic activities of PC2 were analyzed using a synthetic substrate, a significant decrease was observed at early reperfusion hours at which levels of PC2 protein were still high. Also decreased at these reperfusion hours were tissue levels of dynorphin-A(1-8) (DYN-A(1-8)), a PC2 substrate, as determined by radioimmunoassay. Further examination of PC2 protein biosynthesis by metabolic labeling in cultured neuronal cells showed that in ischemic cells, the proteolytic processing of PC2 was greatly attenuated. Finally, in mice, an intracerebroventricular administration of synthetic DYN-A(1-8) significantly reduced the extent of ischemic brain injury. In mice those lack an active PC2, exacerbated brain injury was observed after an otherwise non-lethal focal ischemia. We conclude that brain ischemia attenuates PC2 and PC2-mediated neuropeptide processing. This attenuation may play a role in the pathology of ischemic brain injury.

Na+ mechanism of delta-opioid receptor induced protection from anoxic K+ leakage in the cortex

Activation of delta-opioid receptors (DOR) attenuates anoxic K(+) leakage and protects cortical neurons from anoxic insults by inhibiting Na(+) influx. It is unknown, however, which pathway(s) that mediates the Na(+) influx is the target of DOR signal. In the present work, we found that, in the cortex, (1) DOR protection was largely dependent on the inhibition of anoxic Na(+) influxes mediated by voltage-gated Na(+) channels; (2) DOR activation inhibited Na(+) influx mediated by ionotropic glutamate N-methyl-D-aspartate (NMDA) receptors, but not that by non-NMDA receptors, although both played a role in anoxic K(+) derangement; and (3) DOR activation had little effect on Na(+)/Ca(2+) exchanger-based response to anoxia. We conclude that DOR activation attenuates anoxic K(+) derangement by restricting Na(+) influx mediated by Na(+) channels and NMDA receptors, and that non-NMDA receptors and Na(+)/Ca(2+) exchangers, although involved in anoxic K(+) derangement in certain degrees, are less likely the targets of DOR signal.

Cell-specific loss of kappa-opioid receptors in oligodendrocytes of the dysmyelinating jimpy mouse

Jimpy is a murine mutation in myelin proteolipid protein, leading to premature death of oligodendrocytes and severe central nervous system hypomyelination. Jimpy is a bona fide model of human Pelizaeus-Merzbacher disease. This paper describes a severe reduction in expression of kappa-opioid receptors (KOP) in oligodendrocytes of jimpy mice. A cell-specific reduction of >90% is apparent by 5 days of age. Expression is not reduced in neurons, and mu-opioid receptor expression is normal. Mechanism(s) leading to deficient KOP expression in jimpy mice remain unclear. We speculate that loss of KOP may be related to increased [Ca(2+)](i) and premature death of jimpy oligodendrocytes.

Pituitary adenylate cyclase-activating polypeptide (PACAP) alters parasympathetic neuron gene expression in a time-dependent fashion

Neuropeptides, including pituitary adenylate cyclase-activating polypeptide (PACAP), can influence diverse cellular processes over a broad temporal range. In ciliary ganglion (CG) neurons, for example, PACAP binding to high-affinity PAC1 receptors triggers transduction cascades that both rapidly modulate nicotinic receptors and synapses and support long-term survival. Since PACAP/PAC1 signaling recruits intracellular messengers and effectors that potently alter transcription, we examined its activation of the transcription factor CREB and then tested for changes in gene expression. PACAP/PAC1 signaling rapidly induced prolonged CREB activation in CG neurons by a phospholipase C -independent mechanism supported by Ca2+-influx, adenylate cyclase, and effectors, including protein kinase C (PKC) and possibly PKA. Since PACAP is abundant in the CG and released from depolarized presynaptic terminals, it is well suited to regulate gene expression relevant to neuronal and synaptic development. Gene array screens conducted using RNA from CG cultures grown with PACAP for 1/4, 24, or 96 h revealed a time-dependent pattern of > 600 regulated transcripts, including several encoding proteins implicated in synaptic function, neuronal survival, and development. The results underscore rapid, neuromodulatory, and long-term, neurotrophic consequences of PAC1 signaling in CG neurons and suggest that PACAP exerts such diverse influences by altering the expression of specific gene transcripts in a time-dependent fashion.

Endomorphins and morphine limit anoxia-reoxygenation-induced brain mitochondrial dysfunction in the mouse

The protection of brain mitochondria from oxidative stress is an important therapeutic strategy against ischemia-reperfusion injury and neurodegenerative disorders. Isolated brain mitochondria subjected to a 5 min period of anoxia followed by 5 min reoxygenation mirrored the effect of oxidative stress in the brain. The present study attempts to evaluate the protective effects of endomorphin 1 (EM1), endomorphin 2 (EM2), and morphine (Mor) in an in vitro mouse brain mitochondria anoxia-reoxygenation model. Endomorphins (EM1/2) and Mor were added to mitochondria prior to anoxia or reoxygenation. EM1/2 and Mor markedly improved mitochondrial respiratory activity with a decrease in state 4 and increases in state 3, respiratory control ratio (RCR) and the oxidative phosphorylation efficiency (ADP/O ratio), suggesting that they may play a protective role in mitochondria. These drugs inhibited alterations in mitochondrial membrane fluidity, lipoperoxidation, and cardiolipin (CL) release, which indicates protection of the mitochondrial membranes from oxidative damage. The protective effects of these drugs were concentration-dependent. Furthermore, these drugs blocked the enhanced release of cytochrome c (Cyt c), and consequently inhibited the cell apoptosis induced by the release of Cyt c. Our results suggest that EM1/2 and Mor effectively protect brain mitochondria against oxidative stresses induced by in vitro anoxia-reoxygenation and may play an important role in the prevention of deleterious effects during brain ischemia-reperfusion and neurodegenerative diseases.