Delta-opioid Receptor System Research Articles

Pain, Motivation, Migraine, and the Microbiome: New Frontiers for Opioid Systems and Disease.

For decades the broad role of opioids in addiction, neuropsychiatric disorders, and pain states has been somewhat well established. However, in recent years, with the rise of technological advances, not only is the existing dogma being challenged, but we are identifying new disease areas in which opioids play a critical role. This review highlights four new areas of exploration in the opioid field. The most recent addition to the opioid family, the nociceptin receptor system, shows promise as the missing link in understanding the neurocircuitry of motivation. It is well known that activation of the kappa opioid receptor system modulates negative affect and dysphoria, but recent studies now implicate the kappa opioid system in the modulation of negative affect associated with pain. Opioids are critical in pain management; however, the often-forgotten delta opioid receptor system has been identified as a novel therapeutic target for headache disorders and migraine. Lastly, changes to the gut microbiome have been shown to directly contribute to many of the symptoms of chronic opioid use and opioid related behaviors. This review summarizes the findings from each of these areas with an emphasis on identifying new therapeutic targets. SIGNIFICANCE STATEMENT: The focus of this minireview is to highlight new disease areas or new aspects of disease in which opioids have been implicated; this includes pain, motivation, migraine, and the microbiome. In some cases, this has resulted in the pursuit of a novel therapeutic target and resultant clinical trial. We believe this is very timely and will be a refreshing take on reading about opioids and disease.

PET imaging reveals lower kappa opioid receptor availability in alcoholics but no effect of age

Opioid receptors are implicated in alcoholism, other addictions, withdrawal, and depression, and are considered potential pharmacological targets for treatment. Our goal in the present study was to compare the availability of kappa opioid receptors (KOR) between an alcohol-dependent cohort (AD) and a healthy control cohort (HC). Sixty-four participants—36 AD and 28 HC—underwent PET scans with [11C]LY2795050, a selective kappa antagonist tracer. Partial-volume correction was applied to all PET data to correct for atrophy. Volume of distribution (VT) of the tracer was estimated regionally as a measure of KOR availability. VT values of AD versus HC were compared for 15 defined ROIs. Multivariate analysis showed a main effect of group on VT across these 15 ROIs. Post hoc tests showed that AD had significantly lower VT and thus a lower KOR availability than HC in amygdala and pallidum (corrected for multiple comparisons). Exploratory analysis of change in VT with age was conducted; VT was not found to vary significantly with age in any region. Our findings of lower VT in AD versus HC in multiple regions are in contrast to findings in the mu and delta opioid receptor systems of higher VT in AD versus HC. Although age-related decline in receptors has previously been observed in the mu opioid receptor system, we found that KOR availability does not change with age.

Intermittent Hypoxia Conditioning (IHC) and Delta Opioid Receptor (DOR) Expression in Canine Atrium

Extended hypoxia exposures make hearts vulnerable to damage and reportedly suppress DOR expression in cell models resulting in cell damage. However, 20 days of limited daily IHC produced a robust resistance to ischemia reperfusion injury in canine hearts (Exp Biol Med 2004;229:806‐812). DOR blockade with naltrindole during daily IHC abrogates this cardioprotection. Since DOR stimulation enhances cardioprotective cholinergic systems this study evaluated whether controlled IHC alters DOR, cholinergic and/or adrenergic systems to augment cholinergic control of the heart during ischemia reperfusion. Dogs underwent normoxic sham conditioning, IHC [5‐8 cycles of 5‐10 min hypoxia (9.5‐10% FIO2) + 4 min normoxia], or IHC + daily sc naltrindole before IHC, for 20 d. On day 21, atrial tissue was biopsied for immunoblot analysis of proteins. Relative to sham values, after IHC DORs trended lower but after IHC + naltrindole were higher relative to sham (p<0.05) and IHC (p<0.01). Since DOR blockade abrogates IHC mediated protection, these data suggest that DOR stimulation during IHC down‐regulates DOR with a compensatory increase after DOR blockade. Atrial tyrosine hydroxylase was largely unchanged following IHC, while multiple markers of cholinergic function were mixed, resulting in a potential increase in cholinergic influence.

Cellular localization and adaptive changes of the cardiac delta opioid receptor system in an experimental model of heart failure in rats

The role of the cardiac opioid system in congestive heart failure (CHF) is not fully understood. Therefore, this project investigated the cellular localization of delta opioid receptors (DOR) in left ventricle (LV) myocardium and adaptive changes in DOR and its endogenous ligand, the precursor peptide proenkephalin (PENK), during CHF. Following IRB approval, DOR localization was determined by radioligand binding using [H(3)]Naltrindole and by double immunofluorescence confocal analysis in the LV of male Wistar rats. Additionally, 28 days following an infrarenal aortocaval fistula (ACF) the extent of CHF and adaptions in left ventricular DOR and PENK expression were examined by hemodynamic measurements, RT-PCR, and Western blot. DOR specific membrane binding sites were identified in LV myocardium. DOR were colocalized with L-type Ca(2+)-channels (Cav1.2) as well as with intracellular ryanodine receptors (RyR) of the sarcoplasmatic reticulum. Following ACF severe congestive heart failure developed in all rats and was accompanied by up-regulation of DOR and PENK on mRNA as well as receptor proteins representing consecutive adaptations. These findings might suggest that the cardiac delta opioid system possesses the ability to play a regulatory role in the cardiomyocyte calcium homeostasis, especially in response to heart failure.

The effects of delta‐opioid receptor ligands on the primary and conditioned reinforcing effects of cocaine in rats

Pharmacological and genetic studies suggest that the delta opioid receptor (DOR) system modifies addiction‐related behaviors and neurocircuitry. Therefore, the present study investigated the effects of DOR ligands on the primary and conditioned reinforcing effects of cocaine in rats. DOR agonists and antagonists were administered s.c. immediately before cocaine or food self‐administration sessions or before cue reactivity sessions. The delta opioid agonist SNC80 failed to maintain responding in naïve rats. In rats with a history of cocaine self‐administration, non‐contingent SNC80 administration failed to alter responding for cocaine injections but enhanced responding maintained by stimuli previously associated with cocaine. The delta opioid antagonist naltrindole decreased responding for cocaine‐paired stimuli in the absence of cocaine. Overall, these data suggest that the DOR system may modulate the reinforcing effectiveness of conditioned stimuli associated with drugs of abuse. These studies were supported by the Small Grants Program from the University of Michigan Substance Abuse Research Center.

Hypoxic preconditioning attenuates global cerebral ischemic injury following asphyxial cardiac arrest through regulation of delta opioid receptor system

This study was designed to investigate whether delta opioid receptor (DOR) is involved in the neuroprotective effect induced by hypoxic preconditioning (HPC) in the asphyxial cardiac arrest (CA) rat model. Twenty-four hours after the end of 7-day HPC, the rats were subjected to 8-min asphyxiation and resuscitated with a standardized method. In the asphyxial CA rat model, HPC improved the neurological deficit score (NDS), inhibited neuronal apoptosis, and increased the number of viable hippocampal CA1 neurons at 24 h, 72 h, or 7 days after restoration of spontaneous circulation (ROSC); however, the above-mentioned neuroprotection of HPC was attenuated by naltrindole (a selective DOR antagonist). The expression of hypoxia-inducible factor-1α (HIF-1α) and DOR, and the content of leucine enkephalin (L-ENK) in the brain were also investigated after the end of 7-day HPC. HPC upregulated the neuronal expression of HIF-1α and DOR, and synchronously elevated the content of L-ENK in the rat brain. HIF-1α siRNA was used to further elucidate the relationship between HIF-1α and DOR in the HPC-treated brain. Knockdown of HIF-1α by siRNA markedly abrogated the HPC induced upregulation of HIF-1α and DOR. The present study demonstrates that the expression of DOR in the rat brain is upregulated by HIF-1α following exposure to 7-day HPC, at the same time, HPC also increases the production of endogenous DOR ligand L-ENK in the brain. DOR activation after HPC results in prolonged neuroprotection against subsequent global cerebral ischemic injury, suggesting a new mechanism of HPC-induced neuroprotection on global cerebral ischemia following CA and resuscitation.

Peripheral delta opioid receptors require priming for functional competence in vivo

Although centrally acting opioid analgesics produce profound antinociception under basal conditions, the antinociceptive properties of peripherally restricted opioid analgesics are generally only detectable after inflammation or injection of inflammatory mediators. Despite considerable research, the cellular mechanisms regulating the functional competence of peripheral opioid receptor systems for inhibition of nociception remain unclear. Recent work has demonstrated that brief pre-treatment (priming) with bradykinin, arachidonic acid, protease-activated receptor-2 agonists, or direct activators of protein kinase C (PKC) are capable of inducing the functional competence of the opioid receptor system in cultures of primary sensory neurons in vitro. Here we report that the peripheral delta opioid receptor system also requires PKC-dependent priming to inhibit prostaglandin E 2 (PGE 2)-induced thermal allodynia in the rat. Peripheral hindpaw injection of [D-Pen 2,5]-enkephalin (DPDPE), a selective delta opioid receptor agonist, did not alter PGE 2-induced thermal allodynia. However, following priming (15 min) with bradykinin or arachidonic acid, DPDPE produced a significant reduction in allodynia that was antagonist reversible, peripherally restricted, and exhibited a typical dose–response relationship. Furthermore, the bradykinin priming effect was blocked by the PKC inhibitors, bisindolylmaleimide I and chelerythrine. Collectively, these data support prior in vitro findings that, although present on primary sensory neurons, peripheral opioid receptor systems are functionally inactive under basal conditions and require activation of a PKC- and arachidonic acid-dependent signaling pathway to develop functional competence in vivo.

Targeting endogenous mu- and delta-opioid receptor systems for the treatment of drug addiction.

Drug addiction is a chronic, relapsing disorder that is characterized by a compulsion to take drug regardless of the adverse consequences that may ensue. Although the involvement of mesoaccumbal dopamine neurons in the initiation of drug abuse is well-established, neuroadaptations within the limbic cortical- striatopallidal circuit that occur as a consequence of repeated drug use are thought to lead to the behavioral dysregulation that characterizes addiction. Opioid receptors and their endogenous ligands are enriched in brain regions comprising this system and are, thus, strategically located to modulate neurotransmission therein. This article will review data suggesting an important role of mu-opioid receptor (MOPr) and delta opioid receptor (DOPr) systems in mediating the rewarding effects of several classes of abused drugs and that aberrant activity of these opioid systems may not only contribute to the behavioral dysregulation that characterizes addiction but to individual differences in addiction vulnerability.

The Antidepressant -like Effects of Delta-Opioid Receptor Agonists

Activation of the delta-opioid receptor (DOR) system produces an interesting behavioral profile distinct from that of other opioids. Unlike mu- and kappa-opioid agonists, delta-opioid agonists alone have limited pain-relieving qualities as measured in morphine-sensitive antinociceptive assays. Recent evidence, however, suggests that the DOR system may play a role in regulating mood and emotional states. For example, DOR activation stimulates robust antidepressant-like effects in preclinical assays, suggesting that these compounds may have therapeutic potential for treating human depression. This review discusses the role of the DORs in depression and the antidepressant-like effects of delta-opioid agonists as well as their limitations.

Chronic pain‐induced emotional dysfunction is associated with astrogliosis due to cortical δ‐opioid receptor dysfunction

It has been widely recognized that chronic pain could cause physiological changes at supraspinal levels. The delta-opioidergic system is involved in antinociception, emotionality, immune response and neuron-glia communication. In this study, we show that mice with chronic pain exhibit anxiety-like behavior and an increase of astrocytes in the cingulate cortex due to the dysfunction of cortical delta-opioid receptor systems. Using neural stem cells cultured from the mouse embryonic forebrain, astrocyte differentiation was clearly observed following long-term exposure to the selective delta-opioid receptor antagonist, naltrindole. We also found that micro-injection of either activated astrocyte or astrocyte-conditioned medium into the cingulate cortex of mice aggravated the expression of anxiety-like behavior. Our results indicate that the chronic pain process promotes astrogliosis in the cingulate cortex through the dysfunction of cortical delta-opioid receptors. This phenomenon may lead to emotional disorders including aggravated anxiety under chronic pain-like state.

Delta opioid receptor ligands modulate anxiety‐like behaviors in the rat

The role of the delta opioid receptor in regulating anxiety-like behavior in male Sprague-Dawley rats was examined. Using an elevated plus maze, the effects of the selective delta opioid receptor antagonist naltrindole (1 or 5 mg kg(-1)) and agonist SNC80 (1, 5 or 20 mg kg(-1)) on anxiety-like behavior were measured. Anxiety was also measured following administration of diazepam (3 mg kg(-1)) and amphetamine (1 mg kg(-1)) and compared to the effects of SNC80. Locomotor activity following administration of naltrindole, SNC80, diazepam, and amphetamine was measured. Finally, the defensive burying paradigm was used to confirm the findings from the elevated plus maze. Results demonstrated that SNC80 produced dose-dependent anxiolytic effects similar to that of the classical antianxiety agent, diazepam. Administration of naltrindole caused anxiogenic behavior in rats further supporting the involvement of the delta opioid receptor system in regulating anxiety. Naltrindole also blocked the anxiolytic effects of SNC80. Amphetamine had no effect on anxiety-like behavior. SNC80 induced hyperactivity similar to amphetamine at the doses tested, while naltrindole and diazepam did not significantly affect locomotor activity. Although SNC80 can increase locomotor activity, control experiments reported herein indicate that hyperlocomotion is not sufficient to produce an anxiolytic response on the elevated plus maze. Together with the results from the defensive burying paradigm, this suggests that the effects of SNC80 on reducing anxiety are independent of its effects on locomotion. Collectively these data show that the delta opioid receptor system can regulate anxiety-like behavior in an anxiolytic (agonist) and anxiogenic (antagonist) manner.

Transcriptional Regulation of Mouse δ-Opioid Receptor Gene: ROLE OF Ets-1 IN THE TRANSCRIPTIONAL ACTIVATION OF MOUSE δ-OPIOID RECEPTOR GENE

Previously, we identified a minimum core promoter of the mouse delta-opioid receptor (DOR) gene. The DOR promoter contains an E-box that binds upstream stimulatory factor and is crucial for the DOR promoter activity in NS20Y cells, a mouse neuronal cell line that constitutively expresses DOR. In the present study, we further analyzed the DOR promoter in NS20Y cells and have demonstrated that transcription factor Ets-1 binds to an Ets-1-binding site overlapping the E-box and trans-activates the DOR promoter by synergizing with upstream stimulatory factor in specific DNA binding. In addition, the Ets-1 DNA-binding domain is sufficient to play the functional role of Ets-1 in trans-activating the DOR promoter. Furthermore, through in vivo cross-linking assays and Northern blot analyses, we have demonstrated that Ets-1 binds to the DOR promoter in the neonatal mouse brain and that overexpressed Ets-1 can significantly enhance the expression of DOR mRNA in primary neonatal mouse neuronal cells. Collectively, our data suggest that Ets-1 functions as a trans-activator of the DOR promoter in the neonatal mouse brain and thus may contribute to the development of the mouse brain DOR system.

Hypoxia decreases opioid delta receptor expression in mouse brain.

Delta opioid receptor activation is protective during hypoxic injury. Many adaptive responses occur during exposure to hypoxia to facilitate survival. It is possible that increased activity of the delta opioid receptor system is one such adaptation. We tested the hypothesis that mice exposed to prolonged hypoxia have increased expression of the delta opioid receptor in brain tissue. Prolonged exposure to hypoxia (9% oxygen, balance nitrogen) continuously for seven days selectively decreased delta opioid receptor expression in mouse brain homogenate. The same hypoxic treatment had no effects on either mu or kappa opioid receptor expression, indicating that this response was not due to non-selective degradation of protein. Shorter term hypoxic treatments (one day and three days) did not induce changes in delta opioid receptor expression in whole brain homogenate. Binding assays were also conducted in grossly dissected brain regions (cortex, midbrain, hindbrain) to determine whether the shorter term treatments would induce changes in receptor expression in more discrete areas. No consistent changes in delta opioid receptor expression were detected in these brain regions. These data demonstrate that opioid delta receptors are hypoxia sensitive and may be a part of an adaptive process to increase survival in the organism. One possible cause for the decrease in delta opioid receptor expression following seven days of hypoxic exposure may be receptor down-regulation caused by an increased release of endogenous substances acting at delta receptors. As delta opioid receptor agonists appear promising for therapeutic potential in management of hypoxic injury, changes in delta receptor expression in response to long-term hypoxia could impact potential utilization of delta agonists in patients suffering chronic hypoxia.