The kappa-opioid receptor expressed on the mouse lymphoma cell line R1.1 contains a sulfhydryl group at the binding site

Child transmission of SARS-CoV-2: a systematic review and meta-analysis

Background

Understanding of the role of children in COVID-19 transmission has significant implications for school and childcare policies, as well as appropriate targeting of vaccine campaigns. The objective of this systematic review was to identify the role of children in SARS-CoV-2 transmission to other children and adults.

Methods

MEDLINE, EMBASE, CINAHL, Cochrane Central Register of Controlled Trials, and Web of Science were electronically searched for articles published before March 31, 2021. Studies of child-to-child and child-to-adult transmission and quantified the incidence of index and resulting secondary attack rates of children and adults in schools, households, and other congregate pediatric settings were identified. All articles describing confirmed transmission of SARS-CoV-2 from a child were included. PRISMA guidelines for data abstraction were followed, with each step conducted by two reviewers.

Results

40 of 6110 articles identified met inclusion criteria. Overall, there were 0.8 secondary cases per primary index case, with a secondary attack rate of 8.4% among known contacts. The secondary attack rate was 26.4% among adult contacts versus 5.7% amongst child contacts. The pooled estimate of a contact of a pediatric index case being infected as secondary case was 0.10 (95% CI 0.03-0.25).

Conclusions

Children transmit COVID-19 at a lower rate to children than to adults. Household adults are at highest risk of transmission from an infected child, more so than adults or children in other settings.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12887-022-03175-8.

Reward processing by the opioid system in the brain

The opioid system consists of three receptors, mu, delta, and kappa, which are activated by endogenous opioid peptides processed from three protein precursors, proopiomelanocortin, proenkephalin, and prodynorphin. Opioid receptors are recruited in response to natural rewarding stimuli and drugs of abuse, and both endogenous opioids and their receptors are modified as addiction develops. Mechanisms whereby aberrant activation and modifications of the opioid system contribute to drug craving and relapse remain to be clarified. This review summarizes our present knowledge on brain sites where the endogenous opioid system controls hedonic responses and is modified in response to drugs of abuse in the rodent brain. We review 1) the latest data on the anatomy of the opioid system, 2) the consequences of local intracerebral pharmacological manipulation of the opioid system on reinforced behaviors, 3) the consequences of gene knockout on reinforced behaviors and drug dependence, and 4) the consequences of chronic exposure to drugs of abuse on expression levels of opioid system genes. Future studies will establish key molecular actors of the system and neural sites where opioid peptides and receptors contribute to the onset of addictive disorders. Combined with data from human and nonhuman primate (not reviewed here), research in this extremely active field has implications both for our understanding of the biology of addiction and for therapeutic interventions to treat the disorder.

Ebselen protects both gray and white matter in a rodent model of focal cerebral ischemia

BACKGROUND AND PURPOSE

The neuroprotective efficacy of an intravenous formulation of the antioxidant ebselen has been comprehensively assessed with specific regard to conventional quantitative histopathology, subcortical axonal damage, neurological deficit, and principal mechanism of action.

METHODS

Transient focal ischemia (2 hours of intraluminal thread-induced ischemia with 22 hours of reperfusion) was induced in the rat. Ebselen (1 mg/kg bolus plus 1 mg/kg per hour IV) or vehicle was administered at the start of reperfusion and continued to 24 hours. Neurological deficit was assessed 24 hours after ischemia. Gray matter damage was evaluated by quantitative histopathology. Axonal damage was determined with amyloid precursor protein immunohistochemistry used as a marker of disrupted axonal flow and Tau-1 immunohistochemistry to identify oligodendrocyte pathology. Oxidative damage was determined by 8-hydroxy-2'-deoxyguanosine (8-OHdG) and 4-hydroxynonenal (4-HNE) immunohistochemistry.

RESULTS

Ebselen significantly reduced the volume of gray matter damage in the cerebral hemisphere (by 53.6% compared with vehicle, P<0.02). Axonal damage was reduced by 46.8% (P<0.002) and the volume of oligodendrocyte pathology was reduced by 60.9% (P<0.005). The neurological deficit score was reduced by 40.7% (P<0.05) and the volume of tissue immunopositive for 8-OHdG and 4-HNE was reduced by 65% (P<0.002) and 66% (P<0.001), respectively, in ebselen-treated animals.

CONCLUSIONS

Delayed (2-hour) treatment with intravenous ebselen significantly reduced gray and white matter damage and neurological deficit associated with transient ischemia. The reduction in tissue displaying evidence of oxidative stress suggests that the major mechanism of action is attenuation of free radical damage.

Chronic opioid treatment of the mouse thymoma cell lines R1.G1 and R1EGO leads to down-regulation of the kappa opioid receptor without desensitization of adenylyl cyclase activity

Kappa opioid agonists alter some immune functions of macrophages, and T- and B-lymphocytes. The mouse thymoma cell lines R1.G1 and R1EGO express only kappa-opioid receptors and these kappa-opioid receptors are coupled to an inhibitory GTP-binding regulatory protein. Binding of kappa-opioid agonists to the opioid receptor leads to the inhibition of adenylyl cyclase activity in these cells. In this study, an acute (15 min) and chronic (24 h) treatment of R1.G1 and R1EGO cell with a potent kappa-opioid agonist (-)U50,488 (100 nM) was studied to determine if a kappa-opioid agonist altered receptor number and/or desensitization of adenylyl cyclase activity in these two cell lines. Chronic treatment of both R1.G1 and R1EGO cells with (-)U50,488 lead to down-regulation of the kappa-opioid receptor, measured as a decrease of approximately 50% in the Bmax value for the binding of [3H]U69,593. The binding affinity (Kd value) was not affected after chronic treatment either in R1.G1 or R1EGO cells. There was no difference in the magnitude of inhibition of adenylyl cyclase activity by (-)U50,488 between the acute (15 min) and chronic (24-h) treatment in both cell lines R1.G1 and R1EGO. This study indicates that chronic opioid treatment of mouse thymoma R1.G1 and R1EGO cell lines leads to down-regulation of the receptor, without desensitization. This phenomenon was observed in R1.1 parent mouse thymoma cell line and recently in CHO cells expressing kappa-opioid receptor. This study demonstrates that unlike some neuronal preparations, chronic opioid treatment of the thymoma cell lines resulted in receptor down-regulation without desensitization.

Morphine-6 beta-glucuronide induces potent immunomodulation

The effect of morphine administration on immune parameters is well documented. However, there exists a limited knowledge of the effect of morphine's metabolites on immune status. The present study examines the immunomodulatory effects of the morphine metabolite, morphine-6 beta-glucuronide (M6G), in the rat and provides further evaluation of the antinociceptive effects of M6G. Animals were administered phosphate-buffered saline (PBS) or M6G in doses of 1.0, 3.16, or 10.0 mg/kg (subcutaneous (s.c.)) or 0.1, 0.316, or 1.0 microgram (intracerebroventricular (i.c.v.)). Animals were tested for antinociception in the warm water tail-withdrawal procedure. In a separate set of animals, assessments of splenic natural killer cell activity, lymphocyte proliferative responses to mitogenic stimulation, and production of interferon-gamma were made 1 h following the s.c. or i.c.v. administration of M6G. The results show that M6G induced potent antinociception that was evident for at least 120 min following administration. M6G also produced decreases in natural killer cell activity, lymphocyte proliferation, and interferon-gamma production 1 h following both routes of administration. The difference in potency between immune alterations induced by subcutaneous vs. intracerebroventricular administration suggest central mediation of the immunomodulatory properties of M6G. Thus, M6G produces significant antinociception and immunomodulation in the rat. These findings demonstrate potent immunomodulatory properties of a metabolite of morphine, 1M6G.

Involvement of substance P and central opioid receptors in morphine modulation of the CHS response

Morphine administration prior to challenge with the antigen 2,4-dinitro-fluorobenzene increases the contact hypersensitivity (CHS) response in rats. The present study extended these findings by showing that central, but not systemic, administration of N-methylnaltrexone antagonized the morphine-induced enhancement of the CHS response. The importance of the neuroimmune mediator substance P was shown via the attenuation of the morphine-induced enhancement following both systemic and topical administration of the NK-1 antagonist WIN51,708. Taken together, the findings of the present study provide new data showing that central opioid receptors and peripheral substance P are involved in the morphine-induced enhancement of the CHS response.

Involvement of central mu- but not delta- or kappa-opioid receptors in immunomodulation

Studies completed in both humans and animals have shown that opioids have significant effects on the immune system via pharmacological interactions with the opioid receptor. However, the type of opioid receptor at which morphine binding produces changes in immune status has not been well characterized. To determine the type of opioid receptor involved in opioid-induced immune alterations, the present study assessed the effects of agonists selective for the mu-, delta-, and kappa-opioid receptors. The site of action (i.e., peripheral vs central) at which opioids produce immune changes was investigated by injecting the agonists directly into the left lateral ventricle of the brain. Specifically, Lewis rats received an intracerebroventricular administration of [d-Ala(2),N-Me-Phe(4), Gly-ol(5)]enkephalin (DAMGO), a mu-receptor selective agonist, [D-Pen(2,5)]enkephalin (DPDPE), a delta-opioid receptor agonist, or U69,593, a kappa-receptor agonist. Immune assessments completed 1 h following drug administration showed that the mu-receptor selective agonist DAMGO produced a dose-dependent decrease in natural killer cell activity and T-lymphocyte proliferation to the mitogen concanavalin A (Con A); no immunological changes were found following DPDPE or U69,593 treatment. Calculation of the number of white blood cells per sample showed no differences between rats treated with saline and rats treated with any of the selective agonists. Administration of the opioid antagonist N-methylnaltrexone prior to DAMGO treatment attenuated the DAMGO-induced changes in immune status. Results from the present study indicate that the immunomodulatory effects of opioids can be attributed to interactions with the mu-opioid receptor.

Basal nitric oxide limits immune, nervous and cardiovascular excitation: human endothelia express a mu opiate receptor

Nitric oxide (NO) is a major signaling molecule in the immune, cardiovascular and nervous systems. The synthesizing enzyme, nitric oxide synthase (NOS) occurs in three forms: endothelial (e), neuronal (n) and inducible (i) NOS. The first two are constitutively expressed. We surmise that in many tissues there is a basal level of NO and that the actions of several signaling molecules initiate increases in cNOS-derived NO to enhance momentary basal levels that exerts inhibitory cellular actions, via cellular conformational changes. It is our contention that much of the literature concerning the actions of NO really deal with i-NOS-derived NO. We make the case that cNOS is responsible for a basal or 'tonal' level of NO; that this NO keeps particular types of cells in a state of inhibition and that activation of these cells occurs through disinhibition. Furthermore, naturally occurring signaling molecules such as morphine, anandamide, interleukin-10 and 17-beta-estradiol appear to exert, in part, their beneficial physiological actions, i.e., immune and endothelial down regulation by the stimulation of cNOS. In regard to opiates, we demonstrate the presence of a human endothelial mu opiate receptor by RT-PCR and sequence determination, further substantiating the role of opiates in vascular coupling to NO release. Taken together, cNOS derived NO enhances basal NO actions, i.e., cellular activation state, and these actions are further enhanced by iNOS derived NO.

Heroin modulates the expression of inducible nitric oxide synthase

The use of heroin (diacetylmorphine) is associated with a high incidence of infectious disease, and the immunologic alterations responsible for heroin-induced changes in resistance to infection have not been well characterized. The present study tests the hypothesis that expression of inducible nitric oxide synthase (iNOS) is modulated by the administration of heroin. The initial study using rats showed that heroin administration (0, 0.01, 0.1, or 1.0 mg/kg s.c.) results in a pronounced reduction in lipopolysaccharide (LPS)-induced expression of iNOS mRNA in spleen, lung, and liver tissue as measured by RT-PCR. Heroin also produced a reduction in the level of plasma nitrite/nitrate, the more stable end-product of nitric oxide degradation. In a subsequent study, administration of the opioid receptor antagonist, naltrexone (0.1 mg/kg) prior to the injection of heroin (1.0 mg/kg) blocked the heroin-induced reduction of iNOS expression and plasma nitrite/nitrate levels indicating that the effect is mediated via the opioid-receptor. This study provides the first evidence that heroin induces an alteration of iNOS expression, and suggests that a reduction in nitric oxide production may be involved in the increased incidence of infectious diseases amongst heroin users.

Conformational analysis and automated receptor docking of selective arylacetamide-based kappa-opioid agonists

The three-dimensional structure, dynamics, and binding modes of representative kappa-opioid agonists of the arylacetamide class (U50, 488; U69,593; U62,066; CI-977; ICI199,441; ICI197,067; BRL52,537; and BRL52,656) have been investigated using molecular modeling techniques. Systematic exploration of the conformational space of the ligand combined with molecular dynamics (MD) simulations in water revealed consistent conformational preferences for all the kappa-agonists in this series. The results were further compared with available X-ray and 1D- and 2D-NMR data to identify potential "lead" conformers for molecular docking. Ligand binding modes were initially determined using automated docking of two of the ligands (U50,488 and BRL52,537) to the kappa-opioid receptor. Extrapolation of the predicted binding mode to other members in this ligand series revealed similar docking preferences, with each ligand docked along the receptor helical axis. The binding modes were further refined using MD simulations of the receptor-ligand complexes. The results show a that salt bridge is formed between the amino proton of the ligands and the carboxylate group of Asp138 in TM3. This interaction most likely serves as a key anchoring point for the agonist association. Additional ligand contacts were noted with kappa-specific residues Ile294, Leu295, and Ala298, which may, in part, explain the kappa-selectivity in this series. In comparing the arylacetamides with opiate-based ligands, no evidence was found to link these classes through a common binding motif (except for the ion pair). The binding site model was also applied to explain the enantiomeric preference of U50,488 and to provide insight to the mu/kappa-selectivity of representative ligands in this series. Overall, the results provide a structure-based rationale for ligand recognition that is consistent both with site-directed mutagenesis experiments and structure-function relationship data.

Role of central mu-opioid receptors in the modulation of nitric oxide production by splenocytes

Previous studies have shown that administration of morphine results in alterations of splenic macrophage nitric oxide production. The present studies were conducted to determine the subtype of opioid receptor involved in the modulation of macrophage nitric oxide production. Moreover, the present work was directed at determining whether nitric oxide production is regulated through opioid receptors in the central nervous system (CNS) or via opioid receptors found directly on splenocytes. The study shows that intracerebroventricular (i.c.v.) administration of the mu-selective opioid agonist, DAMGO, to rats dose-dependently increases the production of nitric oxide by splenocytes stimulated with toxic shock syndrome toxin (TSST-1). The effect of DAMGO is blocked by prior i.c.v. administration of N-methylnaltrexone. In contrast, i.c.v. administration of the kappa-selective agonist, U69,593, and the delta-selective agonist, DPDPE, have no significant effect on the production of nitric oxide. Furthermore, the in vitro administration of DAMGO, DPDPE, or U69,593 to splenocytes cultures does not significantly alter the production of nitric oxide by splenocytes. In addition, the present work shows that elevation of nitric oxide production by i.c.v. administration of DAMGO produces functional changes in splenic lymphocytes. Collectively, these results indicate that mu-opioid receptors within the CNS are involved in the regulation of splenic nitric oxide production.

Opiate binding sites in the cellular immune system: expression and regulation

The direct actions of opiates on the mammalian immune system depend on the existence of ligand binding sites either on the surface of the affected cell or in the interior of the cell. With the cloning of various opiate receptors from neuronal tissue, numerous researchers have screened leukocyte cDNA libraries for the expression of these receptors with some positive results. However, the pattern of expression of neuronal opiate receptors in the cellular immune system does not completely explain the biological action of opiates there. Several possibilities could account for this non-congruence including differential expression of the receptors as determined by such factors as cell population or prior history of the cells; the existence of sequence modified versions of the neuronal receptors such that the amplification methods miss their presence; or the opiates act by a different, non-receptor mechanism in the cellular immune system.

Ebselen in acute ischemic stroke: a placebo-controlled, double-blind clinical trial. Ebselen Study Group

BACKGROUND AND PURPOSE

The effect of ebselen, a seleno-organic compound with antioxidant activity through a glutathione peroxidase-like action, on the outcome of acute ischemic stroke was evaluated in a multicenter, placebo-controlled, double-blind clinical trial.

METHODS

Patients diagnosed as having acute ischemic stroke who could receive drug treatment within 48 hours of stroke onset were enrolled. Oral administration of ebselen granules suspended in water (150 mg BID) or placebo was started immediately after admission and was continued for 2 weeks. The major end points were the Glasgow Outcome Scale scores at 1 month and 3 months after the start of treatment. The modified Mathew Scale and modified Barthel Index scores at 1 month and 3 months were also studied as secondary outcome measures.

RESULTS

Three hundred two patients were enrolled in the trial. Intent-to-treat analysis of 300 patients (151 given ebselen and 149 given placebo) revealed that ebselen treatment achieved a significantly better outcome than placebo at 1 month (P = .023, Wilcoxon rank sum test) but not at 3 months (P = .056, Wilcoxon rank sum test). The improvement was significant in patients who started ebselen within 24 hours of stroke onset but not in those who started treatment after 24 hours. There was a corresponding improvement in the modified Mathew Scale and modified Barthel Index scores.

CONCLUSIONS

Early treatment with ebselen improved the outcome of acute ischemic stroke. Ebselen may be a promising neuroprotective agent.

Identification in the mu-opioid receptor of cysteine residues responsible for inactivation of ligand binding by thiol alkylating and reducing agents

Inactivation by thiol reducing and alkylating agents of ligand binding to the human mu-opioid receptor was examined. Dithiothreitol reduced the number of [3H]diprenorphine binding sites. Replacement by seryl residues of either C142 or C219 in extracellular loops 1 and 2 of the mu receptor resulted in a complete loss of opioid binding. A disulfide bound linking C142 to C219 may thus be essential to maintain a functional conformation of the receptor. We also demonstrated that inactivation of ligand binding upon alkylation by N-ethylmaleimide occurred at two sites. Alteration of the more sensitive (IC50 = 20 microM) did not modify antagonists binding but decreased agonist affinity almost 10-fold. Modification of the less reactive site (IC50 = 2 mM) decreased the number of both agonist and antagonist binding sites. The alkylation site of higher sensitivity to N-ethylmaleimide was shown by mutagenesis experiments to be constituted of both C81 and C332 in transmembrane domains 1 and 7 of the mu-opioid receptor.

14 beta-Chlorocinnamoylamino derivatives of metopon: long-term mu-opioid receptor antagonists

The affinity, selectivity and antinociceptive properties of 5 beta-methyl-14 beta-(p-chlorocinnamoylamino)-7,8-dihydromorphinone (MET-Cl-CAMO) and N-cyclopropyl-methyl-5 beta-methyl-14 beta-(p-chlorocinnamoylamino)-7, 8-dihydronormorphinone (N-CPM-MET-Cl-CAMO) for the multiple opioid receptors were characterized. In competition binding assays using bovine striatal membranes, both compounds inhibited the binding of 0.25 nM [3H][D-Ala2, (Me)-Phe4,Gly(ol)5]enkephalin (DAMGO) with IC 50 values of less than 2 nM. Preincubation of membranes with MET-CI-CAMO and N-CPM-MET-Cl-CAMO produced a concentration-dependent, wash-resistant inhibition of mu-opioid receptor binding. Saturation binding experiments with N-CPM-MET-Cl-CAMO showed a reduction in the number of mu-opioid binding sites without a change in affinity. In the mouse 55 degrees C warm-water tail-flick assay, neither MET-Cl-CAMO nor N-CPM-MET-Cl-CAMO at doses up to 100 nmol produced antinociception after intracerebroventricular administration, but morphine-induced antinociception was antagonized in a time- and dose-dependent manner by both compounds. The antagonism produced by 1 nmol of either MET-Cl-CAMO or N-CPM-MET-Cl-CAMO reached a maximal effect after 24 h, and lasted up to 48 h. Analgesia mediated by delta- or kappa-opioids was not altered by either compound. In summary, the data suggest that MET-Cl-CAMO and N-CPM-MET-Cl-CAMO are long-term, mu-opioid receptor antagonists, devoid of agonist properties in the mouse tail-flick assay, and that N-CPM-MET-Cl-CAMO may produce its antagonistic effects by binding irreversibly to the mu-opioid receptor.

High affinity binding of 125I-neurotensin to dispersed cells from chicken liver and brain

Dispersed cells from chicken brain and liver were found to possess cell surface binding sites for 125I-neurotensin (125I-NT). Scatchard analyses indicated the presence of high affinity (K4, 25-80 pM) and low affinity (Kd, 250-450 pM) components in adult tissues. Binding capacity was reduced 25-40% by incubation with pertussis toxin. Ontogenetic studies indicated that NT receptor capacity increased approximately 20-fold from the embryonic stage to adult. Cross-linking of 125I-NT to intact cells labeled one major band (52 kDa, > or = 90%) and two minor bands (40 and 90 kDa, < or = 10%) which could represent distinct NT-receptors or one receptor partly degraded or cross-linked to G-protein(s). The binding of 125I-NT to dispersed cells was enhanced by reduction with dithoithreitol and suppressed by alkylation with N-ethyl-maleimide (NEM), maleimidocaproic acid (MCA) and p-chloromercuribenzenesulfonate (PCMBS). Since MCA and PCMBS do not permeate cells, this suggests that the sulfhydryl group(s) critical to binding are located within the NT receptor itself. Preincubation of cells with NT prior to treatment with NEM diminished its inhibitory effect, suggesting that the critical SH-group(s) were within the NT binding pocket or were protected by an allosteric effect. These results suggest that one or more of the nine cysteine residues in the NT receptor is involved in the NT binding reaction.

Synthesis, biological evaluation, and quantitative receptor docking simulations of 2-[(acylamino)ethyl]-1,4-benzodiazepines as novel tifluadom-like ligands with high affinity and selectivity for kappa-opioid receptors

The synthesis and biological evaluation of a series of 2-substituted 5-phenyl-1,4-benzodiazepines, structurally related to tifluadom (5), the only benzodiazepine that acts simultaneously as a kappa-opioid agonist and a cholecystokinin-A (CCK-A) antagonist, are reported. The radioligand binding models used in these studies were [(125)I](BH)-CCK-8 in rat pancreas (CCK-A), [(3)H]-(MENLE(28,31))-cck-8 in guinea pig cerebral cortex (CCK-B), and [(3)H]U-69593 (kappa(1)), [(3)H]DAMGO (mu), and [(3)H]DADLE (delta) in guinea pig brain. All the title compounds were devoid of significant affinity for both CCK-A and CCK-B receptors, while some of them bound with nanomolar affinity and high selectivity for kappa-opioid receptors. In particular, the 2-thienyl derivative 7A(X = H) with a K(i) = 0.50 nM represents a clear improvement with respect to tifluadom, showing a comparable potency but higher selectivity. The application of computational simulations and linear regression analysis techniques to the complexes between guinea pig kappa (kappa(1))-receptor and the title compounds allowed the identification of the structural determinants for recognition and quantitative elucidation of the structure-affinity relationships in this class of receptors.

Inhibition of primary murine macrophage cytokine production in vitro following treatment with the kappa-opioid agonist U50,488H

Previous work in our laboratory has shown that both mu- and kappa-opioid agonists exhibit immunosuppressive activity for antibody responses in vitro. Our earlier work has suggested that both accessory cells and T cells may be altered following treatment with the kappa-opioid agonist U50,488H. We intend to further determine the identity of the immune cell population(s) which are affected by opioid treatment, and to determine the nature of the opioid receptor type expressed on these cells. In this study, non- elicited peritoneal macrophages were treated simultaneously with the kappa-agonist U50,488H and lipopolysaccharide (LPS), and the levels of the cytokines interleukin (IL)-1, IL-6 and tumor necrosis factor (TNF)-alpha were determined. The results show that U50,488H had a suppressive effect on the production of TNF-alpha and IL-1 at concentrations as low as 1 nM, while IL-6 was suppressed at concentrations as low as 10 nM. Additional experiments utilizing the opiate antagonist naloxone and the kappa-selective antagonist norbinaltorphimine (norBNI) were performed in order to further characterize the opioid receptor involved in the cytokine suppression produced by treatment with U50,488H. Results showed that naloxone was able to partially block U50,488H suppression while norBNI was able to completely reverse the suppression of IL-6 production. These results suggest that macrophage/monocyte function is significantly modulated following activation of the kappa-opioid receptor.