Protein-protein interactions at the adrenergic receptors

Norepinephrine protects against cochlear outer hair cell damage and noise-induced hearing loss via α2A-adrenergic receptor

BACKGROUND

The cochlear sympathetic system plays a key role in auditory function and susceptibility to noise-induced hearing loss (NIHL). The formation of reactive oxygen species (ROS) is a well-documented process in NIHL. In this study, we aimed at investigating the effects of a superior cervical ganglionectomy (SCGx) on NIHL in Sprague-Dawley rats.

METHODS

We explored the effects of unilateral and bilateral Superior Cervical Ganglion (SCG) ablation in the eight-ten weeks old Sprague-Dawley rats of both sexes on NIHL. Auditory function was evaluated by auditory brainstem response (ABR) testing and Distortion product otoacoustic emissions (DPOAEs). Outer hair cells (OHCs) counts and the expression of α2A-adrenergic receptor (AR) in the rat cochlea using immunofluorescence analysis. Cells culture and treatment, CCK-8 assay, Flow cytometry staining and analysis, and western blotting were to explore the mechanisms of SCG fibers may have a protective role in NIHL.

RESULTS

We found that neither bilateral nor unilateral SCGx protected the cochlea against noise exposure. In HEI-OC1 cells, H2O2-induced oxidative damage and cell death were inhibited by the application of norepinephrine (NE). NE may prevent ROS-induced oxidative stress in OHCs and NIHL through the α2A-AR.

CONCLUSION

These results demonstrated that sympathetic innervation mildly affected cochlear susceptibility to acoustic trauma by reducing oxidative damage in OHCs through the α2A-AR. NE may be a potential therapeutic strategy for NIHL prevention.

Microvascular Capillary and Precapillary Cardiovascular Disturbances Strongly Interact to Severely Affect Tissue Perfusion and Mitochondrial Function in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome Evolving from the Post COVID-19 Syndrome

Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) is a frequent, debilitating and still enigmatic disease. There is a broad overlap in the symptomatology of ME/CFS and the Post-COVID-19 Syndrome (PCS). A fraction of the PCS patients develop the full clinical picture of ME/CFS. New observations in microvessels and blood from patients suffering from PCS have appeared and include microclots and malformed pathological blood cells. Capillary blood flow is impaired not only by pathological blood components but also by prothrombotic changes in the vascular wall, endothelial dysfunction, and the expression of adhesion molecules in the capillaries. These disturbances can finally cause a low capillary flow and even capillary stasis. A low cardiac stroke volume due to hypovolemia and the inability of the capacitance vessels to adequately constrict to deliver the necessary cardiac preload generate an unfavorable low precapillary perfusion pressure. Furthermore, a predominance of vasoconstrictor over vasodilator influences exists, in which sympathetic hyperactivity and endothelial dysfunction play a strong role, causing the constriction of resistance vessels and of precapillary sphincters, which leads to a fall in capillary pressure behind the sphincters. The interaction of these two precapillary cardiovascular mechanisms causing a low capillary perfusion pressure is hemodynamically highly unfavorable in the presence of a primary capillary stasis, which is already caused by the pathological blood components and their interaction with the capillary wall, to severely impair organ perfusion. The detrimental coincidence of microcirculatory and precapillary cardiovascular disturbances may constitute the key disturbance of the Post-COVID-19 syndrome and finally lead to ME/CFS in predisposed patients because the interaction causes a particular kind of perfusion disturbance-capillary ischemia/reperfusion-which has a high potential of causing mitochondrial dysfunction by inducing sodium- and calcium-overload in skeletal muscles. The latter, in turn, worsens the vascular situation through the generation of reactive oxygen species to close a vicious cycle from which the patient can hardly escape.

Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) and Comorbidities: Linked by Vascular Pathomechanisms and Vasoactive Mediators?

Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) is often associated with various other syndromes or conditions including mast cell activation (MCA), dysmenorrhea and endometriosis, postural tachycardia (POTS) and small fiber neuropathy (SFN). The causes of these syndromes and the reason for their frequent association are not yet fully understood. We previously published a comprehensive hypothesis of the ME/CFS pathophysiology that explains the majority of symptoms, findings and chronicity of the disease. We wondered whether some of the identified key pathomechanisms in ME/CFS are also operative in MCA, endometriosis and dysmenorrhea, POTS, decreased cerebral blood flow and SFN, and possibly may provide clues on their causes and frequent co-occurrence. Our analysis indeed provides strong arguments in favor of this assumption, and we conclude that the main pathomechanisms responsible for this association are excessive generation and spillover into the systemic circulation of inflammatory and vasoactive tissue mediators, dysfunctional β2AdR, and the mutual triggering of symptomatology and disease initiation. Overall, vascular dysfunction appears to be a strong common denominator in these linkages.

Dexmedetomidine Attenuates Lipopolysaccharide-Induced Sympathetic Activation and Sepsis via Suppressing Superoxide Signaling in Paraventricular Nucleus

Sympathetic overactivity contributes to the pathogenesis of sepsis. The selective α2-adrenergic receptor agonist dexmedetomidine (DEX) is widely used for perioperative sedation and analgesia. We aimed to determine the central roles and mechanisms of DEX in attenuating sympathetic activity and inflammation in sepsis. Sepsis was induced by a single intraperitoneal injection of lipopolysaccharide (LPS) in rats. Effects of DEX were investigated 24 h after injection of LPS. Bilateral microinjection of DEX in the paraventricular nucleus (PVN) attenuated LPS-induced sympathetic overactivity, which was attenuated by the superoxide dismutase inhibitor DETC, cAMP analog db-cAMP or GABA_A receptor antagonist gabazine. Superoxide scavenger tempol, NADPH oxidase inhibitor apocynin, adenylate cyclase inhibitor SQ22536 or PKA inhibitor Rp-cAMP caused similar effects to DEX in attenuating LPS-induced sympathetic activation. DEX inhibited LPS-induced superoxide and cAMP production, as well as NADPH oxidase, adenylate cyclase and PKA activation. The roles of DEX in reducing superoxide production and NADPH oxidase activation were attenuated by db-cAMP or gabazine. Intravenous infusion of DEX inhibited LPS-induced sympathetic overactivity, NOX activation, superoxide production, TNF-α and IL-1β upregulation in the PVN and plasma, as well as lung and renal injury, which were attenuated by the PVN microinjection of yohimbine and DETC. We conclude that activation of α2-adrenergic receptors with DEX in the PVN attenuated LPS-induced sympathetic overactivity by reducing NADPH oxidase-dependent superoxide production via both inhibiting adenylate cyclase-cAMP-PKA signaling and activating GABA_A receptors. The inhibition of NADPH oxidase-dependent superoxide production in the PVN partially contributes to the roles of intravenous infusion of DEX in attenuating LPS-induced sympathetic activation, oxidative stress and inflammation.

A Photoswitchable Ligand Targeting the β 1 ‐Adrenoceptor Enables Light‐Control of the Cardiac Rhythm**

Catecholamine‐triggered β‐adrenoceptor (β‐AR) signaling is essential for the correct functioning of the heart. Although both β₁‐ and β₂‐AR subtypes are expressed in cardiomyocytes, drugs selectively targeting β₁‐AR have proven this receptor as the main target for the therapeutic effects of beta blockers in the heart. Here, we report a new strategy for the light‐control of β₁‐AR activation by means of photoswitchable drugs with a high level of β₁‐/β₂‐AR selectivity. All reported molecules allow for an efficient real‐time optical control of receptor function in vitro. Moreover, using confocal microscopy we demonstrate that the binding of our best hit, pAzo‐2, can be reversibly photocontrolled. Strikingly, pAzo‐2 also enables a dynamic cardiac rhythm management on living zebrafish larvae using light, thus highlighting the therapeutic and research potential of the developed photoswitches. Overall, this work provides the first proof of precise control of the therapeutic target β₁‐AR in native environments using light.

Central Gαi2 Protein Mediated Neuro-Hormonal Control of Blood Pressure and Salt Sensitivity

Hypertension, a major public health issue, is estimated to contribute to 10% of all deaths worldwide. Further, the salt sensitivity of blood pressure is a critical risk factor for the development of hypertension. The hypothalamic paraventricular nucleus (PVN) coordinates neuro-hormonal responses to alterations in plasma sodium and osmolality and multiple G Protein-Coupled Receptors (GPCRs) are involved in fluid and electrolyte homeostasis. In acute animal studies, our laboratory has shown that central Gαi/o subunit protein signal transduction mediates hypotensive and bradycardic responses and that Gz/q, proteins mediate the release of arginine vasopressin (AVP) and subsequent aquaretic responses to acute pharmacological stimuli. Extending these studies, our laboratory has shown that central Gαi2 proteins selectively mediate the hypotensive, sympathoinhibitory and natriuretic responses to acute pharmacological activation of GPCRs and in response to acute physiological challenges to fluid and electrolyte balance. In addition, following chronically elevated dietary sodium intake, salt resistant rats demonstrate site-specific and subunit-specific upregulation of Gαi2 proteins in the PVN, resulting in sympathoinhibition and normotension. In contrast, chronic dietary sodium intake in salt sensitive animals, which fail to upregulate PVN Gαi2 proteins, results in the absence of dietary sodium-evoked sympathoinhibition and salt sensitive hypertension. Using in situ hybridization, we observed that Gαi2 expressing neurons in parvocellular division of the PVN strongly (85%) colocalize with GABAergic neurons. Our data suggest that central Gαi2 protein-dependent responses to an acute isotonic volume expansion (VE) and elevated dietary sodium intake are mediated by the peripheral sensory afferent renal nerves and do not depend on the anteroventral third ventricle (AV3V) sodium sensitive region or the actions of central angiotensin II type 1 receptors. Our translational human genomic studies have identified three G protein subunit alpha I2 (GNAI2) single nucleotide polymorphisms (SNPs) as potential biomarkers in individuals with salt sensitivity and essential hypertension. Collectively, PVN Gαi2 proteins-gated pathways appear to be highly conserved in salt resistance to counter the effects of acute and chronic challenges to fluid and electrolyte homeostasis on blood pressure via a renal sympathetic nerve-dependent mechanism.

Toxoplasmosis: Targeting neurotransmitter systems in psychiatric disorders

The most common form of the disease caused by Toxoplasma gondii (T. gondii) is latent toxoplasmosis due to the formation of tissue cysts in various organs, such as the brain. Latent toxoplasmosis is probably a risk factor in the development of some neuropsychiatric disorders. Behavioral changes after infection are caused by the host immune response, manipulation by the parasite, central nervous system (CNS) inflammation, as well as changes in hormonal and neuromodulator relationships. The present review focused on the exact mechanisms of T. gondii effect on the alteration of behavior and neurotransmitter levels, their catabolites and metabolites, as well as the interaction between immune responses and this parasite in the etiopathogenesis of psychiatric disorders. The dysfunction of neurotransmitters in the neural transmission is associated with several neuropsychiatric disorders. However, further intensive studies are required to determine the effect of this parasite on altering the level of neurotransmitters and the role of neurotransmitters in the etiology of host behavioral changes.

Suppression of P2X3 receptor-mediated currents by the activation of α2A -adrenergic receptors in rat dorsal root ganglion neurons

Aims

The α₂‐adrenergic receptor (α₂‐AR) agonists have been shown to be effective in the treatment of various pain. For example, dexmedetomidine (DEX), a selective α_2A‐AR agonist, can be used for peripheral analgesia. However, it is not yet fully elucidated for the precise molecular mechanisms. P2X3 receptor is a major receptor processing nociceptive information in primary sensory neurons. Herein, we show that a functional interaction of α_2A‐ARs and P2X3 receptors in dorsal root ganglia (DRG) neurons could contribute to peripheral analgesia of DEX.

Methods

Electrophysiological recordings were carried out on rat DRG neurons, and nociceptive behavior was quantified in rats.

Results

The activation of α_2A‐ARs by DEX suppressed P2X3 receptor‐mediated and α,β‐methylene‐ATP (α,β‐meATP)‐evoked inward currents in a concentration‐dependent and voltage‐independent manner. Pre‐application of DEX shifted the α,β‐meATP concentration‐response curve downwards, with a decrease of 50.43 ± 4.75% in the maximal current response of P2X3 receptors to α,β‐meATP in the presence of DEX. Suppression of α,β‐meATP‐evoked currents by DEX was blocked by the α_2A‐AR antagonist BRL44408 and prevented by intracellular application of the G_i/o protein inhibitor pertussis toxin, the adenylate cyclase activator forskolin, and the cAMP analog 8‐Br‐cAMP. DEX also suppressed α,β‐meATP‐evoked action potentials through α_2A‐ARs in rat DRG neurons. Finally, the activation of peripheral α_2A‐ARs by DEX had an analgesic effect on the α,β‐meATP‐induced nociception.

Conclusions

These results suggested that activation of α_2A‐ARs by DEX suppressed P2X3 receptor‐mediated electrophysiological and behavioral activity via a G_i/o proteins and cAMP signaling pathway, which was a novel potential mechanism underlying analgesia of peripheral α_2A‐AR agonists.

Co-Evolution of Opioid and Adrenergic Ligands and Receptors: Shared, Complementary Modules Explain Evolution of Functional Interactions and Suggest Novel Engineering Possibilities

Cross-talk between opioid and adrenergic receptors is well-characterized and involves second messenger systems, the formation of receptor heterodimers, and the presence of extracellular allosteric binding regions for the complementary ligand; however, the evolutionary origins of these interactions have not been investigated. We propose that opioid and adrenergic ligands and receptors co-evolved from a common set of modular precursors so that they share binding functions. We demonstrate the plausibility of this hypothesis through a review of experimental evidence for molecularly complementary modules and report unexpected homologies between the two receptor types. Briefly, opioids form homodimers also bind adrenergic compounds; opioids bind to conserved extracellular regions of adrenergic receptors while adrenergic compounds bind to conserved extracellular regions of opioid receptors; opioid-like modules appear in both sets of receptors within key ligand-binding regions. Transmembrane regions associated with homodimerization of each class of receptors are also highly conserved across receptor types and implicated in heterodimerization. This conservation of multiple functional modules suggests opioid–adrenergic ligand and receptor co-evolution and provides mechanisms for explaining the evolution of their crosstalk. These modules also suggest the structure of a primordial receptor, providing clues for engineering receptor functions.

Pathophysiology of skeletal muscle disturbances in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS)

Chronic Fatigue Syndrome or Myalgic Encephaloymelitis (ME/CFS) is a frequent debilitating disease with an enigmatic etiology. The finding of autoantibodies against ß2-adrenergic receptors (ß2AdR) prompted us to hypothesize that ß2AdR dysfunction is of critical importance in the pathophysiology of ME/CFS. Our hypothesis published previously considers ME/CFS as a disease caused by a dysfunctional autonomic nervous system (ANS) system: sympathetic overactivity in the presence of vascular dysregulation by ß2AdR dysfunction causes predominance of vasoconstrictor influences in brain and skeletal muscles, which in the latter is opposed by the metabolically stimulated release of endogenous vasodilators (functional sympatholysis). An enigmatic bioenergetic disturbance in skeletal muscle strongly contributes to this release. Excessive generation of these vasodilators with algesic properties and spillover into the systemic circulation could explain hypovolemia, suppression of renin (paradoxon) and the enigmatic symptoms. In this hypothesis paper the mechanisms underlying the energetic disturbance in muscles will be explained and merged with the first hypothesis. The key information is that ß2AdR also stimulates the Na+/K+-ATPase in skeletal muscles. Appropriate muscular perfusion as well as function of the Na+/K+-ATPase determine muscle fatigability. We presume that dysfunction of the ß2AdR also leads to an insufficient stimulation of the Na+/K+-ATPase causing sodium overload which reverses the transport direction of the sodium-calcium exchanger (NCX) to import calcium instead of exporting it as is also known from the ischemia-reperfusion paradigm. The ensuing calcium overload affects the mitochondria, cytoplasmatic metabolism and the endothelium which further worsens the energetic situation (vicious circle) to explain postexertional malaise, exercise intolerance and chronification. Reduced Na+/K+-ATPase activity is not the only cause for cellular sodium loading. In poor energetic situations increased proton production raises intracellular sodium via sodium-proton-exchanger subtype-1 (NHE1), the most important proton-extruder in skeletal muscle. Finally, sodium overload is due to diminished sodium outward transport and enhanced cellular sodium loading. As soon as this disturbance would have occurred in a severe manner the threshold for re-induction would be strongly lowered, mainly due to an upregulated NHE1, so that it could repeat at low levels of exercise, even by activities of everyday life, re-inducing mitochondrial, metabolic and vascular dysfunction to perpetuate the disease.

Astrocytes as Perspective Targets of Exercise- and Caloric Restriction-Mimetics

Enhanced mental and physical activity can have positive effects on the function of aging brain, both in the experimental animals and human patients, although cellular mechanisms underlying these effects are currently unclear. There is a growing evidence that pre-clinical stage of many neurodegenerative diseases involves changes in interactions between astrocytes and neurons. Conversely, astrocytes are strategically positioned to mediate the positive influence of physical activity and diet on neuronal function. Thus, development of therapeutic agents which could improve the astroglia-neuron communications in ageing brain is of crucial importance. Recent advances in studies of cellular mechanisms of brain longevity suggest that astrocyte-neuron communications have a vital role in the beneficial effects of caloric restriction, physical exercise and their pharmacological mimetics on synaptic homeostasis and cognitive function. In particular, our recent data indicate that noradrenaline uptake inhibitor atomoxetine can enhance astrocytic Ca²⁺-signaling and astroglia-driven modulation of synaptic plasticity. Similar effects were exhibited by caloric restriction-mimetics metformin and resveratrol. The emerged data also suggest that astrocytes could be involved in the modulatory action of caloric restriction and its mimetics on neuronal autophagy. Still, the efficiency of astrocyte-targeting compounds in preventing age-related cognitive decline is yet to be fully explored, in particular in the animal models of neurodegenerative diseases and autophagy impairment.

Adrenergic signalling in osteoarthritis

Adrenoceptors (ARs) mediate the effects of the sympathetic neurotransmitters norepinephrine (NE) and epinephrine (E) in the human body and play a central role in physiologic and pathologic processes. Therefore, ARs have long been recognized as targets for therapeutic agents, especially in the field of cardiovascular medicine. During the past decades, the contribution of the sympathetic nervous system (SNS) and particularly of its major peripheral catecholamine NE to the pathogenesis of osteoarthritis (OA) attracted growing interest. OA is the most common degenerative joint disorder worldwide and a disease of the whole joint. It is characterized by progressive degradation of articular cartilage, synovial inflammation, osteophyte formation, and subchondral bone sclerosis mostly resulting in chronic pain. The subchondral bone marrow, the periosteum, the synovium, the vascular meniscus and numerous tendons and ligaments are innervated by tyrosine hydroxylase-positive (TH+) sympathetic nerve fibers that release NE into the synovial fluid and cells of all abovementioned joint tissues express at least one out of nine AR subtypes. During the past decades, several in vitro studies explored the AR-mediated effects of NE on different cell types in the joint. So far, only a few studies used animal OA models to investigate the contribution of distinct AR subtypes to OA pathogenesis in vivo. This narrative review shortly summarizes the current background knowledge about ARs and their signalling pathways at first. In the second part, we focus on recent findings in the field of NE-induced AR-mediated signalling in different joint tissues during OA pathogenesis and at the end, we will delineate the potential of targeting the adrenergic signalling for OA prevention or treatment. We used the PubMed bibliographic database to search for keywords such as 'joint' or 'cartilage' or 'synovium' or 'bone' and 'osteoarthritis' and/or 'trauma' and 'sympathetic nerve fibers' and/or 'norepinephrine' and 'adrenergic receptors / adrenoceptors' as well as 'adrenergic therapy'.

Analgesic Efficacy of α2 Adrenergic Receptor Agonists Depends on the Chronic State of Neuropathic Pain: Role of Regulator of G Protein Signaling 4

The analgesic effect of alpha-2 adrenergic receptor (α₂AR) agonists, which relieve chronic neuropathic pain, is highly variable among individuals. Here, we used a mouse model of spared nerve injury (SNI) to show that treatment time after the establishment of neuropathic pain was important for the variability in the analgesic efficacy of α₂AR agonists, which was related to the activity of regulator of G-protein signaling protein 4 (RGS4). Intrathecal treatment with α₂AR agonists, clonidine (0.1-1 nmol) or dexmedetomidine (0.3-1 nmol), relieved mechanical allodynia and thermal hyperalgesia on postoperative day (POD) 14, but their efficacy was weaker on POD28 and absent on POD56. The RGS4 level of plasma membrane was increased on POD56 compared to that on POD14. Moreover, in RGS4-deficient or RGS4 inhibitor (CCG50014)-treated mice, the analgesic effect of the α₂AR agonists was conserved even on POD56. The increased plasma membrane RGS4 expression and the reduced level of active G_αi after clonidine injection on POD56 were completely restored by CCG50014. Higher doses of clonidine (10 nmol) and dexmedetomidine (3 nmol) relieved neuropathic pain on POD56 but were accompanied with serious side effects. Whereas, the coadministration of CCG50014 with clonidine (1 nmol) or dexmedetomidine (1 nmol) did not cause side effects. These findings demonstrated that SNI-induced increase in plasma membrane RGS4 expression was associated with low efficacy of α₂AR agonists in a model of persistent, chronic neuropathic pain. Furthermore, α₂AR agonist administration together with RGS4-targeted intervention represents a novel strategy for the treatment of neuropathic pain to overcome dose-limiting side effects.

A Unifying Hypothesis of the Pathophysiology of Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS): Recognitions from the finding of autoantibodies against ß2-adrenergic receptors

Myalgic Encephalomyelitis or Chronic Fatigue Syndrome (CFS/ME) is a complex and severely disabling disease with a prevalence of 0.3% and no approved treatment and therefore a very high medical need. Following an infectious onset patients suffer from severe central and muscle fatigue, chronic pain, cognitive impairment, and immune and autonomic dysfunction. Although the etiology of CFS/ME is not solved yet, there is numerous evidence for an autoantibody mediated dysregulation of the immune and autonomic nervous system. We found elevated ß2 adrenergic receptor (ß2AdR) and M3 acetylcholine receptor antibodies in a subset of CFS/ME patients. As both ß2AdR and M3 acetylcholine receptor are important vasodilators, we would expect their functional disturbance to result in vasoconstriction and hypoxemia. An impaired circulation and oxygen supply could result in many symptoms of ME/CFS. There are consistent reports of vascular dysfunction in ME/CFS. Muscular and cerebral hypoperfusion has been shown in ME/CFS in various studies and correlated with fatigue. Metabolic changes in ME/CFS are also in line with a concept of hypoxia and ischemia. Here we try to develop a unifying working concept for the complex pathomechanism of ME/CFS based on the presence of dysfunctional autoantibodies against ß2AdR and M3 acetylcholine receptor and extrapolate it to the pathophysiology of ME/CFS without an autoimmune pathogenesis.

The role of neutrophils in neuro-immune modulation

Neutrophils are peripheral immune cells that represent the first recruited innate immune defense against infections and tissue injury. However, these cells can also induce overzealous responses and cause tissue damage. Although the role of neutrophils activating the immune system is well established, only recently their critical implications in neuro-immune interactions are becoming more relevant. Here, we review several aspects of neutrophils in the bidirectional regulation between the nervous and immune systems. First, the role of neutrophils as a diffuse source of acetylcholine and catecholamines is controversial as well as the effects of these neurotransmitters in neutrophil's functions. Second, neutrophils contribute for the activation and sensitization of sensory neurons, and thereby, in events of nociception and pain. In addition, nociceptor activation promotes an axon reflex triggering a local release of neural mediators and provoking neutrophil activation. Third, the recruitment of neutrophils in inflammatory responses in the nervous system suggests these immune cells as innovative targets in the treatment of central infectious, neurological and neurodegenerative disorders. Multidisciplinary studies involving immunologists and neuroscientists are required to define the role of the neurons-neutrophils communication in the pathophysiology of infectious, inflammatory, and neurological disorders.

Fatigue, Sleep, and Autoimmune and Related Disorders

Profound and debilitating fatigue is the most common complaint reported among individuals with autoimmune disease, such as systemic lupus erythematosus, multiple sclerosis, type 1 diabetes, celiac disease, chronic fatigue syndrome, and rheumatoid arthritis. Fatigue is multi-faceted and broadly defined, which makes understanding the cause of its manifestations especially difficult in conditions with diverse pathology including autoimmune diseases. In general, fatigue is defined by debilitating periods of exhaustion that interfere with normal activities. The severity and duration of fatigue episodes vary, but fatigue can cause difficulty for even simple tasks like climbing stairs or crossing the room. The exact mechanisms of fatigue are not well-understood, perhaps due to its broad definition. Nevertheless, physiological processes known to play a role in fatigue include oxygen/nutrient supply, metabolism, mood, motivation, and sleepiness-all which are affected by inflammation. Additionally, an important contributing element to fatigue is the central nervous system-a region impacted either directly or indirectly in numerous autoimmune and related disorders. This review describes how inflammation and the central nervous system contribute to fatigue and suggests potential mechanisms involved in fatigue that are likely exhibited in autoimmune and related diseases.

Detailed insight on β-adrenoceptors as therapeutic targets

Human G protein-coupled receptors (GPCRs), especially adrenoceptors, play a crucial role in maintaining important physiological activities including cardiovascular and pulmonary functions. Among all adrenoceptors, β-adrenoceptors are the best characterized GPCRs and possess distinctive features as drug targets. Similarly, ligands that activate/deactivate β-adrenoceptors also hold a significant position in the field of biomarker identification and drug discovery. Several studies regarding molecular characterization of the β-adrenoceptor ligands have revealed that ligands with abilities to inhibit basal or intrinsic receptor activity or prevent receptor desensitization are particularly important to efficiently manage detrimental health conditions, including chronic heart failure, asthma, chronic obstructive pulmonary disease, obesity, and diabetes. Given the importance of β-adrenoceptors as molecular targets for many pathological conditions, this review aims to provide a detailed insight on the structural and functional aspects of β-adrenoceptors, with a particular emphasis on their importance as biomarkers and therapeutic targets.

Updates in the function and regulation of α1 -adrenoceptors

α₁ -Adrenoceptors are seven transmembrane domain GPCRs involved in numerous physiological functions controlled by the endogenous catecholamines, noradrenaline and adrenaline, and targeted by drugs useful in therapeutics. Three separate genes, whose products are named α_1A -, α_1B -, and α_1D - adrenoceptors, encode these receptors. Although the existence of multiple α₁ -adrenoceptors has been acknowledged for almost 25 years, the specific functions regulated by each subtype are still largely unknown. Despite the limited comprehension, the identification of a single class of subtype-selective ligands for the α_1A - adrenoceptors, the so-called α-blockers for prostate dysfunction, has led to major improvement in therapeutics, demonstrating the need for continued efforts in the field. This review article surveys the tissue distribution of the three α₁ -adrenoceptor subtypes in the cardiovascular system, genitourinary system, and CNS, highlighting the functions already identified as mediated by the predominant activation of specific subtypes. In addition, this review covers the recent advances in the understanding of the molecular mechanisms involved in the regulation of each of the α₁ -adrenoceptor subtypes by phosphorylation and interaction with proteins involved in their desensitization and internalization. LINKED ARTICLES: This article is part of a themed section on Adrenoceptors-New Roles for Old Players. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.14/issuetoc.

Functional characterization of β2-adrenergic and insulin receptor heteromers

This study aimed to functionally characterize β₂-adrenergic (β₂AR) and insulin receptor (IR) heteromers in regard to β-arrestin 2 (βarr2) recruitment and cAMP signaling and to examine the involvement of the cytoplasmic portion of the IR β chain in heteromerization with β₂AR. Evidence for β₂AR:IR:βarr2 complex formation and the specificity of the IR:βarr2 interaction was first provided by bioinfomatics analysis. Receptor-heteromer investigation technology (HIT) then provided functional evidence of β₂AR:IR heterodimerization by showing isoproterenol-induced but not insulin-induced GFP²-βarr2 recruitment to the β₂AR:IR complex; the IR:βarr2 interaction was found to only be constitutive. The constitutive IR:βarr2 BRET signal (BRET_const) was significantly smaller in cells coexpressing IR-RLuc8 and a GFP²-βarr2 1-185 mutant lacking the proposed IR binding domain. β₂AR:IR heteromerization also influenced the pharmacological phenotype of β₂AR, i.e., its efficacy in recruiting βarr2 and activating cAMP signaling. Evidence suggesting involvement of the cytoplasmic portion of the IR β chain in the interaction with β₂AR was provided by BRET² saturation and HIT assays using an IR 1-1271 stop mutant lacking the IR C-terminal tail region. For the complex consisting of IR 1-1271-RLuc8:β₂AR-GFP², saturation was not reached, most likely reflecting random collisions between IR 1-1271 and β₂AR. Furthermore, in the HIT assay, no substantial agonist-induced increase in the BRET² signal was detected that would be indicative of βarr2 recruitment to the IR 1-1271:β₂AR heteromer. Complementary 3D visualization of β₂AR:IR provided supporting evidence for stability of the heterotetramer complex and identified amino acid residues involved in β₂AR:IR heteromerization. This article is part of the Special Issue entitled 'Receptor heteromers and their allosteric receptor-receptor interactions'.

Adrenergic signaling in teleost fish liver, a challenging path

Adrenergic receptors or adrenoceptors (ARs) belong to the huge family of G-protein coupled receptors (GPCRs) that have been well characterized in mammals primarily because of their importance as therapeutic drug targets. ARs are found across vertebrates and this review examines the path to identify and characterize these receptors in fish with emphasis on hepatic metabolism. The absence of reliable and specific pharmacological agents led investigators to define the fish hepatic AR system as relying solely on a β2-AR, cAMP-dependent signaling transduction pathway. The use of calcium-radiometric imaging, purified membranes for ligand-binding studies, and perifused rather than static cultured fish hepatocytes, unequivocally demonstrated that both α1- and β2-AR signaling systems existed in the fish liver consistent with studies in mammals. Additionally, the use of molecular tools and phylogenetic analysis clearly demonstrated the existence of multiple AR-types and -subtypes in hepatic and other tissues of a number of fish species. This review also examines the use of β-blockers as pharmaceuticals and how these drugs that are now in the aquatic environment may be impacting aquatic species including fish and some invertebrates. Clearly there is a large conservation of structure and function within the AR system of vertebrates but there remain a number of key questions that need to be addressed before a clear understanding of these systems can be resolved.

Rediscovery of nefopam for the treatment of neuropathic pain

Nefopam (NFP) is a non-opioid, non-steroidal, centrally acting analgesic drug that is derivative of the non-sedative benzoxazocine, developed and known in 1960s as fenazocine. Although the mechanisms of analgesic action of NFP are not well understood, they are similar to those of triple neurotransmitter (serotonin, norepinephrine, and dopamine) reuptake inhibitors and anticonvulsants. It has been used mainly as an analgesic drug for nociceptive pain, as well as a treatment for the prevention of postoperative shivering and hiccups. Based on NFP's mechanisms of analgesic action, it is more suitable for the treatment of neuropathic pain. Intravenous administration of NFP should be given in single doses of 20 mg slowly over 15-20 min or with continuous infusion of 60-120 mg/d to minimize adverse effects, such as nausea, cold sweating, dizziness, tachycardia, or drowsiness. The usual dose of oral administration is three to six times per day totaling 90-180 mg. The ceiling effect of its analgesia is uncertain depending on the mechanism of pain relief. In conclusion, the recently discovered dual analgesic mechanisms of action, namely, a) descending pain modulation by triple neurotransmitter reuptake inhibition similar to antidepressants, and b) inhibition of long-term potentiation mediated by NMDA from the inhibition of calcium influx like gabapentinoid anticonvulsants or blockade of voltage-sensitive sodium channels like carbamazepine, enable NFP to be used as a therapeutic agent to treat neuropathic pain.

Three basic residues of intracellular loop 3 of the beta-1 adrenergic receptor are required for golgin-160-dependent trafficking

Golgin-160 is a member of the golgin family of proteins, which have been implicated in the maintenance of Golgi structure and in vesicle tethering. Golgin-160 is atypical; it promotes post-Golgi trafficking of specific cargo proteins, including the β-1 adrenergic receptor (β1AR), a G protein-coupled receptor. Here we show that golgin-160 binds directly to the third intracellular loop of β1AR and that this binding depends on three basic residues in this loop. Mutation of the basic residues does not affect trafficking of β1AR from the endoplasmic reticulum through the Golgi complex, but results in reduced steady-state levels at the plasma membrane. We hypothesize that golgin-160 promotes incorporation of β1AR into specific transport carriers at the trans-Golgi network to ensure efficient delivery to the cell surface. These results add to our understanding of the biogenesis of β1AR, and suggest a novel point of regulation for its delivery to the plasma membrane.

Expression of adrenergic receptors in bovine and rabbit oocytes and preimplantation embryos

Catecholamines play an important role in embryogenesis, and data obtained in the rodent model indicate that they can act even during the preimplantation period of development. Using RT-PCR with specific oligonucleotide primers distinguishing among all members of the adrenergic receptor family, we examined expression of adrenergic receptors in bovine and rabbit oocytes, morulas and blastocysts. We found several profiles of adrenoceptor mRNA expression. Transcripts for some receptor subtypes (bovine alpha 2 receptors, rabbit α2A, α2C, β1 and β2 receptors) were detected at all examined stages, which suggests receptor expression throughout (or at most stages) the preimplantation developmental period. Expression in oocytes but not at later stages was found in only one adrenoceptor subtype (rabbit α1B). In contrast, mRNA for several adrenoceptors was found in embryos but not in oocytes (bovine beta adrenoceptors and rabbit α1A). Nucleotide sequences of our PCR products amplified in rabbit oocytes, and preimplantation embryos represent the first published mRNA sequences (partial sequences coding at least one transmembrane region) of rabbit α2C, β1 and β2 adrenoceptors. Our results suggest that the expression of adrenergic receptors can be a general feature of mammalian oocytes and preimplantation embryos. On the other hand, comparison of three mammalian species (cattle, rabbit and mouse) revealed possible interspecies differences in the expression of particular adrenoceptor subtypes. Our results support the opinion that stress mediators can act directly in cells of preimplantation embryos.

β-Adrenergic signaling promotes posteriorization in Xenopus early development

Adrenaline (also known as Epinephrine) is a hormone, which works as major regulator of various biological events such stages of vertebrate, the role of adrenaline for early embryogenesis has been as heart rate, blood vessel and air passage diameters, and metabolic shifts. Although its specific receptors are expressing at the early developmental stage those functions are poorly understood. Here, we show that loss-of-functional effects of adrenergic receptor β-2 (Adrβ2), which was known as the major receptor for adrenaline and highly expressed in embryonic stages, led posterior defects at the tadpole stage of Xenopus embryos, while embryos injected with Adrβ2 mRNA or treated with adrenaline hormone adversely lost anterior structures. This posteriorization effect by adrenaline hormone was dose-dependently increased but effectively rescued by microinjection of antisense morpholino oligomer for Adrβ2 (Adrβ2-MO). Combination of adrenaline treatments and microinjection of Adrβ2 mRNA maximized efficiency in its posteriorizing activity. Interestingly, both gain- and loss-of-functional treatment for β-adrenergic signaling could not influence anterior neural fate induced by overexpression of Chordin mRNA in presumptive ectodermal region, meaning that it worked via mesoderm. Taken together with these results, we conclude that adrenaline is a novel regulator of anteroposterior axis formation in vertebrates.

Pannexin 1 in the regulation of vascular tone

Pannexins are a recently discovered protein family with the isoform Panx1 ubiquitously expressed and therefore extensively studied. Panx1 proteins form membrane channels known to release purines such as ATP. Because ATP and, more generally, purinergic signaling plays an important role in the vasculature, it became evident that Panx1 could have a key role in vascular functions. This article reviews recent findings on the pivotal role of Panx1 in smooth muscle cells in the contraction of arteries as well as recent insights into Panx1 channel regulation.