Endogenous purinergic signaling is required for osmotic volume regulation of retinal glial cells

Recent advances in the study of circadian rhythm disorders that induce diabetic retinopathy

Diabetic retinopathy (DR) is a severe microvascular complication of diabetes mellitus and a major cause of blindness in young adults. Multiple potential factors influence DR; however, the exact mechanisms are poorly understood. Advanced treatments for DR, including laser therapy, vitrectomy, and intraocular drug injections, slow the disease's progression but fail to cure or reverse visual impairment. Therefore, additional effective methods to prevent and treat DR are required. The biological clock plays a crucial role in maintaining balance in the circadian rhythm of the body. Poor lifestyle habits, such as irregular routines and high-fat diets, may disrupt central and limbic circadian rhythms. Disrupted circadian rhythms can result in altered glucose metabolism and obesity. Misaligned central and peripheral clocks lead to a disorder of the rhythm of glucose metabolism, and chronically high sugar levels lead to the development of DR. We observed a disturbance in clock function in patients with diabetes, and a misaligned clock could accelerate the development of DR. In the current study, we examine the relationship between circadian rhythm disorders, diabetes, and DR. We conclude that: 1) abnormal function of the central clock and peripheral clock leads to abnormal glucose metabolism, further causing DR and 2) diabetes causes abnormal circadian rhythms, further exacerbating DR. Thus, our study presents new insights into the prevention and treatment of DR.

Combined molecular, structural and memory data unravel the destructive effect of tramadol on hippocampus

Tramadol is a synthetic analogue of codeine and stimulates neurodegeneration in several parts of the brain that leads to various behavioral impairments. Despite the leading role of hippocampus in learning and memory as well as decreased function of them under influence of tramadol, there are few studies analyzing the effect of tramadol administration on gene expression profiling and structural consequences in hippocampus region. Thus, we sought to determine the effect of tramadol on both PC12 cell line and hippocampal tissue, from gene expression changes to structural alterations. In this respect, we investigated genome-wide mRNA expression using high throughput RNA-seq technology and confirmatory quantitative real-time PCR, accompanied by stereological analysis of hippocampus and behavioral assessment following tramadol exposure. At the cellular level, PC12 cells were exposed to 600μM tramadol for 48 hrs, followed by the assessments of ROS amount and gene expression levels of neurotoxicity associated with neurodegenerative pathways such as apoptosis and autophagy. Moreover, the structural and functional alteration of the hippocampus under chronic exposure to tramadol was also evaluated. In this regard, rats were treated with tramadol at doses of 50 mg/kg for three consecutive weeks. In vitro data revealed that tramadol provoked ROS production and caused the increase in the expression of autophagic and apoptotic genes in PC12 cells. Furthermore, in-vivo results demonstrated that tramadol not only did induce hippocampal atrophy, but it also triggered microgliosis and microglial activation, causing upregulation of apoptotic and inflammatory markers as well as over-activation of neurodegeneration. Tramadol also interrupted spatial learning and memory function along with long-term potentiation (LTP). Taken all together, our data disclosed the neurotoxic effects of tramadol on both in vitro and in-vivo. Moreover, we proposed a potential correlation between disrupted biochemical cascades and memory deficit under tramadol administration.

Metformin Corrects Abnormal Circadian Rhythm and Kir4.1 Channels in Diabetes

Purpose

Diabetic retinopathy (DR) is a leading cause of visual impairment. Müller cells in DR are dysfunctional due to downregulation of the inwardly rectifying potassium channel Kir4.1. Metformin, a commonly used oral antidiabetic drug, is known to elicit its action through 5′ adenosine monophosphate-activated protein kinase (AMPK), a cellular metabolic regulator; however, its effect on Kir4.1 channels is unknown. For this study, we hypothesized that metformin treatment would correct circadian rhythm disruption and Kir4.1 channel dysfunction in db/db mice.

Methods

Metformin was given orally to db/db mice. Wheel-running activity, retinal levels of Kir4.1, and AMPK phosphorylation were determined at study termination. In parallel, rat retinal Müller cell line (rMC-1) cells were treated using metformin and 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) to assess the effect of AMPK activation on the Kir4.1 channel.

Results

The wheel-running activity of the db/db mice was improved following the metformin treatment. The Kir4.1 level in Müller cells was corrected after metformin treatment. Metformin treatment led to an upregulation of clock regulatory genes such as melanopsin (Opn4) and aralkylamine N-acetyltransferase (Aanat). In rMC-1 cells, AMPK activation via AICAR and metformin resulted in increased Kir4.1 and intermediate core clock component Bmal-1 protein expression. The silencing of Prkaa1 (gene for AMPKα1) led to decreased Kir4.1 and Bmal-1 protein expression.

Conclusions

Our findings demonstrate that metformin corrects abnormal circadian rhythm and Kir4.1 channels in db/db mouse a model of type 2 diabetes. Metformin could represent a critical pharmacological agent for preventing Müller cell dysfunction observed in human DR.

Comprehensive gene expression profiling of human astrocytes treated with a hepatic encephalopathy-inducible factor, alpha 1-antichymotripsin

Astrocytes are major glial cells that play a critical role in brain homeostasis. Abnormalities in astrocytic function, such as hepatic encephalopathy (HE) during acute liver failure, can result in brain death following brain edema and the associated astrocyte swelling. Recently, we have identified alpha 1-antichymotripsin (ACT) to be a biomarker candidate for HE. ACT induces astrocyte swelling by upregulating aquaporin 4 (AQP4); however, the causal connection between these proteins is not clear yet. In this study, we utilized a microarray profile to screen the differentially expressed genes (DEGs) in astrocytes treated with ACT. We then performed Gene Ontology, REACTOME, and the comprehensive resource of mammalian protein complexes (CORUM) enrichment analyses of the identified DEGs. The results of these analyses indicated that the DEGs were enriched in pathways activating adenylate cyclase (AC)-coupled G protein-coupled receptors (GPCRs) and therefore were involved in the cyclic adenosine monophosphate (cAMP) signaling. These results indicate that ACT may act as a ligand of Gs-GPCRs and subsequently upregulate cAMP. As cAMP is known to upregulate AQP4 in astrocytes, these results suggest that ACT may upregulate AQP4 by activating AC GPCRs and therefore serve as a therapeutic target for acute HE.

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The expression of AQP4 is induced by alpha 1-antichymotripsin dependent activation of adenylate cyclase.

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Combination treatment of alpha 1-antichymotripsin and arginine vasopressin enhance the activation of adenylate cyclase.

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Alpha 1-antichemotripsin might be a therapeutic target for acute hepatic encephalopathy.

Immune Responses Alter Taste Perceptions: Immunomodulatory Drugs Shape Taste Signals during Treatments

Considering that nutrients are required in health and diseases, the detection and ingestion of food to meet the requirements is attributable to the sense of taste. Altered taste sensations lead to a decreased appetite, which is usually one of the frequent causes of malnutrition in patients with diseases. Ongoing taste research has identified a variety of drug pathways that cause changes in taste perceptions in cancer, increasing our understanding of taste disturbances attributable to aberrant mechanisms of taste sensation. The evidence discussed in this review, which addresses the implications of innate immune responses in the modulation of taste functions, focuses on the adverse effects on taste transmission from taste buds by immune modulators responsible for alterations in the perceived intensity of some taste modalities. Another factor, damage to taste progenitor cells that directly results in local effects on taste buds, must also be considered in relation to taste disturbances in patients with cancer. Recent discoveries discussed have provided new insights into the pathophysiology of taste dysfunctions associated with the specific treatments. SIGNIFICANCE STATEMENT: The paradigm that taste signals transmitted to the brain are determined only by tastant-mediated activation via taste receptors has been challenged by the immune modification of taste transmission through drugs during the processing of gustatory information in taste buds. This article reports the findings in a model system (mouse taste buds) that explain the basis for the taste dysfunctions in patients with cancer that has long been observed but never understood.

Modulation of three key innate immune pathways for the most common retinal degenerative diseases

This review highlights the role of three key immune pathways in the pathophysiology of major retinal degenerative diseases including diabetic retinopathy, age‐related macular degeneration, and rare retinal dystrophies. We first discuss the mechanisms how loss of retinal homeostasis evokes an unbalanced retinal immune reaction involving responses of local microglia and recruited macrophages, activity of the alternative complement system, and inflammasome assembly in the retinal pigment epithelium. Presenting these key mechanisms as complementary targets, we specifically emphasize the concept of immunomodulation as potential treatment strategy to prevent or delay vision loss. Promising molecules are ligands for phagocyte receptors, specific inhibitors of complement activation products, and inflammasome inhibitors. We comprehensively summarize the scientific evidence for this strategy from preclinical animal models, human ocular tissue analyses, and clinical trials evolving in the last few years.

The Astrocytic cAMP Pathway in Health and Disease

Astrocytes are major glial cells that play critical roles in brain homeostasis. Abnormalities in astrocytic functions can lead to brain disorders. Astrocytes also respond to injury and disease through gliosis and immune activation, which can be both protective and detrimental. Thus, it is essential to elucidate the function of astrocytes in order to understand the physiology of the brain to develop therapeutic strategies against brain diseases. Cyclic adenosine monophosphate (cAMP) is a major second messenger that triggers various downstream cellular machinery in a wide variety of cells. The functions of astrocytes have also been suggested as being regulated by cAMP. Here, we summarize the possible roles of cAMP signaling in regulating the functions of astrocytes. Specifically, we introduce the ways in which cAMP pathways are involved in astrocyte functions, including (1) energy supply, (2) maintenance of the extracellular environment, (3) immune response, and (4) a potential role as a provider of trophic factors, and we discuss how these cAMP-regulated processes can affect brain functions in health and disease.

A step closer in defining glycosylphosphatidylinositol anchored proteins role in health and glycosylation disorders

Glycosylphosphatidylinositol anchored proteins (GPI-APs) represent a class of soluble proteins attached to the external leaflet of the plasma membrane by a post-translation modification, the GPI anchor. The 28 genes currently involved in the synthesis and remodelling of the GPI anchor add to the ever-growing class of congenital glycosylation disorders. Recent advances in next generation sequencing technology have led to the discovery of Mabry disease and CHIME syndrome genetic aetiology. Moreover, with each described mutation known phenotypes expand and new ones emerge without clear genotype-phenotype correlation. A protein database search was made for human GPI-APs with defined pathology to help building-up a physio-pathological mechanism from a clinical perspective. GPI-APs function in vitamin-B6 and folate transport, nucleotide metabolism and lipid homeostasis. Defining GPI-APs role in disease bears significant clinical implications.

Astroglial cAMP signalling in space and time

To maintain a high level of specificity and normal cell function, the cyclic adenosine monophosphate (cAMP) pathway is tightly regulated in space and time. Recent advances in cAMP reporter technology have provided insights into spatio-temporal characteristics of cAMP signalling in individual living cells, including astrocytes. Astrocytes are glial cells in the central nervous system with many homeostatic functions. In contrast to neurons, astrocytes are electrically silent, but, in response to extracellular stimuli through activation of surface receptors, they can increase intracellular levels of secondary messengers, e.g. Ca²⁺ and cAMP. This enables them to communicate with neighbouring cells, such as neurons and endothelial cells of blood vessels. The dynamics of receptor-mediated Ca²⁺ signalling in astrocytes has been extensively studied in the past in contrast to cAMP signalling. Here, we present the first insights into the temporal dynamics of cAMP signalling in living astrocytes, which revealed that cAMP signals in astrocytes exhibit tonic dynamics and are slower than Ca²⁺ signals with phasic dynamics. We debate on the heterogeneity of basal cAMP levels in astrocytes and how hypotonicity-induced astrocyte swelling affects temporal dynamics of cAMP signalling. Understanding the spatio-temporal characteristics of cAMP signalling in astrocytes is of extreme importance because cAMP governs many important cellular processes and any malfunctions may lead to pathology.

Increased Ap4A levels and ecto-nucleotidase activity in glaucomatous mice retina

The pathogenesis of glaucoma involves numerous intracellular mechanisms including the purinergic system contribution. Furthermore, the presence and release of nucleotides and dinucleotides during the glaucomatous damage and the maintenance of degradation machinery through ecto-nucleotidase activity are participating in the modulation of the suitable extracellular complex balance. The aim of this study was to investigate the levels of diadenosine tetraphosphate (Ap₄A) and the pattern of ecto-nucleotidase activity expression in glaucomatous retinas during the progress the pathology. Ap₄A levels were analyzed by HPLC in glaucomatous retinas from the DBA/2J mice at 3, 9, 15, and 23 months of age. For that, retinas were dissected as flattened whole-mounts and stimulated in Ringer buffer with or without 59 mM KCl. NPP1 expression was analyzed by RT-PCR and western blot and its distribution was assessed by immunohistochemistry studies examined under confocal microscopy. Glaucomatous mice exhibited Ap₄A values, which changed in stimulated retinas as long as the pathology progressed varying from 0.73 ± 0.04 (3 months) to 0.170 ± 0.05 pmol/mg retina (23 months). Concomitantly, NPP1 expression was significantly increased (82.15%) in the DBA/2J mice at 15 months. Furthermore, immunohistochemical studies showed that NPP1 labeling was stronger in OPL and IPL labeling tangentially in the vitreal part of the retina and was upregulated at 15 months of age. Our findings demonstrate that Ap₄A decreased levels may be related with exacerbated activity of NPP1 protein in glaucomatous degeneration and in this way contributing to elucidate different mechanisms involved in retinal impairment in glaucomatous degeneration.

Role of Purines in Müller Glia

Müller glia, the principal macroglia of the retina, express diverse subtypes of adenosine and metabotropic purinergic (P2Y) receptors. Müller cells of several species, including man, also express ionotropic P2X₇ receptors. ATP is liberated from Müller cells after activation of metabotropic glutamate receptors and during osmotic and mechanical induction of membrane stretch; adenosine is released through equilibrative nucleoside transporters. Müller cell-derived purines modulate the neuronal activity and have autocrine effects, for example, induction of glial calcium waves and regulation of the cellular volume. Glial calcium waves induced by neuron-derived ATP mediate functional hyperemia in the retina. Purinergic signaling contributes to the induction of Müller cell gliosis, for example, of cellular proliferation and downregulation of potassium channels, which are important for the homeostatic functions of Müller cells. Purinergic glial calcium waves may also promote the long-range propagation of gliosis and neuronal degeneration across the retinal tissue. The osmotic ATP release is inhibited under pathological conditions. Inhibition of the ATP release may result in osmotic Müller cell swelling and dysregulation of the water transport through the cells; both may contribute to the development of retinal edema. Suppression of the osmotic ATP release and upregulation of the ecto-apyrase (NTPDase1), which facilitate the extracellular degradation of ATP and the formation of adenosine, may protect neurons and photoreceptors from death due to overactivation of P2X receptors. Pharmacological inhibition of P2X₇ receptors and stimulation of adenosine receptors may represent clinical approaches to prevent retinal cell death and dysregulated cell proliferation, and to treat retinal edema.

Hippocampal and Cortical Pyramidal Neurons Swell in Parallel with Astrocytes during Acute Hypoosmolar Stress

Normal nervous system function is critically dependent on the balance of water and ions in the extracellular space (ECS). Pathological reduction in brain interstitial osmolarity results in osmotically-driven flux of water into cells, causing cellular edema which reduces the ECS and increases neuronal excitability and risk of seizures. Astrocytes are widely considered to be particularly susceptible to cellular edema due to selective expression of the water channel aquaporin-4 (AQP4). The apparent resistance of pyramidal neurons to osmotic swelling has been attributed to lack of functional water channels. In this study we report rapid volume changes in CA1 pyramidal cells in hypoosmolar ACSF (hACSF) that are equivalent to volume changes in astrocytes across a variety of conditions. Astrocyte and neuronal swelling was significant within 1 min of exposure to 17 or 40% hACSF, was rapidly reversible upon return to normosmolar ACSF, and repeatable upon re-exposure to hACSF. Neuronal swelling was not an artifact of patch clamp, occurred deep in tissue, was similar at physiological vs. room temperature, and occurred in both juvenile and adult hippocampal slices. Neuronal swelling was neither inhibited by TTX, nor by antagonists of NMDA or AMPA receptors, suggesting that it was not occurring as a result of excitotoxicity. Surprisingly, genetic deletion of AQP4 did not inhibit, but rather augmented, astrocyte swelling in severe hypoosmolar conditions. Taken together, our results indicate that neurons are not osmoresistant as previously reported, and that osmotic swelling is driven by an AQP4-independent mechanism.

Expression of Vesicular Nucleotide Transporter in the Mouse Retina

Vesicular nucleotide transporter (VNUT) is a membrane protein that is responsible for vesicular storage and subsequent vesicular release of nucleotides, such as ATP, and plays an essential role in purinergic chemical transmission. In the present study, we investigated whether VNUT is present in the rodent retina to define the site(s) of vesicular ATP release. In the mouse retina, reverse transcription polymerase chain reaction (RT-PCR) and immunological analyses using specific anti-VNUT antibodies indicated that VNUT is expressed as a polypeptide with an apparent molecular mass of 59 kDa. VNUT is widely distributed throughout the inner and outer retinal layers, particularly in the outer segment of photoreceptors, outer plexiform layer, inner plexiform layer, and ganglion cell layer. VNUT is colocalized with vesicular glutamate transporter 1 and synaptophysin in photoreceptor cells, while it is colocalized with vesicular γ-aminobutyric acid (GABA) transporter in amacrine cells and bipolar cells. VNUT is also present in astrocytes and Müller cells. The retina from VNUT knockout (VNUT(-/-)) mice showed the loss of VNUT immunoreactivity. The retinal membrane fraction took up radiolabeled ATP in diisothiocyanate stilbene disulfonic acid (DIDS)-, an inhibitor of VNUT, and bafilomycin A1-, a vacuolar adenosine triphosphatase (ATPase) inhibitor, in a sensitive manner, while membranes from VNUT(-/-) mice showed the loss of DIDS-sensitive ATP uptake. Taken together, these results indicate that functional VNUT is expressed in the rodent retina and suggest that ATP is released from photoreceptor cells, bipolar cells, amacrine cells, and astrocytes as well as Müller cells to initiate purinergic chemical transmission.

Comparative electrophysiology of retinal Müller glial cells-A survey on vertebrate species

Müller cells are the dominant macroglial cells in the retina of all vertebrates. They fulfill a variety of functions important for retinal physiology, among them spatial buffering of K⁺ ions and uptake of glutamate and other neurotransmitters. To this end, Müller cells express inwardly rectifying K⁺ channels and electrogenic glutamate transporters. Moreover, a lot of voltage- and ligand-gated ion channels, aquaporin water channels, and electrogenic transporters are expressed in Müller cells, some of them in a species-specific manner. For example, voltage-dependent Na⁺ channels are found exclusively in some but not all mammalian species. Whereas a lot of data exist from amphibians and mammals, the results from other vertebrates are sparse. It is the aim of this review to present a survey on Müller cell electrophysiology covering all classes of vertebrates. The focus is on functional studies, mainly performed using the whole-cell patch-clamp technique. However, data about the expression of membrane channels and transporters from immunohistochemistry are also included. Possible functional roles of membrane channels and transporters are discussed. Obviously, electrophysiological properties involved in the main functions of Müller cells developed early in vertebrate evolution. GLIA 2017;65:533-568.

Two different mechanosensitive calcium responses in Müller glial cells of the guinea pig retina: Differential dependence on purinergic receptor signaling

Tractional forces or mechanical stimulation are known to induce calcium responses in retinal glial cells. The aim of the study was to determine the characteristics of calcium responses in Müller glial cells of the avascular guinea pig retina induced by focal mechanical stimulation. Freshly isolated retinal wholemounts were loaded with Mitotracker Deep Red (to fill Müller cells) and the calcium-sensitive dye Fluo-4/AM. The inner retinal surface was mechanically stimulated with a micropipette tip for 10 ms. Stimulation induced two different cytosolic calcium responses in Müller cells with different kinetics in dependence on the distance from the stimulation site. Müller cells near the stimulation site displayed an immediate and long-lasting calcium response with high amplitude. This response was mediated by calcium influx from the extracellular space likely triggered by activation of ATP-insensitive P2 receptors. More distant Müller cells displayed, with a delay of 2.4 s, transient calcium responses which propagated laterally in a wave-like fashion. Propagating calcium waves were induced by a calcium-independent release of ATP from Müller cells near the stimulation site, and were mediated by a release of calcium from internal stores triggered by ATP, acting in part at P2Y₁ receptors. The data suggest that mechanically stimulated Müller cells of the guinea pig retina release ATP which induces a propagating calcium wave in surrounding Müller cells. Propagating calcium waves may be implicated in the spatial regulation of the neuronal activity and homeostatic glial functions, and may transmit gliosis-inducing signals across the retina. Mechanical stimulation of guinea pig Müller cells induces two calcium responses: an immediate response around the stimulation site and propagating calcium waves. Both responses are differentially mediated by activation of purinergic receptors. GLIA 2016 GLIA 2017;65:62-74.

The role of dinucleoside polyphosphates on the ocular surface and other eye structures

Dinucleoside polyphosphates comprises a group of dinucleotides formed by two nucleosides linked by a variable number of phosphates, abbreviated NpnN (where n represents the number of phosphates). These compounds are naturally occurring substances present in tears, aqueous humour and in the retina. As the consequence of their presence, these dinucleotides contribute to many ocular physiological processes. On the ocular surface, dinucleoside polyphosphates can stimulate tear secretion, mucin release from goblet cells and they help epithelial wound healing by accelerating cell migration rate. These dinucleotides can also stimulate the presence of proteins known to protect the ocular surface against microorganisms, such as lysozyme and lactoferrin. One of the latest discoveries is the ability of some dinucleotides to facilitate the paracellular way on the cornea, therefore allowing the delivery of compounds, such as antiglaucomatous ones, more easily within the eye. The compound Ap₄A has been described being abnormally elevated in patient's tears suffering of dry eye, Sjogren syndrome, congenital aniridia, or after refractive surgery, suggesting this molecule as biomarker for dry eye condition. At the intraocular level, some diadenosine polyphosphates are abnormally elevated in glaucoma patients, and this can be related to the stimulation of a P2Y₂ receptor that increases the chloride efflux and water movement in the ciliary epithelium. In the retina, the dinucleotide dCp₄U, has been proven to be useful to help in the recovery of retinal detachments. Altogether, dinucleoside polyphosphates are a group of compounds which present relevant physiological actions but which also can perform promising therapeutic benefits.

P2Y13 receptors mediate presynaptic inhibition of acetylcholine release induced by adenine nucleotides at the mouse neuromuscular junction

It is known that adenosine 5'-triphosphate (ATP) is released along with the neurotransmitter acetylcholine (ACh) from motor nerve terminals. At mammalian neuromuscular junctions (NMJs), we have previously demonstrated that ATP is able to decrease ACh secretion by activation of P2Y receptors coupled to pertussis toxin-sensitive Gi/o protein. In this group, the receptor subtypes activated by adenine nucleotides are P2Y12 and P2Y13. Here, we investigated, by means of pharmacological and immunohistochemical assays, the P2Y receptor subtype that mediates the modulation of spontaneous and evoked ACh release in mouse phrenic nerve-diaphragm preparations. First, we confirmed that the preferential agonist for P2Y12-13 receptors, 2-methylthioadenosine 5'-diphosphate trisodium salt hydrate (2-MeSADP), reduced MEPP frequency without affecting MEPP amplitude as well as the amplitude and quantal content of end-plate potentials (EPPs). The effect on spontaneous secretion disappeared after the application of the selective P2Y12-13 antagonists AR-C69931MX or 2-methylthioadenosine 5'-monophosphate triethylammonium salt hydrate (2-MeSAMP). 2-MeSADP was more potent than ADP and ATP in reducing MEPP frequency. Then we demonstrated that the selective P2Y13 antagonist MRS-2211 completely prevented the inhibitory effect of 2-MeSADP on MEPP frequency and EPP amplitude, whereas the P2Y12 antagonist MRS-2395 failed to do this. The preferential agonist for P2Y13 receptors inosine 5'-diphosphate sodium salt (IDP) reduced spontaneous and evoked ACh secretion and MRS-2211 abolished IDP-mediated modulation. Immunohistochemical studies confirmed the presence of P2Y13 but not P2Y12 receptors at the end-plate region. Disappearance of P2Y13 receptors after denervation suggests the presynaptic localization of the receptors. We conclude that, at motor nerve terminals, the Gi/o protein-coupled P2Y receptors implicated in presynaptic inhibition of spontaneous and evoked ACh release are of the subtype P2Y13. This study provides new insights into the types of purinergic receptors that contribute to the fine-tuning of cholinergic transmission at mammalian neuromuscular junction.

Impaired Purinergic Regulation of the Glial (Müller) Cell Volume in the Retina of Transgenic Rats Expressing Defective Polycystin-2

Retinal glial (Müller) cells possess an endogenous purinergic signal transduction cascade which normally prevents cellular swelling in osmotic stress. The cascade can be activated by osmotic or glutamate receptor-dependent ATP release. We determined whether activation of this cascade is altered in Müller cells of transgenic rats that suffer from a slow photoreceptor degeneration due to the expression of a truncated human cilia gene polycystin-2 (CMV-PKD21/703 HA). Age-matched Sprague-Dawley rats served as control. Retinal slices were superfused with a hypoosmotic solution (60 % osmolarity). Müller cells in retinas of PKD21/703 rats swelled immediately in hypoosmotic stress; this was not observed in control retinas. Pharmacological blockade of P2Y1 or adenosine A1 receptors induced osmotic swelling of Müller cells from control rats. The swelling induced by the P2Y1 receptor antagonist was mediated by induction of oxidative-nitrosative stress, mitochondrial dysfunction, production of inflammatory lipid mediators, and a sodium influx from the extracellular space. Exogenous VEGF or glutamate prevented the hypoosmotic swelling of Müller cells from PKD21/703 rats; this effect was mediated by activation of the purinergic signaling cascade. In neuroretinas of PKD21/703 rats, the gene expression levels of P2Y1 and A1 receptors, pannexin-1, connexin 45, NTPDases 1 and 2, and various subtypes of nucleoside transporters are elevated compared to control. The data may suggest that the osmotic swelling of Müller cells from PKD21/703 rats is caused by an abrogation of the osmotic ATP release while the glutamate-induced ATP release is functional. In the normal retina, ATP release and autocrine P2Y1 receptor activation serve to inhibit the induction of oxidative-nitrosative stress, mitochondrial dysfunction, and production of inflammatory lipid mediators, which otherwise will induce a sodium influx and cytotoxic Müller cell swelling under anisoosmotic conditions. Purinergic receptors may represent a target for the protection of retinal glial cells from mitochondrial oxidative stress.

Adrenergic activation attenuates astrocyte swelling induced by hypotonicity and neurotrauma

Edema in the central nervous system can rapidly result in life-threatening complications. Vasogenic edema is clinically manageable, but there is no established medical treatment for cytotoxic edema, which affects astrocytes and is a primary trigger of acute post-traumatic neuronal death. To test the hypothesis that adrenergic receptor agonists, including the stress stimulus epinephrine protects neural parenchyma from damage, we characterized its effects on hypotonicity-induced cellular edema in cortical astrocytes by in vivo and in vitro imaging. After epinephrine administration, hypotonicity-induced swelling of astrocytes was markedly reduced and cytosolic 3'-5'-cyclic adenosine monophosphate (cAMP) was increased, as shown by a fluorescence resonance energy transfer nanosensor. Although, the kinetics of epinephrine-induced cAMP signaling was slowed in primary cortical astrocytes exposed to hypotonicity, the swelling reduction by epinephrine was associated with an attenuated hypotonicity-induced cytosolic Ca(2+) excitability, which may be the key to prevent astrocyte swelling. Furthermore, in a rat model of spinal cord injury, epinephrine applied locally markedly reduced neural edema around the contusion epicenter. These findings reveal new targets for the treatment of cellular edema in the central nervous system.

Effects of IP3R2 Receptor Deletion in the Ischemic Mouse Retina

Glial cells in the diseased nervous system undergo a process known as reactive gliosis. Gliosis of retinal Müller glial cells is characterized by an upregulation of glial fibrillary acidic protein and frequently by a reduction of inward K(+) current amplitudes. Purinergic signaling is assumed to be involved in gliotic processes. As previously shown, lack of the nucleotide receptor P2Y1 leads to an altered regulation of K(+) currents in Müller cells of the ischemic retina. Here, we asked first whether this effect is mediated by the IP3 receptor subtype 2 (IP3R2) known as the major downstream signaling target of P2Y1 in Müller cells. The second question was whether lack of IP3R2 affects neuronal survival in the control and ischemic retina. Ischemia was induced in wild type and IP3R2-deficient (IP 3 R2 (-/-)) mice by transient elevation of the intraocular pressure. Immunostaining and TUNEL labelling were used to quantify neuronal cell loss. The downregulation of inward K(+) currents in Müller cells from ischemic IP 3 R2 (-/-) retinae was less strong than in wild type animals. The reduction of the number of cells in the ganglion cell layer and of calretinin- and calbindin-positive cells 7 days after ischemia was similar in wild type and IP 3 R2 (-/-) mice. However, IP3R2 deficiency led to an increased number of TUNEL-positive cells in the outer nuclear layer at 1 day and to an enhanced postischemic loss of photoreceptors 7 days after ischemia. This implies that IP3R2 is involved in some but not all aspects of signaling in Müller cells after an ischemic insult.

Purinergic signaling in retinal degeneration and regeneration

Purinergic signaling is centrally involved in mediating the degeneration of the injured and diseased retina, the induction of retinal gliosis, and the protection of the retinal tissue from degeneration. Dysregulated calcium signaling triggered by overactivation of P2X7 receptors is a crucial step in the induction of neuronal and microvascular cell death under pathogenic conditions like ischemia-hypoxia, elevated intraocular pressure, and diabetes, respectively. Overactivation of P2X7 plays also a pathogenic role in inherited and age-related photoreceptor cell death and in the age-related dysfunction and degeneration of the retinal pigment epithelium. Gliosis of micro- and macroglial cells, which is induced and/or modulated by purinergic signaling and associated with an impaired homeostatic support to neurons, and the ATP-mediated propagation of retinal gliosis from a focal injury into the surrounding noninjured tissue are involved in inducing secondary cell death in the retina. On the other hand, alterations in the glial metabolism of extracellular nucleotides, resulting in a decreased level of ATP and an increased level of adenosine, may be neuroprotective in the diseased retina. Purinergic signals stimulate the proliferation of retinal glial cells which contributes to glial scarring which has protective effects on retinal degeneration and adverse effects on retinal regeneration. Pharmacological modulation of purinergic receptors, e.g., inhibition of P2X and activation of adenosine receptors, may have clinical importance for the prevention of photoreceptor, neuronal, and microvascular cell death in diabetic retinopathy, retinitis pigmentosa, age-related macular degeneration, and glaucoma, respectively, for the clearance of retinal edema, and the inhibition of dysregulated cell proliferation in proliferative retinopathies. This article is part of a Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Differential effects of P2Y1 deletion on glial activation and survival of photoreceptors and amacrine cells in the ischemic mouse retina

Gliosis of retinal Müller glial cells may have both beneficial and detrimental effects on neurons. To investigate the role of purinergic signaling in ischemia-induced reactive gliosis, transient retinal ischemia was evoked by elevation of the intraocular pressure in wild-type (Wt) mice and in mice deficient in the glia-specific nucleotide receptor P2Y₁ (P2Y₁ receptor-deficient (P2Y1R-KO)). While control retinae of P2Y1R-KO mice displayed reduced cell numbers in the ganglion cell and inner nuclear layers, ischemia induced apoptotic death of cells in all retinal layers in both, Wt and P2Y1R-KO mice, but the damage especially on photoreceptors was more pronounced in retinae of P2Y1R-KO mice. In contrast, gene expression profiling and histological data suggest an increased survival of amacrine cells in the postischemic retina of P2Y1R-KO mice. Interestingly, measuring the ischemia-induced downregulation of inwardly rectifying potassium channel (Kir)-mediated K⁺ currents as an indicator, reactive Müller cell gliosis was found to be weaker in P2Y1R-KO (current amplitude decreased by 18%) than in Wt mice (decrease by 68%). The inner retina harbors those neurons generating action potentials, which strongly rely on an intact ion homeostasis. This may explain why especially these cells appear to benefit from the preserved Kir4.1 expression in Müller cells, which should allow them to keep up their function in the context of spatial buffering of potassium. Especially under ischemic conditions, maintenance of this Müller cell function may dampen cytotoxic neuronal hyperexcitation and subsequent neuronal cell loss. In sum, we found that purinergic signaling modulates the gliotic activation pattern of Müller glia and lack of P2Y₁ has janus-faced effects. In the end, the differential effects of a disrupted P2Y₁ signaling onto neuronal survival in the ischemic retina call the putative therapeutical use of P2Y₁-antagonists into question.

Molecular pathways of pannexin1-mediated neurotoxicity

Pannexin1 (Panx1) forms non-selective membrane channels, structurally similar to gap junction hemichannels, and are permeable to ions, nucleotides, and other small molecules below 900 Da. Panx1 activity has been implicated in paracrine signaling and inflammasome regulation. Recent studies in different animal models showed that overactivation of Panx1 correlates with a selective demise of several types of neurons, including retinal ganglion cells, brain pyramidal, and enteric neurons. The list of Panx1 activators includes extracellular ATP, glutamate, high K(+), Zn(2+), fibroblast growth factors (FGFs),pro-inflammatory cytokines, and elevation of intracellular Ca(2+). Most of these molecules are released following mechanical, ischemic, or inflammatory injury of the CNS, and rapidly activate the Panx1 channel. Prolonged opening of Panx1 channel induced by these "danger signals" triggers a cascade of neurotoxic events capable of killing cells. The most vulnerable cell type are neurons that express high levels of Panx1. Experimental evidence suggests that Panx1 channels mediate at least two distinct neurotoxic processes: increased permeability of the plasma membrane and activation of the inflammasome in neurons and glia. Importantly, both pharmacological and genetic inactivation of Panx1 suppresses both these processes, providing a marked protection in several disease and injury models. These findings indicate that external danger signals generated after diverse types of injuries converge to activate Panx1. In this review we discuss molecular mechanisms associated with Panx1 toxicity and the crosstalk between different pathways.

Dynamics of β-adrenergic/cAMP signaling and morphological changes in cultured astrocytes

The morphology of astrocytes, likely regulated by cAMP, determines the structural association between astrocytes and the synapse, consequently modulating synaptic function. β-Adrenergic receptors (β-AR), which increase cytosolic cAMP concentration ([cAMP]i ), may affect cell morphology. However, the real-time dynamics of β-AR-mediated cAMP signaling in single live astrocytes and its effect on cell morphology have not been studied. We used the fluorescence resonance energy transfer (FRET)-based cAMP biosensor Epac1-camps to study time-dependent changes in [cAMP]i ; morphological changes in primary rat astrocytes were monitored by real-time confocal microscopy. Stimulation of β-AR by adrenaline, noradrenaline, and isoprenaline, a specific agonist of β-AR, rapidly increased [cAMP]i (∼15 s). The FRET signal response, mediated via β-AR, was faster than in the presence of forskolin (twofold) and dibutyryl-cAMP (>35-fold), which directly activate adenylyl cyclase and Epac1-camps, respectively, likely due to slow entry of these agents into the cytosol. Oscillations in [cAMP]i have not been recorded, indicating that cAMP-dependent processes operate in a slow time domain. Most Epac1-camps expressing astrocytes revealed a morphological change upon β-AR activation and attained a stellate morphology within 1 h. The morphological changes exhibited a bell-shaped dependency on [cAMP]i . The 5-10% decrease in cell cross-sectional area and the 30-50% increase in cell perimeter are likely due to withdrawal of the cytoplasm to the perinuclear region and the appearance of protrusions on the surface of astrocytes. Because astrocyte processes ensheath neurons, β-AR/cAMP-mediated morphological changes can modify the geometry of the extracellular space, affecting synaptic, neuronal, and astrocyte functions in health and disease.

Disruption of endogenous purinergic signaling inhibits vascular endothelial growth factor- and glutamate-induced osmotic volume regulation of Müller glial cells in knockout mice

BACKGROUND/AIMS

Osmotic swelling of Müller cells is a common phenomenon in animal models of ischemic and diabetic retinopathies. Müller cells possess a swelling-inhibitory purinergic signaling cascade which can be activated by various receptor ligands including vascular endothelial growth factor (VEGF) and glutamate. Here, we investigated whether deletion of P2Y1 (P2Y1R) and adenosine A1 receptors (A1AR), and of inositol-1,4,5-trisphosphate-receptor type 2 (IP3R2), in mice affects the inhibitory action of VEGF and glutamate on Müller cell swelling.

METHODS

The cross-sectional area of Müller cell somata was recorded after a 4-min superfusion of retinal slices with a hypoosmotic solution.

RESULTS

Hypoosmolarity induced a swelling of Müller cells from P2Y1R(-/-), A1AR(-/-) and IP3R2(-/-) mice, but not from wild-type mice. Swelling of wild-type Müller cells was induced by hypoosmotic solution containing barium chloride. Whereas VEGF inhibited the swelling of wild-type Müller cells, it had no swelling-inhibitory effect in cells from A1AR(-/-) and IP3R2(-/-) mice. Glutamate inhibited the swelling of wild-type Müller cells but not of cells from P2Y1R(-/-), A1AR(-/-) and IP3R2(-/-) animals.

CONCLUSION

The swelling-inhibitory effects of VEGF and glutamate in murine Müller cells is mediated by transactivation of P2Y1R and A1AR, as well as by intracellular calcium signaling via activation of IP3R2.

Nucleotides in the eye: focus on functional aspects and therapeutic perspectives

The presence and activity of nucleotides and dinucleotides in the physiology of most, if not all, organisms, from bacteria to humans, have been recognized by the scientific community, and the eye is no exception. Nucleotides in the dynamic fluids interact with many ocular structures, such as the tears and aqueous humor. Moreover, high concentrations of nucleotides in these secretions may reflect disease states such as dry eye and glaucoma. Apart from the nucleotide concentration in these fluids, P2 purinergic receptors have been described on the ocular surface (cornea and conjunctiva), anterior pole (ciliary body, trabecular meshwork), and posterior pole (retina). P2X and P2Y purinergic receptors are essential in maintaining the homeostasis of ocular processes, such as tear secretion, aqueous humor production, or retinal modulation. When they are functioning properly, they allow the eye to do its job (to see), but in some cases, a lack or an excess of nucleotides or a malfunction in the corresponding purinergic receptors leads to disease. This Perspective is focused on the nucleotides and dinucleotides and the P2 purinergic receptors in the eye and how they contribute to normal and disease states. We also emphasize the action of nucleotides and their receptors and antagonists as potential therapeutic agents.

New functions of Müller cells

Müller cells, the major type of glial cells in the retina, are responsible for the homeostatic and metabolic support of retinal neurons. By mediating transcellular ion, water, and bicarbonate transport, Müller cells control the composition of the extracellular space fluid. Müller cells provide trophic and anti-oxidative support of photoreceptors and neurons and regulate the tightness of the blood-retinal barrier. By the uptake of glutamate, Müller cells are more directly involved in the regulation of the synaptic activity in the inner retina. This review gives a survey of recently discoved new functions of Müller cells. Müller cells are living optical fibers that guide light through the inner retinal tissue. Thereby they enhance the signal/noise ratio by minimizing intraretinal light scattering and conserve the spatial distribution of light patterns in the propagating image. Müller cells act as soft, compliant embedding for neurons, protecting them in case of mechanical trauma, and also as soft substrate required for neurite growth and neuronal plasticity. Müller cells release neuroactive signaling molecules which modulate neuronal activity, are implicated in the mediation of neurovascular coupling, and mediate the homeostasis of the extracellular space volume under hypoosmotic conditions which are a characteristic of intense neuronal activity. Under pathological conditions, a subset of Müller cells may differentiate to neural progenitor/stem cells which regenerate lost photoreceptors and neurons. Increasing knowledge of Müller cell function and responses in the normal and diseased retina will have great impact for the development of new therapeutic approaches for retinal diseases.

P2Y1 receptor activation by photolysis of caged ATP enhances neuronal network activity in the developing olfactory bulb

It has recently been shown that adenosine-5'-triphosphate (ATP) is released together with glutamate from sensory axons in the olfactory bulb, where it stimulates calcium signaling in glial cells, while responses in identified neurons to ATP have not been recorded in the olfactory bulb yet. We used photolysis of caged ATP to elicit a rapid rise in ATP and measured whole-cell current responses in mitral cells, the output neurons of the olfactory bulb, in acute mouse brain slices. Wide-field photolysis of caged ATP evoked an increase in synaptic inputs in mitral cells, indicating an ATP-dependent increase in network activity. The increase in synaptic activity was accompanied by calcium transients in the dendritic tuft of the mitral cell, as measured by confocal calcium imaging. The stimulating effect of ATP on the network activity could be mimicked by photo release of caged adenosine 5'-diphosphate, and was inhibited by the P2Y(1) receptor antagonist MRS 2179. Local photolysis of caged ATP in the glomerulus innervated by the dendritic tuft of the recorded mitral cell elicited currents similar to those evoked by wide-field illumination. The results indicate that activation of P2Y(1) receptors in the glomerulus can stimulate network activity in the olfactory bulb.

Using antibodies against P2Y and P2X receptors in purinergic signaling research

The broad expression pattern of the G protein-coupled P2Y receptors has demonstrated that these receptors are fundamental determinants in many physiological responses, including neuromodulation, vasodilation, inflammation, and cell migration. P2Y receptors couple either G(q) or G(i) upon activation, thereby activating different signaling pathways. Ionotropic ATP (P2X) receptors bind extracellular nucleotides, a signal which is transduced within the P2X protein complex into a cation channel opening, which usually leads to intracellular calcium concentration elevation. As such, this family of proteins initiates or shapes several cellular processes including synaptic transmission, gene expression, proliferation, migration, and apoptosis. The ever-growing range of applications for antibodies in the last 30 years attests to their major role in medicine and biological research. Antibodies have been used as therapeutic tools in cancer and inflammatory diseases, as diagnostic reagents (flow cytometry, ELISA, and immunohistochemistry, to name a few applications), and in widespread use in biological research, including Western blot, immunoprecipitation, and ELISPOT. In this article, we will showcase several of the advances that scientists around the world have achieved using the line of antibodies developed at Alomone Labs for P2Y and P2X receptors.

Mechanisms of VEGF- and glutamate-induced inhibition of osmotic swelling of murine retinal glial (Müller) cells: indications for the involvement of vesicular glutamate release and connexin-mediated ATP release

We determined the mechanisms of glutamate and ATP release from murine retinal glial (Müller) cells by pharmacological manipulation of the vascular endothelial growth factor (VEGF)- and glutamate-induced inhibition of cellular swelling under hypoosmotic conditions. It has been shown that exogenous glutamate inhibits hypoosmotic swelling of rat Müller cells via the induction of the release of ATP (Uckermann et al. in J Neurosci Res 83:538-550, 53). VEGF was shown to inhibit hypoosmotic swelling of rat Müller cells by inducing the release of glutamate (Wurm et al. in J Neurochem 104:386-399, 55). The swelling-inhibitory effect of VEGF in murine Müller cells was blocked by an inhibitor of vesicular exocytosis, by a modulator of the allosteric site of vesicular glutamate transporters, and by inhibitors of phospholipase C and protein kinase C. The swelling-inhibitory effect of glutamate in murine Müller cells was prevented by inhibitors of connexin hemichannels. The effects of both VEGF and glutamate were blocked by tetrodotoxin and by an inhibitor of T-type voltage-gated calcium channels. Murine Müller cells display connexin-43 immunoreactivity. The data suggest that Müller cells of the murine retina may release glutamate by vesicular exocytosis, whereas ATP is released through connexin hemichannels.

Müller glial cells in retinal disease

Virtually all pathogenic stimuli activate Müller cells. Reactive Müller cells exert protective and toxic effects on photoreceptors and neurons. They contribute to oxidative stress and glutamate toxicity due to malfunctions of glutamate uptake and glutathione synthesis. Downregulation of potassium conductance disrupts transcellular potassium and water transport, resulting in neuronal hyperexcitability and edema. Protective effects of reactive Müller cells include upregulation of adenosine 5'-triphosphate (ATP)-degrading ectoenzymes, which enhances the extracellular availability of the neuroprotectant adenosine, abrogation of the osmotic release of ATP, which might protect retinal ganglion cells from apoptosis, and the release of antioxidants and neurotrophic factors. The dedifferentiation of reactive Müller cells to progenitor-like cells might have an impact on future therapeutic approaches. A better understanding of the gliotic mechanisms will be helpful in developing efficient therapeutic strategies aiming at increased protective and regenerative properties and decreased toxicity of reactive Müller cells.

Purinergic regulation of high-glucose-induced caspase-1 activation in the rat retinal Müller cell line rMC-1

Chronic activation of proinflammatory caspase-1 in the retinas of diabetic animals and patients in vivo and retinal Müller cells in vitro is well documented. In this study we characterized how elevated glucose and extracellular purines contribute to the activation of caspase-1 in a cultured rat Müller cell (rMC-1) model. The ability of high glucose (25 mM, 24 h) to activate caspase-1 was attenuated by either apyrase, which metabolizes extracellular ATP to AMP, or adenosine deaminase (ADA), which metabolizes extracellular adenosine to inosine. This suggested that autocrine stimulation of ATP-sensing P2 receptors and adenosine-sensing P1 receptors may in part mediate the response to high glucose. Exogenous ATP, 5'-N-ethylcarboxamido-adenosine (NECA), a nonselective P1 receptor agonist, or forskolin (FSK) increased caspase-1 activity in rMC-1 cells cultured in control glucose (5 mM) medium. Accumulation of active caspase-1 was also increased by dipyridamole, which suppresses adenosine reuptake. High-glucose stimulation of caspase-1 was attenuated by suramin, a nonselective P2 antagonist, or A2 adenosine receptor antagonists, but not by antagonism of P2X7 ATP-gated ion channel receptors. Although high glucose increased P2X7 mRNA, neither P2X7 protein nor function was detected in rMC-1 cells. The increased caspase-1 activity stimulated by high glucose, FSK, NECA, or ATP was correlated with increased gene expression of caspase-1 and thioredoxin-interacting-protein (TXNIP). These findings support a novel role for autocrine P1 and P2 purinergic receptors coupled to cAMP signaling cascades and transcriptional induction of caspase-1 in mediating the high-glucose-induced activation of caspase-1 and secretion of IL-1β in a cell culture model of nonhematopoietic retinal Müller cells.

Involvement of oxidative stress and mitochondrial dysfunction in the osmotic swelling of retinal glial cells from diabetic rats

Osmotic swelling of retinal glial (Müller) cells may contribute to the development of edema in diabetic retinopathy. Here, we tested whether oxidative stress and mitochondrial dysfunction are pathogenic factors involved in the osmotic swelling of Müller cells in retinal slices from control and streptozotocin-injected hyperglycemic rats. Hypotonic challenge did not change the size of Müller cell somata from control animals but induced soma swelling in Müller cells of diabetic animals. Administration of a reducing agent blocked the osmotic swelling of Müller cell somata. In retinal tissues from control animals, administration of the reducing agent blocked also the swelling-inducing effects of antagonists of P2Y₁ and adenosine A₁ receptors. In tissues from diabetic animals, inhibition of xanthine oxidase decreased the soma swelling by approximately 50% while inhibition of NADPH oxidase and nitric oxide synthase had no effects. Blockade of mitochondrial oxidative stress by perindopril, as well as of mitochondrial permeability transition by cyclosporin A or minocycline, attenuated the swelling. In addition, activation of mitochondrial K(ATP) channels by pinacidil fully prevented the swelling. The data suggest that oxidative stress produced by xanthine oxidase, as well as the mitochondria, are implicated in the induction of osmotic swelling of Müller cells from diabetic rats.

Altered immune response in mice deficient for the G protein-coupled receptor GPR34

The X-chromosomal GPR34 gene encodes an orphan G(i) protein-coupled receptor that is highly conserved among vertebrates. To evaluate the physiological relevance of GPR34, we generated a GPR34-deficient mouse line. GPR34-deficient mice were vital, reproduced normally, and showed no gross abnormalities in anatomical, histological, laboratory chemistry, or behavioral investigations under standard housing. Because GPR34 is highly expressed in mononuclear cells of the immune system, mice were specifically tested for altered functions of these cell types. Following immunization with methylated BSA, the number of granulocytes and macrophages in spleens was significantly lower in GPR34-deficient mice as in wild-type mice. GPR34-deficient mice showed significantly increased paw swelling in the delayed type hypersensitivity test and higher pathogen burden in extrapulmonary tissues after pulmonary infection with Cryptococcus neoformans compared with wild-type mice. The findings in delayed type hypersensitivity and infection tests were accompanied by significantly different basal and stimulated TNF-α, GM-CSF, and IFN-γ levels in GPR34-deficient animals. Our data point toward a functional role of GPR34 in the cellular response to immunological challenges.

Receptor regulation of osmolyte homeostasis in neural cells

The capacity of cells to correct their volume in response to hyposmotic stress via the efflux of inorganic and organic osmolytes is well documented. However, the ability of cell-surface receptors, in particular G-protein-coupled receptors (GPCRs), to regulate this homeostatic mechanism has received much less attention. Mechanisms that underlie the regulation of cell volume are of particular importance to cells in the central nervous system because of the physical restrictions of the skull and the adverse impact that even small increases in cell volume can have on their function. Increases in brain volume are seen in hyponatraemia, which can arise from a variety of aetiologies and is the most frequently diagnosed electrolyte disorder in clinical practice. In this review we summarize recent evidence that the activation of GPCRs facilitates the volume-dependent efflux of osmolytes from neural cells and permits them to more efficiently respond to small, physiologically relevant, reductions in osmolarity. The characteristics of receptor-regulated osmolyte efflux, the signalling pathways involved and the physiological significance of receptor activation are discussed. In addition, we propose that GPCRs may also regulate the re-uptake of osmolytes into neural cells, but that the influx of organic and inorganic osmolytes is differentially regulated. The ability of neural cells to closely regulate osmolyte homeostasis through receptor-mediated alterations in both efflux and influx mechanisms may explain, in part at least, why the brain selectively retains its complement of inorganic osmolytes during chronic hyponatraemia, whereas its organic osmolytes are depleted.