Ionotropic and metabotropic GABA and glutamate receptors in primate basal ganglia

The functions of glutamate and GABA in the CNS are mediated by ionotropic and metabotropic, G protein-coupled, receptors. Both receptor families are widely expressed in basal ganglia structures in primates and nonprimates. The recent development of highly specific antibodies and/or cDNA probes allowed the better characterization of the cellular localization of various GABA and glutamate receptor subtypes in the primate basal ganglia. Furthermore, the use of high resolution immunogold techniques at the electron microscopic level led to major breakthroughs in our understanding of the subsynaptic and subcellular localization of these receptors in primates. In this review, we will provide a detailed account of the current knowledge of the localization of these receptors in the basal ganglia of humans and monkeys.     

Ionotropic glutamate and GABA receptors in human epileptic neocortical tissue: quantitative in vitro receptor autoradiography

Since a disturbed balance between excitatory and inhibitory amino acid receptors is suggested to be an important condition for epileptogenic cortical activity, the present study has focused on the analysis of the densities of (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), N-methyl-D-aspartate, kainate and GABA subtype A receptors in neocortical tissue surgically removed from patients with focal epilepsy. The mean densities (collapsed over cortical layers I-VI) and the laminar distribution patterns of [3H]AMPA, [3H]MK-801, [3H]kainate and [3H]muscimol binding to AMPA, N-methyl-D-aspartate, kainate and GABAA receptors were determined with quantitative receptor autoradiography in the neocortex of patients with focal epilepsy and controls. The tissue probes used in the present study were functionally characterized by parallel electrophysiological investigations. From that, the different probes could be subdivided into a spontaneously spiking and a non-spontaneously spiking group. The mean density of [3H]AMPA binding sites was significantly increased (+37%) in the group of epileptic brains (n = 10) compared with controls (n = 10), but the mean densities of [3H]MK-801, [3H]kainate and [3H]muscimol binding sites were not significantly altered (-8%, +/-0% and -7%, respectively). The relation between the densities of all four binding sites were simultaneously displayed as polar plots in each single brain ("receptor fingerprints"). The consistent up-regulation of [3H]AMPA binding sites in all epileptic brains was found to be associated with a down-regulation of the N-methyl-D-aspartate receptor in four of the five non-spontaneously spiking cases, and an associated up-regulation of the N-methyl-D-aspartate receptor was seen in all spontaneously spiking cases. Finally, the laminar distribution of binding site densities was analysed, since the mean densities collapsed over all neocortical layers may obscure layer-specific alterations. Layer- and receptor- specific up- or down-regulations were found in epileptic tissue compared with controls. Moreover, the laminar distribution pattern of current sinks associated with epileptiform potentials in a spontaneously spiking cortical slice was found to be co-localized with local maxima of AMPA receptor densities. The present analysis of four ionotropic glutamate and GABA receptor subtypes demonstrates a consistent and significant up-regulation of [3H]AMPA binding sites in all cases of human focal epilepsy, which co-localizes with the occurrence of sinks in current-source-density analysis. The receptor fingerprint analysis suggests a subdivision of focal epilepsy into two subtypes on the basis of neurochemical/functional correlations: (i) a spontaneously spiking subtype with increased N-methyl-D-aspartate receptor density, and (ii) a non-spontaneously spiking subtype with decreased N-methyl-D-aspartate receptor density.     

Glutamate-mediated neuroprotection against N-methyl-D-aspartate toxicity: a role for metabotropic glutamate receptors

We studied N-methyl-D-aspartate-induced cell death in organotypic hippocampal slices from seven-day-old Wistar rat pups cultured for 12-14 days in a medium containing no added glutamate. Propidium iodide fluorescence intensity was used as an indicator of cell death measured with the help of confocal microscopy. Exposure of slices for 2h to L-glutamate (1-500 microM) prior to the N-methyl-D-aspartate challenge significantly reduced N-methyl-D-aspartate-induced cell death. Glutamate at 10 and 500 microM concentrations was highly protective against N-methyl-D-aspartate-induced cell death, but was less protective at the 1 microM concentration. The protection was not blocked by the Na(+) channel blocker tetrodotoxin (1 microM), the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonopentanoic acid (20 microM) or the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (20 microM). 1S, 3R-1-Aminocyclopentane-trans-1,3-dicarboxylic acid, an agonist at metabotropic glutamate receptor types 1, 2/3 and 5, was protective at 100 microM but not at 50 microM. In contrast, the ionotropic glutamate receptor agonist aspartate (250 microM) facilitated N-methyl-D-aspartate toxicity. Treatment of slices with the protein kinase C inhibitor staurosporine (0.2 microM) or antisense oligonucleotide (10nM, 72 h) that selectively inhibits metabotropic glutamate receptor type 5 synthesis significantly reduced glutamate protection.These results suggest that ambient glutamate may reduce nerve cell susceptibility to injury caused by excessive N-methyl-D-aspartate receptor activation by acting at metabotropic glutamate receptors linked to protein kinase C.     

Apoptosis and dependence receptors: a molecular basis for cellular addiction

Classical signal transduction is initiated by ligand-receptor interactions. We have described an alternative form of signal transduction that is initiated by the withdrawal of ligands from specific receptors referred to as dependence receptors. This process is widespread, featuring in developmental cell death, carcinogenesis (especially metastasis), neurodegeneration, and possibly subapoptotic events such as neurite retraction and somal atrophy. Initial mechanistic studies of dependence receptors suggest that these receptors form complexes that include specific caspases. Complex formation appears to be a function of ligand-receptor interaction, and dependence receptors appear to exist in at least two conformational states. Complex formation in the absence of ligand leads to caspase activation by a mechanism that in at least some cases is dependent on caspase cleavage of the receptor itself, releasing proapoptotic peptides. Thus these receptors may serve in caspase amplification, and in so doing create cellular states of dependence on their respective ligands.     

Structural basis for partial agonist action at ionotropic glutamate receptors

An unresolved problem in understanding neurotransmitter receptor function concerns the mechanism(s) by which full and partial agonists elicit different amplitude responses at equal receptor occupancy. The widely held view of 'partial agonism' posits that resting and active states of the receptor are in equilibrium, and partial agonists simply do not shift the equilibrium toward the active state as efficaciously as full agonists. Here we report findings from crystallographic and electrophysiological studies of the mechanism of activation of an AMPA-subtype glutamate receptor ion channel. In these experiments, we used 5-substituted willardiines, a series of partial agonists that differ by only a single atom. Our results show that the GluR2 ligand-binding core can adopt a range of ligand-dependent conformational states, which in turn control the open probability of discrete subconductance states of the intact ion channel. Our findings thus provide a structure-based model of partial agonism.     

The molecular basis of glutamate formiminotransferase deficiency

The original article to which this Erratum refers was published in Human Mutation 22:67–73 Human Mutation(2003) 22(1) 67–73     

The molecular basis of glutamate formiminotransferase deficiency

Glutamate formiminotransferase deficiency, an autosomal recessive disorder and the second most common inborn error of folate metabolism, is presumed to be due to defects in the bifunctional enzyme glutamate formiminotransferase-cyclodeaminase (FTCD). Features of a severe phenotype, first identified in patients of Japanese descent, include elevated levels of formiminoglutamate (FIGLU) in the urine in response to histidine administration, megaloblastic anemia, and mental retardation. Features of a mild phenotype include high urinary excretion of FIGLU in the absence of histidine administration, mild developmental delay, and no hematological abnormalities. We found mutations in the human FTCD gene in three patients with putative glutamate formiminotransferase deficiency. Two siblings were heterozygous for missense mutations, c.457C>T (R135C) and c.940G>C (R299P). Mutagenesis of porcine FTCD and expression in E. coli showed that the R135C mutation reduced formiminotransferase activity to 61% of wild-type, whereas the R299P mutation reduced this activity to 57% of wild-type. The third patient was hemizygous for c.1033insG, with quantitative PCR indicating that the other allele contained a deletion. These mutations are the first identified in glutamate formiminotransferase deficiency and demonstrate that mutations in FTCD represent the molecular basis for the mild phenotype of this disease.     

Ionotropic glutamate receptors and glutamate transporters are involved in necrotic neuronal cell death induced by oxygen-glucose deprivation of hippocampal slice cultures

Organotypic hippocampal slice cultures represent a feasible model for studies of cerebral ischemia and the role of ionotropic glutamate receptors in oxygen-glucose deprivation-induced neurodegeneration. New results and a review of existing data are presented in the first part of this paper. The role of glutamate transporters, with special reference to recent results on inhibition of glutamate transporters under normal and energy-failure (ischemia-like) conditions is reviewed in the last part of the paper. The experimental work is based on hippocampal slice cultures derived from 7 day old rats and grown for about 3 weeks. In such cultures we investigated the subfield neuronal susceptibility to oxygen-glucose deprivation, the type of induced cell death and the involvement of ionotropic glutamate receptors. Hippocampal slice cultures were also used in our studies on glutamate transporters reviewed in the last part of this paper. Neurodegeneration was monitored and/or shown by cellular uptake of propidium iodide, loss of immunocytochemical staining for microtubule-associated protein 2 and staining with Fluoro-Jade B. To distinguish between necrotic vs. apoptotic neuronal cell death we used immunocytochemical staining for active caspase-3 (apoptosis indicator) and Hoechst 33342 staining of nuclear chromatin. Our experimental studies on oxygen-glucose deprivation confirmed that CA1 pyramidal cells were the most susceptible to this ischemia-like condition. Judged by propidium iodide uptake, a selective CA1 lesion, with only minor affection on CA3, occurred in cultures exposed to oxygen-glucose deprivation for 30 min. Nuclear chromatin staining by Hoechst 33342 and staining for active caspase-3 showed that oxygen-glucose deprivation induced necrotic cell death only. Addition of 10 microM of the N-methyl-D-aspartate glutamate receptor antagonist MK-801, and 20 microM of the non-N-methyl-D-aspartate glutamate receptor antagonist 2,3-dihyroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline to the culture medium confirmed that both N-methyl-D-aspartate and non-N-methyl-D-aspartate ionotropic glutamate receptors were involved in the oxygen-glucose deprivation-induced cell death. Glutamate is normally quickly removed, from the extracellular space by sodium-dependent glutamate transporters. Effects of blocking the transporters by addition of the DL-threo-beta-benzyloxyaspartate are reviewed in the last part of the paper. Under normal conditions addition of DL-threo-beta-benzyloxyaspartate in concentrations of 25 microM or more to otherwise untreated hippocampal slice cultures induced neuronal cell death, which was prevented by addition of 2,3-dihyroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline and MK-801. In energy failure situations, like cerebral ischemia and oxygen-glucose deprivation, the transporters are believed to reverse and release glutamate to the extracellular space. Blockade of the transporters by a subtoxic (10 microM) dose of DL-threo-beta-benzyloxyaspartate during oxygen-glucose deprivation (but not during the next 48 h after oxygen-glucose deprivation) significantly reduced the oxygen-glucose deprivation-induced propidium iodide uptake, suggesting a neuroprotective inhibition of reverse transporter activity by DL-threo-beta-benzyloxyaspartate during oxygen-glucose deprivation under these conditions. Adding to this, other results from our laboratory have demonstrated that pre-treatment of the slice cultures with glial cell-line derived neurotrophic factor upregulates glutamate transporters. As a logical, but in some glial cell-line derived neurotrophic factor therapy-related conditions clearly unwanted consequence the susceptibility for oxygen-glucose deprivation-induced glutamate receptor-mediated cell death is increased after glial cell-line derived neurotrophic factor treatment. In summary, we conclude that both ionotropic glutamate receptors and glutamate transporters are involved in oxygen-glucose deprivation-induced necrotic cell death in hippocampal slice cultures, which have proven to be a feasible tool in experimental studies on this topic.     

Ionotropic and metabotropic glutamate receptor mediation of glucocorticoid-induced apoptosis in hippocampal cells and the neuroprotective role of synaptic N-methyl-D-aspartate receptors

Glutamate receptors have been proposed to mediate the apoptotic actions of glucocorticoids in hippocampal cells. To further analyze the role of glutamate receptors in this process, we pretreated primary hippocampal cells from neonatal (postnatal day 4) rats with antagonists of ionotropic glutamate receptor (iGluR) and metabotropic glutamate receptor (mGluR) antagonists before exposure to the specific glucocorticoid receptor agonist dexamethasone (DEX) at a dose of 1 microM. Dizocilpine (MK801; a general N-methyl-D-aspartic acid [NMDA] receptor antagonist, NMDAR antagonist) and ifenprodil (a specific ligand of the NMDAR 2B subunit, NR2B), were used to block iGluR; (RS)-alpha-ethyl-4-carboxyphenylglycine (E4CPG) and (RS)-alpha-cyclopropyl-4-phosphonophenyl-glycine (CPPG) were employed as I/II (E4CPG) and II/III (CPPG) mGluR antagonists. Blockade of iGluR resulted in a significant attenuation of DEX-induced cell death; the finding that ifenprodil exerted a similar potency to MK801 demonstrates the involvement of NR2B receptors in glucocorticoid-induced cell death. Apoptosis accounted for a significant amount of the cell loss observed, as detected by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling histochemistry for the in situ labeling of DNA breaks; apoptotic cells were distinguished from necrosis on the basis of morphological criteria, including chromatin condensation, membrane blebbing and presence of apoptotic bodies. Treatment with E4CPG and CPPG completely abolished the apoptotic response to DEX, thus showing the additional contribution of mGluR to the phenomenon. Further, dose-response studies with NMDA revealed that whereas high (10 microM) doses of NMDA themselves elicit cytotoxic responses, low (1-5 microM) concentrations of NMDA can effectively oppose DEX-induced cell death. Interestingly, the neuroprotective actions of low dose NMDA stimulation were abolished when either synaptic or extrasynaptic NMDA receptors were blocked with MK801 in combination with the GABA receptor antagonist bicuculline (synaptic) or ifenprodil (extrasynaptic). In summary, the present data show that both iGluR and mGluR mediate the neurotoxic effects of glucocorticoids on hippocampal cells and that pre-treatment with low doses of NMDA, by acting on synaptic and extrasynaptic receptors, render hippocampal cells less vulnerable to glucocorticoid insults.     

Ionotropic glutamate receptors regulating labeled acetylcholine release from rat striatal tissue in vitro: possible involvement of receptor modulation in magnesium sensitivity

This study evaluated the role of glutamate ionotropic receptors on the control of [3H]acetylcholine ([3H]ACh) release by the intrinsic striatal cholinergic cells. [3H]-choline previously taken up by chopped striatal tissue and converted to [3H]ACh, was released under stimulation by glutamate, N-methyl-d-aspartate (NMDA), kainate and a-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA). Experiments were conducted in the absence of choline uptake inhibitors or acetylcholinesterase inhibitors. A paradigm of two stimulations was employed, the first in control conditions and the second after 9 min of perfusion with the test agents MK-801, 2-amino-5-phosphonopentanoic acid (AP-5), tetrodotoxin (TTX), 6,7-dinitroquinoxaline-2,3-dione (DNQX), 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo-[f]quinoxaline-7-sulfonamide (NBQX), glycine and magnesium. Our results support that (1) in the absence of Mg2+, NMDA is the most effective agonist to stimulate [3H]ACh release from striatal slices (2) magnesium effectively antagonized kainate and AMPA stimulation suggesting that at least part of the kainate and AMPA effects might be attributed to glutamate release (3) besides NMDA, kainate receptors showed a more direct involvement in [3H]ACh release control based on the smaller dependence on Mg2+ and less inhibition by TTX and (4) stimulation of ionotropic glutamate receptors may induce long lasting biochemical changes in receptor/ion channel function since the effects of TTX and/or Mg2+ ions on [3H]ACh release were modified by previous exposure of the tissue to agonists.     

Glucocorticoid receptors in human airways

Inhaled and intranasal glucocorticoids are the most common and effective drugs for controlling symptoms and airway inflammation in respiratory diseases such as asthma, allergic rhinitis, and nasal polyposis. The last few years have seen a growing understanding of the mechanisms of glucocorticoid action and, in particular, the receptor that mediates glucocorticoid actions, the glucocorticoid receptor (GR). In this revision we present an update on the GR gene, the expression and regulation of its gene products, namely GRalpha and GRbeta, as well as their alterations in pathological states. GRalpha is responsible for the induction and repression of target genes, it is expressed in virtually all human cells and tissues, and its expression is known to be downregulated by glucocorticoids. GRbeta has been found to act as a dominant negative inhibitor of GRalpha-mediated transactivation in in vitro studies with transfected cells, but it does not appear to have a significant inhibitory effect on GRalpha-mediated transrepression. In addition, for most tissues the expression of GRbeta, at least at the mRNA level, is extremely low compared with that of GRalpha. Some pro-inflammatory cytokines appear to upregulate the expression of GRbeta, and increased GRbeta expression has been reported in diseases associated with glucocorticoid resistance or insensitivity, such as bronchial asthma, nasal polyposis, and ulcerative colitis. However, the possible role of GRbeta in modulating glucocorticoid sensitivity and/or resistance in vivo has been highly debated and it is not yet clear.     

Roles of estrogen receptors alpha and beta in sexually dimorphic neuroprotection against glutamate toxicity

Although most agree that 17β-estradiol is neuroprotective via a variety of mechanisms, less is known about the role that biological sex plays in receptor-mediated estradiol neuroprotection. To address this issue we isolated primary cortical neurons from rat pups sorted by sex and assessed the ability of estradiol to protect the neurons from death induced by glutamate. Five-minute pretreatment with 10–50 nM 17β-estradiol protected female but not male neurons from glutamate toxicity 24 h later. Both estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ) are expressed in these cultures. Experiments using an ERα selective agonist or antagonist indicate that this receptor is important for neuroprotection in female cortical neurons. The ERβ selective agonist conveys a small degree of neuroprotection to both male and female cortical neurons. Interestingly, we found that 17α estradiol and the novel membrane estrogen receptor (mER) agonist STX, but not bovine serum albumin conjugated estradiol or the GPR30 agonist G1 were neuroprotective in both male and female neurons. Taken together these data highlight a role for ERα in sexually dimorphic neuroprotection.     

Ionotropic glutamate receptor subunit distribution on hypoglossal motoneuronal pools in the rat

The expression of ionotropic glutamate receptor subunits in the motoneuronal pools of the hypoglossal nucleus was studied using specific antibodies against subunits of the -amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), kainate and N-methyl-D-aspartate (NMDA) subtypes. The highest numbers of intensely immunolabelled motoneurons were found in the dorsal tier and caudoventromedial part of the hypoglossal nucleus with all antibodies except that against the GluR1 AMPA subunit. Labelling for the GluR1 subunit was weak except for caudally located groups of motoneurons which innervate tongue muscles related to respiratory activity. By contrast, most motoneurons were intensely immunostained with antibodies against GluR2/3 and GluR4 subunits of the AMPA subtype. The low staining observed using an antibody specific for the GluR2 subunit (which prevents Ca²⁺-entry through AMPA channels) strongly suggests that AMPA receptors in hypoglossal motoneurons are Ca²⁺-permeable. Immunolabelling for the GluR5/6/7 kainate receptor subunits was found in many motoneuronal somata as well as in thin axon-like profiles and puncta that resembled synaptic boutons. Most motoneurons were intensely immunostained for the NMDA receptor subunit NR1. These results show that the hypoglossal nucleus contains five heterogeneous pools of motoneurons which innervate functionally defined groups of tongue muscles. The uneven expression of the different receptor subunits analysed here could reflect diverse phenotypic properties of hypoglossal motoneurons which might be expected to generate different patterns of motor responses under different physiological or pathological conditions.     

Ionotropic glutamate receptor activated by N-methyl-D-aspartate: a key molecule of conscious life

We want to propose that basic stereotyped integrative functions are the result of sequentially built neuronal circuits in the primitive regions of the brain in whose build-up a particular subunit composition iGluR-NMDA would play a central role. iGluR-NMDA is a multiregulated heteromeric glutamate receptor-ion channel found in plasma membranes of neurons and other cells. iGluR-NMDA may be composed of up to five subunits, depending on the type of cell involved and its location. There are three major types of subunits and there are variations within each type allowing for up to 13 possible subunits, at least in the rat, which differ from each other in amino acid sequence and thus, in tertiary structure. The actual iGluR-NMDA heteropolymer involved in a given function may thus have a great number of subunit composition possibilities which would be the result of the particular genes expressed in a given type of cell. The iGluR-NMDA is an ion channel that opens in response to glutamate in a highly regulated fashion in which different molecules and ions present in the interstitial fluid determine whether or not the channel opens upon glutamate binding. The original function of iGluR-NMDA may have been that of allowing calcium influx to cells. As the brain receives external stimuli through the senses, new circuits will be formed stepwise in the neocortex in which a particular subunit composition iGluR-NMDA will be involved. Differentiation between external stimuli generated circuits and those governing internal functions will allow distinction between self and non-self, thus generating conscious awareness. The role of the particular subunit composition iGluR-NMDA proposed would be that of providing a means of calcium influx to the synapses to be formed and, if the same stimulus is forthcoming, allowing permanent synapses formation through the membrane incorporation of calcium dependent adhesion molecules such as cadherins and cytoskeleton reorganization promoted by nectins.     

Ambient atmospheric particles in the airways of human lungs

Epidemiologic studies linking particulate matter (PM) air pollution and adverse respiratory and cardiovascular effects have focused attention on the interactions of PM and lung cells. Information on the types, numbers, composition, sizes, and distribution of ambient particles in the airways is potentially useful for correlations with pathological and/or physiological changes, but relatively little is known about the extent to which ambient particles actually enter airway epithelial cells and are retained in airway walls and even less information is available about correlations with pathologic changes. Since many ambient particles are colorless and/or well below the level of resolution of light microscopy, definitive evaluation of particle burden in the airway wall requires analytical electron microscopy. Studies from the authors' laboratory suggest that in Vancouver, a city with relatively low PM levels, particle loads in the airway mucosa are nonetheless numerically substantial, typically in the range of 10⁷     

Mechanisms of tolerance and allergic sensitization in the airways and the lungs

The respiratory mucosa is constantly exposed to non-infectious substances that have the potential of triggering inflammation. While many particles are excluded, soluble molecules can reach the epithelium surface, where they can be uptaken by dendritic cells and stimulate an adaptive immune response. Most mucosal responses result in tolerance to subsequent antigen encounters, which is mediated by Foxp3(+) regulatory T cells. Genetic and environmental factors, added to the ability of certain allergens to induce innate responses, can predispose to allergic sensitization. In this review we discuss recent advances in the understanding of the mechanisms of tolerance and allergic sensitization to airborne allergens.     

Morphology of the distal conducting airways in rhesus monkey lungs

This study systematically characterizes the organization and nature of epithelial populations in the distal airways of the adult rhesus monkey. Infusion-fixed lungs were evaluated using airway dissection and scanning and transmission electron microscopy. We found that a true bronchiole free of cartilage and alveolar outpockets was not consistently present. Cartilage and alveolar outpocketings were often observed within the same airway generation. The epithelial population of nonalveolarized terminal conducting airways was pseudostratified columnar, consisting of ciliated, mucous, and basal cells. The respiratory bronchiole found immediately distal to the terminal conducting airways had two clearly demarcated zones of distinctly different epithelial populations. Overlying the pulmonary artery was the same pseudostratified ciliated columnar epithelial population observed in nonalveolarized terminal airways. The epithelial population in the remainder of the respiratory bronchiole, not associated with the pulmonary artery, was simple nonciliated cuboidal with a few squamous cells. The cuboidal nonciliated bronchiolar cell differs from the mucous cell by having few small granules and rough and smooth endoplasmic reticulum. The extension of the ciliated, mucous, and basal cells several generations into the respiratory bronchiole in rhesus monkey has not been observed in rodents and other laboratory mammals. Data from studies of human airways, although not explicit, suggest that the rhesus monkey epithelial cell pattern resembles the pattern in the human terminal airways.     

Ionotropic purinergic receptors P2X in frog and turtle retina: Glial and neuronal localization

Purinergic signaling is represented in both the peripheral and central nervous system (CNS), and in particular in the retina, which may be regarded as a part of the CNS. While purigenic signaling is relatively well studied in mammalian retinas, little is known about it in retinas of lower vertebrates. The aim of present study was to investigate, using immunocytochemistry, the distribution of purinoreceptors P2X in retinas of frog and turtle, which are appropriate models of the brain neuron-to-glia interactions. The results showed widespread expression of all seven ionotropic purinoreceptors (P2X1–P2X7) in both frog and turtle retinas. They were predominantly expressed in Müller cells, the principal glial cells in the retina. All structures typical of Müller cells: the outer and the inner limiting membranes, the cells bodies in the inner nuclear layer, the radial processes in the inner plexiform layer (IPL), and the so called endfeet (frog) or the orthogonal arrays of particles (turtle) in the ganglion cells layer were immunostained. Colocalizations between P2X1–P2X7 and the glial cell marker Vimentin proved that the immunostaining was in the Müller cells. In addition to the glial staining, neuronal staining was also seen as fine puncta in the inner plexiform layer and by small dots and patches in the outer plexiform layer. Some cell bodies of horizontal, amacrine and ganglion cells were also stained. The results obtained imply that the purinergic P2X receptors may significantly contribute to the neuron-to-glia signaling in retinas of the lower vertebrates.