Expression of purinergic receptors in bipolar cells of the rat retina

Ocular P2 receptors and glaucoma

Adenosine triphosphate (ATP), an energy source currency in cells, is released or leaked to the extracellular space under both physiological and pathological conditions. Extracellular ATP functions as an intercellular signaling molecule through activation of purinergic P2 receptors. Ocular tissue and cells release ATP in response to physiological stimuli such as intraocular pressure (IOP), and P2 receptor activation regulates IOP elevation or reduction. Dysregulated purinergic signaling may cause abnormally elevated IOP, which is one of the major risk factors for glaucoma. Glaucoma, a leading cause of blindness worldwide, is characterized by progressive degeneration of optic nerves and retinal ganglion cells (RGCs), which are essential retinal neurons that transduce visual information to the brain. An elevation in IOP may stress RGCs and increase the risk for glaucoma pathogenesis. In the aqueous humor of human patients with glaucoma, the ATP level is significantly elevated. Such excess amount of ATP may directly cause RGC death via a specific subtype of P2 receptors. Dysregulated purinergic signaling may also trigger inflammation, oxidative stress, and excitotoxicity via activating non-neuronal cell types such as glial cells. In this review, we discussed the physiological roles of extracellular nucleotides in the ocular tissue and their potential role in the pathogenesis of glaucoma. This article is part of the Special Issue on 'Purinergic Signaling: 50 years'.

Purinergic Receptors P2X7 and P2X4 as Markers of Disease Progression in the rd10 Mouse Model of Inherited Retinal Dystrophy

The purinergic receptor P2X7 (P2X7R) is implicated in all neurodegenerative diseases of the central nervous system. It is also involved in the retinal degeneration associated with glaucoma, age-related macular degeneration, and diabetic retinopathy, and its overexpression in the retina is evident in these disorders. Retinitis pigmentosa is a progressive degenerative disease that ultimately leads to blindness. Here, we investigated the expression of P2X7R during disease progression in the rd10 mouse model of RP. As the purinergic receptor P2X4 is widely co-expressed with P2X7R, we also studied its expression in the retina of rd10 mice. The expression of P2X7R and P2X4R was examined by immunohistochemistry, flow cytometry, and western blotting. In addition, we analyzed retinal functionality by electroretinographic recordings of visual responses and optomotor tests and retinal morphology. We found that the expression of P2X7R and P2X4R increased in rd10 mice concomitant with disease progression, but with different cellular localization. Our findings suggest that P2X7R and P2X4R might play an important role in RP progression, which should be further analyzed for the pharmacological treatment of inherited retinal dystrophies.

The Role of Purinergic Receptors in the Circadian System

The circadian system is an internal time-keeping system that synchronizes the behavior and physiology of an organism to the 24 h solar day. The master circadian clock, the suprachiasmatic nucleus (SCN), resides in the hypothalamus. It receives information about the environmental light/dark conditions through the eyes and orchestrates peripheral oscillators. Purinergic signaling is mediated by extracellular purines and pyrimidines that bind to purinergic receptors and regulate multiple body functions. In this review, we highlight the interaction between the circadian system and purinergic signaling to provide a better understanding of rhythmic body functions under physiological and pathological conditions.

Purinergic modulation of frog electroretinographic responses: The role of the ionotropic receptor P2X7

The contribution of the purinergic receptors P2X7 (P2X7Rs) to the electroretinographic (ERG) responses was studied by testing the effects of the selective P2X7R antagonist A438079 and the selective P2X7R agonist Bz-ATP on the electroretinograms obtained in perfused frog (Rana ridibunda) eyecup preparations under a variety of stimulation conditions. The P2X7R blockade by 200 µM A438079 diminished the amplitude of the photoreceptor components: the a-wave and the pharmacologically isolated mass receptor potential. In the pure rod-driven and pure cone-driven responses, the amplitude of the postreceptoral ON (b-wave) and OFF (d-wave) components was also diminished. The OFF responses were affected to a greater extent compared to the ON responses. In the mixed rod- and cone-driven responses, obtained in the mesopic intensity range, the b-wave amplitude was increased, while the d-wave amplitude was decreased. The amplitude of the oscillatory potentials was diminished. The relative amplitude changes produced by the P2X7R blockade were greater in the dark-adapted compared to the light-adapted eyes. The application of 100 µM Bz-ATP produced small effects opposite to those of the antagonist, while a prolonged (>20 min) treatment with 1 mM Bz-ATP resulted in a significant amplitude reduction or even abolishment of b- and d-waves. Our results show that endogenous ATP through its P2X7Rs exerts significant, mostly potentiating effects on the ERG photoreceptor and postreceptoral responses. There is a clear ON/OFF asymmetry of the effects on the ERG postreceptoral responses favoring OFF responses: they are always strongly potentiated, while the ON responses are either less potentiated (in the rod-driven and most of the cone-driven responses) or even inhibited (in the mixed rod- and cone-driven responses). The overstimulation of P2X7Rs can produce acute pathological changes, that is, a decrease or abolishment of the ERG responses.

Physiological contribution of P2X receptors in postreceptoral signal processing in the mouse retina

ATP activates P2X receptors and acts as a neurotransmitter in the nervous system. We have previously reported that P2X receptors modulate the firing rate of retinal ganglion cells. Since many subtypes of P2X receptors are distributed in the mouse retina, it is likely that the modulatory effects of P2X receptor-mediated signaling can occur at multiple synaptic levels in the retina. In this study, we investigated whether P2X receptors expressed between the photoreceptor layer and the inner nuclear layer in the mouse retina were physiologically functional, by electroretinography (ERG). In the combined rod-cone ERG and the scotopic ERG, intravitreal injection of PPADS, an antagonist of P2X receptors, had no effects on the amplitude of the a-wave, but decreased the amplitude of the b-wave. In the photopic ERG, intravitreal injection of PPADS significantly decreased the amplitude of both the a-wave and the b-wave. In ex vivo recordings, a decrease in the b-wave amplitude was observed at 20μM PPADS, confirming that the inhibition of the b-wave by intravitreal injection of PPADS is due to the inhibition of P2X receptors. Our findings suggest that P2X receptor-mediated signaling has a physiological effect in both the rod and the cone pathways in postreceptoral processing.

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.

Saffron reduces ATP-induced retinal cytotoxicity by targeting P2X7 receptors

P2X7-type purinergic receptors are distributed throughout the nervous system where they contribute to physiological and pathological functions. In the retina, this receptor is found in both inner and outer cells including microglia modulating signaling and health of retinal cells. It is involved in retinal neurodegenerative disorders such as retinitis pigmentosa and age-related macular degeneration (AMD). Experimental studies demonstrated that saffron protects photoreceptors from light-induced damage preserving both retinal morphology and visual function and improves retinal flicker sensitivity in AMD patients. To evaluate a possible interaction between saffron and P2X7 receptors (P2X7Rs), different cellular models and experimental approaches were used. We found that saffron positively influences the viability of mouse primary retinal cells and photoreceptor-derived 661W cells exposed to ATP, and reduced the ATP-induced intracellular calcium increase in 661W cells. Similar results were obtained on HEK cells transfected with recombinant rat P2X7R but not on cells transfected with rat P2X2R. Finally, patch-clamp experiments showed that saffron inhibited cationic currents in HEK-P2X7R cells. These results point out a novel mechanism through which saffron may exert its protective role in neurodegeneration and support the idea that P2X7-mediated calcium signaling may be a crucial therapeutic target in the treatment of neurodegenerative diseases.

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.

Distribution and development of P2Y1-purinoceptors in the mouse retina

There is increasing evidence that ATP acts on purinergic receptors and mediates synaptic transmission in the retina. In a previous study, we raised the possibility that P2X-purinoceptors, presumably P2X(2)-purinoceptors in OFF-cholinergic amacrine cells, play a key role in the formation of OFF pathway-specific modulation. In this study, we examined whether the P2Y(1)-purinoceptors can function in cholinergic amacrine cells in the mouse retina since cholinergic amacrine cells in the rat retina express P2Y(1)-purinoceptors. P2Y(1)-purinoceptors were shown to be expressed in dendrites of both ON- and OFF-cholinergic amacrine cells in adults. At postnatal day 7, there was immunoreactivity for P2Y(1)-purinoceptors in the soma of cholinergic amacrine cells. At postnatal day 14, weak immunoreactivity for P2Y(1)-purinoceptors was detected in the dendrites but not in the soma of cholinergic amacrine cells. At postnatal day 21, strong immunoreactivity for P2Y(1)-purinoceptors was detected in dendrites of cholinergic amacrine cells. The expression pattern of P2Y(1)-purinoceptors was not affected by visual experience. We concluded that P2Y(1)-purinoceptors are not involved in the OFF-pathway-specific signal transmission in cholinergic amacrine cells of the mouse retina.

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.

Rod and cone pathway signalling is altered in the P2X7 receptor knock out mouse

The P2X7 receptor (P2X7-R) is expressed in the retina and brain and has been implicated in neurodegenerative diseases. However, whether it is expressed by neurons and plays a role as a neurotransmitter receptor has been the subject of controversy. In this study, we first show that the novel vesicular transporter for ATP, VNUT, is expressed in the retina, verifying the presence of the molecular machinery for ATP to act as neurotransmitter at P2X7-Rs. Secondly we show the presence of P2X7-R mRNA and protein in the retina and cortex and absence of the full length variant 1 of the receptor in the P2X7-R knock out (P2X7-KO) mouse. The role of the P2X7-R in neuronal function of the retina was assessed by comparing the electroretinogram response of P2X7-KO with WT mice. The rod photoreceptor response was found to be similar, while both rod and cone pathway post-photoreceptor responses were significantly larger in P2X7-KO mice. This suggests that activation of P2X7-Rs modulates output of second order retinal neurons. In line with this finding, P2X7-Rs were found in the outer plexiform layer and on inner retinal cell classes, including horizontal, amacrine and ganglion cells. The receptor co-localized with conventional synapses in the IPL and was expressed on amacrine cells post-synaptic to rod bipolar ribbon synapses. In view of the changes in visual function in the P2X7-KO mouse and the immunocytochemical location of the receptor in the normal retina, it is likely the P2X7-R provides excitatory input to photoreceptor terminals or to inhibitory cells that shape both the rod and cone pathway response.

Release of ATP from avian Müller glia cells in culture

ATP can be released from neurons and act as a neuromodulator in the nervous system. Besides neurons, cortical astrocytes also are capable of releasing ATP from acidic vesicles in a Ca(2+)-dependent way. In the present work, we investigated the release of ATP from Müller glia cells of the chick embryo retina by examining quinacrine staining and by measuring the extracellular levels of ATP in purified Müller glia cultures. Our data revealed that glial cells could be labeled with quinacrine, a reaction that was prevented by incubation of the cells with 1μM bafilomycin A1 or 2μM Evans blue, potent inhibitors of vacuolar ATPases and of the vesicular nucleotide transporter, respectively. Either 50mM KCl or 1mM glutamate was able to decrease quinacrine staining of the cells, as well as to increase the levels of ATP in the extracellular medium by 77% and 89.5%, respectively, after a 5min incubation of the cells. Glutamate-induced rise in extracellular ATP could be mimicked by 100μM kainate (81.5%) but not by 100μM NMDA in medium without MgCl(2) but with 2mM glycine. However, both glutamate- and kainate-induced increase in extracellular ATP levels were blocked by 50μM of the glutamatergic antagonists DNQX and MK-801, suggesting the involvement of both NMDA and non-NMDA receptors. Extracellular ATP accumulation induced by glutamate was also blocked by incubation of the cells with 30μM BAPTA-AM or 1μM bafilomycin A1. These results suggest that glutamate, through activation of both NMDA and non-NMDA receptors, induces the release of ATP from retinal Müller cells through a calcium-dependent exocytotic mechanism.

Mechanisms of photoreceptor death during retinal degeneration

The development of treatments that slow photoreceptor death could profoundly improve patient wellbeing in those with inherited retinal degenerations. Over recent years, it has emerged that extracellular adenosine-tri-phosphate (ATP) regulates the function of photoreceptors in rodents and primates. Moreover, when the retina is exposed to high levels of ATP, rapid death of photoreceptors occurs, which can be blocked by pretreatment with antagonists to P2X receptors. Compounds that inhibit the action of extracellular ATP slow photoreceptor loss in an animal model of inherited retinal degeneration. In this article, I provide an overview of our work in relation to other research in this area and suggest a model by which ATP contributes to photoreceptor death in inherited retinal degenerations.

Involvement of the PI3K/AKT pathway in ATP-induced proliferation of developing retinal cells in culture

ATP induces the proliferation of chick retinal cells in culture through the activation of P2Y1 receptors, PKC and MAP kinases. Together with MAP kinases, the PI3K/AKT pathway has also been implicated as an important mediator in proliferative events during development. Here we investigated the participation of the PI3K/AKT signal pathway on ATP-induced proliferation of chick embryo retinal cells in culture. When retinal cultures obtained from 7-day-old embryos were cultivated for 1 day and treated with ATP, a transient and dose-dependent phosphorylation of both ERK and AKT was observed, an effect that could be mimicked by 500 microM ADP and blocked by 100 microM PPADS, a P2 receptor antagonist. Maximal stimulation of both enzymes was obtained with 100 microM ATP in 5 min, decreasing thereafter. Activation of these pathways by ATP seemed to be independent, since LY294002 and U0126, inhibitors of PI3K and MEK, did not block the activation of ERK and AKT, respectively, although each compound blocked its respective target. Moreover, when the cultures were incubated with ATP in the presence of LY294002, a decreased incorporation of [(3)H]-thymidine was observed, as compared to cultures treated only with ATP, a decline that was also obtained by incubating the cells with ATP plus 0.5 microM API-59CJ-Ome, an inhibitor of AKT. No decrease in cell viability was observed with this concentration of API-59CJ-Ome. An increase in cyclin D1 expression, that could be inhibited by 10 microM LY 294002 or 20 microM U0126, was observed when cells were incubated with 500 microM ADP. No effect of PI3K and MEK inhibitors was observed in the expression of p27kip1 in the cultures. These results suggest that, besides the involvement of the MAP kinases pathway, ATP-induced cell cycling of late developing retinal progenitors in culture also involves the activation of the PI3K/AKT pathway.

ATP controls cell cycle and induces proliferation in the mouse developing retina

Previous data suggest that nucleotides are important mitogens in the developing chick retina. Here, we extended the study on the mitogenic effect of ATP to newborn mouse retinal explants. Our results showed that P2Y(1) receptors were widely distributed in C57bl/6 mice retina and that the majority of PCNA positive cells co-localized with P2Y(1) receptor. To evaluate proliferation, retinal explants obtained from newborn mice were incubated with 0.5 microCi [(3)H]-thymidine or 3 microM BrDU 1h before the end of culture. Our data showed that ATP induced a dose-dependent increase in [(3)H]-thymidine incorporation, an effect that was mimicked by ADP but not by UTP and was blocked by the P2 antagonist PPADS in a dose-dependent manner. The increase in [(3)H]-thymidine incorporation induced by ATP was only observed in explants cultured for 3 days or less and was mimicked by the ectoapyrase inhibitor ARL 67156. It corresponded to an increase in the number of BrdU(+) cells in the neuroblastic layer (NL) of the tissue, suggesting that ATP, through activation of P2Y(1) receptors, induced proliferation of late developing progenitors in retinal explants of newborn mice. The increase in the number of BrdU(+) cells was observed across the whole NL when explants were incubated with ATP for 24h and no increase in the number of p-histone H3 labeled cells could be noticed at this time point. In longer incubations of 48h with ATP or 24h with ATP followed by a period of 24h in fresh medium, an increase in the number of BrdU(+) cells promoted by ATP was observed only in the middle and outer, but not in the inner NL. In these conditions, an increase in the number of p-histone H3 labeled cells was detected in the outer NL, suggesting that ATP induced cells to enter S and progress to G2 phase of the cell cycle in the first 24h period of incubation. ATP also induced an increase and a decrease in the expression of cyclin D1 and p27(kip1), respectively, in retinal progenitors of the NL. While the increase in the expression of cyclin D1 was observed when retinal explants were incubated for 3h or longer periods of time, the decrease in the expression of p27(kip1) was noticed only after 6h incubation with ATP. Both effects were blocked by the P2 receptor antagonist PPADS. These data suggest that ATP induces cell proliferation in retinal explants by inducing late developing progenitors to progress from G1 to S phase of cell cycle.

Pathway-dependent modulation by P2-purinoceptors in the mouse retina

Adenosine trisphosphate (ATP) activates purinoceptors and acts as a neurotransmitter in the nervous system. In the retina, we previously reported that the immunohistochemical distribution of the subset of P2-purinoceptors differs between the ON and OFF pathways. Here, we investigated whether ATP activates P2-purinoceptors and modulates the physiological function of the mouse retina. We also examined if signal processing by P2-purinoceptors is pathway specific. Results showed that ATP activated both ON- and OFF-cholinergic amacrine cells. However, responses in OFF-cholinergic amacrine cells were greater than those in ON-cholinergic amacrine cells. Pharmacological studies in OFF-cholinergic amacrine cells showed that the response of OFF-cholinergic amacrine cells is mediated P2X(2)-purinoceptors. Further, ATP increased gamma-aminobutyric acid (GABA)ergic inhibitory postsynaptic currents (IPSCs) in OFF- but not ON-cholinergic amacrine cells. The increase in GABAergic IPSCs was mediated by P2-purinoceptors. P2-purinoceptor-mediated signals suppressed OFF ganglion cells but activated ON ganglion cells. Our findings indicate that ATP physiologically modulates signal processing of the ON and OFF pathways in a pathway-specific manner through P2-purinoceptors.

Inhibitory interaction between P2X4 and GABA(C) rho1 receptors

Reciprocal functional inhibition between P2X and GABA(A/C) receptors represents a novel mechanism fine-tuning neuronal excitability. However, the participating receptors and underlying mechanisms are not fully understood. P2X(4) receptor is widely found in neurons that express GABA(C) rho1 receptor. Thus, we co-expressed P2X(4) and rho1 receptors in HEK293 cells and, using patch-clamp recording, examined whether they have mutual functional inhibition. Currents evoked by simultaneous application of ATP and GABA (I(ATP+GABA)) were significantly smaller compared to the addition of I(ATP) and I(GABA). Furthermore, I(ATP) were strongly suppressed during rho1 receptor activation. Similarly, I(GABA) were greatly attenuated during P2X(4) receptor activation. Such mutual inhibition was absent in cells only expressing P2X(4) or rho1 receptor. Taken together, these functional data support negative cross-talk between P2X(4) and rho1 receptors.

Nucleotides in ocular secretions: their role in ocular physiology

The eye is the sense organ that permits the detection of light owing to the existence of a sophisticated neuronal array, called the retina, which is responsive to photons. The correct functioning of this complex system requires the coordination of several intraocular structures that ultimately permit the perfect focusing of images on the neural retina. Light has to pass through different media: the tear, the cornea, aqueous humour, lens, and vitreous humour before it reaches the retina. Moreover, the composition and structure of some of these media can change due to several physiological mechanisms. Nucleotides are active components of the humours bathing relevant ocular structures. The tear contains nucleotides and dinucleotides that control the process of tearing, wound healing and protects of superficial infections. In the inner eye, the aqueous humour also presents a collection of mono and dinucleotides that affect pupil contraction, aqueous humour production and accommodation. Behind the lens and between this structure and the retina the vitreous humour can modify the physiology of the retinal cells, mostly the ganglion cells. By investigating the actions of nucleotides and dinucleotide present in the ocular humours we will be able not only to understand the functioning of the ocular structures but also to develop new pharmacological therapies for pathologies such as dry eye, glaucoma or retinal detachment.

Distribution of immunoreactivity for P2X3, P2X5, and P2X6-purinoceptors in mouse retina

Previous findings have shown that P2X-purinoceptor-mediated signaling pathways regulate the release of ACh in the retina. We previously reported the existence of immunoreactivity for P2X1-, P2X2-, P2X4-, and P2X7-purinoceptors in mouse retina and speculated that P2X2 and P2X7-purinoceptors may modulate the activity of cholinergic amacrine cells. In the present study, we used an immunohistochemical technique to examine whether P2X3-, P2X5, and P2X6-purinoceptors are also important for the modulation of cholinergic amacrine cells in mouse retina. Immunoreactivity for P2X3-, P2X5-, and P2X6-purinoceptors was observed in mouse retina. Immunoreactivity for P2X3- purinoceptors was observed in the dendrites of cholinergic amacrine cells. Immunoreactivity for P2X5-purinoceptors existed in the soma of cholinergic amacrine cells. P2X6-purinoceptor immunoreactivity was not colocalized with the cholinergic amacrine cells. We concluded that, among the three P2X-purinoceptors that were examined, P2X3-purinoceptors seem to affect the function of cholinergic amacrine cells in the mouse retina.

Control of cell proliferation by neurotransmitters in the developing vertebrate retina

In the developing vertebrate retina, precise coordination of retinal progenitor cell proliferation and cell-cycle exit is essential for the formation of a functionally mature retina. Unregulated or disrupted cell proliferation may lead to dysplasia, retinal degeneration or retinoblastoma. Both cell-intrinsic and -extrinsic factors regulate the proliferation of progenitor cells during CNS development. There is now growing evidence that in the developing vertebrate retina, both slow and fast neurotransmitter systems modulate the proliferation of retinal progenitor cells. Classic neurotransmitters, such as GABA (gamma-amino butyric acid), glycine, glutamate, ACh (acetylcholine) and ATP (adenosine triphosphate) are released, via vesicular or non-vesicular mechanisms, into the immature retinal environment. Furthermore, these neurotransmitters signal through functional receptors even before synapses are formed. Recent evidence indicates that the activation of purinergic and muscarinic receptors may regulate the cell-cycle machinery and consequently the expansion of the retinal progenitor pool. Interestingly, GABA and glutamate appear to have opposing roles, inducing retinal progenitor cell-cycle exit. In this review, we present recent findings that begin to elucidate the roles of neurotransmitters as regulators of progenitor cell proliferation at early stages of retinal development. These studies also raise several new questions, including how these neurotransmitters regulate specific cell-cycle pathways and the mechanisms by which retinal progenitor cells integrate the signals from neurotransmitters and other exogenous factors during vertebrate retina development.

Neuronal expression of P2X3 purinoceptors in the rat retina

P2X3 purinoceptors are involved in fast, excitatory neurotransmission in the nervous system, and are expressed predominantly within sensory neurons. In this study, we examined the cellular and synaptic localization of the P2X3 receptor subunit in the retina of the rat using immunofluorescence immunohistochemistry and pre-embedding immunoelectron microscopy. In addition, we investigated the activity of ecto-ATPases in the inner retina using an enzyme cytochemical method. The P2X3 receptor subunit was expressed in the soma of a subset of GABA immunoreactive amacrine cells, some of which also expressed protein kinase C-alpha. In addition, punctate immunoreactivity was observed within both the inner and outer plexiform layers of the retina. Double labeling studies showed that P2X3 receptor puncta were associated with both rod and cone bipolar cell axon terminals in the inner plexiform layer. Ultrastructural studies indicated that P2X3 receptor subunits were expressed on putative A17 amacrine cells at sites of reciprocal synaptic input to the rod bipolar cell axon terminal. Moreover, we observed P2X3 immunolabeling on amacrine cell processes that were associated with cone bipolar cell axon terminals and other conventional synapses. In the outer retina, P2X3 immunoreactivity was observed on specialized junctions made by putative interplexiform cells. Ecto-ATPase activity was localized to the inner plexiform layer on the extracellular side of all plasma membranes, but was not apparent in the ganglion cell layer or the inner nuclear layer, suggesting that ATP dephosphorylation occurs exclusively in synaptic regions of the inner retina. These data provide further evidence that purines participate in retinal transmission, particularly within the rod pathway.

Evidence for the involvement of purinergic P2X receptors in outer retinal processing

Extracellular ATP mediates fast excitatory neurotransmission in many regions of the central nervous system through activation of P2X receptors. Although several P2X receptor subunits have been identified in the mammalian retina, little is known about the functional role of these receptors in retinal signalling. The purpose of the present study was to investigate whether purinergic P2X(7) receptors are involved in outer retinal processing by assessing receptor localization, degradation of extracellular ATP and the effect of functional activation of P2X(7) receptors on the electroretinogram (ERG). Using light and electron microscopy, we demonstrated that P2X(7) receptors are expressed postsynaptically on horizontal cell processes as well as presynaptically on photoreceptor synaptic terminals in both the rat and marmoset retina. Using an enzyme cytochemical method, we showed that ecto-ATPases are active in the outer plexiform layer of the rat retina, providing a mechanism by which purinergic synaptic transmission can be rapidly terminated. Finally, we evaluated the role of P2X(7) receptors in retinal function by assessing changes to the ERG response of rats after intravitreal delivery of the P2X(7) receptor agonist benzoyl benzoyl ATP (BzATP). Intravitreal injection of BzATP resulted in a sustained increase (up to 58%) in the amplitude of the photoreceptor-derived a-wave of the ERG. In contrast, BzATP caused a transient reduction in the rod- and cone-derived postreceptoral responses. These results provide three lines of evidence for the involvement of extracellular purines in outer retinal processing.

Purinergic P2 receptors as targets for novel analgesics

Following hints in the early literature about adenosine 5'-triphosphate (ATP) injections producing pain, an ion-channel nucleotide receptor was cloned in 1995, P2X3 subtype, which was shown to be localized predominantly on small nociceptive sensory nerves. Since then, there has been an increasing number of papers exploring the role of P2X3 homomultimer and P2X2/3 heteromultimer receptors on sensory nerves in a wide range of organs, including skin, tongue, tooth pulp, intestine, bladder, and ureter that mediate the initiation of pain. Purinergic mechanosensory transduction has been proposed for visceral pain, where ATP released from epithelial cells lining the bladder, ureter, and intestine during distension acts on P2X3 and P2X2/3, and possibly P2Y, receptors on subepithelial sensory nerve fibers to send messages to the pain centers in the brain as well as initiating local reflexes. P1, P2X, and P2Y receptors also appear to be involved in nociceptive neural pathways in the spinal cord. P2X4 receptors on spinal microglia have been implicated in allodynia. The involvement of purinergic signaling in long-term neuropathic pain and inflammation as well as acute pain is discussed as well as the development of P2 receptor antagonists as novel analgesics.

P2Y isoforms operative in mouse taste cells

Recent functional evidence indicates that mouse taste cells express P2Y receptors coupled to IP(3) production and Ca(2+) mobilization. Our studies of the expression profile of particular P2Y isoforms in the taste tissue of the mouse have revealed that ATP and UTP equipotently mobilize intracellular Ca(2+) at saturating concentrations, suggesting that common receptors for both nucleotides, i.e., P2Y(2) and P2Y(4) subtypes, might be involved. Reverse transcription/polymerase chain reaction and immunohistochemistry have confirmed the presence of P2Y(2) and P2Y(4) receptors in a population of taste bud cells from the circumvallate and foliate papillae. Transcripts for the P2Y(1) and P2Y(6) isoforms have also been detected in taste tissue preparations, this observation being consistent with the ADP and UDP responsiveness of taste cells. Together, our data suggest that P2Y(2) and P2Y(4) receptors play a predominant role in mediating taste cell responses to ATP and UTP.

P2X(7) receptor-mRNA and -protein in the mouse retina; changes during retinal degeneration in BALBCrds mice

A combined real-time PCR/immunohistochemistry study was carried out to investigate whether P2X(7) receptors, known to induce apoptosis and necrosis, may be causally related to the process of retinal degeneration in BALBCrds mice. In the retinae of BALBCrds mice, P2X(7) receptor-mRNA was the highest at an age of 20-40 days, and declined afterwards. At the same time, the P2X(7) receptor-message was constantly low in the retina of control BALBC mice until postnatal day 100. The receptor-mRNA in total brain tissue of both strains of mice was comparable with that of BALBCrds retinae. Double immunofluorescence in combination with laser scanning microscopy was used to study the distribution of P2X(7) receptor-immunoreactivity (IR) on neurons and different glial cell types of the retina. An exclusively neuronal localization of P2X(7)-IR in the ganglion cell layer was found by using either anti-neuronal nuclei or microtubule associated protein-2 as neuronal markers. There was a slight age-dependent decrease in the abundance of neuronal P2X(7)-IR both in BALBCrds or BALBC mice. P2X(7)-IR failed to co-localize with any of the non-neuronal markers used to stain microglial or Müller glial cells. No P2X(7) receptor-IR was found in the retinal ganglion cell layer of P2X(7)(-/-) animals, when compared with the control littermates. Hence, we suggest that, in BALBCrds mice, an early up-regulation of neuronal P2X(7) receptors may cause injury of retinal neurons and thereby functionally contribute to the retinal damage.

ATP-induced non-neuronal cell permeabilization in the rat inner retina

The P2X7 subtype holds a special position among P2X receptors because of its ability to act both as a classical, ligand-gated ion channel, and as a permeabilization pore that can induce cell death under prolonged activation by ATP. We have shown previously that, in rat retina, P2X7 receptors are located in the inner nuclear layer and ganglion cell layer (GCL). The present study was aimed at finding whether retinal P2X7 receptors can act as a mediator of cell permeabilization and, if so, at identifying the cellular target(s) of this effect. As an indicator of cell permeabilization, we used the fluorescent dye YO-PRO-1 (molecular weight, 375 Da), which enters cells only through large pores like those opened by prolonged or sustained stimulation of P2X(7) receptors and binds to DNA, providing a stable labeling of the activated cells. Different agonists for P2 receptors were tested for their ability to cause cell permeabilization in flat-mounted rat retinas. Among them, only high concentrations of ATP (500 microM) and BzATP (2',3'-O-(4-benzoyl-benzoyl)-ATP triethylammonium) (100 microM) were able to induce accumulation of YO-PRO-1 in the GCL and in the nerve fiber layer, suggesting that different cell types were responding to P2X7 stimulation. This effect was blocked by the P2 antagonists suramin and PPADS (pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid) and by the P2X7-selective inhibitor Brilliant Blue G. To identify the retinal cell types affected by ATP-induced permeabilization, we used in vivo labeling techniques. Our data clearly reveal that prolonged stimulation of P2X7 receptors elicits permeabilization exclusively in microglial cells but not in neurons of the inner retina.

Control of programmed cell death by neurotransmitters and neuropeptides in the developing mammalian retina

It has long been known that a barrage of signals from neighboring and connecting cells, as well as components of the extracellular matrix, control cell survival. Given the extensive repertoire of retinal neurotransmitters, neuromodulators and neurotrophic factors, and the exhuberant interconnectivity of retinal interneurons, it is likely that various classes of released neuroactive substances may be involved in the control of sensitivity to retinal cell death. The aim of this article is to review evidence that neurotransmitters and neuropeptides control the sensitivity to programmed cell death in the developing retina. Whereas the best understood mechanism of execution of cell death is that of caspase-mediated apoptosis, current evidence shows that not only there are many parallel pathways to apoptotic cell death, but non-apoptotic programs of execution of cell death are also available, and may be triggered either in isolation or combined with apoptosis. The experimental data show that many upstream signaling pathways can modulate cell death, including those dependent on the second messengers cAMP-PKA, calcium and nitric oxide. Evidence for anterograde neurotrophic control is provided by a variety of models of the central nervous system, and the data reviewed here indicate that an early function of certain neurotransmitters, such as glutamate and dopamine, as well as neuropeptides such as pituitary adenylyl cyclase-activating polypeptide and vasoactive intestinal peptide is the trophic support of cell populations in the developing retina. This may have implications both regarding the mechanisms of retinal organogenesis, as well as pathological conditions leading to retinal dystrophies and to dysfunctional cellular behavior.

Distribution of metabotropic P2Y receptors in the rat retina: a single-cell RT-PCR study

P2Y receptors are metabotropic G-protein linked purinergic receptors, which are especially widespread in the central nervous system. The purpose of the present study was to determine the distribution patterns of P2Y receptors in distinct retinal cell types in the adult retina. Retinal ganglion cells (RGC), bipolar cells (BPC) and Muller cells (MC) of adult pigmented rats were analyzed for their expression of P2Y-receptor subtypes P2Y1, P2Y2, P2Y4, and P2Y6 by single-cell reverse transcription polymerase chain reaction (SC-RT-PCR). SC-RT-PCR resulted in a positive amplification signal for all P2Y-receptor subtype mRNAs in all cell types examined. However, subtype distribution differed among the different cell types. The percentage of cells expressing a distinct P2Y subtype was: (a) for RGCs: 80% with P2Y1, 100% with P2Y2, 30% with P2Y4 and 50% with P2Y6, (b) for BPCs: 60% with P2Y1, 40% with P2Y2, 20% with P2Y4 and 80% with P2Y6, and (c) for MCs: 60% with P2Y1, 80% with P2Y2, 60% with P2Y4 and 100% with P2Y6. Our data show that different subtypes of P2Y receptors (P2Y1, P2Y2, P2Y4 and P2Y6) are expressed in various retinal cells and indicate that extracellular purines and pyrimidines act on RGCs, BPCs and MCs via different P2Y receptors.

Synaptic localization of P2X7 receptors in the rat retina

The distribution of P2X(7) receptor (P2X(7)R) subunits was studied in the rat retina using a subunit-specific antiserum. Punctate immunofluorescence was observed in the inner and outer plexiform layers. Double labeling of P2X(7) and the horizontal cell marker, calbindin, revealed extensive colocalization in the outer plexiform layer (OPL). Significant colocalization of P2X(7)R and kinesin, a marker of photoreceptor ribbons, was also observed, indicating that this receptor may be expressed at photoreceptor terminals. Furthermore, another band of P2X(7)R puncta was identified below the level of the photoreceptor terminals, adjacent to the inner nuclear layer (INL). This band of P2X(7)R puncta colocalized with the active-zone protein, bassoon, suggesting that "synapse-like" structures exist outside photoreceptor terminals. Preembedding immunoelectron microscopy demonstrated P2X(7)R labeling of photoreceptor terminals adjacent to ribbons. In addition, some horizontal cell dendrites and putative "desmosome-like" junctions below cone pedicles were labeled. In the inner plexiform layer (IPL), P2X(7)R puncta were observed surrounding terminals immunoreactive for protein kinase C-alpha, a marker of rod bipolar cells. Double labeling with bassoon in the IPL revealed extensive colocalization, indicating that P2X(7)R is likely to be found at conventional cell synapses. This finding was confirmed at the ultrastructural level: only processes presynaptic to rod bipolar cells were found to be labeled for the P2X(7)R, as well as other conventional synapses. These findings suggest that purines play a significant role in neurotransmission within the retina, and may modulate both photoreceptor and rod bipolar cell responses.

Cross-talk and co-trafficking between rho1/GABA receptors and ATP-gated channels

Gamma-aminobutyric-acid (GABA) and ATP ionotropic receptors represent two structurally and functionally different classes of neurotransmitter-gated channels involved in fast synaptic transmission. We demonstrate here that, when the inhibitory rho1/GABA and the excitatory P2X2 receptor channels are co-expressed in Xenopus oocytes, activation of one channel reduces the currents mediated by the other one. This reciprocal inhibitory cross-talk is a receptor-mediated phenomenon independent of agonist cross-modulation, membrane potential, direction of ionic flux, or channel densities. Functional interaction is disrupted when the cytoplasmic C-terminal domain of P2X2 is deleted or in competition experiments with minigenes coding for the C-terminal domain of P2X2 or the main intracellular loop of rho1 subunits. We also show a physical interaction between P2X2 and rho1 receptors expressed in oocytes and the co-clustering of these receptors in transfected hippocampal neurons. Co-expression with P2X2 induces retargeting and recruitment of mainly intracellular rho1/GABA receptors to surface clusters. Therefore, molecular and functional cross-talk between inhibitory and excitatory ligand-gated channels may regulate synaptic strength both by activity-dependent current occlusion and synaptic receptors co-trafficking.

Evidence for multiple P2Y receptors in trabecular meshwork cells

The purpose of this study was to determine whether functional purinergic P2 receptors are present in trabecular meshwork cells. The human trabecular cell line HTM-3 and cultured bovine trabecular cells were used to assess the effects of P2 agonists on intracellular Ca(2+) levels, extracellular signal-regulated kinase (ERK1/2) activation, and P2Y receptor expression. ATP, UTP, ADP, and 2-methyl-thio-adenosine triphosphate (2-MeS-ATP) each produced a concentration-dependent increase in intracellular Ca(2+) in bovine trabecular cells and the HTM-3 cell line. The addition of UDP did not produce any detectable rise in intracellular Ca(2+). Pretreatment with the P2Y(1) receptor antagonist 2'-deoxy-N(6)-methyladenosine-3',5'-diphosphate (MRS-2179) blocked the ADP- and 2-MeS-ATP-induced rise in intracellular Ca(2+). However, the ATP- or UTP-induced rise in intracellular Ca(2+) was not inhibited by MRS-2179 pretreatment. The addition of ADP, 2-MeS-ATP, ATP, or UTP were also found to activate the ERK1/2 signaling pathway. This activation of ERK1/2 was blocked by pretreatment with the mitogen-activated protein kinase kinase inhibitor 1,4-diamino-2,3-dicyano-1,4-bis(o-aminophenylmercapto)butadiene (U-0126) or the protein kinase C inhibitor chelerythrine chloride, but not by MRS-2179. Analysis of mRNA from HTM-3 cells by reverse transcription-polymerase chain reaction revealed the expression of P2Y(1), P2Y(4), and P2Y(11) receptor subtypes. These data demonstrate that multiple P2Y receptors are present in trabecular cells. Our results are consistent with the idea that the mobilization of intracellular Ca(2+)results from the activation of P2Y(1) and P2Y(4) receptors, whereas the activation of the ERK1/2 pathway results from the activation of P2Y(4) receptors alone. However, a role for the P2Y(11) receptors in mobilization of Ca(2+), or activation of the ERK1/2 pathway, cannot be discounted.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Vanilloid receptor like 1 (VRL1) immunoreactivity in mammalian retina: Colocalization with somatostatin and purinergic P2X1 receptors

The distribution of vanilloid receptor like1 immunoreactivity (VRL1-IR) in the retinas of rat, cat, and monkey was studied by single- and double-labeling immunocytochemistry. The patterns were similar for all three species in that VRL1-IR was most prominent in the inner plexiform layer, with scattered compact projections to the outer plexiform layer (OPL). VRL1-immunoreactive cell bodies were present throughout the rat retina, represented by amacrine cells in the inner nuclear layer and ganglion cell layer (GCL). In cat and monkey retinas, VRL1-immunoreactive cell bodies were restricted to the GCL in the inferior retina. Occasional cell bodies were associated with retinal blood vessels, but their identity as pericytes, glia, or neurons is uncertain. All VRL1-immunoreactive cells and processes colocalized with somatostatin and purinergic P2X1 receptor-IR but not with tyrosine hydroxylase-IR. VRL1-immunoreactive processes in the OPL did not label with antisera against synaptic vesicle 2 (SV2), suggesting that they were dendritic and did not derive from interplexiform cells. However, VRL1-immunoreactive processes in the far periphery toward the pars plana labeled for SV2, suggesting that these processes were presynaptic. The VRL1-immunoreactive cell bodies in the monkey GCL were not calbindin-immunoreactive, demonstrating that they were not displaced H2 horizontal cells. The VRL1-immunoreactive cells in cat and monkey could represent biplexiform and/or associational ganglion cells that receive input in the OPL throughout the retina and direct output to the far periphery. The presence of P2X1 receptors and vanilloid receptor like 1 protein on somatostatin-containing neurons in mammalian retina adds to the growing complexity regarding the chemical control of retinal function that is likely to include the microcirculation.

Neuron-specific distribution of P2X7 purinergic receptors in the monkey retina

Extracellular ATP is a signaling molecule, working through P2X purinoceptors in the nervous system. P2X7 is a major subtype of the purinoceptors in the brain, where it is expressed mostly in glia cells and considered to work as a trigger of cytolysis. In the rodent retina, however, P2X7 is expressed in several classes of neurons including ganglion cells. In the present study we identified cells immunopositive for P2X7 by double immunolabeling. Immunoreactivity for P2X7 was observed in the inner nuclear layer (INL), the inner plexiform layer (IPL), and the ganglion cell layer (GCL). In the INL, strongly immunopositive cells corresponded to the subpopulation of horizontal cells. In the IPL, fine processes were immunopositive. In the GCL, most of the ganglion cells showed P2X7 immunoreactivity. At the ultrastructural level, immunoreactivity was confirmed in the cytoplasm of ganglion cells. No P2X7 immunoreactivity was found in non-neural cells, i.e., Müller cells or microglia. The immunohistochemical distribution of other purinoceptor subtypes (P2X1, P2X2, and P2X4) was also examined in the monkey retina. Immunoreactivity for P2X1 was strongly detected in a band, in sublamina a of the IPL. The band existed at almost the same level as tyrosine hydroxylase immunoreactivity, but did not seem to actually overlap. P2X2 was not expressed in the retina, and P2X4 was only faintly expressed at the scleral margin of the INL. Because P2X7 in the primate retina is expressed exclusively in neurons, it may in this location be involved in neural transmission rather than in cytolysis, as found for glia cells.

Pharmacological characterization of P2Y receptor subtypes on isolated tiger salamander Müller cells

Müller cells express a variety of neurotransmitter receptors that permit them to "sense" the extracellular environment within the retina. We have used a battery of agonists and antagonists to characterize the purinergic receptor subtypes expressed on isolated tiger salamander Müller cells. Changes in intracellular calcium ion concentration ([Ca(2+)](i)) in Müller cells were measured using the Ca(2+) indicator dye Fura-2 and digital imaging microscopy. ATP, 2-methylthio-ATP, 2-methylthio-ADP, ADP, UTP, UDP, deoxyATP, and 3'-O-(4-benzoyl)benzoyl ATP evoked increases in [Ca(2+)](i) in both the presence and absence of extracellular Ca(2+). Therefore, the increases we observed were likely due to intracellular Ca(2+) release mediated by G-protein-coupled P2Y receptor activation, rather than Ca(2+) influx via P2X receptor channels. The P2Y(1) receptor agonists 2-methylthio-ATP, 2-methylthio-ADP, and ADP evoked increases in [Ca(2+)](i) that were inhibited by the P2Y(1) receptor antagonists adenosine 3'-phosphate 5'-phosphosulfate and 2'-deoxy-N(6)-methyleneadenosine-3',5'-bisphosphate. Responses to ADP were not completely inhibited by the P2Y(1) receptor antagonists. The residual response to ADP could be mediated by P2Y(13) receptors. UTP evoked an increase in [Ca(2+)](i) that was partially inhibited by suramin, suggesting that Müller cells express P2Y(2) and P2Y(4) receptors. The P2Y(6) receptor agonist UDP, and the P2Y(11) receptor agonists deoxyATP, and 3'-O-(4-benzoyl)benzoyl ATP, evoked increases in [Ca(2+)](i) in Müller cells. We conclude that isolated tiger salamander Müller cells express P2Y(1), P2Y(2), P2Y(6), P2Y(11), and possibly P2Y(4) and P2Y(13) receptors. Therefore, the physiological release of ATP, ADP, UTP, and UDP and/or their accumulation in the retina under pathological conditions could stimulate increases in [Ca(2+)](i) in Müller cells.

Molecular physiology of P2X receptors

P2X receptors are membrane ion channels that open in response to the binding of extracellular ATP. Seven genes in vertebrates encode P2X receptor subunits, which are 40-50% identical in amino acid sequence. Each subunit has two transmembrane domains, separated by an extracellular domain (approximately 280 amino acids). Channels form as multimers of several subunits. Homomeric P2X1, P2X2, P2X3, P2X4, P2X5, and P2X7 channels and heteromeric P2X2/3 and P2X1/5 channels have been most fully characterized following heterologous expression. Some agonists (e.g., alphabeta-methylene ATP) and antagonists [e.g., 2',3'-O-(2,4,6-trinitrophenyl)-ATP] are strongly selective for receptors containing P2X1 and P2X3 subunits. All P2X receptors are permeable to small monovalent cations; some have significant calcium or anion permeability. In many cells, activation of homomeric P2X7 receptors induces a permeability increase to larger organic cations including some fluorescent dyes and also signals to the cytoskeleton; these changes probably involve additional interacting proteins. P2X receptors are abundantly distributed, and functional responses are seen in neurons, glia, epithelia, endothelia, bone, muscle, and hemopoietic tissues. The molecular composition of native receptors is becoming understood, and some cells express more than one type of P2X receptor. On smooth muscles, P2X receptors respond to ATP released from sympathetic motor nerves (e.g., in ejaculation). On sensory nerves, they are involved in the initiation of afferent signals in several viscera (e.g., bladder, intestine) and play a key role in sensing tissue-damaging and inflammatory stimuli. Paracrine roles for ATP signaling through P2X receptors are likely in neurohypophysis, ducted glands, airway epithelia, kidney, bone, and hemopoietic tissues. In the last case, P2X7 receptor activation stimulates cytokine release by engaging intracellular signaling pathways.

Vitamin B6 protects primate retinal neurons from ischemic injury

Vitamin B6 derivatives protect the retinal neurons from excitotoxic injury in vitro. However, their in vivo role in a process involving excitotoxicity, such as ischemia, remains unknown. We studied potential protective effects of pyridoxal 5'-phosphate (PLP) and pyridoxal hydrochloride (pyridoxal) on the retinal neurons in a monkey model of transient global ischemia. Daily intravenous injections (15 mg/kg) of pyridoxal and PLP were performed for consecutive 10 days. On the sixth day, whole brain complete ischemia was produced by clipping the innominate and the left subclavian arteries for 20 min. The monkeys were sacrificed 5 days after ischemia and their retinas were processed for histological analysis. The ischemia induced a marked cellular injury in the retina as shown by the loss of ganglion cells and the reduction of thickness of the ganglion cell, inner plexiform, and inner nuclear layers. PLP significantly prevented the ganglion cell loss and the reduction of thickness of the ganglion cell layer. Pyridoxal significantly prevented the ganglion cell loss as well as the reduction of thickness of ganglion cell, inner plexiform and inner nuclear layers. These results suggest that PLP and pyridoxal counteract the postischemic neuronal death in the adult primate retina, offering a potential for a novel pharmacotherapy of retinal ischemic injury.

ATP induces proliferation of retinal cells in culture via activation of PKC and extracellular signal-regulated kinase cascade

Both ATP and acetylcholine can induce the mobilization of intracellular calcium in the early developing chick embryo retina, a response that decreases during retinal development. In this study, the effects of these transmitters on the turnover of phosphoinositides and proliferation of developing retinal cells in culture were characterized. While ATP, UTP or carbachol were able to induce a >400% accumulation of phosphoinositides in retinal cell cultures, only ATP promoted a dose-dependent increase in [(3)H]-thymidine incorporation in cultured cells (EC(50)=8.6 microM), a response that was inhibited by the P2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) (0.1 or 0.25 mM). ADP, but not UTP or adenosine, also stimulated the proliferation of retinal cells (EC(50)=5.8 microM), indicating that activation of P2Y1 receptors mediates the proliferative response of retinal cells to ATP. The mitogenic effect of ATP was completely prevented by the PKC inhibitor chelerythrine chloride (0.5 microM) and the phospholipase C (PLC) inhibitor U73122 (0.5 microM). PD 98059 (25 or 50 microM), an inhibitor of the activation of extracellular signal-regulated kinases (ERKs) also blocked the increase in [(3)H]-thymidine incorporation induced by ATP. Moreover, the effect of ATP was pronounced in cultures obtained from retinas at embryonic days 6-8, but not at day 9. Since Müller and bipolar cells are the predominant cell types that proliferate at these embryonic stages, our data suggest that ATP, through activation of P2Y1 receptors coupled to phospholipase C, PKC and MAP kinases, affects DNA synthesis in one or both of these cell types in culture.

Identification of purinergic receptors in retinal ganglion cells

P2X receptors are ligand-gated ion channels activated by adenosine triphosphate and expressed in a broad variety of tissues. The present study demonstrates the expression of various types of purinergic P2X receptors in identified retinal ganglion cells (RGCs) of the adult rat retina. Single-cell reverse transcription polymerase chain reaction (SC-RT-PCR) resulted in a positive amplification signal for all P2X receptor subunit mRNAs examined (P2X(3-5), P2X(7)). Immunohistochemistry with P2X(3,4) receptor subunit-specific antibodies showed a labelling of neurons in the ganglion cell layer and inner nuclear layer. Our data suggest that extracellular ATP acts directly on RGCs via several types of P2X receptors and may provide neuromodulatory influences on information processing in the retina.

ATP-gated ion channel P2X(3) is increased in human inflammatory bowel disease

P2X(3) is a novel ATP-gated cation channel that is selectively expressed by small-diameter sensory neurones in rodents, and may play a role in nociception by binding ATP released from damaged or inflamed tissues. We have studied, for the first time, P2X(3) immunoreactivity in human inflammatory bowel disease, using Western blotting and immunohistochemistry. A major 66-kDa specific protein was found by Western blotting in all colon extracts. In the inflamed group there was a significant two-fold increase in the relative optical density of the 66-kDa band (21.2 +/- 3.1; n=8) compared to controls (11.4 +/- 3.7; n=8; P=0.009). In the control colon, P2X(3)-immunoreactive neurones were scattered throughout the myenteric and submucosal plexuses, with some neurones showing immunopositive axons/dendrites. The pattern of immunostaining was similar to the neuronal marker peripherin. In general, the intensity of the staining was greater in myenteric than submucosal neurones. The number of P2X(3)-immunoreactive neurones was significantly increased in the myenteric plexus of inflamed colon compared to controls (n=13; P=0.01). In humans, unlike rodents, P2X(3) is thus not restricted to sensory neurones. Increased P2X(3) in inflamed intestine suggests a potential role in dysmotility and pain, for which it represents a new therapeutic target.