Xiong-Li Yang

Natriuretic peptides and their receptors in the central nervous system.

Natriuretic peptides (NPs), including atrial, brain and C-type NPs, are a family of structurally related but genetically distinct peptides. These peptides, along with their receptors (NPRs), are long known to be involved in the regulation of various physiological functions, such as diuresis, natriuresis, and blood flow. Recently, abundant evidence shows that NPs and NPRs are widely distributed in the central nervous system (CNS), suggesting possible roles of NPs in modulating physiological functions of the CNS. This review starts with a brief summary of relevant background information, such as molecular structures of NPs and NPRs and general intracellular mechanisms after activation of NPRs. We then provide a detailed description of the expression profiles of NPs and NPRs in the CNS and an in-depth discussion of how NPs are involved in neural development, neurotransmitter release, synaptic transmission and neuroprotection through activation of NPRs.

Melatonin potentiates glycine currents through a PLC/PKC signalling pathway in rat retinal ganglion cells

In vertebrate retina, melatonin regulates various physiological functions. In this work we investigated the mechanisms underlying melatonin‐induced potentiation of glycine currents in rat retinal ganglion cells (RGCs). Immunofluorescence double labelling showed that rat RGCs were solely immunoreactive to melatonin MT2 receptors. Melatonin potentiated glycine currents of RGCs, which was reversed by the MT2 receptor antagonist 4‐P‐PDOT. The melatonin effect was blocked by intracellular dialysis of GDP‐β‐S. Either preincubation with pertussis toxin or application of the phosphatidylcholine (PC)‐specific phospholipase C (PLC) inhibitor D609, but not the phosphatidylinositol (PI)‐PLC inhibitor U73122, blocked the melatonin effect. The protein kinase C (PKC) activator PMA potentiated the glycine currents and in the presence of PMA melatonin failed to cause further potentiation of the currents, whereas application of the PKC inhibitor bisindolylmaleimide IV abolished the melatonin‐induced potentiation. The melatonin effect persisted when [Ca2+]i was chelated by BAPTA, and melatonin induced no increase in [Ca2+]i. Neither cAMP‐PKA nor cGMP‐PKG signalling pathways seemed to be involved because 8‐Br‐cAMP or 8‐Br‐cGMP failed to cause potentiation of the glycine currents and both the PKA inhibitor H‐89 and the PKG inhibitor KT5823 did not block the melatonin‐induced potentiation. In consequence, a distinct PC‐PLC/PKC signalling pathway, following the activation of Gi/o‐coupled MT2 receptors, is most likely responsible for the melatonin‐induced potentiation of glycine currents of rat RGCs. Furthermore, in rat retinal slices melatonin potentiated light‐evoked glycine receptor‐mediated inhibitory postsynaptic currents in RGCs. These results suggest that melatonin, being at higher levels at night, may help animals to detect positive or negative contrast in night vision by modulating inhibitory signals largely mediated by glycinergic amacrine cells in the inner retina.

Modulation by melatonin of glutamatergic synaptic transmission in the carp retina

Melatonin is involved in a variety of physiological functions through activating specific receptors coupled to GTP‐binding protein. Melatonin and its receptors are abundant in the retina. Here we show for the first time that melatonin modulates glutamatergic synaptic transmission from cones to horizontal cells (HCs) in carp retina. Immunocytochemical data revealed the expression of the MT1 receptor on carp HCs. Whole‐cell recordings further showed that melatonin of physiological concentrations potentiated glutamate‐induced currents from isolated cone‐driven HCs (H1 cells) in a dose‐dependent manner, by increasing the efficacy and apparent affinity of the glutamate receptor. The effects of melatonin were reversed by luzindole, but not by K 185, indicating the involvement of the MT1 receptor. Like melatonin, methylene blue (MB), a guanylate cyclase inhibitor, also potentiated the glutamate currents, but internal infusion of cGMP suppressed them. The effects of melatonin were not observed in cGMP‐filled and MB‐incubated HCs. These results suggest that the melatonin effects may be mediated by decreasing the intracellular concentration of cGMP. Consistent with these observations, melatonin depolarized the membrane potential of H1 cells and reduced their light responses, which could also be blocked by luzindole. These effects of melatonin persisted in the presence of the antagonists of receptors for dopamine, GABA and glycine, indicating a direct action of melatonin on H1 cells. Such modulation by melatonin of glutamatergic transmission from cones to HCs is thought to be in part responsible for circadian changes in light responsiveness of cone HCs in teleost retina.

Group I mGluR-Mediated Inhibition of Kir Channels Contributes to Retinal Müller Cell Gliosis in a Rat Chronic Ocular Hypertension Model

Müller cell gliosis, which is characterized by upregulated expression of glial fibrillary acidic protein (GFAP), is a universal response in many retinal pathological conditions. Whether down-regulation of inward rectifying K+ (Kir) channels, which commonly accompanies the enhanced GFAP expression, could contribute to Müller cell gliosis is poorly understood. We investigated changes of Kir currents, GFAP and Kir4.1 protein expression in Müller cells in a rat chronic ocular hypertension (COH) model, and explored the mechanisms underlying Müller cell gliosis. We show that Kir currents and Kir4.1 protein expression in Müller cells were reduced significantly, while GFAP expression was increased in COH rats, and these changes were eliminated by MPEP, a group I metabotropic glutamate receptors (mGluR I) subtype mGluR5 antagonist. In normal isolated Müller cells, the mGluR I agonist (S)-3,5-dihydroxyphenylglycine (DHPG) suppressed the Kir currents and the suppression was blocked by MPEP. The DHPG effect was mediated by the intracellular Ca2+-dependent PLC/IP3-ryanodine/PKC signaling pathway, but the cAMP-PKA pathway was not involved. Moreover, intravitreal injection of DHPG in normal rats induced changes in Müller cells, similar to those observed in COH rats. The DHPG-induced increase of GFAP expression in Müller cells was obstructed by Ba2+, suggesting the involvement of Kir channels. We conclude that overactivation of mGluR5 by excessive extracellular glutamate in COH rats could contribute to Müller cell gliosis by suppressing Kir channels.

Involvement of Calpain/p35-p25/Cdk5/NMDAR Signaling Pathway in Glutamate-Induced Neurotoxicity in Cultured Rat Retinal Neurons

We investigated possible involvement of a calpain/p35-p25/cyclin-dependent kinase 5 (Cdk5) signaling pathway in modifying NMDA receptors (NMDARs) in glutamate-induced injury of cultured rat retinal neurons. Glutamate treatment decreased cell viability and induced cell apoptosis, which was accompanied by an increase in Cdk5 and p-Cdk5T15 protein levels. The Cdk5 inhibitor roscovitine rescued the cell viability and inhibited the cell apoptosis. In addition, the protein levels of both calpain 2 and calpain-specific alpha-spectrin breakdown products (SBDPs), which are both Ca2+-dependent, were elevated in glutamate-induced cell injury. The protein levels of Cdk5, p-Cdk5T15, calpain 2 and SBDPs tended to decline with glutamate treatments of more than 9 h. Furthermore, the elevation of SBDPs was attenuated by either D-APV, a NMDAR antagonist, or CNQX, a non-NMDAR antagonist, but was hardly changed by the inhibitors of intracellular calcium stores dantrolene and xestospongin. Moreover, the Cdk5 co-activator p35 was significantly up-regulated, whereas its cleaved product p25 expression showed a transient increase. Glutamate treatment for less than 9 h also considerably enhanced the ratio of the Cdk5-phosphorylated NMDAR subunit NR2A at Ser1232 site (p-NR2AS1232) and NR2A (p-NR2AS1232/NR2A), and caused a translocation of p-NR2AS1232 from the cytosol to the plasma membrane. The enhanced p-NR2AS1232 was inhibited by roscovitine, but augmented by over-expression of Cdk5. Calcium imaging experiments further showed that intracellular Ca2+ concentrations ([Ca2+]i) of retinal cells were steadily increased following glutamate treatments of 2 h, 6 h and 9 h. All these results suggest that the activation of the calpain/p35-p25/Cdk5 signaling pathway may contribute to glutamate neurotoxicity in the retina by up-regulating p-NR2AS1232 expression.

Expression of glycine receptor and transporter on bullfrog retinal Müller cells

The expression of the glycine receptor (GlyR) alpha1, alpha2 and beta subunits and glycine transporter (GlyT) on Müller cells was studied in bullfrog retina using double immunofluorescence labeling and confocal scanning microscopy. Double labeling of glial fibrillary acidic protein (GFAP), a specific marker for Müller cells, and the GlyR subunits showed that almost all Müller cells moderately expressed GlyR alpha1 and weakly GlyR beta, whereas no immunoreactivity for GlyR alpha2 was observed. The labeling for GlyR alpha1 and GlyR beta appeared in somata, major processes, endfeet and branchlets of the Müller cells. Müller cells were also GlyT1-labeled. Consistent with previous electrophysiological results, these findings suggest that Müller cells may be involved in modulation of glycinergic transmission by reciprocal interactions with retinal neurons through GlyR and GlyT.

Neuromodulatory role of melatonin in retinal information processing

The neurohormone melatonin is implicated in a variety of physiological processes. In the retina, a major source for melatonin production, melatonin is involved in modulation of neuronal activities. In this article we review recent advances in this research field, which is preceded by a concise account of general information about melatonin, melatonin receptors and intracellular signaling pathways for melatonin actions. Melatonin is mainly synthesized in and released from photoreceptors in the retina. Different subtypes of melatonin receptors are present on major types of retinal neurons, and the expression of these receptors is highly species- and neuron subtype-dependent. By activating different melatonin receptor subtypes, melatonin modulates activities of retinal neurons. In the outer retina, melatonin regulates the activity of photoreceptors. In addition, melatonin reduces the light responsiveness of cone-driven horizontal cells, but potentiates rod signal to rod-dominant ON type bipolar cells in teleost fish or inhibits the TEA-sensitive potassium channel of rod-driven ON type bipolar cells in rats. In the inner retina, melatonin potentiates inputs from glycinergic amacrine cells to ganglion cells in rats. These actions of melatonin on retinal neurons are mediated by distinct intracellular signaling pathways via different subtypes of melatonin receptors and all serve to improve visual performance in a world of changing ambient illumination. The topics, concerning allosteric action of melatonin, interplay between melatonin and dopamine systems, and potential interaction between melatonin and melanopsin systems, are also discussed. An in-depth discussion of future directions in this research field is presented.

Melatonin potentiates rod signals to ON type bipolar cells in fish retina

Melatonin is involved in regulation of a variety of physiological functions through activation of specific G‐protein coupled receptors. However, the neuromodulatory role of melatonin, released from photoreceptors in the retina, is poorly understood. Here we show that melatonin enhances the sensitivity of the rod signal pathway by potentiating signal transfer from rod photoreceptors to ON bipolar cells (Rod‐ON‐BCs). Whole‐cell patch‐clamp recordings showed that melatonin induced a sustained inward current from Rod‐ON‐BCs, through activation of the melatonin MT2 receptor, which was identified as one mediated by a cGMP‐dependent cation channel. Consistent with this, melatonin was found, using immunocytochemistry, to increase intracellular cGMP levels, which was identified due to an inhibition of phosphodiesterase. Physiologically, melatonin potentiated responses of Rod‐ON‐BCs to simulated light flashes (brief puffs of CPPG, an mGluR6 antagonist, in the presence of l‐AP4, an mGluR6 agonist), which was mediated by cGMP‐dependent kinase, and increased the amplitude of the scotopic electroretinographic b‐wave, a reflection of Rod‐ON‐BC activity. These results suggest that melatonin, being at a higher level at night, may improve the signal/noise ratio for rod signals in the outer retina by enhancing signal transfer from rods to BCs.

GluA2 Trafficking Is Involved in Apoptosis of Retinal Ganglion Cells Induced by Activation of EphB/EphrinB Reverse Signaling in a Rat Chronic Ocular Hypertension Model

EphB1, expressed in Müller cells, and ephrinB2, expressed in both Müller cells and retinal ganglion cells (RGCs), constitute an EphB/ephrinB reverse signaling in RGCs. Whether and how this reverse signaling is involved in RGC apoptosis in a rat chronic ocular hypertension (COH) model was investigated. In the COH model, both EphB1 and ephrinB2 were significantly increased and the reverse signaling was activated, which was accompanied by increased protein levels of phosphorylated (p) src, GluA2, and p-GluA2. Intravitreal injection of EphB2-Fc, an activator of ephrinB2, induced an increase in TUNEL-positive signals in normal retinae. A coimmunoprecipitation assay demonstrated direct interactions among ephrinB2, p-src, and GluA2. Moreover, in COH rats the expression of GluA2 proteins on the surface of retinal cells was decreased. Such GluA2 endocytosis could be prevented by preoperational intravitreal injection of 4-amino-3-(4-chlorophenyl)-1-(t-butyl)-1H-pyrazolo [3,4-d] pyrimidine (PP2), an inhibitor of src family tyrosine kinases, and possibly involved the protein interacting with C kinase 1 and phosphorylation of GluA2. In normal rats, intravitreal injection of EphB2-Fc caused changes in these protein levels similar to those observed in COH rats, which all could be avoided by preinjection of PP2. Patch-clamp experiments further showed that the current–voltage relationship of AMPA receptor-mediated EPSCs of RGCs exhibited stronger inward rectification in EphB2-Fc-injected rats. Furthermore, preinjection of PP2 or N-[3-[[4-[(3-aminopropyl)amino]butyl]amino]propyl]-1-naphthaleneacetamide trihydrochloride) (Naspm), a Ca2+-permeable GluA2-lacking AMPA receptor inhibitor, remarkably inhibited RGC apoptosis in either EphB2-Fc-injected or COH rats. Together, elevated GluA2 trafficking induced by activated EphB2/ephrinB2 reverse signaling likely contributes to RGC apoptosis in COH rats.

Melatonin modulates glycine currents of retinal ganglion cells in rat

Melatonin is a hormone participating in the modulation of various physiological functions via binding to specific melatonin receptors. In the retina, melatonin is synthesized and released by photoreceptors and may play a neuromodulatory role. By using patch clamp techniques, we demonstrate for the first time that glycine-induced currents from a population of isolated ganglion cells in the rat retina are potentiated by melatonin of nanomolar concentrations by increasing the efficacy and the channel conductance of the strychnine-sensitive glycine receptor. The melatonin effect is blocked by 4-P-PDOT, indicating the mediation of the MT2 receptor. These results suggest that melatonin, along with the MT2 receptor, may be involved in retinal information processing by modulating glycine receptor-mediated inhibition.