Characterization of glycinergic synapses in vertebrate retinas

An Overview of Glycine Transporter Subtype 1 Inhibitors Under Preclinical and Clinical Evaluation for the Treatment of Alcohol Abuse

Abstract:

Being a historical issue that withstands multiple societal control measures, alcohol abuse remains a major healthcare problem. Despite worldwide efforts to limit consumption and educate people about its effects, consumption rates remain unchanged. Alcohol abuse arises from chronic alcohol exposure-caused permanent synaptic plasticity changes in the brain. These manifest in life-threatening withdrawal symptoms and drive relapse even after detoxification and treatment. Since ethanol has multiple targets in the human brain, it warrants a multiapproach therapy; here we introduce the potential therapeutic effects of glycine transporter subtype 1 inhibitors. We have listed the various glycine transporter 1 inhibitors used in studies of alcoholism and how they influenced glycine release from rat hippocampus was demonstrated in a preliminary study. Glycine transporters modulate both glutamatergic and glycinergic pathways: (i) glutamatergic neurotransmission plays an important role in the development of chronic changes in alcoholism as daily alcohol administration was shown to increase N-methyl-D-aspartic acid receptor activity long-term, and (ii) ethanol has access to the dopaminergic reward system via glycine receptors, being an allosteric modulator of glycine receptors. This manuscript summarises the progress and development of glycine transporter 1 inhibitors, characterizing them by their mode of action, adverse effects, and discusses their clinical applicability. Furthermore, we highlight the progress in the latest clinical trials, outline currently applied treatment methods, and offer suggestions for implementing glycine transporter 1 inhibitors into the long-term treatment of alcohol abuse.

Prox1 Is a Marker for AII Amacrine Cells in the Mouse Retina

The transcription factor Prox1 is expressed in multiple cells in the retina during eye development. This study has focused on neuronal Prox1 expression in the inner nuclear layer (INL) of the adult mouse retina. Prox1 immunostaining was evaluated in vertical retinal sections and whole mount preparations using a specific antibody directed to the C-terminus of Prox1. Strong immunostaining was observed in numerous amacrine cell bodies and in all horizontal cell bodies in the proximal and distal INL, respectively. Some bipolar cells were also weakly immunostained. Prox1-immunoreactive amacrine cells expressed glycine, and they formed 35 ± 3% of all glycinergic amacrine cells. Intracellular Neurobiotin injections into AII amacrine cells showed that all gap junction-coupled AII amacrine cells express Prox1, and no other Prox1-immunostained amacrine cells were in the immediate area surrounding the injected AII amacrine cell. Prox1-immunoreactive amacrine cell bodies were distributed across the retina, with their highest density (3887 ± 160 cells/mm²) in the central retina, 0.5 mm from the optic nerve head, and their lowest density (3133 ± 350 cells/mm²) in the mid-peripheral retina, 2 mm from the optic nerve head. Prox1-immunoreactive amacrine cell bodies comprised ~9.8% of the total amacrine cell population, and they formed a non-random mosaic with a regularity index (RI) of 3.4, similar to AII amacrine cells in the retinas of other mammals. Together, these findings indicate that AII amacrine cells are the predominant and likely only amacrine cell type strongly expressing Prox1 in the adult mouse retina, and establish Prox1 as a marker of AII amacrine cells.

Temporal properties of dual-peak responses of mouse retinal ganglion cells and effects of inhibitory pathways

Dual-peak responses of retinal ganglion cells (RGCs) are observed in various species, previous researches suggested that both response peaks were involved in retinal information coding. In the present study, we investigated the temporal properties of the dual-peak responses recorded in mouse RGCs elicited by spatially homogeneous light flashes and the effect of the inhibitory inputs mediated by GABAergic and/or glycinergic pathways. We found that the two peaks in the dual-peak responses exhibited distinct temporal dynamics, similar to that of short-latency and long-latency single-peak responses respectively. Pharmacological studies demonstrated that the application of exogenous GABA or glycine greatly suppressed or even eliminated the second peak of the cells' firing activities, while little change was induced in the first peak. Co-application of glycine and GABA led to complete elimination of the second peak. Moreover, application of picrotoxin or strychnine induced dual-peak responses in some cells with transient responses by unmasking a second response phase. These results suggest that both GABAergic and glycinergic pathways are involved in the dual-peak responses of the mouse RGCs, and the two response peaks may arise from distinct pathways that would converge on the ganglion cells.

Disinhibitory recruitment of NMDA receptor pathways in retina

Glutamate release at bipolar to ganglion cell synapses activates NMDA and AMPA/kainic acid (KA) ionotropic glutamate receptors. Their relative strength determines the output signals of the retina. We found that this balance is tightly regulated by presynaptic inhibition that preferentially suppresses NMDA receptor (NMDAR) activation. In transient ON-OFF neurons, block of GABA and glycine feedback enhanced total NMDAR charge by 35-fold in the ON response and 9-fold in the OFF compared with a 1.7-fold enhancement of AMPA/KA receptors. Blocking only glycine receptors enhanced the NMDAR excitatory postsynaptic current 10-fold in the ON and 2-fold in the OFF pathway. Blocking GABA(A) or GABA(C) receptors (GABA(C)Rs or GABA(A)Rs) produced small changes in total NMDAR charge. When both GABA(A)Rs and GABA(C)Rs were blocked, the total NMDAR charge increased ninefold in the ON and fivefold in the OFF pathway. This exposed a strong GABA(C)R feedback to bipolar cells that was suppressed by serial amacrine cell synapses mediated by GABA(A)Rs. The results indicate that NMDAR currents are large but latent, held in check by dual GABA and glycine presynaptic inhibition. One example of this controlled NMDAR activation is the cross talk between ON and OFF pathways. Blocking the ON pathway increased NMDAR relative strength in the OFF pathway. Stimulus prolongation similarly increased the NMDAR relative strength in the OFF response. This NMDAR enhancement was produced by a diminution in GABA and glycine feedback. Thus the retinal network recruits NMDAR pathways through presynaptic disinhibition.

Early stage development of the glycine-1 re-uptake inhibitor SCH 900435: central nervous system effects compared with placebo in healthy men

AIMS

To report the first three studies with SCH 900435, a selective glycine-1 re-uptake inhibitor in development for treating schizophrenia, using systematic evaluations of pharmacodynamics to understand the observed effects.

METHODS

Three double-blind, placebo-controlled studies (single, visual effect and multiple dose) were performed. In the single and multiple dose study SCH 900435 (0.5-30 mg) was given to healthy males and frequent pharmacokinetic and pharmacodynamic measurements were performed. The visual effects study incorporated visual electrophysiological measures of macular, retinal and intracranial visual pathway function.

RESULTS

In the single dose study (highest difference, 95% CI, P) increases in smooth pursuit eye movements (8, 12 mg (-6.09, 10.14, -2.04, 0.013), 30 mg), pupil : iris ratio (20 and 30 mg (-0.065, 0.09, -0.04, <0.0001)), VAS colour perception (30 mg (-9.48, 13.05, -5.91, <0.0001)) and changes in spontaneous reports of visual disturbance were found, while FSH (8 mg (0.42, 0.18, 0.66, 0.0015), 12, 20 mg), LH (8-30 mg (1.35, 0.65, 2.05, 0.0003)) and EEG alpha2 activity decreased (12, 20, 30 mg (0.27, 0.14, 0.41, 0.0002)). A subsequent dedicated visual effects study demonstrated that visual effects were transient without underlying electrophysiological changes. This provided enough safety information for starting a multiple ascending dose study, showing less visual symptoms after twice daily dosing and titration, possibly due to tolerance.

CONCLUSIONS

Several central nervous system (CNS) effects and gonadotropic changes resulted from administration of 8 mg and higher, providing evidence for CNS penetration and pharmacological activity of SCH 900435. Antipsychotic activity in patients, specificity of the reported effects for this drug class and possible tolerance to visual symptoms remain to be established.

Application of human persistent fetal vasculature neural progenitors for transplantation in the inner retina

Persistent fetal vasculature (PFV) is a potentially serious developmental anomaly in human eyes, which results from a failure of the primary vitreous and the hyaloid vascular systems to regress during development. Recent findings from our laboratory indicate that fibrovascular membranes harvested from subjects with PFV contain neural progenitor cells (herein called NPPFV cells). Our studies on successful isolation, culture, and characterization of NPPFV cells have shown that they highly express neuronal progenitor markers (nestin, Pax6, and Ki67) as well as retinal neuronal markers (β-III-tubulin and Brn3a). In the presence of retinoic acid and neurotrophins, these cells acquire a neural morphological appearance in vitro, including a round soma and extensive neurites, and express mature neuronal markers (β-III-tubulin and NF200). Further experiments, including real-time qRT-PCR to quantify characteristic gene expression profiles of these cells and Ca(2+) imaging to evaluate the response to stimulation with different neurotransmitters, indicate that NPPFV cells may resemble a more advanced stage of retinal development and show more differentiation toward inner retinal neurons rather than photoreceptors. To explore the potential of inner retinal transplantation, NPPFV cells were transplanted intravitreally into the eyes of adult C57BL/6 mice. Engrafted NPPFV cells survived well in the intraocular environment in presence of rapamycin and some cells migrated into the inner nuclear layer of the retina 1 week posttransplantation. Three weeks after transplantation, NPPFV cells were observed to migrate and integrate in the inner retina. In response to daily intraperitoneal injections of retinoic acid, a portion of transplanted NPPFV cells exhibited retinal ganglion cell-like morphology and expressed mature neuronal markers (β-III-tubulin and synaptophysin). These findings indicate that fibrovascular membranes from human PFV harbor a population of neuronal progenitors that may be potential candidates for cell-based therapy for degenerative diseases of the inner retina.

Regulation of synaptic transmission at the photoreceptor terminal: a novel role for the cation-chloride co-transporter NKCC1

The Na(+)-K(+)-2Cl(-) co-transporter type 1 (NKCC1) is localized primarily throughout the outer plexiform layer (OPL) of the distal retina, a synaptic lamina that is comprised of the axon terminals of photoreceptors and the dendrites of horizontal and bipolar cells. Although known to play a key role in development, signal transmission and the gating of sensory signals in other regions of the retina and in the CNS, the contribution of NKCC1 to synaptic transmission within the OPL is largely unknown. In the present study, we investigated the function of NKCC1 at the photoreceptor-horizontal cell synapse by recording the electrical responses of photoreceptors and horizontal cells before and after blocking the activity of the transporter with bumetanide (BMN). Because NKCC1 co-transports 1 Na(+), 1 K(+) and 2 Cl(-), it is electroneutral and its activation had little effect on membrane conductance. However, recordings from postsynaptic horizontal cells revealed that inhibiting NKCC1 with BMN greatly increased glutamate release from both rod and cone terminals. In addition, we found that NKCC1 directly regulates Ca(2+)-dependent exocytosis at the photoreceptor synapse, raising the possibility that NKCC1 serves to suppress bulk release of glutamate vesicles from photoreceptor terminals in the dark and at light offset. Interestingly, NKCC1 gene and protein expressions were upregulated by light, which we attribute to the light-induced release of dopamine acting on D1-like receptors. In sum, our study reveals a new role for NKCC1 in the regulation of synaptic transmission in photoreceptors.

Dopamine modulates the off pathway in light-adapted mouse retina

DL-2-Amino-4-phosphonobutyric acid (APB) is often used as a tool to block On pathways in studies of interactions between On and Off pathways in retinas. APB is an agonist of mGluR6 receptors and hyperpolarizes the On cone bipolar cells and rod bipolar cells. How APB affects Off responses of retinal ganglion cells (RGCs) in mouse retinas under dark and light adaptation is not clear. The light-evoked excitatory postsynaptic currents (light-evoked EPSCs) from Off and On-Off RGCs cells were recorded using whole-cell patch-clamp recording to assess how APB affects Off responses (light-evoked Off EPSCs) of RGCs in dark- and light-adapted mouse retinas. We found that APB differentially affected Off responses of RGCs in dark- and light-adapted mouse retinas. Under dark adaptation, while the APB-sensitive Off responses were blocked, APB increased the remaining Off responses (mainly from the secondary rod Off pathways) via removal of inhibition from On pathways to Off pathways. Under light adaptation, APB decreased Off responses. Glycinergic and GABAergic antagonists did not prevent the APB-induced reduction of Off responses of RGCs; however, a dopaminergic type 1 receptor (D(1)) blocker (SCH 23390) and a hyperpolarization-activated cyclic nucleotide-gated (HCN) channel blocker (ZD 7288) prevented the APB-induced reduction of Off responses of RGCs under light adaptation. The results indicated afunctional circuit: On cone bipolar cells to Off cone bipolar cells via D(1) receptors and HCN channels.

Inhibition of nitric oxide synthase desensitizes retinal ganglion cells to light by diminishing their excitatory synaptic currents under light adaptation

The effect of inhibiting nitric oxide synthase (NOS) on the visual responses of mouse retinal ganglion cells (RGCs) was studied under light adaptation by using patch-clamp recordings. The results demonstrated that NOS inhibitor, l-NAME, reduced the sensitivity of RGCs to light under light adaptation at different ambient light conditions. These observations were seen in all cells that recordings were made from. l-NAME diminished the excitatory synaptic currents (EPSCs), rather than increasing the inhibitory synaptic currents, of RGCs to reduce the sensitivity of RGCs to light. Cones may be the sites that l-NAME acted to diminish the EPSCs of RGCs.

Characteristics of glycine transport across the inner blood-retinal barrier

Although glycine plays a pivotal role in neurotransmission and neuromodulation in the retina and is present in high concentration in the retina, the source of retinal glycine is still unclear. The purpose of the present study was to investigate glycine transport across the inner blood-retinal barrier (inner BRB). [(14)C]Glycine transport at the inner BRB was characterized using a conditionally immortalized rat retinal capillary endothelial cell line (TR-iBRB2 cells) as an in vitro model of the inner BRB and in vivo vascular injection techniques. [(14)C]Glycine uptake by TR-iBRB2 cells was Na(+)- and Cl(-)-dependent, and concentration-dependent with Michaelis-Menten constants of 55.4 microM and 8.02 mM, and inhibited by glycine transporter 1 (GlyT1) and system A inhibitors. These uptake studies suggest that GlyT1 and system A are involved in [(14)C]glycine uptake by TR-iBRB2 cells. RT-PCR analysis demonstrated that GlyT1 and system A (encoding ATA 1 and ATA2) mRNA are expressed in TR-iBRB2 cells. An in vivo study suggested that [(14)C]glycine is transported from blood to the retina whereas [(14)C]alpha-methylaminoisobutyric acid, a selective substrate for system A, is not. In conclusion, GlyT1 most likely mediates glycine transport at the inner BRB and is expected to play an important role in regulating the glycine concentration in the neural retina.

Functional expression of the glycine transporter 1 on bullfrog retinal cones

Using patch clamp techniques, we characterized glycine-induced currents from cones in bullfrog retinal slices. Application of glycine to cone terminals induced an inward current, which was in part suppressed by strychnine. The remaining strychnine-resistant current component, which did not show polarity reversion in a range of -120 mV to +40 mV, was blocked by N[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl] sarcosine, an antagonist of glycine transporter 1 (GlyT1), but not affected by amoxapine, an inhibitor of glycine transporter 2. Application of sarcosine, an agonist of GlyT1, to cone terminals induced an inward current that was completely suppressed by N[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl] sarcosine or when external Na in Ringer's was replaced by choline. All these results show for the first time the functional expression of GlyT1 on bullfrog cones.

Synaptic inhibition by glycine acting at a metabotropic receptor in tiger salamander retina

Glycine is the lone fast neurotransmitter for which a metabotropic pathway has not been identified. In retina, we found a strychnine-insensitive glycine response in bipolar and ganglion cells. This glycine response reduced high voltage-activated calcium current. It was G-protein mediated and protein kinase A dependent. The EC(50) of the metabotropic glycine response is 3 mum, an order of magnitude lower than the ionotropic glycine receptor in the same retina. The bipolar cell glutamatergic input to ganglion cells was suppressed by metabotropic glycine action. The synaptic output of about two-thirds of bipolar cells and calcium current in two-thirds of ganglion cells are sensitive to the action of glycine at metabotropic receptors, suggesting this signal regulates specific synaptic pathways in proximal retina. This study resolves the curious absence of a metabotropic glycine pathway in the nervous system and reveals that the major fast inhibitory neurotransmitters, GABA and glycine, both activate G-protein-coupled pathways as well.