Regulation of transmitter release from retinal bipolar cells

Trophoblast glycoprotein is required for efficient synaptic vesicle exocytosis from retinal rod bipolar cells

Introduction

Rod bipolar cells (RBCs) faithfully transmit light-driven signals from rod photoreceptors in the outer retina to third order neurons in the inner retina. Recently, significant work has focused on the role of leucine-rich repeat (LRR) proteins in synaptic development and signal transduction at RBC synapses. We previously identified trophoblast glycoprotein (TPBG) as a novel transmembrane LRR protein localized to the dendrites and axon terminals of RBCs.

Methods

We examined the effects on RBC physiology and retinal processing of TPBG genetic knockout in mice using immunofluorescence and electron microscopy, electroretinogram recording, patch-clamp electrophysiology, and time-resolved membrane capacitance measurements.

Results

The scotopic electroretinogram showed a modest increase in the b-wave and a marked attenuation in oscillatory potentials in the TPBG knockout. No effect of TPBG knockout was observed on the RBC dendritic morphology, TRPM1 currents, or RBC excitability. Because scotopic oscillatory potentials primarily reflect RBC-driven rhythmic activity of the inner retina, we investigated the contribution of TPBG to downstream transmission from RBCs to third-order neurons. Using electron microscopy, we found shorter synaptic ribbons in TPBG knockout axon terminals in RBCs. Time-resolved capacitance measurements indicated that TPBG knockout reduces synaptic vesicle exocytosis and subsequent GABAergic reciprocal feedback without altering voltage-gated Ca2+ currents.

Discussion

TPBG is required for normal synaptic ribbon development and efficient neurotransmitter release from RBCs to downstream cells. Our results highlight a novel synaptic role for TPBG at RBC ribbon synapses and support further examination into the mechanisms by which TPBG regulates RBC physiology and circuit function.

Bipolar cells containing protein kinase Cα mediate attentional facilitation of the avian retinal ganglion cells by the retinopetal signal

Birds have a well-developed retinopetal system projecting from the midbrain to the contralateral retina. The signal sent to the retina through the retinopetal system facilitates visual responses of the retinal ganglion cells (RGCs), and the retinopetal signals function as attentional signals during visual search. Thus, the retinopetal signal somehow reaches and facilitates visual responses of the RGCs. However, the tertiary neuron of the retinopetal system, the isthmo-optic target cell (IOTC), is unlikely to contact most RGCs directly, because the IOTCs form axon terminals localized in the outermost sublayer (lamina 1) of the inner plexiform layer (IPL) where few RGC dendrites terminate. Therefore, some other intrinsic retinal neurons must be involved in the centrifugal attentional enhancement of visual responses of the RGCs. We investigated connections of the target cells of the IOTCs in chicken and quail, using light and electron microscopic immunohistochemistry. We show that axon terminals of the IOTC make synaptic contacts with protein kinase Cα (PKCα)-immunoreactive (ir) bipolar cells (PKCα-BCs) in lamina 1 of the IPL. Furthermore, with prolonged electrical stimulation of the isthmo-optic nucleus (ION) on one side, whose neurons send their axons to the contralateral retina and make synaptic contacts there with IOTCs, phosphorylation of cAMP response element-binding protein was observed in the PKCα-BCs in the contralateral retina, but not in the ipsilateral retina. This suggests that electrical stimulation of ION activated PKCα-BCs through synapses from IOTCs to PKCα-BCs, thus stimulating transcription in PKCα-BCs. Thus, centrifugal attentional signals may facilitate visual responses of RGCs via the PKCα-BCs.

Quantifying the effect of light activated outer and inner retinal inhibitory pathways on glutamate release from mixed bipolar cells

Inhibition mediated by horizontal and amacrine cells in the outer and inner retina, respectively, are fundamental components of visual processing. Here, our purpose was to determine how these different inhibitory processes affect glutamate release from ON bipolar cells when the retina is stimulated with full-field light of various intensities. Light-evoked membrane potential changes (ΔV_m ) were recorded directly from axon terminals of intact bipolar cells receiving mixed rod and cone inputs (Mbs) in slices of dark-adapted goldfish retina. Inner and outer retinal inhibition to Mbs was blocked with bath applied picrotoxin (PTX) and NBQX, respectively. Then, control and pharmacologically modified light responses were injected into axotomized Mb terminals as command potentials to induce voltage-gated Ca²⁺ influx (Q^Ca ) and consequent glutamate release. Stimulus-evoked glutamate release was quantified by the increase in membrane capacitance (ΔC_m ). Increasing depolarization of Mb terminals upon removal of inner and outer retinal inhibition enhanced the ΔV_m /Q^Ca ratio equally at a given light intensity and inhibition did not alter the overall relation between Q^Ca and ΔC_m . However, relative to control, light responses recorded in the presence of PTX and PTX + NBQX increased ΔC_m unevenly across different stimulus intensities: at dim stimulus intensities predominantly the inner retinal GABAergic inhibition controlled release from Mbs, whereas the inner and outer retinal inhibition affected release equally in response to bright stimuli. Furthermore, our results suggest that non-linear relationship between Q^Ca and glutamate release can influence the efficacy of inner and outer retinal inhibitory pathways to mediate Mb output at different light intensities.

Calcium spike-mediated digital signaling increases glutamate output at the visual threshold of retinal bipolar cells

Most retinal bipolar cells (BCs) transmit visual input from photoreceptors to ganglion cells using graded potentials, but some also generate calcium or sodium spikes. Sodium spikes are thought to increase temporal precision of light-evoked BC signaling; however, the role of calcium spikes in BCs is not fully understood. Here we studied how calcium spikes and graded responses mediate neurotransmitter release from Mb-type BCs, known to produce both. In dark-adapted goldfish retinal slices, light induced spikes in 40% of the axon terminals of intact Mbs; in the rest, light generated graded responses. These light-evoked membrane potentials were used to depolarize axotomized Mb terminals where depolarization-evoked calcium current (ICa) and consequent exocytosis-associated membrane capacitance increases (ΔCm) could be precisely measured. When evoked by identical dim light intensities, spiking responses transferred more calcium (Q(Ca)) and triggered larger exocytosis with higher efficiency (ΔCm/Q(Ca)) than graded potentials. Q(Ca) was translated into exocytosis linearly when transferred with spikes and supralinearly when transferred with graded responses. At the Mb output (ΔCm), spiking responses coded light intensity with numbers and amplitude whereas graded responses coded with amplitude, duration, and steepness. Importantly, spiking responses saturated exocytosis within scotopic range but graded potentials did not. We propose that calcium spikes in Mbs increase signal input-output ratio by boosting Mb glutamate release at threshold intensities. Therefore, spiking Mb responses are suitable to transfer low-light-intensity signals to ganglion cells with higher gain, whereas graded potentials signal for light over a wider range of intensities at the Mb output.

Nitric oxide mediates activity-dependent plasticity of retinal bipolar cell output via S-nitrosylation

Coding a wide range of light intensities in natural scenes poses a challenge for the retina: adaptation to bright light should not compromise sensitivity to dim light. Here we report a novel form of activity-dependent synaptic plasticity, specifically, a "weighted potentiation" that selectively increases output of Mb-type bipolar cells in the goldfish retina in response to weak inputs but leaves the input-output ratio for strong stimuli unaffected. In retinal slice preparation, strong depolarization of bipolar terminals significantly lowered the threshold for calcium spike initiation, which originated from a shift in activation of voltage-gated calcium currents (ICa) to more negative potentials. The process depended upon glutamate-evoked retrograde nitric oxide (NO) signaling as it was eliminated by pretreatment with an NO synthase blocker, TRIM. The NO-dependent ICa modulation was cGMP independent but could be blocked by N-ethylmaleimide (NEM), indicating that NO acted via an S-nitrosylation mechanism. Importantly, the NO action resulted in a weighted potentiation of Mb output in response to small (≤-30 mV) depolarizations. Coincidentally, light flashes with intensity ≥ 2.4 × 10(8) photons/cm(2)/s lowered the latency of scotopic (≤ 2.4 × 10(8) photons/cm(2)/s) light-evoked calcium spikes in Mb axon terminals in an NEM-sensitive manner, but light responses above cone threshold (≥ 3.5 × 10(9) photons/cm(2)/s) were unaltered. Under bright scotopic/mesopic conditions, this novel form of Mb output potentiation selectively amplifies dim retinal inputs at Mb → ganglion cell synapses. We propose that this process might counteract decreases in retinal sensitivity during light adaptation by preventing the loss of visual information carried by dim scotopic signals.

Amperometric resolution of a prespike stammer and evoked phases of fast release from retinal bipolar cells

The neurotransmitter glutamate is used by most neurons in the brain to activate a multitude of different types of glutamate receptors and transporters involved in fast and relatively slower signaling. Synaptic ribbons are large presynaptic structures found in neurons involved in vision, balance, and hearing, which use a large number of glutamate-filled synaptic vesicles to meet their signaling demands. To directly measure synaptic vesicle release events, the ribbon-type presynaptic terminals of goldfish retinal bipolar cells were coaxed to release a false transmitter that could be monitored with amperometry by placing the carbon fiber directly on the larger synaptic terminal. Spontaneous secretion events formed a unimodal charge distribution, but single spike properties were heterogeneous. Larger events rose exponentially without interruption (τ ∼ 30 μs), and smaller events exhibited a stammer in their rising phase that is interpreted as a brief pause in pore dilation, a characteristic commonly associated with large dense core granule fusion pores. These events were entirely Ca(2+)-dependent. Holding the cells at -60 mV halted spontaneous release; and when the voltage was stepped to >-40 mV, secretion ensued. When stepping the voltage to 0 mV, novel kinetic phases of vesicle recruitment were revealed. Approximately 14 vesicles were released per ribbon in two kinetic phases with time constants of 1.5 and 16 ms, which are proposed to represent different primed states within the population of docked vesicles.

Independent control of reciprocal and lateral inhibition at the axon terminal of retinal bipolar cells

Bipolar cells (BCs), the second order neurons in the vertebrate retina, receive two types of GABAergic feedback inhibition at their axon terminal: reciprocal and lateral inhibition. It has been suggested that two types of inhibition may be mediated by different pathways. However, how each inhibition is controlled by excitatory BC output remains to be clarified. Here, we applied single/dual whole cell recording techniques to the axon terminal of electrically coupled BCs in slice preparation of the goldfish retina, and found that each inhibition was regulated independently. Activation voltage of each inhibition was different: strong output from a single BC activated reciprocal inhibition, but could not activate lateral inhibition. Outputs from multiple BCs were essential for activation of lateral inhibition. Pharmacological examinations revealed that composition of transmitter receptors and localization of Na(+) channels were different between two inhibitory pathways, suggesting that different amacrine cells may mediate each inhibition. Depending on visual inputs, each inhibition could be driven independently. Model simulation showed that reciprocal and lateral inhibition cooperatively reduced BC outputs as well as background noise, thereby preserving high signal-to-noise ratio. Therefore, we conclude that excitatory BC output is efficiently regulated by the dual operating mechanisms of feedback inhibition without deteriorating the quality of visual signals.

Selective glycine receptor α2 subunit control of crossover inhibition between the on and off retinal pathways

In the retina, the receptive fields (RFs) of almost all ganglion cells (GCs) are comprised of an excitatory center and a suppressive surround. The RF center arises from local excitatory bipolar cell (BC) inputs and the surround from lateral inhibitory inputs. Selective antagonists have been used to define the roles of GABA(A) and GABA(C) receptor-mediated input in RF organization. In contrast, the role of glycine receptor (GlyR) subunit-specific inhibition is less clear because the only antagonist, strychnine, blocks all GlyR subunit combinations. We used mice lacking the GlyRα2 (Glra2(-/-)) and GlyRα3 (Glra3(-/-)) subunits, or both (Glra2/3(-/-)), to explore their roles in GC RF organization. By comparing spontaneous and visually evoked responses of WT with Glra2(-/-), Glra3(-/-) and Glra2/3(-/-) ON- and OFF-center GCs, we found that both GlyRα2 and GlyRα3 modulate local RF interactions. In the On pathway, both receptors enhance the excitatory center response; however, the underlying inhibitory mechanisms differ. GlyRα2 participates in crossover inhibition, whereas GlyRα3 mediates serial inhibition. In the Off pathway, GlyRα2 plays a similar role, again using crossover inhibition and enhancing excitatory responses within the RF center. Comparisons of single and double KOs indicate that GlyRα2 and GlyRα3 inhibition are independent and additive, consistent with the finding that they use different inhibitory circuitry. These findings are the first to define GlyR subunit-specific control of visual function and GlyRα2 subunit-specific control of crossover inhibition in the retina.

Synaptic vesicle cycling is not impaired in a glutamatergic and a cholinergic synapse that exhibit deficits in acidification and filling

The purpose of the present work was to investigate synaptic vesicle trafficking when vesicles exhibit alterations in filling and acidification in two different synapses: a cholinergic frog neuromuscular junction and a glutamatergic ribbon-type nerve terminal in the retina. These synapses display remarkable structural and functional differences, and the mechanisms regulating synaptic vesicle cycling might also differ between them. The lipophilic styryl dye FM1-43 was used to monitor vesicle trafficking. Both preparations were exposed to pharmacological agents that collapse ΔpH (NH4Cl and methylamine) or the whole ΔµH+ (bafilomycin), a necessary situation to provide the driving force for neurotransmitter accumulation into synaptic vesicles. The results showed that FM1-43 loading and unloading in neuromuscular junctions did not differ statistically between control and experimental conditions (P > 0.05). Also, FM1-43 labeling in bipolar cell terminals proved highly similar under all conditions tested. Despite remarkable differences in both experimental models, the present findings show that acidification and filling are not required for normal vesicle trafficking in either synapse.

PKC{alpha} is essential for the proper activation and termination of rod bipolar cell response

PURPOSE

Protein kinase (PKC)-α is abundant in retinal bipolar cells. This study was performed to explore its role in visual processing.

METHODS

PKCα-knockout (Prkca(-/-)) mice and control animals were examined by using electroretinography (ERG), light microscopy, and immunocytochemistry.

RESULTS

The Prkca(-/-) mice showed no signs of retinal degeneration up to 12 months of age, but ERG measurements indicated a decelerated increase in the ascending limb of the scotopic (rod-sensitive) b-wave as well as a delayed return to baseline. These results suggest that PKCα is an important modulator that affects bipolar cell signal transduction and termination. Confocal microscopy of retinal sections showed that PKCα co-localized with calbindin, which indicates a PKCα localization in close proximity to the horizontal cell terminals. In addition, the implicit time of the ERG c-wave originating from the retinal pigment epithelium (RPE) and the recovery of photoreceptors from bleaching conditions were substantially faster in the knockout mice than in the wild-type control animals.

CONCLUSIONS

These results suggest that PKCα is a modulator of rod-bipolar cell function by accelerating glutamate-driven signal transduction and termination. This modulation is of importance in the switch between scotopic and photopic vision. Furthermore, PKCα seems to play a role in RPE function.

Ectopic synaptic ribbons in dendrites of mouse retinal ON- and OFF-bipolar cells

The ectopic distribution of synaptic ribbons in dendrites of mouse retinal bipolar cells was examined by using genetic ablation of metabotropic glutamate receptor subtype 6 (mGluR6), electron microscopy, and immunocytochemistry. Ectopic ribbons were observed in dendrites of rod and ON-cone bipolar cells in the mGluR6-deficient mouse but not in those of wild-type mice. The number of rod spherules facing the ectopic ribbons in mGluR6-deficient rod bipolar dendrites increased gradually during early growth and reached a plateau level of about 20% at 12 weeks. These ectopic ribbons were immunopositive for RIBEYE, a ribbon-specific protein, but the associated vesicles were immunonegative for synaptophysin, a synaptic-vesicle-specific protein. The presence of ectopic ribbons was correlated with an increase in the roundness of the invaginating dendrites of the rod bipolar cells. We further confirmed ectopic ribbons in dendrites of OFF-cone bipolar cells in wild-type retinas. Of the four types of OFF-cone bipolar cells (T1-T4), only the T2-type, which had a greater number of synaptic ribbons at the axon terminal and a thicker axon cylinder than the other types, had ectopic ribbons. Light-adapted experiments revealed that, in wild-type mice under enhanced-light adaptation (considered similar to the mGluR6-deficient state), the roundness in the invaginating dendrites and axon terminals of rod bipolar cells increased, but no ectopic ribbons were detected. Based on these findings and known mechanisms for neurotransmitter release and protein trafficking, the possible mechanisms underlying the ectopic ribbons are discussed on the basis of intracellular transport for the replenishment of synaptic proteins.

Protein kinase C modulates synaptic vesicle acidification in a ribbon type nerve terminal in the retina

The driving force for neurotransmitter accumulation into synaptic vesicles is provided by the generation of a transmembrane electrochemical gradient (DeltamicroH+) that has two components: a chemical gradient (DeltapH, inside acidic) and an electrical potential across the vesicular membrane (DeltaPsi, inside positive). This gradient is generated in situ by the electrogenic vacuolar H(+)-ATPase, which is responsible for the acidification and positive membrane potential of the vesicle lumen. Here, we investigate the modulation of vesicle acidification by using the acidic-organelle probe LysoTracker and the pH-sensitive probe LysoSensor at goldfish Mb-type bipolar cell terminals. Since phosphorylation can modulate secretory granule acidification in neuroendocrine cells, we investigated if drugs that affect protein kinases modulate LysoTracker staining of bipolar cell terminals. We find that protein kinase C (PKC) activation induces an increase in LysoTracker-fluorescence. By contrast, protein kinase A (PKA) or calcium/calmodulin kinase II (CaMKII) activation or inhibition did not change LysoTracker-fluorescence. Using a pH-dependent fluorescent dye (LysoSensor) we show that the PKC activation with PMA induces an increase in LysoSensor-fluorescence, whereas the inactive analog 4alpha-PMA was unable to cause the same effect. This increase induced by PMA was blocked by PKC inhibitors, calphostin C and staurosporine. These results suggest that phosphorylation by PKC may increase synaptic vesicle acidification in retinal bipolar cells and therefore has the potential to modulate glutamate concentrations inside synaptic vesicles.

Novel processes invaginate the pre-synaptic terminal of retinal bipolar cells

Mixed-rod cone bipolar (Mb) cells of goldfish retina have large synaptic terminals (10 microm in diameter) that make 60-90 ribbon synapses mostly onto amacrine cells and rarely onto ganglion cells and, in return, receive 300-400 synapses from gamma-aminobutyric acid (GABA)-ergic amacrine cells. Tissue viewed by electron microscopy revealed the presence of double-membrane-bound processes deep within Mb terminals. No membrane specializations were apparent on these invaginating processes, although rare vesicular fusion was observed. These invaginating dendrites were termed "InDents". Mb bipolar cells were identified by their immunoreactivity for protein kinase C. Double-label immunofluorescence with other cell-type-specific labels eliminated Müller cells, efferent fibers, other Mb bipolar cells, dopaminergic interplexiform cells, and somatostatin amacrine cells as a source of the InDents. Confocal analysis of double-labeled tissue clearly showed dendrites of GABA amacrine cells, backfilled ganglion cells, and dendrites containing PanNa immunoreactivity extending into and passing through Mb terminals. Nearly all Mb terminals showed evidence for the presence of InDents, indicating their common presence in goldfish retina. No PanNa immunoreactivity was found on GABA or ganglion cell InDents, suggesting that a subtype of glycine amacrine cell contained voltage-gated Na channels. Thus, potassium and calcium voltage-gated channels might be present on the InDents and on the Mb terminal membrane opposed to the InDents. In addition to synaptic signaling at ribbon and conventional synapses, Mb bipolar cells may exchange information with InDents by an alternative signaling mechanism.

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.

Progesterone potentiates IP(3)-mediated calcium signaling through Akt/PKB

The activity of cells critically depends on the control of their cytosolic free calcium ion (Ca(2+)) concentration. The objective of the present study was to identify mechanisms of action underlying the control of the gain of intracellular Ca(2+) release by circulating gonadal steroid hormones. Acute stimulation of isolated neurons with progesterone led to IP(3)R-mediated Ca(2+) transients that depend on the activation of the PI3 kinase/Akt/PKB signaling pathway. These results were confirmed at the molecular level and phosphorylation of IP(3)R type 1 by Akt/PKB was identified as the mechanism of action. Hence, it is likely that circulating gonadal steroid hormones control neuronal activity including phosporylation status through receptor- and kinase-mediated signaling. With a direct control of the gain of the Ca(2+) second messenger system as a signaling gatekeeper for neuronal activity the present study identifies a novel pathway for interaction of the endocrine and central nervous system.

Synaptic transmission mediated by internal calcium stores in rod photoreceptors

Retinal rod photoreceptors are depolarized in darkness to approximately -40 mV, a state in which they maintain sustained glutamate release despite low levels of calcium channel activation. Blocking voltage-gated calcium channels or ryanodine receptors (RyRs) at the rod presynaptic terminal suppressed synaptic communication to bipolar cells. Spontaneous synaptic events were also inhibited when either of these pathways was blocked. This indicates that both calcium influx and calcium release from internal stores are required for the normal release of transmitter of the rod. RyR-independent release can be evoked by depolarization of a rod to a supraphysiological potential (-20 mV) that activates a large fraction of voltage-gated channels. However, this calcium channel-mediated release depletes rapidly if RyRs are blocked, indicating that RyRs support prolonged glutamate release. Thus, the rod synapse couples a small transmembrane calcium influx with a RyR-dependent amplification mechanism to support continuous vesicle release.

Spontaneous oscillatory activity of starburst amacrine cells in the mouse retina

Using patch-clamp techniques, we investigated the characteristics of the spontaneous oscillatory activity displayed by starburst amacrine cells in the mouse retina. At a holding potential of -70 mV, oscillations appeared as spontaneous, rhythmic inward currents with a frequency of approximately 3.5 Hz and an average maximal amplitude of approximately 120 pA. Application of TEA, a potassium channel blocker, increased the amplitude of oscillatory currents by >70% but reduced their frequency by approximately 17%. The TEA effects did not appear to result from direct actions on starburst cells, but rather a modulation of their synaptic inputs. Oscillatory currents were inhibited by 6-cyano-7-nitroquinoxalene-2,3-dione (CNQX), an antagonist of AMPA/kainate receptors, indicating that they were dependent on a periodic glutamatergic input likely from presynaptic bipolar cells. The oscillations were also inhibited by the calcium channel blockers cadmium and nifedipine, suggesting that the glutamate release was calcium dependent. Application of AP4, an agonist of mGluR6 receptors on on-center bipolar cells, blocked the oscillatory currents in starburst cells. However, application of TEA overcame the AP4 blockade, suggesting that the periodic glutamate release from bipolar cells is intrinsic to the inner plexiform layer in that, under experimental conditions, it can occur independent of photoreceptor input. The GABA receptor antagonists picrotoxin and bicuculline enhanced the amplitude of oscillations in starburst cells prestimulated with TEA. Our results suggest that this enhancement was due to a reduction of a GABAergic feedback inhibition from amacrine cells to bipolar cells and the resultant increased glutamate release. Finally, we found that some ganglion cells and other types of amacrine cell also displayed rhythmic activity, suggesting that oscillatory behavior is expressed by a number of inner retinal neurons.

Characterization of receptors for glutamate and GABA in retinal neurons

Glutamate and gamma-aminobutyric acid (GABA) are major excitatory and inhibitory neurotransmitters in the vertebrate retina, "a genuine neural center" (Ramón y Cajal, 1964, Recollections of My Life, C.E. Horne (Translater) MIT Press, Cambridge, MA). Photoreceptors, generating visual signals, and bipolar cells, mediating signal transfer from photoreceptors to ganglion cells, both release glutamate, which induces and/or changes the activity of the post-synaptic neurons (horizontal and bipolar cells for photoreceptors; amacrine and ganglion cells for bipolar cells). Horizontal and amacrine cells, which mediate lateral interaction in the outer and inner retina respectively, use GABA as a principal neurotransmitter. In recent years, glutamate receptors and GABA receptors in the retina have been extensively studied, using multi-disciplinary approaches. In this article some important advances in this field are reviewed, with special reference to retinal information processing. Photoreceptors possess metabotropic glutamate receptors and several subtypes of GABA receptors. Most horizontal cells express AMPA receptors, which may be predominantly assembled from flop slice variants. In addition, these cells also express GABAA and GABAC receptors. Signal transfer from photoreceptors to bipolar cells is rather complicated. Whereas AMPA/KA receptors mediate transmission for OFF type bipolar cells, several subtypes of glutamate receptors, both ionotropic and metabotropic, are involved in the generation of light responses of ON type bipolar cells. GABAA and GABAC receptors with distinct kinetics are differentially expressed on dendrites and axon terminals of both ON and OFF bipolar cells, mediating inhibition from horizontal cells and amacrine cells. Amacrine cells possess ionotropic glutamate receptors, whereas ganglion cells express both ionotropic and metabotropic glutamate receptors. GABAA receptors exist in amacrine and ganglion cells. Physiological data further suggest that GABAC receptors may be involved in the activity of these neurons. Moreover, responses of these retinal third order neurons are modulated by GABAB receptors, and in ganglion cells there exist several subtypes of GABAB receptors. A variety of glutamate receptor and GABA receptor subtypes found in the retina perform distinct functions, thus providing a wide range of neural integration and versatility of synaptic transmission. Perspectives in this research field are presented.

A computer model of field potential responses for the study of short-term plasticity in hippocampus

Activity-dependent synaptic plasticity has important implications for network function. The previously developed model of the hippocampal CA1 area, which contained pyramidal cells (PC) and two types of interneurons involved in feed-forward and recurrent inhibition, respectively, and received synaptic inputs from CA3 neurons via the Schaffer collaterals, was enhanced by incorporating dynamic synaptic connections capable of changing their weights depending on presynaptic activation history. The model output was presented as field potentials, which were compared with those derived experimentally. The parameters of Schaffer collateral-PC excitatory model synapse were determined, with which the model successfully reproduced the complicated dynamics of train-stimulation sequential potentiation/depression observed in experimentally recorded field responses. It was found that the model better reproduces the time course of experimental field potentials if the inhibitory synapses on PC are also made dynamic, with expressed properties of frequency-dependent depression. This finding supports experimental evidence that these synapses are subject to activity-dependent depression. The model field potentials in response to various randomly generated and real (derived from recorded CA3 unit activity) long stimulating trains were calculated, illustrating that short-term plasticity with the observed characteristics could play specific roles in frequency processing in hippocampus and thus providing a new tool for the theoretical study of activity-dependent synaptic plasticity.

A model synapse that incorporates the properties of short- and long-term synaptic plasticity

We propose a general computer model of a synapse, which incorporates mechanisms responsible for the realization of both short- and long-term synaptic plasticity-the two forms of experimentally observed plasticity that seem to be very significant for the performance of neuronal networks. The model consists of a presynaptic part based on the earlier 'double barrier synapse' model, and a postsynaptic compartment which is connected to the presynaptic terminal via a feedback, the sign and magnitude of which depend on postsynaptic Ca(2+) concentration. The feedback increases or decreases the amount of neurotransmitter which is in a ready for release state. The model adequately reproduced the phenomena of short- and long-term plasticity observed experimentally in hippocampal slices for CA3-CA1 synapses. The proposed model may be used in the investigation of certain real synapses to estimate their physiological parameters, and in the construction of realistic neuronal networks.

Vesicular neurotransmitter transporter expression in developing postnatal rodent retina: GABA and glycine precede glutamate

Vesicular transporters regulate the amount and type of neurotransmitter sequestered into synaptic vesicles and, hence, the kind of signal transmitted to postsynaptic neurons. Glutamate is the prominent excitatory neurotransmitter in retina; GABA and glycine are the main inhibitory neurotransmitters. Little is known about the ontogeny of vesicular neurotransmission in retina. We investigated expression of glutamatergic [vesicular glutamate transporter 1 (VGLUT1)] and GABA/glycinergic [vesicular GABA/glycine transporter (VGAT)] vesicular transporters in postnatal retina. VGLUT1 labels glutamatergic synapses. VGLUT1 and synaptic vesicle 2 colocalized to photoreceptor terminals. VGLUT1 colocalized with PKC to rod bipolar terminals and to ON bipolar terminals in metabotropic glutamate receptor 6+/- mice. Developmentally, VGAT expression precedes VGLUT1. In rat and mouse retina, VGAT occurred in the inner retina by postnatal day 1 (P1). In rat retina, VGLUT1 was in the outer retina by P5-P7 and the inner retina by P7. In the mouse retina, VGLUT1 expression was in the outer retina by P3 and the inner retina by P5. Both rat and mouse retina had an adult pattern of VGLUT1 expression by P14. VGLUT1 expression precedes ribbon synapses, which are first observed in the inner retina at P11 (Fisher, 1979) in mouse and P13 (Horsburgh and Sefton, 1987) in rat. The ribbon synapse marker RIBEYE was not detected in inner retina of P5 or P7 rat. Spontaneous EPSCs in mouse ganglion cells were recorded as early as P7. Together, these findings indicate that vesicular GABA and glycine transmission precedes vesicular glutamate transmission in developing rodent retina. Furthermore, vesicular glutamate transmission likely occurs before ribbon synapse formation in the inner retina.

Effects of beta-adrenergic blockers on glutamate-induced calcium signals in adult mouse retinal ganglion cells

Betaxolol, a selective beta(1)-adrenoceptor antagonist, is an antiglaucoma drug commonly used to lower the intraocular pressure (IOP) in treatment of glaucoma. Recent evidence has also shown that it attenuates ligand- and voltage-gated currents in retinal ganglion cells, which may lead to reduction of intracellular calcium and prevention of glutamate-induced ganglion cell damage in glaucoma. In the present study, we examined the effectiveness of betaxolol and other beta-adrenergic blockers on glutamate-induced calcium signals. Dissociated adult mouse retinal ganglion cells were immuno-labeled with antibody CD90.2 and loaded with Fura-2AM. Calcium signals were recorded with optical recording techniques. Low doses of glutamate cause an increase in intracellular calcium that may result in pathological changes in ganglion cells. The action of glutamate could be reversibly suppressed by beta-adrenergic blockers and the order of inhibitory potency is (s)(-)-propranolol>betaxolol>>timolol, with average IC(50) of 78.05, 235.7 and 2167.05, microM, respectively. Betaxolol compressed the dose-response curve of glutamate. The EC(50) of glutamate was shifted from 6.19 to 23.53 microM, indicating that betaxolol acts as a non-competitive inhibitor of glutamate response in retinal ganglion cells. Our data are consistent with previous reports that betaxolol and other beta-adrenergic blockers may exert its neuroprotective action by suppression of glutamate-induced intracellular calcium increase in retinal ganglion cells.

Increase in the pool size of releasable synaptic vesicles by the activation of protein kinase C in goldfish retinal bipolar cells

Secretion from neurons and neuroendocrine cells is enhanced by the activation of protein kinase C (PKC) in various preparations. We have already reported that transmitter (glutamate) release from Mb1 bipolar cells in the goldfish retina is potentiated by the activation of PKC. However, it is not yet settled whether the potentiation is ascribed to the increase in the pool size of releasable synaptic vesicles or in release probability. In the present study, Ca2+ influx and exocytosis were simultaneously monitored by measuring the presynaptic Ca2+ current and membrane capacitance changes, respectively, in a terminal detached from the bipolar cell. The double pulse protocol was used to estimate separately the changes in the pool size and release probability. The activation of PKC by phorbol 12-myristate 13-acetate (PMA) specifically increased the pool size but not the release probability. PKC was activated by PMA even after the Ca2+ influx was blocked by Co2+. In bipolar cells the releasable pool can be divided into two components: one is small and rapidly exhausted, and the other is large and slowly exocytosed. To identify which component is responsible for the increase in the pool size, the effects of PMA and a PKC-specific inhibitor, bisindolylmaleimide I (BIS), on each component were examined. The slow component was selectively increased by PMA and reduced by BIS. Thus, we conclude that the activation of PKC in Mb1 bipolar cells potentiates glutamate release by increasing the pool size of the slow component.

Synaptic distribution of ionotropic glutamate receptors in the inner plexiform layer of the primate retina

The distribution and synaptic clustering of glutamate receptors (GluRs) were studied in the inner plexiform layer (IPL) of the macaque monkey retina by using subunit specific antisera. A punctate immunofluorescence pattern was observed in the IPL for all subunits tested, and electron microscopy confirmed that the immunoreactive puncta represent clustering of receptors at sites postsynaptic to the bipolar cell ribbon synapses (dyads). Usually only one of the two postsynaptic processes at the dyads expressed a given subunit. Immunoreactive GluR2, GluR2/3, and GluR4 puncta were found at high density throughout the IPL and are probably expressed at every dyad. The GluR1 subunit was expressed at lower density. The N-methyl-D-aspartate (NMDA) receptor subunits NR2A and NR1C2' were restricted to synapses localized in two broad bands in the center of the IPL. They were often colocalized with GluR2/3 and GluR4 subunits. The orphan receptor subunits delta 1/2 predominated in three horizontal bands. The kainate receptor subunits GluR6/7 were clustered in large postsynaptic densities adjacent to bipolar cell axon terminals but lacking a synaptic ribbon on the presynaptic side. This might represent a conventional synapse made by a bipolar axon terminal. The results suggest that GluR2/3 and GluR4, together with NMDA receptors, are preferentially expressed on ganglion cell dendrites, whereas kainate receptors and the delta 1/2 subunits are mostly localized on amacrine cell processes.

Glutamate receptors in the rod pathway of the mammalian retina

Rod bipolar (RB) cells of the mammalian retina release glutamate in a graded, light-dependent fashion from 20 to 40 ribbon synapses (dyads). At the dyads, two classes of amacrine cells, the AI and AII cells, are the postsynaptic partners. We examined the glutamate receptors (GluRs) that are expressed by AI and AII cells using immunocytochemistry with specific antibodies against GluR subunits. Sections of macaque monkey and rabbit retina were examined by confocal microscopy. AII amacrine cells were selectively labeled for calretinin, and AI cells in rabbits were labeled for 5-HT uptake. Thus, double- and triple-labeling for these markers and GluR subunits was possible. Electron microscopy using postembedding immunocytochemistry and double-labeling was applied to show the synaptic expression of GluRs. We also studied the synaptic localization of the two postsynaptic density proteins PSD-95 and glutamate receptor-interacting protein (GRIP). We found that AII amacrine cells express the AMPA receptor subunits GluR2/3 and GluR4 at the RB cell dyads, and they are clustered together with PSD-95. In contrast, AI amacrine cells express the delta1/2 subunits that appear to be associated with kainate receptor subunits and to be clustered together with GRIP. The RB cell dyad is therefore a synapse that initiates two functionally and molecularly distinct pathways: a "through conducting" pathway based on AMPA receptors and a modulatory pathway mediated by a combination of delta1/2 subunits and kainate receptors.

Effect of Ca(2+)-free and Mg(2+)-free BSS Plus solution on the retinal pigment epithelium and retina in rabbits

PURPOSE

To determine whether intravitreal irrigation with Ca(2+)-free and Mg(2+)-free BSS Plus (Alcon Laboratory, Fort Worth, Texas) solution alters the adhesiveness between the retinal pigment epithelium and the retina of rabbits.

METHODS

Thirty-four eyes of 34 Dutch pigmented rabbits underwent lensectomy and vitrectomy. Subsequently, the vitreous cavity of 24 eyes was irrigated with Ca(2+)-free and Mg(2+)-free BSS Plus solution for 10 or 20 minutes. The other 10 eyes were irrigated with BSS Plus solution for 20 minutes as controls. To determine the adhesiveness between the retinal pigment epithelium and retina, a retinal detachment was produced in 12 of the 34 eyes. The apical surface of the retinal pigment epithelium and the photoreceptor outer segments were examined by scanning electron microscopy. Retinal physiology was assessed by electroretinography and retinal morphology by light microscopy.

RESULTS

After retinal detachment was produced, the number of cone sheaths on the apical surface of the retinal pigment epithelium after irrigation with Ca(2+)-free and Mg(2+)-free BSS Plus solution for 20 minutes (33 +/- 15, mean +/- SD) was significantly less than the number of cone sheaths on the apical surface of the retinal pigment epithelium of eyes after irrigation with BSS Plus solution for 20 minutes (120 +/- 50) or the number of cone sheaths on the apical surface of the retinal pigment epithelium of eyes after 10 minutes of irrigation with Ca(2+)-free and Mg(2+)-free BSS Plus solution (115 +/- 49; P =.02). The b-wave amplitudes in the eyes irrigated with Ca(2+)-free and Mg(2+)-free BSS Plus solution for 20 minutes were depressed compared with the b-waves in eyes irrigated with BSS Plus solution for 20 minutes on the first postoperative day (P =.03). After the third postoperative day, there was no significant difference in the b-waves (P >.06). Light microscopy demonstrated no morphologic abnormalities after the use of both solutions.

CONCLUSIONS

Intravitreal irrigation with Ca(2+)-free and Mg(2+)-free BSS Plus solution for 20 minutes altered the adhesion between the retinal pigment epithelium microvilli and retinal outer segments and made the creation of retinal detachment less traumatic. These results suggest that Ca(2+)-free and Mg(2+)-free BSS Plus solution may be of clinical value for the creation of an intentional retinal detachment for foveal translocation surgery.