The influence of HEPES on light responses of rabbit horizontal cells

Lateral interactions in the outer retina

Lateral interactions in the outer retina, particularly negative feedback from horizontal cells to cones and direct feed-forward input from horizontal cells to bipolar cells, play a number of important roles in early visual processing, such as generating center-surround receptive fields that enhance spatial discrimination. These circuits may also contribute to post-receptoral light adaptation and the generation of color opponency. In this review, we examine the contributions of horizontal cell feedback and feed-forward pathways to early visual processing. We begin by reviewing the properties of bipolar cell receptive fields, especially with respect to modulation of the bipolar receptive field surround by the ambient light level and to the contribution of horizontal cells to the surround. We then review evidence for and against three proposed mechanisms for negative feedback from horizontal cells to cones: 1) GABA release by horizontal cells, 2) ephaptic modulation of the cone pedicle membrane potential generated by currents flowing through hemigap junctions in horizontal cell dendrites, and 3) modulation of cone calcium currents (I(Ca)) by changes in synaptic cleft proton levels. We also consider evidence for the presence of direct horizontal cell feed-forward input to bipolar cells and discuss a possible role for GABA at this synapse. We summarize proposed functions of horizontal cell feedback and feed-forward pathways. Finally, we examine the mechanisms and functions of two other forms of lateral interaction in the outer retina: negative feedback from horizontal cells to rods and positive feedback from horizontal cells to cones.

Mesopic background lights enhance dark-adapted cone ERG flash responses in the intact mouse retina: a possible role for gap junctional decoupling

The cone-driven flash responses of mouse electroretinogram (ERG) increase as much as twofold over the course of several minutes during adaptation to a rod-compressing background light. The origins of this phenomenon were investigated in the present work by recording preflash-isolated (M-)cone flash responses ex vivo in darkness and during application of various steady background lights. In this protocol, the cone stimulating flash was preceded by a preflash that maintains rods under saturation (hyperpolarized) to allow selective stimulation of the cones at varying background light levels. The light-induced growth was found to represent true enhancement of cone flash responses with respect to their dark-adapted state. It developed within minutes, and its overall magnitude was a graded function of the background light intensity. The threshold intensity of cone response growth was observed with lights in the low mesopic luminance region, at which rod responses are partly compressed. Maximal effect was reached at intensities sufficient to suppress ∼ 90% of the rod responses. Light-induced enhancement of the cone photoresponses was not sensitive to antagonists and agonists of glutamatergic transmission. However, applying gap junction blockers to the dark-adapted retina produced qualitatively similar changes in the cone flash responses as did background light and prevented further growth during subsequent light-adaptation. These results are consistent with the idea that cone ERG photoresponses are suppressed in the dark-adapted mouse retina by gap junctional coupling between rods and cones. This coupling would then be gradually and reversibly removed by mesopic background lights, allowing larger functional range for the cone light responses.

Intracellular pH modulates inner segment calcium homeostasis in vertebrate photoreceptors

Neuronal metabolic and electrical activity is associated with shifts in intracellular pH (pH(i)) proton activity and state-dependent changes in activation of signaling pathways in the plasma membrane, cytosol, and intracellular compartments. We investigated interactions between two intracellular messenger ions, protons and calcium (Ca²(+)), in salamander photoreceptor inner segments loaded with Ca²(+) and pH indicator dyes. Resting cytosolic pH in rods and cones in HEPES-based saline was acidified by ∼0.4 pH units with respect to pH of the superfusing saline (pH = 7.6), indicating that dissociated inner segments experience continuous acid loading. Cytosolic alkalinization with ammonium chloride (NH₄Cl) depolarized photoreceptors and stimulated Ca²(+) release from internal stores, yet paradoxically also evoked dose-dependent, reversible decreases in [Ca²(+)](i). Alkalinization-evoked [Ca²(+)](i) decreases were independent of voltage-operated and store-operated Ca²(+) entry, plasma membrane Ca²(+) extrusion, and Ca²(+) sequestration into internal stores. The [Ca²(+)](i)-suppressive effects of alkalinization were antagonized by the fast Ca²(+) buffer BAPTA, suggesting that pH(i) directly regulates Ca²(+) binding to internal anionic sites. In summary, this data suggest that endogenously produced protons continually modulate the membrane potential, release from Ca²(+) stores, and intracellular Ca²(+) buffering in rod and cone inner segments.

Hemichannel-mediated and pH-based feedback from horizontal cells to cones in the vertebrate retina

BACKGROUND

Recent studies designed to identify the mechanism by which retinal horizontal cells communicate with cones have implicated two processes. According to one account, horizontal cell hyperpolarization induces an increase in pH within the synaptic cleft that activates the calcium current (Ca(2+)-current) in cones, enhancing transmitter release. An alternative account suggests that horizontal cell hyperpolarization increases the Ca(2+)-current to promote transmitter release through a hemichannel-mediated ephaptic mechanism.

METHODOLOGY/PRINCIPAL FINDINGS

To distinguish between these mechanisms, we interfered with the pH regulating systems in the retina and studied the effects on the feedback responses of cones and horizontal cells. We found that the pH buffers HEPES and Tris partially inhibit feedback responses in cones and horizontal cells and lead to intracellular acidification of neurons. Application of 25 mM acetate, which does not change the extracellular pH buffer capacity, does lead to both intracellular acidification and inhibition of feedback. Because intracellular acidification is known to inhibit hemichannels, the key experiment used to test the pH hypothesis, i.e. increasing the extracellular pH buffer capacity, does not discriminate between a pH-based feedback system and a hemichannel-mediated feedback system. To test the pH hypothesis in a manner independent of artificial pH-buffer systems, we studied the effect of interfering with the endogenous pH buffer, the bicarbonate/carbonic anhydrase system. Inhibition of carbonic anhydrase allowed for large changes in pH in the synaptic cleft of bipolar cell terminals and cone terminals, but the predicted enhancement of the cone feedback responses, according to the pH-hypothesis, was not observed. These experiments thus failed to support a proton mediated feedback mechanism. The alternative hypothesis, the hemichannel-mediated ephaptic feedback mechanism, was therefore studied experimentally, and its feasibility was buttressed by means of a quantitative computer model of the cone/horizontal cell synapse.

CONCLUSION

We conclude that the data presented in this paper offers further support for physiologically relevant ephaptic interactions in the retina.

The effect of different zwitterionic buffers and PBS used for out-of-incubator procedures during standard in vitro embryo production on development, morphology and gene expression of bovine embryos

The effect of the zwitterionic buffers HEPES, TES and MOPS and of PBS used for out-of-incubator procedures during standard in vitro embryo production on bovine oocytes and embryo development, morphology and on the expression patterns of eight selected genes: Fgf-4, Lama1, Ube2a, Gsta4, Il6, Sod1, Prss11 and Hspb1, was evaluated. All buffers were prepared at a concentration of 10 mM in TALP medium, with the exception of PBS. The total time of oocyte/embryo exposure to each buffer was approximately 41 min. The cleavage rates and number of embryos that developed to > or =8 cells at day 4 were no different among the buffers tested, however, more blastocysts developed at day 7, 8 and 9 in HEPES and MOPS treatments than in PBS and TES (P<0.05). No difference between buffers in total and apoptotic cell number was found. Except for Hspb1 and Ube2a genes, the levels of expression of the six remaining transcripts were higher in in vivo than in in vitro embryos irrespective of buffer used (P<0.05). In addition, higher expression of Hspb1 and lower expression of Ube2a and Lama1 were observed in PBS and TES than in MOPS and HEPES treatments (P<0.05). Expression of Fgf-4 and Gsta4 in the in vitro embryos was lower in PBS than in the remaining three buffers (P<0.05) and the level of expression of the Il6 gene was not affected by any buffer tested but was lower in in vitro than in in vivo derived embryos. Expression of both Sod1 and Prss11 genes in MOPS were at the level of the in vivo embryos. These results showed that the choice of buffer and short exposure time of approximately 41 min, affects mRNA expression of in vitro produced bovine embryos.

Modulation of extracellular proton fluxes from retinal horizontal cells of the catfish by depolarization and glutamate

Self-referencing H(+)-selective microelectrodes were used to measure extracellular proton fluxes from cone-driven horizontal cells isolated from the retina of the catfish (Ictalurus punctatus). The neurotransmitter glutamate induced an alkalinization of the area adjacent to the external face of the cell membrane. The effect of glutamate occurred regardless of whether the external solution was buffered with 1 mM HEPES, 3 mM phosphate, or 24 mM bicarbonate. The AMPA/kainate receptor agonist kainate and the NMDA receptor agonist N-methyl-D-aspartate both mimicked the effect of glutamate. The effect of kainate on proton flux was inhibited by the AMPA/kainate receptor blocker CNQX, and the effect of NMDA was abolished by the NMDA receptor antagonist DAP-5. Metabotropic glutamate receptor agonists produced no alteration in proton fluxes from horizontal cells. Depolarization of cells either by increasing extracellular potassium or directly by voltage clamp also produced an alkalinization adjacent to the cell membrane. The effects of depolarization on proton flux were blocked by 10 microM nifedipine, an inhibitor of L-type calcium channels. The plasmalemma Ca(2+/)H(+) ATPase (PMCA) blocker 5(6)-carboxyeosin also significantly reduced proton flux modulation by glutamate. Our results are consistent with the hypothesis that glutamate-induced extracellular alkalinizations arise from activation of the PMCA pump following increased intracellular calcium entry into cells. This process might help to relieve suppression of photoreceptor neurotransmitter release that results from exocytosed protons from photoreceptor synaptic terminals. Our findings argue strongly against the hypothesis that protons released by horizontal cells act as the inhibitory feedback neurotransmitter that creates the surround portion of the receptive fields of retinal neurons.

The complexity of the afterpotential of rabbit A-type horizontal cells

Afterpotentials of A-type horizontal cells (HCs) are believed to be rod-induced. They are, however, complex potentials and evidently of multiple causation. That part of the HC potential immediately after light-off is not entirely rod-determined because it has the same spectral sensitivity as the response to light-on, which is cone-induced with only some rod influence. It persists during a strong blue adapting light, which suppresses rod activity. The afterpotential may also be influenced by feedback from HCs to photoreceptors. The later part of the afterpotentials of A-type HCs is, however, rod dominated, as are the afterpotentials of axon terminals of B-type HCs.

Depolarizing effect of GABA in horizontal cells of the rabbit retina

Gamma-amino butyric acid (GABA) has been characterized as an inhibitory neurotransmitter acting through chloride mediated channels in the adult nervous system. Using gramicidin-perforated patch clamp recordings from horizontal cells dissociated from the retinas of adult rabbits, we found that GABA is able to induce cell depolarization. Ionic currents induced by GABA in dissociated horizontal cells showed a reversal potential close to -30 mV. This value is more positive than the resting potential of these cells (ca. -70 mV). Therefore, according to the Nernst equation, the intracellular chloride concentration in horizontal cells was estimated to be of 44 mM. The depolarizing effect of GABA at the dendrites of horizontal cells may serve to shape the center-surround organization of the receptive fields in retinal cells, thereby securing the shape discrimination of visual input.

Cobalt ions inhibit negative feedback in the outer retina by blocking hemichannels on horizontal cells

In goldfish, negative feedback from horizontal cells to cones shifts the activation function of the Ca2+ current of the cones to more negative potentials. This shift increases the amount of Ca2+ flowing into the cones, resulting in an increase in glutamate release. The increased glutamate release forms the basis of the feedback-mediated responses in second-order neurons, such as the surround-induced responses of bipolar cells and the spectral coding of horizontal cells. Low concentrations of Co2+ block these feedback-mediated responses in turtle retina. The mechanism by which this is accomplished is unknown. We studied the effects of Co2+ on the cone/horizontal network of goldfish retina and found that Co2+ greatly reduced the feedback-mediated responses in both cones and horizontal cells in a GABA-independent way. The reduction of the feedback-mediated responses is accompanied by a small shift of the Ca2+ current of the cones to positive potentials. We have previously shown that hemichannels on the tips of the horizontal cell dendrites are involved in the modulation of the Ca2+ current in cones. Both the absence of this Co2+-induced shift of the Ca2+ current in the absence of a hemichannel conductance and the sensitivity of Cx26 hemichannels to low concentrations of Co2+ are consistent with a role for hemichannels in negative feedback from horizontal cells to cones.

The involvement of glutamate-gated channels in negative feedback from horizontal cells to cones

Photoreceptors are the light sensitive cells in the retina. They project to horizontal cells and bipolar cells via a glutamatergic feed forward pathway. Horizontal cells are strongly electrically coupled and integrate in that way the input from the photoreceptors. Horizontal cells feedback to cones negatively. The combined signal from the photoreceptors and the horizontal cells is sent to the bipolar cells. The feedback pathway from horizontal cells to cones is thought to form the basis for the center/surround organization of bipolar cells. The nature of the feedback pathway is an issue of intense debate. It was thought for a long time that this feedback pathway was GABAergic, because cones have GABA-receptors and horizontal cells release GABA via a GABA-transporter working in the reversed direction. However, recently we showed in goldfish that horizontal cells feed back to cones via an alternative mechanism. In goldfish, negative feedback from horizontal cells to cones shifts the calcium current of the cone to more negative potentials. This feedback pathway is independent of GABA, since feedback cannot be blocked by either saturating concentrations of PTX, the GABA-transporter blocker SKF89976A, or application of GABA. The mechanism of negative feedback from horizontal cells to cones involves hemichannels located at the tips of the invaginating horizontal cells, just opposite to the calcium channels of the cones. Current flowing through these hemichannels changes the extracellular potential deep in the synaptic cleft and in that way modulates the calcium current of the cones. Such a modulation of the extracellular potential is called ephaptic. If negative feedback from horizontal cells to cones is indeed ephaptic, other channels present in the synapse should also be able to act as a current source, i.e., should also be able to change the output of the cone. We showed that glutamate-gated channels present at the tips of the horizontal cell dendrites can also mediate feedback responses. Surprisingly, although the glutamate-gated conductance of the horizontal cells is eight times the hemichannel conductance, glutamate-gated channels are not the major current source in negative feedback from horizontal cells to cones. In this chapter we present evidence that this is due to the more focal localization of the hemichannels, compared to a diffuse and extrasynaptic localization of the glutamate-gated channels.

The effect of GABA and the GABA-uptake-blocker NO-711 on the b-wave of the ERG and the responses of horizontal cells to light

BACKGROUND

The effects of GABA in the retina have now become of special interest because the anti-epileptic drug vigabatrin, a GABA analogue, can cause visual field loss in humans. Vigabatrin inhibits the GABA-aminotransferase, which finally results in GABA accumulation in the extracellular space. The b-wave of the electroretinogram (ERG), which originates partly in on-bipolar cells, is influenced by both GABAergic horizontal cells (HCs) and GABAergic amacrine cells (ACs). Their influences, however, are difficult to separate. In an attempt to isolate the effect of GABAergic ACs, use has been made of the specific effect of the GABA-uptake-blocker NO-711, which blocks only the GABA transporter GAT1 of GABAergic ACs.

METHODS

The ERG and the intracellular responses of HCs to light were recorded in the isolated rabbit retina, and the effects of GABA and NO-711, when added separately to the superfusate, were determined.

RESULTS

GABA reduced significantly both the light responses of HCs and the b-wave. NO-711 enlarged the b-wave drastically, but did not affect the responses of HCs to light.

CONCLUSIONS

An increase in the extracellular GABA concentration decreases the b-wave; an impairment of the function of ACs increases the b-wave. These conditions are discussed in the context of the lack of consistent changes to the b-wave during therapy with vigabatrin.

Control of intracellular chloride concentration and GABA response polarity in rat retinal ON bipolar cells

GABAergic modulation of retinal bipolar cells plays a crucial role in early visual processing. It helps to form centre-surround receptive fields which filter the visual signal spatially at the bipolar cell dendrites in the outer retina, and it produces temporal filtering at the bipolar cell synaptic terminals in the inner retina. The observed chloride transporter distribution in ON bipolar cells has been predicted to produce an intracellular chloride concentration, [Cl(-)](i), that is significantly higher in the dendrites than in the synaptic terminals. This would allow dendritic GABA-gated Cl(-) channels to generate the depolarization needed for forming the lateral inhibitory surround of the cell's receptive field, while synaptic terminal GABA-gated Cl(-) channels generate the hyperpolarization needed for temporal shaping of the light response. In contrast to this idea, we show here that in ON bipolar cells [Cl(-)](i) is only slightly higher in the dendrites than in the synaptic terminals, and that GABA-gated channels in the dendrites may generate a hyperpolarization rather than a depolarization. We also show that [Cl(-)](i) is controlled by movement of Cl(-) through ion channels in addition to transporters, that changes of [K(+)](o) alter [Cl(-)](i) and that voltage-dependent equilibration of [Cl(-)](i) in bipolar cells will produce a time-dependent adaptation of GABAergic modulation with a time constant of 8 s after illumination-evoked changes of membrane potential. Time-dependent adaptation of [Cl(-)](i) to voltage changes in retinal bipolar cells may add a previously unsuspected layer of temporal processing to signals as they pass through the retina.

The effects of GABA and vigabatrin on horizontal cell responses to light and the effect of vigabatrin on the electroretinogram

When used as an antiepileptic drug in humans vigabatrin, which is a GABA analogue and an inhibitor of GABA-aminotransferase, often causes peripheral visual field loss. This could result from increases in endogenous GABA levels. Accordingly we have investigated the effects of GABA on horizontal cells (HCs) of the rabbit retina, and of vigabatrin, when applied for only a few minutes, on HCs and on the electroretinogram (ERG). The intracellular HC and ERG records were first obtained from isolated rabbit retinas during perfusion with a physiological solution. The perfusate was then changed to one containing GABA (2 mM) or vigabatrin (25, 40 or 150 microM) for at least 5 min, and then returned to the control solution. 2 mM GABA significantly but reversibly reduced the light responses of HCs elicited by diffuse light (at -4 log intensity) to 52 +/- 17% (SD, n = 7). Vigabatrin had no significant effect on the light responses of HCs (n = 7), and no effect on the b-wave (n = 4), but the PIII-component of the ERG was slightly but significantly reduced to 84 +/- 5% (SD, n = 5). The high dosage of GABA needed to affect the light responses of HCs could be due to strong GABA uptake systems in the intact rabbit retina. It is, however, possible that in humans receiving long-term treatment with vigabatrin, high levels of GABA occur because of the inhibition of GABA- aminotransferase. It seems, from these observations, that neurons like on-bipolar cells, which are contributors to the b-wave, and HCs are uninfluenced by vigabatrin in short-term experiments. The slightly reduced slow PIII-component, however, indicates an influence on the glial Müller cells which are the main contributors to the slow PIII-component.