Properties of the cGMP-activated channel of retinal on-bipolar cells

PDE9A is expressed in the inner retina and contributes to the normal shape of the photopic ERG waveform

The ubiquitous second messenger cGMP is synthesized by guanylyl cyclase and hydrolyzed by phosphodiesterase (PDE). cGMP mediates numerous signaling pathways in multiple tissues. In the retina, cGMP regulates signaling in nearly every cell class including photoreceptors, bipolar cells, amacrine cells, and ganglion cells. In order to understand the specific role of cGMP and its regulating enzymes in different cell types, it is first necessary to localize these components and dissect their influence on the circuits. Here we tested the contribution of PDE9A to retinal processing by recording the electroretinograms (ERG) of PDE9A^™/™ (KO) mice and by localizing the enzyme. We found that while the scotopic ERG of KO was the same as that of wild type (WT) in both amplitude and kinetics, the photopic ERG was greatly affected. The greatest effect was on the recovery of the b-wave; the falling phase and the b-wave duration were significantly longer in the KO mice for all photopic stimuli (UV, green, or saturating white flashes). The rising phase was slower in KO than in WT for UV and green stimuli. For certain stimuli, amplitudes of both the a- and b-waves were smaller than in WT. Using Lac-Z expression in KO retinas as a reporter for PDE9A expression pattern, we found that PDE9A is localized to GABA-positive and GABA-negative amacrine cells, and likely also to certain types of ganglion cells. Our results indicate that PDE9A, by controlling the level of cGMP, modulates inhibitory processes within the cone pathway. We speculate that these circuits involve NO/cGMP signaling pathways.

TRPM1

The transient receptor potential (TRP) channels play a wide variety of essential roles in the sensory systems of various species, both invertebrates and vertebrates. The TRP channel was first identified as a molecule required for proper light response in Drosophila melanogaster. We and another group recently revealed that TRPM1, the founding member of the melanoma-related transient receptor potential (TRPM) subfamily, is required for the photoresponse in mouse retinal ON-bipolar cells. We further demonstrated that Trpm1 is a component of the transduction cation channel negatively regulated by the metabotropic glutamate receptor 6 (mGulR6) cascade in ON-bipolar cells through a reconstitution experiment using CHO cells expressing Trpm1, mGluR6, and Goα. Furthermore, human TRPM1 mutations are associated with congenital stationary night blindness (CSNB), whose patients lack rod function and suffer from night blindness starting in early childhood. In addition to the function of transduction cation channel, TRPM1 is one of the retinal autoantigens in some paraneoplastic retinopathy (PR) associated with retinal ON-bipolar cell dysfunction. In this chapter, we describe physiological functions of the TRPM1 channel and its underlying biochemical mechanisms in retinal ON-bipolar cells in association with CSNB and PR.

Next-generation sequencing analysis of gene regulation in the rat model of retinopathy of prematurity

PURPOSE

The purpose of this study was to identify the genes, biochemical signaling pathways, and biological themes involved in the pathogenesis of retinopathy of prematurity (ROP).

METHODS

Next-generation sequencing (NGS) was performed on the RNA transcriptome of rats with the Penn et al. (Pediatr Res 36:724-731, 1994) oxygen-induced retinopathy model of ROP at the height of vascular abnormality, postnatal day (P) 19, and normalized to age-matched, room-air-reared littermate controls. Eight custom-developed pathways with potential relevance to known ROP sequelae were evaluated for significant regulation in ROP: The three major Wnt signaling pathways, canonical, planar cell polarity (PCP), and Wnt/Ca(2+); two signaling pathways mediated by the Rho GTPases RhoA and Cdc42, which are, respectively, thought to intersect with canonical and non-canonical Wnt signaling; nitric oxide signaling pathways mediated by two nitric oxide synthase (NOS) enzymes, neuronal (nNOS) and endothelial (eNOS); and the retinoic acid (RA) signaling pathway. Regulation of other biological pathways and themes was detected by gene ontology using the Kyoto Encyclopedia of Genes and Genomes and the NIH's Database for Annotation, Visualization, and Integrated Discovery's GO terms databases.

RESULTS

Canonical Wnt signaling was found to be regulated, but the non-canonical PCP and Wnt/Ca(2+) pathways were not. Nitric oxide signaling, as measured by the activation of nNOS and eNOS, was also regulated, as was RA signaling. Biological themes related to protein translation (ribosomes), neural signaling, inflammation and immunity, cell cycle, and cell death were (among others) highly regulated in ROP rats.

CONCLUSIONS

These several genes and pathways identified by NGS might provide novel targets for intervention in ROP.

Efficiency of synaptic transmission of single-photon events from rod photoreceptor to rod bipolar dendrite

A rod transmits absorption of a single photon by what appears to be a small reduction in the small number of quanta of neurotransmitter (Q(count)) that it releases within the integration period ( approximately 0.1 s) of a rod bipolar dendrite. Due to the quantal and stochastic nature of release, discrete distributions of Q(count) for darkness versus one isomerization of rhodopsin (R*) overlap. We suggested that release must be regular to narrow these distributions, reduce overlap, reduce the rate of false positives, and increase transmission efficiency (the fraction of R* events that are identified as light). Unsurprisingly, higher quantal release rates (Q(rates)) yield higher efficiencies. Focusing here on the effect of small changes in Q(rate), we find that a slightly higher Q(rate) yields greatly reduced efficiency, due to a necessarily fixed quantal-count threshold. To stabilize efficiency in the face of drift in Q(rate), the dendrite needs to regulate the biochemical realization of its quantal-count threshold with respect to its Q(count). These considerations reveal the mathematical role of calcium-based negative feedback and suggest a helpful role for spontaneous R*. In addition, to stabilize efficiency in the face of drift in degree of regularity, efficiency should be approximately 50%, similar to measurements.

The pharmacology of cyclic nucleotide-gated channels: emerging from the darkness

Cyclic nucleotide-gated (CNG) ion channels play a central role in vision and olfaction, generating the electrical responses to light in photoreceptors and to odorants in olfactory receptors. These channels have been detected in many other tissues where their functions are largely unclear. The use of gene knockouts and other methods have yielded some information, but there is a pressing need for potent and specific pharmacological agents directed at CNG channels. To date there has been very little systematic effort in this direction - most of what can be termed CNG channel pharmacology arose from testing reagents known to target protein kinases or other ion channels, or by accident when researchers were investigating other intracellular pathways that may regulate the activity of CNG channels. Predictably, these studies have not produced selective agents. However, taking advantage of emerging structural information and the increasing knowledge of the biophysical properties of these channels, some promising compounds and strategies have begun to emerge. In this review we discuss progress on two fronts, cyclic nucleotide analogs as both activators and competitive inhibitors, and inhibitors that target the pore or gating machinery of the channel. We also discuss the potential of these compounds for treating certain forms of retinal degeneration.

Postsynaptic calcium feedback between rods and rod bipolar cells in the mouse retina

Light-evoked currents were recorded from rod bipolar cells in a dark-adapted mouse retinal slice preparation. Low-intensity light steps evoked a sustained inward current. Saturating light steps evoked an inward current with an initial peak that inactivated, with a time constant of about 60-70 ms, to a steady plateau level that was maintained for the duration of the step. The inactivation was strongest at hyperpolarized potentials, and absent at positive potentials. Inactivation was mediated by an increase in the intracellular calcium concentration, as it was abolished in cells dialyzed with 10 mM BAPTA, but was present in cells dialyzed with 1 mM EGTA. Moreover, responses to brief flashes of light were broader in the presence of intracellular BAPTA indicating that the calcium feedback actively shapes the time course of the light responses. Recovery from inactivation observed for paired-pulse stimuli occurred with a time constant of about 375 ms. Calcium feedback could act to increase the dynamic range of the bipolar cells, and to reduce variability in the amplitude and duration of the single-photon signal. This may be important for nonlinear processing at downstream sites of convergence from rod bipolar cells to AII amacrine cells. A model in which intracellular calcium rapidly binds to the light-gated channel and reduces the conductance can account for the results.

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 retinal-specific regulator of G-protein signaling interacts with Galpha(o) and accelerates an expressed metabotropic glutamate receptor 6 cascade

G(o) is the most abundant G-protein in the brain, but its regulators are essentially unknown. In retina, Galpha(o1) is obligatory in mediating the metabotropic glutamate receptor 6 (mGluR6)-initiated ON response. To identify the interactors of G(o), we conducted a yeast two-hybrid screen with constituitively active Galpha(o) as a bait. The screen frequently identified a regulator of G-protein signaling (RGS), Ret-RGS1, the interaction of which we confirmed by coimmunoprecipitation with Galpha(o) in transfected cells and in retina. Ret-RGS1 localized to the dendritic tips of ON bipolar neurons, along with mGluR6 and Galpha(o1). When Ret-RGS1 was coexpressed in Xenopus oocytes with mGluR6, Galpha(o1), and a GIRK (G-protein-gated inwardly rectifying K+) channel, it accelerated the deactivation of the channel response to glutamate in a concentration-dependent manner. Because light onset suppresses glutamate release from photoreceptors onto the ON bipolar dendrites, Ret-RGS1 should accelerate the rising phase of the light response of the ON bipolar cell. This would tend to match its kinetics to that of the OFF bipolar that arises directly from ligand-gated channels.

Pharmacological strategies to block rod photoreceptor apoptosis caused by calcium overload: a mechanistic target-site approach to neuroprotection

PURPOSE

Photoreceptor apoptosis and resultant visual deficits occur in humans and animals with inherited, and disease-, injury- and chemical-induced retinal degeneration. Our aims were three-fold: 1) to determine the kinetics of rod apoptosis and Ca2+ overload in Pde6b9rd1) mice and developmentally lead-exposed rats, 2) to establish a pathophysiologically-relevant model of Ca2+ overload/rod-selective apoptosis in isolated rat retina and 3) to examine different mechanistic based neuroprotective strategies that would abrogate or mollify rod Ca2+ overload/apoptosis.

METHODS

Retinal morphometry and elemental calcium content ([Ca]) determined the kinetics of rod apoptosis and Ca2+ overload. A multiparametric analysis of apoptosis including rod [Ca], a live/dead assay, rod oxygen consumption, cytochrome c immunoblots and caspase assays was combined with pharmacological studies of an isolated rat retinal model of rod-selective Ca2+ overload/apoptosis.

RESULTS

Ca2+ overload preceded rod apoptosis in mice and rats, although the extent and kinetics in each differed significantly. The isolated rat model of rod Ca2+ overload/apoptosis showed that blockade of Ca2+ entry through rod cGMP-activated channels with L-cis diltiazem was partially neuroprotective, whereas blockade of Ca2+ entry into rods through L-type Ca2+ channels with D-cis diltiazem or verapamil provided no protection. Inhibition of the mitochondrial Na+/Ca2+ exchanger with D-cis diltiazem provided no protection. CsA and NIM811, mitochondrial permeability transition pore (mPTP) inhibitors, blocked all Ca(2+)-induced apoptosis, whereas the caspase-3 inhibitor DEVD-fmk only blocked the downstream cytochrome c-induced apoptosis.

CONCLUSIONS

The successful pharmacological neuroprotective strategies for rod Ca2+ overload/apoptosis targeted the rod cGMP-activated channels or mPTP, but not the rod L-type Ca2+ channels.

Dopamine and nitric oxide control both flickering and steady-light-induced cone contraction and horizontal cell spinule formation in the teleost (carp) retina: serial interaction of dopamine and nitric oxide

Adaptation to ambient light, which is an important characteristic of the vertebrate visual system, involves cellular and subcellular (synaptic) plasticity of the retina. The present study investigated dopamine (DA) and nitric oxide (NO) as possible neurochemical modulators controlling cone photomechanical movements (PMMs) and horizontal cell (HC) spinules in relation to steady and flickering light adaptation in the carp retina. Haloperidol (HAL; a nonspecific DA receptor blocker) or cPTIO (a NO scavenger) largely inhibited the cone PMMs and HC spinule formation induced by either steady or flickering light. These results suggested that both DA and NO could be involved in the light-adaptation changes induced by either pattern of input and that DA and NO effects may not be completely independent. The possibility that NO and DA interact serially was evaluated pharmacologically by cross-antagonist application (i.e., DA + cPTIO or NO + HAL). When a NO donor was coapplied with HAL to dark-adapted eyecups, normal light-adaptive cone PMMs and HC spinules occurred. In contrast, when DA was applied in the presence of cPTIO, the dark-adapted state persisted. It was concluded 1) that DA and NO are both light-adaptive neurochemicals, released in the retina during either steady or flickering light; 2) that the effects of DA and NO on light-adaptive cone PMMs and HC spinules do not occur in parallel; and 3) that NO and DA act mainly in series, specifically as follows: Light --> DA --> NO --> Cone PMMs + HC spinules.

Potentiation of 'on' bipolar cell flash responses by dim background light and cGMP in dogfish retinal slices

The high sensitivity of the vertebrate visual system results from amplification inherent in phototransduction in rods and from the amplification of rod signals on their synaptic transfer at the first synapse with 'on' bipolar cells. These cells possess a metabotropic glutamate receptor linked via a cGMP cascade to the control of cGMP-activated channels. In the study presented here, we show that very dim background light, isomerising only one rhodopsin in 1 out of 10 rods per second, potentiates 'on' bipolar cell responses to superimposed flashes. Responses to dim flashes, which were undetectable above the noise in the dark, were boosted above the increased noise level induced by the background. This potentiation could be reproduced by elevating cGMP, which increases with light, or by dialysing the cells with a non-hydrolysable cGMP analogue. Inhibition of tyrosine kinase activity also reproduced the effect and induced a speeding up of the rising phase of the flash response, similar to the action of dim background light. Conversely, inhibition of tyrosine phosphatase activity blocked the potentiation. These results suggest that cGMP promotes tyrosine-site dephosphorylation of 'on' bipolar cell cGMP-activated channels, resulting in a rise in the sensitivity to cGMP, as has recently been demonstrated for rod cGMP-activated channels. This constitutes a positive feedback mechanism such that as cGMP increases with light, the sensitivity of the channels to cGMP increases and boosts the signal above background noise. This mechanism would allow stochastic resonance to occur, facilitating single-photon detection when dark-adapted, and may therefore lead to improved discrimination.

Cyclic nucleotide-gated ion channels

Cyclic nucleotide-gated (CNG) channels are nonselective cation channels first identified in retinal photoreceptors and olfactory sensory neurons (OSNs). They are opened by the direct binding of cyclic nucleotides, cAMP and cGMP. Although their activity shows very little voltage dependence, CNG channels belong to the superfamily of voltage-gated ion channels. Like their cousins the voltage-gated K+ channels, CNG channels form heterotetrameric complexes consisting of two or three different types of subunits. Six different genes encoding CNG channels, four A subunits (A1 to A4) and two B subunits (B1 and B3), give rise to three different channels in rod and cone photoreceptors and in OSNs. Important functional features of these channels, i.e., ligand sensitivity and selectivity, ion permeation, and gating, are determined by the subunit composition of the respective channel complex. The function of CNG channels has been firmly established in retinal photoreceptors and in OSNs. Studies on their presence in other sensory and nonsensory cells have produced mixed results, and their purported roles in neuronal pathfinding or synaptic plasticity are not as well understood as their role in sensory neurons. Similarly, the function of invertebrate homologs found in Caenorhabditis elegans, Drosophila, and Limulus is largely unknown, except for two subunits of C. elegans that play a role in chemosensation. CNG channels are nonselective cation channels that do not discriminate well between alkali ions and even pass divalent cations, in particular Ca2+. Ca2+ entry through CNG channels is important for both excitation and adaptation of sensory cells. CNG channel activity is modulated by Ca2+/calmodulin and by phosphorylation. Other factors may also be involved in channel regulation. Mutations in CNG channel genes give rise to retinal degeneration and color blindness. In particular, mutations in the A and B subunits of the CNG channel expressed in human cones cause various forms of complete and incomplete achromatopsia.

Rectification of cGMP-activated channels induced by phosphorylation in dogfish retinal 'on' bipolar cells

1. Whole-cell current responses to brief flashes were obtained from voltage-clamped 'on' bipolar cells in dark-adapted dogfish retinal slices. When internal Ca2+ was buffered to low levels, the current-voltage (I-V) relation of their flash responses was linear, with a reversal potential near 0 mV. 2. On elevating internal Ca2+ the light-dependent I-V relation showed outward rectification, such that the current response to a flash decreased e-fold for a hyperpolarization of 22 mV. 3. Inclusion of a CaMKII inhibitory peptide in the patch-pipette solution removed the rectification even in the presence of 50 microM Ca2+. 4. These results are consistent with CaMKII phosphorylation of cGMP-activated channels leading to a voltage-dependent reduction in conductance (outward rectification) and a reduced light response. The voltage-dependent property suggests that phosphorylation creates an energy barrier near the outer part of the channel, reducing the flow principally of monovalent cations. 5. This is the first reported instance of CaMKII phosphorylation acting to change the electrical characteristics of a membrane channel from linear to rectifying. 6. Ca2+-dependent desensitization by background light and channel rectification may underlie the change in centre-surround organization of the visual system with light adaptation.

On-bipolar cells and depolarising third-order neurons as the origin of the ERG-b-wave in the RCS rat

In the retinas of Royal College of Surgeons (RCS) rats light induces an increase in distal extracellular potassium irrespective of the age, between days 19-24 and days 29-35 postpartum, but by days 29-35 the ERG b-wave has become reduced. The synaptic blocker 2-amino-4-phosphonobutyric acid (APB) causes the abolition of both the b-wave and the potassium increase at any age. MgCl2 greatly reduces the b-wave at all ages and abolishes the potassium increase in older rats, but in younger rats the potassium increase is enlarged. Since this increase occurs in the absence of the b-wave it is unlikely that the on-bipolar cells are the only sources of the b-wave. Because the NMDA receptor blocker ketamine reduces the b-wave, third order neurons, which possess NMDA receptors, could contribute to the b-wave.

Activation of Ca2+--calmodulin kinase II induces desensitization by background light in dogfish retinal 'on' bipolar cells

Retinal 'on' bipolar cells possess a metabotropic glutamate receptor (mGluR6) linked to the control of a G-protein and cGMP-activated channels which functions to generate high synaptic amplification of rod signals under dark-adapted conditions. Desensitization of 'on' bipolar cells is initiated by a rise in Ca2+ during background light too weak to adapt rod photoreceptors. Desensitization could also be elicited by raising intracellular Ca2+ above 1 microM. In order to investigate the mechanism of desensitization, whole-cell current responses to brief flashes and to steps of light were obtained from voltage-clamped 'on' bipolar cells in dark-adapted dogfish retinal slices. The inclusion of Ca2+-calmodulin kinase II (CaMKII) inhibitor peptides in the patch pipette solutions not only blocked desensitization of 'on' bipolar cells by dim background light and by 50 microM Ca2+, but also increased their flash sensitivity. The substrate of phosphorylation by CaMKII is the 'on' bipolar cell cGMP-activated channels. Desensitization probably results from a reduction in their sensitivity to cGMP and a voltage-dependent decrease in their conductance. A role for protein kinase C (PKC) in this process was excluded since activating PKC independently of Ca2+ with the phorbol ester PMA failed to induce desensitization of 'on' bipolar cells.

Regulation of the on bipolar cell mGluR6 pathway by Ca2+

Glutamate produces a hyperpolarizing synaptic potential in On bipolar cells by binding to the metabotropic glutamate receptor mGluR6, leading to closure of a cation channel. Here it is demonstrated that this cation channel is regulated by intracellular Ca(2+). Glutamate-evoked currents were recorded from On bipolar cells in light-adapted salamander retinal slices in the presence of 2 mm external Ca(2+). When glutamate was applied almost continuously, interrupted only briefly to measure the size of the response, the glutamate response remained robust. However, currents elicited by intermittent and brief applications of glutamate exhibited time-dependent run down. Run down of the glutamate response was also voltage dependent, because it was accelerated by membrane hyperpolarization. Run down was triggered, at least in part, by a rise in intracellular Ca(2+); measured as a function of time or voltage, it was attenuated by intracellular buffering of Ca(2+) with BAPTA or by omitting Ca(2+) from the bathing solution. Current-voltage measurements demonstrated that Ca(2+) induced run down of the glutamate response by downregulating cation channel function, rather than by preventing closure of the channel by glutamate and mGluR6. A major source of the Ca(2+) that mediated this inhibition is the cation channel itself, which was found to be permeable to Ca(2+), accounting for the use dependence of the run down. These results suggest that Ca(2+) influx through the cation channel during background illumination could provide a signal to close the cation channel and repolarize the membrane toward its dark potential, an adaptive mechanism for coping with changes in ambient light.

Lead and calcium produce rod photoreceptor cell apoptosis by opening the mitochondrial permeability transition pore

Calcium overload is suggested to play a fundamental role in the process of rod apoptosis in chemical-induced and inherited retinal degenerations. However, this hypothesis has not been tested directly. We developed an in vitro model utilizing isolated rat retinas to determine the mechanisms underlying Ca(2+)- and/or Pb(2+)-induced retinal degeneration. Confocal microscopy, histological, and biochemical studies established that the elevated [Ca(2+)] and/or [Pb(2+)] were localized to photoreceptors and produced rod-selective apoptosis. Ca(2+) and/or Pb(2+) induced mitochondrial depolarization, swelling, and cytochrome c release. Subsequently caspase-9 and caspase-3 were sequentially activated. Caspase-7 and caspase-8 were not activated. The effects of Ca(2+) and Pb(2+) were additive and blocked completely by the mitochondrial permeability transition pore (PTP) inhibitor cyclosporin A, whereas the calcineurin inhibitor FK506 had no effect. The caspase inhibitors carbobenzoxy-Leu-Glu-His-Asp-CH(2)F and carbobenzoxy-Asp-Glu-Val-Asp-CH(2)F, but not carbobenzoxy-Ile-Glu-Thr-Asp-CH(2)F, differentially blocked post-mitochondrial events. The levels of reduced and oxidized glutathione and pyridine nucleotides in rods were unchanged. Our results demonstrate that rod mitochondria are the target site for Ca(2+) and Pb(2+). Moreover, they suggest that Ca(2+) and Pb(2+) bind to the internal metal (Me(2+)) binding site of the PTP and subsequently open the PTP, which initiates the cytochrome c-caspase cascade of apoptosis in rods.

Molecular cloning and functional characterization of a new modulatory cyclic nucleotide-gated channel subunit from mouse retina

Cyclic nucleotide-gated (CNG) channels play a key role in olfactory and visual transduction. Native CNG channels are heteromeric complexes consisting of the principal alpha subunits (CNG1-3), which can form functional channels by themselves, and the modulatory beta subunits (CNG4-5). The individual alpha and beta subunits that combine to form the CNG channels in rod photoreceptors (CNG1 + CNG4) and olfactory neurons (CNG2 + CNG4 + CNG5) have been characterized. In contrast, only an alpha subunit (CNG3) has been identified so far in cone photoreceptors. Here we report the molecular cloning of a new CNG channel subunit (CNG6) from mouse retina. The cDNA of CNG6 encodes a peptide of 694 amino acids with a predicted molecular weight of 80 kDa. Among the CNG channel subunits, CNG6 has the highest overall similarity to the CNG4 beta subunit (47% sequence identity). CNG6 transcripts are present in a small subset of retinal photoreceptor cells and also in testis. Heterologous expression of CNG6 in human embryonic kidney 293 cells did not lead to detectable currents. However, when coexpressed with the cone photoreceptor alpha subunit, CNG6 induced a flickering channel gating, weakened the outward rectification in the presence of extracellular Ca(2+), increased the sensitivity for L-cis diltiazem, and enhanced the cAMP efficacy of the channel. Taken together, the data indicate that CNG6 represents a new CNG channel beta subunit that may associate with the CNG3 alpha subunit to form the native cone channel.

Effects of nitric oxide, light adaptation and APB on spectral characteristics of H1 horizontal cells in carp retina

The spectral characteristics of cone-driven horizontal cells of H1 subtype (H1 HCs) receiving main synaptic input from red-sensitive cones were studied in light- and dark-adapted retinae of carp. The spectral sensitivity profile of H1 HCs in dark-adapted retinae was practically the same as the absorption spectrum of red-sensitive cones. Light-adaptation decreased the sensitivity preferentially in the short-wavelength (blue/green) region, resulting in a relative enhancement of the 617 nm peak. Application of nitric oxide (NO) donors, sodium nitroprusside (SNP) and S-nitrosoglutathione (SNOG or GSNO), or dopamine to dark-adapted retinae decreased the sensitivity preferentially in blue/green region, an effect similar to that of light-adaptation. Application of haemoglobin (Hb, an NO scavenger) or 2-amino-4-phosphonobutyrate (APB, a metabotropic glutamate receptor agonist), to light-adapted retinae increased the sensitivity preferentially in the blue/green region, an effect similar to dark-adaptation. The photoresponses of H1 HCs were univariant in dark-adapted retinae as well as Hb-treated light-adapted retinae. In light-adapted retinae with normal Ringer, however, the univariance did not hold. These results suggested that the photoresponses of H1 HCs to short-wavelength stimuli contain a depolarising (sign-reversing) component, which can be activated by light-adaptation or application of NO and dopamine, and inactivated by dark-adaptation or deprivation of NO or application of APB.

The metabotropic receptor mGluR6 may signal through G(o), but not phosphodiesterase, in retinal bipolar cells

Bipolar cells are retinal interneurons that receive synaptic input from photoreceptors. Glutamate, the photoreceptor transmitter, hyperpolarizes On bipolar cells by closing nonselective cation channels, an effect mediated by the metabotropic receptor mGluR6. Previous studies of mGluR6 transduction have suggested that the receptor couples to a phosphodiesterase (PDE) that preferentially hydrolyzes cGMP, and that cGMP directly gates the nonselective cation channel. This hypothesis was tested by dialyzing On bipolar cells with nonhydrolyzable analogs of cGMP. Whole-cell recordings were obtained from On bipolar cells in slices of larval tiger salamander retina. Surprisingly, On bipolar cells dialyzed with 8-(4-chlorophenylthio)-cyclic GMP (8-pCPT-cGMP), or 8-bromo-cyclic GMP (8-Br-cGMP) responded normally to glutamate or L-2-amino-4-phosphonobutyrate (L-APB). Response amplitudes and kinetics were not significantly altered compared with cells dialyzed with cGMP alone. Comparable results were obtained with the PDE inhibitor 3-isobutyl-1-methyl-xanthine (IBMX) or with 8-pCPT-cGMP and IBMX together, indicating that PDE is not required for mGluR6 signal transduction. Addition of the G-protein subunit G(o)alpha to the pipette solution suppressed the cation current and occluded the glutamate response, whereas dialysis with G(i)alpha or with transducin Gbetagamma had no significant effect on either the cation current or the response. Dialysis of an antibody directed against G(o)alpha also reduced the glutamate response, indicating a functional role for endogenous G(o)alpha. These results indicate that mGluR6 may signal through G(o), rather than a transducin-like G-protein.

Stimulation with N-methyl-D-aspartate or kainic acid increases cyclic guanosine monophosphate-like immunoreactivity in turtle retina: involvement of nitric oxide synthase

In brain and retina, stimulation with excitatory amino acids (EAA) can generate nitric oxide (NO) and increase levels of cyclic guanosine monophosphate (cGMP). Because nitric oxide synthase (NOS) has been found in retinas of all species examined to date, an NO signal-transduction pathway is likely to be present in all retinas. We tested the hypothesis that stimulation of ionotropic glutamate receptors in turtle retina would result in increases in cGMP through an NOS/NO/cGMP pathway. Following in vitro incubations of turtle eye cups with the glutamate receptor agonists, N-methyl-D-aspartate (NMDA) or kainic acid (KA), we quantified the increases in cGMP-like immunoreactivity (cGMP-LI) by using enzyme-linked immunosorbant assay (ELISA) and localized the increased cGMP-LI by using an antibody against cGMP. Stimulation with NMDA or KA increased cGMP-LI in bipolar and amacrine cells as well as in some somata in the ganglion cell layer. Either KA or NMDA produced statistically significant increases in total retinal cGMP-LI by ELISA. To test the involvement of NO, we used the NOS inhibitors 7-nitroindazole and L-nitroarginine. Both inhibitors blocked virtually all of the KA- or NMDA-stimulated increases in cGMP-LI. These results indicate that activation of ionotropic glutamate receptors can increase cGMP in select retinal neurons. Differences between the agonist-evoked increases of retinal cGMP-LI suggest that there can be specificity in the activation of the NOS/NO/cGMP signal-transduction pathway by glutamate. This suggests that, in addition to short-term electrical changes, activation of ionotropic glutamate receptors also may produce longer term modulatory or metabolic effects involving NO/cGMP.

A rise in intracellular Ca2+ underlies light adaptation in dogfish retinal 'on' bipolar cells

1. This investigation was to determine the extent to which retinal 'on' bipolar cells contribute to the adaptive changes that occur with light, which enable the rod visual system to operate over a wide range of ambient light intensities, and to elucidate the underlying adaptive mechanism. 2. Whole-cell voltage clamp recordings were obtained from bipolar cells in dark-adapted dogfish retinal slices. Current responses to brief flashes and steps of light were analysed. 'On' bipolar cell inward current light responses are mediated by a metabotropic glutamate receptor linked to the control of a cGMP cascade, with cGMP opening cation channels. Outward current responses to light of 'off' bipolar cells are mediated by the closure of ionotropic glutamate receptor channels. 3. When Ca2+ buffer was omitted from the patch pipette solution, 'on' bipolar cells rapidly desensitized to steps of light as dim as one rhodopsin molecule bleached per rod per second (1 Rh* s-1), whereas 'off' bipolar cells did not desensitize. Responses of 'on' bipolar cells to flashes in the presence of dim backgrounds recovered after a delay, but with diminished sensitivity, i.e. the cells adapted. 4. With the Ca2+ chelator BAPTA in the patch pipette solution, step responses of 'on' bipolar cells were sustained and flash responses following steps showed rapid recovery. Buffering Ca2+ in the patch pipette solution to 1 microM prevented desensitization, whereas 50 microM free Ca2+ reduced the 'on' bipolar cell flash responses, suppressed inward dark current and decreased input conductance. 5. We conclude that a major component of adaptation of the visual system is due to a reduction in gain at the rod-'on' bipolar cell synapse as a result of Ca2+ loading of the dendrites when their cGMP-gated cation channels open with light.

Localization of nNOS in photoreceptor, bipolar and horizontal cells in turtle and rat retinas

Neuronal nitric oxide synthase (nNOS), an enzyme that synthesizes NO, has been found in the outer retina using light microscopic immunocytochemistry, but its subcellular localization is unknown. We used electron immunocytochemistry to examine nNOS-like immunoreactivity (nNOS-LI) in the outer plexiform layer of turtle and rat retinas. In turtle, nNOS-LI was present in some bipolar and horizontal cell processes at photoreceptor ribbon synapses and at basal junctions between photoreceptors. In rat, nNOS-LI was present in some rod bipolar and B-type horizontal cell axon terminals at rod ribbon synapses. These results indicate that in vertebrates, NO is produced by all of the major nerve cell types in the outer retina at specific synaptic contacts.

Postsynaptic responses of horizontal cells in the tiger salamander retina are mediated by AMPA-preferring receptors

The postsynaptic responses of sign-preserving second-order retinal neurons (horizontal cells (HCs) and off-bipolar cells) are mediated by CNQX-sensitive AMPA/KA glutamate receptors. In this study we used receptor-specific allosteric regulators of desensitization and selected antagonists to determine the glutamate receptor subtypes in tiger salamander horizontal cells. Two approaches were employed in this study. The first was to measure postsynaptic currents induced by exogenously applied glutamate under voltage clamp conditions in living retinal slices; and the second was to record voltage responses controlled by endogenous glutamate released from photoreceptors in whole retinas. Application of 100 microM cyclothiazide (a specific AMPA receptor desensitization blocker) enhanced the glutamate-induced current by about 5 fold. In contrast, 300 microgram ml-1 Co nA (a specific kainate receptor desensitization blocker), had no effect. GYKI 52466 (a specific AMPA receptor antagonist) at 30 microM almost completely suppressed the glutamate-induced inward current in HCs. Cyclothiazide at 100 microM depolarized the HC dark membrane potential by about 5 mV and reduced the amplitudes of the voltage responses to dim lights, but enhanced the voltage responses to bright lights. Cyclothiazide had no effect on either the dark potential or the light responses of rods and cones. Con A at 300 microgram ml-1 had no effect on either the dark potential or the light responses of the HC. GYKI 52466 (30 microM) hyperpolarized the HC dark membrane potential by about 55 mV and almost completely suppressed the light responses. We conclude from these results that the postsynaptic glutamate- and light-induced responses in the tiger salamander retinal horizontal cells are mediated by AMPA-preferring, and not kainate-preferring glutamate receptors. The functional roles of AMPA receptors and their desensitization kinetics in visual information processing are discussed.

Recoverin and hippocalcin distribution in the lamprey (Lampreta fluviatilis) retina

Recoverin is a calcium-sensing protein which is involved in the transduction of light in vertebrate photoreceptors. It is also detected in other retina cell types in which its function is not yet elucidated, and is an autoantigen in a cancer-associated degenerative disease of the retina. Recently, hippocalcin, an homologous protein of recoverin, belonging to the same family of fatty acylated EF-hand calcium binding proteins was described in mammals. The immunohistochemical studies presented in this paper demonstrate, that, in the retina of the lamprey, an Agnathan considered the living ancestor of actual jawed vertebrates, recoverin was present in all photoreceptors and, to a lesser extent in subpopulations of amacrine and ganglion cells whereas hippocalcin was detected in numerous amacrine and ganglion cells and in the inner segments of long photoreceptors. The existence of these calcium-binding proteins shows that they have a high degree of conservation during evolution. Their presence in the same cells that in jawed vertebrates (photoreceptors and ganglion cells for recoverin; amacrine and ganglion cells for hippocalcin) suggests that some retinal functions are well conserved but because they were also found in different cell types than in other species (amacrine for recoverin; photoreceptors for hippocalcin), they may have functions more specific to the lamprey retina.

Functional coupling of a human retinal metabotropic glutamate receptor (hmGluR6) to bovine rod transducin and rat Go in an in vitro reconstitution system

The cDNA encoding hmGluR6, appended with a 15-amino acid antibody epitope (1D4), was transiently transfected in COS-7 cells. The receptor was purified from COS cell membranes using an antibody affinity column. The purified receptor was then reconstituted into lipid vesicles, and its ability to activate either transducin, the rod photoreceptor-specific GTP-binding protein, or the alpha subunit of Go was assayed in vitro using a guanosine 5'-3-O-(thio)triphosphate binding assay. Activation of both transducin and Go was observed. The rate of Go activation was 18-fold greater than the rate of transducin activation. This indicates that the coupling of mGluR6 to Go is more efficient and suggests that Go may be involved in coupling to mGluR6 in ON-bipolar cells.

Immunocytochemical and histochemical localization of nitric oxide synthase in the turtle retina

Recent interest in nitric oxide and its relationship to cGMP has produced many attempts to anatomically localize the enzyme synthesizing nitric oxide, nitric oxide synthase. In the retina, numerous previous studies have used the NADPH-diaphorase enzyme activity of nitric oxide synthase as a histochemical method to localize nitric oxide synthase. However, all NADPH-diaphorase activity is not necessarily nitric oxide synthase, because several enzymes have similar biochemical activity. Additionally, various histochemical methods have been used to demonstrate NADPH-diaphorase activity, which makes comparisons between studies difficult. The purpose of this study was twofold. First, we wanted to examine the histochemical labeling of NADPH-diaphorase in the turtle retina to allow comparisons to previous studies. Second, we wanted to compare the histochemical localization of NADPH-diaphorase activity to the immunocytochemical localization of nitric oxide synthase in the turtle retina. Our histochemical localization of NADPH-diaphorase activity and our localization of nitric oxide synthase-like immunoreactivity in the turtle retina both produced similar results. Both the histochemistry and immunocytochemistry consistently labeled photoreceptor inner segments, at least three amacrine cell types, and processes in the inner plexiform layer. In optimized double-labeled preparations, all cells with NADPH-diaphorase activity were also positive for nitric oxide synthase-like immunoreactivity, although some somata in the ganglion cell layer only had nitric oxide synthase-like immunoreactivity. The immunocytochemical localization of nitric oxide synthase in photoreceptors, amacrine cells, and putative ganglion cells indicates that nitric oxide may function at several levels of visual processing in the turtle retina.

ON cone bipolar cells in rat express the metabotropic receptor mGluR6

The rod bipolar cell and about five types of ON cone bipolar cells depolarize to light by employing a sign-reversing metabotropic glutamate receptor. Glutamate responses are similar in both rod bipolar and cone bipolar cells, but the receptor mediating this response (mGluR6) was so far demonstrated only in rod bipolar cells. To test if ON cone bipolar cells also express mGluR6, we immunoreacted rat retina with an antibody specific for mGluR6, and studied the staining from serial ultrathin sections. We demonstrate that mGluR6 is indeed expressed in the dendritic tips of cone bipolar cells, the majority of which receive a ribbon synapse, and thus probably are ON cone bipolar cells. We further show that half of the dendritic tips contacting the cones stain for mGluR6, thus implying that all ON cone bipolar cell types express mGluR6.

cGMP-mediated effects on the physiology of bovine and human retinal Müller (glial) cells

1. Whole-cell currents of freshly dissociated or cultured Müller cells from human and bovine retinas were studied using the perforated-patch and standard whole-cell recording techniques. 2. We found that internal perfusion of cGMP or external exposure to 8-bromo-cGMP activated a calcium permeable, non-selective cation current in Müller cells, the principal glial cells of the retina. In addition, the activity of calcium-activated potassium channels increased markedly. These currents were minimally affected by cAMP. 3. Molecular studies using the reverse transcription-polymerase chain reaction demonstrated that human müller cells in culture contain transcripts closely related to the rod cyclic nucleotide-gated (CNG) channel. 4. Since guanylate cyclase is a known target for nitric oxide (NO), we tested the effect of NO donors on Müller cell currents. These agents induced currents that were qualitatively similar to those activated by cGMP. 5. Our experiments support the idea that the NO-cGMP pathway regulates the physiology of Müller cells and may play a role in integrating neuron-glia interactions in the retina.

Contrast adaptation and excitatory amino acid receptors in cat striate cortex

We have employed two paradigms to investigate the mechanisms of contrast gain control in cat striate cortex. In the first paradigm, optimal drifting gratings were presented in three consecutive periods. The contrast was near threshold in the first and third periods and accompanied by iontophoretic pulses of glutamate or glutamate receptor (GluR) agonists. The contrast was set to evoke a higher firing rate in the second period. Although both visual and iontophoretic conditions were identical in the first and third periods, responses to glutamate, N-methyl-D-aspartic acid (NMDA), and (IS,3R)-1-Aminocyclopentane-1,3-dicarboxylic acid (ACPD) were reduced following the adapting interval. (S)-alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) responses were not reduced. Administration of ionotropic GluR antagonists did not affect adaptation to the high-contrast grating. The metabotropic GluR antagonist (+/-)-alpha-Methyl-4-carboxyphenylglycine (MCPG), which acts at presynaptic glutamate autoreceptors, decreased the degree of adaptation exhibited by striate cells. In a second paradigm, contrast response functions (CRFs) were obtained at various adapting contrasts and least-squares fits to a hyperbolic ratio equation generated for each adapting level. Similar to previous reports, DL-2-amino-5-phosphonovaleric acid (APV) reduced the slope of the CRF and increased the responsiveness of the cells but did not affect the semisaturation constant, sigma, or the exponent of the CRF, n. Only MCPG significantly altered the distribution of sigma and n for 19 cells. The effect on sigma suggests that this drug can interfere with the cell's ability to shift its operating point to match the adapting contrast. These results suggest the involvement of a presynaptic mechanism for contrast adaptation. The decrease in neuronal responsiveness immediately following the high-contrast period may reflect an additional, postsynaptic effect in which there is a decrease in the NMDA-mediated component of the visual response.

Glutamate responses of bipolar cells in a slice preparation of the rat retina

Whole-cell currents from >70 voltage-clamped bipolar cells were recorded in a slice preparation of the rat retina. The recorded cells were identified and classified by intracellular staining with Lucifer yellow. Glutamate, the specific agonists (+/-)-2-amino-4-phosphonobutyric acid (AP-4) and kainate (KA), and the antagonist 6-cyanoquinoxaline-2,3-dione (CNQX) were applied. The cells could be isolated from presynaptic influences by the co-application of bicuculline, strychnine, and cobalt ions. Responses to AP-4 were elicited only from bipolar cells with axons stratifying in the inner part of the inner plexiform layer (IPL). AP-4 caused an outward current in these cells attributable to the closure of nonspecific cation channels. Responses to kainate representing a direct action of the drug on the recorded cells were observed only in bipolar cells with axons stratifying in the outer part of the IPL. KA caused a CNQX-sensitive inward current in these cells, associated with openings of nonspecific cation channels. The results predict that cone bipolar (CB) cells with axons terminating in the outer IPL are OFF-bipolars, whereas those with axons terminating in the inner IPL are ON-bipolars. Most of the cells expressed GABA-gated Cl- conductances. In rod bipolar and in some CB cells, only part of the GABA-induced currents could be blocked by the application of bicuculline, suggesting the presence of GABAc receptors in addition to GABAA receptors.

Responses of rod bipolar cells isolated from dogfish retinal slices to concentration-jumps of glutamate

Rod on-bipolar cell light responses are mediated by a class of metabotropic glutamate receptor which is coupled via a G-protein to the control of a cGMP cascade, with cGMP acting to open cation channels, whilst off-bipolar cells possess ionotropic glutamate receptors. Whole-cell voltage-clamp recordings were obtained from on- and off-bipolar cells of dark-adapted dogfish retinal slices, identified by their light responses. Isolated cells were exposed to concentration-jumps of glutamate. At negative voltage-clamp potentials, on-bipolar cells responded to glutamate with outward currents with a mean delay of 10.8 ms, whilst off-bipolar cells responded with inward currents without any delay. Neither cell type showed desensitization to applied steps of glutamate. The dose-response relation for on-bipolar cells showed no gradual saturation, but increased linearly with a sharp cutoff above 200 microM glutamate. This dose-response relation could be fitted with a theoretical expression assuming Michaelis-Menten kinetics for the action of glutamate on receptors and a linear relation between the concentration of receptors bound to glutamate and the fall in cGMP this induces. The dose-response relation of off-bipolar cells showed saturation with a limiting slope of 2 at low glutamate concentrations, suggesting that two molecules of glutamate are required to open each channel by a cooperative mechanism. The glutamate receptor coupled cGMP cascade of rod on-bipolar cells can account for high synaptic voltage gain.

Immunocytochemical analysis of bipolar cells in the macaque monkey retina

Transfer of visual information from photoreceptors to ganglion cells within the retina is mediated by specialized groups of bipolar cells. At least 10 different morphological types of bipolar cells have been distinguished in Golgi studies of primate retina. In the present study, bipolar cell populations in the macaque monkey retina were identified by their differential immunoreactivity to a spectrum of antibody markers. This enabled their spatial density and photoreceptor connections to be analysed. An antibody against the beta isozyme of protein kinase C (PKCA beta) labelled many cone bipolar cells. Invaginating (presumed ON) cone bipolar cells and rod bipolar cells were preferentially labelled with a monoclonal antibody raised against rabbit olfactory bulb. Flat (presumed OFF) bipolar cells were labelled with an antiserum against the glutamate transporter protein (GLT-1). Different populations of diffuse cone bipolar cells, which contact 5-10 cones, could be distinguished. The GLT-1 antiserum preferentially labelled the flat diffuse bipolar cell type DB2 (Boycott and Wässle, 1991, Eur. J. Neurosci. 3:1069-1088) as well as flat midget bipolar cells. Antibodies to calbindin (CaBP D-28K) labelled the flat diffuse bipolar cell type DB3 and (possibly) the invaginating diffuse bipolar cell type DB5. An antibody against the alpha isozyme of PKC labelled an invaginating diffuse bipolar cell type (DB4) as well as rod bipolar cells. Comparison of the spatial density of cone bipolar cell populations with that of photoreceptors suggests that each bipolar cell class provides a complete coverage of the cone array (each cone is contacted by at least one member of every bipolar cell class). These results support the classification scheme of Boycott and Wässle (1991) by showing that different diffuse bipolar cell classes express different patterns of immunoreactivity, and they reinforce the view that different spatial and temporal components of the signal from the photoreceptor array are processed in parallel within the primate retina.

Retinal synapses. Glutamate receptors for signal amplification

The localization of metabotropic glutamate receptors in retinal bipolar cells supports the view that these receptors are involved in the synaptic amplification that allows the detection of dim-light stimuli.

A new subunit of the cyclic nucleotide-gated cation channel in retinal rods

Retinal rods respond to light with a membrane hyperpolarization produced by a G-protein-mediated signalling cascade that leads to cyclic GMP hydrolysis and the consequent closure of a cGMP-gated channel that is open in darkness. A protein that forms this channel has recently been purified from bovine retina and molecularly cloned, suggesting that the native cGMP-gated channel might be a homo-oligomer. Here we report the cloning of another protein from human retina which has only about 30% overall identity to the rod channel subunit. This protein, immunocytochemically localized to rod outer segments, does not form functional channels by itself. However, when co-expressed with the cloned human rod channel protein, it introduces rapid flickers to the channel openings that are characteristic of the native channel. The hetero-oligomeric channel is also highly sensitive to the blocker L-cis-diltiazem, like the native channel. This new protein thus seems to be another subunit of the native rod channel. The hetero-oligomeric nature of the rod channel means that it is no exception to a common motif shared by other ligand-gated channels.

Recoverin immunoreactivity in mammalian cone bipolar cells

Human, macaque monkey, and rat retinas were immunostained with a polyclonal antibody preparation against purified recoverin, a 23-kD calcium-binding protein isolated from bovine retina that localizes to rods and cones (Dizhoor et al., 1991). In addition to immunoreactive photoreceptors, we have identified subpopulations of recoverin-positive bipolar cells in all three species. Results from immunostaining with progressive dilutions of anti-recoverin and preadsorption of the antibody with a dilution series of purified recoverin showed that photoreceptors and bipolar cells had similar affinities for the antibody and suggested that the molecule recognized by the antibody in both cell types is recoverin. Immunoreactivity for recoverin and protein kinase C, a selective marker for all rod bipolar cells, was found in separate bipolar cell populations. Recoverin immunoreactivity is therefore a characteristic of certain cone bipolar cell types. In rat retina, anti-recoverin labeled two morphologically distinct subpopulations of cone bipolar cells whose axonal arbors stratified at different depths in the inner plexiform layer (IPL). The bipolar cells labeled with anti-recoverin did not correspond to those that were reactive for calbindin, another cone bipolar cell marker. Human and monkey retinas also had two populations of cone bipolar cells that were recoverin-positive. One population showed a distinct pattern of narrow bistratification at the outer border of the IPL and a regular mosaic arrangement of its axonal arbors, suggesting that the entire population of a single cone bipolar type was labeled. Cell density, dendritic morphology, and axonal-field size and stratification indicate that anti-recoverin selectively strains the flat midget (presumed OFF-center) cone bipolar cell type observed previously in Golgi preparations. By contrast the second bipolar cell population had axonal stratification in the inner half of the IPL and showed an unusual but consistent morphology and spatial distribution. Individual cells were intensely stained but were present at an extremely low density (approximately 2-5 cells/mm2). These cells had multibranched dendritic trees characteristic of the diffuse bipolar cell class, but very small axonal fields in the size range of the midget bipolar class. Neither of the two recoverin-positive bipolar cell types in monkey was labeled with anti-calbindin or anti-cholecystokinin. An antibody preparation against bovine pineal hydroxyindole-O-methyltransferase (HIOMT) labeled photoreceptors and bipolar cells that closely resembled the recoverin-positive bipolar cells in human and rat retinas.(ABSTRACT TRUNCATED AT 400 WORDS)