Immunocytochemical localisation of L-glutamic acid decarboxylase in the goldfish retina

Drift-diffusion simulation of the ephaptic effect in the triad synapse of the retina

Experimental evidence suggests the existence of a negative feedback pathway between horizontal cells and cone photoreceptors in the outer plexiform layer of the retina that modulates the flow of calcium ions into the synaptic terminals of cones. However, the underlying mechanism for this feedback is controversial and there are currently three competing hypotheses: the ephaptic hypothesis, the pH hypothesis, and the GABA hypothesis. The goal of this investigation is to demonstrate the ephaptic hypothesis by means of detailed numerical simulations. The drift-diffusion (Poisson-Nernst-Planck) model with membrane boundary current equations is applied to a realistic two-dimensional cross-section of the triad synapse in the goldfish retina to verify the existence of strictly electrical feedback, as predicted by the ephaptic hypothesis. The effect on electrical feedback from the behavior of the bipolar cell membrane potential is also explored. The computed steady-state cone calcium transmembrane current-voltage curves for several cases are presented and compared with experimental data on goldfish. The results provide convincing evidence that an ephaptic mechanism can produce the feedback effect seen in experiments. The model and numerical methods presented here can be applied to any neuronal circuit where dendritic spines are invaginated in presynaptic terminals or boutons.

Acidification of the synaptic cleft of cone photoreceptor terminal controls the amount of transmitter release, thereby forming the receptive field surround in the vertebrate retina

In the vertebrate retina, feedback from horizontal cells (HCs) to cone photoreceptors plays a key role in the formation of the center-surround receptive field of retinal cells, which induces contrast enhancement of visual images. The mechanism underlying surround inhibition is not fully understood. In this review, we discuss this issue, focusing on our recent hypothesis that acidification of the synaptic cleft of the cone photoreceptor terminal causes this inhibition by modulating the Ca channel of the terminals. We present evidence that the acidification is caused by proton excretion from HCs by a vacuolar type H(+) pump. Recent publications supporting or opposing our hypothesis are discussed.

Transporter mediated GABA release in the retina: Role of excitatory amino acids and dopamine

In general, the release of neurotransmitters in the central nervous system is accomplished by a calcium-dependent process which constitutes a common feature of exocytosis, a conserved mechanism for transmitter release in all species. However, neurotransmitters can also be released by the reversal of their transporters. In the retina, a large portion of GABA is released by this mechanism, which is under the control of neuroactive agents, such as excitatory amino acids and dopamine. In this review, we will focus on the transporter mediated GABA release and the role played by excitatory amino acids and dopamine in this process. First, we will discuss the works that used radiolabeled GABA to study the outflow of the neurotransmitter and then the works that took into consideration the endogenous pool of GABA and the topography of GABAergic circuits influenced by excitatory amino acids and dopamine.

pH changes in the invaginating synaptic cleft mediate feedback from horizontal cells to cone photoreceptors by modulating Ca2+ channels

Feedback from horizontal cells (HCs) to cone photoreceptors plays a key role in the center-surround–receptive field organization of retinal neurons. Recordings from cone photoreceptors in newt retinal slices were obtained by the whole-cell patch-clamp technique, using a superfusate containing a GABA antagonist (100 μM picrotoxin). Surround illumination of the receptive field increased the voltage-dependent calcium current (I_Ca) in the cones, and shifted the activation voltage of I_Ca to negative voltages. External alkalinization also increased cone I_Ca and shifted its activation voltage toward negative voltages. Enrichment of the pH buffering capacity of the extracellular solution increased cone I_Ca, and blocked any additional increase in cone I_Ca by surround illumination. Hyperpolarization of the HCs by a glutamate receptor antagonist-augmented cone I_Ca, whereas depolarization of the HCs by kainate suppressed cone I_Ca. From these results, we propose the hypothesis that pH changes in the synaptic clefts, which are intimately related to the membrane voltage of the HCs, mediate the feedback from the HCs to cone photoreceptors. The feedback mediated by pH changes in the synaptic cleft may serve as an additional mechanism for the center-surround organization of the receptive field in the outer retina.

Circadian activation of bullfrog retinal mitogen-activated protein kinase associates with oscillator function

The vertebrate retina retains a circadian oscillator, and its oscillation is self-sustained with a period close to 24 h under constant environmental conditions. Here we show that bullfrog retinal mitogen-activated protein kinase (MAPK) exhibits an in vivo circadian rhythm in phosphorylation with a peak at night in a light/dark cycle. The phosphorylation rhythm of MAPK persists in constant darkness with a peak at subjective night, and this self-sustained rhythm is also observed in cultured retinas, indicating its close interaction with the retinal oscillator. The rhythmically phosphorylated MAPK is detected only in a discrete subset of amacrine cells despite ubiquitous distribution of MAPK throughout the retinal layers. Treatment of the cultured retinas with MAPK kinase (MEK) inhibitor PD98059 suppresses MAPK phosphorylation during the subjective night, and this pulse perturbation of MEK activity induces a significant phase delay (4-8 h) of the retinal circadian rhythm in MAPK and MEK phosphorylation. These observations strongly suggest that the site-specific and time-of-day-specific activation of MAPK contributes to the circadian time-keeping mechanism of the retinal clock system.

Vertebrate ancient-long opsin: a green-sensitive photoreceptive molecule present in zebrafish deep brain and retinal horizontal cells

Nonretinal/nonpineal photosensitivity has been found in the brain of vertebrates, but the molecular basis for such a "deep brain" photoreception system remains unclear. We conducted an extensive search for brain opsin cDNAs of the zebrafish (Danio rerio), a useful animal model for genetic studies, and we have isolated a partial cDNA clone encoding an ortholog of vertebrate ancient (VA) opsin, the function of which is unknown. Subsequent characterization revealed the occurrence of two kinds of mRNAs encoding putative splicing variants, VA and VA-Long (VAL) opsin, the latter of which is a novel variant of the former. Both opsins shared a common core sequence in the membrane-spanning domains, but VAL-opsin had a C-terminal tail much longer than that of VA-opsin. Functional reconstitution experiments on the recombinant proteins showed that VAL-opsin with bound 11-cis-retinal is a green-sensitive pigment (lambdamax approximately 500 nm), whereas VA-opsin exhibited no photosensitivity even in the presence of 11-cis-retinal. Immunoreactivity specific to this functionally active VAL-opsin was localized at a limited number of cells surrounding the diencephalic ventricle of central thalamus, and these cells were distributed over approximately 200 micrometer along the rostrocaudal axis. Taken together with the previous study on the locus of the teleost brain photosensitivity (von Frisch K, 1911), it is strongly suggested that the VAL-positive cells in the zebrafish brain represent the deep brain photoreceptors. The VAL-specific immunoreactivity was also detected in a subset of non-GABAergic horizontal cells in the zebrafish retina. The existence of VAL-opsin, a new member of the rhodopsin superfamily, in these tissues may indicate its multiple roles in visual and nonvisual photosensory physiology.

Atypical effect of dopamine in modulating the functional inhibition of NMDA receptors of cultured retina cells

Cultured retina cells released accumulated [3H]GABA (gamma-aminobutyric acid) when stimulated by L-glutamate, N-methyl-D-aspartate (NMDA) and kainate. In the absence of Mg2+, dopamine at 200 microM (IC50 60 microM), inhibited in more than 50% the release of [3H]GABA induced by L-glutamate and NMDA, but not by kainate. This effect was not blocked by the D1-like dopamine receptor antagonist, R-(+)-7-chloro-8-hydroxy-3-methyl- -phenyl-2,3,4,5-tetrahydro- H-3-benzazepine hydrochloride (SCH 23390), neither by haloperidol nor spiroperidol (dopamine D2-like receptor antagonists). The dopamine D1-like receptor agonist R(+)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,diol hydrochloride (SKF 38393) at 50 microM, but not its enantiomer, also inhibited the release of [3H]GABA induced by NMDA, but not by kainate; an effect that was not prevented by the antagonists mentioned above. (+/-)-6-Chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepin e hydrobromide (SKF 812497) had no effect. Neither 8BrcAMP (5 mM) nor forskolin (10 microM) inhibited the release of [3H]GABA. Our results suggest that dopamine and (+)-SKF 38393 inhibit the glutamate and NMDA-evoked [3H]GABA release through mechanisms that seem not to involve known dopaminergic receptor systems.

Neurochemical signatures revealed by glutamine labeling in the chicken retina

Postembedding immunocytochemistry was used to determine the retinal distribution of the amino acid glutamine, and characterize amino acid signatures in the avian retinal ganglion cell layer. Glutamine is a potential precursor of glutamate and some glutamatergic neurons may use this amino acid to sustain production of glutamate for neurotransmission. Ganglion cells, cells in the inner nuclear layer, and some photoreceptors exhibited glutamine immunoreactivity of varying intensity. Ganglion cells demonstrated the highest level of immunoreactivity which indicates either slow glutamine turnover or active maintenance of a large standing glutamine pool relative to other glutamatergic neurons. Müller's cells in the avian retina are involved in glutamate uptake and carbon recycling by the rapid conversion of glutamate to glutamine, thus explaining the low glutamate and high glutamine immunoreactivity found throughout Müller's cells. Most chicken retinal ganglion cells are glutamate (E) and glutamine (Q) immunoreactive but display diverse signatures with presumed functional subsets of cells displaying admixtures of E and Q with GABA (gamma) and/or glycine (G). The four major ganglion cell signatures are (1) EQ; (2) EQ gamma; (3) EQG; and (4) EQ gamma G.

Depolarization elicits, while hyperpolarization blocks uptake of endogenous glutamate by retinal horizontal cells of the turtle

We have employed an immunoreaction against glutamate to qualitatively demonstrate varying levels of glutamate in retinal horizontal cells of the turtle. Glutamate-like immunoreactivity (GLI) in horizontal cells could be demonstrated after glutamate decarboxylase was inhibited by aminooxyacetic acid (AOAA) and its degradation to GABA was blocked. Depolarization of horizontal cells by kainic acid (KA) induces strong glutamate immunoreactivity in these cells, whereas hyperpolarization by 2,3-cis piperidine dicarboxylate (PDA) abolishes glutamate-like immunoreactivity in horizontal cells. When glutamate release from cones and bipolar cells is blocked in the absence of calcium, or when glutamate uptake is blocked by DL-threo beta-hydroxy aspartate, KA/AOAA treatment of the retina does not induce GLI in horizontal cells. Our data show that horizontal cells are capable of taking up glutamate from the endogenous retinal pool in an activity dependent way. Our interpretation of these findings is that retinal horizontal cells are capable of regulating glutamate levels in the extracellular space of the cone pedicle complex by an activity-dependent uptake system. We suggest that inhibition of glutamate uptake upon hyperpolarization rather than inhibition of GABA release may evoke the antagonistic surround response of retinal bipolar cells.

Modulation of a sustained calcium current by intracellular pH in horizontal cells of fish retina

A sustained high voltage-activated (HVA), nifedipine- and cadmium-sensitive calcium current and a sustained calcium action potential (AP) were recorded from horizontal cells isolated from catfish retina. pH indicator dyes showed that superfusion with NH4Cl alkalinized these cells and that washout of NH4Cl or superfusion with Na-acetate acidified them. HVA current was slightly enhanced during superfusion of NH4Cl but was suppressed upon NH4Cl washout or application of Na-acetate. When 25 mM HEPES was added to the patch pipette to increase intracellular pH buffering, the effects of NH4Cl and Na-acetate on HVA current were reduced. These results indicated that intracellular acidification reduces HVA calcium current and alkalinization increases it. Sustained APs, recorded with high resistance, small diameter microelectrodes, were blocked by cobalt and cadmium and their magnitude varied with extracellular calcium concentration. These results provide confirmatory evidence that the HVA current is a major component of the AP and indicate that the AP can be used as a measure of how the HVA current can be modified in intact, undialyzed cells. The duration of APs was increased by superfusion with NH4Cl and reduced by washout of NH4Cl or superfusion with Na-acetate. The Na-acetate and NH4Cl washout-dependent shortening of the APs was observed in the presence of intracellular BAPTA, a calcium chelator, IBMX, a phosphodiesterase inhibitor, and in Na-free or TEA-enriched saline. These findings provide supportive evidence that intracellular acidification may directly suppress the HVA calcium current in intact cells. Intracellular pH changes would thereby be expected to modulate not only the resting membrane potential of these cells in darkness, but calcium-dependent release of neurotransmitter from these cells as well. Furthermore, this acidification-dependent suppression of calcium current could serve a protective role by reducing calcium entry during retinal ischemia, which is usually thought to be accompanied by intracellular acidosis.

Three types of GABA-immunoreactive cone horizontal cells in teleost retina

Peroxidase-anti-peroxidase immunocytochemistry, applied on serial semithin epoxy resin sections, was used to examine the localization of endogenous GABA in horizontal cells in the retina of a marine teleost, the dragonet (Callionymus lyra L.). The immunostaining shows that not only the external H1 cone horizontal cells label with antibodies against GABA, but also the H2 and H3 cone horizontal cells in the inner nuclear layer. The distribution of the H1 cells corresponds to that of the single cones. They are square-patterned and in the dorsal retina their density equals 20,000 cells/mm2. The estimated density of the immunostained H2 and the H3 cells in the dorsal retina is 9500 and 1300 cells/mm2, respectively. The H2 and H3 cells are not geometrically arranged, but nearest-neighbor analysis shows that these horizontal cell types do have a very regular disposition. We suggest that GABA is the likely neurotransmitter substance used by all cone horizontal cell types in teleost retina.

Binding of the benzodiazepine ligand [3H]-RO 15-1788 to membrane preparations of the rabbit and turtle retina

1. We have studied the binding of [3H]-RO 15-1788 to membrane preparations of the retina of rabbit (Lepus cunicula) and turtle (Pseudemys scripta elegans). 2. In both species, [3H]-RO 15-1788 binding was maximal at 0 degrees C and decreased with increasing temperature. It was saturable, protein concentration-dependent and specific. Flunitrazepam, unlabelled RO 15-1788 and ethyl-beta-carboline were the most effective displacers, whereas RO 5,4864 was ineffective. 3. In both turtle and rabbit retina, Scatchard analysis indicated the presence of a single binding site for [3H]-RO 15-1788. The KD was 0.75 nM in both turtle and rabbit, while the Bmax were 520 and 250 fmol/mg protein in turtle and rabbit respectively. A study of the association rate of [3H]-RO 15-1788 binding revealed faster kinetics in turtle, as compared to rabbit.

Localization of GAD- and GABA-like immunoreactivity in ground squirrel retina: retrograde labeling demonstrates GAD-positive ganglion cells

Glutamic acid decarboxylase (GAD)- and gamma-aminobutyric acid (GABA)-like immunoreactivity was examined in the retina of the 13-lined ground squirrel (Spermophilus tridecemlineatus). Labeling was observed in the inner nuclear layer (INL), inner plexiform layer (IPL) and ganglion cell layer (GCL). The immunoreactive cell bodies in the inner third of the INL were 6-13 microns in diameter and, because of their size and location it was considered that these were amacrine cells. Labeling in the IPL was concentrated in 5 bands corresponding to laminae 1a, 1c, 2, 4 and 5. In the GCL a heterogeneous population of neurons exhibited GAD- and GABA-like immunoreactivity. The soma diameters of the GCL cells ranged from 5 to 17 microns. These may represent displaced amacrines and/or ganglion cells. To determine if any of the immunoreactive cells in the GCL were ganglion cells, double labeling experiments were performed using rhodamine latex microspheres ('beads') as retrograde neuronal tracers. Rhodamine beads were injected into the superior colliculus, and retinas with retrogradely labeled ganglion cells were subsequently incubated with the anti-GAD antiserum. These experiments revealed a small population of GAD-positive ganglion cells, setting a lower limit for the total number of GABAergic ganglion cells.

GABA-like immunoreactivity in the developing chick retina: differentiation of GABAergic horizontal cell and its possible contacts with photoreceptors

The morphology of gamma-aminobutyric acid (GABA)-containing horizontal cells was examined in mature and developing chick retinas by GABA immunocytochemistry. In the outer plexiform layer of the mature retina, GABA-immunoreactive components were located in three different sublayers. In the inner (vitreal) layer most positively-stained fibres were laterally oriented processes from horizontal cells. Thick processes were found in the middle layer, and the relatively thin fibres in the outer (scleral) layer showed a concave curvature, suggesting their termination on photoreceptor terminals. By electron microscopy it was found that the principal cone pedicles were usually indented by immunoreactive lateral neurites of horizontal cells but that rod spherules faced only occasionally immunoreactive fibres. Accessory cones and single cones were also not usually indented by immunoreactive fibres. These observations may indicate that horizontal cells regulate the excitation of cone photoreceptors by several different inhibitory mechanisms. During retinal development, horizontal cells begin to extend lateral fibres by the ninth embryonic day, and some GABAergic horizontal cells also possess inwardly extending fibres until embryonic day 11. Between embryonic days 13 and 15, some immunoreactive cells were found among the bipolar cells, suggesting that they were still migrating to their final position. On embryonic day 17, the staining pattern was very similar to that of the mature retina. These results suggest that GABA immunohistochemistry may be an excellent tool for studying horizontal cell differentiation.

gamma-Aminobutyric acid system in developing and degenerating mouse retina

Freeze-dried sections (14 microns thick) of retinal layers were prepared from mice with retinal degeneration (C3H strain) and control mice (C57BL strain). The weighed sections (2-30 ng dry weight) were analyzed using our microassay methods. In the control retina, gamma-aminobutyric acid (GABA) concentration and glutamate decarboxylase (GAD) activity, on a dry weight basis, increased from birth to 9 weeks of age and decreased slightly at 20 weeks. In the degenerated retina, the levels of GABA and GAD activity were higher at birth than in the control retina, and continued to increase until 20 weeks of age, at which time the GAD activity reached a markedly high level. This increase was found when the total GABA and GAD levels per retina were determined. In the normal retinal layers, GABA and GAD were confined primarily to the inner plexiform layer. In the degenerated retina, GAD activity gradually increased in the inner layers during postnatal development, but by 20 weeks the increase was most prominent in the inner part of inner nuclear layer and in the outer part of inner plexiform layer. GABA transaminase activity and its distribution were not much different in both normal and degenerated retinas during development.

GABA-ergic and glycinergic pathways in the inner plexiform layer of the goldfish retina

GABA-ergic and glycinergic circuitry in the inner plexiform layer of the goldfish retina was evaluated by electron microscopic autoradiography of 3H-GABA and 3H-glycine uptake, combined with retrograde horseradish peroxidase (HRP) labeling of ganglion cells. GABA-ergic and glycinergic synapses were found on labeled ganglion cells throughout the inner plexiform layer. This reinforces the idea that physiological evidence of GABA-ergic and glycinergic influence on a variety of ganglion cells in goldfish and carp often reflects direct inputs. Double-labeled synapses are presented as evidence of direct type Ab amacrine cell input to on-center ganglion cells. At least one population of type Aa sustained-off GABA-ergic amacrine cell is proposed, on the basis of profuse GABA-ergic inputs onto bipolar cells in sublamina a. Similar GABA-labeled profiles are shown to synapse onto HRP-labeled probable off-center ganglion cells. Thus GABA-ergic amacrine cells not only provide the predominant feedback to depolarizing (on-center) and hyperpolarizing (off-center) bipolar cells but also provide feed-forward inputs to on- and off-center ganglion cells. Large-caliber GABA-ergic dendrites present in both sublaminae a and b resemble those expected of a previously described bistratified, transient amacrine cell. These processes synapse onto HRP-labeled ganglion cell profiles in both sublaminae. Two morphologies of glycinergic amacrine cell are proposed on the basis of light microscopic autoradiography, 1) the previously described small pyriform cell and 2) a multipolar cell. The differential connectivity of the glycinergic neurons described, however, remains indistinguishable. Whereas abundant glycinergic inputs to ganglion cells occur throughout the inner plexiform layer, contacts between glycinergic profiles and bipolar cells are extremely rare. Therefore, interpreting the meaning of glycinergic input to ganglion cells will require further study of amacrine cell circuitry.

Localization of GABA- and GAD-like immunoreactivity in the turtle retina

gamma-aminobutyric acid (GABA) has been reported to be an important neurotransmitter in the retinas of many species. This immunocytochemical study detailed the localization of antigens resembling GABA and glutamic acid decarboxylase (GAD, an enzyme involved in the synthesis of GABA), in retinal neurons in the turtle, Pseudemys scripta elegans. GABA-like immunoreactivity was present within somata in the inner and outer regions of the inner nuclear layer, within somata in the ganglion cell layer, and in processes in the outer plexiform layer, inner plexiform layer, and ganglion cell axon layer. GAD-like immunoreactivity was found in somata in the inner and outer regions of the inner nuclear layer and in processes in the inner and outer plexiform layers. Cell counts indicated more somata with GABA-like than GAD-like immunoreactivity in the inner nuclear layer. Double-label studies showed that every somata in the inner nuclear layer which had GAD-like immunoreactivity also had GABA-like immunoreactivity, but that many somata had only GABA-like immunoreactivity. The stratification of immunoreactivity within the inner plexiform layer was analyzed using a scanning densitometer. We described the strate within the inner plexiform layer such that S0 represented the inner nuclear layer/inner plexiform layer border and S100 represented the inner plexiform layer/ganglion cell layer border. Analysis of GAD-like labeling yielded seven distinct strata with peak densities at positions S8, S19, S28, S42, S59, S75, and S93. GABA-like labeling provided five distinct strata with peak densities at positions S17, S28, S67, S84, and S95. The strata with peaks of GABA-like immunoreactivity at S17 and S28 were in statistically identical locations to corresponding strata with GAD-like immunoreactivity. The strata with GABA-like immunoreactivity at S67, S84, and S95 did not have statistically identical peaks of correlated GAD-like immunoreactivity, although there were corresponding strata with GAD-like immunoreactivity nearby. Antiserum directed against GABA failed to produce labeled strata at positions corresponding to the strata with GAD-like immunoreactivity at S8 and S42. In summary, our results indicated that the antisera we used, which were directed against GABA and GAD, produced significantly different labeling in the inner nuclear layer, inner plexiform layer, and the ganglion cell body and axon layers of the turtle retina. Until the physiological significance of these differences is resolved, studies employing these markers to investigate the function of GABA in the turtle retina should be interpreted with caution.

Binding of [3H] muscimol to the retina of rabbit and turtle

1. We studied the binding of [3H]muscimol to membrane preparations of the retina of rabbit (Lepus cuniculus) and turtle (Pseudemys scripta elegans). 2. In both species, [3H]muscimol binding was maximal at 0 degrees C and decreased with increasing temperature, it was saturable, protein concentration dependent and specific. Muscimol, GABA and bicuculline were the most effective displacers, whereas baclofen and diaminobutyric acid were ineffective. 3. In the turtle retina, Scatchard analysis indicated the presence of a single site with a KD of 20.81 nM, and a Bmax of 3.620 pmol/mg prot. 4. In the rabbit, a single site could be identified in the nanomolar concentration range (KD of 12.8 nM, Bmax of 1.327). A study of the association rate of [3H]muscimol binding revealed a faster kinetics in turtle, as compared to rabbit.

GABA as a potential transmitter in lizard photoreceptors: immunocytochemical and biochemical evidence

The retina of the desert night lizard, Xantusia vigilis, was examined for immunoreactivity to antibodies against gamma-aminobutyric acid and L-glutamate decarboxylase. At the electron microscopic level it was found that a distinct population of the photoreceptor cells was immunoreactive to both antibodies. Computer-assisted reconstruction of serial sections positively identified the immunoreactive receptors as cones. These cones constituted 15% of the photoreceptors in the retinal sections, and they were morphologically distinct. The mean diameter of the labeled cone synaptic pedicles was 5.8 micron whereas that of the unlabeled pedicles was 7.9 micron, a statistically significant difference. L-glutamate decarboxylase was extracted from the lizard brain, positively identified radiometrically, and shown by immunodiffusion to crossreact with the antibody used for localization. The authors suggest that the immunoreactive cones synthesize and accumulate gamma-aminobutyric acid. Whether or not it is used by those cones as a neurotransmitter should be tested directly.

The effects of gabaculine in vivo on the distribution of GABA-like immunoreactivity in the rat retina

gamma-Aminobutyric acid (GABA) is an inhibitory transmitter found in the retinae of mammals largely within certain amacrine cells. In previous studies from this laboratory, subcutaneous administration to rats of gabaculine, an enzyme-activated irreversible inhibitor of gamma-aminobutyric acid (GABA)-transaminase, produced large, rapid and long-lasting increases in levels of retinal GABA. We employed immunocytochemistry to determine whether such changes in the levels of retinal GABA are accompanied by changes in the cellular distribution of GABA. Using a recently developed antiserum to a GABA-protein conjugate, and the peroxidase-antiperoxidase method, we examined retinae from control rats and from rats 2 or 8 h after administration of 10 mg/kg gabaculine. From previous work, retinal levels of GABA were respectively elevated 3- or 6-fold at those postgabaculine times. In the present study, marked changes in the distribution of GABA-like immunoreactivity (GABA-LIR) were apparent by 2 h after injection of gabaculine, and were more striking at 8 h postgabaculine. The pattern of staining for GABA-LIR strongly suggested that much of the GABA in gabaculine-treated retinae was within Müller glial cells. That observation provides evidence for the importance of those cells in the uptake and degradation of GABA after its release from retinal neurons.

The action of gamma-aminobutyric acid on the horizontal cells of the skate retina

The effects of gamma-aminobutyric acid (GABA) were studied in the superfused retina of the skate. Intracellular recordings were made from horizontal cells. After application of 500 microM GABA there was a depolarization of the membrane potential, a decrease in the light-evoked amplitude of the response and an increase in the duration of the waveform.

Retinal bipolar cells receive negative feedback input from GABAergic amacrine cells

Bipolar cells make reciprocal synapses with amacrine cells in the inner plexiform layer; both feedforward connections and feedback connections are present. The physiological properties of the feedback synapse have not been well described. Since some amacrine cells are thought to be GABAergic, we examined bipolar cells for feedback input from gamma-aminobtyric acid (GABA)ergic amacrine cells. Solitary bipolar cells were dissociated enzymatically from the goldfish retina. Cells were voltage clamped with a patch pipette and their GABA sensitivity was examined. GABA evoked responses in all bipolar cells with a large axon terminal, which were identified to be the rod dominant ON type, and in some bipolar cells with a small axon terminal. The highest GABA sensitivity was located at the axon terminal. The least effective dose was as low as 100 nM. A small insignificant response of high threshold was evoked when GABA was applied to the dendrite and soma. GABA increased the Cl conductance and caused membrane hyperpolarization. The bipolar cells had the GABAA receptor coupled with a benzodiazepine receptor. The GABA-evoked response was not susceptible to Co ions, which suppressed the GABA-induced responses in turtle cones by 50% at 5 microM concentration. Incomplete desensitization was observed, suggesting that the GABAergic pathway seems capable of transmitting signals tonically. The present results strongly indicate that the rod-dominant ON-type bipolar cells and some bipolar cells with a small axon terminal receive negative feedback inputs from GABAergic amacrine cells.

Neurotoxic effect of 1-methyl-4-phenylpyridinium ion on dopaminergic neurons of the retina of goldfish

Dopaminergic neurons of the goldfish retina were selectively destroyed after a single intravitreal injection of 1-methyl-4-phenylpyridinium ion (MPP+). The ultrastructural analysis of the retina 3 days after toxin administration shows darkening of some retinal neurons present in the inner nuclear layer including their cytoplasmic processes. Both uptake and release of dopamine were reduced in the toxin-injected retina, whereas choline acetyltransferase (ChAT) and glutamic acid decarboxylase (GAD) activities, as well as the uptake of D-[3H]aspartate were not affected.

Neuronal uptake and laminar distribution of tritiated aspartate, glutamate, gamma-aminobutyrate and glycine in the prestriate cortex of squirrel monkeys: correlation with levels of cytochrome oxidase activity and their uptake in area 17

The neuronal uptake and laminar distribution of cortically injected tritium-labeled gamma-aminobutyrate (GABA), aspartic acid, glutamate and glycine was examined in the prestriate cortex of squirrel monkeys. The intent of this investigation was not to examine the role of these amino acids as neurotransmitters, but to correlate the distribution of tritium-labeled neurons with their levels of cytochrome oxidase activity. A comparison of the number of these labeled neurons was made between the metabolically active "puff" and the less active "nonpuff" regions. In addition, these results were contrasted with the findings in area 17. With each tritiated amino acid tested, labeled neurons that had either high or low levels of cytochrome oxidase activity were present in all laminae. However, the density of labeled neurons varied between lamina for a given amino acid as well as between different amino acids. While many neurons that were cytochrome oxidase-reactive were also tritium-labeled, cytochrome oxidase activity was not a prerequisite for the sequestering of tritium label. In fact, many of the labeled neurons exhibited relatively low levels of cytochrome oxidase activity. Similar to area 17, few aspartate- or glutamate-labeled neurons were present in laminae II-III. The number of labeled neurons for both amino acids increased in laminae IV-VI, with the greatest increase observed in laminae V-VI. Gamma-aminobutyrate-labeled neurons were more prevalent in laminae I and upper II than in the other laminae, whereas in area 17, a greater proportion of the labeled neurons were found in laminae V-VI. With the exception of the uppermost laminae, where GABA-labeled neurons were more abundant, the number of glycine-labeled neurons was significantly greater throughout most laminae than with the other amino acids examined. The density of glycine-labeled neurons in lamina IV, however, was significantly less than the number observed in lamina III even though lamina III was farther away from the injection site which was at the boundary between laminae V-VI. Glycine-labeled neurons were, on average, larger than those labeled with any other amino acid. Similar to area 17, more GABA- and glycine-labeled neurons were observed within the puff regions than in nonpuff regions. No puff/nonpuff differences were observed in the distribution of leucine-injected controls. Labeled neurons for each amino acid included stellate-, fusiform- and pyramidal-shaped cells, each of varying sizes. However, outside the intensely labeled injection sites, no GABA-labeled pyramidal cells were observed.(ABSTRACT TRUNCATED AT 400 WORDS)

GABA concentration and GAD activity levels in normal and degenerated retinas from mice

Freeze-dried sections (14 microns thick) were prepared from mice with normal (C57BL strain) and degenerated (C3H strain) retinas. GABA concentration and GAD activity were determined in the microsamples (1.8-20 ng dry weight) of retinal layers and sublayers, using an enzymatic amplication reaction, NADP cycling. GABA was distributed over all layers of normal retina with a broad concentration peak covering both inner nuclear and plexiform layers. In contrast, GAD activity was mostly localized in the inner plexiform layer. GABA concentration was similar in one-fourth of the sublayers of each inner nuclear or plexiform layer. GAD activity was highest in the innermost sublayer of the inner nuclear layer. An increasing gradient of GAD activity was present in the inward direction in the inner plexiform layer. In the degenerated retina, lacking in photoreceptors, the inner nuclear and plexiform layers remained, and GABA and GAD levels in these layers were similar to those in normal retina.

Demonstration of histaminergic neurons in horizontal cells of guinea pig retina

The existence of L-histidine decarboxylase (HDC, EC 4.1.1.22)-like immunoreactive (HDC-I) cells in guinea pig retina was demonstrated using antiserum raised against HDC purified from fetal rat liver. The anti-HDC antiserum partially cross-reacted guinea pig L-DOPA decarboxylase (DDC, EC 4.1.1.28), so the histaminergic neurons were carefully identified. Comparison of HDC-I and DDC-like immunoreactive (DDC-I) cell types in adjacent sections revealed that HDC-I structures were found in some horizontal cells and amacrine cells, and double-staining procedures with anti-HDC antiserum and monoclonal anti-DDC antibody showed that HDC-I horizontal cells had no DDC-I structures, but all the HDC-I amacrine cells had DDC-I structures. From the results, some horizontal cells (with HDC-like immunoreactivities but without DDC-like immunoreactivities) were concluded to be histaminergic.

gamma-Aminobutyric acid exerts a local inhibitory action on the axon terminal of bipolar cells: evidence for negative feedback from amacrine cells

It is well-established morphologically that bipolar cells, the second-order neurons in the vertebrate retina, make reciprocal synapses with amacrine cells in the inner plexiform layer. However, neither the property nor the physiological function of the feedback synapse is understood. Autoradiographic and immunohistochemical studies suggest the presence of gamma-aminobutyric acid (GABA)-ergic amacrine cells, and therefore the bipolar cells are thought to receive GABAergic inputs from amacrine cells. This possibility was investigated in the present study, in which we used solitary bipolar cells dissociated from the goldfish retina enzymatically. Dissociated solitary bipolar cells showed a large variety in morphology. In the present study, we selected the bipolar cells with a huge bulbous axon terminal. Bipolar cells of this subtype were identical in morphology to the on-center cells with rod-dominant inputs as revealed in earlier studies by intracellular staining. Membrane currents were measured under voltage clamp with a patch pipette in the whole cell configuration. In some experiments, GABA-sensitive membrane was excised as an outside-out patch from the axon terminal bulb of solitary bipolar cells. All cells of this type responded to GABA. The highest sensitivity was located at the axon terminal. The minimal effective dose was on the order of 10(-7) M. GABA increased the chloride conductance and evoked a membrane hyperpolarization. Partial desensitization was observed during the application of GABA. The bipolar cells had GABA type A receptors. These results are consistent with the idea that the rod-dominant on-center bipolar cells receive negative feedback inputs from GABAergic amacrine cells.

Gamma-aminobutyric acid- and glutamic acid decarboxylase-immunoreactive neurons in the retina of different vertebrates

The localization of gamma-aminobutyric acid (GABA)- and L-glutamate 1 carboxy-lyase (GAD)-immunoreactive neurons was compared in the skate, frog, pigeon, chicken, rabbit, and man. Horizontal cells show both GABA and GAD immunoreactivity in the skate, frog, and bird. Certain amacrine cells show GABA and GAD immunoreactivity in all species. The distribution of GABA- and GAD-immunoreactive cell bodies and cell processes was very similar, if not identical, in the skate and man. In the other species, cell populations with GAD immunoreactivity also showed GABA immunoreactivity. However, in the bird, frog, and rabbit, the GABA-immunoreactive amacrine cells were at least twice as numerous as the GAD-immunoreactive cells. In birds, the distributions of the GAD and GABA immunoreactivities were different in the sublayers of the inner plexiform layer. The reason for the difference is currently unknown. GABA-immunoreactive bipolar-like cells were seen in the frog.

Immunocytochemical and autoradiographic localization of GABAergic neurons in the goldfish retina

The putative neurotransmitter gamma-aminobutyric acid (GABA) was localized in goldfish retina by using an antiserum directed against GABA itself. The same types of cells were stained with this antibody as were labelled with an antiserum directed against the synthesizing enzyme for GABA, glutamic acid decarboxylase. Stained neurites of these cells were located throughout the inner plexiform layer (IPL) but staining was more intense in the proximal IPL. The GABA-immunoreactive staining could be reduced or completely abolished by preabsorbing the primary antibody with GABA. Uptake of [3H]-GABA or the GABA agonist [3H]-muscimol was localized in GABA-stained retinas using light microscope autoradiography. These experiments demonstrated that all types of GABA-immunoreactive amacrine cells had high-affinity uptake mechanisms for both [3H]-GABA and -muscimol. Thirty percent of proximal inner nuclear layer (INL) and some cells in the ganglion cell layer (GCL) were labelled by all three GABAergic markers. Most GABA-immunoreactive amacrine cells were lightly labelled due to [3H]-GABA uptake but a few amacrines (Ab) were heavily labelled. These findings demonstrate that the autoradiographic localization of [3H]-GABA or [3H]-muscimol uptake and the immunocytochemical localization of GAD or GABA are appropriate methods for localizing GABAergic neurons in the retina. Few GABA-immunoreactive amacrine cells accumulated the putative amino acid transmitter [3H]-glycine, verifying that the goldfish retina contains distinct subpopulations of glycinergic and GABAergic amacrine cells.

The self-regulating synapse: a functional role for the co-existence of neuroactive substances

Although the co-localizations of neuroactive substances, such as transmitters and peptides, in identified neurons is now a common histochemical phenomenon, the physiological roles and functional significance of such co-existence are largely unknown. Using the vertebrate retina as a model for the central nervous system, we have examined the relationship between co-existence and co-function. We propose here that the co-localization of neuroactive substances in a synaptic terminal provides the structural configuration to ensure the co-release of two or more predetermined substances into the same synaptic cleft, resulting in the capability of the presynaptic neuron to stringently regulate its own activities and output.

Light and dark adaptation influences GABA receptor sites in the chick retina

The aim of the present study was to investigate the effect of environmental conditions such as light-and-dark-adaptation on the plasticity of GABA receptor sites in the chick retina. In chicks exposed to light for 5 hr (light-adapted), specific [3H]GABA binding was increased by 35% in comparison to the binding found in chicks maintained in darkness (dark-adapted). Conversely, in the retina of chicks exposed to darkness for 5 hr, specific [3H]GABA binding was decreased by 28% with respect to that found in chicks kept in the light. Scatchard analysis of the binding data revealed that the affinity of GABA for its receptor binding site was higher in the retinas of light-adapted chicks than in those of dark-adapted chicks (Kd values of 19.20 +/- 1.23 and 27.20 +/- 1.47 nM, respectively). On the contrary, the maximal number of binding sites (Bmax) remained unchanged in light- and dark-adapted chicks (5.2 +/- 0.10 and 5.3 +/- 0.15 pmol/mg protein, respectively). These results suggest the involvement of GABA receptors in the regulation of visual function.

GABA-like immunoreactivity in the vertebrate retina: a species comparison

Rabbit antisera directed against gamma-amino butyric acid (GABA) conjugated to bovine serum albumin was used to localize neurons containing GABA-like immunoreactivity in the retinas of nine species of animals: human, cat, rabbit, rat, chicken, turtle, frog, mudpuppy and goldfish. The retinas of all species contained GABA-like labeling in several populations of amacrine cells in the inner nuclear layer, cells in the ganglion-cell layer that may include displaced amacrine cells and in fibers in the inner plexiform layer and in the optic nerve fiber layer. Labeled horizontal cells were found in cat and in all non-mammalian retinas. Labeled interplexiform cells were found in rat, rabbit, cat and human retinas. Labeled bipolar cells were restricted to frog and mudpuppy retinas. The distribution of anti-GABA is usually similar to that of anti-glutamic acid decarboxylase and neuronal [3H]GABA uptake, indicating good correspondence between these 'GABAergic' markers. However, several significant differences among these markers are discussed.

Autoradiographic localization of high affinity GABA, benzodiazepine, dopaminergic, adrenergic and muscarinic cholinergic receptors in the rat, monkey and human retina

High affinity gamma-aminobutyric acid, benzodiazepine, strychnine (glycine), dopamine, spirodecanone, alpha 1-adrenergic, alpha 2-adrenergic, beta-adrenergic and muscarinic cholinergic binding sites were localized by semiquantitative autoradiography in rat and, in some instances, in monkey and human retinae using [3H]muscimol, [3H]flunitrazepam, [3H]strychnine, [3H]spiperone, [3H]prazosin, [3H]para-aminoclonidine, [3H]dihydroalprenolol and [3H]quinuclidinyl benzylate, respectively. In nearly every case, the inner plexiform layer (IP) contained a high receptor density. The distribution of alpha 1 sites was unusual in that binding was concentrated in the outer plexiform layer (OP). Dopaminergic and, to a lesser extent, beta-adrenergic binding was diffusely distributed in the outer nuclear layer, the OP, the inner nuclear layer and the IP. The ganglion cell layer displayed significant benzodiazepine binding. The intraretinal distribution of pre- and postsynaptic markers of these neurotransmitters is discussed.

GABA and GAD immunoreactivity of photoreceptor terminals in primate retina

Within the vertebrate retina, two types of photoreceptor cells--the rods and cones--transduce visual signals and convey this information through synapses with bipolar and horizontal cells. Although the neurotransmitter at these first-order synapses has not been identified, electrophysiological studies suggest that it might be excitatory. In the present study, however, we have found photoreceptor terminals in the rhesus monkey retina which are immunoreactive with antibodies to either gamma-aminobutyric acid (GABA) or L-glutamic acid decarboxylase (GAD, an enzyme involved in the synthesis of GABA). In the perifoveal region of the retina, approximately 25% of presynaptic profiles having ultrastructural characteristics of either rod or cone terminals are immunoreactive with one or the other antibody. This evidence for a putatively inhibitory neurotransmitter in photoreceptor terminals challenges present understanding of retinal synaptic function.

Dopaminergic regulation of cone retinomotor movement in isolated teleost retinas: II. Modulation by gamma-aminobutyric acid and serotonin

In the accompanying paper we reported that 3,4-dihydroxyphenylethylamine (dopamine) induced light-adaptive retinomotor movements in teleost photoreceptors and that this effect was mediated by D2 dopamine receptors located on the photoreceptors themselves. In this study, we investigated the effects on cone retinomotor movement of three agents that have been reported by others to modulate retinal dopamine release: gamma-aminobutyric acid (GABA), 5-hydroxytryptamine (5-HT, serotonin), and melatonin. We report here that the GABA antagonists bicuculline and picrotoxin induced light-adaptive cone contraction in dark-adapted green sunfish retinas cultured in constant darkness; thus they mimic the effect of light or exogenously applied dopamine. Since their effects were blocked by either the D2 dopamine antagonist sulpiride or by Co2+, it seems likely that these agents act by enhancing retinal dopamine release. The GABA agonist muscimol produced effects opposite to those of GABA antagonists. Muscimol inhibited light-induced cone contraction in previously dark-adapted retinas and induced dark-adaptive cone elongation in light-adapted retinas. These results suggest that in green sunfish retinas, as has been reported for other retinas, GABA inhibits dopamine release. 5-HT induced light-adaptive cone contraction in dark-adapted retinas; thus 5-HT also mimics the effect of light or exogenously applied dopamine. The effect of 5-HT was blocked by sulpiride, Co2+, or the 5-HT antagonist mianserin. These results suggest that 5-HT induces cone contraction by stimulating dopamine release. Melatonin neither inhibited dopamine-induced cone contraction in retinas cultured in the dark nor induced cone elongation in retinas cultured in the light. Our results suggest that both GABA and 5-HT (but not melatonin) affect cone retinomotor movements in green sunfish by modulating dopamine release: GABA by inhibiting and 5-HT by stimulating dopamine release. We report in the companion paper that dopamine induced contraction in isolated cone fragments. Together these observations strongly suggest that dopamine serves as the final extracellular messenger directly inducing light-adaptive cone retinomotor movement, and that GABA and 5-HT affect these movements by modulating dopamine release.

Blocking effects of cobalt and related ions on the gamma-aminobutyric acid-induced current in turtle retinal cones

Red-sensitive cone photoreceptors were isolated from the turtle retina, and GABA-induced currents were recorded under voltage clamp. The effect of Co2+, widely used as a blocker of chemical synapses, on the GABA-induced current was studied. Co2+ blocked the GABA-induced current evoked by local application either at the synaptic region (cone pedicle) or at the extra-synaptic region (cell body). 5 microM-Co2+ suppressed the GABA-induced current by 50%, and a few hundred microM-Co2+ blocked it almost completely. Co2+ suppressed the GABA-induced current non-competitively: the saturating response amplitude decreased without a change in the threshold or saturating dose of GABA. The blocking was not voltage dependent in the physiological range of the membrane potential. Ni2+ and Cd2+ also blocked the GABA-induced current non-competitively, and were as effective as Co2+. Tetraethylammonium (25 mM) showed a similar but weaker blocking effect. On the other hand, Mg2+ (20 mM), Mn2+, Sr2+, Ba2+ (10-100 microM each), D-600 (10 microM) or Cs+ (10 mM) did not affect the GABA-induced current. The Ca current in the turtle cones was blocked almost completely by 20 mM-Mg2+ or 4 mM-Co2+, or strongly suppressed by 10 microM-D-600. However, Cd2+ and Ni2+ (10 microM each) blocked the Ca current by ca. 50%, and Co2+ and Mn2+ (10 microM each) suppressed it only partially. The blocking of the GABA-induced current by these agents was, therefore, not directly related to the blocking of the Ca current and/or Ca-mediated currents. These observations present a warning on the use of some divalent cations, such as Co2+, Ni2+ or Cd2+, as a presynaptic blocker at the GABAergic synapse. High concentrations of Mg2+ are recommended as a more appropriate blocker.

Effects of gamma-aminobutyric acid on isolated cone photoreceptors of the turtle retina

Isolated cones dissociated from the retina of the freshwater turtle were voltage clamped using a single 'patch' pipette electrode. gamma-Aminobutyric acid (GABA) applied ionophoretically to the axon terminal evoked an inward current in cells held at -66 mV when they were recorded with patch pipettes filled with the 'control' pipette solution containing 120 mM-Cl-. Polarity of the GABA-induced current reversed near 0 mV when examined with the pipette filled with the control pipette solution. The reversal potential depended strongly on both external and intrapipette Cl- concentrations ([Cl-]o, and [Cl-]p). The reversal potential agreed with the equilibrium potential for Cl- calculated by the Nernst equation with given [Cl-]o and [Cl-]p. The reversal potential was not affected by concentrations of either external Na or K ions. Voltage responses evoked by GABA were hyperpolarizing from its resting level of about -50 mV immediately after the rupture of the patch membrane. The response polarity reversed into depolarization in a few seconds when [Cl-]p was greater than 24 mM, while hyperpolarizing responses persisted when [Cl-]p was less than 12 mM. Thus, the intracellular Cl- concentration of undisturbed isolated cones was estimated to be between 12 and 24 mM. Cones were desensitized to GABA in the presence of GABA (greater than 100 nM) in the medium, or by a prolonged ionophoretic application. The maximum reduction in response amplitude was about 70% in both experiments. Muscimol was as potent as GABA, while beta-p-chlorophenyl-GABA (baclofen) was ineffective even at 100 microM. GABA was antagonized by bicuculline competitively, and by picrotoxin non-competitively. These observations suggest that turtle cones have GABAA receptors which associate with chloride channels. The present results suggest that GABA, presumably released continuously from monophasic horizontal cells in the dark, would exert a tonic hyperpolarization in red-sensitive and green-sensitive cones. Suppression by light of tonic GABA release would depolarize these types of cones by disinhibition. Disinhibitory depolarization in cones may contribute to the centre surround antagonism in retinal neurones, and to the biphasic colour responses recorded in a subtype of horizontal cells.

Comparative distribution of 3H-GABA uptake and GAD immunoreactivity in goldfish retinal amacrine cells: a double-label analysis

The comparative distribution of 3H-GABA uptake and glutamic acid decarboxylase immunoreactivity (GAD-IR) in amacrine cells of goldfish retina was studied simultaneously by using a combined autoradiographic/immunocytochemical technique in order to determine the degree of colocalization of these two markers of GABAergic neurons; 3H-GABA was taken up most intensely by large pyriform Ab amacrine cells (3% of inner nuclear layer (INL) somata), and less intensely by smaller, polyform amacrine cells (12% of INL somata), cell bodies in the ganglion cell layer (one-half as common as Ab cells), and cell bodies in the inner plexiform layer (very rare). GAD-IR was observed in 25% of amacrine cells in the INL, the vast majority of which were polyform in shape, and cell bodies in the ganglion cell layer. Twice as many cells were labeled for GAD-IR as for 3H-GABA uptake. Of the cells that took up 3H-GABA, colocalization of 3H-GABA uptake and GAD-IR was observed in 90% of the polyform amacrine cells, 80% of the cells in the ganglion cell layer, and none of the pyriform Ab amacrine cells or cells in the inner plexiform layer. We suggest that the polyform cells compose the major population of GABAergic amacrine cells in the goldfish retina, rather than the pyriform Ab cells. GABAergic displaced amacrine cells or ganglion cells are also indicated by our data. The implications of these data with regard to the physiology of the goldfish retina are discussed as well.