N-methyl-D-aspartate-mediated glutamate toxicity in the developing rabbit retina

Extracellular levels of endogenous glutamate are relatively high in the developing rabbit retina but nonetheless appear to promote cell survival and developmental processes at concentrations considered toxic in the adult. We wished to examine the development of retinal susceptibility to glutamate toxicity as well as the protective effects of two N-methyl-D-aspartate (NMDA) antagonists, 2-amino-5-phosphono-5-valeric acid (APV) and dextromethorphan (Dex), and the nitric oxide synthase (NOS) inhibitor, NG-methyl-L-arginine (metARG). One day in vitro retinal explants of adult and neonatal rabbits were incubated with various agonists and antagonists, and stained with trypan blue to visualize necrotic cells. The density of the necrotic cells was analyzed using the Zeiss Videoplan 2. Immature neurons were approximately 10-fold less sensitive to NMDA toxicity compared to the adult. Although both NMDA antagonists and metARG provided marked protection for adult retinal neurons against glutamate toxicity, the modest susceptibility of the immature neuron was blocked only by Dex and not APV or metARG. At least two factors may contribute to the ability of the neonatal retina to survive in the presence of high levels of endogenous extracellular glutamate. First, the 10-fold developmental increase in NMDA toxicity occurs simultaneously with a 12-15-fold downregulation of extracellular glutamate, probably through the actions of maturing Muller cells. Second, the NMDA/NO excitotoxic pathway may not be active at birth since an NOS inhibitor had little effect at this stage and our previous morphological data demonstrate that NOS-containing cells are not present in their mature configuration until the second postnatal week.

GABA and GABA-A receptors are maximally expressed in association with cone synaptogenesis in neonatal rabbit retina

Previous studies have shown the cone photoreceptors form reciprocal synapses with horizontal cells during the first week after birth in rabbits. These synapses constitute pioneering elements of the developing outer plexiform layer. We now report that antibodies against the alpha-1 and against the beta-2/3 subunits of the GABA-A receptor label a highly restricted sublamina in the developing outer plexiform layer known to contain nascent cone photoreceptor terminals. Staining is relatively weak at birth, increases to maximal levels between postnatal days 5 and 7, and is significantly reduced in the adult. These results support recent calcium imaging studies which have shown that the activation of GABA-A receptors causes an increase in intracellular free calcium in cones, an effect which is observed only at 3-9 days after birth. The transient expression of GABA-A receptors in this region coincides with the period of peak expression of GABA immunoreactivity in horizontal cells. A direct functional link between GABAergic transmission and cone synaptogenesis is suggested by previous reports that GABA-A receptor antagonists cause disruption of cone synaptogenesis. Together these findings support the notion that GABA functions as a developmental neurotransmitter which is produced by horizontal cells and interacts with developing cone axons in order to facilitate synaptic linkage between these two cells types.

GABA-induced increases in [Ca2+]i in retinal neurons of postnatal rabbits

Previous studies have indicated that gamma-aminobutyric acid (GABA) plays an important trophic role in the synapse formation between horizontal cells and photoreceptors in postnatal rabbit retina. However, the mechanism of the GABA effect has not been identified. Using fluo-3 Ca2+ imaging and confocal laser scanning microscopy we examined the effect of GABA on [Ca2+]i during postnatal retinal development. GABA (100 microM) evoked a fast and transient increase of [Ca2+]i in selected populations of freshly dissociated retinal cells from postnatal rabbits. This increase was apparent on postnatal day 1 and reached a maximum on day 5. Little increase in [Ca2+]i was observed in retinal cells isolated from adult rabbits. GABA receptor antagonists, picrotoxin and bicuculline, significantly reduced the response. The GABAB agonist, baclofen, did not evoke any [Ca2+]i changes. The GABA-induced increase in [Ca2+]i was observed in all retinal layers in neonatal retinal whole-mount explants. In the outer retina, the increase was seen in cone photoreceptors which were specifically labeled with peanut agglutinin (PNA). The GABA-induced increase in [Ca2+]i may provide an important mechanism for regulating cone synaptogenesis in the outer plexiform layer of the postnatal retina.

Expression and downregulation of the GABAergic phenotype in explants of cultured rabbit retina

PURPOSE

To study the morphologic and neurochemical development of the rabbit retina in explant culture.

METHODS

Explants of retina from newborn rabbits were cultured in defined medium in the absence of serum or soluble growth factors. The morphology of the explant and the neurochemical development of the GABAergic system were examined by light microscopy, autoradiography, and immunohistochemistry for 7 days and compared to those of the postnatal rabbit retina in vivo.

RESULTS

Cultured explants from newborn rabbit retina develop and maintain well-defined plexiform and cellular layers up to 7 days. Exogenous 3H-gamma-aminobutyric acid (GABA) and antibodies against GABA labeled a population of horizontal, amacrine and displaced amacrine cells in the ganglion cell layer during the first 3 days in culture. After 4 days in culture, the extent of uptake and immunolabeling was diminished among all three cell types, but labeled horizontal cells were markedly rare. At 7 days in culture, uptake and GABA-like immunoreactivity could not be detected in horizontal cells, but antibodies to calbindin-D reacted with horizontal and amacrine cells in the appropriate retinal layers. Peanut agglutinin lectin binding studies revealed a mosaic of cone photoreceptor inner segments indistinguishable from that of neonatal retina in vivo.

CONCLUSIONS

The experiments show that the maturation of cellular layers and the developmental expression of the GABAergic phenotype can be observed in retinal explants cultured under chemically defined conditions. Histochemical evidence is presented that indicates cultured explants of newborn rabbit retinas express markers of the GABAergic phenotype in a manner consistent with that observed in vivo. The authors show that horizontal cells continue to survive in culture after the diminution in GABA immunoreactivity.

High levels of extracellular glutamate are present in retina during neonatal development

The three major classes of neurons which comprise the primary visual pathway in retina are glutamatergic. These cells are generated in two separate developmental stages, with one subclass of photoreceptors (cones) and ganglion cells generated before birth; and the other subclass of photoreceptors (rods) and bipolar cells generated during the first week after birth. Gas chromatography/mass spectroscopy analysis coupled with a new method for collecting small samples of extracellular fluids from retina were used to determine the levels of endogenous glutamate present during differentiation and synaptogenesis of these different cell types. As expected the total retinal content of glutamate increased during the postnatal period in synchrony with the generation and maturation of glutamatergic cells. However, a significant proportion of the endogenous pool was found extracellularly at birth. Intracellular glutamate is localized within cell bodies and growing processes of cones and ganglion cells at this time but few glutamatergic synapses are present. The extracellular concentration of glutamate actually declined during the most active period of synaptogenesis, reaching very low levels in the adult. The high concentrations of extracellular glutamate in neonatal retina could play an important role in a variety of developmental events such as dendritic pruning, programmed cell death and neurite sprouting.

Calbindin immunoreactivity of horizontal cells in the developing rabbit retina

Horizontal cells are retinal interneurons that establish inhibitory feedback loops within the outer plexiform layer of the primary visual pathway. Most mammalian retinas contain two types of horizontal cells. A-type horizontal cells have neuritic branches that contact cone photoreceptors exclusively, while the B-type horizontal cells have dendritic branches that contact cones, in addition to axons that form synapses with rod photoreceptors. Immunoreactivity for calbindin, a calcium binding protein involved in calcium transport, was used as a marker for horizontal cells during post-natal development of the rabbit retina. On post-natal days 1, 3 and 5, calbindin immunoreactivity is limited to a single population of A-type horizontal cells. They appear as a monolayer of cells with broad tapering processes, establishing the proximal border of the nascent outer plexiform layer and forming a target for ingrowing cone photoreceptor terminals. The size and density of the cell bodies and the length of neuritic processes are essentially unchanged during this period, which corresponds to the time of peak expression of GABAergic markers in horizontal cells. Coincident with a decrease in GABAergic markers and the completion of cone-to-horizontal cell synaptogenesis by day 7, changes within the horizontal cell mosaic are detected morphometrically. A delayed phase of overall cell growth results in a 70% increase in average somal diameter (representing a 3.7-fold increase in spherical volume), a six-fold increase in mean neurite length and a decrease in cell density to one-third of that found in the newborn. We conclude that the process of terminal differentiation of horizontal cells is not complete until some time after the second post-natal week. Furthermore, the expression of GABAergic markers is associated primarily with early maturational events, whereas expression of calbindin is sustained throughout post-natal development, suggesting a prominent role for calcium dependent mechanisms at all development stages.

Modulatory actions of gamma aminobutyric acid (GABA) on GABA type A receptor subunit expression and function

Gamma aminobutyric acid (GABA) is present in the central nervous system (CNS) during very early embryogenesis. It is therefore likely to play a role not only as a neurotransmitter but also as a signal molecule for neuronal differentiation, growth, and development. It has been firmly established that formation of synapses is strengthened by GABA, and the expression of certain subunits of the GABA type A (GABAA) receptor complex is clearly promoted by GABA. This latter effect of GABA may have profound implications for the functional activity of GABAergic synapses since the pharmacological properties of GABAA receptors are governed by the subunit composition of the receptor complex. Dynamic changes in GABAA receptor expression and diversity during development and differentiation may therefore play important roles for the inhibitory potential of the CNS during mature stages.

The role of GABA during development of the outer retina in the rabbit

Horizontal cells are among the first to mature in the neonatal mammalian retina and they are the first to establish the position of the outer synaptic layer which is subsequently formed by invading terminals of both rod and cone photoreceptors. During the period of cone synaptogenesis, horizontal cells transiently express the full complement of GABAergic properties (uptake, release, synthesis and storage of GABA); later during development of rod terminals, these properties are down-regulated. Given the reports of GABA's role in other developing neuronal systems (for review: 10), we have examined the effect that GABA, produced from horizontal cells, might have on photoreceptor maturation in rabbit retina. Results from our previous studies show that lesioning the horizontal cell with kainic acid in vivo leads to a displacement of cone photoreceptor cells and a disappearance of their synaptic terminals, while rod cells maintain their normal position and produce an overabundance of terminals. Similar effects are seen with the GABA-A receptor antagonists, picrotoxin and bicuculline. New evidence from 3H-thymidine studies suggests that the effects of kainic acid are specific and that cell division, migration and differentiation in other cell types do not appear to be affected. This body of work is summarized and possible mechanisms of action are suggested which could account for the apparent ability of GABA to help maintain the normal position of cone cell bodies and regulate cone synaptogenesis.

gamma-Aminobutyric acid immunoreactivity in multiple cell types of the developing rabbit retina

We have previously demonstrated that the neonatal rabbit retina contains a larger complement of cells that accumulate [3H]-GABA than does the adult. In order for these neurons to be classified as GABAergic, they must also contain endogenous GABA. We now report that these same neonatal cell populations are also immunoreactive to GABA antisera. In frozen sections from rabbit retina, treated with GABA antisera, immunoreactive processes in both synaptic layers were observed at postnatal day 1. The appearance of immunofluorescent fibers precedes that of photoreceptor and bipolar cell terminals in the outer plexiform layer and is diminished by postnatal day 5. Also noted, was a 50% decrease in the density of GABA-immunoreactive cell bodies in the inner nuclear and ganglion cell layers, accompanied by an increase in cell volume and a shift toward a more spherical cell shape of the remaining cells. At postnatal day 1 and 3, we also observed immunoreactive cells having the characteristic morphology of interplexiform cells. This cell type sends branches to both the outer and inner plexiform layers, thus a morphological basis for communication between the two developing plexiform layers is present as early as postnatal day 1. Thus, retinas from neonatal rabbits have a larger complement of cells that stain for endogenous GABA than does the adult. These results coupled with our previous studies suggest that GABAergic properties are expressed by a larger number of cell types in the neonate than in the adult and are consistent with the general hypothesis that GABA functions as a trophic agent during development.

Development of the glutamate system in rabbit retina

We have investigated two characteristics of the glutamate system in the developing rabbit retina. 1) Glutamate immunoreactivity was observed at birth within developing processes of four cell types; two of which, photoreceptors and ganglion cells, are known to be glutamatergic in the adult. Two other cell types, type A horizontal cells and amacrine cells, are immunoreactive to both glutamate and GABA at birth, suggesting that endogenous pools of glutamate in GABAergic neurons serve as precursor for GABA synthesis. Thus it appears that endogenous glutamate pools are present within neurons prior to synaptogenesis as part of the early expression of either the glutamate or GABA transmitter phenotype. 2) Analysis of 3H-glutamate metabolism during retinal development showed that rapid conversion of glutamate to glutamine does not occur until the second postnatal week, coincident with the expression of Muller (glial) cell activity. In the absence of glial metabolism in the neonate, extracellular concentrations of glutamate remain relatively high and are likely to have major effects on neuronal maturation.

Melatonin inhibits ACh release from rabbit retina

Previous studies have suggested that melatonin, released from photoreceptors, may modulate retinal dark-adaptive responses by inhibition of dopamine release from retinal interneurons. We have broadened these studies to examine the effect of melatonin on release of another retinal neurotransmitter, acetylcholine (ACh). The ACh system in rabbit retina has been localized to starburst amacrine cells, which release ACh in response to a variety of experimental stimuli, including direct potassium depolarization, flashing light, and glutamatergic as well as GABAergic inputs. The effect of melatonin on release of endogenously synthesized [3H]-ACh was measured in perfusates from retinas or retinal synaptosomes preloaded with [3H]-choline chloride. Melatonin significantly inhibited ACh release stimulated by potassium in intact retina but not in synaptosomes. Stimulation of intact retina by flashing light or by the glutamate receptor agonist, kainic acid, was also inhibited by melatonin. In contrast, there was no significant effect of melatonin on picrotoxin-induced release. These findings suggest that melatonin does have an inhibitory effect on ACh release, either by direct interaction with the cholinergic amacrine cell, or indirectly via GABAergic but not glutamatergic neurons.

Acetylcholine release from the rabbit retina mediated by kainate receptors

The cholinergic amacrine cells of the rabbit retina may be labeled with 3H-choline (3H-Ch), and the activity of the cholinergic population may be monitored by following the release of 3H-ACh. Glutamate analogs caused massive ACh release, up to 50 times the basal efflux, with the following rank order of potency: alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) greater than quisqualate (QQ) = kainate (KA) much greater than NMDA (in magnesium-free medium) much greater than glutamate greater than aspartate. In contrast, the release of 3H-Ch was unchanged. Submaximal doses of each agonist were used to establish the specifity of glutamate antagonists. Kynurenic acid was selective for KA much greater than QQ, and 6,7-dinitroquinoxaline-2,3-dione (DNQX) was selective for KA greater than QQ much greater than NMDA. At low doses, which selectively blocked the response to KA, both antagonists blocked the light-evoked release of ACh. These results suggest that ACh release may be produced via several glutamate receptors, but the physiological input to the cholinergic amacrine cells is mediated by KA receptors. Because these cells receive direct input from cone bipolar cells, this work supports previous evidence that the bipolar cell transmitter is glutamate.

Evidence for functionally distinct subclasses of gamma-aminobutyric acid receptors in rabbit retina

gamma-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the mammalian retina, where it serves many roles in establishing complex response characteristics of ganglion cells. We now provide biochemical and physiological evidence that at least three subclasses of GABA receptors (A1, A2, and B) contribute to different types of synaptic integration. Receptor binding studies indicate that approximately three-fourths of the total number of [3H]GABA binding sites in retina are displaced by the GABAA receptor antagonist, bicuculline, whereas one-fourth are displaced by the GABA-B receptor agonist, baclofen. GABAA receptors can be described by a three-site binding model with KD values of 19 nM, 122 nM, and 5.7 microM. Benzodiazepines and barbiturates potentiate binding to the GABAA site, which suggests that significant numbers of GABAA receptors are coupled to regulatory sites for these compounds and thus are classified as GABAA1 receptors. The response to pentobarbital appears to involve a conversion of low-affinity sites to higher-affinity sites, and is reflected in changes in the densities of sites at different affinities. Functional studies were used to establish which of the different receptor subclasses regulate release from cholinergic amacrine cells. Our results show that GABA suppresses light-evoked [3H]acetylcholine release via GABAA2 receptors not coupled to a benzodiazepine or barbiturate regulatory site, and enhances release via GABAB receptors. GABAA1 sites do not appear to control acetylcholine release in rabbit retina.

Kainic acid lesioning alters development of the outer plexiform layer in neonatal rabbit retina

The first synaptic relay in the primary visual pathway occurs between terminals of photoreceptors and second-order neurons within the outer plexiform layer of the retina. During development, one of these types of second-order neurons, the type A horizontal cell, differentiates and assumes mature characteristics several days before any other cells ramifying in that synaptic layer. In neonates, horizontal cells appear to be GABAergic during the first 5 days of postnatal life and in addition they also are responsive to kainic acid. We have previously suggested that they may play a pioneering role in the postnatal development of the outer plexiform layer, perhaps providing structural guidance or trophic substances such as GABA, for synaptic development. To test this hypothesis, we first demonstrated that a single intraocular injection of kainic acid within 24 hr of birth results in a permanent and selective loss of type A horizontal cells in the outer retina. Retinas from animals maintained for 5 days postinjection were harvested for analysis of postnatal development of the outer plexiform layer in the absence of horizontal cells. One of the major findings was that kainic acid treatment caused a reversal of the normal complement of photoreceptor cell types, resulting in an abnormally high rod/cone ratio. The distribution of cell processes within the outer plexiform layer was also altered and normal synaptic connections were not made. In spite of these changes in the constituents of the outer plexiform layer, the normal position of the synaptic layer was not affected by the loss of horizontal cells. These results rule out the possibility that horizontal cells provide a structural barrier which is an absolute requirement for establishing the location of the outer plexiform layer. Rather, these cells may be more involved in cell differentiation and synaptogenesis.

Darkness stimulates rapid synthesis and release of melatonin in rat retina

The presence of melatonin in retina has been widely reported for over two decades although studies of its functional importance within the retina have only recently been emphasized. We have analyzed the biochemical characteristics of melatonin synthesis and release, focusing on rapid changes in response to light/dark conditions. Our major findings are consistent with the following conclusions: (1) melatonin synthesis is stimulated within minutes after exposure to darkness, and may reflect an increase in N-acetyl transferase activity; (2) melatonin is not stored, but rather it diffuses freely throughout the retina immediately after it is synthesized; and (3) the dark-induced increase in retinal melatonin release is a synthesis-coupled response and does not involve separate secretion mechanisms. The characteristics of melatonin synthesis and release described herein would be consistent with the proposed role of melatonin as a local paracrine effector of dark-adaptive responses in retina.

AP4 inhibits chloride-dependent binding and uptake of [3H]glutamate in rabbit retina

Glutamate is one of the major neurotransmitters used by primary and secondary neurons of the visual pathway in retina. AP4(2-amino-4-phosphonobutyric acid) preferentially blocks the activity of one functional subclass of retinal neurons, ON bipolar cells, apparently by acting as an agonist at a hyperpolarizing glutamate receptor. We have used in vitro binding assays to examine different subclasses of presumptive glutamate receptors in retinal membrane fractions. One subclass consists of AP4-sensitive binding sites which require calcium and chloride for maximal binding and which are inhibited by freeze-thaw procedures. In addition, AP4 inhibits chloride-dependent [3H]glutamate uptake into retinal synaptosomes and intact retina. [3H]glutamate which is accumulated via the AP4-sensitive mechanism can be subsequently released by depolarizing levels of potassium. The pharmacological selectivity of AP4-sensitive glutamate receptors on ON bipolar cells measured electrophysiologically is very similar to that of AP4-sensitive, [3H]glutamate binding and uptake, measured biochemically in subcellular fractions. These results raise the possibility that AP4-sensitive glutamate recognition sites in retina may be linked to two separate effectors, one which gates ion channels and leads to hyperpolarization, and another which acts as a glutamate transporter.

Neurotransmitter systems in the outer plexiform layer of mammalian retina

Melatonin represents a second type of chemical signal released from photoreceptors in response to increased darkness, one with characteristics which are significantly different from those of glutamate. Concise spatial and temporal aspects of the photoreceptor signal are conserved through discrete glutamatergic synapses. Different classes of post-synaptic neurons each have appropriate subclasses of glutamate receptors which transmit sign conserving or sign inverting images of the visual mosaic. In contrast, melatonin, because of its highly lipophilic nature is not released by stimulus-coupled secretion mechanisms, but rather by simple diffusion. Thus control of melatonin "release" may be less concise than glutamate. In addition, melatonin may diffuse beyond the confines of the synaptic area to target cells throughout the retina. Effects of melatonin in retina are not well understood; however, current hypotheses suggest that, perhaps via its control of dopamine systems in the inner retina, melatonin plays an important role in dark adaptation and in various retinal processes which exhibit a circadian rhythm. Melatonin and glutamate may represent "co-transmitters" which provide the visual pathway with two types of signals, with melatonin providing widespread modulatory influences on the discrete visual information conveyed via glutamatergic circuits.

Synaptosomal neurotransmitter uptake systems in the retina and brain nuclei of light- and dark-adapted rabbits

High affinity uptake rate for [14C]aspartate and [3H]dopamine by retinal homogenate (H), Pl (outer plexiform layer, OPL), and P2 (inner plexiform layer, IPL) retinal synaptosomal fractions were not significantly different between light- and dark-adapted rabbits. However, there were significant increases in the dark in [3H]gamma-aminobutyric acid high affinity uptake rate by retinal H and P2 but not that of Pl. There was a significantly higher [3H]choline uptake rate by retinal H, Pl and P2 in the dark-adapted compared to light-adapted rabbits, but there was no significant change in this rate for synaptosomal fractions from the lateral geniculate body, superior colliculus, visual cortex (VA I + II), caudate nucleus (CN) and hippocampus (HP). Data obtained in this study, along with reports of others, indicate that the change in retinal neurotransmission functions may not always be parallel with the change in high affinity uptake rates of neurotransmitters by retinal synaptosomal fractions. Data obtained indicate an increase in retinal cholinergic neuronal activities in the dark and indicate that optic nerves are not cholinergic and cholinergic neurons in brain nuclei, such as VA, CN and HP, are not significantly influenced by optic nerve inputs in light and dark conditions.

An indoleamine system in photoreceptor cell terminals of the Long-Evans rat retina

Uptake of 3H-serotonin is localized to the outer plexiform layer in Long-Evans rat retinas. Autoradiographic accumulation is seen only after in vitro incubation in the light, with retinas isolated from the underlying sclera. Potassium stimulates the release of 3H-serotonin. In this species, amacrine cells do not accumulate these compounds; thus the outer plexiform layer appears to be the only site of uptake and release of this indoleamine. The age-related loss of 3H-serotonin accumulation in the outer plexiform layer of retinal dystrophic rats coincides temporally with the spontaneous degeneration of photoreceptor cells that occurs in this species. Electron-microscopic autoradiography of 3H-serotonin accumulation further confirms that uptake is localized to rod and cone terminals in the outer plexiform layer. The specific accumulation of indoleamines into rod and cone terminals that is observed in the light but is absent in darkness suggests that indoles have an important physiological role in photoreceptors.

Glutamate receptor agonists release [3H]GABA preferentially from horizontal cells

A total of 5-6 different cell types in vertebrate retinas accumulate [3H]gamma-aminobutyric acid (GABA). In frog retina, specific populations of cells in the horizontal, amacrine and ganglion cell layers are labeled autoradiographically after a 15-min in vitro incubation with [3H]GABA. Cells which may be bipolar or interplexiform cells are also labeled. Similar autoradiographic patterns are observed in chick retina except for the absence of labeled bipolar or interplexiform cells. In rat retinas, [3H]GABA uptake is limited primarily to Muller and amacrine cells. Depolarizing glutamate receptor agonists (glutamate, aspartate and kainic acid) applied in an in vitro perfusion system, stimulated massive release of [3H]GABA from frog and chick retina but not from rat retina. Under these conditions, autoradiographic labeling of horizontal cells was virtually depleted, while labeling of other cell types remained robust. In contrast, potassium caused release of the label from all 3 types of retina, and loss of autoradiographic labeling occurred uniformly in all cell types. We conclude that [3H]GABA-accumulating horizontal cells possess depolarizing glutamate receptors and that activation of these receptors leads to a release of GABA stores. On the other hand, Muller cells and the various subclasses of [3H]GABA-accumulating amacrine, bipolar and/or interplexiform cells, do not release GABA in response to glutamate receptor stimulation and thus appear to be relatively insensitive to excitatory amino acids.

Postnatal development of 3H-GABA-accumulating cells in rabbit retina

Light and electron microscopic autoradiography demonstrates that 3H-GABA is accumulated by horizontal cells in neonatal rabbit retina but not in the adult. A specific population of horizontal cells appears to be mature at birth and they avidly accumulate 3H-GABA during a 15-minute incubation period in vitro. Uptake into horizontal cells is not observed after the fifth postnatal day; 3H-GABA-accumulating horizontal cell bodies and their processes are the first identifiable components that clearly mark the future location of the outer plexiform layer at birth and as such, may be considered pioneering elements. Our observations raise the interesting possibility that the pioneering horizontal cell may provide structural and/or chemical factors necessary for the subsequent development of the outer plexiform layer of the retina. Labeling patterns of other retinal cells also show varying degrees of change during development. A population of amacrine cells accumulate 3H-GABA at birth. These cells show little change in their morphological or 3H-GABA uptake properties from birth to adulthood. Müller cells show weak accumulation of 3H-GABA at birth. Subsequent to this time, labeling of Müller cells is significantly more robust, resulting in Müller cell domination of retinal autoradiographic patterns in more mature retinas. Every cell body in the ganglion cell layer accumulates 3H-GABA at birth. The number of labeled cells declines during postnatal development, resulting in a very limited adult population. We conclude that the ability of retinal cells to accumulate 3H-GABA does not remain constant during postnatal development; rather each cell population displays a unique maturation sequence that results in a dramatic developmental shift in the number and types of GABA-accumulating cells present in the retina.

Light evoked release of acetylcholine in response to a single flash: cholinergic amacrine cells receive ON and OFF input

By pharmacologically blocking the inhibitory inputs to the cholinergic amacrine cells of the rabbit retina, we have been able to detect the light-evoked release of ACh in response to a single flash. Under these conditions ACh is released equally at light ON and light OFF. This implies that: the cholinergic amacrine cells receive ON and OFF input; they respond to light with depolarizing transients; and the inputs to this system have a basic symmetry.

Analysis of pre- and postsynaptic factors of the serotonin system in rabbit retina

[3H]Serotonin is accumulated by a specific set of amacrine cells in the rabbit retina. These cells also accumulate the neurotoxin, 5,7-dihydroxytryptamine, and show signs of necrosis within 4 h of in vivo exposure to the drug. Biochemical analysis of [3H]serotonin uptake reveal a sodium- and temperature-dependent, high affinity uptake system with a Km of 0.94 microM and Vmax of 1.08 pmol/mg protein/min. [3H]Tryptophan is also accumulated in rabbit retinal homogenates by a high affinity process. Accumulated [3H]serotonin is released in response to potassium-induced depolarization of intact, isolated retinas. In vitro binding studies of rabbit retinal homogenate membranes demonstrate specific sets of binding sites with characteristics of the postsynaptic serotonin receptor. These data strongly suggest that rabbit retina contains virtually all of the molecular components required for a functional serotonergic neurotransmitter system. The only significant difference between the serotonin system in rabbit retina and that in the well-established serotonin transmitter systems in nonmammalin retinas and in brains of most species is the relatively low concentration of endogenous serotonin in rabbit retinas, as demonstrated by high-performance liquid chromatography, histofluorescence, or immunocytochemistry.

Serotonin systems in the inner and outer plexiform layers of the vertebrate retina

In retinas of certain nonmammalian vertebrate species such as frog, pigeon, and chick, serotonin appears to function as the neurotransmitter of a specific population of amacrine cells. Neurochemical and morphological studies have demonstrated high endogenous levels of 5-hydroxytryptamine (5-HT) as well as uptake, release, and receptor-binding activity restricted to the inner plexiform layer. In retinas from most mammalian species, uptake, release, and receptor-binding activity have also been localized to amacrine cell terminals in the inner plexiform layer. However, serotonin content in mammalian retinas is low, and attempts to localize the endogenous store of 5-HT have failed. Thus the status of serotonin as a candidate in mammalian retina is still open to question. Our more recent studies have revealed a light-sensitive serotonin system associated with photoreceptor terminals in retinas of Long-Evans rats. Uptake, synthesis, and release of [3H]serotonin have been demonstrated. Endogenous levels of 5-HT decrease in the dark and increase in the light. Electrophysiological studies are needed to illucidate the functional role(s) of serotonin within retinas of different species.

Kainic acid-induced denervation supersensitivity of nicotinic, cholinergic receptors in ganglion cells of the rat retina

The rat retina contains both nicotinic and muscarinic cholinergic receptor sites as demonstrated by specific, high affinity binding of the nicotinic ligand, [3H]-alpha-bungarotoxin, and the muscarinic ligand, [3H]-quinuclidinyl benzylate. Seven days after an intraocular injection of 5 nmol of kainic acid, nicotinic binding was increased three-fold. We suggest that nicotinic sites may be located on ganglion cells because previous studies have shown that many ganglion cells are spared after kainic acid treatment and in fact, have an increased physiological response to ACh under these conditions. The increase in nicotinic sites may reflect a supersensitivity response to the loss of acetylcholine input after the kainic acid lesion. In contrast, muscarinic binding was decreased by 70% after kainic acid treatment. These data suggest that muscarinic sites are located on amacrine cells since these cells are destroyed by kainic acid treatment and some are known to be cholino-receptive. Some of the retinal muscarinic sites may function as inhibitory autoreceptors which regulate acetylcholine release from cholinergic amacrine cells.

Permanent alterations in muscarinic receptors and pupil size produced by chronic atropinization in kittens

Chronic mydriasis was induced in six kittens (four monocular, two binocular) and two adult cats (both monocular) by the daily topical application of atropine. Both the kittens and the adult cats were atropinized for a 13-week period with the treatment regimen beginning at the time of eye opening for the kittens. Pupil size measurements, obtained 1 year after the atropinization were discontinued, revealed that, although the pupils of the adult cats were normal, the pupils of the kittens' treated eyes were consistently smaller than pupils in control eyes. The status of the muscarinic receptors in the kittens' irides was investigated using 3H-QNB binding assays. In comparison with iris muscle homogenates from the control eyes, those from the treated eyes demonstrated an eightfold increase in the number of receptor binding sites. The results indicate that pupil size can be altered permanently by chronic mydriasis initiated early in the life of a kitten and that the permanent change in pupil size may result, in part, from a type of permanent supersensitivity response in the muscle following chronic blockade of muscarinic transmission by atropine.

GABA as a trophic factor during development

The process of synaptogenesis has been studied by many investigators to determine the factors which regulate synapse formation. We have used neonatal rabbit retina to investigate the role of the gamma-aminobutyric acid (GABA) neurotransmitter system during development. By utilizing an in vitro incubation treatment of isolated eyecups we found that treatment with nipecotic acid, a GABA uptake blocker, resulted in a 4-fold increase in the amount of specific 3H-muscimol binding. In addition, incubation of the tissue in the presence of the GABA agonists muscimol, 4,5,6,7-tetrahydroisoxazolo [5,4-c]pyridine-3-ol (THIP), or GABA itself led to similar increases in specific 3H-muscimol binding. The findings support the conclusion from previous studies that the induction of GABA receptors observed after in vivo treatment of 1-day-old rabbits with nipecotic acid resulted from an increase in the extracellular concentration of GABA. A possible role for GABA in the regulation of GABAergic synapse formation is presented in this report.

Intraocular injections of nipecotic acid produce a preferential block of neuronal 3H-GABA accumulation in adult rabbit retina

A procedure by which the activity of the retinal GABA uptake system can be manipulated in vivo has been developed. Intraocular injections of nipecotic acid, a proported GABA uptake blocker, were administered to adult rabbits every 48 hours for a two-week period. No behavioral or systemic changes were observed. Injections were well-tolerated with less than 10% loss of the tissue caused by physical damage or injection. Biochemical analyses demonstrated a dose-dependent inhibition of 14C-GABA uptake into retinal tissue. No effect on uptake was observed for saline-treated tissue. Autoradiographic analyses showed that in vivo treatment with nipecotic acid preferentially blocked accumulation of 3H-GABA into the amacrine cell bodies and processes in the inner plexiform layer. This treatment may be especially useful in assessing the functional significance of GABA transport in vivo.

Synaptic interactions in the GABA system during postnatal development in retina

Using biochemical analyses, we have demonstrated the presence of a high-affinity, sodium- and temperature-dependent uptake system for GABA in the retinas of newborn rabbits. The activity of this system two days after birth is approximately 70 percent of adult values, slowly increasing to adult level by postnatal day 6-8. An intraocular injection nipecotic acid (final concentration = 10 mM) into one-day-old rabbit pups resulted in a 60 percent inhibition in uptake activity. In order to study the possible role of the GABA uptake system in retinal development, we have determined the consequences of blocking GABA uptake with nipecotic acid on the postnatal development of post-synaptic GABA receptors, as measured by 3H-muscimol binding. Nipecotic acid treatment caused a significant increase in receptor binding in retinas prior to eye opening, with the maximal stimulation being one day after the intraocular injection. Our data indicate that the development of GABA receptor sites is influenced by the activity of the GABA uptake system and suggest that GABA may function as a trophic factor in the developing rabbit retina.

Autoradiographic analysis of 3H-glutamate, 3H-dopamine, and 3H-GABA accumulation in rabbit retina after kainic acid treatment

We have previously reported that exposure of isolated rabbit retina to 10(-3) M kainic acid produces profound morphological changes in specific retinal neurons (Hampton et al, 1981). We noted specific swelling of horizontal cell bodies and neurites, necrosis of cell bodies in the amacrine and ganglion cell layers, and swelling of elements in the inner plexiform layer. We now report a differential sensitivity to kainic acid of specific subclasses of amacrine cells autoradiographically labeled with 3H-glutamate, 3H-GABA, or 3H-dopamine. Three different effects were observed: (1) Labeling of neurons after incubation in 3H-glutamate was uniformly reduced while labeling of glia was much less affected. (2) The accumulation of 3H-dopamine was also decreased by kainic acid in two of the three labeled bands of the inner plexiform layer. The outermost labeled band was insensitive to kainic acid at the highest concentration tested (10(-2) M). These findings provide a basis for the subclassification of dopaminergic amacrine cells into at least two subclasses based on their sensitivity to kainic acid. (3) Kainic acid caused a dramatic increase in the labeling of GABAergic amacrine cell bodies and their terminals. This increased intensity may reflect a compensatory increase in uptake activity in response to kainic acid-induced depletion of endogenous GABA stores. These results confirm the highly toxic nature of kainic acid and demonstrate a high degree of specificity and complexity in its action in the retina.

The effects of 2-amino-4-phosphonobutyric acid (APB) on the ERG and ganglion cell discharge of rabbit retina

Perfusion of 100 microM 2-amino-4-phosphonobutyric acid (APB) into the in vivo rabbit eye-cup selectively and reversibly blocked the b-wave of the ERG and all On responses from retinal ganglion cells. In contrast, Off responses were occasionally enhanced, sometimes dramatically. The antagonistic surround inputs to Off ganglion cells, identified by their latency to light stimulation and magnesium sensitivity, were unchanged by APB. These observations suggest that APB selectively blocks depolarizing bipolar cells in rabbit retina in close agreement with the results of Slaughter and Miller (1981) from mudpuppy retina. We conclude that APB may be useful as a pharmacological tool to differentiate On and Off pathways in the rabbit visual system.

The cholinergic amacrine cells of rabbit retina receive on and off input: an analysis of [3H]-ACh release using 2-amino-4-phosphonobutyric acid (APB) and chloride free medium

Using the in vitro rabbit eye-cup we have examined the light-evoked release of ACh from cholinergic amacrine cells under conditions known to eliminate On responses in the retina. APB (100 microM), which blocks the photoreceptor/depolarizing bipolar cell synapse, reduced the light-evoked release of ACh by 80% but a small light-evoked response remained, which was potentiated by bicuculline. Depolarizing bipolar cells are also Cl- dependent. Cl- free medium caused a tenfold Ca2+-dependent increase in the release of ACh but some small light evoked release remained. These results indicate that the cholinergic amacrine cells receive On and Off input. Our findings are consistent with anatomical and electrophysiological evidence which suggests that the displaced cholinergic amacrine cells are On cells and the conventionally placed cholinergic amacrine cells are Off cells.

A tonic gamma-aminobutyric acid-mediated inhibition of cholinergic amacrine cells in rabbit retina

Using the in vivo rabbit eyecup, we have studied the light-evoked release of acetylcholine (ACh) which is presumed to indicate the activity of cholinergic amacrine cells. Gamma-Aminobutyric acid (GABA) inhibited the light-evoked release of ACh (IC50 congruent to 1 mM), but the GABA antagonists bicuculline (5 micro M) and picrotoxin (20 micro M) potentiated the light-evoked release and markedly increased the resting release of ACh. This bicuculline/picrotoxin-evoked release was calcium dependent and the effects of bicuculline, but not picrotoxin, were blocked by muscimol, a potent GABA agonist. Muscimol also inhibited the light-evoked release of ACh (IC50 less than 1 micro M) and was at least 1000 times more potent than GABA. Nipecotic acid (1 mM), a GABA transport blocker, also inhibited the light-evoked release of ACh, but the effect was slow in onset and recovery was prompt. We conclude that the cholinergic amacrine cells of rabbit retina are inhibited by GABA. The relatively weak action of GABA, compared to muscimol, may be due to the presence of avid GABA transport systems. We ascribe the excitatory effects of bicuculline and picrotoxin to the antagonism of endogenous GABA, suggesting that the cholinergic cells are influenced by a tonic release of GABA. This is consistent with the effects of nipecotic acid. Although we are unable to specify the synaptic arrangements involved, we suggest that the most likely interaction is directly between GABA amacrine cells and the cholinergic amacrine cells and/or their presumed bipolar cell inputs.

[3H]GABA binding in developing rabbit retina

We have studied the developmental sequence of the GABA system in the rabbit retina using an in vitro binding assay to monitor developmental changes in the post-synaptic receptor. A variety of tissue treatments including perchlorate and Triton X-100 were employed to optimize binding and remove endogenous factors which inhibit binding. Pre-treatment of the tissue with 0.05% Triton X-100 revealed high affinity binding for [3H]GABA which increased in a sigmoidal fashion with the post-natal age of the animal. A constant level of binding, at about 16% of adult levels, was noted until day 8, at which time a rapid increase occurred. At 16 days post-natal, the amount of specific binding reached a plateau near adult levels. Kinetic analysis of the GABA receptor showed an increase in the number of receptors (Bmax) with little or no change in the apparent affinity (KD). Our results suggest that the onset of post-synaptic receptor activity is delayed approximately 1 to 2 days, relative to the pre-synaptic components, and the period of rapid increase in GABA receptor binding coincides with the period of maximum increase in retinal synaptic density.

2-amino-4-phosphonobutyric acid and N-methyl-D-aspartate differentiate between "3H]glutamate and [3H] aspartate binding sites in bovine retina

Three pharmacologically and kinetically distinct binding sites for acidic amino acids were observed in synaptic membranes from bovine retina. One site preferentially binds aspartate with an apparent Kd of 6.3 nM and Bmax of 0.033 pmol/mg protein. [3H] Glutamate binds preferentially to two sites which have KdS of approximately 10 and 800 nM; and BmaxS of 0.148 and 0.417 pmol/mg protein, respectively. Only one of these sites, the 800 nM glutamate site, displayed a pharmacological specificity which was in extensive agreement with that reported for the putative glutamate receptor previously localized to a specific set of retinal neurons, the on-center, depolarizing bipolar cells.

3H-muscimol binding in synaptosomal fractions from bovine and developing rabbit retinas

Bovine and rabbit retina synaptosomal fractions bind the potent and specific GABA agonist, [3H]-muscimol with high affinity and limited capacity. The high degree of pharmacological specificity and the subcellular distribution of these binding sites are similar to those reported for [3H]-GABA binding sites. These observations suggest that these sites represent the recognition sites for GABA receptors. The specific binding of [3H]-muscimol in retinal homogenates from different aged rabbits reveal a distinct developmental profile with a fivefold to six fold increase in binding between days 5 and 13. Thus, it appears that GABA receptor development continues after eye opening in rabbits (day 9-10) and that receptor maturation is delayed by at least two to three days with respect to published of GABA uptake and evoked release.

Localization of kainic acid-sensitive cells in mammalian retina

Short-term (15 minutes) in vitro exposure to kainic acid (KA), a rigid structural analog of L-glutamic acid (Glu), caused two morphologically distinct neuronal lesions in retinas of several species. In rabbit retina, one type of lesion was characterized by rapid swelling after exposure to low concentrations of KA (10(-4)M). This lesion was observed in elements of both plexiform layers and, more specifically, in cell bodies and neurites of horizontal cells that contact cones. A few cell bodies from the amacrine cell layer showed some limited swelling. The swelling was completely blocked when sodium was removed from the incubation medium. The second type of lesion was generally seen after longer exposures of after exposure to higher concentrations of KA and was evidenced by degeneration of neurons in the amacrine and ganglion cell layers. One exception was noted in that a few cells from the ganglion cell layer degenerated even under low exposure conditions. The second type of lesion was not blocked by removal of sodium ions. Photoreceptor cells appeared resistant to all effects of KA. The results suggest that a correlation may exist between certain KA-induced lesions of the retina and putative glutamoreceptive neurons. At the same time, the two types of retinal lesions produced by KA are morphologically and chemically differentiable and may be useful in elucidating the differences between specific, Glu-related toxicity and nonspecific toxicity of KA.

Bacterial collagenase. Proposed adjunct to vitrectomy with membranectomy

Clostridiopeptidase A digested preretinal cicatricial tissue without causing morphological alteration of normal retina during a 30-minute incubation in the rabbit. Light and transmission electron microscopy were used to determine effects on the inner limiting membrane and retinal ganglion and Müller's cells and to evaluate enzyme digestion of preretinal scarring. Removal of the injected collangenase by vitrectomy resulted in normal electroretinograms and retinal morphology 48 hours postoperatively. If the enzyme was left in the eye for 24 hours, lens opacities, partial erosion of the inner limiting membrane, and extensive hemorrhage resulted. The specificity of action of the collagenase is due to the high degree of purity of the enzyme used and a substantial biochemical difference between scar collagen and basement membrane collagen. The injection of purified collagenase capable of digesting vitreal scar tissue while leaving the retina undamaged could represent a new approach to vitrectomy, specifically to facilitate certain cases of membranectomy.

Isolation of synaptosomal fractions from rabbit retina

Homogenization of rabbit retina in isotonic sucrose and differential rate centrifugation yielded two morphologically distinct synaptosomal fractions. One fraction was enriched in photoreceptor cell synaptosomes; the second fraction contained small synaptosomes derived from conventional sized synapses most abundant in the inner plexiform layer. Attempts to further purify these fractions using a variety of density gradients proved unsuccessful due to poor viability of photoreceptor cell synaptosomes. The synaptosomes prepared by our method are functionally stable as they demonstrate high affinity uptake for putative retinal neurotransmitters, neurotransmitter-sensitive adenylate cyclase activity, and calcium-dependent, potassium-stimulated release of [14C]GABA and [3H]dopamine.

The effects of intravitreal irrigation during vitrectomy on the electroretinogram

Intravitreal irrigation with Balanced Salt Solution Plus (BSS Plus) produced less decrease in b-wave amplitude than either normal saline solution or Balanced Salt Solution. BSS Plus was more suitable for intravitreal irrigation because it contained the appropriate bicarbonate, pH, and ionic composition necessary for maintenance of normal retinal electrical activity, and it contained glutathione, which is necessary for maintenance of endothelial cell adenosine triphosphatase and for protection against free radical damage and oxidative stress.