GABAergic ganglion cells in the rabbit retina

GABAergic Retinal Ganglion Cells Projecting to the Superior Colliculus Mediate the Looming-Evoked Flight Response

The looming stimulus-evoked flight response to approaching predators is a defensive behavior in most animals. However, how looming stimuli are detected in the retina and transmitted to the brain remains unclear. Here, we report that a group of GABAergic retinal ganglion cells (RGCs) projecting to the superior colliculus (SC) transmit looming signals from the retina to the brain, mediating the looming-evoked flight behavior by releasing GABA. GAD2-Cre and vGAT-Cre transgenic mice were used in combination with Cre-activated anterograde or retrograde tracer viruses to map the inputs to specific GABAergic RGC circuits. Optogenetic technology was used to assess the function of SC-projecting GABAergic RGCs (scpgRGCs) in the SC. FDIO-DTA (Flp-dependent Double-Floxed Inverted Open reading frame-Diphtheria toxin) combined with the FLP (Florfenicol, Lincomycin & Prednisolone) approach was used to ablate or silence scpgRGCs. In the mouse retina, GABAergic RGCs project to different brain areas, including the SC. ScpgRGCs are monosynaptically connected to parvalbumin-positive SC neurons known to be required for the looming-evoked flight response. Optogenetic activation of scpgRGCs triggers GABA-mediated inhibition in SC neurons. Ablation or silencing of scpgRGCs compromises looming-evoked flight responses without affecting image-forming functions. Our study reveals that scpgRGCs control the looming-evoked flight response by regulating SC neurons via GABA, providing novel insight into the regulation of innate defensive behaviors.

The Interplay between Neurotransmitters and Calcium Dynamics in Retinal Synapses during Development, Health, and Disease

The intricate functionality of the vertebrate retina relies on the interplay between neurotransmitter activity and calcium (Ca2+) dynamics, offering important insights into developmental processes, physiological functioning, and disease progression. Neurotransmitters orchestrate cellular processes to shape the behavior of the retina under diverse circumstances. Despite research to elucidate the roles of individual neurotransmitters in the visual system, there remains a gap in our understanding of the holistic integration of their interplay with Ca2+ dynamics in the broader context of neuronal development, health, and disease. To address this gap, the present review explores the mechanisms used by the neurotransmitters glutamate, gamma-aminobutyric acid (GABA), glycine, dopamine, and acetylcholine (ACh) and their interplay with Ca2+ dynamics. This conceptual outline is intended to inform and guide future research, underpinning novel therapeutic avenues for retinal-associated disorders.

Diverse modes of binocular interactions in the mouse superior colliculus

The superior colliculus (SC) integrates visual and other sensory information to regulate critical reflexive and innate behaviors, such as prey capture. In the mouse, the vast majority of retinal ganglion cells (RGCs) innervate the SC, including inputs from both the contralateral (contra-RGCs) and ipsilateral (ipsi-RGCs) eye. Despite this, previous studies revealed minimal neuronal responses to ipsilateral stimulation and few binocular interactions in the mouse SC. More recent work suggests that ipsi-RGC function and innervation of the SC are critical for efficient prey capture, raising the possibility that binocular interactions in the mouse SC may be more prevalent than previously thought. To explore this possibility, we investigated eye-specific and binocular influences on visual responses and tuning of SC neurons, focusing on the anteromedial region. Although the majority of SC neurons were primarily driven by contralateral eye stimulation, we observed that a substantial proportion of units were influenced or driven by ipsilateral stimulation. Clustering based on differential responses to eye-specific stimulus presentation revealed five distinct putative subpopulations and multiple modes of binocular interaction, including facilitation, summation, and suppression. Each of the putative subpopulations exhibited selectivity for orientation, and differences in spatial frequency tuning and spatial summation properties were observed between subpopulations. Further analysis of orientation tuning under different ocular conditions supported differential modes of binocular interaction between putative subtypes. Taken together, these data suggest that binocular interactions in the mouse SC may be more prevalent and diverse than previously understood.NEW & NOTEWORTHY The mouse superior colliculus (SC) receives binocular inputs, which inform complex behavioral programs. However, we know surprisingly little about binocular tuning in the rodent SC. Here, we characterize responses to eye-specific presentations of visual stimuli and reveal a previously unappreciated diversity of binocularly modulated neurons in the SC. This foundational work broadens our understanding of visual processing in the SC and sets the stage for future studies interrogating the circuit mechanisms underlying binocular tuning.

GABAergic retinal ganglion cells regulate innate defensive responses

Gamma-aminobutyric acid (GABA) is regarded as the most important inhibitory neurotransmitter in the central nervous system, including the retina. However, the roles of GABA-immunolabeled retinal ganglion cells (RGCs) have not been explored. Here, we report the expression of GABAergic RGCs that project to many brain areas in mice, including the superior colliculus. Selective ablation of the superior colliculus-projecting GABAergic RGCs, leaving other GABAergic RGCs intact, reduces the looming stimulus-induced defensive response without affecting image-forming functions; it also significantly enhances glucose metabolism in the superior colliculus, as determined by [18F]-fluorodeoxyglucose PET (FDG PET). Our findings demonstrate that superior colliculus-projecting GABAergic RGCs control the visually active defensive response by regulating superior colliculus neurons.

Ionotropic GABA Receptors and Distal Retinal ON and OFF Responses

In the vertebrate retina, visual signals are segregated into parallel ON and OFF pathways, which provide information for light increments and decrements. The segregation is first evident at the level of the ON and OFF bipolar cells in distal retina. The activity of large populations of ON and OFF bipolar cells is reflected in the b- and d-waves of the diffuse electroretinogram (ERG). The role of gamma-aminobutyric acid (GABA), acting through ionotropic GABA receptors in shaping the ON and OFF responses in distal retina, is a matter of debate. This review summarized current knowledge about the types of the GABAergic neurons and ionotropic GABA receptors in the retina as well as the effects of GABA and specific GABAA and GABAC receptor antagonists on the activity of the ON and OFF bipolar cells in both nonmammalian and mammalian retina. Special emphasis is put on the effects on b- and d-waves of the ERG as a useful tool for assessment of the overall function of distal retinal ON and OFF channels. The role of GABAergic system in establishing the ON-OFF asymmetry concerning the time course and absolute and relative sensitivity of the ERG responses under different conditions of light adaptation in amphibian retina is also discussed.

Mapping kainate activation of inner neurons in the rat retina

Kainate receptors mediate fast, excitatory synaptic transmission for a range of inner neurons in the mammalian retina. However, allocation of functional kainate receptors to known cell types and their sensitivity remains unresolved. Using the cation channel probe 1-amino-4-guanidobutane agmatine (AGB), we investigated kainate sensitivity of neurochemically identified cell populations within the structurally intact rat retina. Most inner retinal neuron populations responded to kainate in a concentration-dependent manner. OFF cone bipolar cells demonstrated the highest sensitivity of all inner neurons to kainate. Immunocytochemical localization of AGB and macromolecular markers confirmed that type 2 bipolar cells were part of this kainate-sensitive population. The majority of amacrine (ACs) and ganglion cells (GCs) showed kainate responses with different sensitivities between major neurochemical classes (γ-aminobutyric acid [GABA]/glycine ACs > glycine ACs > GABA ACs; glutamate [Glu]/weakly GABA GCs > Glu GCs). Conventional and displaced cholinergic ACs were highly responsive to kainate, whereas dopaminergic ACs do not appear to express functional kainate receptors. These findings further contribute to our understanding of neuronal networks in complex multicellular tissues.

Early development of GABAergic cells of the retina in sharks: an immunohistochemical study with GABA and GAD antibodies

We studied the ontogeny and organization of GABAergic cells in the retina of two elasmobranches, the lesser-spotted dogfish (Scyliorhinus canicula) and the brown shyshark (Haploblepharus fuscus) by using immunohistochemistry for gamma-aminobutyric acid (GABA) and glutamic acid decarboxylase (GAD). Both antibodies revealed the same pattern of immunoreactivity and both species showed similar organization of GABAergic cells. GABAergic cells were first detected in neural retina of embryos at stage 26, which showed a neuroepithelial appearance without any layering. In stages 27-29 the retina showed similar organization but the number of neuroblastic GABAergic cells increased. When layering became apparent in the central retina (stage-30 embryos), GABAergic cells mainly appeared organized in the outer and inner retina, and GABAergic processes and fibres were seen in the primordial inner plexiform layer (IPL), optic fibre layer and optic nerve stalk. In stage-32 embryos, layering was completed in the central retina, where immunoreactivity appeared in perikarya of the horizontal cell layer, inner nuclear layer and ganglion cell layer, and in numerous processes coursing in the IPL, optic fibre layer and optic nerve. From stage 32 to hatching (stage 34), the layered retina extends from centre-to-periphery, recapitulating that observed in the central retina at earlier stages. In adults, GABA/GAD immunoreactivity disappears from the horizontal cell layer except in the marginal retina. Our results indicate that the source of GABA in the shark retina can be explained by its synthesis by GAD. Such synthesis precedes layering and synaptogenesis, thus supporting a developmental role for GABA in addition to act as neurotransmitter and neuromodulator.

Expression of alpha 7 nicotinic acetylcholine receptors by bipolar, amacrine, and ganglion cells of the rabbit retina

Cholinergic agents affect the light responses of many ganglion cells (GCs) in the mammalian retina by activating nicotinic acetylcholine receptors (nAChRs). Whereas retinal neurons that express beta2 subunit-containing nAChRs have been characterized in the rabbit retina, expression patterns of other nAChR subtypes remain unclear. Therefore, we evaluated the expression of alpha7 nAChRs in retinal neurons by means of single-, double-, and triple-label immunohistochemistry. Our data demonstrate that, in the rabbit retina, several types of bipolar cells, amacrine cells, and cells in the GC layer express alpha7 nAChRs. At least three different populations of cone bipolar cells exhibited alpha7 labeling, whereas glycine-immunoreactive amacrine cells comprised the majority of alpha7-positive amacrine cells. Some GABAergic amacrine cells also displayed alpha7 immunoreactivity; alpha7 labeling was never detected in rod bipolar cells or rod amacrine cells (AII amacrine cells). Our data suggest that activation of alpha7 nAChRs by acetylcholine (ACh) or choline may affect glutamate release from several types of cone bipolar cells, modulating GC responses. ACh-induced excitation of inhibitory amacrine cells might cause either inhibition or disinhibition of other amacrine and GC circuits. Finally, ACh may act on alpha7 nAChRs expressed by GCs themselves.

Spatial and temporal patterns of proliferation and differentiation in the developing turtle eye

Here we show for the first time different aspects of the pattern of neurogenesis in the developing turtle retina by using different morphological and molecular clues. We show the chronotopographical fashion of occurrence of three major aspects of retinal development: (1) morphogenesis of the optic primordia and emergence of the different retinal layers, (2) the temporal progression of neurogenesis by the cessation of proliferative activity, and (3) the apparition and cellular localization of different antigens and neuroactive substances. Retinal cells were generated in a conserved temporal order with ganglion cells born first, followed by amacrine, photoreceptor, horizontal and bipolar/Müller cells. While eventually expressed in many types of retinal neurons, Islet1 was permanently expressed in differentiating and mature ganglion cells. Calbindin-immunoreactive elements were found in the ganglion cell layer and the inner nuclear layer. Interestingly, at later stages the amount of expressing cells in these layers was reduced dramatically. On the contrary, the number of calbindin-immunoreactive photoreceptors increased as development proceeded. In addition, calretinin expressing cells were prominent in the horizontal cell bodies, and their processes extending into the outer plexiform layer were also strongly labeled. Finally, the synthesis of gamma-aminobutyric acid (GABA) was detected in developing and matured horizontal and amacrine cells. All these maturational features began in the dorso-central area, in a region slightly displaced towards the temporal retina.

Inner retinal neurons display differential responses to N-methyl-D-aspartate receptor activation

The N-methyl-D-aspartate (NMDA) responses of neurons from within the inner rabbit retina were mapped using a channel permeable cation, 1-amino-4-guanidobutane (agmatine, AGB). Serial sections were subsequently probed with immunoglobulins targeting AGB, glutamate, gamma-aminobutyric acid (GABA), and glycine to visualize the NMDA responses of neurochemical subpopulations of neurons. Most inner retinal subpopulations of neurons demonstrated an NMDA concentration-dependent increase in activation. This NMDA-induced activation displayed a distinct pattern, with the most sensitive class to least sensitive class ranking being GC > GABA cAC > GABA/Gly cAC > Gly cAC > GABA dAC (GC, ganglion cells; AC, amacrine cells; c, conventional; d, displaced; Gly, glycine). The variable NMDA response may reflect differences in NMDA receptor subunit disposition or differences in receptor density. In addition to the variable NMDA activation pattern, we found that virtually all ganglion cells (87%) showed NMDA-gated AGB entry, compared with only 58% of amacrine cells. We conclude that a large cohort of amacrine cells do not possess functional NMDA receptors. In addition to most ganglion cells being activated by NMDA, a large subpopulation displayed the highest sensitivity to NMDA application. The functional significance of this finding is that the ganglion cell population will be the first neuronal class to be susceptible to glutamate-induced neurotoxicity mediated through the NMDA receptor. The addition of betaxolol significantly reduced NMDA-mediated AGB entry into most neuronal groups (ganglion cells, GABA, and glycine amacrine cells), with the greatest effect being on ganglion cells. Betaxolol had no significant effect on NMDA-gated entry of AGB on the GABA/Gly amacrine cell population.

Localization and Synaptic Release of N-acetylaspartylglutamate in the Chick Retina and Optic Tectum

The neuropeptide, N-acetylaspartylglutamate (NAAG), was identified in the chick retina (1.4 nmol/retina) by HPLC, radioimmunoassay and immunohistochemistry. This acidic dipeptide was found within retinal ganglion cell bodies and their neurites in the optic fibre layer of the retina. Substantial, but less intense, immunoreactivity was detected in many amacrine-like cells in the inner nuclear layer and in multiple bands within the inner plexiform layer. In addition, NAAG immunoreactivity was observed in the optic fibre layer and in the neuropil of the superficial layers of the optic tectum, as well as in many cell bodies in the tectum. Using a newly developed, specific and highly sensitive (3 fmol/50 microl) radioimmunoassay for NAAG, peptide release was detected in isolated retinas upon depolarization with 55 mM extracellular potassium. This assay also permitted detection of peptide release from the optic tectum following stimulation of action potentials in retinal ganglion cell axons of the optic tract. Both of these release processes required the presence of extracellular calcium. Electrically stimulated release from the tectum was reversibly blocked by extracellular cadmium. These findings suggest that NAAG serves an extracellular function following depolarization-induced release from retinal amacrine neurons and from ganglion cell axon endings in the chick optic tectum. These data support the hypothesis that NAAG functions in synaptic communication between neurons in the visual system.

Colocalization of choline acetyltransferase and gamma-aminobutyric acid in the developing and adult turtle retinas

Acetylcholine and gamma -aminobutyric acid (GABA) are putative neurotransmitters in the adult vertebrate retina. In this study, cells that coexpress choline acetyltransferase (ChAT) and GABA or glutamic acid decarboxylase (GAD) were investigated in turtle retinas from stage 14 (S14) to adulthood by using a double-labeling immunofluorescence technique. ChAT immunoreactivity was observed at S15 and included not only the presumptive starburst cholinergic amacrine cells but also a population in the ganglion cell layer (GCL) that expressed ChAT transiently during the embryonic stages (see the accompanying paper: Nguyen et al. [2000] J. Comp. Neurol. 420:512-526).

Differences in the retinal GABA system among control, spastic mutant and retinal degeneration mutant mice

Immunocytochemical methods were used to compare the GABA system in control mice and two mutant strains: spastic which has reduced glycine receptors and retinal degeneration mutant in which the photoreceptors degenerate and reportedly have increased GABA and GAD levels. We found that the spastic mutant retina had reduced GABA-immunoreactivity (IR) in the proximal retina, reduced staining for GAD-1440 in the OPL, and reduced GABAA receptor staining in the OPL, compared to control. The retinal degeneration mutant retinas had enhanced GABA-IR throughout the retina, particularly in Müller cells, bipolar cells and IPL, and enhancement of GABAA receptor staining in the OPL, compared to control. The distributions of GABA-IR, GAD-1440-IR and GABAA receptor-IR in retinas of spastic mutant mice that also expressed the retinal degeneration phenotype resembled those found in retinas of mice that expressed only the retinal degeneration phenotype rather than those that expressed only the spastic mutation. No differences were observed among the conditions for GAD-65, GAD-67 or GABA-T. Our results with the spastic and retinal degeneration mutant mice demonstrate that attenuation in the glycinergic system and photoreceptor degeneration, respectively, is accompanied by alterations in different aspects of the GABA system, giving impetus for caution in the interpretation of experiments involving genetic manipulation of complex phenotypes.

Calretinin in neurochemically well-defined cell populations of rabbit retina

In the rabbit retina, parvalbumin has been localized selectively to AII amacrine cells, while 28 kDa calbindin could be detected in horizontal cells, in one type of depolarizing cone bipolar cell and a population of wide-field amacrine cells. The distribution of the third neuronal calcium binding protein, calretinin, however, has not been studied to date in detail in the rabbit retina. Therefore in this study we aimed to describe the overall distribution of calretinin in the different retinal layers and the possible colocalization pattern with other neurochemical marker molecules. A few cone photoreceptor cells were found to be labeled, whereas the outer plexiform layer was free from immunoreactive elements. In the most proximal row of the inner nuclear layer amacrine cells were labeled, while more distally a few cells emitted beaded axon-like processes toward the outer retina. There were large (18-28 microm in diameter) cells labeled in the ganglion cell layer, of which many apparently had their axon stained. Some of the calretinin immunoreactive amacrine cells (the AII neurons) also contained parvalbumin. Colocalization of calretinin and 28 kDa calbindin could not be ascertained in the same amacrine cell populations, nor was tyrosine hydroxylase present in calretinin-containing cells. There was partial colocalization of calretinin in the gamma-aminobutyric acid-positive amacrine cell population. Parvalbumin containing ganglion cells were also positive for calretinin; however, the calretinin-positive ganglion cells were more numerous. gamma-Aminobutyric acid could be colocalized in some calretinin-positive neurons of the ganglion cell layer.

Analysis of the distribution of glycine and GABA in amacrine cells of the developing rabbit retina: a comparison with the ontogeny of a functional GABA transport system in retinal neurons

The objectives of this study were to (1) determine whether the glycinergic and GABAergic amacrine cells in the developing rabbit retina were neurochemically distinct at birth, (2) determine if the ratio of GABAergic to glycinergic amacrine cells was constant during development, (3) determine whether the capacity to take up a GABA analogue was restricted to GABAergic neurons, and (4) whether initiation of GABA transport into GABAergic neurons preceded the presence of a content of GABA in these neurons. We have used a novel strategy to immunolocalize a non-endogenous GABA analogue, gamma-vinyl GABA, which is taken up into neurons by a GABA transporter. Examination of serial semithin resin-embedded sections of neonatal rabbit retinae that had been immunolabelled for glycine, GABA or gamma-vinyl GABA revealed that at 1 day postnatum, 60% of amacrine cells contain glycine but not GABA and did not accumulate gamma-vinyl GABA, which is similar to the percentage of glycinergic amacrine cells in the adult retina. The vast majority of the remaining amacrine cells contained GABA and many also transported gamma-vinyl GABA; however, a significant number of GABA-containing cells failed to accumulate gamma-vinyl GABA suggesting that possession of a content of GABA did not have to be preceded by, or be concomitant with, the presence of a GABA transport system. By 10 days postnatum, over 99% of GABA-containing amacrine cells also transported gamma-vinyl GABA indicating their functional maturity. Analysis of the horizontal cells revealed no evidence for uptake of gamma-vinyl GABA, but another GABA analogue, diaminobutyric acid, which is a substrate both for the neuron-associated GABA transporter and the glial GABA transporter, was accumulated into some horizontal cells at 21 days postnatum, a time point when these cells also contain endogenous GABA. We conclude that amacrine cells are committed to being GABAergic or glycinergic at, or prior to birth, and that in some amacrine cells, expression of a content of GABA may occur prior to the capacity to transport GABA. Conversely, in some ganglion cells transport of gamma-vinyl GABA may precede a content of GABA.

Enrichment of glutamate-like immunoreactivity in the retinotectal terminals of the viper Vipera aspis: an electron microscope quantitative immunogold study

A post-embedding immunogold study was carried out to estimate the immunoreactivity to glutamate in retinal terminals, P axon terminals and dendrites containing synaptic vesicles in the superficial layers of the optic tectum of Vipera. Retinal terminals, identified following either intraocular injection of tritiated proline, horseradish peroxidase (HRP) or short-term survivals after retinal ablation, were observed to be highly glutamate-immunoreactive. A detailed quantitative analysis showed that about 50% of glutamate immunoreactivity was localized over the synaptic vesicles, 35.8% over mitochondria and 14.2% over the axoplasmic matrix. The close association of immunoreactivity with the synaptic vesicles could indicate that Vipera retino-tectal terminals may use glutamate as their neurotransmitter. P axon terminals and dendrites containing synaptic vesicles, strongly gamma-aminobutyric (GABA)-immunoreactive, were shown to be also moderately glutamate-immunoreactive, but two to three times less than retinal terminals. Moreover, in P axon terminals, the glutamate immunoreactivity was denser over mitochondria than over synaptic vesicles, possibly reflecting the 'metabolic' pool of glutamate, which serves as a precursor in the formation of GABA.

Expression of GABA in the fetal, postnatal, and adult human retinas: an immunohistochemical study

The expression of GABA in the human fetal (12-25 weeks of gestation), postnatal (five-month-old), and adult (35-year-old) retinas was investigated by immunohistochemistry. GABA expression was seen as early as 12 weeks in the undifferentiated cells of the inner neuroblast zone; a few optic nerve fiber layer axons were clearly labeled, suggesting that some of the stained cell bodies were prospective ganglion cells, others could be displaced amacrine cells. From 16-17 to 24-25 weeks, intense labeling was found in the amacrine, displaced amacrine, and some ganglion cells. During this time period, horizontal cells (identified by calbindin immunohistochemistry), undergoing migration (periphery) and differentiation (center), expressed GABA prominently. In the postnatal retina, some horizontal cells were moderately labeled, but very weakly in a few cells, in the adult. The Müller cells developed immunoreactivity first weakly at 12 weeks and then moderately from 16-17 weeks onward. The staining was also evident in the postnatal and adult retinas, showing labeled processes of these glial cells. Virtually no axons in the adult optic nerve and nerve fiber layer were stained; the staining was restricted to a few, large ganglion cells and displaced amacrine cells: Some amacrines were also labeled. The possibility that GABA might play a role in horizontal cell differentiation and maturation is highlighted. Other evidences suggest that GABA might play a role in metabolism during retinal development.

Dopaminergic and GABAergic retinal cell populations in mammals

A number of modern techniques now allow histologists to characterize subpopulations of retinal neurons by their neurotransmitters. The morphologies and connections of these chemically defined neurons can be analyzed precisely at both light and electron microscope levels and lead to a better understanding of retinal circuitry. The dopaminergic neurons form a loose population of special wide-field amacrine cells bearing intraretinal axons within the inner plexiform layer. One subtype, the interplexiform cell, sends an axon to the outer plexiform and outer nuclear layers. The number of interplexiform cells is variable throughout mammalian species. The GABAergic neurons form a dense and heterogeneous population of amacrine cells branching at all levels of the inner plexiform layer. The presence of GABA in horizontal cells seems to be species-dependent. Close relationships occur between dopaminergic and GABAergic cells. GABA antagonizes a number of dopaminergic actions by inhibiting both the release and synthesis of dopamine. This inhibition can be supported by GABA synapses onto dopaminergic cells, but GABA can also diffuse to its targets. Finally, GABA is also contained and synthesized in dopaminergic cells. This colocalization might be the basis of an intracellular modulation of dopamine by GABA.

Amino acid signatures in the primate retina

Pattern recognition of amino acid signals partitions virtually all of the macaque retina into 16 separable biochemical theme classes, some further divisible by additional criteria. The photoreceptor-->bipolar cell-->ganglion cell pathway is composed of six separable theme classes, each possessing a characteristic glutamate signature. Neuronal aspartate and glutamine levels are always positively correlated with glutamate signals, implying that they largely represent glutamate precursor pools. Amacrine cells may be parsed into four glycine-dominated (including one glycine/GABA immunoreactive population) and four GABA-dominated populations. Horizontal cells in central retina possess a distinctive GABA signature, although their GABA content is constitutively lower than that of amacrine cells and shows both regional and sample variability. Finally, a taurine-glutamine signature defines Müller's cells. We thus have established the fundamental biochemical signatures of the primate retina along with multiple metabolic subtypes for each neurochemical class and demonstrated that virtually all neuronal space can be accounted for by cells bearing characteristic glutamate, GABA, or glycine signatures.

Retinal and non-retinal inputs upon retinopetal RMA neurons in the lamprey: a light and electron microscopic study combining HRP axonal tracing and GABA immunocytochemistry

A light and electron microscopic study, combining HRP axonal tracing or degeneration and GABA immunocytochemistry, was performed in the lamprey Lampetra fluviatilis in order to analyze retinal and non-retinal inputs upon the retinopetal neurons localized in the reticular mesencephalic area (RMA). The iontophoretic deposit of HRP onto the central stump of the cut optic nerve produced a dense anterograde labeling in the retino-recipient strata marginale and cellular externum of the optic tectum as well as the retrograde labeling of retinopetal neurons in the mesencephalic tegmentum. The large ascending proximal dendrites of the retinopetal neurons constituted a distinct bundle coursing first dorso-laterally in the dorsal mesencephalic tegmentum, and then dorso-medially in the strata fibrosum centrale and cellulare et fibrosum internum of the optic tectum before their distal portions penetrated the retino-recipient tectal layers. The distribution of GABA immunoreactivity was also investigated in the tectal layers and dorsal mesencephalic tegmentum with both pre- and post-embedding methods. The retinal terminals, identified either following HRP iontophoresis in the optic nerve or in early phases of degeneration after short-term survivals following retinal lesion, contained rounded-shaped synaptic vesicles and were always GABA immunonegative. They established asymmetrical synaptic contacts on the distal dendrites of RMA neurons and represented 11.4% of all terminals contacting such neurons (15% of these neurons were GABA immunopositive). The dense extra-retinal input upon the retinopetal RMA neurons was composed of five types of axon terminal profiles, either GABA-immunopositive or -immunonegative. Considering the different cytochemical types of axon terminals contacting RMA neurons, as well as the characteristics of the retinal targets of these neurons, we suggest that, globally, the effects of RMA neurons upon the retina are mainly inhibitory.

GABA immunopositive axons in the optic nerve and optic tract of macaque monkeys

Using an antibody to gamma-aminobutyric acid (GABA), we examined the optic nerves and optic tracts from macaque monkeys at the light and electron microscopic levels to determine if there is a possible inhibitory projection from the retina to the brain. All of the monkeys (n = 5) had GABA immunopositive axons that were evenly distributed in their optic nerves. These immunopositive axons were slightly larger than the axons around them and comprised an average of 2.6% of the axons in the nerves. Thus, their estimated total was about 44,000 axons per nerve. In the optic tracts, the GABA immunopositive axons were not distributed evenly, but were concentrated mostly in the ventromedial part, indicating that this retinal pathway probably goes to a midbrain destination such as the superior colliculus. The present findings provide further evidence that there is a GABAergic retinal projection to the brain in primates with currently unknown physiological influences.

Immunocytochemical localization of three subtypes of GABA transporter in rat retina

Cellular distributions of three subtypes of GABA transporter (GAT1, GAT2, GAT3) in the eye were examined using polyclonal antisera for each subtype. GAT1 was present in the inner plexiform layer and proximal part of the inner nuclear layer, while GAT3 was distributed throughout the entire sensory retina, being predominant in the distal part of the inner plexiform layer and in the outer plexiform layer. GAT2 immunoreactivity was seen in the retina, including the retinal pigment epithelium layer and nerve fiber layer, also in the ciliary body epithelium. Confocal scanning laser fluorescence microscopy disclosed that the GAT1 immunoreactivity consisted of a number of small deposits in the inner plexiform layer and that GAT1-immunoreactive dots encircle immunonegative neurons in the inner nuclear layer. GAT2 immunoreactivity was present in the fiber bundle of the optic nerve and in the retinal pigment epithelium within the retina. GAT3 immunoreactive cells had long processes running vertically throughout the sensory retina. GAT1 is suggested to be present mainly in the processes of amacrine cells and GAT3 to be distributed in Müller cells. We conclude that GAT1, GAT2 and GAT3 are expressed in different cells, that they are involved in distinct GABAergic transmission in the retina, and that GAT2 may be involved in non-neuronal functions in the eye.

Immunocytochemical evidence for an axonal localization of GABA in the optic nerves of rabbits, rats, and cats

We have examined, by light-microscopic immunocytochemistry, the distribution of GABA in the optic nerves of adult rabbits, rats, and cats. Within the optic nerves, immunoreactivity for GABA was restricted to a small subset of axons; some axons were strongly labelled, others weakly labelled, whilst most axons were unlabelled. Glia and other non-neuronal elements were always unlabelled. Our ability to detect GABA in optic nerve axons of adult mammals contrasts with previous reports that indicate a lack of GABA immunoreactivity in such axons. We suggest that this discrepancy may be due to the sensitivity of our immunocytochemical techniques which enable us to detect low concentrations of GABA.

Early development of GABA-like immunoreactive cells in the retina of turtle embryos

Gamma aminobutyric acid (GABA) is one of the earliest neuroactive substances appearing in the developing central nervous system. The distribution and the time course of the appearance of GABA-like immunoreactivity in the retina of the turtle Emys orbicularis were investigated from embryonic stage 13 to hatching. The first GABA-like immunoreactive cells were observed at stage 14. These cells were located in both the scleral third of the neuroblastic layer and the inner layers of the retina. They were identified as presumptive immature horizontal cells and amacrine cells, respectively. The observation of numerous labelled fibers in the nerve fiber layer suggests that some of the GABA-like immunoreactive cells in the layers were ganglion cells. The development of GABA-like immunoreactive cells followed a gradient of maturation from central to peripheral retina. At hatching, the central retina appeared nearly morphologically mature. In conclusion, GABA is present before the morphofunctional maturation of the retina and this precocious existence supports the idea of its involvement in a neurotrophic role preceding the establishment of synaptic connections and neurotransmitter function.

Early appearance and transient expression of putative amino acid neurotransmitters and related molecules in the developing rabbit retina: an immunocytochemical study

We have studied, by immunocytochemistry, the ontogeny of GABA, glycine, glutamate, glutamine, and taurine-containing cells in the rabbit retina. Amacrine cells show GABA immunoreactivity by embryonic day 25 (E25) and throughout postnatal life. By contrast, ganglion cells and horizontal cells are only transiently GABA-immunoreactive (-IR); few appear GABA-IR by the third postnatal week. At maturity, glycine is present in amacrine cells and in some bipolar cells. During development, putative ganglion cells transiently contained glycine between E25 and postnatal day 3 (P3), whereas immunolabelling in presumed amacrine cells and bipolar cells persists after birth. Ganglion cells, bipolar cells, photoreceptors, and some amacrine cells are glutamate-IR in the adult retina. Glutamate immunoreactivity first appears in the somata and processes of cytoblastic cells by E20 and is prominent by E25. Surprisingly, ganglion cells are not strongly glutamate-IR until just before eye-opening, at postnatal day 10 (P10), coincident with the appearance of glutamine in their somata and in Müller glial cells. Bipolar cells are glutamate-IR before they or Müller cells contain high levels of glutamine (at P10). Glutamate immunoreactivity in photoreceptors is progressively restricted to the inner segments by eye-opening. At no stage are presumed horizontal cells glutamate-IR or glutamine-IR, but some amacrine cells show glutamate- and glutamine-IR by P10. Taurine is localized to photoreceptors and Müller glial in the adult retina. Some cytoblasts are taurine-IR at E20; with ensuing development, taurine labelling becomes restricted primarily to Müller cells and photoreceptors; some putative bipolar cells may also be labelled. However, for a few days around birth, cells resembling horizontal cells, also show taurine immunoreactivity. The early appearance and often transient expression of these amino acids in retinal cells suggests that these neuroactive molecules may be involved in the structural and functional development of the retina.

Diversity of neuronal phenotypes expressed in monolayer cultures from immature rabbit retina

We have used monolayer cultures prepared from early postnatal rabbit retinae (days 2-5) by the sandwich technique to study the capacity of immature neurons to express specific neuronal phenotypes in a homogeneous in vitro environment. Applying morphological, immunocytochemical, and autoradiographic criteria, we demonstrate that a variety of phenotypes could be distinguished after 7-14 days in vitro, and correlated with known retinal cell types. Bipolar cell-like neurons (approximately 4% of total cell number) were identified by cell type-specific monoclonal antibodies (115A10) and their characteristic bipolar morphology. Small subpopulations (about 1%) of GABA-immunoreactive neurons acquired elaborate morphologies strikingly similar to those of A- and B-type horizontal cells. Amongst putative amacrine cells several different subpopulations could be classified. GABA-immunoreactive amacrine-like neurons (6.5%), which also showed high affinity [3H]-GABA uptake, comprised cells of varying size and shape and could be subdivided into subpopulations with respect to their response to different glutamate receptor agonists (NMDA, kainic acid, quisqualic acid). In addition, a small percentage of [3H]-GABA accumulating cells with large dendritic fields showed tyrosine-hydroxylase immunoreactivity. Presumptive glycinergic amacrine cells (18.5%) were rather uniform in shape and had small dendritic fields. Release of [3H]-glycine from these neurons was evoked by kainic and quisqualic acid but not by NMDA. Small [3H]-glutamate accumulating neurons with few short processes were the most frequent cell type (73%). This cell type also exhibited opsin immunoreactivity and probably represented incompletely differentiated photoreceptor cells. Summing the numbers of characterized cells indicated that we were able to attribute a defined retinal phenotype to most, if not all of the cultured neurons. Thus, we have demonstrated that immature neuronal cells growing in monolayer cultures, in the absence of a structured environment, are capable of maintaining or producing specific morphological and functional properties corresponding to those expressed in vivo. These results stress the importance of intrinsic factors for the regulation of neuronal differentiation. On the other hand, morphological differentiation was far from perfect indicating the requirement for regulatory factors.

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.

Interaction between enkephalin and gamma-aminobutyric acid in the chicken retina: a double-label immunoelectron microscopic analysis

In the present study, double-label immunoelectron microscopy was used to examine the synaptic relationships between amacrine cell populations in the chicken retina that contain either enkephalin or gamma-aminobutyric acid (GABA) or both enkephalin and GABA. The objectives of the present study were twofold. First, the ultrastructural features and synaptic organization of enkephalin and enkephalin/GABA amacrine cells were compared. Second, the synaptic interactions between these populations and the population of GABA amacrine cells were examined. A total of 475 synaptic arrangements were observed to involved enkephalin or enkephalin/GABA amacrine cell processes. The synaptic relationships of enkephalin and enkephalin/GABA amacrine cells were quite similar. Each population was pre- and postsynaptic to amacrine cells, postsynaptic to bipolar cells, and presynaptic to processes possibly originating from ganglion cells. A substantial percentage of each population's pre- and postsynaptic relationships were with the processes of GABAergic amacrine cells. Moreover, when enkephalin and enkephalin/GABA amacrine cell processes were postsynaptic to bipolar cells, their dyadic partner was observed frequently to be a GABA amacrine cell process. The present study suggests a diversity in the population of chicken enkephalin amacrine cells with respect to their expression of the classical inhibitory transmitter GABA. Moreover, a functional relationship between enkephalinergic and GABAergic pathways is indicated by studies showing that both enkephalin and enkephalin/GABA amacrine cells exhibit substantial synaptic interaction with GABA amacrine cells.

Localization of substance P and GABA in retinotectal ganglion cells of the larval tiger salamander

The present study was performed as part of a systematic examination of the transmitter specificity of neuronal populations in the larval tiger salamander retina. Backfill-labeling of ganglion cells from the optic tectum was combined with double-label immunofluorescence histochemistry to determine if substance P and GABA are localized to ganglion cell populations in the tiger salamander retina. The triple-label analysis revealed the presence of substance P- and GABA-ganglion cells in both central and peripheral regions of the retina. Substance P-immunoreactive ganglion cells comprised 2% of the total population of backfill-labeled ganglion cells, while less than 1% of backfill-labeled ganglion cells expressed GABA immunoreactivity. Ganglion cells were not found to co-label for both substance P and GABA. Backfill-labeled displaced ganglion cells, which comprised 1.4% of the ganglion cell population, were not observed to be immunoreactive for either substance P or GABA. Forty-six point nine percent of substance P-cells in the ganglion cell layer were backfill-labeled and were identified as ganglion cells. GABA ganglion cells comprised less than 1% of GABA-immunoreactive cells in the ganglion cell layer. Therefore, the present study provides evidence for the presence of small populations of substance P- and GABA-ganglion cells in the larval tiger salamander retina. These observations suggest a functional diversity in the population of tiger salamander ganglion cells relative to their unique transmitter specificities.

Serotonin-like immunoreactivity in the adult and developing retina of the leopard frog Rana pipiens

Recent work in nonmammalian vertebrate retinas has suggested that other cell types besides the generally accepted amacrine cells may contain serotonin. We have used immunocytochemical methods to study serotonin-like immunoreactivity (5-HTLI) in the retina of the developing and mature frog Rana pipiens. In the adult, two types of serotonin immunoreactive (5-HT-ir) cells were found in the inner nuclear layer (INL) of the retina. Additionally, a large population of cells in the retinal ganglion cell layer (RGCL) had 5-HTLI. These cells were grouped into three types based on their soma size and their primary dendritic branching pattern. The optic nerve fiber layer was also intensely stained with serotonin antisera although staining intensity decreased progressively as the fibers approached the optic nerve head. Severing the optic nerve resulted in 5-HT-ir elements that extended up the optic nerve shaft from the lesion site toward the retina. Both regional and temporal changes in the pattern of 5-HTLI were seen. In middle regions of retina, approximately 30% of the cells in the RGCL were 5-HT-ir. Nasal and temporal regions of central retina had significantly fewer 5-HT-ir cells. Early in development only scattered cells in the RGCL were 5-HT-ir. As the animals matured there was an increase in both the proportion and the staining intensity of these cells. Our results suggest that in studying the function and development of the visual system in this animal, the role of serotonin must be examined.

Immunocytochemical localization of GABA and glycine in amacrine and displaced amacrine cells of macaque monkey retina

Recent studies have varied widely in the percentages of GABA- and glycine-immunoreactive (GABA+, GLY+) amacrines reported for primate retina. We compared the distributions of GABA+ and GLY+ amacrines and displaced amacrines at seven locations along the horizontal meridian of macaque retina using postembedding immunogold labeling with silver intensification. The percentage of GABA+ amacrine profiles was higher in central retina (50-55%) than peripheral retina (30-40%), whereas the percentage of GLY+ amacrine profiles did not vary much with eccentricity (52-57%). GABA and glycine were colocalized in 5-20% of amacrines, depending on the eccentricity, whereas 5-30% of amacrines were not immunoreactive for either neurotransmitter. GABA+ amacrines were slightly larger than GLY+ amacrines or Müller cells. In the ganglion cell layer, 5-20% of neurons were labeled for either GABA or glycine and were identified as displaced amacrines. Of these, 53% were GABA+ only, 11% were GLY+ only, and 37% were double-labeled. A few large, very lightly labeled GABA+ cells were identified as ganglion cells. Other features that varied with eccentricity included the linear density of GABA+ and GLY+ amacrines, and the ratio of amacrines to Müller cells.

Localization of GABAA receptors in the rat retina

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian retina. The present paper describes the localization of GABAA receptors in the rat retina as revealed by in situ hybridization and immunocytochemistry. In situ hybridization with probes against various alpha subunits revealed a marked differential expression pattern. The alpha 1 subunit gene is expressed mainly in the bipolar and horizontal cell layer, the alpha 2 gene in the amacrine and ganglion cell layer, and the alpha 4 gene in a subpopulation of amacrine cells. beta subunit mRNA is present diffusely throughout the entire inner nuclear layer and in the ganglion cell layer. The monoclonal antibody bd 17 (against beta 2/beta 3 subunits) stained subpopulations of GABAergic and glycinergic amacrine cells as well as some ganglion cells and bipolar cells. Immunoreactivity was not restricted to synaptic input sites. In the outer plexiform layer bipolar cell dendrites were immunoreactive; in the inner plexiform layer mainly amacrine and ganglion cell processes were labeled, and bipolar cell axons appeared unstained. The results demonstrate a strong heterogeneity of GABAA receptors in the retina.

Effects of retinal deafferentation on serotonin receptor types in the superficial grey layer of the superior colliculus of the rat

The effects of retinal axon terminal degeneration on the serotonin-1A, -1B, -2, nuerokinin-1 and gamma-amionobutyric acid-A high affinity binding sites in the superficial grey layer of the superior colliculus were tested with quantitative autoradiography on rat brain sections. The binding to serotonin-2, neurokinin-1 and gamma-aminobutyric acid-A high affinity receptors was not changed in the deafferented superficial grey layer of the superior colliculus after unilateral enucleation. By contrast, we demonstrate that the previously described 21% decrease in the binding of [3H]serotonin to serotonin-1 receptors observed in the deafferented superficial grey layer of the superior colliculus after enucleation, was not due to a decrease in the affinity of the serotonin-1 receptors for the radioligand, but to a decrease in the number of binding sites. Of the different serotonin-1 receptor subtypes, only the serotonin-1B was lost. This signifies that these receptors are probably located on the optic fibre terminals. Visual cortex lesion caused no apparent regulation of the serotonin-1 binding sites in the superficial grey layer of the superior colliculus. A bilateral enucleation produced a smaller decrease in serotonin-1 receptor density than that observed after unilateral enucleation, suggesting the existence of a compensatory mechanism.

Synaptic organization of dopaminergic amacrine cells in the larval tiger salamander retina

The ultrastructural features and synaptic interactions of tyrosine hydroxylase-like-immuno-reactive amacrine cells in the larval tiger salamander retina were examined using routine immunoelectron microscopy. The somas of tyrosine hydroxylase-like-immunoreactive amacrine cells were immunostained evenly throughout their cytoplasm. Their nuclei were generally unstained and possessed indented nuclear membranes. The processes of tyrosine hydroxylase-like-immunoreactive amacrine cells were homogeneously stained with the exception of their mitochondria, whose morphology was often disrupted by the staining procedure. Tyrosine hydroxylase-like-immunoreactive amacrine cell processes were characterized by an occasional dense-cored vesicle(s), in addition to a generally homogeneous population of small, round, agranular synaptic vesicles. They formed conventional synaptic junctions that were characterized by symmetrical synaptic membrane densities. A total of 168 synapses were observed that involved tyrosine hydroxylase-like-immunoreactive amacrine cell processes. A large percentage (79.8%) of these synaptic arrangements were found in sublayer 1 of the inner plexiform layer, while substantially lower percentages were observed in sublayers 3 (9.5%) and 5 (10.7%). They served as pre- and postsynaptic elements 63.1 and 36.9% of the time, respectively. Tyrosine hydroxylase-like-immunoreactive amacrine cell processes were presynaptic to amacrine cell processes (36.9% of total synaptic involvement) and processes that lack synaptic vesicles and whose origin remains uncertain (26.2%). They received synaptic input primarily from amacrine cell processes (31.0%). Tyrosine hydroxylase-like-immunoreactive amacrine cell processes also received a few ribbon synapses from bipolar cells (5.9%). Each of these synaptic relationships were observed in each of sublayers 1, 3 and 5 of the inner plexiform layer, with the majority of each arrangement being found in sublayer 1.

Quantitative analyses of the coexistence of gamma-aminobutyric acid in substance P-amacrine cells of the larval tiger salamander retina

The present study was performed as part of a systematic examination of gamma-aminobutyric acid's (GABA) coexistence with other classical transmitters and neuropeptides in neuronal populations of the larval tiger salamander retina. Substance P immunocytochemistry was combined with either GABA immunocytochemistry or autoradiography of high-affinity GABA uptake to examine for the presence of GABA in substance P-amacrine cells of the larval tiger salamander retina. Double-label analyses revealed two populations of substance P-amacrine cells that express both markers of GABA activity. One population was situated in the innermost cell row of the inner nuclear layer, while the other population was located in the ganglion cell layer. In both cases, these double-labelled cells accounted for approximately 10% of substance P-amacrine cells in their respective layers. The present study demonstrates, therefore, that substance P-amacrine cells in the larval tiger salamander retina can be categorized on the basis of their coexisting/non-coexisting relationships with GABA and suggests a possible functional diversity in the population of substance P-amacrine cells.

Enkephalin-immunoreactive ganglion cells in the pigeon retina

A small number of enkephalin-like immunoreactive cells were observed in the ganglion cell layer of the pigeon retina. Many of these neurons were identified as ganglion cells, since they were retrogradely labeled after injections of fluorescent latex microspheres in the contralateral optic tectum. These ganglion cells were mainly distributed in the inferior retina, and their soma sizes ranged from 12-26 microns in the largest axis. The enkephalin-containing ganglion cells appear to represent only a very small percentage of the ganglion cells projecting to the optic tectum (less than 0.1%). Two to 7 weeks after removal of the neural retina, there was an almost complete elimination of an enkephalin-like immunoreactive plexus in layer 3 of the contralateral, rostrodorsal optic tectum. These data provide evidence for the existence of a population of enkephalinergic retinal ganglion cells with projections to the optic tectum.

All indoleamine-accumulating cells in the rabbit retina contain GABA

The indoleamine-accumulating amacrine cells of the rabbit retina are wide-field and numerous. They form a dense plexus in sublamina 5 of the inner plexiform layer where they make reciprocal synapses with rod bipolar cells. To provide a quantitative test for the colocalization of serotonin (5-HT) and gamma-aminobutyric acid (GABA) in the rabbit retina, we designed two parallel double-label experiments. In the first series, the indoleamine-accumulating cells were labeled with 5,7-dihydroxytryptamine (5,7-DHT), which was subsequently visualized by photooxidation in the presence of diaminobenzidine. This was combined with autoradiography for 3H-muscimol. In the second and complementary series, 3H-5-HT uptake was combined with postembedding GABA immunocytochemistry. These two experiments provided essentially identical results: over 98% of the indoleamine-accumulating amacrine cells were double-labeled. This means that, within the limit of experimental error, all the indoleamine-accumulating amacrine cells are GABAergic. The indoleamine-accumulating amacrine cells account for 15-20% of a large diverse group of GABA amacrine cells. In addition, the rare type 3 indoleamine-accumulating cells and fine processes running in the optic fiber layer were double-labeled. If there is insufficient 5-HT to support a transmitter role in the rabbit retina, our results suggest that the indoleamine-accumulating cells may use GABA as a neurotransmitter. Thus, rod bipolar cells, in common with other bipolar cell types, receive extensive negative feedback at GABA-mediated reciprocal synapses.

A double-label study demonstrating that all serotonin-like immunoreactive amacrine cells in the larval tiger salamander retina express GABA-like immunoreactivity

A previous study localized serotonin-like immunoreactivity to amacrine cell populations in the larval tiger salamander retina. The present double-label immunocytochemical analysis of the tiger salamander retina was performed to determine if gamma-aminobutyric acid (GABA)-like immunoreactivity is expressed by serotonin-immunoreactive amacrine cells. More than 3,000 serotonin-amacrine cells were observed in double-label preparations, and all were found to express GABA-like immunoreactivity. This finding extends previous studies of serotonin-GABA coexistence in the retina by providing the first report of the co-localization of endogenous serotonin and GABA-like compounds in a retinal neuron.

Neurotransmitter-specific identification and characterization of neurons in the all-cone retina of Anolis carolinensis, I: Gamma-aminobutyric acid

The inhibitory amino-acid neurotransmitter, gamma-aminobutyric acid (GABA), was localized in the pure cone retina of the lizard Anolis carolinensis by autoradiographic and immunocytochemical techniques. Uptake of [3H]-GABA labeled horizontal cells, amacrine cells, numerous cells in the ganglion cell layer, both plexiform layers, and the nerve fiber layer. Label in the inner plexiform layer showed distinct lamination. The pattern of GABA immunoreactivity was similar to the pattern of [3H]-GABA uptake, although some differences, particularly in labeling of amacrine and ganglion cells, were observed. Immunocytochemistry revealed endogenous stores of GABA in a set of horizontal cells, amacrine cells, and cells in the ganglion cell layer. Both plexiform layers were labeled by the GABA antisera. Labeling in the inner plexiform layer (IPL) was highly stratified and GABA-immunoreactive strata were present in both sublaminae a and b. Six subtypes of conventionally placed GABA-immunoreactive amacrine cells and one displaced amacrine cell subtype were identified. Three of the six conventional amacrine cell subtypes were of pyriform morphology and three subtypes were of multipolar morphology. GABA-immunoreactive interstitial cells also were observed. Under certain conditions the GABA antiserum labeled the cones. Etching the resin eliminated cone labeling, suggesting that GABA in the cones is present in a labile pool, unlike GABA in horizontal or amacrine cells, or the observed labeling was not due to endogenous GABA. Cones did not demonstrate [3H]-GABA uptake.

Development of gamma-aminobutyric acid-immunoreactive neurons in normal and intracranially transplanted retinas in rats

Retinas from embryonic day 14 Sprague-Dawley rats were transplanted intracranially to the midbrain or cortex of newborn (P0) rats with right eyes enucleated at the time of transplantation and the gamma-aminobutyric acid (GABA) immunoreactivity in developing retinal transplants, host as well as normal retinas, was studied. The results showed that GABA-immunoreactive neurons were identified in retinas of normal and host rats from the day of birth (P0) onward and that their somata were distributed primarily in the inner half of the internal nuclear layer and in the ganglion cell layer. The adult pattern of GABA immunoreactivity was first observed at P16 when several immunoreactive sublaminae were clearly identifiable in the inner plexiform layer. In contrast, gamma-aminobutyric acid-immunoreactive somata could not be identified in retinal transplants until P4, with a significant reduction in the density and number of GABAergic neurons detected by P12. Moreover, only two immunoreactive sublaminae were observed in the inner plexiform layer in all transplants at P12, as well as in more mature stages. These results suggest that significant changes occurred in the GABA system of the transplanted retina, despite the fact that the overall pattern of organization of the GABAergic neurons and their processes in the retinal transplants was comparable to that of the normal retina.

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.

GABA-like immunoreactive neurons in the retina of Bufo marinus: evidence for the presence of GABA-containing ganglion cells

gamma-Aminobutyric acid (GABA)-like immunoreactive (IR) neurons in the retina of the cane toad Bufo marinus were revealed using immunohistochemistry on retinal wholemount preparation and sectioned material. GABA-IR neurons included horizontal, bipolar and amacrine cells in the inner nuclear layer and small to medium sized cells in the ganglion cell layer. A few IR axons were seen in the optic fiber layer of the retina. Following the injection of the carbocyanine dye, DiI into the optic tectum ganglion cells were retrogradely filled. A small population of DiI-filled ganglion cells (2.8%) was found to be GABA-IR. GABA-IR neurons in the ganglion cell layer without DiI label were considered to be displaced amacrine cells of which 45.3% were GABA positive. It is proposed that GABA-containing ganglion cells may form an inhibitory projection to visual centers of the anuran brain.

Localization of GABA, glycine, glutamate and tyrosine hydroxylase in the human retina

A light microscope study using postembedding immunocytochemistry techniques to demonstrate the common neurotransmitter candidates gamma-aminobutyric acid (GABA), glycine, glutamate, and tyrosine hydroxylase for dopamine has been done on human retina. By using an antiserum to GABA, we found GABA-immunoreactivity (GABA-IR) to be primarily in amacrine cells lying in the inner nuclear layer (INL) or displaced to the ganglion cell layer (GCL). A few stained cells in the INL, which are probably interplexiform cells, were observed to project thin processes towards the outer plexiform layer (OPL). There were heavily stained bands of immunoreactivity in strata 1, 3 and 5 of the inner plexiform layer (IPL). An occasional ganglion cell was also GABA-IR. By using an antiserum to glycine, stained cells were observed at all levels of the INL. Most of these were amacrines, but a few bipolar cells were also glycine-IR. Displaced amacrine cells and large-bodied cells, which are probably ganglion cells, stained in the GCL. The bipolar cells that stained appeared to include both diffuse and midget varieties. The AII amacrine cell of the rod pathway was clearly stained in our material but at a lower intensity than two other amacrine cell types tentatively identified as A8 and A3 or A4. Again, there was stratified staining in the IPL, with strata 2 and 4 being most immunoreactive. An antiserum to glutamate revealed that most of the neurons of the vertical pathways in the human retina were glutamate-IR. Rod and cone photoreceptor synaptic endings labeled as did the majority of bipolar and ganglion cells. The rod photoreceptor stained more heavily than the cone photoreceptor in our material. While both midget and diffuse cone bipolar cell types were clearly glutamate-IR, rod bipolars were not noticeably stained. The most strongly staining glutamate-IR processes of the IPL lay in the outer half, in sublamina a. The antiserum to tyrosine hydroxylase (TOH) revealed two different amacrine cell types. Strongly immunoreactive cells (TOH1) had their cell bodies in the INL and their dendrites ramified in a dense plexus in stratum 1 of the IPL. Fine processes arising from their cell bodies or from the stratum 1 plexus passed through the INL to reach the OPL but did not produce long-ranging ramifications therein. The less immunoreactive amacrines (TOH2) lay in the INL, the center of the IPL or the GCL and emitted thick dendrites that were monostratified in stratum 3 of the IPL.

Interrelationship between retinal ischaemic damage and turnover and metabolism of putative amino acid neurotransmitters, glutamate and GABA

Conditions causing a reduction of oxygen availability (anoxia), such as stroke or diabetes, result in drastic changes in ion movements, levels of neurotransmitters and metabolites and subsequent neural death. Currently, there is no clinically available treatment for anoxia induced neural cell death resulting in drastic and permanent central nervous system dysfunction. However, there have been some exciting developments in experimentally induced anoxic conditions where several classes of drugs appear to significantly reduce neural cell death. This report aims to provide the foundations for understanding both the basic mechanisms involved in retinal ischaemic damage and experimental treatments used to prevent such damage. We discuss the normal release, actions and uptake of the fast retinal neurotransmitters, glutamate and GABA, in the vertebrate retina. Immunocytochemistry is used to demonstrate that both glutamate and GABA are found in the macaque retina. Following this is a discussion on how ischaemia may enhance neurotransmitter release or disrupt its uptake, thus causing an increase in extracellular concentration of these neurotransmitters and subsequent neuronal damage. The mechanisms involved in glutamate neurotoxicity are reviewed, because excess glutamate is the likely cause of retinal ischaemic damage. Finally, the mechanisms behind four possible modes of treatment of neurotransmitter toxicity and their advantages and disadvantages are discussed. Hopefully, further research in this area will lead to the development of a rational therapy for retinal, as well as cerebral ischaemia.