Bipolar Cell Terminals Research Articles

Neuropharmacological analysis of the role of indoleamine-accumulating amacrine cells in the rabbit retina.

In order to elucidate the role of putative indoleaminergic amacrine cells in visual processing, we have employed pharmacological agents specific for the two classes of serotonin receptor, 5-HT2 and 5-HT1, which have been identified in both the retina and brain. Perfusion of the rabbit retina with 5-HT2 selective antagonists decreases the ON-excitation of all classes of ganglion cell as well as the spontaneous activity of these cells. The effect on OFF-responses depends on the cell type: OFF-excitation of center-surround brisk and sluggish cells is increased or not affected by these drugs, but OFF excitation of ON/OFF direction selective cells is reduced. No disruption of the trigger features of direction selective or orientation selective cells was discovered, suggesting that indoleaminergic amacrine cells do not play a role in the generation of the complex properties of these cells. 5-HT1 receptors are heterogeneous and classified as a, b, or c subtypes. Since no selective antagonists are available for these sites, we have employed specific agonists. The most specific of these are for the 5-HT1A receptor. Perfusion with these agonists had physiological effects similar to those with perfusion of 5-HT2 antagonists. Thus, we have suggested that these two classes of serotonin receptors mediate opponent processes in the neural pathway. Since indoleaminergic cells make reciprocal synaptic connections with rod bipolar cell terminals, which are depolarizing in the rabbit retina, we hypothesize that 5-HT2 receptors facilitate the synaptic transmission from the depolarizing rod bipolar cell thus facilitating ON-excitation in the retinal network while 5-HT1A receptors mediate an inhibitory process. Similar self-opponent processing appears to take place in the hypothalamic and hippocampal serotonergic systems as well as in the dopaminergic retinal system. Thus, it is likely that this organization is a general feature of monoamine signal transmission in the central nervous system.

Synaptic organization of dopaminergic interplexiform cells in the goldfish retina.

The synaptic organization of dopaminergic interplexiform cells (DA-IPC) in the goldfish retina was studied by a combined double-label electron-microscopical (EM) immunocytochemical/autoradiographical study. DA-IPCs were labeled with antisera against tyrosine hydroxylase. The possibility of synaptic contact with GABAergic amacrine cells in the proximal inner plexiform layer (IPL) was studied by using 3H-GABA uptake. Most synaptic input and output from DA-IPC processes involved amacrine cell processes. In addition, synaptic interactions were observed between DA-IPC processes and bipolar cell terminals, other DA-IPC processes, very small dendrites in the IPL, ganglion cell and optic fiber layers (OFL), and cell bodies in the ganglion cell layer (GCL). Input and output synapses with GABAergic amacrine processes also were observed. Two-thirds of the DA-IPC boutons in the proximal IPL were involved in "junctional appositions," that is, the junctions appeared to be specialized but they were different than classical chemical synapses. The synaptic organization of DA-IPCs in the goldfish IPL appears to be far more complex than previously thought. Although earlier studies have attempted to explain the action of dopamine in terms of interaction only with amacrine cells, the present study shows that effects involving bipolar cells, other DA-IPCs, unidentified processes and cell bodies in the GCL and OFL must be considered as well.

Behavioural pleiotropy of the murine staggerer gene: Implications for the thermo-regulation

Previous studies in a mouse model of retinitis pigmentosa indicate that the GABAergic system in the retina may be overactive. GABA is known to act on GABAC receptors present on the axon terminals of bipolar cells to inhibit the release of excitatory neurotransmitter from these cells. The present study examined the effects of a GABAC receptor antagonist on the light-evoked responses of retinal ganglion cells (RGCs) in a rat model of retinitis pigmentosa. Extracellular recordings were made from RGCs in retinas isolated from P23H transgenic rats and non-dystrophic Sprague–Dawley (SD) rats. Spike activity of RGCs was measured in response to brief flashes of light over a range of light intensities. Intensity–response curves were evaluated prior to and during bath application of the GABAC receptor antagonist TPMPA. I found that TPMPA consistently increased the sensitivity of P23H rat RGCs to light flashes. For ON-center RGCs (n = 21), the average increase in light sensitivity was 0.63 log unit. For OFF-center RGCs (n = 6), the average increase was 0.38 log unit. TPMPA increased the maximum peak response of ON-center RGCs by 22% and OFF-center RGCs by 11%. However, the increase in maximum peak response of OFF-center RGCs was not statistically significant. TPMPA had no significant effect on the dynamic operating range of either ON-center or OFF-center RGCs. Nine ON-center SD rat RGCs were also tested. In contrast to what was observed for P23H rat RGCs, TPMPA decreased the sensitivity of these RGCs to light flashes, on average by 0.20 log unit. In conclusion, GABAC receptors may be novel targets for therapeutic interventions aimed at increasing light responsiveness in patients with retinitis pigmentosa or other diseases involving degeneration of photoreceptors.

GABAergic input to the synaptic terminals of mb 1 bipolar cells in the goldfish retina

An EM-autoradiographical/immunocytochemical technique was used to study amacrine cell synapses onto mb 1 bipolar cell terminals in goldfish retina. Tissue was double labeled for [ 3H]GABA uptake and glutamate decarboxylase (GAD) immunolocalization. Nearly 90% of the amacrine cell synaptic processes onto both proximal and distal halves of mb 1 terminals were labeled with either [ 3H]GABA or GAD-immunoreactivity (IR). Proximal half: 73% of the amacrine synapses were labeled with [ 3H]GABA uptake and 82% with GAD-IR; 88% of [ 3H]GABA labeled contacts were double labeled. Distal half: 17% of the amacrine synapses were labeled with [ 3H]GABA uptake and 67% with GAD-IR; 63% of [ 3H]GABA lbeled contacts wre double labeled. After consideration of the possible sources of [ 3H]GABA labeled synapses onto mb 1 terminals, we concluded that the synaptic terminals of pyriform Ab amacrine cells double label for [ 3H]GABA and GAD-IR despite our previous report that Ab cell bodies do not stain for anti-catfish brain GAD antiserum. We suggest that Ab cells contain isoenzymes of GAD which differ in subcellular distribution, thereby accounting for the differential staining of the cell bodies and dendrites obtained with the GAD antiserum we used.

Direct excitatory and lateral inhibitory synaptic inputs to amacrine cells in the tiger salamander retina

Two distinct synaptic currents in amacrine cells were measured using whole cell patch clamp in retinal slices. Synaptic activity was elicited with transretinal current passed from photoreceptors to ganglion cells to depolarize bipolar cell terminals. Recordings made in line with the stimulating electrodes revealed inward synaptic currents that reversed near 0 mV, positive to spike threshold. Recording 400 μm lateral to the stimulating electrodes revealed synaptic currents that reversed near-60 mV, negative to spike threshold. The lateral signal was blocked by tetrodotoxin and strychnine. A current with revesal potential similar to the lateral input was elicited by direct application of glycine to the amacrine cells. The results suggest that bipolar, but not amacrine input can elicit spikes in neighboring amacrine cells, that amacrine cells are mutually inhibitory and the inhibitory lateral transmission is limited to the extent of processes of the excited, spiking cell.

Calmodulin-dependent protein phosphatase: immunocytochemical localization in chick retina.

Calmodulin-dependent protein phosphatase, previously called CaM-BP80 or calcineurin, is present in high concentrations in the central nervous system. The level of the phosphatase has been shown by radioimmunoassay to increase during development in the retinas of embryonic and hatching chicks (Tallant, E.A., and W.Y. Cheung, 1983, Biochemistry, 22:3630-3635). The aims of this study are to immunocytochemically localize the phosphatase in developing and mature retinas and to determine if the phosphatase is present in fractions of retinal synaptic membranes and synaptic junctions. Vibratome slices of fixed chick retina and Western blots of detergent-solubilized retinal fractions are both treated sequentially with rabbit primary antisera and goat anti-rabbit Fab fragments conjugated to peroxidase, and then reacted with hydrogen peroxide and diaminobenzidine. The tissue slices are further processed for electron microscopy. This paper demonstrates the presence of peroxidase reaction product in the retina just before synapse formation. In the outer plexiform layer the product is confined to photoreceptor synaptic terminals, whereas in the inner plexiform layer it is present in synaptic terminals of bipolar cells and in dendrites of ganglion cells. In this latter site the product is present postsynaptically at bipolar and amacrine synapses. The phosphatase is detected in Western blots of both synaptic plasma membrane and synaptic junction fractions.

Opioid pathways in an avian retina. II. Synaptic organization of enkephalin-immunoreactive amacrine cells

Peroxidase-antiperoxidase immunocytochemistry was utilized to investigate the synaptic organization of enkephalin-like immunoreactive amacrine (Enk-amacrine) cells in the chicken retina. An initial light microscopic analysis revealed that immunostained cell bodies were situated in either the second or third tier of cells from the border of the inner nuclear and inner plexiform layers. The processes of such cells extended into the inner plexiform layer where they ramified as a fine plexus in sublaminae 1 and 3 to 5. At the ultrastructural level, enkephalin-positive somas exhibited a rather dense and evenly distributed peroxidase reaction product throughout their cytoplasm. Furthermore, the nucleus of Enk-amacrine cells was characterized by a round, unindented nuclear membrane. Immunoreactive processes of such cells were found to receive synaptic input from unstained amacrine and bipolar cells. Immunolabeled varicosities formed conventional synaptic contacts onto other vesicle-filled, nonimmunoreactive profiles tentatively suggested to originate from amacrine cells. Bipolar cell terminals did not receive synaptic input from stained varicosities of enkephalin-immunoreactive amacrine cells. Moreover, each of the above synaptic relationships were identified in each of sublaminae 1, and 3 to 5 of the inner plexiform layer. In addition, labeled varicosities of Enk-amacrine cells synapsed upon unstained processes which lacked synaptic vesicles and possibly arise from ganglion cells. Enkephalin-positive processes were also observed to synapse upon the vitreal pole of unstained somas situated in the innermost cell row of the inner nuclear layer. Lastly, large immunoreactive varicosities of Enk-amacrine cells were often characterized by the presence of large dense-core vesicles in addition to typical, small agranular synaptic vesicles.

Bar synapses and gap junctions in the inner plexiform layer: synaptic relationships of the interstitial amacrine cell of the retina of the cichlid fish, Astronotus ocellatus.

The retina of the cichlid fish, Astronotus ocellatus, contains an unusual class of amacrine cell, the interstitial amacrine cell, which has its soma and processes restricted to a sublamina of the proximal inner plexiform layer. The interstitial amacrine cell is unique in making synapses which contain a presynaptic dense bar specialization. The interstitial amacrine cell makes reciprocal synapses with bipolar cell terminals and is presynaptic to other amacrine cells and to ganglion cell dendrites. Processes of interstitial amacrine cells are connected to each other by large gap junctions.

Immunocytochemical and autoradiographic localization of GABA system in the vertebrate retina.

The localization of γ-aminobutyric acid (GABA) neurons in the goldfish and the rabbit retina has been studied by immunocytochemical localization of the GABA-synthesizing enzyme L-glutamate decarboxylase (GAD, L-glutamate 1-carboxy-lase, EC 4.1.1.15) and by [3H] GABA uptake autoradiography. In the goldfish retina, GAD is localized in some horizontal cells (H1 type), a few amacrine cells and sublamina b of the inner plexiform layer. Results from immunocytochemical studies of GAD-containing neurons and autoradiographic studies of GABA uptake reveals a marked similarity in the labeling pattern suggesting that in goldfish retina, the neurons which possess a high-affinity system for GABA uptake also contain significant levels of GAD. In the rabbit retina, when Triton X-100 was included in immunocytochemical incubations with a modified protein A-peroxidase-antiperoxidase method, reaction product was found in four broad, evenly spaced laminae within the inner plexiform layer. In the absence of the detergent, these laminae were seen to be composed of small, punctate deposits. When colchicine was injected intravitreally before glutamate decarboxylase staining, cell bodies with the characteristic shape and location of amacrine cells were found to be immunochemically labeled. Electron microscopic examination showed that these processes were presynaptic to ganglion cell dendrites (infrequently), amacrine cell telodendrons, and bipolar cell terminals. Often, bipolar cell terminals were found which were densely innervated by several GAD-positive processes. No definite synapses were observed in which a GAD-positive process represented the postsynaptic element. In autoradiographic studies by intravitreal injection of [3H] GABA a diffuse labeling of the inner plexiform layer and a dense labeling of certain amacrine cell bodies in the inner nuclear layer was observed. Both immunocytochemical and autoradiographic results support the notion that certain, if not all, amacrine cells use GABA as their neurotransmitter.

Immunocytochemical localization of GABA neurons in the rabbit and frog retina

The visualization of γ-aminobutyric acid (GABA) neurons in rabbit and frog retinas has been carried out, using an immunocytochemical technique for the localization of the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD). In the rabbit, immunoreactivity was restricted to a small group of amacrine cell bodies and their laminated processes in the inner plexiform layer. Electron microscopic examination showed that these processes were presynaptic to ganglion cell dendrites (infrequently), amacrine cell telodendrons, and bipolar cell terminals. Often, bipolar cell terminals were found which were densely innervated by several GAD-positive processes. No definite synapses were observed in which a GAD-positive process represented the postsynaptic element. In the frog, dense GAD immunoreactivity was observed in the inner plexiform layer, both as punctate deposits and as filled processes of stratified and diffuse amacrine cells; in the inner nuclear layer, where many cell bodies were labeled, including those of some horizontal cells; and diffusely in the outer plexiform layer.

The interplexiform cell system. I. Synapses of the dopaminergic neurons of the goldfish retina.

Interplexiform cells are a class of retinal neuron that extends processes widely in both plexiform layers. In goldfish they contain dopamine and readily take up certain biogenic amines. Two of these amines, 6-hydroxyopamine (6-HDA) and 5, 6-dihydroxytryptamine (5,6-DHT), induce fine structural changes in the neurons that accumulate them, allowing the processes of the cells to be recognized by electron microscopy. Typically, the synaptic vesicles within the processes show electron-dense cores. The terminal cytoplasm may also show increased density, as may the cellular and cytoplasmic membranes, presumably an indication of degenerative changes induced by the drugs. 5, 6-DHT gives more readily observable changes than 6-HDA but labels both dopaminergic and indoleamine-accumulating neurons. The terminals of the indoleamine-accumulating terminals were therefore removed by intraocular injections of 5, 7-dihydroxytryptamine (5, 7-DHT) prior to the labelling with 5, 6-DHT. This procedure allowed an analysis of the dopaminergic terminals without interference by the terminals of the indoleamine-accumulating cells. The dopaminergic neurons were found to make synapses of the conventional type. In the outer plexiform layer they contacted both external horizontal cells and bipolar cell dendrites, but not hotoreceptor terminals or intermediate (rod) horizontal cells. No synapses onto the dopaminergic processes were found in the outer plexiform layer despite an extensive search. In the inner plexiform layer the dopaminergic processes were observed to be both pre- and postsynaptic to amacrine cells and their processes. No synaptic contacts between dopaminergic processes and bipolar cell terminals or ganglion cell dendrites were seen. We conclude that the dopaminergic interplexiform cells provide a centri­fugal pathway for information flow in the retina from inner to outer plexiform layer.

Structural basis for ON-and OFF-center responses in retinal ganglion cells.

The inner plexiform layer of the mammalian retina has a bisublaminar organization determined by restricted branching of the terminals of cone bipolar cells and dendrites of class I (large) and class II (small) ganglion cells. Comparison of dendritic field diameters and receptive fiedl center sizes of large ganglion cells suggests that neural circuitry in sublamina a conveys "OFF"-center properties and connections in sublamina b "ON"-center properties to retinal ganglion cells.

Subcellular fractionation of the rabbit retina: the isolation of synaptic pedicles and inner segments of photoreceptor cells

The effect of mechanical or hand homogenization on the integrity of rabbit retinal photoreceptors has been investigated by electron microscopy. Four regions of these cells can be isolated: photoreceptor synaptic pedicles, nucleic and the inner and outer segments. All were sedimented in a low-speed nuclear pellet and have been further purified by centrifugation on a sucrose density gradient. Bipolar cell terminals and conventional nerve endings from amacrine and horizontal cells were sedimented at higher speeds in a crude mitochondrial pellet.

Synapses made by myelinated fibers running to teleost and elasmobranch retinas.

AbstractMyelinated fibers are described in teleost (carp, goldfish) and elasmobranch (dogfish) retinas which make synapses onto amacrine cell bodies and bipolar cell terminals. The fibers run in parallel with bundles of optic nerve fibers radiating from the optic disk until at some point they traverse the inner plexiform layer, turning again horizontally to course under the layer of amacrine cell bodies for 0.5–0.75 mm. En route each fiber makes several en passant synapses characterized by a cessation of the myelin sheath, a local increase in axonal diameter, an accumulation of agranular vesicles 300–400 Å in diameter and increased densification of pre‐ and post‐synaptic membranes. These fibers are not post‐synaptic to any retinal element. The evidence that the myelinated fibers described form an efferent system is based on their resemblance in many particulars to the centrifugal fiber system innervating the pigeon retina. Degeneration experiments, however, could not distinguish conclusively between a true efferent system, originating in some central nucleus, and a recurrent collateral system branching from afferent optic nerve axons.

Organization of the primate retina: electron microscopy.

The retinae of monkey and man have been studied by electron microscopy to identify cell types, their processes and synaptic contacts. In the inner plexiform layer, the morphological characteristics of the three types of cells (bipolar, ganglion and amacrine) are described and seven synaptic relationships are identified. The bipolar terminals contain ribbons at points of synaptic contact, and, at these points, there are typically two postsynaptic processes, one a ganglion cell dendrite, the other an amacrine cell process. This synaptic arrangement is here termed a dyad. The amacrine cell processes themselves make synaptic contacts with ganglion cell dendrites and somata, other amacrine cell processes, and, most frequently, with the bipolar cell terminals. Often, the amacrine-bipolar contact is adjacent to a bipolar-amacrine junction, forming a reciprocal synaptic arrangement between the bipolar and the amacrine. In the more peripheral retina, large bipolar cell terminals (probably of rod bipolars) are occasionally observed adjacent to the perikarya of the ganglion cells. At these junctions, areas of fusion between the plasma membranes are seen, suggesting that such axosomatic junctions could be electrical. In the outer plexiform layer, synapses have been identified only in the receptor cell bases where receptor cells contact bipolar and horizontal cell processes. Synaptic contacts of the horizontal cells have not been clearly identified, but their strategic terminations in the receptor cell ending are described and interpreted as possibly synaptic. A model of the retina, based on the described anatomy, is presented and correlated with ganglion cell physiology.