Anatomical and electrophysiological evidence for GABAergic bipolar cells in tiger salamander retina

The projective field of retinal bipolar cells and its modulation by visual context

The receptive field of a sensory neuron spells out all the receptor inputs it receives. To understand a neuron's role in the circuit, one also needs to know its projective field, namely the outputs it sends to all downstream cells. Here we present the projective fields of the primary excitatory neurons in a sensory circuit. We stimulated single bipolar cells of the salamander retina and recorded simultaneously from a population of ganglion cells. Individual bipolar cell signals diverge through polysynaptic pathways into ganglion cells of many different types and over surprisingly large distance. However, the strength and polarity of the projection depend on the cell types involved. Furthermore, visual stimulation strongly modulates the bipolar cell projective field, in opposite direction for different cell types. In this way, the context from distant parts of the visual field can control the routing of signals in the inner retina.

Immunocytochemical evidence that monkey rod bipolar cells use GABA

Certain bipolar cells in most species immunostain for GABA or its synthesizing enzyme glutamic acid decarboxylase. However, it is unknown whether they actually release GABA and, if so, from which cellular compartment and by what release mechanism. We investigated these questions in monkey retina where rod bipolar cells immunostain for GABA. We found that rod bipolar cells immunostain for one isoform of GAD (GAD65) in their somas, dendrites and axon terminals. Near the fovea, the somatic stain of rod bipolar cells is weaker than that of horizontal cells but, at the periphery, it is stronger. Staining for the vesicular GABA transporter in monkey rod bipolar cells is negative. However, staining for the GABA transporter GAT3 is positive in the soma and primary dendrites (but not in the axon terminals). Staining for GAT3 is also positive in horizontal cells. Double staining of rod bipolar cells and the alpha subunit of the GABAA receptor reveals scarce GABAA puncta that appose rod bipolar dendrites. We conclude that monkey rod bipolar cells use GABA and discuss the possibility that they tonically release GABA from their dendrites using a reverse action of GAT3.

Transporter-mediated GABA responses in horizontal and bipolar cells of zebrafish retina

GABA-mediated interactions between horizontal cells (HCs) and bipolar cells (BCs) transform signals within the image-processing circuitry of distal retina. To further understand this process, we have studied the GABA-driven membrane responses from isolated retinal neurons. Papain-dissociated retinal cells from adult zebrafish were exposed to GABAergic ligands while transmembrane potentials were monitored with a fluorescent voltage-sensitive dye (oxonol, DiBaC4(5)). In HCs hyperpolarizing, ionotropic GABA responses were almost never seen, nor were responses to baclofen or glycine. A GABA-induced depolarization followed by after hyperpolarization (dep/AHP) occurred in 38% of HCs. The median fluorescence increase (dep component) was 0.17 log units, about 22 mV. HC dep/AHP was not blocked by bicuculline or picrotoxin. Muscimol rarely evoked dep/AHP responses. In BCs picrotoxin sensitive, hyperpolarizing, ionotropic GABA and muscimol responses occurred in most cells. A picrotoxin insensitive dep/AHP response was seen in about 5% of BCs. The median fluorescence increase (dep component) was 0.18 log units, about 23 mV. Some BCs expressed both muscimol-induced hyperpolarizations and GABA-induced dep/AHP responses. For all cells, the pooled Hill fit to median dep amplitudes, in response to treatments with a GABA concentration series, gave an apparent k of 0.61 muM and an n of 1.1. The dep/AHP responses of all cells required both extracellular Na+ and Cl(-), as dep/AHP was blocked reversibly by Li+ substituted for Na+ and irreversibly by isethionate substituted for Cl(-). All cells with dep/AHP responses in zebrafish have the membrane physiology of neurons expressing GABA transporters. These cells likely accumulate GABA, a characteristic of GABAergic neurons. We suggest Na+ drives GABA into these cells, depolarizing the plasma membrane and triggering Na+, K+-dependent ATPase. The ATPase activity generates AHP. In addition to a GABA clearance function, these large-amplitude transporter responses may provide an outer plexiform layer GABA sensor mechanism.

GABA-immunoreactive photoreceptors in the retina of an anuran, Pelobates fuscus

We have recently started to unravel the retinal neurochemistry of an anuran species, the spadefoot toad (Pelobates fuscus), because of its unique lifestyle. The immunolabelling experiments included tests to localize the major inhibitory transmitter, gamma-aminobutyric acid (GABA) to subsets of retinal neurons, using commercially available antibodies. Apart from the regular GABA-immunoreactive pattern observed formerly in other anurans, certain structures in the photoreceptor layer were also regularly labeled for GABA. The soma diameter of the labeled cells is 5-6 microm and the outer segment seems to be unlabeled. In resin-embedded preparations GABA-positive photoreceptor cells were identified as cones based on their sparse distribution and short outer segments. If these cells release GABA as a transmitter, it may act on the second order cells, from which certain horizontal and bipolar cells have functional GABA receptors. Alternatively, GABA may influence the cones themselves through autoreceptors.

Synaptic organization of GABAergic amacrine cells in the salamander retina

The synaptic organization of GABA-immunoreactive (GABA-IR) amacrine cells in the inner plexiform layer (IPL) of salamander retina was studied with the use of postembedding immuno-electron microscopy. A total of 457 GABA-IR amacrine synapses, with identified postsynaptic elements, were analyzed on photomontages of electron micrographs covering 3,618 microm2 of the IPL. GABA-IR amacrine synapses were distributed throughout the IPL, with a small peak at the proximal margin of sublamina a. The majority of the output targets (81%) were GABA(-) neurons. Most of the contacts were simple synapses with one postsynaptic element identified as a process of an amacrine cell (55%), bipolar cell (19%) or ganglion cell (26%), and serial synapses were very rare. Of the 89 postsynaptic bipolar terminals, 63% participated in a reciprocal feedback synapse with the same presynaptic GABA-IR amacrine profile. There appeared to be no preference between GABA-IR amacrine contacts with rod- or cone-dominated bipolar cells (9.1% vs. 8.9%) or in the total number of amacrine synapses in sublaminas a and b (52% vs. 47%). The preponderance of amacrine cell input to bipolar cells in the OFF layer was derived from GABA-IR cells. These findings provide ultrastructural support to the existing physiological studies regarding the functional roles of the GABAergic amacrine cells in this species. Our results have added to the data base demonstrating that, in contrast to mammals, GABA-IR amacrine cells in amphibians and other nonmammals contact other amacrine cells more frequently, suggesting greater involvement of GABAergic amacrine cells in modulating lateral inhibitory pathways.

Evidence that certain retinal bipolar cells use both glutamate and GABA

Retinal bipolar neurons release the excitatory transmitter, glutamate. However, certain bipolar cells contain GABA, raising the question whether a neuron might release both transmitters and, if so, what function might the inhibitory transmitter play in a particular circuit? Here we identify a subset of cone bipolar cells in cat retina that contain glutamate, plus its vesicular transporter (VGLUT1), and GABA, plus its synthetic enzyme (GAD(65)) and its vesicular transporter (VGAT). These cells are negative for a marker of ON bipolar cells and restrict their axons to the OFF strata of the inner synaptic layer. They do not colocalize with the neurokinin 3 receptor that stains a type (or two) of OFF bipolar cells. By "targeted injection," we identified two types of OFF bipolar cell with the machinery to make and package both transmitters. One of these types costratifies with a dopamine plexus.

Differential synaptic organization of GABAergic bipolar cells and non-GABAergic (glutamatergic) bipolar cells in the tiger salamander retina

The synaptic organizations of gamma-aminobutyric acid-immunoreactive (GABA-IR, GABAergic) and non-GABA-IR (non-IR, glutamatergic) bipolar cells in salamander retina were compared by postembedding immunoelectron microscopy. A total of 238 presynaptic bipolar cell synapses were studied; 61 were GABA-IR and 177 were non-IR. Both groups were similar in that (1). they made asymmetrical ribbon synapses as well as asymmetrical non-ribbon synapses; (2). they made ribbon synapses at dyads, triads, and monads; and (3). the vast majority of ribbon synapses ( approximately 90%) were with dyads. The differences were that synapses of GABA-IR bipolar cells had a higher proportion of (1). direct contact with ganglion cells, (2). non-ribbon synapses, (3). output to GABA-IR amacrine cells, and (4). output in sublamina a. Overall, the output of GABA-IR ribbons was equally split between amacrine and ganglion cell processes, whereas for non-IR ribbons, it was approximately 2:1 in favor of amacrine cells. The ribbon:non-ribbon synapse ratio was approximately 1.2:1 (33:28) for GABA-IR but approximately 2:1 (118:59) for non-IR terminals. Thus, GABA-IR bipolar cells made more direct contacts with ganglion cells and used a higher proportion of non-ribbon synapses. GABA-IR dyads were more likely to contact GABA-IR amacrine profiles (52% vs. 38%). Finally, GABA-IR ribbon synapses were more common in sublamina a than sublamina b (2:1), whereas non-IR synapses were equally present in sublaminas a and b. This differential targeting of ganglion cells and amacrine cells in the OFF vs. ON layers indicates a difference in the role of bipolar cells in the generation of receptive field properties, depending on whether or not they use GABA as well as glutamate for their transmitter.