Synaptic circuitry of serotonin-synthesizing and serotonin-accumulating amacrine cells in the retina of the cane toad, Bufo marinus

Serotonin released from amacrine neurons is scavenged and degraded in bipolar neurons in the retina

The neurotransmitter serotonin is synthesized in the retina by one type of amacrine neuron but accumulates in bipolar neurons in many vertebrates. The mechanisms, functions and purpose underlying serotonin accumulation in bipolar cells remain unknown. Here, we demonstrate that exogenous serotonin transiently accumulates in a distinct type of bipolar neuron. KCl-mediated depolarization causes the depletion of serotonin from amacrine neurons and, subsequently, serotonin is taken-up by bipolar neurons. The accumulation of endogenous and exogenous serotonin by bipolar neurons is blocked by selective reuptake inhibitors. Exogenous serotonin is specifically taken-up by bipolar neurons even when serotonin-synthesizing amacrine neurons are destroyed; excluding the possibility that serotonin diffuses through gap junctions from amacrine into bipolar neurons. Further, inhibition of monoamine oxidase A prevents the degradation of serotonin in bipolar neurons, suggesting that monoamine oxidase A is present in these neurons. However, the vesicular monoamine transporter 2 is present only in amacrine cells suggesting that serotonin is not transported into synaptic vesicles and reused as a transmitter in the bipolar neurons. We conclude that the serotonin-accumulating bipolar neurons perform glial functions in the retina by actively transporting and degrading serotonin that is synthesized in neighboring amacrine cells.

Presence of thyrotropin-releasing-hormone-immunoreactive (TRHir) amacrine cells in the retina of anuran and urodele amphibians

The presence of thyrotropin-releasing-hormone-immunoreactive (TRH-ir) amacrine cells in the retina of amphibians is reported for the first time. The anuran and urodele retinas studied exhibit major differences in the distribution of TRH-ir cells. In the two urodele species investigated, most TRH-ir amacrine cells were located in the ganglion cell layer (GCL). These pear-shaped cells originate a dense TRH-ir dendritic plexus in strata 4-5 of the inner plexiform layer (IPL). A small number of TRH-ir amacrine cells were observed in the inner nuclear layer (INL). Most of these INL TRH-ir cells were multipolar neurons with radiating dendrites that originate a loose plexus in the IPL stratum 1. In the three anuran species investigated, most TRH-ir amacrine cells were located in the INL. Distribution of TRH-ir processes in the IPL of anurans was not so clearly layered as in urodeles, dendrites being observed throughout strata 1-5. In the toad retina THR-ir material was also observed in the outer plexiform layer, which suggests that toads may have some TRH-ir interplexiform neurons. In the frog and toad, TRH-ir fibers were also observed in the optic nerve, although their origin could not be ascertained. The number of TRH-ir amacrine cells per whole retina was higher in anurans than in urodeles, though urodeles have higher cell densities. The marked differences in distribution of TRH-ir amacrine cells observed between anurans and urodeles, and among the three anuran species, suggest different functions of TRH in retinal processing, perhaps related to the different specializations of the visual systems of these species.

Synaptology of the inner plexiform layer in the anuran retina

The inner plexiform layer of the retina is a synaptic layer mostly devoid of perikarya. It contains the processes of three major neuron types: the bipolar cells, which carry information from the photoreceptors, the ganglion cells, which are the output elements of the retina, and the amacrine cells, which are able to influence the communication between the former two. Since amacrine cells are the most diverse retinal neurons, they are in a position to carve out and delineate the neural circuits of the inner retina. The aim of this review is to offer a summary of findings related to the general synaptology of the inner retina in frogs and also to provide some insight into the synaptic organization of neurochemically identified amacrine cells. The main conclusions of this paper are as follows: (i) Most contacts are formed between amacrine cells. (2) Direct bipolar to ganglion cell synapses exist, but are rare in the anuran retina. (3) All neurochemically identified amacrine cell types receive inputs from bipolar cells, but not all of them form reciprocal contacts with bipolar cell axon terminals. (4) A major inhibitory transmitter, gamma-aminobutyric acid, is involved in more than 50% of the synapses. Since contacts between inhibitory elements were often observed, disinhibitory circuits must also play a role in retinal information processing. (5) Reciprocal relationship between dopaminergic and gamma-aminobutyric acid-containing cells have been confirmed. Similar situation was observed in case of serotoninergic and gamma-aminobutyric acid-positive elements. No contacts were verified between serotoninergic and dopaminergic elements. (6) Both monoamine- and neuropeptide-containing amacrine cells establish direct contacts with ganglion cell dendrites, providing a morphological basis for neuromodulatory influence on the output elements of the retina. Unfortunately, only a handful of studies have been carried out to identify the synaptic connections between neurochemically identified cells in the anuran retina. Double-label studies at the electron microscope level to reveal the synaptic relationship of cell populations containing two different transmitters/modulators are extremely rare. Further insight into retinal synaptic circuitries could be gained with a combination of electrophysiology and morphology at the electron microscopic level. These studies must also involve identification of the transmitter receptors on identified cell types. Only after this step can the function of different synaptic circuitries be better approximated.

Synaptic inputs to retrogradely labeled ganglion cells in the retina of the cane toad, Bufo marinus

The entire population of ganglion cells in the retina of the toad Bufo marinus was labeled by retrograde transport of a lysine-fixable biotinylated dextran amine of 3000 molecular weight. Synaptic connections between bipolar, amacrine, and ganglion cells in the inner plexiform layer were quantitatively analyzed, with emphasis on synaptic inputs to labeled ganglion cell dendrites. Synapses onto ganglion cell dendrites comprised 47% of a total of 1234 identified synapses in the inner plexiform layer. Approximately half of the bipolar or amacrine cell synapses were directed onto ganglion cell dendrites, while the rest were made mainly onto amacrine cell dendrites. Most of the synaptic inputs to ganglion cell dendrites derived from amacrine cell dendrites (84%), with the rest from bipolar cell terminals. Synaptic inputs to ganglion cell dendrites were distributed relatively uniformly throughout all sublaminae of the inner plexiform layer. The present study provides unambiguous identification of ganglion cell dendrites including very fine processes, enabling a detailed analysis of the types and distribution of synaptic inputs from the bipolar and amacrine cell to the ganglion cells. The retrograde tracing technique used in the present study will prove to be a useful tool for identifying synaptic inputs to ganglion cell dendrites from neurochemically identified bipolar and amacrine cell types in the retina.

The number and distribution of bipolar to ganglion cell synapses in the inner plexiform layer of the anuran retina

The main route of information flow through the vertebrate retina is from the photoreceptors towards the ganglion cells whose axons form the optic nerve. Bipolar cells of the frog have been so far reported to contact mostly amacrine cells and the majority of input to ganglion cells comes from the amacrines. In this study, ganglion cells of frogs from two species (Bufo marinus, Xenopus laevis) were filled retrogradely with horseradish peroxidase. After visualization of the tracer, light-microscopic cross sections showed massive labeling of the somata in the ganglion cell layer as well as their dendrites in the inner plexiform layer. In cross sections, bipolar output and ganglion cell input synapses were counted in the electron microscope. Each synapse was assigned to one of the five equal sublayers (SLs) of the inner plexiform layer. In both species, bipolar cells were most often seen to form their characteristic synaptic dyads with two amacrine cells. In some cases, however, the dyads were directed to one amacrine and one ganglion cell dendrite. This type of synapse was unevenly distributed within the inner plexiform layer with the highest occurrence in SL2 both in Bufo and Xenopus. In addition, SL4 contained also a high number of this type of synapse in Xenopus. In both species, we found no or few bipolar to ganglion cell synapses in the marginal sublayers (SLs 1 and 5). In Xenopus, 22% of the bipolar cell output synapses went onto ganglion cells, whereas in Bufo this was only 10%. We conclude that direct bipolar to ganglion cell information transfer exists also in frogs although its occurrence is not as obvious and regular as in mammals. The characteristic distribution of these synapses, however, suggests that specific type of the bipolar and ganglion cells participate in this process. These contacts may play a role in the formation of simple ganglion cell receptive fields.

Synaptic circuitry of neuropeptide-containing amacrine cells in the retina of the cane toad, Bufo marinus

Synaptic connections of amacrine cells with substance P-like or neuropeptide Y-like immunoreactivity (SP-LI or NPY-LI) in the retina of the cane toad, Bufo marinus, were investigated using ultrastructural immunocytochemistry. The perikarya of SP-LI or NPY-LI amacrine cells were located in the innermost row of the inner nuclear layer. The synapses associated with SP-LI amacrine cells were distributed mainly in sublaminae 3 and 4 with about 10% in sublamina 1 of the inner plexiform layer. The synapses formed by NPY-LI amacrine cells were found in sublaminae 1, 2, and 4 with approximately equal frequency. Of a total of 175 SP-LI profiles, 56% were in presynaptic positions and 44% in postsynaptic positions. The synaptic inputs to SP-LI profiles predominantly derived from other unlabeled amacrine cell dendrites, and to a lesser extent, from bipolar cell terminals. The majority of synaptic outputs from SP-LI amacrine cell dendrites were directed onto unlabeled amacrine cell processes. The SP-LI profiles also made synapses onto bipolar cell terminals and formed synapses onto presumed ganglion cell dendrites. Of a total of 200 NPY-LI profiles, 48% were in presynaptic positions and 52% in postsynaptic positions. The profiles of NPY-LI amacrine cells mainly received their synaptic inputs from other unlabeled amacrine cell processes, and to a lesser extent, from bipolar cell terminals. The majority of NPY-LI amacrine cell profiles gave their synaptic outputs onto unlabeled amacrine cell dendrites, and others formed synapses onto presumed ganglion cell processes. These results suggest that these two populations of neuropeptide-containing amacrine cells in the Bufo retina are involved in different synaptic circuits.