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.
Department of General Zoology and Neurobiology, University of Pécs, Ifjúság u. 6., H-7632, Hungary. gabriel@ttk.pte.hu
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Gábriel
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Rábl
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Veisenberger
D000818:Animals; D001001:Anura / Q000033:anatomy & histology* / Q000502:physiology; D004298:Dopamine / Q000378:metabolism; D017950:Ganglia, Sensory / Q000502:physiology / Q000648:ultrastructure; D009474:Neurons / Q000502:physiology / Q000648:ultrastructure; D009479:Neuropeptides / Q000378:metabolism; D012160:Retina / Q000166:cytology / Q000502:physiology / Q000648:ultrastructure; D012701:Serotonin / Q000378:metabolism; D013569:Synapses / Q000502:physiology / Q000648:ultrastructure; D005680:gamma-Aminobutyric Acid / Q000378:metabolism
D009479:Neuropeptides; D012701:Serotonin; D005680:gamma-Aminobutyric Acid; D004298:Dopamine