Synaptology of the inner plexiform layer in the anuran retina

The inner plexiform layer of the retina is a synaptic layer mostly devoid o f perikarya. It contains the processes of three major neuron types: the bip olar cells, which carry information from the photoreceptors, the ganglion c ells, 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 positio n to carve out and delineate the neural circuits of the inner retina. The a im 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 i nto the synaptic organization of neurochemically identified amacrine cells. The main conclusions of this paper are as follows: (i) Most contacts are f ormed between amacrine cells. (2) Direct bipolar to ganglion cell synapses exist, but are rare in the anuran retina. (3) All neurochemically identifie d amacrine cell types receive inputs from bipolar cells, but not all of the m form reciprocal contacts with bipolar cell axon terminals. (4) A major in hibitory transmitter, gamma-aminobutyric acid, is involved in more than 50% of the synapses. Since contacts between inhibitory elements were often obs erved, disinhibitory circuits must also play a role in retinal information processing. (5) Reciprocal relationship between dopaminergic and gamma-amin obutyric acid-containing cells have been confirmed. Similar situation was o bserved in case of serotoninergic and gamma-aminobutyric acid-positive elem ents. No contacts were verified between serotoninergic and dopaminergic ele ments. (6) Both monoamine- and neuropeptide-containing amacrine cells estab lish direct contacts with ganglion cell dendrites, providing a morphologica l 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 a nuran retina. Double-label studies at the electron microscope level to reve al the synaptic relationship of cell populations containing two different t ransmitters/modulators are extremely rare. Further insight into retinal syn aptic circuitries could be gained with a combination of electrophysiology a nd morphology at the electron microscopic level. These studies must also in volve identification of the transmitter receptors on identified cell types. Only after this step can the function of different synaptic circuitries be better approximated. (C) 2000 Wiley-Liss, Inc.