Abstract
Glycine and γ-aminobutyric acid (GABA) are the major inhibitory neurotransmitters in the retina. Approximately half of the amacrine cells release glycine at their synapses with bipolar, other amacrine, and ganglion cells. Glycinergic amacrine cells are small-field amacrine cells with vertically oriented dendrites and comprise more than 10 different morphological types. The retinal distributions of glycine receptor (GlyR) α1, α2, α3 and α4 subtypes have been mapped with subunit-specific antibodies. GlyRs were clustered at postsynaptic hot spots which showed selective distributions for the different subunits. As a rule, only one α subunit was expressed at a given postsynaptic site. The kinetic properties of GlyRs were measured by recording spontaneous inhibitory postsynaptic currents (sIPSCs) from identified retinal neurons in wild-type, Glra1spd-ot, Glra2 and Glra3 knockout mice. From observed differences of sIPSCs in wild-type and mutant mice, the cell-type specific subunit composition of GlyRs could be defined. OFF-cone bipolar cells and A-type ganglion cells receive prominent glycinergic input with fast kinetics that is mainly mediated by α1β GlyRs (decay time constant τ ∼ 5 ms). By contrast, AII amacrine cells express α3β GlyRs with medium fast kinetics (τ ∼ 11 ms). Narrow-field (NF) and wide-field amacrine cells contain predominantly α2β GlyRs with slow kinetics (τ ∼ 27 ms). Lastly, ON-starburst, narrow-field and wide-field amacrine cells in Glra2 knockout mice express α4β GlyRs with very slow kinetics (τ ∼ 70 ms).
Highlights
Glycine and Glycine and γ-aminobutyric acid (GABA), the major inhibitory transmitters of the mammalian retina, are preferentially localized in different types of amacrine cells which fulfill specific roles in the processing of visual signals (Pourcho, 1996)
Strong glycine immunoreactivity can be observed in amacrine cell bodies and their dendrites descending into the inner plexiform layer (IPL) (Figure 1A)
Weak glycine expression is found in putative ON-cone bipolar cells in the centre of the inner nuclear layer (INL)
Summary
Glycine and GABA, the major inhibitory transmitters of the mammalian retina, are preferentially localized in different types of amacrine cells which fulfill specific roles in the processing of visual signals (Pourcho, 1996). The GlyR α1 subunit is expressed in a sparse population of puncta in the OPL, which represent synapses between glycinergic interplexiform processes and bipolar cell dendrites (Figures 3A,C). In the case of AII cells, the decay time constants measured in wild-type, Glra1spd-ot and Glra2−/− mice were not significantly different.
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