A tale of two neurotransmitters

Abstract

Amacrine cells are a diverse set of local circuit neurons of the inner retina, and they all release either GABA or glycine, amino acid neurotransmitters that are generally inhibitory. But some types of amacrine cells have another function besides inhibiting other neurons. One glycinergic amacrine cell, the Aii type, excites a subset of bipolar cells via extensive gap junctions while inhibiting others at chemical synapses. Many types of GABAergic amacrine cells also release monoamines, acetylcholine, or neuropeptides. There is now good evidence that another type of amacrine cell releases glycine at some of its synapses and releases the excitatory amino acid glutamate at others. The glutamatergic synapses are made onto a subset of retinal ganglion cells and amacrine cells and have the asymmetric postsynaptic densities characteristic of central excitatory synapses. The glycinergic synapses are made onto other types of ganglion cells and have the symmetric postsynaptic densities characteristic of central inhibitory synapses. These amacrine cells, which contain vesicular glutamate transporter 3, will be the focus of this brief review.

Keywords: Amacrine; GABA; Glycine; Retina; Vesicular glutamate transporter.

Fig. 1

Schematic diagram showing the effects of the vGluT3 cells (GAC) on various types of ganglion cells in mouse retina. These amacrine cells mediate glycinergic inhibition of the suppressed-by-contrast, also known as uniformity detectors (UD), types and glutamatergic excitation of several types whose responses are enhanced-by-contrast, including: OFF Alpha, ON and ON–OFF direction-selective (DSGC), and a subpopulation of W3 cells. Modified from Fig. 4G of Lee et al., (2016).