Glutamate immunoreactivity in the tiger salamander retina differentiates between GABA-immunoreactive and...

A higher proportion of retinal amacrine cells, largely thought to be inhibitory, in tiger salamander retina are immunoreactive (+) for the excitatory amino acid glutamate than for either of the inhibitory amino acids, GABA and glycine. The colocalizations of glutamate immunoreactivity, GABA immunoreactivity and glycine immunoreactivity were studied in the proximal retina of tiger salamander using postembedding immunocytochemical methods in consecutive semithin sections. A considerable degree of colocalization was observed between glutamate immunoreactivity and GABA immunoreactivity and to a lesser extent with glycine immunoreactivity. No glutamate-immunoreactive cells were single labelled in INL3 (proximal inner nuclear), 12% were single labelled in INL2 (mid INL; these are bipolar cells), 62% were singled labelled in the ganglion cell layer (GCL) and likely to be ganglion cells. Single-labelled glycine-immunoreactive cells comprised 16% in INL3, 2%; INL2 and GCL, 0%). Glutamate immunoreactivity co-localized four times as often with GABA immunoreactivity than glycine immunoreactivity in INL3 (77% versus 19%), comparably in INL2 (37% and 32%) and six times as often in the GCL (29% versus 5%). A computerized image analysis of the relative intensities of glutamate immunoreactivity in GABA-immunoreactive/glutamate-immunoreactive and glycine-immunoreactive/glutamate-immunoreactive double labelled amacrine cells in INL3 revealed that all glycine-immunoreactive cells contained low intensities of glutamate immunoreactivity; whereas 35% of GABA-positive cells had medium to high intensities of glutamate immunoreactivity. There was a significant correlation (r = 0.44; p < 0.01) between the intensities of glutamate immunoreactivity and GABA immunoreactivity in GABA-immunoreactive/glutamate-immunoreactive amacrine cells. The common colocalization of glutamate immunoreactivity with GABA immunoreactivity and the positive correlation of their relative intensities suggests that glutamate is likely to serve as a precursor for GABA synthesis. However, intense glutamate immunoreactivity in 5% of the amacrine cells suggests a transmitter function. Glycine-immunoreactive amacrine cells contained little or no glutamate immunoreactivity, suggesting a minimal role for glutamate as a neurotransmitter in these cells. The major distribution of intense glutamate immunoreactivity is in bipolar cells and ganglion cells, consistent with data suggesting that glutamate is the transmitter in the retinal through pathway.