Expression of glycine and the glycine transporter Glyt-1 in the developingrat retina

Previous studies show that glycine transporter-1 (glyt-l) is a consistent m embrane marker of adult retinal neurons that are likely to release glycine at their synaptic terminals (Pow, 1998; Vaney et al., 1998; Pow & Hendricks on, 1999). The current study investigated when glyt-l immunoreactivity appe ared in the postnatal rat retina, and whether all glycine-containing neuron s also labelled for glyt-1. Ganglion cells, horizontal cells, and photorece ptors showed transient labelling. Many cells in the ganglion cell layer are immunoreactive for both glycine and glyt-1 at postnatal day (Pd) 1 but bot h are minimal by Pd5. Transient immunoreactivity for both glyt-1 and glycin e was observed in presumptive horizontal cells between Pd5 and Pd10. At Pd1 many cells in the outer part of the retina which resembled immature photor eceptors were heavily labelled for glycine, but did not express glyt-1; the se disappeared at older ages. These findings suggest diverse mechanisms and transient roles for glycine in the developing rat retina. In the adult rat retina, a subpopulation of amacrine cells are prominently immunoreactive f or both glycine and glyt-1. These cells labelled for glycine at Pd1, but di d not express significant levels of glyt-1 until Pd5. Processes from these amacrine cells did not reach the inner half of the inner plexiform layer un til Pd10-14. Bipolar cells became glycine-IR between Pd10 and Pd14, but con sistently lacked any glyt-1 immunoreactivity. This temporal pattern of labe lling strongly indicates that bipolar cells label for glycine when gap junc tions become functional between glycine/glyt-1 immunoreactive amacrine cell s and cone bipolar cells.