Localized binding of [3H]muscimol to synapses in chicken retina

Binding sites for [3H]muscimol, an analogue of gamma-aminobutyric acid (GABA) were localized in the synaptic layers of chicken retina by light microscopic and electron microscopic autoradiography. Light microscopic autoradiography of cryostat sections incubated in [3H]muscimol or [3H]GABA revealed identical binding patterns: a band over the inner plexiform layer (IPL) and a band over the outer plexiform layer (OPL). This binding pattern differed from the uptake pattern for [3H]GABA: labeling over horizontal, amacrine, and ganglion cell bodies as well as very intense labeling over lamina 5 in the proximal IPL. Statistical analysis of electron microscopic autoradiography data from the IPL indicated that only amacrine synapses bind [3H]muscimol (i.e., make GABAergic synapses). Processes of amacrine, bipolar, or ganglion cells can be postsynaptic to these amacrine synapses. The highest concentration of synapses binding [3H]muscimol occurred in laminae 2 and 4 of the IPL and not in lamina 5 as might be expected from the density of [3H]GABA uptake. In the OPL, [3H]muscimol binding occurred over specialized junctions proximal to photoreceptor terminals. In cone receptor terminals, [3H]muscimol binding was suspected near horizontal cell dendrite/receptor terminal membranes lateral to the synaptic ribbon, supporting the hypothesis that horizontal cells are involved in a GABAergic feedback loop with cone terminals. We conclude that the synaptic binding pattern provides a more accurate concept of GABAergic synaptic interaction than does the uptake pattern for [3H]GABA because the two patterns in the IPL are not related.