GABAergic synapses in the goldfish retina: an autoradiographic study of 3H-muscimol and 3H-GABA binding.

The localization of synaptic receptor sites for gamma-amino butyric acid (GABA) in the goldfish retina was studied by light microscopic (LM) and electron microscopic autoradiography (EM-ARG) of 3H-muscimol and 3H-GABA binding. Sodium-independent binding of both 3H-muscimol and 3H-GABA was localized to a uniform band throughout the inner plexiform layer (IPL) by LM-ARG of cryostat sections incubated in these compounds. No binding was observed over the outer plexiform layer. This binding pattern differs from the uptake pattern of 3H-GABA in the IPL, which shows very heavy labeling in the most proximal IPL and little labeling throughout the rest of the IPL. Statistical analysis of EM-ARGs of 3H-muscimol labeling in the IPL suggests that only amacrine synapses bind 3H-muscimol (i.e., make GABAergic synapses). Labeled amacrine-to-amacrine synapses are mostly concentrated in the 20-80% levels of the IPL, whereas amacrine-to-bipolar synapses are concentrated at about the 70-90% depth. Maximal 3H-GABA uptake, however, occurs at the 90-100% depth. This difference in the location of GABAergic synaptic binding and GABAergic uptake leads to the conclusion that the density of uptake label provides little information about the density of synaptic contacts. Thus, in order to localize GABAergic interactions, synaptic binding rather than neuronal uptake appears to provide the more appropriate description.