Ultrastructure and GABA Immunoreactivity in Layers 8 and 9 of the Optic Tectum of Xenopus laevis

This study presents an ultrastructural analysis of layers 8 and 9 in the optic tectum of Xenopus laevis. Retinotectal axons were labelled with horseradish peroxidase and tectal cells were labelled with antibody to GABA. Four distinct axonal and dendritic structures were identified. GABA‐negative axon terminals formed asymmetric synapses and were categorized as type a‐1 (which included retinotectal axons), characterized by medium size synaptic vesicles and pale mitochondria, and type a‐2 (non‐retinotectal) with large vesicles and dense mitochondria. GABA‐negative dendrites (type d) contained dense mitochondria, microtubules in the dendritic shafts, and dendritic spines devoid of microtubules. GABA‐positive structures contained small synaptic vesicles and dense mitochondria. Some dendrites (type D) were not only postsynaptic but were also presynaptic elements, as defined by the presence of vesicles and distinct synaptic clefts with symmetric specializations. GABA‐positive presynaptic structures were mostly located in vesicle‐filled, bulbous extensions of dendritic shafts and usually terminated onto dendritic spines. Some type D dendrites were the middle element in serial synapses, with input from either GABA‐positive or GABA‐negative structures and output to GABA‐negative structures. Retinotectal terminals were identified as one of the synaptic inputs to GABA‐positive processes. Glia were characterized by granular cytoplasm and large mitochondria, often displaying a crystalline matrix structure. These results indicate that GABA‐positive neurons are a prominent component of circuitry in the superficial layers of the tectum of Xenopus and that, as in mammals, they participate in serial synaptic arrangements in which retinotectal axons are the first element. These arrangements are consistent with complex processing of visual input to the tectum and a central role for inhibitory processes in the shaping of tectal responses.