Figure 3. GABA immunoreactivity in optic tectum of stage 42 and stage 47 tadpoles (horizontal plane).

A. Stage 42: Left: Schematic indicating relative positions of horizontal sections through the dorsal midbrain and locations of major brain regions (top left). Schematic of a horizontal section through the brain with locations of brain regions labeled. Blue is the cell body area; white is the neuropil area. An image of a GABA-immunolabeled right hemisection is superimposed on the schematic (bottom left). Sections (1–5) show GABA-immunoreactivity (green) counterstained with PI (blue). B. Schematic of horizontal brain section (left) showing regions of high magnification images, shown to the right. Higher magnification single optical sections from stage 42 midbrain. B1. Intense GABA immunolabeling of axons in the tecto-tegmental commissure (ttc) and posterior commissure (pc) (solid arrows). B2. Clustered GABA-immunoreactive neurons in the optic tectum (solid arrows) extend processes toward the neuropil. B2a, b. Enlargements of boxed regions in B2 showing GABA-immunoreactive processes (arrows) extending from labeled cell bodies (arrowheads in 2a,b). C. Stage 47: Left. Schematics comparable to A. Sections (1–5) of GABA-immunoreactivity (green) and PI counterstain (blue). GABA-immunoreactivity becomes more broadly distributed across the optic tectal cell body layer (arrowheads) and neuropil. D. Higher magnification (single optical sections) showing strong GABA labeling in the lateral forebrain bundle (lfb, D1), and sparse GABA-positive somata in the caudolateral optic tectum (D2a, arrowheads) extending GABA-positive processes toward the neuropil (D2a, solid arrows). D3. The border between the caudal optic tectum and the medial hindbrain (HB) shows that the proliferative zone in caudal tectum is negative for GABA immunostaining (arrows), whereas neuronal cell bodies and processes in the medial HB are GABA-immunolabeled (arrowheads). Scale bars, A, C: 50 µm; B1, 2: 20 µm; B2a,b: 10 µm: D1,3: 30 µm; D2a,b: 20 µm.