Localization and source of gamma aminobutyric acid immunoreactivity in theisthmic nucleus of the frog Rana esculenta

The distribution of gamma-aminobutyric acid (GABA)containing neurons and ne rve fibers was studied in the isthmic nucleus of the frog Rana esculenta us ing light and electron microscopical immunohistochemical techniques. Approx imately 0.5% of isthmic cells showed GABA immunopositivity, and the majorit y of these cells was found in the anterior one-third of the nucleus. A mesh work of GABA-immunostained fine beaded axons filled the entire isthmic nucl eus. The GABA-immunoreactive terminals formed pericellular basket-like stru ctures around a few cells both in the medulla and the cortex of the isthmic nucleus. To determine the source of GABA-positive fibers in the isthmic nu cleus lesion experiments were carried out. After unilateral tectal ablation no change was observed in GABA immunoreactivity. Hemisectioning the tegmen tum close to the anterior border of the isthmic nucleus, transection of the caudal tectal commissure and decussatio veli, or electrical lesioning of t he anterodorsal tegmental nucleus all resulted in a moderate decrease in th e density of GABA-positive fibers. Our results suggest that the majority of GABA-positive fibers derives from local GABA-positive cells, but some GABA ergic afferents seem to arise in the tegmentum, (C) 1999 Elsevier Science I nc.