IMMUNOHISTOCHEMICAL INVESTIGATION OF GAMMA-AMINOBUTYRIC-ACID ONTOGENYAND TRANSIENT EXPRESSION IN THE CENTRAL-NERVOUS-SYSTEM OF XENOPUS-LAEVIS TADPOLES

The ontogeny of the gamma-aminobutyric acid (GABA)-positive neurons in the brain of Xenopus laevis tadpoles was investigated by means of imm unohistochemistry, using specific antibodies both against GABA and its biosynthetic enzyme, glutamate decarboxylase (GAD). The results obtai ned with the two antisera were comparable. The GABA system differentia tes very early during development. At stages 35/36, numerous GABA-posi tive neurons were seen throughout the prosencephalon and formed two ma in bilateral clusters within the lateral walls of the forebrain that r an caudally toward the hindbrain. Other GABA-immunolabeled cell bodies , together with a conspicuous network of GABAergic fibers, were seen i n the posterior hypothalamus. In the spinal cord, the lateral marginal zone was GABA-positive, as were Rohon-Beard neurons, interneurons, an d Kolmer-Agdhur cells. A very rich GABA innervation was observed in th e pars intermedia of the pituitary. At stage 50, plentiful immunoposit ive neurons and fibers were found in the telencephalic hemispheres, th e diencephalon, and the mesencephalon (optic tectum and tegmentum). By stage 54, the number of GABA-immunoreactive neurons in the posterior hypothalamus had decreased, so that, at stage 58, there were very few GABA-labeled cell bodies in the dorsolateral walls of the infundibulum , despite a strong GABAergic innervation within the median eminence an d the pars intermedia. From stage 58 to stage 66, the distribution pat tern was very similar to that described in the adult X. Laevis and in other amphibian species. These results point to transient GABA express ion within the hypothalamus, possibly related to either 1) a naturally occurring cell death or 2) a phenotypic switch. (C) 1996 Wiley-Liss, Inc.