The early development of neurons with GABA immunoreactivity in the CNS of Xenopus laevis embryos.

We have used an antibody to glutaraldehyde fixation complexes of gamma-amino butyric acid (GABA) to stain the developing central nervous system of Xenopus laevis embryos. Neuronal somata, growth cones, axons, and dendrites were found with GABA-like immunoreactivity. Transmission electron microscope (TEM) observations were made of axons and synapses. By observation of the earliest stages of differentiation of neurons, seven classes of putative GABAergic interneurons were discerned. 1) Ascending neurons are first stained in the hindbrain at stage 26 and later extend caudally in the spinal cord. They have ascending ipsilateral axons. 2) Midhindbrain reticulospinal neurons are first stained at stage 25 and develop as a compact group with descending ipsilateral and contralateral axons. 3) Vestibular complex commissural neurons are first stained at stage 29/30 in a dorsal position near the entry of the seventh and eighth cranial nerves. They have ventral commissural axons that descend contralaterally and their somata form a compact mass. 4) Rostral hindbrain commissural neurons are first stained at stage 33/34 just rostral to the entry of the trigeminal nerve. They each have a decussating projection. 5) Rostral midbrain neurons are first stained in the midbrain at stage 29/30 and are later associated with prominent dorsal and ventral commissures. 6) Optic tract and 7) rostral forebrain neurons are found in the forebrain associated with strongly stained axon tracts. The direction of axonal growth from its earliest stages was distinct for each class of hindbrain and spinal cord neuron.