DISTRIBUTION OF GABA IMMUNOREACTIVITY IN THE CENTRAL AND PERIPHERAL NERVOUS-SYSTEM OF AMPHIOXUS (BRANCHIOSTOMA-LANCEOLATUM PALLAS)

On the basis of labeling with an anti-gamma-aminobutyric acid (GABA) a ntibody, we report for the first time the presence and distribution of GABA-immunoreactive cells in the central and peripheral nervous syste m of amphioxus. In the nerve cord, there is a large dorsorostral group of cerebrospinal-fluid-contacting (CSFc) cells at the caudal end of t he brain vesicle that gives rise to a large ventral commissure and neu ropilar region. In the middle and caudal region of the brain, numerous commissural and CSFc neurons are situated below the region of large d orsal cells. In the spinal cord, several types of GABA-immunoreactive neurons of different size, appearance, and distribution were observed. In the dorsalmost region, very small commissural cells are scattered regularly along the cord. More ventrally in the cord, GABAergic neuron s, both of commissural and CSFc cell types, form segmental groups, but scattered cells are observed throughout. These cells give rise to den se longitudinal fascicles of GABAergic fibers and to scattered commiss ural fibers. The caudal ampulla lacks GABAergic cells and fibers. Some of the fibers of the most rostral and caudal peripheral (sensory) ner ves, as well as some sensory cells of the rostral and caudal epidermis , are GABA immunoreactive. The significance of these results for the u nderstanding of the evolution of GABAergic systems of vertebrates is d iscussed. J. Comp. Neurol. 401:293-307, 1998. (C) 1998 Wiley-Liss, Inc .