ONTOGENY OF GABA-IMMUNOREACTIVE NEURONS IN THE CENTRAL-NERVOUS-SYSTEMIN A TELEOST, GASTEROSTEUS-ACULEATUS L

The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) is know n to exert various neurotrophic actions in the developing nervous syst em, but little is known about its distribution in the central nervous system during early development. We have studied the development of GA BA-immunoreactive (GABAir) neurons during embryogenesis of a teleost f ish, the three-spined stickleback. As early as 51 h postfertilization (PF; hatching occurs 144-168 h PF, and the first monoaminergic neurons appear around 72 h PF) GABAir neurons appear in the ventral prosencep halon caudal to the optic recess, in the ventral mesencephalon, and in the spinal cord. Then, there is a gradual addition of GABAir cell gro ups in the rostral prosencephalon and ventral rhombencephalon (66 h PF ), dorsal and caudal hypothalamus and pretectum (72 h PF), ventral hyp othalamus (78 h PF), preoptic region, thalamus, and in the mesencephal on and rhombencephalon (96 h PF). GABAir axons appear in the spinal co rd already at 51 h PF, and then gradually appear in the various tracts of the early axonal scaffold of pathfinding fibers, so that by 96 h P F the entire axonal scaffold contains GABAir fibers. It appears likely that GABAergic axons contribute a major population to the formation o f the axonal scaffold. Moreover, in the prosencephalon GABAir neurons are arranged in clusters that may reflect a neuromeric organization wi th six prosencephalic neuromeres.