SEQUENCE AND EXPRESSION OF GLUTAMIC-ACID DECARBOXYLASE ISOFORMS IN THE DEVELOPING ZEBRAFISH

We describe the isolation two glutamic acid decarboxylase (GAD) cDNAs from zebrafish with over 84% identity to human GAD65 and GAD67. In sit u hybridization studies revealed that both GAD65 and GAD67 were expres sed in the early zebrafish embryo during the period of axonogenesis, s uggesting a role for GABA prior to synapse formation. Both GAD genes w ere detected in the telencephalon, in the nucleus of the medial longit udinal fasciculus in the midbrain, and at the border regions of the rh ombomeres in the rostral hindbrain. In the caudal hindbrain, only GAD6 7 was detected (in neurons with large-caliber axons). In the spinal co rd, both GAD genes were detected in dorsal longitudinal neurons, commi ssural secondary ascending neurons, ventral longitudinal neurons, and Kolmer-Agduhr neurons. Immunohistochemistry for gamma-aminobutyric aci d (GABA) revealed that GABA is produced at all sites of GAD expression , including the novel cells in the caudal hindbrain. These results are discussed in the context of the hindbrain circuitry that supports the escape response. We conclude that fish, like mammals, have two GAD ge nes. The zebrafish GAD65 and GAD67 are present in identified neurons i n the forebrain, midbrain, hindbrain, and spinal cord, and they cataly ze the production of GABA in the developing embryo. (C) 1998 Wiley-Lis s, Inc.