Characterization of zebrafish GABA receptor subunits.

γ-Aminobutyric acid (GABA), the major inhibitory neurotransmitter in the central nervous system, exerts its effect through the activation of GABA receptors. GABA receptors are ligand-gated chloride channels composed of five subunit proteins. Mammals have 19 different GABA receptor subunits (α1-6, β1-3, γ1-3, δ, ε, π, θ, and ρ1-3), the physiological properties of which have been assayed by electrophysiology. However, the evolutionary conservation of the physiological characteristics of diverged GABA receptor subunits remains unclear. Zebrafish have 23 subunits (α1, α2a, α2b, α3-5, α6a, α6b, β1-4, γ1-3, δ, π, ζ, ρ1, ρ2a, ρ2b, ρ3a, and ρ3b), but the electrophysiological properties of these subunits have not been explored. In this study, we cloned the coding sequences for zebrafish GABA receptor subunits and investigated their expression patterns in larval zebrafish by whole-mount in situ hybridization. We also performed electrophysiological recordings of GABA-evoked currents from Xenopus oocytes injected with one or multiple zebrafish GABA receptor subunit cRNAs and calculated the half-maximal effective concentrations (EC50s) for each. Our results revealed the spatial expressions and electrophysiological GABA sensitivities of zebrafish GABA receptors, suggesting that the properties of GABA receptor subunits are conserved among vertebrates.