RESUMO
In the vertebrate brain, inhibition is largely mediated by γ-aminobutyric acid (GABA). This neurotransmitter comprises a signaling machinery of GABAA, GABAB receptors, transporters, glutamate decarboxylases (gads) and 4-aminobutyrate aminotransferase (abat), and associated proteins. Chloride is intimately related to GABAA receptor conductance, GABA uptake, and GADs activity. The response of target neurons to GABA stimuli is shaped by chloride-cation co-transporters (CCCs), which strictly control Cl- gradient across plasma membranes. This research profiled the expression of forty genes involved in GABA signaling in the zebrafish (Danio rerio) brain, grouped brain regions and retinas. Primer pairs were developed for reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The mRNA levels of the zebrafish GABA system share similarities with that of mammals, and confirm previous studies in non-mammalian species. Proposed GABAA receptors are α1ß2γ2, α1ß2δ, α2bß3γ2, α2bß3δ, α4ß2γ2, α4ß2δ, α6bß2γ2 and α6bß2δ. Regional brain differences were documented. Retinal hetero- or homomeric ρ-composed GABAA receptors could exist, accompanying α1ßyγ2, α1ßyδ, α6aßyγ2, α6aßyδ. Expression patterns of α6a and α6b were opposite, with the former being more abundant in retinas, the latter in brains. Given the stoichiometry α6wßyγz, α6a- or α6b-containing receptors likely have different regulatory mechanisms. Different gene isoforms could originate after the rounds of genome duplication during teleost evolution. This research depicts that one isoform is generally more abundantly expressed than the other. Such observations also apply to GABAB receptors, GABA transporters, GABA-related enzymes, CCCs and GABAA receptor-associated proteins, whose presence further strengthens the proof of a GABA system in zebrafish.
