Expression of genes involved in brain GABAergic neurotransmission in three-spined stickleback exposed to near-future CO(2)

Change in the activity of the main inhibitory receptor, GABA(A), has been suggested to be a general mechanism behind the behavioural alterations reported in ocean acidification studies on fish. It has been proposed that regulatory acid–base mechanisms in response to high CO(2) alter the neuronal Cl(−) and HCO(3)(−) gradients that are important for GABA(A) receptor function. Here, we report a comprehensive analysis of gene expression of GABA(A) receptor subunits and of genes involved in GABAergic transmission in the brain of fish exposed to near-future CO(2). Altogether, 56 mRNA transcripts were quantified in brains of three-spined stickleback (Gasterosteus aculeatus) kept in control pCO(2) (333 ± 30 μatm CO(2)) or at high pCO(2) levels (991 ± 57 μatm) for 43 days. The gene expression analysis included GABA(A) receptor subunits (α1–6, β1–3, γ1–3, δ, π and ρ1–3), enzymes and transporters involved in GABA metabolism (GAD1–2, GABAT and GAT1–3), GABA(A) receptor-associated proteins (GABARAP and GABARAPL), ion cotransporters (KCC1–4, NKCC1, ClC21–3, AE3 and NDAE) and carbonic anhydrase (CAII). Exposure to high CO(2) had only minor effects on the expression of genes involved in GABAergic neurotransmission. There were significant increases in the mRNA levels of α family subunits of the GABA(A) receptor, with a more pronounced expression of α1(2), α3, α4 and α6b. No changes were detected in the expression of other GABA(A) subunits or in genes related to receptor turnover, GABA metabolism or ion transport. Although the minor changes seen for mRNA levels might reflect compensatory mechanisms in the high-CO(2) conditions, these were apparently insufficient to restore normal neural function, because the behavioural changes persisted within the time frame studied.