Expression of genes involved in brain GABAergic neurotransmission in three-spined stickleback exposed to near-future C[O.sub.2]

Change in the activity of the main inhibitory receptor, [GABA.sub.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 C[O.sub.2] alter the neuronal [Cl.sup.-] and HC[O.sub.3.sup.-] gradients that are important for [GABA.sub.A] receptor function. Here, we report a comprehensive analysis of gene expression of [GABA.sub.A] receptor subunits and of genes involved in GABAergic transmission in the brain of fish exposed to near-future C[O.sub.2]. Altogether, 56 mRNA transcripts were quantified in brains of three-spined stickleback (Gasterosteus aculeatus) kept in control pC[O.sub.2] (333 [+ or -] 30 [micro]atm C[O.sub.2]) or at high pC[O.sub.2] levels (991 [+ or -] 57 [micro]atm) for 43 days. The gene expression analysis included [GABA.sub.A] receptor subunits ([alpha]1-6, [beta]1-3, [gamma]1-3, [delta], [pi] and [rho]1-3), enzymes and transporters involved in GABA metabolism (GAD1-2, GABAT and GAT1-3), [GABA.sub.A] receptorassociated proteins (GABARAP and GABARAPL), ion cotransporters (KCC1-4, NKCC1, ClC21-3, AE3 and NDAE) and carbonic anhydrase (CAII). Exposure to high C[O.sub.2] had only minor effects on the expression of genes involved in GABAergic neurotransmission. There were significant increases in the mRNA levels of a family subunits of the [GABA.sub.A] receptor, with a more pronounced expression of [[alpha]1.sub.2], [alpha]3, [alpha]4 and [alpha]6b. No changes were detected in the expression of other [GABA.sub.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-C[O.sub.2] conditions, these were apparently insufficient to restore normal neural function, because the behavioural changes persisted within the time frame studied.