Expression of genes involved in GABAergic neurotransmission in anoxic crucian carp brain (Carassius carassius).

The crucian carp, Carassius carassius, survives days to months without oxygen, depending on temperature. In the anoxic crucian carp brain, increased GABAergic inhibition, mediated by increased extracellular levels of GABA, has been shown to suppress electric activity and ATP consumption. To investigate an involvement of gene expression in this response, we utilized real-time RT-PCR to test the effect of 1 and 7 days anoxia (8 degrees C) on the expression of 22 genes, including nine GABA(A) receptor subunits (alpha(1-6), beta(2), delta, and gamma(2)), three GABA(B) receptor subunits (G(B)1a-1b and G(B)2), three enzymes involved in GABA metabolism (GAD65 and GAD67, GABAT), four GABA transporters (GAT1, 2a-b and 3), two GABA(A) receptor-associated proteins (GABARAP 1 and 2), and the K(+)/Cl(-) cotransporter KCC2. While the expression of GABA(A) receptor subunits was dominated by alpha(4)-, alpha(6)-, and delta-subunits, all of which are located to extrasynaptic sites in mammalian brains and respond to elevations in extracellular levels of GABA by showing tonic activity patterns, the expression of GABA transporters was dominated by GAT2 (a and b) and GAT3, which also show extrasynaptic location in mammals. These expression patterns differ from those observed in mammals and may be a prerequisite for GABAergic inhibition of anoxic metabolic rate in crucian carp. Furthermore, while the expression of the majority of the genes was largely unaltered by anoxia, the expression of GAT2 and GAT3 decreased to 20%. This suggests impairment of GABA transport, which could be a mechanism behind the accumulation of extracellular GABA and the increased GABAergic inhibition.