Differential expression of alpha1, alpha2, alpha3, and alpha5 GABAA receptor subunits in seizure-prone and seizure-resistant rat models of temporal lobe epilepsy

Abstract

Temporal lobe epilepsy remains one of the most widespread seizure disorders in man, the etiology of which is controversial. Using new rat models of temporal lobe epilepsy that are either prone or resistant to develop complex partial seizures, we provide evidence that this seizure susceptibility may arise from arrested development of the GABAA receptor system. In seizure-prone (Fast kindling) and seizure-resistant (Slow kindling) rat models, both the mRNA and protein levels of the major alpha subunit expressed in adult brain (alpha1), as well as those highly expressed during development (alpha2, alpha3, and alpha5), were differentially expressed in both models compared with normal controls. We found that alpha1 subunit mRNA expression in the Fast kindling strain was approximately half the abundance of control rats, whereas in the Slow kindling strain, it was approximately 70% greater than that of controls. However, Fast rats overexpressed the alpha2, alpha3, and alpha5 ("embryonic") subunits, having a density 50-70% greater than controls depending on brain area, whereas the converse was true of Slow rats. Using subunit-specific antibodies to alpha1 and alpha5 subunits, quantitative immunoblots and immunocytochemistry revealed a concordance with the mRNA levels. alpha1 protein expression was approximately 50% less than controls in the Fast strain, whereas it was 200% greater in the Slow strain. In contrast, alpha5 subunit protein expression was greater in the Fast strain than either the control or Slow strain. These data suggest that a major predispositional factor in the development of temporal lobe epilepsy could be a failure to complete the normal switch from the GABAA receptor alpha subunits highly expressed during development (alpha2, alpha3, and alpha5) to those highly expressed in adulthood (alpha1).

Fig. 1.

In situ hybridization histochemistry reveals differences in the expression of GABA_A receptor subunit mRNA expression in the lateral and basolateral amygdala. Dark-field photomicrograph of lateral and basolateral amygdala shows that α₁ subunit transcript density in these two areas is less in the Fast kindling rats than in normal control rats, which is less than in Slow rats. A similar photomicrograph shows a reciprocal expression pattern between the three strains for α₂ subunit expression. Ce, Central amygdala.

Fig. 2.

α₁, α₃, and α₅ subunit mRNA expression in amygdala and the endopiriform nucleus. Bright-field photographs show lower α₁ subunit mRNA density in the LA of Fast rats compared with normal control and Slow rats. In contrast, α₃ and α₅ subunit mRNA expression in the endopiriform and BLA nuclei, respectively, are elevated in Fast compared with normal control and Slow rats.

Fig. 3.

α₁ and α₅ subunit mRNA expression in the hippocampus is not different between the strains.

Fig. 4.

A, Quantitative immunoblot of α₁ and α₅ subunit protein expression. In concordance with α₁ subunit mRNA expression, α₁ subunit protein expression is lower in Fast rats compared with normal control and Slow rats. Complimentary results were obtained for α₅ subunit protein in which the latter was greater in Fast rats than in normal control and Slow rats.B, α₁ labeling shows subunit protein expression in layers I–III of the piriform cortex. Little labeling occurred in layer I (top arrow) in Fast kindling rats in which clear labeling is observed in normal control and Slow kindling rats. The cell body layer (layer II, bottom arrow) and all of layer III (below bottom arrow) in Slow kindling rats also is intensely stained in contrast to normal control and Fast kindling rats. At higher magnification, α₅ subunit expression in the BLA is shown. Note the intensely stained neuropil (filled arrow) and one of two positive cells in a Fast rat compared with a weaker stained neuropil and one of several negative cells (open arrow) in a normal control rat. In Slow rats, both the neuropil and the labeled cell bodies show less immunoreactivity than in either normal control or Fast rats.