Early phylogenetic value of the major GABA_A receptor subunit mRNAs in the telencephalon

Abstract.

In an effort to establish distinct phylogenetic values for the major GABA_A receptor subunit mRNAs (α₁, β₂ and γ₂) in the telencephalon, the cellular expression capacity of these mRNAs was evaluated in this brain region of two distantly related mammals (hedgehog and rat). Application of emulsion hybridization histochemistry and quantitative film autoradiography approaches allowed us to detect elevated (P<0.01) β₂ mRNA levels in primitive telencephalic neuronal populations such as those of the glomerular olfactory layer of the hedgehog. Conversely, notable levels of the α₁-subunit mRNA were also registered for the higher telencephalic regions such as the oriens-pyramidalis layer of the hippocampus although elevated levels were mostly obtained for the rat. Interestingly, it was the high number of giant pyramidal neurons that were actively involved in the expression of this subunit. A similar pattern was also detected for pyramidal-like cells of the rat lateral amygdala region expressing the γ₂-subunit mRNA. Taken together, these results suggest an early phylogenetic success of GABAergic neuronal mechanisms relying mainly on elevated β₂-subunit levels. On the other hand, elevated quantities of γ₂- and, above all, of α₁-subunit mRNAs in the rat seem instead to be related to the highly advanced behavioral and mnemonic strategies that are under the control of these telencephalic regions.