We evaluated whether tolerance to the antagonism of bicuculline-induced seizures by diazepam is associated with changes (i) in the content of mRNAs encoding for γ-aminobutyric acidA (GABAA) receptor subunits, (ii) in the expression density of these subunits, and (iii) in the 1,4-benzodiazepine binding site characteristics in discrete neocortical structures. We found that in diazepam-tolerant rats, the content of the mRNA encoding for the α1 subunit of the GABAA receptor decreased in the frontoparietal motor (FrPaM) cortex and in the hippocampus (42% and 20%, respectively) but not in the frontoparietal somatosensory (FrPaSS) cortex, striatum, olfactory bulb, and cerebellum. In the FrPaM cortex, γ2S and γ2L subunit mRNA contents were also decreased (48% and 30%, respectively), whereas that of α5 was increased (30%). In the FrPaM and FrPaSS cortices as well as in cerebellum of diazepam-tolerant rats, the content of α2, α3, α6, β2, and δ subunit mRNA was unchanged, as was the content of α2, α5, γ1, and γ2S subunit mRNA in the hippocampus. Furthermore, the reduction in α1 subunit mRNA content in the FrPaM cortex and the anticonvulsant tolerance to diazepam returned to control values 72 hr after termination of the protracted diazepam treatment. Rats receiving a treatment with imidazenil in doses equipotent and with a schedule identical to that of diazepam failed to exhibit tolerance to the anticonvulsant action of this drug or cross-tolerance to diazepam. In these rats, the content of mRNA encoding for α1, α2, α3, α5, α6, γ1, γ2S, γ2L, and δ GABAA receptor subunits failed to change in the FrPaM and FrPaSS cortices, in the hippocampus, and in the other brain areas that were studied in diazepam-tolerant rats. Although the density and affinity of[3H]flumazenil and [3H]imidazenil binding failed to change in the FrPaM and FrPaSS cortices of diazepam-tolerant rats, the expression density of α1 subunit immunogold labeling decreased by 37%, whereas that of α5, γ2L/S, and β2/3 increased by 158%, 50%, and 47%, respectively, in the FrPaM cortex, and the density of the α5 subunit selectively increased (209%) in the FrPaSS cortex. In contrast, the immunogold labeling density of the α1, α5, γ2L/S, and β2/3 subunits failed to change in either the FrPaM or FrPaSS cortex of rats receiving protracted imidazenil treatment.
|Number of pages||10|
|Publication status||Published - May 1996|
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