The effect of kainate, an agonist selective for ionotropic AMPA/kainate
type of glutamate receptors, on GABA_A receptor subunit expression in
cultured mouse cerebellar granule cells was studied using quantitative
RT-PCR, ligand binding and electrophysiology. Chronic kainate treatment,
without producing excitotoxicity, resulted in preferential, dose- and
time-dependent down-regulation of alpha1, alpha6 and beta2 subunit mRNA
expression, the expression of beta3, gamma2 and delta subunit mRNAs
being less affected. The down-regulation was reversed by DNQX, an
AMPA/kainate-selective glutamate receptor antagonist. A 14-day kainate
treatment resulted in 46% decrease of total [³H]Ro 15-4513 binding to
the benzodiazepine sites. Diazepam-insensitive [³H]Ro 15-4513 binding
was decreased by 89% in accordance with very low amount of alpha6
subunit mRNA present. Diazepam-sensitive [³H]Ro 15-4513 binding was
decreased only by 40%, contrasting >90% decrease in alpha1 subunit
mRNA expression. However, this was consistent with lower potentiation of
GABA-evoked currents in kainate-treated than control cells by the
alpha1-selective benzodiazepine site ligand zolpidem, suggesting
compensatory expression of alpha5 (and/or alpha2 or alpha3) subunits
producing diazepam-sensitive but zolpidem-insensitive receptor subtypes.
In conclusion, chronic kainate treatment of cerebellar granule cells
selectively down-regulates oil, alpha6 and beta2 subunits resulting in
altered GABA_A receptor pharmacology.