Acute pentylenetetrazol injection reduces rat GABAA receptor mRNA levels and GABA stimulation of benzodiazepine binding with No effect on benzodiazepine binding site density

Abstract

The effects of a single convulsive dose of pentylenetetrazol (PTZ, 45 mg/kg i.p.) on rat brain gamma-aminobutyric acid type A (GABAA) receptors were studied. Selected GABAA receptor subunit mRNAs were measured by Northern blot analysis (with beta-actin mRNA as a standard). Four hours after PTZ, the GABAA receptor gamma2-mRNA was decreased in hippocampus, cerebral cortex, and cerebellum; alpha1-mRNA was decreased in cerebellum; and beta2 subunit mRNA was decreased in cortex and cerebellum. The alpha5 subunit mRNA level was not altered. Those mRNAs that had been reduced were increased in some brain regions at the 24-h time point, and these changes reverted to control levels by 48 h. PTZ effect on GABAA receptors was also studied by autoradiographic binding assay with the benzodiazepine agonist [3H]flunitrazepam (FNP), the GABAA agonist [3H]muscimol, and the benzodiazepine antagonist [3H]flumazenil. There was an overall decrease in [3H]FNP binding 12 but not 24 h after PTZ treatment. In contrast, [3H]muscimol binding was minimally affected, and [3H]flumazenil binding was unchanged after PTZ treatment. Additional binding studies were performed with well-washed cerebral cortical homogenates to minimize the amount of endogenous GABA. There was no PTZ effect on specific [3H]FNP binding. However, there was a significant reduction in the stimulation of [3H]FNP binding by GABA. The results showed that an acute injection of PTZ caused transient changes in GABAA receptor mRNA levels without altering receptor number but affected the coupling mechanism between the GABA and benzodiazepine sites of the GABAA receptor.