Reduction in potency of selective gamma-aminobutyric acidA agonists and diazepam in CA1 region of in vitro hippocampal slices from chronic flurazepam-treated rats.

The potency and efficacy of selective gamma-aminobutyric acidA (GABAA) agonists (GABA, muscimol, isoguvacine and 4,5,6,7-tetrahydroisoxazolo-[5,4-c]-pyridin-3-ol), the GABAB agonist, baclofen, and the benzodiazepine agonist, diazepam, were examined using extracellular recording techniques in in vitro hippocampal slices from rats sacrificed 2 days after 1 week of flurazepam treatment. Population spikes elicited by stimulation of Schaffer collaterals were recorded in the CA1 pyramidal cell region with NaCl-containing glass micropipettes. GABA agonists were superfused in increasing concentrations for 5 min. Drug responses, averaged over the last 2 min for each concentration, were compared to the predrug base line. GABAA agonists, but not baclofen, showed a significant, 2-fold, decrease in potency, but not efficacy, to reduce CA1-evoked responses in treated vs. control slices. The benzodiazepine effect was evaluated by the shift in the isoguvacine dose-response curve in the absence, then presence, of diazepam. A reduction in diazepam potency was demonstrated in vitro by a significantly reduced shift in the isoguvacine curve by 300 nM, but not 1 microM, diazepam after chronic but not acute in vivo pretreatment. The results indicated a selective GABAA agonist subsensitivity and diazepam tolerance in hippocampus after 1 week of flurazepam treatment and establish the hippocampal slice preparation as a valuable substrate for investigating synaptic mechanisms of benzodiazepine tolerance.