Antagonist-induced reversal of functional and structural measures of hippocampal benzodiazepine tolerance

One week oral flurazepam (FZP) administration in rats results in anticonvul sant tolerance in vivo, tolerance measured in vitro in hippocampal CA1 pyra midal cells, and regulation of hippocampal gamma-aminobutyric acid(A)-recep tor subunit protein expression. A single injection (4 or 20 mg/kg i.p) of t he benzodiazepine antagonist flumazenil (FLM) was given 1 day after FZP tre atment, and tolerance and subunit protein expression were evaluated 1 day l ater. In vivo tolerance was measured by a reduced ability of the alpha(1)-s ubunit-selective agonist zolpidem to suppress pentylenetetrazole-induced se izures. This tolerance was reversed by 20 but not 4 mg/kg FLM. In in vitro hippocampal slices, there was tolerance to the effect of zolpidem to prolon g the decay of pyramidal cell miniature inhibitory postsynaptic currents, w hich was reversed by FLM (4 mg/kg) pretreatment. A reduction in miniature i nhibitory postsynaptic current amplitude (similar to 50%) was also restored by FLM injection. [H-3]Zolpidem binding measured 0, 2, and 7 days after FZ P treatment was significantly decreased in the hippocampus and cortex at 0 days but not thereafter. Changes in alpha(1)- and beta(3)-subunit protein e xpression were examined via quantitative immunohistochemical techniques. al pha(1)-Subunit protein levels were down-regulated in the CA1 stratum oriens and beta subunit levels were up-regulated in the stratum oriens and stratu m radiatum of the CA3 region. Chronic FZP effects on alpha(1)- and beta(3)- subunit protein levels were also reversed by prior FLM injection. FLM's eff ect on both functional and structural correlates of benzodiazepine toleranc e suggests that each of these measures plays an interdependent role in medi ating benzodiazepine tolerance.