Role of bicarbonate ion in mediating decreased synaptic conductance in benzodiazepine tolerant hippocampal CA1 pyramidal neurons.

Chronic flurazepam treatment substantially impairs the function of GABAergic synapses on hippocampal CA1 pyramidal cells. Previous findings included a significant decrease in the synaptic and unitary conductance of CA1 pyramidal neuron GABA(A) receptor channels and the appearance of a GABA(A)-receptor mediated depolarizing potential. To investigate the ionic basis of the decreased conductance, whole-cell voltage-clamp techniques were used to record evoked, GABA(A) receptor-mediated IPSCs carried by HCO(3)(-)-Cl(-) or Cl(-) alone. Hippocampal slices were prepared from rats administered flurazepam orally for 1 week, 2 days after ending drug treatment. Slices were superfused with HCO(3)(-)-aCSF or with HEPES-aCSF (without HCO(3)(-)) plus 50 microM APV and 10 microM DNQX. The micropipette contained 130 mM CsCl and 1 microM QX-314. GABA(A) receptors located on pyramidal cell somata or dendrites were activated monosynaptically by maximal stimulation of GABAergic terminals at the stratum oriens-pyramidale (SO-SP) or stratum lacunosum-molecular (S-L-M) border, respectively. In HCO(3)(-)-aCSF, there was a significant reduction in synaptic-conductance in flurazepam-treated neurons following both SO-SP (control: 1058 pS, flurazepam: 226 pS, P<0.01) and S-L-M (control 998 pS, flurazepam: 179 pS, P<0.01) stimulation, as well as the total charge transfer, indicating a decreased HCO(3)(-)-Cl(-) flux. In HEPES-aCSF, the synaptic conductance and total charge transfer, and thus Cl(-) flux, was unchanged in flurazepam-treated neurons (SO-SP: control 588 pS, flurazepam: 580 pS, P>0.05; S-L-M: control 595 pS, flurazepam: 527 pS, P>0.05). Taken together, these findings suggest that a reduction in HCO(3)(-) flux may play a prominent role in mediating the action of GABA and that a loss of HCO(3)(-) conductance may significantly contribute to impaired GABA(A) receptor function after chronic benzodiazepine treatment.