title: Role of bicarbonate ion in mediating decreased synaptic conductance in benzodiazepine tolerant hippocampal CA1 pyramidal neurons.
abstract: 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.
authors.affiliation: Department of Pharmacology, Medical College of Ohio, Block Health Science Building, 3035 Arlington Ave., Toledo, OH 43614-5804, USA.
authors.forename: X J
authors.initials: XJ
authors.lastname: Zeng
authors.forename: E I
authors.initials: EI
authors.lastname: Tietz
mesh_terms: D000818:Animals; D001569:Benzodiazepines / Q000494:pharmacology*; D001639:Bicarbonates / Q000378:metabolism* / Q000494:pharmacology; D002555:Cerebrospinal Fluid / Q000378:metabolism; D018118:Chloride Channels / Q000187:drug effects / Q000378:metabolism; D004361:Drug Tolerance / Q000502:physiology*; D004558:Electric Stimulation; D006531:HEPES / Q000494:pharmacology; D006624:Hippocampus / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D066298:In Vitro Techniques; D008297:Male; D009433:Neural Inhibition / Q000187:drug effects / Q000502:physiology; D017966:Pyramidal Cells / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D011963:Receptors, GABA-A / Q000187:drug effects* / Q000378:metabolism; D013569:Synapses / Q000187:drug effects / Q000378:metabolism; D009435:Synaptic Transmission / Q000187:drug effects* / Q000502:physiology
chemical_list: D001639:Bicarbonates; D018118:Chloride Channels; D011963:Receptors, GABA-A; D001569:Benzodiazepines; D006531:HEPES