Benzodiazepine tolerance at GABAergic synapses on hippocampal CA1 pyramidal cells.
Modulation of GABA function following 1 week oral administration of flurazepam (FZP) was investigated in chloride-loaded, rat hippocampal CA1 pyramidal neurons. Rats were sacrificed 2 or 7 days after ending drug treatment, when anticonvulsant tolerance was present or absent in vivo, respectively. Spontaneous (s)IPSCs and miniature (m)IPSCs were recorded using whole-cell voltage-clamp techniques. s/mIPSCs were bicuculline-sensitive, voltage-dependent, and reversed their polarity at 0 mV, the predicted E(Cl-). Comparisons of s/mIPSCs between FZP-treated and control groups were made at Vh = -90, -70, and -50 mV. The frequency of sIPSCs, but not mIPSCs, was significantly decreased in FZP-treated neurons 2 days, but not 7 days, after FZP treatment, suggesting a decrease in interneuron activity. These conclusions were supported by the negative findings of additional studies of [3H]GABA release from hippocampal slices and [3H]GABA uptake from hippocampal synaptosomes. The lack of change in the paired-pulse depression of GABA(B)-mediated IPSPs suggested that autoreceptor function was also not impaired following chronic FZP treatment. A large reduction in both sIPSC and mIPSC amplitude (60%) in FZP-treated neurons, the absence of mIPSCs in one-third of FZP-treated cells, and a measurable reduction in synaptic and unitary conductance confirmed that postsynaptic GABA(A) receptor function was profoundly impaired in FZP-treated CA1 neurons. Zolpidem, an alpha1-selective benzodiazepine receptor ligand, enhanced mIPSC amplitude and decay, but its ability to prolong mIPSC decay was reduced in FZP-treated neurons. Several pre- and postsynaptic changes at GABAergic synapses on CA1 pyramidal cells might be related to the decreased tonic GABA inhibition in FZP-treated CA1 neurons associated with the expression of benzodiazepine anticonvulsant tolerance.
Zeng
X J
XJ
Department of Pharmacology, Medical College of Ohio, Toledo 43614-5804, USA.
Tietz
E I
EI
D000818:Animals; D014151:Anti-Anxiety Agents / Q000494:pharmacology*; D004361:Drug Tolerance; D004594:Electrophysiology; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects / Q000502:physiology; D005479:Flurazepam / Q000494:pharmacology*; D018757:GABA Modulators / Q000494:pharmacology*; D006624:Hippocampus / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D006993:Hypnotics and Sedatives / Q000494:pharmacology; D066298:In Vitro Techniques; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009431:Neural Conduction / Q000187:drug effects; D018408:Patch-Clamp Techniques; D017966:Pyramidal Cells / Q000187:drug effects* / Q000378:metabolism; D011725:Pyridines / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D013569:Synapses / Q000187:drug effects* / Q000378:metabolism; D000077334:Zolpidem; D005680:gamma-Aminobutyric Acid / Q000378:metabolism / Q000502:physiology*
D014151:Anti-Anxiety Agents; D018757:GABA Modulators; D006993:Hypnotics and Sedatives; D011725:Pyridines; D005680:gamma-Aminobutyric Acid; D000077334:Zolpidem; D005479:Flurazepam