Analysis of voltage-gated and synaptic conductances contributing to network excitability defects in the mutant mouse tottering.
1. Intracellular current- and voltage-clamp recordings were carried out in CA3 pyramidal neurons from hippocampal slices of adult tg/tg mice and their coisogenic C57BL/6J (+/+) controls with the use of the single-electrode switch-clamp technique. The principal aim of this study was to investigate the mechanisms responsible for the tg gene-linked prolongation (mean 60%) of a giant synaptic response, the potassium-induced paroxysmal depolarizing shift (PDS) at depolarized membrane potentials (Vm -47 to -54 mV) during synchronous network bursting induced by 10 mM potassium ([K+]o). 2. To examine the role of intrinsic voltage-dependent conductances underlying the mutant PDS prolongation, neurons were voltage clamped by the use of microelectrodes filled with 100 mM QX-314 or QX-222 chloride (voltage-gated sodium channel blockers) and 2 M cesium sulphate (potassium channel blocker). The whole-cell currents active during the PDS showed a significantly prolonged duration (mean 34%) at depolarized Vms in tg/tg compared with +/+ cells, indicating that a defect in voltage-dependent conductances is unlikely to completely account for the mutant phenotype. 3. Bath application of 40 microM (DL)-2-aminophosphonovalerate (DL-APV) produced a 30% reduction in PDS duration in both genotypes but failed to significantly alter the tg gene-linked prolongation compared with the wild type. These data indicate that the mutant PDS abnormality does not result from a selective increase of the N-methyl-D-aspartate (NMDA) receptor-mediated excitatory synaptic component. 4. Blockade of gamma-aminobutyric acid-A (GABAA) transmission with picrotoxin (50 microM) or bicuculline (1-5 microM) completely eliminated the difference in PDS duration between the genotypes. Furthermore, although both GABAA receptor antagonists increased the mean PDS duration in +/+ neurons, they did not significantly alter it in tg/tg neurons. These findings are consistent with a reduction in GABAA receptor-mediated synaptic inhibition during bursting in the tg CA3 hippocampal network. 5. To test this hypothesis, bursting CA3 pyramidal neurons were loaded intracellularly with chloride by the use of KCl-filled microelectrodes to examine the effect of reversing the hyperpolarizing chloride-dependent GABAA receptor-mediated inhibitory postsynaptic component of the PDS. Chloride loading prolonged PDS duration in both genotypes, but the increase was greater in +/+ than in tg/tg neurons, indicating that a smaller GABAA inhibitory postsynaptic potential (IPSP) component was reversed in the mutant.(ABSTRACT TRUNCATED AT 400 WORDS)
Helekar
S A
SA
Department of Neurology, Baylor College of Medicine, Houston, Texas 77030.
Noebels
J L
JL
D000818:Animals; D002712:Chlorides / Q000378:metabolism; D004558:Electric Stimulation; D004594:Electrophysiology; D006624:Hippocampus / Q000187:drug effects / Q000502:physiology; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects; D051379:Mice; D008810:Mice, Inbred C57BL; D008818:Mice, Neurologic Mutants; D009415:Nerve Net / Q000502:physiology*; D009431:Neural Conduction / Q000502:physiology*; D011188:Potassium / Q000494:pharmacology; D017966:Pyramidal Cells / Q000187:drug effects / Q000502:physiology; D018091:Receptors, AMPA / Q000037:antagonists & inhibitors / Q000187:drug effects / Q000502:physiology; D011963:Receptors, GABA-A / Q000502:physiology; D016194:Receptors, N-Methyl-D-Aspartate / Q000502:physiology; D013569:Synapses / Q000502:physiology*; D013570:Synaptic Membranes / Q000502:physiology
D002712:Chlorides; D018091:Receptors, AMPA; D011963:Receptors, GABA-A; D016194:Receptors, N-Methyl-D-Aspartate; D011188:Potassium