Modulation of decay kinetics and frequency of GABAA receptor-mediated spontaneous inhibitory postsynaptic currents in hippocampal neurons.
Inhibitory postsynaptic currents mediated by spontaneous activation of GABAA receptors were studied using whole-cell voltage-clamp recordings in granule cells of the adult rat (postnatal day 60+) dentate gyrus in 400-microns-thick coronal half-brain slices maintained at 34-35 degrees C. The average amplitude of spontaneous inhibitory postsynaptic currents remained constant during a given recording period (i.e. no rundown was noted). The spontaneous currents had an average conductance between 200-400 pS, were mediated by Cl- flux through GABAA receptor/channels since they reversed at the Cl- equilibrium potential and were blocked by bicuculline or picrotoxin. Their mono-exponential decay time-constants (range: 4.2-7.2 ms) were prolonged by midazolam and pentobarbital in a dose-dependent manner. The effect of midazolam was reversed by the benzodiazepine receptor antagonist flumazenil (RO 15-1788) which, by itself, had no effect on the decay time-constant. The decay time-constant was also dependent on membrane voltage and on temperature. A 132-mV change in membrane potential produced an e-fold prolongation of the decay while the Q10 (between 22-37 degrees C) of the decay rate was 2.1. Within a given neuron, the frequency of spontaneous GABAergic events was remarkably constant over long time-periods, though the mean frequency among different cells showed large variability. Spontaneous miniature inhibitory postsynaptic currents also persisted under experimental conditions such as the presence of extracellular tetrodotoxin (1 microM), Cd2+ (200 microM) or lowered extracellular Ca2+/elevated Mg2+, which effectively abolished all stimulus-evoked GABAergic neurotransmission. The frequency of tetrodotoxin-resistant miniature events was increased by elevating extracellular K+ concentration and was diminished by the GABAB receptor agonist (-)baclofen only at a dose (50 microM) which was an order of magnitude larger than that required to depress stimulus-evoked responses. These findings are consistent with different mechanisms being responsible for the spontaneous and stimulus-evoked release of GABA from interneuron terminals and also identify pre- and postsynaptic modulatory factors of the endogenous, action-potential-independent, GABAergic neurotransmission as being important determinants of the excitability level of mammalian CNS neurons.
Otis
T S
TS
Department of Neurology and Neurological Sciences, Stanford University School of Medicine, CA 94305.
Mody
I
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D015763:2-Amino-5-phosphonovalerate / Q000494:pharmacology; D018750:6-Cyano-7-nitroquinoxaline-2,3-dione; D000375:Aging; D000779:Anesthetics, Local; D000818:Animals; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology; D005071:Evoked Potentials / Q000187:drug effects; D005442:Flumazenil / Q000494:pharmacology; D006624:Hippocampus / Q000254:growth & development / Q000502:physiology*; D007700:Kinetics; D008012:Lidocaine / Q000031:analogs & derivatives / Q000494:pharmacology; D008433:Mathematics; D008536:Meglumine / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D008874:Midazolam / Q000494:pharmacology; D008959:Models, Neurological; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D010424:Pentobarbital / Q000494:pharmacology; D011336:Probability; D011810:Quinoxalines / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D011963:Receptors, GABA-A / Q000187:drug effects / Q000502:physiology*; D013569:Synapses / Q000187:drug effects / Q000502:physiology*; D013779:Tetrodotoxin / Q000494:pharmacology
D000779:Anesthetics, Local; D015220:Calcium Channels; D011810:Quinoxalines; D011963:Receptors, GABA-A; C012647:QX-314; D005442:Flumazenil; D013779:Tetrodotoxin; D008536:Meglumine; D018750:6-Cyano-7-nitroquinoxaline-2,3-dione; D015763:2-Amino-5-phosphonovalerate; D008012:Lidocaine; D010424:Pentobarbital; D008874:Midazolam