title: Differential presynaptic modulation of excitatory and inhibitory autaptic currents in cultured hippocampal neurons.
abstract: Short-term synaptic plasticity has an important role in higher cortical function. Hyperpolarization may effect a form of short-term plasticity by promoting recovery from sodium channel inactivation or by activating axonal A-type potassium channels. To determine whether one or both processes occur, we examined the effect of hyperpolarizing prepulses on autaptic currents in cultured postnatal rat hippocampal neurons. As expected of enhanced recovery from sodium channel inactivation, hyperpolarizing prepulses reversibly increased fast excitatory autaptic currents (eacs) mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), slow eacs mediated by N-methyl-D-aspartate receptors (NMDARs), and inhibitory autaptic currents (iacs) mediated by gamma-aminobutyric acidA receptors (GABAARs). Hyperpolarizing prepulses augmented nearly all fast and slow eacs but only half of the iacs. This change occurred without a change in autaptic current kinetics. Of note, hyperpolarizing prepulses did not significantly reduce autaptic currents in any neuron studied. The rapidly dissociating competitive antagonists kynurenate and L-2-amino-5-phosphonovaleric acid (LAPV) inhibited fast and slow eacs, respectively, to the same extent with and without a hyperpolarizing prepulse. In addition, hyperpolarizing prepulses revealed a slow eac even after the slow eac evoked without a prepulse was completely blocked by the open channel blocker, (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801). Finally, hyperpolarizing prepulses did not alter currents evoked by exogenous applications of glutamate and GABA. These findings suggest that hyperpolarizing prepulses preferentially enhance eacs over iacs, and that they do so, in part, by overcoming conduction block or by activating silent synapses.
authors.affiliation: Department of Neurology and the Center for the Study of Nervous System Injury, Washington University School of Medicine, 660 South Euclid Avenue, Box 8111, St. Louis, MO 63110, USA.
authors.forename: Liu Lin
authors.initials: LL
authors.lastname: Thio
authors.forename: Kelvin A
authors.initials: KA
authors.lastname: Yamada
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D016291:Dizocilpine Maleate / Q000494:pharmacology; D006624:Hippocampus / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D007736:Kynurenic Acid / Q000494:pharmacology; D009433:Neural Inhibition / Q000187:drug effects / Q000502:physiology*; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D017729:Presynaptic Terminals / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology*
chemical_list: D016291:Dizocilpine Maleate; D007736:Kynurenic Acid