Modulation of synaptic plasticity by theta rhythm and structure-function relationships in a single ion channel

Abstract

A few studies support the idea that the theta rhythm modulates synaptic plasticity by demonstrating that the phase of the theta cycle at which the induction stimuli are delivered determines the nature of the resulting synaptic plasticity. These studies were conducted in urethane-anesthetized animals and in vitro slice preparations where the theta rhythm is artificially generated. Our goal was to find whether and how natural theta activity affects synaptic plasticity in the hippocampus of adult and old freely behaving animals. In both adult and aged, memory-impaired rats, LTP lasting at least 48 h was induced when stimuli were delivered at theta peak. No change in synaptic strength was observed when stimuli were delivered at theta trough. These observations indicate that the naturally occurring theta rhythm modulates synaptic plasticity, and suggest a mechanism by which the phase of firing could contain meaningful information. The degree of LTP, however, was significantly smaller in the old animals. To better understand the conformational changes and the dynamic interactions that govern ion-channel kinetics we developed a new approach using simultaneous single-channel patch-clamp recording and single-molecule fluorescence microscopy. Gramicidin monomers were tagged with a fluorescence dye and single-channel current was recorded from gramicidin channels in the bilayer that was formed at the tip of a patch pipette. Co-localization and fluorescence resonance energy transfer (FRET) within a single gramicidin dimer were probed. The new technique made it possible to directly capture the conformational dynamics between the two gramicidin monomers by observing the changes in the distance between the attached dye molecules. The molecular interactions of the NMDA receptor with its ligands determine the dynamic properties of activation and desensitization that in turn shape NMDA receptor mediated currents. We have monitored the occurrence and intensity changes of FRET between two fluorescence-labeled agonists at the glutamate binding site of the receptor, simultaneously with single channel current recording. These observations can be translated to dissociation/association rates and aid in our understanding of the mechanisms that underlie the transitions of the receptor between different kinetic states.