Membrane Potential Ripple High-frequency Oscillations Of Pyramidal Cells In The Mouse Hippocampal CA1Region

Different types of rhythm oscillations in the brain are considered to carrying certain coding information in the learning and memory behavior. Oscillations in the hippocampus include theta rhythms (4-12Hz), gamma oscillations (40-100Hz), and high-frequency ripples (100-250Hz). Theta and gamma rhythms occur in exploration activities or rapid-eye-movement sleep period in animals, while ripple high-frequency oscillations occur in the period of inactive awake animals, completion of the exercise behavior or period of slow wave sleep.Ripples are composed of5-15low-amplitude (less than500μV) sinusoidal waves, and the frequency of these sinusoidal waves is about200Hz (approximately5ms wave width). The magnitude of ripple waves is the largest in the pyramidal cell layer and these high-frequency network oscillations commonly cooccur with large amplitude sharp waves. Ripple rhythm of local field potential in the hippocampus is thought to be involved in hippocampus-dependent memory conversion from short-term to long-term memory. Although there is ample evidence for the involvement of ripples in mnemonic processes, the precise mechanisms underlying the generation of ripples are not clear. To further study the neural mechanisms of ripple high-frequency oscillation, we recorded the membrane potential activity of hippocampal CA1pyramidal neurons using in vivo whole-cell patch-clamp technique in anesthesia mice, and simultaneously record local filed potential activity of hippocampus, and then analyzed the relationship between hippocampal CA1pyramidal cell membrane potential and ripple oscillation.Our findings indicate that the frequnecy of ripples under anesthesia (80-150Hz) is slower than in the awake rat(100-250Hz), and there is certain synchronization of the pyramidal cell membrane potential and extracellular local field potential in the ripple band. When ripple high-frequency oscillation occurs, there is a depolarization on the pyramidal cell membrane potential. Ripple rhythm oscillations of membrane potential is high-frequency oscillations superimposed on the peak of the membrane polarization. And with the different levels of the membrane potential, the magnitude of the polarization changes correspondingly:the lower level of the membrane potential, the larger magnitude of membrane depolarization, the higher level of the membrane potential, the smaller magnitude of membrane depolarization. Furthermore, When the levels of membrane potential are elevated by cell depolarization, phase relationship of intracellular ripple oscillations appears to reversal relative to the extracellular field potential ripple rhythm.These findings suggest that the local ripple field potential in hippocampal CA1region may be derived from the membrane potential ripple fluctuations of pyramidal cell. Ripple rhythm oscillations of membrane potential is high-frequency oscillations superimposed on the peak of the membrane polarization which may reflect the excitatory input from synchronized discharge of CA3pyramidal cells to CA1pyramidal cells by the schaffer collateral and is the source of extracellular local sharp wave field potential. The ripple high-frequency oscillation on the basis of membrane potential depolarization may be relate to the release of GABA inhibitory neurotransmitter of hippocampus interneurons.