Membrane Potential Of Pyramidal Cells And Ripple Oscillations In The Hippocampal CA1 Region

Hippocampal ripple oscillations (100-250 Hz) in local field potential occur during slow-wave sleep and behavioral immobility and are thought to represent stored information that is transferred to the neocortex during memory consolidation. Utilizing multi-Channel recording techniques, we are able to record hippocampal local field potential and unit activity simultaneously. Among all neurons recorded, we identified two types of fast firing interneurons that are associate with extracellular ripple oscillations, both show high firing rate during each epoch of extracellular ripple. One of them fires just one spike to each sub-cycle of any ripple epoch, while another shows up two action potentials within the same time window, respectfully. Granger Causality test suggests that highly organized firing of these two types of interneurons are among the driving force of ripple oscillations in the local field potential.To further investigate the cellular mechanism of ripple oscillations, the somatic membrane potentials of CA1 pyramidal cells during the network oscillation was studied under anesthesia using in vivo whole-cell patch-clamp recording, while simultaneously recording local filed potential. Our finding indicates that the membrane potentials in the CA1 pyramidal cells are highly related to the local field potential within the ripple band (130-200Hz). During the occurrence of a ripple oscillation in the local field potential, there is a depolarization tendency in the somatic membrane potentials of CA1 pyramidal cells though out the whole event. And the extent of this membrane depolarization varies according to the level of cellular membrane potential; interestingly, the depolarization increases as the membrane potential becomes more negative. We postulate that this phenomenon is related to the inhibitory neurotransmitter release from certain group of interneurons. Furthermore, a phase change between the intracellular and extracellular ripple oscillation occurs when the somatic membrane potentials of a pyramidal cell alters. The phase of intracellular oscillation relative to the extracellular ripple reversed when the membrane was hyperpolarized more than-80 mV.These findings suggest that the intracellularly recorded ripple rhythm in the CAl pyramidal cells might result from the inhibitory neurotransmitter activity of some specific hippocampal interneurons.