Network mechanisms underlying the initiation and generation of sharp-wave-associated ripple oscillations.

Editor's Note: These short, critical reviews of recent papers in the Journal, written exclusively by graduate students or postdoctoral fellows, are intended to summarize the important findings of the paper and provide additional insight and commentary. For more information on the format and purpose of the Journal Club, please see Network oscillations of different amplitudes , frequencies, and durations occur during different states and behaviors and are hypothesized to coordinate information processing across brain areas. Hippocampal sharp-wave-associated ripple (SWR) oscillations (ϳ140 –200 Hz) are the largest recorded brain signals of physiological significance (Buzsáki et al., 1992). Spontaneous SWRs occur as 50 –100 ms epochs during slow-wave sleep (SWS), awake im-mobility, and consummatory behaviors like eating, drinking, and rearing. During each SWR, ϳ10 –20% (ϳ50,000 –100,000) of all pyramidal neurons in the rat hippocampal complex produce action potentials (Csics-vari et al., 2000). Moreover, spontaneous SWRs have been recorded from all regions of the hippocampal complex including CA1, CA3, subiculum, presubiculum, para-subiculum and the deep layers of the ento-rhinal cortex (Chrobak and Buzsáki, 1996), across rodents (Buzsáki et al., 1992), nonhu-man primates (Skaggs et al., 2007), and humans (Bragin et al., 1999). It is hypothesized that such synchronous population patterns are essential for the faithful transfer of information from the hippocampus to downstream cortical structures (Buzsáki et al., 1983). Furthermore, several experiments have shown that these intermittent high-frequency oscillatory events of the hip-pocampus subserve the role of memory consolidation (Girardeau et al., 2009; Jad-hav et al., 2012; see also O'Neill et al., 2010). Despite clear evidence for the physiological role of SWR oscillations and past work on the mechanistic aspects of SWRs in the hippocampus, the underlying cellular and network mechanisms for the generation and initiation of SWR oscillations are still debated. Two recent papers use electrophysiology and pharmacology along with selective manipulation of neuronal subtypes using optogenetics to advance our understanding of the cellular network mechanisms underlying the generation and initiation of hippocampal SWR oscillations (Stark et al., 2014; Schlingloff et al., 2014). The studies were performed under varying conditions: one examined the CA1 region of the hippocampus in vivo (Stark et al., 2014) and the other investigated the CA3 region in vitro (Schlingloff et al., 2014). But the objective of both studies was to clarify and limit existing models of the cellular network mechanisms underlying the initiation and generation of SWR oscillations in the hippocampus. Despite inherent differences in CA1 …