Using a combination of high-density extracellular recordings, multisite/multicolor closed-loop optical stimulation, and pharmacological intervention in freely behaving and urethane-anesthetized mice and rats, we examined the mechanisms of SPW-induced fast gamma/ripple generation. Our principal findings are as follows: (1) activation of a small group of pyramidal cells is sufficient to generate iHFOs. (2) Fast GABA-mediated inhibition is critical for the generation of iHFOs. (3) Pyramidal cell activity is critical for the maintenance of ripples. (4) PV interneurons can pace spiking in local populations. (5) Multisite activation induces temporally coherent ripples mediated by phase-locked interneuron spiking. These findings are consistent with a model of ripple generation based on PYR-INT-INT interactions ( Figure 1 D).

Mechanisms of Ripple Generation In Vivo

A -receptor-mediated inhibition is an additional requisite for the generation of high-frequency oscillations. The necessary and sufficient requirements are further illustrated by the induction of iHFOs in deep neocortical layers and in the dentate gyrus. The neocortical iHFOs may be related to LFP ripples reported in deep neocortical layers upon strongly synchronized population bursts of activity ( Kandel and Buzsáki, 1997 Kandel A.

Buzsáki G. Cellular-synaptic generation of sleep spindles, spike-and-wave discharges, and evoked thalamocortical responses in the neocortex of the rat. Grenier et al., 2001 Grenier F.

Timofeev I.

Steriade M. Focal synchronization of ripples (80-200 Hz) in neocortex and their neuronal correlates. However, differences were also noted, such as a relatively lower mean frequency of iHFOs associated with a relatively larger phase separation between pyramidal cells and interneurons during iHFOs. Such differences might be explained by the activating mechanisms: during spontaneous ripples, INT receive excitatory input from diverse CA3 loci and CA1 pyramidal cells, whereas during iHFOs they are driven only by the local CA1 PYR. In transgenic mice we cannot exclude the possibility that terminals of the CA2/CA3 inputs were also activated by light, but the consistent observations in virus-injected wild-type animals indicate that direct activation of CA1 pyramidal cells is the main cause of iHFOs. Our experiments identified two cardinal components for ripple generation. First, activity of a few dozen pyramidal neurons is necessary for ripple generation. Second, fast GABA-receptor-mediated inhibition is an additional requisite for the generation of high-frequency oscillations. The necessary and sufficient requirements are further illustrated by the induction of iHFOs in deep neocortical layers and in the dentate gyrus. The neocortical iHFOs may be related to LFP ripples reported in deep neocortical layers upon strongly synchronized population bursts of activity (). How the optogenetically induced HFOs affect the spike content, incidence, and sequential neuronal activity during native ripples remains to be addressed.