Figure 1.

Gamma oscillations are exemplified from the CA3 network of the hippocampus. (A) The hippocampal formation consists of the dentate gyrus (DG) with excitatory granule cells (white ovals) and subfields, CA3 and CA1 with excitatory pyramidal cells (orange triangles). Parvalbumin-positive (PV+) interneurons are also illustrated (intermittent blue spheres). Red arrows indicate main routes of information processing in the hippocampal formation. (B) Example trace of persistent gamma oscillations that occur in local field potential (LFP) recordings in the presence of cholinergic or glutamatergic receptor agonists. The power spectrum (PS) was calculated from the same trace. (C) and (D) Immunohistochemical stainings of PV+ interneurons, visualized with 3,3'-diaminobenzidine (DAB) (C) or fluorescent secondary antibody (D). The images show parts of the CA3 subfield. Note both location of somata (C, black dots) and numerous synaptic contacts of PV+ interneurons to the perisomatic region of pyramidal cells (D, turquoise meshwork). Nuclei of neurons and glial cells are counterstained with 4',6-diamidino-2-phenylindole (DAPI) (D, grey dots). (E) A single PV+ interneuron contacts numerous pyramidal cells (PC) via GABAergic synapses. Excitatory synapses from pyramidal cells onto the interneuron are not shown. (F) Rhythmic action potentials (blue vertical bars) of PV+, fast-spiking interneurons (first line) evoke synchronous inhibitory postsynaptic potentials in pyramidal cells that are reflected by oscillations in the local field potential (LFP) (second line). This rhythmic perisomatic inhibition provides a temporal matrix for individual pyramidal cells (orange triangles) to generate action potentials (red vertical bars) during brief time windows, resulting in a well-defined aggregate sequence of action potentials (black vertical bars) from a given pyramidal cell group and thus precise information processing in neuronal networks (bottom line). Scale bar, 100 μm (C and D).