Cell type-specific tuning of hippocampal interneuron firing during gamma oscillations in vivo

Abstract

Cortical gamma oscillations contribute to cognitive processing and are thought to be supported by perisomatic-innervating GABAergic interneurons. We performed extracellular recordings of identified interneurons in the hippocampal CA1 area of anesthetized rats, revealing that the firing patterns of five distinct interneuron types are differentially correlated to spontaneous gamma oscillations. The firing of bistratified cells, which target dendrites of pyramidal cells coaligned with the glutamatergic input from hippocampal area CA3, is strongly phase locked to field gamma oscillations. Parvalbumin-expressing basket, axo-axonic, and cholecystokinin-expressing interneurons exhibit moderate gamma modulation, whereas the spike timing of distal dendrite-innervating oriens-lacunosum moleculare interneurons is not correlated to field gamma oscillations. Cholecystokinin-expressing interneurons fire earliest in the gamma cycle, a finding that is consistent with their suggested function of thresholding individual pyramidal cells. Furthermore, we show that field gamma amplitude correlates with interneuronal spike-timing precision and firing rate. Overall, our recordings suggest that gamma synchronization in vivo is assisted by temporal- and domain-specific GABAergic inputs to pyramidal cells and is initiated in pyramidal cell dendrites in addition to somata and axon initial segments.

Figure 1.

The spike timing of a bistratified interneuron is strongly coupled to the ascending phase of spontaneous gamma oscillations. A, Reconstruction of the Neurobiotin-labeled soma and dendrites (red; shown complete) and axons (yellow; shown from only 5 70-μm-thick sections). A pyramidal cell is superimposed in blue for orientation (different scale and animal). B, Light micrograph of putative dendritic self-innervation sites (red arrow in A). Black and red arrowheads mark axon and dendrite, respectively. C, Electron micrograph showing a terminal of the same bistratified cell (black) making a type 2 synapse (arrow) with an oblique pyramidal dendrite (dend). D, Extracellularly recorded APs (top) and LFPs (bottom; recorded extracellularly with a second electrode in SP) with filtered gamma oscillations (bandpass, 30–80 Hz; middle). The bistratified cell fires preferentially at the ascending gamma phase. E, Average discharge rates as a function of gamma phase (per 36° bin; 2 cycles are shown with troughs at 0, 360, and 720°). SO, Stratum oriens. Scale bars: A, 100 μm; B, 10 μm; C, 0.2 μm. Calibration: D, horizontal, 0.05 s; vertical, top, 2 mV; middle and bottom, 0.2 mV.

Figure 2.

Spike timing of an O-LM cell is not coupled to field gamma oscillations. A, Reconstructions of the Neurobiotin-labeled soma and dendrites (red; shown complete) and axons (yellow; shown from only 12 70-μm-thick sections). B, The Neurobiotin-labeled O-LM cell is immunopositive for PV, somatostatin (SOM), and mGluR1α. C, Extracellularly recorded APs (top) and LFPs (bottom; recorded extracellularly with a second electrode in SP) with filtered gamma oscillations (bandpass, 30–80 Hz; middle). The O-LM cell firing is not coupled to any particular gamma phase. D, Average discharge rates as a function of gamma phase (per 36° bin; 2 cycles are shown). SO, Stratum oriens. Scale bars: A, 100 μm; B, 10 μm. Calibration: C, horizontal, 0.1 s; vertical, top, 0.2 mV; middle, 0.1 mV; bottom, 0.4 mV.

Figure 3.

Discharge of distinct interneuron classes is differentially modulated during field gamma oscillations. A, Gamma-modulated firing of interneurons characterized by the depth of modulation, r, and the mean preferred gamma phase angle for each identified cell (n = 29). Different symbols mark different cell types as indicated; for each cell type, numbers in parentheses indicate the number of significantly modulated cells (Rayleigh test, p < 0.05)/total number of cells per cell class. Gray symbols represent cells not modulated by gamma. The trough and peak of the field gamma cycle in SP are at 0 and 180°, respectively. B, Bistratified cell firing was more strongly gamma modulated than O-LM and CCK cell firing; PV basket cell firing was also more strongly gamma modulated than O-LM cell firing. Median and interquartile range of all cells are shown, including cells not significantly modulated. **p < 0.01; *p < 0.02 (exact KW test). C, D, Interneurons show significantly stronger phase coupling (C) and higher discharge rates (D) during strong gamma cycles with high field gamma amplitude than during weak gamma cycles with lower amplitude. *p < 0.05; **p < 0.001 (paired one-tailed Wilcoxon signed-rank test). E, Schematic spike timing of sequentially active pyramidal place cells (colored spikes) during theta-nested gamma oscillations and their reverse and compressed replay during subsequent ripple oscillations (Buzsaki, 1989; Foster and Wilson, 2006). Pyramidal cells fire at the trough and ascending phase of gamma and ripple oscillations, with similar phase relation relative to bistratified cells (black spikes).