Gamma Oscillations in Rat Hippocampal Subregions Dentate Gyrus, CA3, CA1, and Subiculum Underlie Associative Memory Encoding

Abstract

Neuronal oscillations in the rat hippocampus relate to both memory and locomotion, raising the question of how these cognitive and behavioral correlates interact to determine the oscillatory network state of this region. Here, rats freely locomoted while performing an object-location task designed to test hippocampus-dependent spatial associative memory. Rhythmic activity in theta, beta, slow gamma, and fast gamma frequency ranges were observed in both action potentials and local field potentials (LFPs) across four main hippocampal subregions. Several patterns of LFP oscillations corresponded to overt behavior (e.g., increased dentate gyrus-CA3 beta coherence during stationary moments and CA1-subiculum theta coherence during locomotion). In comparison, slow gamma (∼40 Hz) oscillations throughout the hippocampus related most specifically to object-location associative memory encoding rather than overt behavior. The results help to untangle how hippocampal oscillations relate to both memory and motion and single out slow gamma oscillations as a distinguishing correlate of spatial associative memory.

Keywords: CA1; CA3; dentate gyrus; electrophysiology; gamma oscillations; hippocampus; memory; object; subiculum; synchrony.

Figure 1. Slow gamma coherence increased while rats explored novel objects

(A) Illustration of the serial connections of the hippocampal subregions [DG, CA3, CA1, and SUB (subiculum)] as well as its connections with the entorhinal cortex (EC). (B) Coronal hippocampal section showing LFP recording locations (circles) for each rat (different colors) in each of the four targeted subregions. (C) Example LFP data as a rat approached (< 0 s) and explored ( > 0 s) a novel object. (D) Moving window spectrograms for each hippocampal subregion time-locked to the initiation of novel object exploration (0 s). Minimum and maximum power values in decibels are noted on each spectrogram. (E) Moving window coherograms for each pair of directly connected hippocampal subregions time-locked to the initiation of novel object exploration (0 s). Increased coherence and power in the slow gamma range were apparent for DG/CA3 and CA3/CA1 during Exploration relative to Approach.

Figure 2. Object exploration was accompanied by a distinct spectral profile

(A) Illustration of the four behavioral states analyzed. (B) Top: Spectral power for each hippocampal subregion (and averaged across subregions) for each behavioral state. Bottom: Spectral power plotted as the difference from average across behavioral states. (C) Top: Coherence for each directly connected pair of hippocampal subregions (and averaged across subregion pairs). Bottom: Coherence plotted as the difference from average across behavioral states. Throughout the figure, gray rectangles mark frequency ranges exhibiting significant interactions between behavioral state and subregion. Black horizontal lines bookended by dagger symbols (†) indicate frequency ranges differing significantly (p<.05) across behavioral states, and those bookended by asterisks indicate significant differences after Bonferroni correction for multiple comparisons (here, 5 for power and 4 for coherence). Colored lines indicate mean (darker shading) ± SEM (lighter shading). See also Figures S1, S2, and S3.

Figure 3. Gamma power and coherence during exploration was not explained as the product of the cessation of locomotion

(A) Speed of movement for 4-s epochs surrounding the transition from novel object Approach to Exploration (purple to blue) and from Run to Stationary (green to red). (B) Slow gamma power (top) and coherence (bottom) plotted as the difference between behavioral state transitions (Approach to Exploration minus Run to Stationary). For example, for CA3 slow gamma power, power was reduced for Approach relative to Run but increased during Exploration relative to Stationary. (C) Fast gamma power (top) and coherence (bottom) plotted as the difference between behavioral state transitions (Approach to Exploration minus Run to Stationary). Indicators of statistical significance throughout are the same as in Figure 2 except that Bonferroni-correction involved four and three comparisons here for power and coherence, respectively. Colored lines indicate mean (darker shading) ± SEM (lighter shading). See also Tables S1 and S2.

Figure 4. Principal cell firing aligned strongly to local oscillations in distinct frequency bands

(A) Peri-event raster of spike times by subregion time-locked to the initiation of object exploration events (0 s) [Approach (< 0 s); Exploration (> 0 s)]. Each dot indicates an action potential. Each row shows all action potentials from a single neuron. Rows alternate between gray and black dots for better visibility. (B) Mean distributions of action potentials in each subregion relative to the phase (P = peak; F = falling; T = trough; R = rising) of distinct oscillatory rhythms (denoted at top) recorded from that same subregion. Averages and error (SEM) are for those neurons found to be significantly phase modulated (see text). (C) Distributions across significantly phase modulated neurons of mean preferred oscillatory phase for spiking. Data is plotted twice in panels B and C, replicated across the oscillatory cycle, to aid visualization of periodicity. See also Figures S3, S4, and S5 and Table S3.

Figure 5. Rats demonstrated memory for objects and objects’ locations

(A) Schematic of memory task. Each trial consisted of three laps around a circular track. On lap 1 of each trial, rats encountered two novel objects. On lap 2, rats encountered duplicates of those same two objects in the same positions. On lap 3, one of two trial-type manipulations were presented. Either: (1) one object was replaced with a duplicate while the other object was replaced by a novel object (REPEAT OBJECT/NOVEL OBJECT Trial) or (2) the objects were replaced by duplicates in swapped locations (SWAP OBJECTS Trial). (B) A significant reduction in average exploration time from lap 1 to lap 2 evidenced rats’ memory for the novel objects presented on lap 1. Asterisks indicate p<0.05. (C) On lap 3, rats explored Novel objects longer than Swap objects, and Swap objects longer than Repeat objects, indicating memory for the objects’ locations. Asterisks indicate p<0.05. (D) Diagram of how object+location, object, and poor subsequent memory conditions were defined (also see Experimental Procedures). (E) Average exploration times across laps sorted by subsequent memory conditions and plotted as percent of lap 1 exploration time using colors indicated in panel D. Error bars throughout the figure show standard error of the mean across rats.

Figure 6. Slow gamma during lap 3 object exploration related to object-location memory condition

(A) Power by subregion (and the average across subregions) plotted as the difference from mean across memory conditions (denoted throughout figure by colors indicated in legend). (B) Coherence for each directly connected subregion pair plotted as the difference from mean across conditions. (C) Average slow gamma and fast gamma power for each subregion (and averaged across subregions; AVG) standardized to the mean across conditions and plotted as Z score. (D) Average slow gamma and fast gamma coherence for each directly connected subregion pair (and averaged across subregion pairs; AVG) standardized to the mean across conditions and plotted as Z score. (E) Average slow gamma and fast gamma non-normalized directed transfer function standardized to the mean across conditions and plotted as Z score. Colored lines in panels A and B indicate mean (darker shading) ± SEM (lighter shading). Error bars in panels C, D, and E show SEM. Indicators of statistical significance in panels A and B are the same as in Figure 2. Similarly, diagonal lines in panels C, D, and E indicate statistical significance of linear trends, and symbols next the region labels on the x axes indicate statistical significance for that region of one-way repeated measures ANOVAs across object conditions. See also Figure S6 and Tables S4 and S5 for detailed statistics.

Figure 7. Slow gamma during novel object exploration related to subsequent object-location associative memory

(A) Power by subregion (and the average across subregions) plotted as the difference from mean across memory conditions (denoted throughout figure by colors indicated in legend). (B) Coherence for each directly connected subregion pair plotted as the difference from mean across conditions. (C) Average slow gamma and fast gamma power for each subregion (and averaged across subregions; AVG) standardized to the mean across conditions and plotted as Z score. (D) Average slow gamma and fast gamma coherence for each directly connected subregion pair (and averaged across subregion pairs; AVG) standardized to the mean across conditions and plotted as Z score. (E) Average slow gamma and fast gamma non-normalized directed transfer function standardized to the mean across conditions and plotted as Z score. Colored lines in panels A and B indicate mean (darker shading) ± SEM (lighter shading). Error bars in panels C, D, and E show SEM. Indicators of statistical significance in panels A and B are the same as in Figure 2. Similarly, diagonal lines in panels C, D, and E indicate statistical significance of linear trends, and symbols next the region labels on the x axes indicate statistical significance for that region of one-way repeated measures ANOVAs across object conditions. See also Figure S7 and Tables S6 and S7 for detailed statistics.