Anxiety Cells in a Hippocampal-Hypothalamic Circuit

Abstract

The hippocampus is traditionally thought to transmit contextual information to limbic structures where it acquires valence. Using freely moving calcium imaging and optogenetics, we show that while the dorsal CA1 subregion of the hippocampus is enriched in place cells, ventral CA1 (vCA1) is enriched in anxiety cells that are activated by anxiogenic environments and required for avoidance behavior. Imaging cells defined by their projection target revealed that anxiety cells were enriched in the vCA1 population projecting to the lateral hypothalamic area (LHA) but not to the basal amygdala (BA). Consistent with this selectivity, optogenetic activation of vCA1 terminals in LHA but not BA increased anxiety and avoidance, while activation of terminals in BA but not LHA impaired contextual fear memory. Thus, the hippocampus encodes not only neutral but also valence-related contextual information, and the vCA1-LHA pathway is a direct route by which the hippocampus can rapidly influence innate anxiety behavior.

Keywords: anxiety; calcium imaging; ventral hippocampus.

Figure 1. Representations of anxiety-related information in vCA1

(A) Experimental design for vCA1 freely-moving Ca²⁺ imaging. GCaMP6f was virally expressed and a GRIN lens implanted to target the CA1 pyramidal layer (middle image). Right, representative cell contours from a segmented Ca²⁺ video FOV. Right bottom, extracted Ca²⁺ transients from 5 example vCA1 neurons. (B) Left, normalized Ca²⁺ activity from individual vCA1 neurons in the Elevated Plus Maze (EPM) in an example FOV, binned by distance (cm) from the EPM center point (red dot on EPM diagram) into either the open or closed arms. The majority of cells are more active in the open arm compartment (cells are sorted by location of peak Ca²⁺ activity bin (Closed arm: cells 1-26, Open arm/center: cells 27: 146), and activity spread throughout the maze (# of bins with 20% of peak Ca²⁺ activity)). Right, vCA1 Ca²⁺ transients from an example FOV during a behavioral transition between the closed and open arm compartment (transition at time 0 sec). Red asterisks indicate timing and peak of identified Ca²⁺ transient events. (C) Rate of vCA1 neuron Ca²⁺ transients was significantly higher in the EPM open arms compared to closed arms (Wilcoxon sign rank, Z=−17.013, p<0.0001, N_cells=2,137). (D) Left, example behavioral trajectories between arm compartments. Right, heatmaps of vCA1 normalized Ca²⁺ activity during those trajectories (each heatmap-trajectory pair is from the same FOV). Red dotted lines indicate a head-dip behavioral event in the open arm (also marked with arrows). (E) Top, Novel object task design. Bottom, mice spend significantly more time exploring the novel object than the neutral zone (paired t-test, t₄=−8.594, p<0.01, N_mice=5). (F) vCA1 Ca²⁺ transient rates are not significantly different between novel object and neutral zone (Wilcoxon sign rank, Z=−0.125, p=0.90, N_cells=677). (G) Mean Ca²⁺ transient rate difference (open-closed) across an FOV is correlated with the anxiety state of the subject (% time in open arm) (linear regression, F(1,10)=13.467, p<0.01, R²=0.57, N_mice=12). (H) Rate of Ca²⁺ transients is higher during headdip behaviors relative to open arm (Wilcoxon sign rank, Z=−7.251, p<0.0001, N_cells=400). All data error bars represent mean +/− SEM

Figure 2. Real-time control of avoidance behavior by vCA1

(A) Bilateral optogenetic silencing of CamKII-Arch expressing vCA1 neurons. Bottom, Representative placement of fiber optic. (B) Left, In the EPM, the laser was triggered-on when mice entered the open arm only, and open arm silencing significantly increased % time exploration of the open arm, while silencing in the EPM closed arm (right panel) had no effect (EPM ANOVA F_(1,14)=6.184, p<0.05, N_eYFP=9, N_Arch=7; EPM ANOVA F_(1,19)=3.465, p=0.08, N_eYFP=9, N_Arch=12). (C) RTPP, laser was triggered-on when mice entered one side of the identical 2-chamber arena, and mice showed no preference for % time exploration of the stimulation side. (ANOVA F_(1,13)=0.435, p=0.52, N_eYFP=8, N_Arch=7) All data error bars represent mean +/− SEM

Figure 3. Differential representations of anxiety-related information along the dorsoventral axis of CA1

(A) Experimental design for dCA1 Ca²⁺ imaging. Left image, dCA1 GCaMP6f and GRIN representative lens placement. Right image, Representative contours of identified cellular units from a dCA1 imaging FOV. (B) dCA1 rate of Ca²⁺ transients in the EPM. dCA1 neurons did not significantly increase their rate of Ca²⁺ transients to the open arm compartment (Wilcoxon signed rank Z=−1.451 p=0.15, N_dCA1=408). (C) vCA1 is enriched in cells that are significantly selective for the EPM open arm compared to shuffle (orange pie chart; see methods), relative to dCA1 (Chi squared test of proportions χ²(2)= 43.984, p<0.0001 N_dCA1=408, N_vCA1=2,137). (D) Analysis design. Top, imaging sessions from individual mice (imaged at a constant FOV) were concatenated into one large video prior to motion correction and cell segmentation to allow for cross-session cell tracking in different tasks (video frame edge-color denotes different imaging sessions combined; EPM: orange, OFT: purple, Novel Object task: green). Bottom, EPM open arm selective cells were then defined as in Fig. 3C, and their Ca²⁺ transient rates were compared across other imaging sessions. (E) The Ca²⁺ transient rates of EPM open arm selective cells in vCA1 (left panels) and dCA1 (right panels) were compared in the EPM, OFT, and Novel Object tasks. vCA1 open arm selective cells were significantly more active to the EPM open arm and OFT center zones compared to the safe closed arm and periphery compartments (left two bar graphs), but did not change activity to exploration of a Novel Object (right bar graph) (vCA1 open arm cells: EPM open versus closed Wilcoxon signed rank Z=−12.016, p<0.0001; OFT center versus periphery rates Wilcoxon signed rank Z=−2.103, p<0.05; Novel Object Wilcoxon signed rank Z=−1.069, p=0.28; N_cells=192). In contrast, dCA1 open arm selective cells did not exhibit any changes in Ca²⁺ transient rate in the OFT center and Novel Object task, indicating that while vCA1 open arm neurons exhibit heightened activity across multiple tasks of innate anxiety, dCA1 open arm selective neurons are context-specific (dCA1 open arm cells Wilcoxon signed rank: EPM open versus closed Z=−10.338, p<0.0001; OFT center versus periphery Z=−1.651, p=0.10; Novel Object rates Z=−1.242, p=0.21; N_cells=142). All data error bars represent mean +/− SEM

Figure 4. vCA1 inhibitory interneurons are not recruited during open arm exploration

(A) Experimental design for vCA1-vGAT Ca²⁺ imaging. Bottom left image, vCA1-vGAT-Cre/flex-GCaMP6f expression and representative GRIN lens placement; Bottom right image is a magnified inset of left. Top panel, extracted Ca²⁺ transients from an example vCA1-vGAT FOV. (B) vCA1-vGAT Ca²⁺ activity in the EPM is not different between arm-types (N_cells=70 for all analyses). Left: Ca²⁺ transient area under the curve (AUC)/second (Wilcoxon sign rank Z=−0.781, p=0.43). Middle: mean Ca²⁺ transient amplitude (Wilcoxon sign rank Z=−1.505, p=0.13). Right: rate of Ca²⁺ transients (Wilcoxon sign rank Z=−0.451, p=0.65). (C) vCA1-vGAT neurons are enriched in closed-arm selective cells (exceeding shuffle distribution rates) in the EPM. Scatter plot showing individual vCA1-vGAT neuron open vs closed rates, colored based on arm-type selectivity determined by exceeding shuffle distribution. Left pie chart inset is a summary of the scatter plot data.

Figure 5. Separate populations of vCA1 neurons project to the Basal Amygdala and Lateral Hypothalamus

(A) Anterograde tracing of vCA1 axon terminals. CamKII-ChR2-eYFP virus was injected into vCA1 (left), and ChR2-eYFP axon terminals were visualized in the BA (middle) and LHA subfields (right). (B) Average fluorescence of ChR2-eYFP terminal fields were similar between BA and LHA fields (paired t-test, t₍₄₎=1.886, p=0.13, N_mice=5) (C) In vitro slice recordings of BA cells in ChR2-eYFP vCA1 injected mice. 5ms 473nm light pulses elicited large optical EPSC (mean onset latency= 0.93 ms +/− 0.12 SEM; purple example trace, middle bar graph, N_cells=10), which were abolished by APV/NBQX infusion (black example trace; right bar graph Mann Whitney U=0.00, p<0.05, N_cells=4) (D) In vitro slice recordings of LHA cells in ChR2-eYFP vCA1 injected mice. 5ms 473nm light pulses elicited large optical EPSC (mean onset latency= 1.56 ms +/− 0.09 SEM; purple example trace, middle bar graph, N_cells=10), which were abolished by APV/NBQX infusion (black example trace; right bar graph Mann Whitney U=0.00, p<0.01, N_cells=5) (E) Ctb retrograde labeling of vCA1 projections to the BA (Ctb-555) and LHA (Ctb-488). Left, representative images of CTB injection sites in the BA (bottom) and LHA (top). Right, representative image of retrogradely labeled vCA1 neurons projecting to BA (red cells), LHA (green cells) or both (yellow cells, white arrows). (F) Quantification of retrogradely labeled vCA1 neurons. vCA1-BA and LHA projecting neurons are largely non-overlapping as only ~3% of counted cells were dual-labeled (paired t-tests BA or LHA vs Dual, BA vs Dual t(10)=4.309, LHA vs Dual t(10)=5.832, p<0.01 for both). (G) Left diagram, CA1 deep and superficial lamination. Right, cumulative distribution of vCA1-BA (red line) or vCA1-LHA (green line) labeled neuron distances from the inner radial border. The vCA1-LHA neuron distance distribution was significantly right shifted relative to vCA1-BA neurons, indicating that LHA neurons are organized deeper in the CA1 pyramidal layer (KS test, p<0.0001, KS stat=0.2999). All data error bars represent mean +/− SEM

Figure 6. vCA1-Amygdala and vCA1-LHA projectors differentially contribute to anxiety-related behavior and learned fear

(A) Experimental design of bilateral vCA1-amygdala terminal ChR2 optogenetic stimulation (targeting the basal amygdala). (B) vCA1-amygdala ChR2-eYFP optogenetic terminal stimulation (473nm 10hz, 5ms pulses) during CFC encoding (left panel, training light on) and CFC retrieval (right panel, testing light on). Stimulation of vCA1-BA terminals on training day 1 or testing day 2 reduces % time freezing, indicating a disruption of both CFC encoding and retrieval (repeated measures anova, % time freezing*genotype interaction; training light on (left panel) F_(1,24)=6.358, p<0.05, N_eYFP=15, N_ChR2=11; testing light on (right panel) F_(1,15)=21.10, p-value<0.001, N_eYFP=9, N_ChR2=8). (C) vCA1-BA ChR2-eYFP optogenetic terminal stimulation in OFT in 3 min laser epochs (light off-on-off; light on: 473nm 20hz, 5ms pulses) had no impact on % center distance (Repeated measures ANOVA, F_(1,16)=0.497, p=0.61, N_eYFP=7, N_ChR2=11). (D) vCA1-amygdala ChR2-eYFP optogenetic terminal stimulation in RTPP (473nm 20hz, 5ms pulses, laser triggered on in one chamber only). Left panel, RTPP chamber occupancy heatmaps of representative eYFP and ChR2 mice. Right graph, ChR2 stimulation did not impact % time on stimulation side, indicating that the stimulation was neither appetitive nor aversive (5 min bins, ANOVA % time stim*genotype, F_(1,11)=0.573, p=0.46, N_eYFP=8, N_ChR2=5). (E) Experimental design of bilateral vCA1-LHA terminal ChR2 optogenetic stimulation. (F) vCA1-LHA ChR2-eYFP optogenetic terminal stimulation (473nm 10hz, 5ms pulses) during CFC encoding (left bar graph, training light on) and CFC retrieval (right bar graph, testing light on), had no impact on % time freezing on either testing day (Repeated measures ANOVA, F_(1,12)=0.216, p=0.81, N_eYFP=7, N_ChR2=7). (G) vCA1-LHA ChR2-eYFP optogenetic terminal stimulation in OFT in 3 min laser epochs (light off-on-off; light on: 473nm 20hz, 5ms pulses) significantly reduced % center distance relative to controls (ChR2: blue line, Repeated measures ANOVA; % center distance*genotype interaction, F_(1,19)=7.635, p<0.01; light on ANOVA F_(1,19)=9.356, p<0.01; light off epoch 3 ANOVA F_(1,19)=7.981, p<0.05; N_eYFP=9, N_ChR2=12). (H) vCA1-LHA ChR2-eYFP optogenetic terminal stimulation in RTPP (473nm 20hz, 5ms pulses, laser triggered on in one chamber only). Left panel, RTPP chamber occupancy heatmaps of representative eYFP and ChR2 mice. Right graph, ChR2 stimulation significantly decreased % time on stimulation side, indicating that the stimulation was aversive (5 min bins, ANOVA, % time stim*genotype F_(1,15)=8.403, p<0.05; N_eYFP=9, N_ChR2=8). All data error bars represent mean +/− SEM

Figure 7. The vCA1-LHA projection is enriched in anxiety cells

(A) Projection-specific Ca²⁺ imaging experimental design, CAV2-Cre was injected into either BA (top) or LHA (bottom), and flex-GCaMP6f was injected into vCA1 to express GCaMP6f in vCA1-BA or vCA1-LHA projection neurons specifically. (B) Dual viral targeting of projection specific vCA1 neurons allowed for selective expression of GCaMP6f in vCA1-BA (top panel) or vCA1-LHA (bottom panel) neurons only. This was confirmed by visualization of fluorescent terminals in the BA but not LHA subfield of vCA1-BA labeled mice (top middle and right panels), and fluorescent terminals in the LHA but not BA subfields in vCA1-LHA labeled mice (bottom middle and right panels). (C) Representative GRIN lens FOV from vCA1-BA (top) and vCA1-LHA GCaMP6f (bottom) labeled mice. Right, example of extracted Ca²⁺ transients from projection specific vCA1-BA (top, red traces), and vCA1-LHA (bottom, green traces) labeled FOV. (D) Ca²⁺ transient rate difference (open-closed) in EPM between projection-specific populations. vCA1-LHA projection neurons exhibited significantly greater rate changes in the EPM open arm compartments (Mann Whitney U=1097.00, p<0.05. N_BA=36, N_LHA=80). (E) vCA1-LHA projection neurons are enriched in cells that are significantly selective for the EPM open arm compared to shuffle (orange pie chart; see Fig S6C and methods), relative to vCA1-BA projectors (Chi squared test of proportions, χ²(2)=11.45, p<0.01 N_BA=36, N_LHA=80). (F) Experimental design, CAV2-Cre was injected into the LHA, and Cre-dependent ArchT-GFP was injected into vCA1 to selectivity express Arch in vCA1-LHA projections bilaterally. Optogenetic fibers were implanted at the vCA1 injection site. (G) vCA1-LHA projections were silenced during EPM open arm exploration bouts (laser turned on in open arm only), which significantly decreased open arm avoidance relative to eYFP controls (ANOVA mins 10-20; F_(1,17)=5.195, p<0.05, N_eYFP=10, N_Arch=9). All data error bars represent mean +/− SEM