A contextual fear conditioning paradigm in head-fixed mice exploring virtual reality

Abstract

Contextual fear conditioning is a classical laboratory task that tests associative memory formation and recall. Techniques such as multi-photon microscopy and holographic stimulation offer tremendous opportunities to understand the neural underpinnings of these memories. However, these techniques generally require animals to be head-fixed. There are few paradigms that test contextual fear conditioning in head-fixed mice, and none where the behavioral outcome following fear conditioning is freezing, the most common measure of fear in freely moving animals. To address this gap, we developed a contextual fear conditioning paradigm in head-fixed mice using virtual reality (VR) environments. We designed an apparatus to deliver tail shocks (unconditioned stimulus, US) while mice navigated a VR environment (conditioned stimulus, CS). The acquisition of contextual fear was tested when the mice were reintroduced to the shock-paired VR environment the following day. We tested three different versions of this paradigm and, in all of them, observed an increased conditioned fear response characterized by increased freezing behavior. This was especially prominent during the first trial in the shock-paired VR environment, compared to a neutral environment where the mice received no shocks. Our results demonstrate that head-fixed mice can be fear conditioned in VR, discriminate between a feared and neutral VR context, and display freezing as a conditioned response, similar to freely behaving animals. Furthermore, using a two-photon microscope, we imaged from large populations of hippocampal CA1 neurons before, during, and following contextual fear conditioning. Our findings reconfirmed those from the literature on freely moving animals, showing that CA1 place cells undergo remapping and show narrower place fields following fear conditioning. Our approach offers new opportunities to study the neural mechanisms underlying the formation, recall, and extinction of contextual fear memories. As the head-fixed preparation is compatible with multi-photon microscopy and holographic stimulation, it enables long-term tracking and manipulation of cells throughout distinct memory stages and provides subcellular resolution for investigating axonal, dendritic, and synaptic dynamics in real-time.

Figure 1.. A contextual fear conditioning paradigm for head-fixed mice navigating virtual reality environments.

(A) Experimental setup created with BioRender.com. Mice were head-restrained with their feet resting on a cylindrical treadmill. Five large monitors surrounded the mice that displayed virtual reality (VR) environments. Movement on the treadmill advanced the VR display, allowing for context exploration. (B) Mice were water-restricted and trained to run laps in the VR for water rewards. VR environments were 2m-long linear tracks. Mice were trained to achieve >3 laps per minute, which took ~10 to 14 days (Stage 1). Once well-trained, a “tail-coat” was added to their tails (Stage 2), followed by the removal of the water reward the next day (Stage 3). (C) For Paradigm 1, mice underwent the fear conditioning protocol the following day after the water reward was removed. On the first experiment day (Day 0), mice spent 10 minutes in the training VR (Familiar VR) and then another 10 minutes in a new VR (CFC VR). After the initial exploration, mice received mild electric shocks on the tail (4–12 shocks, 0.5–1.2mA in amplitude, 1s long). The next day (Day 1), mice were tested for memory recall by placing them in the Familiar VR and the CFC VR for 5 minutes each in a counterbalanced manner. (D) Schematic of the tail-coat used for delivering mild electric shocks to the mouse’s tail. (Top) View from the top (Bottom) side view. Dimensions are provided for a typical 12-week-old male mouse weighing ~30g before water restriction (see Methods for more details).

Figure 2.. Paradigm 1: Head-fixed mice show increased freezing behavior following contextual fear conditioning in the CFC VR.

(A) A single example mouse's lap running behavior on experiment days, Day 0 and Day 1. Behavior is shown for approximately 3 minutes in all sessions except during CFC, which is shown for the 6 minutes that the session lasted. Frames where freezing was detected (instantaneous velocity 0cm/s) are marked with black dots. A zoomed-in portion on the right in CFC VR highlights these freezing epochs as periods of minimal movement on the treadmill (i.e. instantaneous velocity 0cm/s), which are less visible in the full-scale view. The traces on the right show that this mouse increased freezing, decreased velocity, and moved backward (shown in gray) in the CFC VR (red traces) but not in the Familiar VR (blue traces) on Recall Day. This mouse received six shocks at 1mA intensity at 60 s inter-stimulus interval (ISI). (B) First two minutes of recall behavior in more mice (n = 5) in Familiar VR versus CFC VR. (C-D) (Left Average freezing percentage on recall day in the very first lap (C) and all laps (D) during the 5 minutes that mice explored the Familiar (blue) and CFC (red) VR. Freezing (%) was calculated as the number of frames where freezing was detected in a lap divided by the total number of frames in each lap. (Right) Delta calculated as the difference in the amount of freezing in the CFC VR compared to the Familiar VR before CFC (Day 0) and after CFC (Day 1). The dashed line represents 0. (E-F) Same as C-D but for the time taken to complete the first lap (E) and all laps (F). Mice displayed an increase in freezing and in time taken to complete a lap in the first lap and, on average, in the CFC VR. In C-F, circles and pluses represent individual mice (n = 27, 25 male and two female mice). In C-F, data was pooled from mice receiving different numbers of shocks (4, 6, 12) at varying intensities (0.5 mA, 0.6mA, 0.8 mA, 1 mA, and 1.2 mA), which is separately displayed in Supplementary Figure 2. Lines join data from the same mouse. P-values were calculated using a paired t-test.

Figure 3.. Using a novel VR as the neutral environment instead of a familiar VR in a second paradigm results in modest increases in freezing.

(A) The training paradigm for Paradigm 2 is similar to Paradigm 1. On experiment day, mice were introduced to two novel VRs, one of which would be associated with the shock (CFC VR) and the other wouldn’t (Control VR). In this paradigm, there was an added habituation day, where mice were exposed to the two VRs for ten minutes. The next day, mice ran in the two VRs again before receiving mild electric shocks in the CFC VR. The recall test occurred the next day. In this cohort, all mice received six shocks at 1 mA intensity. (B-D) These panels show the lap running behavior of a single mouse on experiment days −1, 0, and 1. Behavior is displayed for about 3 minutes in all sessions except during CFC. Frames with freezing detected by a threshold (instantaneous velocity 0cm/s) are marked by black dots. In (D), this mouse shows an increase in freezing, a decrease in velocity, and some backward movement when in the CFC VR (red traces). (E) Comparison between freezing in the first lap (left) and all laps (right) in Paradigm 1 (blue and red) versus Paradigm 2 (green and red). In Paradigm 1, only animals that received six shocks at 1 mA intensity are included. n = 12 mice were used in both paradigms (10 male and two female mice). The scale bar for panels B-D is indicated in B. P-values were calculated using a paired t-test.

Figure 4.. In Paradigm 2, the most significant freezing and reduced speed occurred in the first lap of the CFC VR compared to the Control VR.

(A) First two minutes of recall behavior in five mice in Control VR (green) versus CFC VR (red). (B-C) The left panels show the amount of freezing on the recall day in (B) the very first lap and (C) all laps during the 5 minutes when the mice explored the Control (green) and CFC (red) VR. Freezing (%) is calculated as the number of frames where freezing was detected in a lap divided by the total number of frames in each lap. On the right, the delta is calculated as the difference in the amount of freezing in the CFC VR compared to the Control VR before (Day 0) and after CFC (Day 1). The dashed line represents 0. (D-E) Same as B-C but for the time taken to complete the first lap (D) and all laps (E). In B-E, circles and pluses represent individual mice (n = 12). Lines join data from the same mouse. P-values were calculated using a paired t-test.

Figure 5.. A third paradigm that uses a novel VR as the neutral environment but keeps the tail-coat on during memory recall led to increased freezing in the CFC Environment.

(A) The training and experiment paradigm was similar to Paradigm 2 except that the shocks were administered closer together (20-25 s ISI), and the tail-coat was kept on during recall days. (B) First two minutes of recall behavior in Paradigm 3 in five mice in Control VR (green) versus CFC VR (red). (C-D) Comparison between freezing in the first lap (C) and all laps (D) in Paradigm 1 (blue and red) versus Paradigms 2 and 3 (green and red). n = 12 mice were used in Paradigms 1 and 2 (10 male and two female mice), and n = 20 male mice in Paradigm 3. P-values were calculated using a paired t-test. In Paradigm 3, by keeping the tail-coat on during the recall days, we observed an increase in freezing in the CFC VR in the first lap and across all laps compared to Paradigm 2.

Figure 6.. Mice exhibited increased freezing and overall reduced speed in the CFC VR in Paradigm 3

(A-B) The right panels show the amount of freezing on the recall day in (A) the very first lap and (B) all laps during the 5 minutes when the mice explored the Control (green) and CFC (red) VR. Freezing (%) is calculated as the number of frames where freezing was detected in a lap divided by the total number of frames in each lap. On the left, the delta is calculated as the difference in the amount of freezing in the CFC VR compared to the Control VR before (Day 0) and after CFC (Day 1). The dashed line represents 0. (C-D) Same as A-B but for time taken to complete the first lap (C) and all laps (D). Circles and pluses represent individual mice (n = 20). Lines join data from the same mouse. P-values were calculated using a paired t-test.

Figure 7.. Fear conditioning results in place cell remapping in both Familiar and CFC VR.

(A) A schematic of the procedure for two-photon imaging of large populations of the same CA1 neurons over multiple days, created with BioRender.com (B) An example field of view across the two experiment days. The same field of view was aligned and imaged across days. (C) Examples of a few place cells in the CFC VR across sessions on Day 0 (before CFC, during CFC) and Day 1 (Memory Recall). White lines separate laps in each session. Some place cells maintain stable fields across days, while others remap. (D) Place fields defined on Day 0 in Familiar VR before CFC are plotted across Day 1 during fear memory recall (n = 8 mice). (E) Place fields defined on Day 0 in CFC VR before CFC are plotted across the fear conditioning session and on Day 1 during fear memory recall. (F) On the left is a scatter plot of the center of mass of place fields defined in Familiar VR before CFC on Day 0 (x-axis) compared to their center of mass on Day 1 during fear memory recall (y-axis). On the right is a boxplot of correlation coefficients between mean place fields defined in Familiar VR before CFC on Day 0 and memory recall on Day 1 (Day 0: Day 1). The within-session correlation coefficients serve as control and were calculated between mean place fields in the first half and second half of the Familiar VR before the CFC session. (G) A scatter plot of the center of mass of place fields defined in CFC VR before CFC on Day 0 (x-axis) compared to their center of mass on (right) Day 0 during CFC and (middle) Day 1 during memory recall. On the left is a distribution of correlation coefficients between mean place fields defined in Familiar VR before CFC on Day 0 and during CFC (Day 0: Day 0) and memory recall on Day 1 (Day 0: Day 1). The within-session correlation was calculated between mean place fields in the first half and second half of the CFC VR before the fear conditioning session. Asterisk (*) indicates significant P values (KS test, P < 0.01). Our findings show that place fields present in the before CFC sessions in both Familiar and CFC VR showed significant remapping following fear conditioning.

Figure 8.. The widths of place fields in CA1 narrow during memory recall.

(A) Place fields were defined and sorted by track length in each session, pooled from all mice (n = 8 mice). Each cell’s activity was normalized to its peak, and cells were sorted by their center of mass along the track. (B) The percentage of place cells is calculated as the number of place cells divided by the total number of recorded cells. More place cells were identified by our algorithm in the CFC VR on Day 1. (C-E) Parameters of place fields: (C) Width, (D) Place Field (PF) Reliability, and (E) out/in field firing ratio in both VRs on Day 0 and Day 1. The width of the place fields in the CFC VR significantly decreased on Day 1. No other parameters significantly varied across days. P-values were calculated using a paired t-test.