Subjects:
One hundred and seven experimentally naive, male and female c57bl/6 mice (2-3 months of age) from Charles River Laboratories were used. Animals were housed in groups of up to 5 mice per cage. The animal vivarium was maintained on a 12:12-hour light cycle (lights on at 07:00). Mice were water deprived one day prior to training, with all animals receiving at least 1 mL of water per day throughout the entire duration of being water deprived. Animals were provided with ad-libitum access to food, with mice in engram-tagging groups being placed on a doxycycline (Dox) diet prior to surgery. Mice were given at least 14 days after surgery to recover prior to the start of any behavior. Dox was replaced with standard mouse chow 24 hours prior to behavior to open a time window of activity-dependent labeling (Liu 2013). The colony room was maintained on a 12-hour light/dark cycle. Behavior was run under red light and testing occurred at a consistent time to avoid temporal activity confounds. Experimental procedures were approved by IACUC.
Stereotaxic Surgery:
Mice were anesthetized using isoflurane (inducted at 4%, lowered to 2-3% for the maintenance during surgery) and placed in the stereotaxic frame atop a heating pad to maintain body temperature. Hair was removed with a hair removal cream and ophthalmic ointment was generously applied to both eyes to prevent corneal drying. The surgical site was cleaned with alternating application of betadine and ethanol. An incision was made with a scalpel to expose the skull. The brain was then zeroed relative to the skull, and the following coordinates were used: 2.0 anteroposterior (AP), 0.3 mediolateral (ML), and -1.8 dorsoventral (DV) for the PL.
Mice were bilaterally injected with 300 ul of a given viral vector at a rate of 120 ul/min. When performing injections, the needle was lowered to -2.05 DV and left to rest for 3 minutes prior to being pulled up to -2.0 and injected. Then, the needle remained at -2.0 for five minutes before being removed. For photometry, mice received unilateral optic fiber implants 0.1 mm above the site of infusion (Doric Lenses). The implant was secured to the skull with a layer of adhesive cement (C&M Metabond) followed by multiple layers of dental cement (Stoelting). Following surgery, mice were injected with a 0.1mg/kg intraperitoneal (IP) dose of buprenorphine. Virus was given at least 14 days to express prior to the start of the experiment.
Viral vectors:
For overlap experiments, pAAV9-cFos-tTA, pAAV9-TRE-eYFP were constructed as previously described (Ramirez et al., 2015). AAV9-c-Fos-tTA was combined with AAV9-TRE-eYFP (UMass Vector core) prior to injection at a 1:1 ratio. Similarly, for silencing experiments, AAV9-c-Fos-tTA was combined with AAV9-TRE-HM4Di or AAV9-TRE-mCherry (UMass Vector core) prior to injection at a 1:1 ratio. For photometry experiments, AAV9-hsyn-GCaMP6f (AddGene) was used.
Behavior:
Training was conducted in Med Associates operant conditioning boxes controlled by Trans V software. A dispenser with 10% sucrose water connected to a nose port was located at one end of the chamber, along with a light and speaker. Prior to training, mice were placed on water deprivation to increase their motivation to obtain reward. Mice first underwent one-to-one water training, in which the light was constantly on and they could freely gain access to sucrose water at any point during the 30 min session by performing a nose poke. Animals went through three days of one-to-one training and all got at least one hundred rewarded nose pokes within the 15 minute trial duration before advancing in training. In the Reward training phase, sucrose water was no longer freely available during the task but rather during select periods indicated by the light cue. When the light cue turned on, the mouse then had 30 seconds in which it could poke for sucrose water. After time was up, sucrose water was unavailable for the 60s inter-trial interval (ITI). After five days of Reward training, mice underwent three days of Interleaved Avoidance training. Here, light-cue trials were interleaved with tone-cue trials, separated by a 60s ITI. Mice learned, a 30s tone-cue terminated with a footshock (0.4 mA). Mice could avoid this footshock by stepping into a nearby plastic platform which was located opposite to the reward port to ensure mice could not get reward while on the platform. Following, mice underwent Conflict training for seven days. Where the tone and light cues were combined in order to create a motivational conflict.
Behavioral acquisition:
Video data throughout training was taken using GoPro cameras. For behavioral curves, behavior was manually annotated and cross-verified. For photometry and silencing experiments, behavior was analyzed by manually annotating and using video tracking software (EthoVision).
Histology:
Mice were sacrificed and perfused transcardially with phosphate buffered saline (PBS) followed by 4% paraformaldehyde (PFA) in PBS. Brains were then extracted and stored in PFA for at least 24 hours. Brains were sliced coronally at increments of 50 um using a vibratome and stored at 4°C in 0.01% sodium azide in PBS. When staining was performed for cFos overlaps, slices were washed with PBS for 3 washes of 10 minutes, then blocked on a shaker for 1.5 hours using 5% bovine albumin serum (BSA). Slices were then moved to wells with primary antibodies in 1% BSA (1:1000 rabbit anti-cFos [Abcam] and chicken anti-GFP [ThermoFisher]) and allowed to incubate for 48 hours on a shaker at 4°C. Two days later, slices were washed in PBS for 3 washes of 5 minutes each, then were allowed to incubate for 1.5 hours with secondary antibodies in 1% BSA (1:200 Alexa Fluor 555 goat anti-rabbit [ThermoFisher] and Alexa Fluor 488 goat anti-chicken [ThermoFisher]) before being washed with PBS once more for 4 washes of 10 minutes each. For silencing experiments, a different set of primary antibodies (1:1000 each of guinea pig anti-RFP [SySy] and rat anti-cFos [Millipore]) and secondary antibodies (1:200 each of Goat Anti-Rat 488 Alexa Fluor [Abcam] and Goat anti Guinea Pig 555 Alexa Fluor [Abcam]) was used. Slices were then mounted on slides using Vectashield HardSet Mounting Medium with DAPI (Vector Laboratories, Inc), coverslipped, and dried at room temperature overnight before being moved into the fridge.
LifeCanvas Technologies
Brains for brain-wide cFos analyses were stored in PFA for 24 h after perfusion and extraction. They were then stored in 0.02% sodium azide solution before being sent to LifeCanvas Technologies for brain-wide cFos detection. Once there, brains undergo a series of preservation and clearing steps using SHIELD and SmartClear Pro technology, respectively. Next, the samples are washed and prepped for organ-scale immunolabeling using SmartLabel reagents. Samples are batch labeled in 3.5 μg rabbit anti-cFos per brain using SmartBatch and are left to incubate for roughly 18 h. Then, samples undergo a series of washes and fixation steps over4 • days before being incubated in secondary solutions. Finally, brains are mounted in agarose+EasyIndex solution for image preparation. Brain-wide images are acquired using a SmartSPIM microscope equipped with a 3.6× objective with a 1.8 μm×1.8 μm pixel size and a z-step size of 4 μm. The axial resolution of the images is <4.0 μm. The samples are imaged using two channels: 488 nm (autofluorescence/NeuN) and 642 nm (cFos). The autofluorescence channel is used to align the images to the Allen Brain Atlas (Allen Institute for Brain Science: https://portal.brain-map. org/). LifeCanvas Technologies carries out this alignment process in two phases. The first phase is an automated process that samples 1020 atlas-aligned reference samples for each brain sample using a variety of SimpleElastix warping algorithms. An average alignment was computed for all other intermediate images. To confirm the efficacy of the alignment algorithm, the second phase uses a custom Neuroglancer interface (Nuggt: https://github.com/chunglabmit/nuggt) for manual confirmation of the automated alignment algorithm. Once the images were aligned, cell populations were then mapped onto the atlas for region-specific quantification. LifeCanvas Technologies developed a custom convolutional neural network using the TensorFlow Python package. Cell detection was performed by two networks in sequence. Once the cells were aligned and quantified, cFos data were aggregated into .csv files and sent back to the Ramirez group for further analyses as described below.
Image Acquisition and Analysis:
We acquired images using an LSM-800 confocal microscope with a 10x objective lens (Carl Zeiss AG). Images were captured either manually with no focus strategy or were automated using the software autofocus feature in Zen Blue (ver. 2.3) to detect the most intense fluorescent pixels within the defined z-stack. Using ImageJ, one composite image was created per slice using maximum projections of each channel. Overlapping cells between different channels in composite were analyzed using the QuPath software.
Fiber photometry acquisition:
A 470-nm LED (Neurophotometrics FP3002) delivered an excitation wavelength of light to PL neurons expressing GCaMP6f via a single fiber optic implant. The emitted 530-nm signal from the indicator was collected via this same fiber and patch cord (Doric Lenses), spectrally-separated using a dichroic mirror, passed through a series of filters, and was focused on a scientific camera. Isosbestic signals were simultaneously captured by alternating excitation with 415-nm LED to dissociate motion, tissue autofluorescence, and photobleaching from true changes in fluorescence. All wavelengths were interleaved and collected simultaneously using Bonsai interfacing with the Neurophotometrics system. The sampling rate for the calcium signals was 10 Hz per channel.
Chemogenetic Manipulation:
Deschloroclozapine (DCZ) was used as the ligand for selectively activating the injected DREADDs (designer receptors exclusively activated by designer drugs). DCZ was made within 24 hours of experimental use. Doses were administered to be equivalent to 1 ug/kg per animal and were prepared as previously described 50. Administration of DCZ was performed 20 minutes prior to beginning of behavior.
Behavioral analysis:
Behavioral analysis was performed using custom code in Python 3.9 and Prism. For calculating time spent in the avoidance zone, we delineated the platform zone utilizing EthoVision software and considered time spent in the reward zone as any time in which the animal has his nose inside the reward port. For quantifying the percentages of trials that a given strategy is used, we considered a trial to be a Seek trial if animals spent time in the reward zone and received shock, an Avoiding trial if it spent 0s in the reward port but did not receive shock, and a Timing trial if animals spent time in the reward port but did not receive shock. Group averages were created and shown from both the time in zones and percent strategies chosen. For each animal, transition matrices were computed using the formula Pij = nijNi, where Pij represents the probability of transitioning from strategy i to strategy j, nij is the number of transitions from i to j, and Ni is the total number of transitions from strategy i.
Whole-brain-cFos analysis:
Whole-brain cFos density values were obtained for control and experimental groups and all statistical tests related to distributions were performed and plotted using Cumming plots from the DABEST package and Python 3.9 51. 95% confidence intervals were calculated for all brain regions from 5000 bootstrap re-samples. Any p-value reported is the probability of observing the effect size (or greater), assuming the null hypothesis of zero difference is true.
Graph network creation:
Whole-brain cFos density for each brain region were averaged across both hemispheres leaving out measured fiber tracts and layer information. Spearman correlation values across regions were calculated from these density metrics across all animals in the respective conditions. We thresholded the correlation matrix keeping only coefficients with a p < 0.01. These Spearman correlation coefficients were then used to create a Graph. Graphs were created using Networkx and python 3.9. We defined a Graph (G) as a collection of nodes and edges (N, E), where N is the set of nodes that make up the graph and E is the set of edges that connect the nodes, stated as:
G=(N,E),V = {n | n [ G},E= {(ei,ej) | (ei,ej) [ N ei= ej}
In our Graph objects each brain region represents a node and their pairwise Spearman correlation coefficients represent the edges. We constructed null Graphs by randomly shuffling the edges while preserving the total number of edges each node has. To reduce noise we then subtracted the edge weights in our null Graph from the edges in our real Graph for each condition. If an edge was present in the real Graph but not in the null Graph, the edge was retained with its original weight from the real Graph. Next we subtracted the edges from control Graphs from edges of the experimental Graphs for male and female mice respectively, resulting in Graphs that reflect the unique relationships only present during our experiment. The resulting Graph was then used for further analysis. Networks were plotted using Gephi.In order to identify ‘Hubs’ we chose four network statistics as done in prior work 18: Eigenvector centrality (i.e., how often a node is connected to highly connected nodes), Shortest path length (i.e., average length of the shortest possible path between a node and every other node in the graph), Clustering coefficient (i.e., how often a node is connected to another node that forms a local clique), and Betweenness (i.e., how often a node is in the shortest path between all pairs of nodes). We considered a node significantly important for a given statistic depending on if they were in the top 20% for Eigenvector centrality and Betweenness or bottom 20% for Shortest path length and Clustering coefficient 52. We considered a hub any node which was significantly important in at least one of these network statistics.
Fiber photometry analysis:
All fiber photometry analysis was performed using the GuPPy pipeline 53. The data was filtered using a zero-phase moving average linear digital filter of a window of 5 bins for event detection. For event triggered averages we z-scored the entire trace and aggregated all within-animal signals and then marked any bin that exceeds 1.96 z’s as a significant peri-event.
Statistical analysis:
Data are presented as mean ± s.e.m. unless otherwise indicated. All data analyses were performed in custom code in Python 3.9 and GraphPad (Prism 9 for Windows, GraphPad Software, San Diego, California USA). Data sets were tested for normality using the Shapiro-Wilk test and analyzed using either t-tests or ordinary one-way ANOVAs for normally distributed data. P < 0.05 was considered statistically significant.