Mice
All procedures were performed in accordance with the Guide for the Care and Use of Laboratory Animals and were approved by the National Institute of Mental Health (NIMH) Animal Care and Use Committee. Mice used in this study were group housed under a 12-h light/dark cycle (6:00–18:00 light), at temperatures of 70–74 °F and 40–65% humidity, with food and water available ad libitum. After surgery, mice were singly housed. C57BL/6NJ (005304), Gad2-Cre (019022), and Pv-Cre (017320) were obtained from The Jackson Laboratory. Drd2-Cre (founder line ER44), Rbp4-Cre (founder line KL100), and Syt6-Cre (founder line KI148, 109, or 130) were obtained from GENSAT/MMRRC. Both male and female mice 8–16 weeks of age were used for all experiments. Animals were randomly allocated to the different experimental conditions reported in this study.
Viral vectors
AAV2-CaMKII-eNpHR3.0-mCherry (Deisseroth), AAV2-Ef1α-DIO-hChR2(H134R)-EYFP (Deisseroth), AAV5-CaMKII-ChR2(H134R)-EYFP (Deisseroth), AAV2-CaMKII-EYFP (Deisseroth), AAV2-Ef1α-DIO-eNpHR3.0-mCherry (Deisseroth), and AAV2-syn-FLEX-ChrimsonR-tdTomato (Boyden) were produced by the Vector Core of the University of North Carolina. AAV9-hSyn-Flex-GCaMP8s-WPRE (plasmid no. 162377), AAV9-syn-ChrimsonR -tdTomato (plasmid no. 59171), AAV2-CaMKII-mCherry (plasmid no. 114469), AAV2(retro)-CAG-iCre (plasmid no. 81070), AAV2-hSyn-DIO-hM4Di-mCherry (plasmid no. 44362), AAV2-hSyn-DIO-mCherry (plasmid no. 50459), AAV9-syn-FLEX-jGCaMP7s-WPRE (plasmid no. 104491), and AAV1-hSyn-FLEX-iGABASnFR (plasmid no. 112163) were purchased from Addgene. CAV-Cre was produced by the Institute of Molecular Genetics of Montpellier (Montpellier, France). AAV9-EF1a-FLEX-TVA-mCherry (Addgene, plasmid no. 38044) and AAV9-CAG-FLEX-RG (Addgene, plasmid no. 38043) were produced by Vigene Biosciences. EnvA-SAD-ΔG-eGFP (Addgene, plasmid no. 32635) was produced by the Viral Vector Core of the Salk Institute for Biological Studies. All viral vectors were stored in aliquots at -80°C until use.
Stereotaxic surgery.
All viral injections were performed using previously described procedures1 and an AngleTwo stereotaxic device (Leica Biosystems) at the following stereotaxic coordinates: PL, -1.90 mm from bregma, ±0.55 mm lateral from midline and -2.30 mm vertical from cortical surface; avTRN, -0.70 mm from bregma, ±1.00 mm lateral from midline and -4.20 mm vertical from cortical surface; pPVT, -1.60 mm from bregma, 0.06 mm lateral from midline and -3.30 mm vertical from cortical surface, 6.12° angle for both fiber photometry and optogenetics; NAc, 1.70 mm from bregma, ±0.60 mm lateral from midline and -4.80 mm vertical from cortical surface.
For fiber photometry and optogenetic experiments, an optical fiber (400 µm for photometry, Doric Lenses; 200 µm for optogenetics, ThorLabs) was implanted 200-300 µm above the target of interest and immediately following viral injections, and cemented using Metabond Cement System (Parkell) and Jet Brand dental acrylic (Lang Dental Manufacturing).
For retrograde tracing of TRN-projecting and PVT-projecting PL cells CTB-488 and CTB-555 (1.0% in PBS, Thermo Fisher Scientific) were injected into the TRN (0.5 µL) and PVT (1.0 µL), respectively, and allowed 4 d for retrograde transport. For retrograde tracing of PVT-projecting TRN cells unconjugated CTB (List Labs Product No. 104) was injected into the PVT (1.0 µL). For retrograde labeling of PVT-projecting TRN cells for RNAscope, retrobeads (LumaFluor, Inc.) were injected into the PVT and allowed 7 d for retrograde transport.
After all surgical procedures, animals were returned to their home cages and placed on a heating pad for 24 h for post-surgical recovery and monitoring. Animals received subcutaneous injections with Metacam (meloxicam, 1–2 mg kg-1) for analgesia and anti-inflammatory purposes. Mice without correct targeting of optical fibers, tracers, or vectors were excluded from this study.
Fiber photometry
Fiber photometry was performed as previously described2. Briefly, mice were allowed to habituate to the fiber patch cord in their home cage for approximately 5 min before each behavior test. GCaMP fluorescence and isosbestic autofluorescence signals were excited by the fiber photometry system (Doric Lenses) using two sinusoidally modulated LEDs (473 nm at 211 Hz and 405 nm at 531 Hz) controlled by a standalone driver (DC4100, ThorLabs). Both LEDs were combined via a commercial Mini Cube fiber photometry apparatus (Doric Lenses) into a fiber patch cord (400-µm core, 0.48 NA) connected to the brain implant in each mouse. The light intensity at the interface between the fiber tip and the animal was adjusted from 10 µW to 20 µW (but was constant throughout each test session for each mouse). An RZ5P fiber photometry acquisition system with Synapse software (Tucker-Davis Technologies) collected and saved real-time demodulated emission signals and behavior-relevant TTL inputs. For each trial, GCaMP signals (F473 nm) were compared with autofluorescence signals (F405 nm) to control for movement and bleaching artifacts. Signal data were de-trended by first applying a least-squares linear fit to produce Ffitted 405 nm, and dF/F was calculated as (F473 nm – Ffitted 405 nm)/Ffitted 405 nm. All GCaMP signal data are presented as the z-score of the dF/F from baseline (pre-WS) segments.
Two-way active avoidance (2AA)
Mice were trained on the 2AA task as previously described3. Briefly, the behavioral apparatus consisted of a custom-built shuttle box (18 cm × 36 cm × 30 cm) that contained two identical chambers separated by a hurdle (17.5 cm × 6 cm). The hurdle projected 3 cm above the floor and allowed mice easy access to both chambers. The floor consisted of electrifiable metal rods (H10-11M-TC-SF, Coulbourn Instruments) and was connected to a shock generator (H13-15, Coulbourn Instruments). Before each subject was trained/tested, the shuttle box was wiped clean with 70% ethanol. The mouse’s behavior was captured with a USB camera during each session. A speaker located on the top of the shuttle box (50 cm high) was used to deliver the WS. Subjects’ movement and TTLs of WS, US and optogenetic stimulation were recorded by ANY-maze version 5 (Stoelting).
After a 5-min habituation period, mice were trained with daily sessions of 2AA, each consisting of 30 presentations of the WS (4 kHz, 75dB, lasting up to 15 s each). Trials in which subjects failed to shuttle to the adjacent chamber before the termination of the WS resulted in the presentation of the US (0.6 mA foot shock lasting up to 15 s each) until subjects escaped to the opposite chamber (escape trials). No subject failed to escape the US. For trials in which subjects shuttled to the opposite chamber during the WS, the WS was abruptly terminated, and the US was also prevented (avoidance trials). The inter-trial interval (ITI) was 30 s. Avoidance rate was calculated as the percentage of the number of avoidance trials over the total number of trials. For optogenetic and fiber photometry experiments fiber patch cords were attached every session of training. For all experiments, subjects that did not reach 30% avoidance rates by Day 5 were excluded from data analysis.
Fiber photometry during 2AA. In Fig. 1a-d, Fig. 4a-c, Extended Data Fig. 2 and Extended Data Fig. 6a-d, mice were subjected to five 2AA sessions (one session per day), and the GCaMP signal was collected on Day 5 as described above. In Fig. 5a-d, Extended Data Fig. 7, Extended Data Fig. 8a-f, and Extended Data Fig. 9, mice were subjected to five 2AA sessions (one session per day), and the GCaMP or iGABASnFR signal was collected on Days 4 and 5 as described above.
Optogenetic manipulations during 2AA. In Fig. 1e-f, Fig. 4g-h, Fig. 5e-f, Extended Data Fig. 4, Extended Data Fig. 6g-k, Extended Data Fig. 8g-i, Supplementary Fig. 1, Supplementary Fig. 4b-c and Supplementary Fig. 6a-e, mice were subjected to five 2AA sessions (one session per day), and on Day 4 a yellow light (Ce:YAG + LED Driver, Doric) was presented coinciding with the WS. In Fig. 4e-f, Extended Data Fig. 6e-f and Supplementary Fig. 4a, mice were subjected to five 2AA sessions (one session per day), and on Day 4 the yellow light was presented at the onset of the WS but terminated after 2s. In Supplementary Fig. 2, mice were subjected to five 2AA sessions (one session per day), and on Day 4 a yellow light (Ce:YAG + LED Driver, Doric) was presented coinciding with the ITI. For all mice the light intensity at the at the interface between the fiber tip and the brain implant was ~10 mW. For optogenetic excitation (ChR) the light (10 Hz, 20% duty cycle) was presented at the certain duration as each experiment. For optogenetic inhibition (Halo) the light was presented continuously for the certain duration as each experiment. In Extended Data Fig. 8n-q and Supplementary Fig. 6f-j, mice were subjected to five 2AA sessions (one session per day), and on Day 4 the yellow light (~10 mW, 50Hz, 10% duty) was presented at the onset of the WS but terminated 2s later.
Fiber photometry with optogenetic manipulations during 2AA. In Fig. 1g-l, Fig. 5g-l, Extended Data Fig. 3, and Extended Data Fig. 8j-m, mice were subjected to seven 2AA sessions (one session per day), the GCaMP signal was collected on Days 4-7 and optogenetic inhibition was performed on Days 4 and 5 with constant light presentation that began 5s prior to WS onset and culminated 5s after the offset of the WS.
Data analysis and behavioral tracking for 2AA
Analysis of the 2AA behavioral tracking was done as previously described3. We performed post hoc position tracking of the animal’s nose and body center from video in the software TopScan (CleverSys). WS and US times from ANY-maze and raw video tracking position values from TopScan were exported, and analysis was performed with custom routines in the R statistical computing environment (R Core Team 2019, R Foundation). Missing positions up to ten successive frames were linearly interpolated with custom routines in R. For imaging sessions, video tracking and ANY-maze TTL pulse timestamps were zero corrected to align behavioral and calcium signal timestamps. Next, calcium signals and/or position frames during US and WS were flagged by matching the relevant timestamps to TTL pulse times from ANY-maze, and the frame-by-frame distance traveled for the nose and body center was calculated for the tracking data. To minimize the effects of noise in the tracking data, we calculated the 40% quantile of the frame-by-frame distance traveled by the animal’s nose and body center for each session; in all cases, this yielded a distance value of 0 or 1 mm. This quantile value served as a movement threshold—that is, an inter-frame distance traveled less than or equal to the quantile value was considered non-movement. We then created a binary vector, and frames with coincident immobility of the nose and center body were set to 1. Changepoint analysis4 (R package version 2.2.2 (https://CRAN.R-project.org/package=changepoint)), with a minimum segment length of 30 video frames, was then applied to this vector. This approach allowed us to statistically determine when transitions to (and from) coincident periods of non-movement of nose and body occurred, which were used as a proxy for freezing behavior. Next, each sustained bout of non-movement was isolated, and we probed whether there was any movement that lasted for >5 consecutive video frames. If such movement did occur, we truncated the bout of immobility at the start of movement. Finally, immobility bouts with a duration ≥1 s were considered freezing.
We isolated freezing events (see freeze detection section above) that occurred during the WS as WS freezing and those that occurred during ITI as ITI freezing. For each trial, we calculated the time interval between the moment of animal crossing the hurdle and the WS onset during trials where the animal avoided the footshock, named latency to avoid. WS, ITI freezing or latency to avoid were average within session and then within each group and plotted as mean ±s.e.m.
For fiber photometry, GCaMP data were normalized as dF/F. Next, we used the behavioral flags calculated from the video tracking to create average peri-event time histograms (PETHs) time-locked to the onset of the behavior events of interest, including WS onset, highest movement velocity during the WS (Max Velocity), escape or avoidance movement onset (Shuttle Initiate), escape or avoidance moment (Shuttle) and freezing onset during the WS (WS Freezing). All trials in each session were separated into avoidance and escape trials as described above. For each trial type, the z-score from 10 s before to 30 s after WS onset was plotted in heat maps for all trials in test sessions. The mean of all recorded activity for each trial type was plotted below the corresponding heat map. We calculated and plotted the AUC of the z-scores before WS onset as a baseline, during the WS, and post WS period. We isolated the peri-event calcium signal by a certain time window (2 sec for the onset of the WS and 5 s for all other behavioral events) from avoidance and escape trials separately, then we calculated z-scores based on pre-event signal for each trial and area under the curve (AUC) of the z-score from 1 s bins throughout each behavior event. Lastly, we plotted the mean of signal transitions and AUC for each event type from each trial type and compared the differences of AUCs of 1) adjacent seconds in same trial type and 2) same second between trial types or groups. All 2AA photometric signals and behavioral performance were analyzed blinded. All codes are available at the following repository: https://github.com/Penzolab/Data-analysis-of-Two-way-active-avoidance-task.git.
Fiber photometry with optogenetic and chemogenetic manipulations
In Fig. 3 and Supplementary Fig. 3, mice with mCherry or DREADDs (Gi) unilaterally expressed in the TRN and channelrhodopsin (ChR2) or YFP unilaterally expressed in the PL were injected with either clozapine N-oxide (CNO) or saline (Sal) and subjected to 15 trials of optogenetic stimulation (~10mW, 10 Hz, 20% duty cycle, 15 s) with 30s ITI while simultaneously recording GCaMP signal as described above in NAc-projecting pPVT cells. Both CNO (10mg/kg; Enzo Life Sciences) and Sal injections were given to all mice 30 min prior to recording GCaMP signal and the injection order was counterbalanced on separate days.
Monosynaptic rabies tracing of inputs to PVT-projecting TRN cells
To limit monosynaptic rabies tracing to PVT-projecting neurons of the TRN, CAV-Cre virus was unilaterally injected into the PVT (1.0 µl) of C57BL/6NJ mice. Within the same surgical procedure, a virus mixture of AAV9-EF1a-FLEX-TVA-mCherry and AAV9-CAG-FLEX-RG at a 1:1 ratio was injected into the avTRN (0.6 µl), followed by an injection of the pseudotyped rabies virus EnvA-SAD-ΔG-eGFP (1.2 ul) in the same location of avTRN 2 weeks later. Mouse brain tissues were collected and subjected to analysis 1 week later. Representative images in Supplementary Fig. 5 were scanned through Zeiss 780 confocal.
Histology and immunofluorescence
Animals were deeply anesthetized with euthanasia solution (Vet One) and transcardially perfused with PBS (pH 7.4, 4 °C), followed by paraformaldehyde (PFA) solution (4% in PBS, 4 °C). After extraction, brains were post-fixed in 4% PFA at 4 °C for a minimum of 2 h and subsequently cryoprotected by transferring to a 30% PBS-buffered sucrose solution until brains were saturated (for over 24 h). Coronal brain sections (50 μm) were cut using a freezing microtome (SM 2010R, Leica). For immunofluorescence staining, brain sections were incubated in PBS (pH 7.4) with 10% normal goat serum and 0.1% Triton X-100 (Sigma-Aldrich) for 1 h and then incubated using the following antibody: anti-PV (1:1000, rabbit, Swant, PV 27) (overnight, at 4 °C); anti-CTB (1:##, goat, List Labs, Product No. 703)( 48 h, at 4 °C). After washing, Alexa Fluor-conjugated secondary antibodies (1:500, goat anti-mouse, Thermos Fisher Scientific A-11001; 1:500, goat anti-rabbit, Thermo Fisher Scientific A-21245). Finally, sections were subsequently mounted onto glass slides for imaging (LSM 780 laser scanning confocal microscope, Carl Zeiss). Image analysis and cell counting were performed using ImageJ software (Fiji, version 1.52p). Optical fiber placements for all mice included in this study are presented in Figs 2a, 3c, 4b, 5c, 6b and 7c and Extended Data Figs. 1a, 4b,l, 5b, 6b, 8b,m and 9a.
Sample preparation and ISH procedure for RNAscope. Fresh-frozen brains from adult male C57BL/6NJ mice (8-12 weeks old) were sectioned at a thickness of 16 µm using a Cryostat (Leica Biosystems). Sections were collected onto Superfrost Plus slides (Daigger Scientific), immediately placed on dry ice and subsequently transferred to a -80 °C freezer. The Spp1 and Ecel1 mRNA signal was detected using the RNAscope fluorescent kit (Advanced Cell Diagnostics). Specifically, slides with sections covering the entire anteroposterior spread of the PVT were removed from the -80 °C freezer, fixed with freshly prepared ice-chilled 4% PFA for 15 min at 4 °C and then dehydrated using a series of ethanol solutions at different concentrations (5 min each, room temperature): 1 50%, 1 70% and 2 100%. Next, sections were treated with Protease IV (Advanced Cell Diagnostics) at room temperature for 30 min. Slides were then washed with PBS twice (1 min each) and dried for 5 min at room temperature, and sections were circled with an ImmEdge Hydrophobic Barrier PAP Pen (Vector Laboratories). Hybridization was performed on a HybEZ oven for 2 h at 40 °c using a Spp1 or Ecel1 probe (Advanced Cell Diagnostics). After this, the slides were washed twice with washing buffer (2 min each), then incubated with Hybridize Amp 1-FL for 30 min, Hybridize Amp 2-FL for 15 min, Hybridize Amp 3-FL for 30 min and Hybridize Amp 4-FL for 15 min. Next, the slides were washed twice with washing buffer (2 min each) and coverslips added using Diamond Prolong antifade mounting medium with DAPI (Thermo Fisher Scientific).
Signal detection and analysis for RNAscope. Dried slides were examined on an LSM 780 laser scanning confocal microscope (Carl Zeiss) using 20X objective 24 h after the amplification procedure. Signal was subsequently quantified with CellProfiler 3.1.8 using a freely available pipeline (macros) for RNAscope5. A protocol with a step-by-step description of how to implement this pipeline for analyzing RNAscope data was recently published6. All RNAscope data were analyzed in a blinded manner.
Whole-cell patch-clamp slice electrophysiology
For electrophysiological experiments, slices were prepared as previously described7. Briefly, mice were anesthetized with isoflurane and transcardially perfused with an ice-cold NMDG cutting solution (92 mM N-Methyl-D-glucamine, 2.5 mM KCl, 1.25 mM NaH2PO4, 10 mM MgSO4, 0.5 mM CaCl2, 30 mM NaHCO3, 20 mM glucose, 20 mM HEPES, 2 mM thiourea, 5 mM Na-ascorbate, 3 mM Na-pyruvate, at 7.3-7.4 pH gassed with 95% O2 and 5% CO2). Coronal sections (300-µm thick) containing the avTRN or PVT were cut in the ice-cold NMDG cutting solution using a VT1200S automated vibrating-blade microtome (Leica Biosystems), and were subsequently transferred to a heated incubation chamber containing the NMDG cutting solution at 34-35 °C. After approximately 12 min, slices were transferred to a room temperature (20-24°C) holding chamber containing a HEPES-modified artificial cerebrospinal fluid (92 nM NaCl, 2.5 mM KCl, 1.25 mM NaH2PO4, 2 mM MgSO4, 2 mM CaCl2, 30 mM NaHCO3, 25 mM glucose, 20 mM HEPES, 2 mM thiourea, 5 mM Na-ascorbate, 3 mM Na-pyruvate, at 7.3 pH, gassed with 95% O2 and 5% CO2) and remained in the holding chamber until needed. For recordings, slices were transferred to the recording chamber and constantly supplied with a room-temperature ACSF (118 mM NaCl, 2.5 mM KCl, 26.2 mM NaHCO3, 1 mM NaH2PO4, 20 mM glucose, 2 mM MgCl2, and 2 mM CaCl2, at pH 7.4, gassed with 95% O2 and 5% CO2).
Tetrodotoxin (TTX, Cat. No. 1078), 4-aminopyridine (4AP, Cat. No. 0940), 2,3-Dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX, Cat. No. 0373), D-2-amino-5-phosphovalerate (APV, Cat. No 0106), picrotoxin (PTX, Cat. No. 1128) were obtained from Tocris Bio-Techne, all other salts were obtained from MilleporeSigma.
Whole-cell patch-clamp recordings from avTRN and/or PVT neurons were obtained with a Multiclamp 700B amplifier (Molecular Devices). Recordings were done under visual guidance using an Olympus BX51 microscope with transmitted light illumination. Recordings were made in ACSF and pharmacological antagonists were added to the ACSF and bath applied. All recordings were made with borosilicate glass pipettes with tip resistance of 3-6 MΩ. Access resistance was monitored throughout all recordings and recordings where access resistance increased above 30 MΩ were not included in analyses.
Optogenetically evoked synaptic responses were achieved by shining a blue LED (470 nm, pE-300white, CoolLED) over acute slices to drive ChR2-expressing terminals. All recordings were done in voltage-clamp configuration. Cells were kept at a holding potential of -70 mV and recorded using a Cs-based internal solution containing 117 mM Cs methanesulphate, 10 mM HEPES, 2.5 mM MgCl2, 2 mM Na2-ATP, 0.4 mM Na2-GTP, 10 mM Na2-phosphocreatine, 0.6 mM EGTA, 5 mM QX-314 at pH 7.2 and 288-290 mOSM. Retrogradely-labeled cells (CTB 555) were identified bases on their red fluorescence using a green LED (pE-300white, CoolLED). To isolate monosynaptic responses, recordings were done in the presence of TTX and 4AP.
For dual opsin stimulation experiments, two distinct excitatory opsins were selectively expressed in PL and TRN. Opsins for each target were counterbalanced across subjects. For 5 mice (8 slices) AAV2-syn-FLEX-ChrimsonR-tdTomato was injected in the TRN and AAV5-CaMKII-ChR2(H134R)-EYFP was injected into in the PL. For 2 mice (4 slices) AAV9-syn-ChrimsonR-tdTomato was injected into the PL and AAV2-Ef1α-DIO-hChR2(H134R)-EYFP was injected into the TRN. Recordings were done in voltage clamp mode and using Cs-based internal solution as described above, with TTX and 4AP present in the bath. The holding potential was varied between -70 mV or 0 mV to isolate PL-mediated EPSCs and TRN-mediated IPSCs respectively. A blue light was used to excite ChR2 and a fiber optic cable from a red laser was attached to the recording chamber (630 nm, Opto Engine LLC) to excite ChrimsonR. Given that red-shifted opsins like ChrimsonR can be excited by blue-shifted light (i.e., 470 nm)[NO_PRINTED_FORM], and that the two inputs onto PVT neurons were either excitatory or inhibitory, we implemented a protocol that minimized potential contamination of postsynaptic currents driven by blue light activation of ChrimsonR. First, cells were held at a membrane potential necessary to isolate the postsynaptic current (PSCs) dependent on the location of ChrimsonR expression (e.g., 0 mV for ChrimsonR expression in TRN). After a baseline recording was established, the appropriate antagonists were bath applied to block the PSCs (e.g., PTX). Once the current was reliably blocked, the holding potential was changed to isolate the PSC driven by the neurons expressing ChR2 (e.g., -70 mV for ChR2 expression in PL).
Statistical analysis and reproducibility
All data were plotted and analyzed with OriginPro version 2016 and version 2018 (OriginLab) and GraphPad Prism (version 8.0.1, GraphPad Software). All data are presented as mean ± s.e.m. There were no assumptions or corrections made before data analysis. Differences between two groups were tested with a two-tailed Student’s t-test; differences among multiple groups were examined with analysis of variance (ANOVA, one-way and two-way repeated-measures) followed by two-stage linear step-up procedure of Benjamini, Krieger and Yekutieli; P < 0.05 was considered significant. The sample sizes used in our study, such as the numbers of animals, are typically the same or exceed those estimated by power analysis (power = 0.80, α= 0.05). For tracing experiments, the sample size is 2–5 mice. For fiber photometry experiments alone, the sample size is 4–6 mice. For fiber photometry experiments with optogenetic and/or chemogenetic malipulations, the sample size is 3–4 mice. For optogenetic behavior experiments, the sample size is 6–13 mice. For ex vivo electrophysiology experiments, the sample size was 3-8 mice and 1-3 slices per mouse. All experiments were replicated at least once, and similar results were obtained. All experiments were randomized. For all behavior experiments, investigators were blinded to allocation during experiments. Data distribution was assumed to be normal, but this was not formally tested.
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