Mohammed R. Milad is supported by NIMH grant R01 MH097880 and institutional funds from the Department of Psychiatry at MGH.

NOTE: All procedures in this protocol have been approved by the Subcommittee on Research Animal Care, which serves as the Institutional Animal Care and Use Committee (IACUC) for the Massachusetts General Hospital, and are in compliance with the National Institutes of Health guidelines.
1. Animal Housing and Handling Procedure
<ol>
	<li>Upon arrival, house adult Sprague Dawley female rats about 8-10 weeks of age (weighing 200-225 g) in groups of 3-4 inch plastic bins with woodchip bedding. Maintain them on a diet of ad libitum access to chow and water and a 12 hr light/dark cycle. Leave rats to acclimate to the housing conditions and animal facility for a minimum of 1 week. 
	NOTE: All procedures in this manuscript are conducted during the light cycle.</li>
	<li>Following this acclimation period, handle all animals for 5 min each for 2-3 days. Properly pick up the rats one at a time (supported by the torso and not by the tail), hold them securely, and pet them during this time. Handle animals that appear more stressed, jumpy, or anxious for longer periods of time. Do this to minimize handling stress, which could affect the following steps in this protocol. 
	NOTE: The number of days and amount of time handling the animals can vary between different labs; however, these should be consistent between experiments within the same lab.</li>
</ol>
2. Monitoring the Estrous Cycle: Vaginal Swab Smearing and Staining Procedure
<ol>
	<li>Make 0.9% saline solution (sodium chloride dissolved in distilled water). Moisten a cotton-tipped applicator in this solution. Blot the tip using a paper towel to avoid oversaturation, which could lead to inadequate sample collection or loss of cells. Holding the tail upward away from the vaginal opening, gently insert the cotton tip into the vaginal canal and roll around the walls to capture loose vaginal cells.</li>
	<li>If the animal urinates on the cotton tip, replace with a new one, as urine contamination will make it difficult for phase identification. Be extremely careful as to not cause distress to the animal during this procedure because exposure to stress will disrupt the estrous cycle. When swabbing, aim to achieve a swift insertion and withdrawal that is not too deep as to minimize extra stimulation to the cervix, which can induce pseudopregnancy. 
	NOTE: Pseudopregnancy can be identified as an acyclic estrous cycle that typically lasts for about 12 days, i.e. consecutive days of diestrus25.</li>
	<li>Carefully remove the applicator tip and roll it onto a pre-labeled microscope slide. Avoid pressing down too hard as too much fluid on the slide can make phase identification difficult once dry. 
	NOTE: One slide can contain multiple days for each animal, although this can be done according to individual preferences.</li>
	<li>Make sure the sample transfers onto the slide. Collect samples for several consecutive days on one slide to be able to better track and identify daily estrous phases changes. 
	NOTE: If possible, it is also ideal if the people collecting samples remain the same for all animals.</li>
	<li>Once the slide is dry, begin the staining protocol for phase identification. Using the staining kit (see Table of Materials), immerse slides 10 times in the fixative (light blue), 10 times in stain solution (pink), and 5 times in counterstain solution (purple). Rinse slides gently under running water. Let slides completely air dry before viewing under microscope. 
	NOTE: The typically 4-5 day long estrous cycle consists of (in this order): estrus (E), metestrus (M), diestrus (D), and proestrus (P)26. Estrus is characterized by blue, cornified cells, metestrus by leukocytes and a combination of cornified and nucleated cells, diestrus is similar to metestrus in presence of cell types except they are very sparse, and proestrus by purple-stained aggregates of nucleated cells (Figure 1). There are various staining procedures, in addition to those described above, that are equally effective for vaginal smear cytology.</li>
	<li>Collect samples for at least two complete cycles (~8-10 days) in order to monitor the phases accurately prior to experimentation. Be consistent with the time of day the samples are taken. Collect vaginal smear samples daily throughout and until the end of the experiment.</li>
</ol>
3. Pre-exposure
<ol>
	<li>Prior to the initiation of the experiment, pre-expose the animals to the conditioning chambers allow the animals to acclimate to the context (conditioning chambers) before any behavioral testing.
	<ol>
		<li>Pre-expose the females to the conditioning chambers (located in sound-attenuating boxes) with the house lights on for 20-30 min per day for 3 days prior to any experimentation. Assign the box they are pre-exposed to as their chamber for the study. Thoroughly clean the boxes (walls and trays) between sessions and animals to remove odors since they can affect behavior and estrous cycle.</li>
	</ol>
	</li>
</ol>
4. Day 1: Habituation/Fear Conditioning
<ol>
	<li>Once the animals have been verified to be in the estrus phase of their estrous cycles, begin the habituation/conditioning phase of the 3-day experiment.
	<ol>
		<li>Take pre-tone baseline measures of freezing prior to the first tone conditioned stimulus(CS) presentations on each day of the experiment. Do this by acquiring percent freezing scores(calculated by dividing the seconds spent immobile by the duration of trial, multiplied by 100) during the stimulus-absent time interval at the beginning of the first CS-alone trial before the CS onset. 
		NOTE: These measures can be used to assess and compare baseline freezing levels with those during habituation, conditioning, extinction, and recall.</li>
		<li>To conduct extinction training during the metestrus phase, condition the rats in their last day of estrus. Swab and identify estrous phase (step 2) each day before starting the experimental protocol (conditioning/extinction/recall). As the estrus phase may last longer than one day, use cycle history to predict length of phases to determine when the animals are ready to begin training.</li>
	</ol>
	</li>
	<li>In the conditioning chambers, connect the foot-shock grid floor to an electric stimulation generator. Use a stimulation level of 0.5-0.6 mA as the unconditioned stimulus (US). Use a 4 kHz, 80 dB tone as the conditioned stimulus (CS). 
	NOTE: These parameters may vary across laboratories and may be modified for optimal conditioning.
	<ol>
		<li>If the freezing analysis program requires visual cues (as opposed to temporal), co-activate an LED light with the tone. Position the light on the outside of the operant chamber, within the video frame. At the start of each trial, activate the LED light and keep it illuminated until the end of the trial. 
		NOTE: This light is not visible by the animal and only serves as a marker for the initiation and end of the trial within the recorded video files (to be used to measure freezing behavior).</li>
	</ol>
	</li>
	<li>Place the animals into the conditioning chambers for 5 CS-alone trials of habituation. Immediately following the habituation session, perform 7 paired CS-US trials of fear conditioning. Ensure that the CS tone lasts for 30 sec and coterminates with the 0.5 sec US shock. Run trials in all sessions (habituation, conditioning, extinction, and recall) with a variable intertrial interval averaging 3 minutes.</li>
	<li>At the end of the conditioning session, return the animals to their home cages and to the animal facility until the next day.</li>
</ol>
5. Day 2: Estradiol Preparation
<ol>
	<li>Swab and identify the estrous phase for all animals that underwent Day 1 of this protocol. Prepare estradiol (also known as E2, 17beta-estradiol, or beta-estradiol) for a 15 &#956;g/kg dose subcutaneous injection using sesame oil or 0.9% saline as vehicle.</li>
	<li>To calculate the amount of estradiol to prepare for injections, use the body weights of all animals, which were measured and recorded when they were conditioned on Day 1 of this protocol.</li>
	<li>Make calculations to standardize the administered volume of estradiol (i.e. 0.2 ml) per rat. Since estradiol is difficult to dissolve due to its water insolubility and requires a large volume of solvent for the small dose, prepare a stock solution by mixing estradiol in sesame oil over heat until dissolved. The solution can then be passed through a 0.22 &#956;m filter to remove contaminants. For example, add a calculated volume of a 3,000 &#956;g estradiol/1 ml sesame oil stock solution to the total needed volume of sesame oil in order to achieve a 15 &#956;g/kg dose in a 0.2 ml injection volume per animal. 
	NOTE: Alternatively, use other forms of estrogen that are water-soluble and easier to dissolve.</li>
</ol>
6. Day 2: Estradiol Administration/Extinction Training
<ol>
	<li>Administer the estradiol subcutaneously. Lift the loose skin between the scapula by the neck in a gentle pinch and insert the syringe needle into the triangle that is formed by the skin folds.</li>
	<li>30 minutes following subcutaneous injections of estradiol, place the animals in the conditioning chambers again and begin extinction training. In this extinction session, perform 20 CS-alone trials, with each trial consisting of a 30 sec, 4 kHz, 80 dB tone, presented at a variable intertrial interval averaging 3 min. 
	NOTE: The effect of estradiol on recall occurs when estradiol is administered prior to extinction training or immediately after extinction. The effect is not observed 4 hr post-extinction, so it is recommended to inject close in time to the beginning or end of the training session15.</li>
</ol>
7. Day 3: Extinction Recall
<ol>
	<li>24 hr after the extinction training, swab the animals that completed Day 1 and 2 of this protocol.</li>
	<li>Place the animals into their assigned conditioning chambers (same as Day 1 and 2). Begin the extinction recall session by presenting 3 CS-alone trials (the three trials of extinction recall), which consists of only 3.80 dB CS-alone trials with intertrial intervals similar to the 20 trials administered for extinction training as described in step 6.2.</li>
	<li>Upon completion of the recall session, return the animals to their home cages.</li>
</ol>
8. Data Analysis
<ol>
	<li>Acquire behavioral data through video and analyze them using computer software. 
	NOTE: Freezing scores can also be manually counted by an experimenter that is blind to the drug treatment by timing freezing bouts during the CS presentation of each trial, expressed as percentage of the time spent freezing during the tone. Freezing is defined by complete lack of movement with the exception of breathing during the 30 sec CS-US trials. Calculate percent freezing by dividing the seconds spent freezing (immobile) by 30 seconds (duration of trial) multiplied by 100.</li>
</ol>

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The authors of this manuscript have no competing financial interests or other conflicts of interest to disclose.

Fear extinction has been studied extensively in male rats with the neural mechanisms mediating conditioned fear extinction identified and assessed under various manipulations. Relatively few studies have examined female rats or the role of gonadal hormones in fear extinction. To specifically study the effects of estrogen on fear extinction, naturally cycling female rats are subjected to a three-day behavioral paradigm. This procedure consists of habituation/conditioning, extinction, and extinction recall phases. Using th...

Inherent sex differences are observed across various cognitive behaviors and learning paradigms in both humans and rodents. For example, it has been reported that women generally have stronger verbal and detail-oriented abilities, whereas men have better spatial abilities1-3. These sex differences may be due in part to the influence of gonadal hormones. High estradiol levels improve performance on tasks in which women are better, but worsen performance on tasks that men typically do well4-6. Although this evidence is compelling, the limited ability to completely control experimental environments in human studies makes it difficult to determine if these effects on behavior can be attributed specifically to hormones. Animal studies, in contrast, allow for fully controlled circumstances.
Although the basic neural mechanisms of fear extinction have been identified and are well studied in males, it is unclear whether these systems are the same in females or how they change across the estrous cycle. Fear conditioning and extinction are widely used behavioral paradigms in rodents and humans to conduct studies related to anxiety disorders. Given that women have a higher risk for anxiety disorders as well as higher symptom duration and severity7-13, it is critical to include females in these studies. The underrepresentation of females in this research may be attributed to the challenges of estrous phase monitoring and accounting for the behavioral effects of sex hormones. Laboratories that do examine females in this regard encounter issues that often go unreported or are not described in the methods for their studies.
Emerging evidence in rodents suggests that sex differences in fear extinction are modulated by gonadal hormones14-18. In fear conditioning, animals are trained to fear a particular stimulus. After a number of unreinforced presentations of the stimulus, the animals learn not to fear the cue, a process called extinction. How well the animal learns and consolidates the memory of learning this task can be observed during an extinction recall test given some time after the extinction training. Less fear expression during extinction recall demonstrates good extinction memory consolidation. Recent findings from our laboratory suggest that estrogen can modulate the consolidation of fear extinction memory and improve extinction recall. Specifically, female rats that are extinguished in proestrus, the phase of the estrous cycle during which circulating estrogen levels peak, exhibit enhanced retention of extinction memory. In contrast, females that are extinguished in the low-estrogen metestrus phase display relatively poor extinction recall, which can be improved with exogenous estradiol administration prior to or immediately after extinction training15,16. The neural mechanisms of extinction memory consolidation (including the role of gonadal hormones) in females are not clear.
In laboratory animals, the role of hormones can be investigated using invasive surgical removal procedures such as castration and ovariectomy. Subsequent to recovery from surgery, gonadal hormones are often exogenously manipulated during the performance of a behavioral task19. This approach has provided critical information about sex hormones and is useful because it allows for well-controlled manipulations of gonadal hormones (in terms of timing and dose)20-23. This approach, however, does not assess the influence of the naturally occurring fluctuations that occur across the estrous cycle nor do they represent &#8220;normal&#8221; animals, thereby limiting the translational potential to human studies. It has been well-documented that female sex hormone levels peak and decline at specific phases of the estrous cycle, and estrogen receptor expression changes within the cycle, and after ovariectomy24. Thus, there is a need to conduct studies on females with intact gonads and refine experimental designs to reliably study the effects that high and low estrogen states may have on females throughout their lifespan.
This protocol focuses on the effect of estrogen on the neurobiological systems involved in fear extinction. It describes how to: 1) carefully monitor the estrous cycle, 2) prepare effective doses of estradiol for systemic administration, and 3) follow a behavioral paradigm that includes fear conditioning, extinction, and recall in naturally cycling female rats. This protocol can be modified with other pharmacological manipulations and cellular and molecular tools to aid studies to better understand sex differences that are observed in conditioned fear extinction behavior. Note that the procedures described below are those utilized within our laboratory, and there exist a number of variations of these procedures in the literature.

<table><tbody><tr><td>Fear conditioning chamber</td><td>Coulbourn Instruments</td><td></td><td></td></tr><tr><td>Graphic State</td><td>Coulbourn Instruments</td><td></td><td></td></tr><tr><td>Sound-attenuating box</td><td>Med Associates, Inc.</td><td>NIR-022MD</td><td></td></tr><tr><td>Estradiol</td><td>Sigma-Aldrich</td><td>E1024</td><td>In sesame oil for subcutaneous injection</td></tr><tr><td>Sesame oil</td><td>Sigma-Aldrich</td><td>S3547-250ML</td><td></td></tr><tr><td>Freezescan</td><td>Cleversys, Inc.</td><td></td><td></td></tr><tr><td>Dip quick stain</td><td>Jorgensen Laboratories, Inc.</td><td>J0322A1, J0322A2, J0322A3</td><td></td></tr><tr><td>Cotton-tipped applicators</td><td>Fisher Scientific</td><td>23-400-114</td><td>6-inch, sterile</td></tr><tr><td>0.9% saline</td><td>LabChem, Inc.</td><td>LC23460-2</td><td>Sodium chloride w/v</td></tr><tr><td>Selectfrost microscope slides</td><td>Fisher Scientific</td><td>12-550-003</td><td></td></tr><tr><td>Virex II 256</td><td>Diversey, Inc.</td><td>5019317</td><td>Dilute 1:256 with water</td></tr><tr><td>Luer-Lok Tip 1ml Syringes</td><td>Becton Dickinson</td><td>309628</td><td></td></tr><tr><td>Hypodermic disposable needles</td><td>Exelint International, Co.</td><td>26402</td><td>26-gauge</td></tr></tbody></table>

protocol for studying extinction of conditioned fear in naturally cycling female rats

Extinction of conditioned fear has been extensively studied in male rodents. Recently, there have been an increasing number of studies indicating that neural mechanisms for certain behavioral tasks and response behaviors are different in females and males. Using females in research studies can represent a challenge because of the variation of gonadal hormones during their estrous cycle. This protocol describes well-established procedures that are useful in investigating the role of estrogen in fear extinction memory consolidation in female rats. Phase of the estrous cycle and exogenous estrogen administration prior to extinction training can influence extinction recall 24 hr later. The vaginal swabbing technique for estrous phase identification described here aids the examination and manipulation of naturally cycling gonadal hormones. The use of this basic rodent model may further delineate the mechanisms by which estrogen can modulate fear extinction memory in females.

In this fear extinction recall protocol, percent freezing was measured as an indicator of fear. Animals that extinguished well and retained the memory of the extinction training exhibited low fear on the last day of the behavioral testing during extinction recall. Male and female rats do not significantly differ in conditioned fear expression during the conditioning, extinction, and recall phases (Figure 2). However, a sex difference becomes evident when the animals are analyzed separately as high- and l...

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Protocol for Studying Extinction of Conditioned Fear in Naturally Cycling Female Rats

Gonadal hormones such as estrogen modulate memory formation in a number of experimental paradigms including fear extinction memory. This protocol describes a set of methods for investigating the influence of gonadal hormones specifically during extinction in naturally cycling females, including estrous cycle monitoring and exogenous estrogen administration.

Extinction of conditioned fear has been extensively studied in male rodents. Recently, there have been an increasing number of studies indicating that ...

protocol-for-studying-extinction-conditioned-fear-naturally-cycling

Research

JoVE Journal

Behavior

13.3K Views.  Massachusetts General Hospital. Gonadal hormones such as estrogen modulate memory formation in a number of experimental paradigms including fear extinction memory. This protocol describes a set of methods for investigating the influence of gonadal hormones specifically during extinction in naturally cycling females, including estrous cycle monitoring and exogenous estrogen administration.

Video: Protocol for Studying Extinction of Conditioned Fear in Naturally Cycling Female Rats

Trace Fear Conditioning in Mice

In this experiment we present a technique to measure learning and memory. In the trace fear conditioning protocol presented here there are five pairings between a neutral stimulus and an unconditioned stimulus. There is a 20 sec trace period that separates each conditioning trial. On the following day freezing is measured during presentation of the conditioned stimulus (CS) and trace period. On the third day there is an 8 min test to measure contextual memory. The representative results are from mice that were presented with the aversive unconditioned stimulus (shock) compared to mice that received the tone presentations without the unconditioned stimulus. Trace fear conditioning has been successfully used to detect subtle learning and memory deficits and enhancements in mice that are not found with other fear conditioning methods. This type of fear conditioning is believed to be dependent upon connections between the medial prefrontal cortex and the hippocampus. One current controversy is whether this method is believed to be amygdala-independent. Therefore, other fear conditioning testing is needed to examine amygdala-dependent learning and memory effects, such as through the delay fear conditioning.

In the following experiment we describe a protocol for trace fear conditioning in mice. This type of associative memory includes a trace period that separates the neutral stimulus and the unconditioned stimulus.

In this experiment we present a technique to measure learning and memory. In the trace fear conditioning protocol presented here there are five pairings ...

A Procedure for Studying the Footshock-Induced Reinstatement of Cocaine Seeking in Laboratory Rats 

The most insidious aspect of drug addiction is the high propensity for relapse. Animal models of relapse, known as reinstatement procedures, have been used extensively to study the neurobiology and phenomenology of relapse to drug use. Although procedural variations have emerged over the past several decades, the most conventional reinstatement procedures are based on the drug self-administration (SA) model. In this model, an animal is trained to perform an operant response to obtain drug. Subsequently, the behavior is extinguished by withholding response-contingent reinforcement. Reinstatement of drug seeking is then triggered by a discrete event, such as an injection of the training drug, re-exposure to drug-associated cues, or exposure to a stressor 1.
Reinstatement procedures were originally developed to study the ability of acute non-contingent exposure to the training drug to reinstate drug seeking in rats and monkeys 1, 2. Reinstatement procedures have since been modified to study the role of environmental stimuli, including drug-associated cues and exposure to various forms of stress, in relapse to drug seeking 1, 3, 4.
Over the past 15 years, a major focus of the reinstatement literature has been on the role of stress in drug relapse. One of the most commonly used forms of stress for studying this relationship is acute exposures to mild, intermittent, electric footshocks. The ability of footshock stress to induce reinstatement of drug seeking was originally demonstrated by Shaham and colleagues (1995) in rats with a history of intravenous heroin SA5. Subsequently, the effect was generalized to rats with histories of intravenous cocaine, methamphetamine, and nicotine SA, as well as oral ethanol SA 3, 6.
Although footshock-induced reinstatement of drug seeking can be achieved reliably and robustly, it is an effect that tends to be sensitive to certain parametrical variables. These include the arrangement of extinction and reinstatement test sessions, the intensity and duration of footshock stress, and the presence of drug-associated cues during extinction and testing for reinstatement. Here we present a protocol for footshock-induced reinstatement of cocaine seeking that we have used with consistent success to study the relationship between stress and cocaine seeking.

A Procedure for Studying the Footshock-Induced Reinstatement of Cocaine Seeking in Laboratory Rats

Animal models of relapse, known as reinstatement procedures, have been used extensively to study the role of stress in relapse to drug seeking. Here, we report on a method for inducing the reinstatement of cocaine seeking in laboratory rats via acute exposures to mild, intermittent electric footshock.

The most insidious aspect of drug addiction is the high propensity for relapse.  Animal models of relapse, known as reinstatement procedures, have been ...

Neuroscience

Quantifying Cognitive Decrements Caused by Cranial Radiotherapy

With the exception of survival, cognitive impairment stemming from the clinical management of cancer is a major factor dictating therapeutic outcome. For many patients afflicted with CNS and non-CNS malignancies, radiotherapy and chemotherapy offer the best options for disease control. These treatments however come at a cost, and nearly all cancer survivors (~11 million in the US alone as of 2006) incur some risk for developing cognitive dysfunction, with the most severe cases found in patients subjected to cranial radiotherapy (~200,000/yr) for the control of primary and metastatic brain tumors1. Particularly problematic are pediatric cases, whose long-term survival plagued with marked cognitive decrements results in significant socioeconomic burdens2. To date, there are still no satisfactory solutions to this significant clinical problem.
We have addressed this serious health concern using transplanted stem cells to combat radiation-induced cognitive decline in athymic rats subjected to cranial irradiation3. Details of the stereotaxic irradiation and the in vitro culturing and transplantation of human neural stem cells (hNSCs) can be found in our companion paper (Acharya et al., JoVE reference). Following irradiation and transplantation surgery, rats are then assessed for changes in cognition, grafted cell survival and expression of differentiation-specific markers 1 and 4-months after irradiation. To critically evaluate the success or failure of any potential intervention designed to ameliorate radiation-induced cognitive sequelae, a rigorous series of quantitative cognitive tasks must be performed. To accomplish this, we subject our animals to a suite of cognitive testing paradigms including novel place recognition, water maze, elevated plus maze and fear conditioning, in order to quantify hippocampal and non-hippocampal learning and memory. We have demonstrated the utility of these tests for quantifying specific types of cognitive decrements in irradiated animals, and used them to show that animals engrafted with hNSCs exhibit significant improvements in cognitive function3.
The cognitive benefits derived from engrafted human stem cells suggest that similar strategies may one day provide much needed clinical recourse to cancer survivors suffering from impaired cognition. Accordingly, we have provided written and visual documentation of the critical steps used in our cognitive testing paradigms to facilitate the translation of our promising results into the clinic.

Cognitive impairment resulting from the radiotherapeutic management of brain tumors represents a clinically intractable condition that adversely impacts quality of life. The capability to critically evaluate potential interventions for ameliorating radiation-induced cognitive decrements ultimately depends on the capability to undertake rigorous quantitative assessments of cognition.

With the exception of survival, cognitive impairment stemming from the clinical management of cancer is a major factor dictating therapeutic outcome. For ...

Medicine

Vagus Nerve Stimulation as a Tool to Induce Plasticity in Pathways Relevant for Extinction Learning

Extinction describes the process of attenuating behavioral responses to neutral stimuli when they no longer provide the reinforcement that has been maintaining the behavior. There is close correspondence between fear and human anxiety, and therefore studies of extinction learning might provide insight into the biological nature of anxiety-related disorders such as post-traumatic stress disorder, and they might help to develop strategies to treat them. Preclinical research aims to aid extinction learning and to induce targeted plasticity in extinction circuits to consolidate the newly formed memory. Vagus nerve stimulation (VNS) is a powerful approach that provides tight temporal and circuit-specific release of neurotransmitters, resulting in modulation of neuronal networks engaged in an ongoing task. VNS enhances memory consolidation in both rats and humans, and pairing VNS with exposure to conditioned cues enhances the consolidation of extinction learning in rats. Here, we provide a detailed protocol for the preparation of custom-made parts and the surgical procedures required for VNS in rats. Using this protocol we show how VNS can facilitate the extinction of conditioned fear responses in an auditory fear conditioning task. In addition, we provide evidence that VNS modulates synaptic plasticity in the pathway between the infralimbic (IL) medial prefrontal cortex and the basolateral complex of the amygdala (BLA), which is involved in the expression and modulation of extinction memory.

Vagus nerve stimulation (VNS) has emerged as a tool to induce targeted synaptic plasticity in the forebrain to modify a range of behaviors. This protocol describes how to implement VNS to facilitate the consolidation of fear extinction memory.

Extinction describes the process of attenuating behavioral responses to neutral stimuli when they no longer provide the reinforcement that has been ...

Extinction Training During the Reconsolidation Window Prevents Recovery of Fear

Fear is maladaptive when it persists long after circumstances have become safe. It is therefore crucial to develop an approach that persistently prevents the return of fear. Pavlovian fear-conditioning paradigms are commonly employed to create a controlled, novel fear association in the laboratory. After pairing an innocuous stimulus (conditioned stimulus, CS) with an aversive outcome (unconditioned stimulus, US) we can elicit a fear response (conditioned response, or CR) by presenting just the stimulus alone1,2 . Once fear is acquired, it can be diminished using extinction training, whereby the conditioned stimulus is repeatedly presented without the aversive outcome until fear is no longer expressed3. This inhibitory learning creates a new, safe representation for the CS, which competes for expression with the original fear memory4. Although extinction is effective at inhibiting fear, it is not permanent. Fear can spontaneously recover with the passage of time. Exposure to stress or returning to the context of initial learning can also cause fear to resurface3,4.
Our protocol addresses the transient nature of extinction by targeting the reconsolidation window to modify emotional memory in a more permanent manner. Ample evidence suggests that reactivating a consolidated memory returns it to a labile state, during which the memory is again susceptible to interference5-9. This window of opportunity appears to open shortly after reactivation and close approximately 6hrs later5,11,16, although this may vary depending on the strength and age of the memory15. By allowing new information to incorporate into the original memory trace, this memory may be updated as it reconsolidates10,11. Studies involving non-human animals have successfully blocked the expression of fear memory by introducing pharmacological manipulations within the reconsolidation window, however, most agents used are either toxic to humans or show equivocal effects when used in human studies12-14. Our protocol addresses these challenges by offering an effective, yet non-invasive, behavioral manipulation that is safe for humans.
By prompting fear memory retrieval prior to extinction, we essentially trigger the reconsolidation process, allowing new safety information (i.e., extinction) to be incorporated while the fear memory is still susceptible to interference. A recent study employing this behavioral manipulation in rats has successfully blocked fear memory using these temporal parameters11. Additional studies in humans have demonstrated that introducing new information after the retrieval of previously consolidated motor16, episodic17, or declarative18 memories leads to interference with the original memory trace14. We outline below a novel protocol used to block fear recovery in humans.

Conditioned fear can be diminished through an inhibitory process called extinction, but can resurface under conditions such as the passage of time or exposure to stress. Our protocol presents a novel way of preventing fear recovery by introducing extinction during the reconsolidation window (the re-storage phase of a reactivated memory).

Fear is maladaptive when it persists long after circumstances have become safe. It is therefore crucial to develop an approach that persistently prevents ...

Stress-Enhanced Fear Learning, a Robust Rodent Model of Post-Traumatic Stress Disorder

Fear behaviors are important for survival, but disproportionately high levels of fear can increase the vulnerability for developing psychiatric disorders such as post-traumatic stress disorder (PTSD). To understand the biological mechanisms of fear dysregulation in PTSD, it is important to start with a valid animal model of the disorder. This protocol describes the methodology required to conduct stress-enhanced fear learning (SEFL) experiments, a preclinical model of PTSD, in both rats and mice. SEFL was developed to recapitulate critical aspects of PTSD, including long-term sensitization of fear learning caused by an acute stressor. SEFL uses aspects of Pavlovian fear conditioning but produces a distinct and robust sensitized fear response far greater than normal conditional fear responses. The trauma procedure involves placing a rodent in a conditioning chamber and administering 15 unsignaled shocks randomly distributed over 90 minutes (for rat experiments; for mouse experiments, 10 unsignaled shocks randomly distributed over 60 minutes are used). On day 2, rodents are placed in a novel conditioning context where they receive a single shock; then, on day 3 they are placed back in the same context as on day 2 and tested for changes in freezing levels. Rodents that previously received the trauma display enhanced levels of freezing on the test day compared to those that received no shocks on the first day. Thus, with this model, a single highly stressful experience (the trauma) produces extreme fear of the stimuli associated with the traumatic event.

Here we describe the detailed methodology required to conduct stress-enhanced fear learning (SEFL) experiments, a preclinical model of post-traumatic stress disorder, in both rats and mice. The model utilizes aspects of Pavlovian fear conditioning and freezing as an index of enhanced fear in rodents.

Fear behaviors are important for survival, but disproportionately high levels of fear can increase the vulnerability for developing psychiatric disorders ...

Fear Incubation Using an Extended Fear-Conditioning Protocol for Rats

Emotional memory has been primarily studied with fear-conditioning paradigms. Fear conditioning is a form of learning through which individuals learn the relationships between aversive events and otherwise neutral stimuli. The most-widely utilized procedures for studying emotional memories entail fear conditioning in rats. In these tasks, the unconditioned stimulus (US) is a footshock presented once or several times across single or several sessions, and the conditioned response (CR) is freezing. In a version of these procedures, called cued fear conditioning, a tone (conditioned stimulus, CS) is paired with footshocks (US) during the training phase. During the first test, animals are exposed to the same context in which training took place, and freezing responses are tested in the absence of footshocks and tones (i.e., a context test). During the second test, freezing is measured when the context is changed (e.g., by manipulating the smell and walls of the experimental chamber) and the tone is presented in the absence of footshocks (i.e., a cue test). Most cued fear conditioning procedures entail few tone-shock pairings (e.g., 1-3 trials in a single session). There is a growing interest in less common versions involving an extensive number of pairings (i.e., overtraining) related to the long-lasting effect called fear incubation (i.e., fear responses increase over time without further exposure to aversive events or conditioned stimuli). Extended fear-conditioning tasks have been key to the understanding of fear incubation&#8217;s behavioral and neurobiological aspects, including its relationship with other psychological phenomena (e.g., post-traumatic stress disorder). Here, we describe an extended fear-conditioning protocol that produces overtraining and fear incubation in rats. This protocol entails a single training session with 25 tone-shock pairings (i.e., overtraining) and a comparison of conditioned freezing responses during context and cue tests 48 h (short-term) and 6 weeks (long-term) after training.

We describe an extended fear-conditioning protocol that produces overtraining and fear incubation in rats. This protocol entails a single training session with 25 tone-shock pairings (i.e., overtraining) and a comparison of conditioned freezing responses during context and cue tests 48 h (short-term) and 6 weeks (long-term) after training.

Emotional memory has been primarily studied with fear-conditioning paradigms. Fear conditioning is a form of learning through which individuals learn the ...

A Novel Pavlovian Fear Conditioning Paradigm to Study Freezing and Flight Behavior

Fear- and anxiety-related behaviors significantly contribute to an organism&#8217;s survival. However, exaggerated defensive responses to perceived threat are characteristic of various anxiety disorders, which are the most prevalent form of mental illness in the United States. Discovering the neurobiological mechanisms responsible for defensive behaviors will aid in the development of novel therapeutic interventions. Pavlovian fear conditioning is a widely used laboratory paradigm to study fear-related learning and memory. A major limitation of traditional Pavlovian fear conditioning paradigms is that freezing is the only defensive behavior monitored. We recently developed a modified Pavlovian fear conditioning paradigm that allows us to study both conditioned freezing and flight (also known as escape) behavior within individual subjects. This model employs higher intensity footshocks and a greater number of pairings between the conditioned stimulus and unconditioned stimulus. Additionally, this conditioned flight paradigm utilizes serial presentation of pure tone and white noise auditory stimuli as the conditioned stimulus. Following conditioning in this paradigm, mice exhibit freezing behavior in response to the tone stimulus, and flight responses during the white noise. This conditioning model can be applied to the study of rapid and flexible transitions between behavioral responses necessary for survival.

Defensive behavioral responses are contingent upon threat intensity, proximity, and context of exposure. Based on these factors, we developed a classical conditioning paradigm that elicits clear transitions between conditioned freezing and flight behavior within individual subjects. This model is crucial for the understanding the pathologies involved in anxiety, panic, and post-traumatic stress disorders.

Fear- and anxiety-related behaviors significantly contribute to an organism&#8217;s survival. However, exaggerated defensive responses to perceived threat ...

An Inescapable Cat Odor Exposure Protocol for Studying Innate and Contextual Threat Conditioning in Rats

Animals respond to threatening situations by exhibiting a number of defensive behaviors, including avoidance, freezing, and risk assessment. An animal model with an ethological approach offers a deeper insight into the biological mechanisms underlying threat responses. This paper describes a methodology to measure defensive behaviors toward both innate and learned aversive stimuli in rats. Animals were individually exposed to predator odor in an inescapable chamber to elicit a measurable, sustained, defensive state. The experimental design involved placing a rat in a familiar chamber for 10 min followed by exposure to cat odor for another 10 min in the same context. The next day, the rats were re-exposed for 10 min to the same context chamber where cat odor exposure occurred. Sessions were video-recorded and defensive behaviors were assessed on both days. 
The behavioral test was coupled with reversible functional inactivation and c-Fos immunohistochemistry techniques to determine the role of the interoceptive cortex in threat responses. Rats exposed to cat odor on the first day and re-exposed to the context chamber on the second day displayed higher levels of defensive behaviors, and that cat odor elicited a robust increase in the neural activity of the interoceptive cortex. Moreover, muscimol inactivation of the interoceptive cortex reduced the expression of defensive behaviors in response to cat odor and impaired contextual threat memory. These results show that this behavioral assay is a useful tool for studying neural mechanisms of defensive behaviors and may offer insight into mechanisms that mediate fear in humans and its related disorders.

Cat skin/fur odor exposure provides a reliable method to investigate neural circuits and mechanisms of defensive responses in rodents and may offer insight into mechanisms that mediate fear in humans. Here, we describe a protocol for investigating the role of the interoceptive cortex in threat responses in rats.

Animals respond to threatening situations by exhibiting a number of defensive behaviors, including avoidance, freezing, and risk assessment. An animal ...

Studying Active Defensive Behaviors in Mice with Modified Fear Conditioning

Modified Fear Conditioning for Inducing Flight Behaviors in Mice

The appropriate manifestation of defensive behavior in a threatening situation is critical for survival. The prevailing theory suggests that an active defensive behavior, such as jumping or rapid darting, is expressed under high threat imminence or actual threat, whereas passive defensive behavior, such as freezing, is expressed when the threat is predicted, but the threat imminence is relatively low. In classical fear conditioning, subjects typically exhibit freezing as a conditioned defensive response, with little expression of active defensive behavior in most cases. Here, we introduce a modified fear conditioning procedure for mice to observe the transition from freezing to flight and vice versa, involving five repetitive pairings of conditioned stimuli (CS; continuous tone, 8 kHz, 95 dB SPL (sound pressure levels)) and unconditioned stimuli (US; foot shock, 0.9 mA, 1.0 s) over two days. This modified fear conditioning procedure requires a relatively large number of conditioning sessions and conditioning days but does not necessitate a high-intensity foot shock for modest expression of flight behavior. Using the same context for conditioning and salient CS presentations is essential to elicit flight behaviors. This modified fear conditioning procedure is a reliable method for observing active defensive behaviors in mice, providing an opportunity to elucidate the fine mechanisms and characteristics of such behaviors in a fearful context.

Author Spotlight: Exploring Neural Correlates of Defensive Behaviors in Fear Learning and Extinction

To study flight behaviors in a fearful context, we introduce a modified fear conditioning protocol. This protocol ensures that mice consistently exhibit flight behaviors during cue presentation in the fear conditioning.

The appropriate manifestation of defensive behavior in a threatening situation is critical for survival. The prevailing theory suggests that an active ...

Protocol for Studying Extinction of Conditioned Fear in Naturally Cycling Female Rats

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