The goal of the protocol described in this paper is to induce compulsive-like behavior in rats for the study of obsessive-compulsive disorder (OCD). This behavior is precipitated by attenuating a signal indicating that a lever-press response was effective in producing food.
In the signal attenuation rat model of obsessive-compulsive disorder (OCD), lever-pressing for food is followed by the presentation of a compound stimulus which serves as a feedback cue. This feedback is later attenuated by repeated presentations of the stimulus without food (without the rat emitting the lever-press response). In the next stage, lever-pressing is assessed under extinction conditions (i.e., no food is delivered). At this stage rats display two types of lever-presses, those that are followed by an attempt to collect a reward, and those that are not. The latter are the measure of compulsive-like behavior in the model. A control procedure in which rats do not experience the attenuation of the feedback cue serves to distinguish between the effects of signal attenuation and of extinction. The signal attenuation model is a highly validated model of OCD and differentiates between compulsive-like behaviors and behaviors that are repetitive but not compulsive. In addition the measures collected during the procedure eliminate alternative explanations for differences between the groups being tested, and are quantitative, unbiased and unaffected by inter-experimenter variability. The major disadvantages of this model are the costly equipment, the fact that it requires some technical know-how and the fact that it is time-consuming compared to other models of OCD (11 days). The model may be used for detecting the anti- or pro-compulsive effects of pharmacological and non-pharmacological manipulations and for studying the neural substrate of compulsive behavior.
Obsessive-compulsive disorder (OCD) is a major psychiatric disorder that manifests in 1 – 3% of the general population1,2. People suffering from OCD have recurrent, intrusive and unwanted thoughts (obsessions) and/or repetitive ritualistic behaviors (compulsions)3. The specific neuropathological mechanisms underlying OCD are still not fully understood. However, the involvement of the serotonergic4-7, dopaminergic8,9 and glutamatergic10 systems has been demonstrated in this disorder. In addition, the orbitofrontal cortex, the cingulate cortex, the basal ganglia and regions within the parietal lobe have been implicated in its pathophysiology7,11-13. Finally, life events related to fluctuations in the level of ovarian hormones (e.g., child birth, ovulation) have been reported to trigger or exacerbate OCD in women patients14-16, suggesting that ovarian hormones play a modulatory role in OCD17.
Because the mechanisms underlying OCD are poorly understood, the use of appropriate animal models that closely mimic its behavioral and neural manifestations is essential for advancing our knowledge of its biological basis. In addition, such models contribute to the development of new lines of treatment. This is especially relevant in the case of OCD, because many patients are either treatment-resistant or experience only a partial alleviation of symptoms18,19. Indeed, in recent years, genetic, pharmacological and behavioral animal models of OCD (reviewed in 20-28) have expanded and advanced our knowledge of this disorder.
One of the most extensively used behavioral animal models of OCD is the signal attenuation rat model (for review, see 29). The theoretical assumption behind the model is that a deficit in the feedback associated with a successful performance of goal-directed behaviors leads to compulsive responses30-37. The model, developed by Joel and colleagues28, is based on operant behavior in rats. During initial training, rats are rewarded with a food pellet after pressing a lever. A successful lever-press triggers in addition the onset of a magazine light and a tone. This provides the rat with feedback that the lever-press response has led to the delivery of food. Next, the ability of the stimulus to signal the delivery of the reward is intentionally decreased by repeatedly presenting it without reward (importantly, there are no levers in the box at this stage). Compulsive-like behavior emerges on the last stage of training. During this test stage, which is carried out under extinction conditions, a lever press is followed by the presentation of the stimulus but not of the food reward. “Compulsive” behavior is expressed as multiple lever-presses after which the rat does not try to collect the reward. An anti/pro-compulsive effect is expressed as a decrease/increase in the number of “compulsive” lever-presses. Since signal attenuation involves extinction, it is important to distinguish between the effects of signal attenuation and of extinction per se. Therefore in a control group (the Regular extinction group) the compound stimulus is not attenuated prior to the test stage. Treatment that has an anti/pro compulsive effect should not alter the number of “compulsive” lever-presses in this group. (for additional details, see 29).
“Compulsive” lever-presses mimic the exaggerated and unnecessary nature of compulsive behaviors displayed by OCD patients. Therefore, the signal attenuation model displays good face validity. In addition, studies conducted with this model show that it has good predictive and construct validity (reviewed in 20,21). The model’s predictive validity derives from studies showing that the compulsive lever-pressing is attenuated by drugs known to ameliorate obsessive-compulsive symptoms38,39, as well as by high frequency stimulation of the subthalamic nucleus40, which has been found to have an anti-compulsive effect in human OCD patients41,42. Moreover, several drugs that are inefficient in the treatment of OCD have been found not to exert an anti-compulsive effect in the model38,39. The model also displays good construct validity, because studies indicate that similar neural mechanisms are involved both in OCD symptomatology and in the compulsive-like behavior induced by signal attenuation in rats. Thus, the involvement of the serotonergic43-46, dopaminergic39,46 and glutamatergic47 systems, as well as the involvement of OCD-related brain areas40,44,48-50 has been demonstrated in compulsive lever-pressing. In addition, ovarian hormones have been found to modulate compulsive lever-pressing in females51. Therefore, the signal attenuation model is a powerful tool for exploring the neural substrates of OCD and for screening novel anti-compulsive therapies. For a thorough discussion of the signal attenuation model’s clinical correlates and its usefulness and application in OCD research, see 20-22,29.
NOTE: All experimental protocols conformed to the guidelines of the Institutional Animal Care and Use Committee of Tel Aviv University, Israel, and to the guidelines of the NIH. All efforts were made to minimize the number of animals used and their suffering.
1. Animal Preparation
2. Set-up
3. Handling and Food Restriction
4. Training Procedure
The following results are based on Brimberg et al., 200752. All figures are re-printed with permission from Elsevier.
In this study we tested the behavior of Sprague Dawley (SD) male rats in the signal attenuation model. First, in experiment 1, we tested the effects of 3 doses of the selective serotonin reuptake inhibitor (SSRI) paroxetine in the PTSA procedure (n per group = 10). In the Test, paroxetine dose-dependently decreased the number of ELP-C (Figure 1A; ANOVA yielded a significant main effect of Dose, F(3,22) = 5.15, p < 0.01) and ELP-U (Figure 1B; ANOVA yielded a significant main effect of Dose, F(3,22) = 7.99, p < 0.001).
Figure 1. This figure shows a representative dose response experiment comparing the effects of various doses of the SSRI Paroxetine on ELP-C and ELP-U of male rats following signal attenuation. Mean and standard error of the number of extra lever presses that (A) were followed by magazine entry (extra lever presses in completed trials; ELP-C) and (B) were not followed by magazine entry (extra lever presses in uncompleted trials; ELP-U) of rats treated with vehicle or 1, 5 or 10 mg/kg of paroxetine on the test day of the PTSA procedure. Re-printed with permission from 52.
In experiment 2 we tested the drug dose that was the most effective in experiment 1 (5 mg/kg), in both the PTSA and RE procedures (n per group = 10). In the Test, paroxetine decreased the number of ELP-C in both the PTSA and RE procedures (Figure 2A; two-way ANOVA, main effect of Procedure, F(1,32) = 6.50, p < 0.05; main effect of Drug, F(1,32) = 8.69, p < 0.01; Procedure X Drug interaction, F(1,32) = 0.43, p = 0.52) and in addition exerted an anti-compulsive effect, i.e., decreased the number of ELP-U in the PTSA but not in the RE procedure (Figure 2B; main effect of Procedure, F(1,32) = 9.60, p < 0.005; main effect of Drug, F(1,32) = 5.75, p < 0.05; Procedure X Drug interaction, F(1,32) = 4.83, p < 0.05).
Figure 2. This figure shows a representative experiment comparing the effects of signal attenuation and regular extinction on ELP-C and ELP-U of saline- and paroxetine-exposed male rats. Mean and standard error of the number of (A) ELP-C and (B) ELP-U of rats treated with vehicle or 5 mg/kg of paroxetine on the test day of the PTSA and RE procedures. Re-printed with permission from 52.
The signal attenuation rat model of OCD is a powerful behavioral model for the study of compulsive-like behavior. The model displays high face, predictive and construct validity20,21, and has been extensively used to study the neural substrates of this behavior39,43-45,48, its response to pharmacological manipulations38,39,43,47,53,54 and to deep brain stimulation40,46,50 and its modulation by ovarian hormones51. Thus, this model is a useful animal model for the study of OCD.
Compulsive lever-pressing in the signal attenuation model has several advantages over other experimentally induced repetitive behaviors (such as extinction burst and perseverative behaviors). First, the relevance of compulsive lever-pressing to compulsive behaviors in humans has been well established whereas the validity of other repetitive behaviors, which are often referred to as compulsive-like, is low or has never been tested20-22. Notably, behavioral repetition/perseveration is a phenomenon shared by various psychiatric disorders55-62 and therefore, proper validation of the target behavior as compulsive-like is crucial. In addition, the various behavioral measures collected during the PTSA procedure (i.e., the number of presses on the non-reinforced lever or the general number of nose-pokes the rats perform during the Test stage) help in eliminating alternative explanations for differences in compulsive lever-pressing between the groups being tested. For example, excessive lever-pressing can reflect a general increase in motor activity, in which case it will most likely be accompanied by an increase in the number of presses on the non-reinforced lever (thus, this measure also eliminates the need to test the rats in additional procedures such as the open-field test). On the other hand, manipulations which lead to a general increase in the number of nose-pokes the rats perform are likely to lead to a reduction in compulsive lever-pressing, even if they do not possess a genuine anti-compulsive effect. Additional measures collected even before the Test (excessive lever-presses during the lever-press training stage, completed trials during the signal attenuation stage) allows the experimenter to eliminate the possibility that any differences between the groups on the Test stage stem from prior differences in learning. Notably, all measures collected during the various stages of the procedure are quantitative, and therefore unbiased, not given to subjective interpretation and unaffected by inter-experimenter variability.
A disadvantage of the signal attenuation model is the fact that it requires special equipment (computer-operated operant boxes, appropriate software for the operation of these boxes, etc.). This makes it both costly and somewhat complex to perform, requiring skilled personnel, proficient both in ad-hock troubleshooting and in the day-to-day maintenance of the equipment. In addition, because the model is based on learned rather than spontaneous behavior, and because it consists of multiple stages, it is relatively time-consuming (11 days) when compared to some of the other animal models of OCD. However, in our experience, with the proper training the expertise required for performing the procedure are quite easily acquired. Also, because all equipment is computer-controlled and almost fully automatic, large groups of rats can be run efficiently and simultaneously, reducing its time-cost. In addition, results are easily calculated and do not require manual coding or any special processing. Finally, operant boxes are highly versatile, and once acquired, they can be used for various behavioral procedures in addition to signal attenuation, making them extremely cost-effective.
Another consideration, which should be taken into account when using the model, is that due to its lengthy and multi-staged nature, it may not be well suited for chronic treatments or developmental studies. In order not to affect rats’ learning in the initial stages of the behavioral procedure, administration of a chronic treatment requires a break in the procedure, which makes the procedure even more time-costly. Moreover this break cannot take place immediately before the test stage, and thus, rats administered the chronic treatment will undergo the signal attenuation stage while under the influence of the treatment, which may alter their behavior even before the test stage and make any interpretation of the results problematic. Regarding developmental studies, again, because of the model’s lengthy nature, it is impossible to use it for extremely young rats (e.g., younger than 46-day old rats on test day). In addition, rats cannot be re-tested, making in necessary to train new rats at each age studied, and excluding the possibility of using longitudinal designs.
An important aspect of the signal attenuation model which has been mentioned above is the fact that compulsive lever-pressing is modulated by fluctuations in ovarian hormone levels along the rat estrous cycle51. This aspect is important for researchers interested in studying the mechanisms by which female gonadal hormones affect compulsive behaviors. Although the effects of male gonadal hormones on compulsive lever-pressing have not been tested, these or other factors are affecting male performance in the model, as the variability of the different response measures in the model is similar in male and female rats51. Therefore, researchers, who do not aim to study the role of gonadal hormones, may use both male and female rats without measuring the level of these hormones.
In summary, despite some shortcomings of the signal attenuation rat model of OCD such as its length and the fact that it requires special equipment and some technical knowledge, it provides a sensitive and reliable way of assessing compulsive behaviors in rats. Moreover, it can differentiate between these behaviors and other repetitive/perseverative behaviors, which are not truly compulsive in nature. As such, it is an excellent model for the assessment of putative anti-compulsive therapies, and studies employing it can be used to expand our knowledge of the neural substrates of OCD, which are still not well understood.
The authors have nothing to disclose.
This research was supported by the Israel Science Foundation (grant No. 592/12) to DJ
Name of the Material/Equipment | Company | Catalog Number | Comments/ Description |
Modular Test Chamber for Rats | Campden Instruments Ltd. | Model 80003M | |
Pellet Trough for Modular Chamber with Head Entry and Door | Campden Instruments Ltd. | Model 80210M-R | |
Low Profile Retractable Response Lever | Campden Instruments Ltd. | Model CI4460-M | 2 levers per chamber |
Stimulus Lights | Campden Instruments Ltd. | Model 80221 | |
Pellet Dispenser with 45mg Interchangeable Pellet Size Wheel | Campden Instruments Ltd. | Model 80209-45 | |
Mouse Nosepoke with Stimulus Light | Campden Instruments Ltd. | Model 80116S | |
Sonalert Audible Stimulus System | Campden Instruments Ltd. | Model SC628 | |
ABET II Complete Starter Package with 220VAC/50Hz Power Supply | Campden Instruments Ltd. | Model 88501*C | |
Sound Attenuating Chamber | Campden Instruments Ltd. | Model 80600A-SAC | Equipped with a peephole and a 28 volt DC ventilation fan pannel |
Animal Behavior Environment Test system (ABET) II | Lafayette Instrument Neuroscience, Indiana, USA | Model 89501 | |
Personal computer with a minimum 1.8 GHz Processor | Running Microsoft Windows XP (SP3), or Win7 | ||
45-mg dust-free precision pellets | PMI Nutrition International, Indiana, USA | Formula. P/AlN-76A | Keep the containers tightly closed to protect from moisture. |