The Orofacial Pain Assessment Device (OPAD) was developed by Neubert et al., (2005) as an operant system of pain assessment that relies on voluntary behavior. Assays using the OPAD offer a unidimensional assessment of pain. Responses involve executive functioning and other experiences, thus relying simply on reflex and innate responses does not provide the complete picture. Therefore, a conflict-based paradigm proves to be a more sensitive method that enables in-depth analysis of pain.

The OPAD system is composed of a thermode and metal wires that allow assessment of thermal and mechanical sensitivity. The starved subject is placed in the operant chamber where it can access the food reward only when it contacts the thermal and mechanical stimuli. Essentially, the subject is tasked with choosing to tolerate the stimuli to gain the reward. Unlike traditional assays that are typically based on reflex behaviors, the OPAD allows the subject to choose its own stimuli threshold in order to attain the reward. This reward-conflict paradigm offers better face, content, and predictive validity. Additionally, the OPAD also encompasses psychological and affective dimensions of pain as observed in humans.

Similar devices include the Electric von Frey Filament, the Tail Flick Test, and the Hargreaves Plantar Test. (For more click here).

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Documentation

2.1 Origin

Most pain assays used in preclinical research rely on simple reflexes or innate behaviors to measure pain. These methods, though simple and objective, do not take into consideration other factors associated with pain behavior. Neubert et al. (2005) aimed to develop a system that would allow a better assessment of pain and associated behaviors. The Orofacial Pain Assessment Device was designed to provide the subject with a conflict to either experience the pain to gain a reward or to avoid the aversive stimuli and gain no reward. This system enabled the observation of behaviors that involved different levels of the nervous system. Thus, the OPAD assay improves the translatability of the observation for human applications.

Neubert et al. (2006) investigation revealed that at temperatures 42° C and 45° C significant allodynic and hyperalgesic effects induced by capsaicin could be observed. Another study by Rossi et al. (2006) employed the OPAD to investigate the cold sensitivity of male and female rats. When subjects were evaluated at above-freezing temperatures of 24° C, 10° C, and 2°C, the difference in behavioral performances of males and females could be observed. However, statistical significance was reached only at 10° C temperature. Males were observed to lick less often and consume less milk in comparison to the females.

Since the OPAD relies on affective responses, Rossi and Neubert (2008) used it to evaluate the effects of environmental enrichment on thermal sensitivity. When 7-week-old male hairless Sprague-Dawley rats were maintained in an enriched environment that included objects like cardboard boxes, hammocks, exercise wheels, and other enrichments, an effect on thermal sensitivity could be seen. In comparison to standard cage-maintained rats, enriched rats showed better performance in the OPAD at temperatures 2° C, 24° C, and 45°C. However, though an enriched environment can potentially improve tolerance, the effects do not occur at strongly aversive or pain threshold temperatures.

2.2 Developments

The initial design of the OPAD only employed thermal stimulations. Nolan et al. (2011), further adapted the OPAD to include nickel-titanium wires to act as mechanical stimuli. The cables also conducted electricity, thereby, allowing the experimenters to detect and record actual facial contacts. This addition of a removable mechanical stimulation allowed experimenters to evaluate the pain response to different combinations of stimuli.

Pain alleviation in humans is influenced by transient receptor potential (TRP) cation channels involved in the perception of hot and cold pain. TRPV1 and TRPM8/TRPA1 were altered by capsaicin and menthol administration by Anderson et al. (2014) to investigate the effects of these channels on nociceptive behaviors. OPAD performance observations showed that the co-activation of TRPM8/TRPA1 and TRPV1 made cool temperatures less nociceptive.

2.3 Recent Developments

Nag and Mokha (2016) tested the performances of male and ovariectomized (OVX) Sprague-Dawley rats in the OPAD. The investigation aimed to understand the effects of the activation of the trigeminal α2-adrenoceptor on nociception and hyperalgesia. The investigation was able to highlight the sex-dependent antinociception and antihyperalgesic effects of α2-adrenoceptor. Further, the effects of hormones on the control of pain were also shown suggesting the need for sex-specific therapeutic treatments.

The effect of diets on the reduction of pain was evaluated by Bowden et al. (2017). The study involved the assessment of cocoa flavanols on orofacial pain of male and female Sprague-Dawley rats. It was observed that the subjects that were maintained on cocoa-enriched diets had better pain thresholds when evaluated in the OPAD. The observations led to the suggestion that diet modifications can be potentially used for the management of pain.

The OPAD consists of a clear acrylic cage with a metal base. The chamber base has an additional metal grate. On one side of the chamber, an opening to the rewarding feeder is lined with a pair of thermode and nickel-titanium wires to provide thermal stimulations and mechanical stimulations to the orofacial region, respectively. The distance between the thermodes can be varied to accommodate the size of the animal. Further, the temperature (4° to 75°C temperature range) and experimental parameters of the set-up can be controlled and managed using the associated software and the LCD screen. The apparatus is equipped with a reward bottle having a metal spout. Access to the reward is possible only with simultaneous contact with the thermode or mechanical protrusions. The device allows automated data recording.

Clean the Orofacial Pain Assessment Device thoroughly before and after every use and between subjects. Appropriately illuminate the arena. Ensure that the presence of unnecessary external stimuli is minimized. In addition to the OPAD system’s data acquisition, tracking, and video system can also be used.

Pretraining

Fast the subjects for an appropriate period before training in the OPAD. Place the subject in the chamber and set the thermodes to a non-aversive temperature. Allow the subjects enough time to explore the apparatus. Perform at least 3 trials per week for 2 weeks or until consistent behavior is observed.

Operant procedures are best applied to hairless rodents. In case the rodents have facial hair, it is highly recommended that appropriate methods are used to remove it. Ensure that only the buccal hair and not the vibrissae pad/whiskers are removed to avoid affecting the subject’s behavior. Perform the hair removal procedure 1 to 2 days prior to the testing for accurate results. Apply the treatments before the testing.

Orofacial Pain Assessment

Fill the reward liquid in the reward bottle. Place the reward bottle on the stand such that the subject must make contact with the thermodes on its buccal region to be able to lick the spout. Place the subject in the chamber. Adjust the position of the reward bottle further if the subject’s vibrissa touches it, or the buccal regions do not touch the thermodes. Set the thermodes to the desired temperature and begin the trial set to a predetermined trial length. Repeat the trials as required.

The initial Operant Pain Assessment Device was designed only to assess thermal sensitivity. The device involved a pair of thermodes which were constructed using grounded aluminum tubing lining the opening to the reward feeder. Thermal stimulations were induced by pumping water heated or cooled to the required stimulus temperature via flexible polyethylene tubing.

The OPAD design was modified to include a mechanical component. Two rows of nickel-titanium wires were added to the opening to the reward to assess mechanical allodynia, hyperalgesia, and pain in the orofacial region. The effects of varying the diameters of the wires on performance were tested. The thickness of the wire had an impact on the licking/facial contact ratio, with the thicker wire significantly decreasing it compared to the thinner wire. The effects of wire thickness were further assessed after capsaicin and treatments for pain, with different outcomes observed.

The mechanical component of the OPAD was further modified by Rohrs et al. (2015) by using a 360-degree array of looped stainless steel wires. This adaptation aimed to effectively assess mechanical sensitivity by minimizing animal avoidance behavior.

The Orofacial Pain Assessment Device uses a reward-conflict paradigm to quantify pain behavior. The simple device allows the assessment of pain parameters based on responses to temperatures as well as mechanical stimuli. Thus, allowing a comparison of pain responses within a single system. The apparatus allows the comparison of performances between treated versus untreated groups, and the performances of different disease and lesion groups.

The following measures can be obtained in the OPAD:

• Number of licks (successful attempts)
• Number of contact with thermode/wire
• Reward intake (g)
• Face contact duration/Event ratio
• Reward/Stimulus contact ratio
• The temperature of the thermode during a session
• Latency to approach the reward
• Time spent in contact with thermode/wire

Strengths

The Operant Orofacial Assessment Devices allow the assessment of mechanical and thermal pain sensitivity within a single system. The setup allows direct comparisons of the performances in both modalities under the same experimental parameters. Unlike traditional pain assays, the OPAD relies on the subject’s voluntary behavior. Further, since the device doesn’t require any restraining, it is relatively less stressful for the subject. The investigator-independent nature of the OPAD offers an effective analysis of pain parameters that is not influenced by investigator bias. The device also encompasses psychological and affective dimensions of pain, thus providing sensitive measures of pain that are reproducible. The simplicity of the system makes it easily adaptable for different investigations.

Limitations

Since the OPAD system relies on voluntary behaviors, training, and testing can be time-consuming. Further, subjects that are not motivated enough may not perform the task. The reliance on voluntary behavior also may result in task duration differences within a group, which can impact the accuracy of the data. The task requires the subjects to be fasted prior to training. This introduces an appetitive factor into the performance. The thermal simulations are effective on hairless skin. Thus, subjects with fur or hair must have it removed prior to testing, which adds additional labor. The rows of wires may not equally apply the mechanical stimuli. Subjects may distribute the effects of the mechanical stimuli by tilting their head. Pain sensitivities of subjects can vary based on their strain, age, and gender. Further, external factors such as olfactory, auditory, or visual stimuli can affect task performance.