Evaluating analgesic efficacy and administration route following craniotomy in mice using the grimace scale

www.nature.com/scientificreports OPEN Received: 1 August 2018 Accepted: 28 November 2018 Published: xx xx xxxx Evaluating analgesic efficacy and administration route following craniotomy in mice using the grimace scale Chulmin Cho1, Vassilia Michalidis2, Irene Lecker3, Chereen Collymore4, David Hanwell5, Mary Loka1, Matthew Danesh1, Christine Pham3, Paige Urban3, Robert P. Bonin3 & Loren J. Martin1,2 Most research laboratories abide by guidelines and mandates set by their research institution regarding the administration of analgesics to control pain during the postoperative period. Unfortunately, measuring pain originating from the head is difficult, making adequate decisions regarding pain control following stereotaxic surgery problematic. In addition, most postsurgical analgesia protocols require multiple injections over several days, which may cause stress and distress during a critical recovery period. Here we sought to (1) assess the degree of postoperative pain following craniotomy in mice, (2) compare the efficacy of three common rodent analgesics (carprofen, meloxicam and buprenorphine) for reducing this pain and (3) determine whether the route of administration (injected or selfadministered through the drinking supply) influenced pain relief post-craniotomy. Using the mouse grimace scale (MGS), we found that injectable analgesics were significantly more effective at relieving post-craniotomy pain, however, both routes of administration decreased pain scores in the first 24 h postsurgery. Specifically, buprenorphine administered independently of administration route was the most effective at reducing MGS scores, however, female mice showed greater sensitivity to carprofen when administered through the water supply. Although it is necessary to provide laboratory animals with analgesics after an invasive procedure, there remains a gap in the literature regarding the degree of craniotomy-related pain in rodents and the efficacy of alternative routes of administration. Our study highlights the limitations of administering drugs through the drinking supply, even at doses that are considered to be higher than those currently recommended by most research institutions for treating pain of mild to moderate severity. In neuroscience, there are a variety of approaches and techniques that require direct access to the rodent brain. Consequently, it has become increasingly common for many neuroscience labs to perform craniotomies, an invasive brain surgery in which the brain is accessed via removal of a section of the skull, so that studies involving intracranial injections, cannulations, electrical stimulation, or optical implants can be conducted1–3. However, the pharmacological management of post-craniotomy pain has yet to be standardized and there are no current evidence-based recommendations for the alleviation of craniotomy pain in mice. Given that the insufficient management of acute pain following surgery may lead to depression and anxiety4,5 – behaviors directly studied by many neuroscience labs – finely-tuned pain control following craniotomy should be prioritized and properly evaluated. The establishment of a standardized analgesic regiment for post-craniotomy pain in animals has been hindered by the difficulty in assessing the degree of pain postsurgery and the efficacy of chosen analgesics. Traditional 1 Dept. of Psychology, University of Toronto Mississauga, Mississauga, ON, M9A1C5, Canada. 2Cell and Systems Biology, University of Toronto Mississauga, Mississauga, ON, M9A1C5, Canada. 3Leslie Dan Faculty of Pharmacy, University of Toronto, Toronto, ON, M5S 3M2, Canada. 4Division of Comparative Medicine, Faculty of Medicine, University of Toronto, Toronto, ON, M5S 1A8, Canada. 5Research Oversight and Compliance Office, University of Toronto, Toronto, Canada. Chulmin Cho and Vassilia Michalidis contributed equally. Correspondence and requests for materials should be addressed to R.P.B. (email: ) or L.J.M. (email: ) Scientific RePorts | (2019) 9:359 | DOI:10.1038/s41598-018-36897-w 1 www.nature.com/scientificreports/ Figure 1. Experimental timeline outlining the pre- and postsurgical periods. For all mice, tap water was replaced with the MediDrop solution 48 hrs prior to surgery. Analgesic drugs were then added to drinking water, as appropriate, beginning 48 hrs prior to surgery. MGS baseline scores were recorded 24 h before craniotomy and at the indicated time points following surgery. pain measurement in laboratory rodents has heavily relied on evoked pain behaviors. However, these assays are impractical for assessing pain originating from surgical operations involving the head and scalp (see6 for a review). Non-selective proxy measures such as cardiovascular changes, food and water intake, locomotion, and nest construction have been used to measure ‘spontaneous’ pain following injury with varying degrees of success and large inter-subject variability7–9. In addition, learning paradigms such as conditioned place preference have also been used to evaluate postoperative pain and pain relief following surgery10, but these studies are often limited by one-trial learning making the assessment of pain over time difficult. The recent implementation of the grimace scales has expanded our ability to assess pain in rodents11,12 and potentially addresses some of the issues associated with evoked or proxy measures of pain in laboratory animals6. The grimace scale was originally developed for mice based on the facial action coding system for pain in infants and non-verbal humans13. The mouse grimace scale (MGS) measures changes in facial musculature – orbital tightening, nose bulge, cheek bulge, ear change, and whisker change – following injury11. Each feature is given a score of 0 (not present), 1 (moderate) or 2 (severe), which can be scored with remarkably high reliability and accuracy. Although the grimace scale has been readily evaluated in mice11,14,15 and rats12,16, other grimace scales have been developed to capture ‘pain faces’ in horses17, cats18, rabbits19, sheep20 and ferrets21, which have helped to evaluate postoperative pain across species. The sensitivity of the MGS allows for the detection of postoperative pain upwards of 48 h following surgery14. There is a strong positive correlation between MGS scores and pain-associated behaviors in mice22, suggesting that facial expressions of pain could provide a rapid and reliable clinical pain assessment modality. Because the mouse grimace scale has been shown to be a reliable indicator of pain postsurgery14,15,22, we used it as an assessment tool to characterize the extent and duration of postoperative pain in mice following craniotomy. We evaluated the efficacy of common rodent analgesics: the μ-opioid receptor partial agonist buprenorphine and nonsteroidal anti-inflammatory drugs (NSAID) carprofen and meloxicam. Further, since multiple injections of analgesics are often required for pain management in ro (...truncated)