Abstract Rationale Inhalation of noxious chemicals like chlorine induces extreme distress, pain, and irritation of the exposed individual, yet methods to evaluate the pain-related behavioral response are absent. It is also unknown whether analgesics can be used to alleviate pain and physical discomfort induced by inhalation of noxious chemicals. The grimace scale (GS), which evaluates facial expression features such as orbital tightening (OT), serves as a valuable indicator for pain and distress in animals. However, conventional GS approaches are labor-intensive, prone to subjectivity, and lack quantitative precision. Methods In this study, we employed machine learning with DeepLabCut to annotate key facial landmarks in video recordings of rats exposed to chlorine. Focusing on the superior and inferior eyelid margins, as well as the medial and lateral canthi, we quantified eyelid distance and palpebral fissure width as measures of OT. Rigorous inclusion and exclusion criteria for annotated images were established to ensure accurate and reproducible quantification. We administered an opioid drug i.e. buprenorphine to assess its analgesic effect, and we validated quantitative grimace scale (qGS) in rats subjected to chlorine gas exposure. Results With the help of sheltering tube method, we observed a significant reduction in eyelid distance and palpebral fissure width in chlorine exposed rats compared to baseline control animals not exposed to nociceptive stimuli. Administration of an opioid analgesic buprenorphine significantly reduced the orbital tightening caused by chlorine as measured by this method. Conclusions This study establishes a robust, quantitative method for assessing orbital tightening in chlorine exposed rats using DeepLabCut, offering a scalable and objective tool for assessing pain induced by inhalation of noxious chemicals in preclinical research. This study also suggests that opioids can alleviate pain and physical discomfort induced by inhalation of noxious chemicals, providing a new therapeutic strategy for managing the respiratory hazard of noxious chemicals. This abstract is funded by: National Institutes of Health Office of the Director (NIH OD), U01ES033263
Jadhav et al. (Fri,) studied this question.