Orbital tightening assessment to evaluate pain and physical discomfort in chlorine-exposed rats: A machine learning based measurement approach

Noxious chemicals like chlorine induce extreme distress, pain, and irritation in exposed individuals, yet methods to evaluate pain-related behavioral responses are absent. It is also unknown whether analgesics would alleviate pain and physical discomfort induced by such noxious chemicals. The grimace scale (GS), which evaluates facial expression features such as orbital tightening (OT), is a valuable indicator of pain and distress in animals. However, conventional GS approaches are labor-intensive, prone to subjectivity, and lack quantitative precision. In this study, we employed machine learning with DeepLabCut to annotate key facial landmarks in video recordings of chlorine-exposed rats. Focusing on the superior and inferior eyelid margins and 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 accuracy and reproducibility. The quantitative GS in rats subjected to chlorine exposure was validated. Significant reductions in eyelid distance and palpebral fissure width were observed upon chlorine exposure as compared to unexposed control animals. Administration of the opioid analgesic buprenorphine significantly reduced the OT caused by chlorine. This study establishes a robust, quantitative method for assessing OT in chlorine-exposed rats using DeepLabCut, providing a scalable, objective tool for assessing pain induced by 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.