The Intraplantar CFA Inflammatory Pain Model Incompletely Captures Pain Comorbidities Across Sexes

In humans, chronic pain is characterized by a combination of heightened nociceptive sensitivity along with emotional comorbidities, with more women impacted than men. A reported limitation of preclinical models is that they often fail to encapsulate this complexity, and few studies have directly compared behavioral outcomes between male and female mice. The intraplantar Complete Freund's adjuvant (CFA) inflammatory pain model is a widely used preclinical model of persistent mechanical and thermal hypersensitivity, which triggers sex-dependent adaptations in neuronal signaling. However, the literature regarding the duration of pain hypersensitivity and the presence or absence of associated locomotor and anxiety-like changes is inconsistent. Therefore, this study aimed to systematically assess the CFA model's capacity to duplicate pain comorbidities in male and female mice. CFA-injected or saline control male and female mice were assessed over 3 weeks for mechanical and cold nociceptive sensitivity (von Frey and acetone test), locomotion (open field test), and anxiety-like behaviors (light-dark and elevated plus maze tests). CFA-injected mice of both sexes reliably developed mechanical hypersensitivity and cold allodynia. Transient locomotor deficits were observed only in male mice at early timepoints, and no changes in anxiety-like behaviors in either males or females were observed. Hence, the CFA persistent pain model is a practical tool for inducing pronociceptive states in male and female mice but does not reliably produce anxiety-like comorbidities over the tested time periods, suggesting that additional factors may be necessary to elicit these behavioral outcomes.