Behavioral Voluntary and Social Bioassays Enabling Identification of Complex and Sex-Dependent Pain-(-Related) Phenotypes in Rats with Bone Cancer

Simple Summary Bone metastases are one of the most common complications in patients with advanced cancer that result in pain, which is usually severe, thereby significantly reducing the patient's quality of life. Although preclinical pain research in rodents is improving, the pain phenotyping methods currently used have been criticized. This study aimed to identify in detail pain phenotypes of cancer-induced bone pain (CIBP) in both sexes of rats. CIBP in the splint bone on one side results in a distinct CIBP-related phenotype characterized by mechanical hypersensitivity, resting pain, and antalgic gait in both sexes. Progression of tumor growth leads to the establishment of the CIBP phenotype that appears earlier in male than in female rats and affects rat-specific social behaviors in both sexes. We demonstrate social transfer of pain in a bone cancer model in both sexes, resulting in mechanical and, in females, also heat hypervigilance in non-tumor bearing control rats. Cancer-induced bone pain (CIBP) is a common and devastating symptom with limited treatment options in patients, significantly affecting their quality of life. The use of rodent models is the most common approach to uncovering the mechanisms underlying CIBP; however, the translation of results to the clinic may be hindered because the assessment of pain-related behavior is often based exclusively on reflexive-based methods, which are only partially indicative of relevant pain in patients. To improve the accuracy and strength of the preclinical, experimental model of CIBP in rodents, we used a battery of multimodal behavioral tests that were also aimed at identifying rodent-specific behavioral components by using a home-cage monitoring assay (HCM). Rats of all sexes received an injection with either heat-deactivated (sham-group) or potent mammary gland carcinoma Walker 256 cells into the tibia. By integrating multimodal datasets, we assessed pain-related behavioral trajectories of the CIBP-phenotype, including evoked and non-evoked based assays and HCM. Using principal component analysis (PCA), we discovered sex-specific differences in establishing the CIBP-phenotype, which occurred earlier (and differently) in males. Additionally, HCM phenotyping revealed the occurrence of sensory-affective states manifested by mechanical hypersensitivity in sham when housed with a tumor-bearing cagemate (CIBP) of the same sex. This multimodal battery allows for an in-depth characterization of the CIBP-phenotype under social aspects in rats. The detailed, sex-specific, and rat-specific social phenotyping of CIBP enabled by PCA provides the basis for mechanism-driven studies to ensure robustness and generalizability of results and provide information for targeted drug development in the future.