Neonatal maternal separation leads to a dampening of inflammation-related mechanical and thermal hypersensitivity in juvenile rats.

Early life stress (ELS) constitutes a risk factor for the later development of psychopathological disorders partly displaying pain comorbidity. Since ELS may hence be expected to have an impact on pain processing the present study investigates whether ELS could be a factor of vulnerability or resilience against an enhancement of noxious sensitivity in the framework of inflammatory pain in later life. Rats were exposed to the maternal separation (MS) paradigm, an established ELS model. At adulthood, corticosterone levels and anxiety-related behaviors were evaluated. Subsequently, paw edema, noxious mechanical and thermal sensitivity were measured prior and during an inflammation induced by intraplantar injection of Complete Freunds Adjuvant (CFA). The open field test and the corticosterone measures showed no effect of MS. MS did not change mechanical thresholds prior to inflammation but reduced mechanical hyperalgesia after CFA-injection. MS animals did however present shorter latencies to display nocifensive behaviors compared to controls (CON). Furthermore, in CON but not MS, the repetitive noxious heat testing induced a decrease in reaction latencies. Moreover, MS dampened CFA-induced heat hyperalgesia. Altogether, our results suggest that ELS may have a protective impact on inflammatory pain.