Genetic diversity contributes to abnormalities in pain behaviors between young and old rats

Aging has profound yet unpredictable effects on pain perception and incidence of anxiety disorders. However, the mechanisms underlying age-related pathologies are confounded by contradictory observations in rodent models. Therefore, the goal of our study was to test the hypothesis that genetic variability contributes to age-related pain behaviors and susceptibility to anxiety. To address this hypothesis, we examined pain and anxiety-like behavior in young or old Brown Norway (BN), Fisher 344, and BN/F344 (F1), three rat strains used in studies to evaluate the effect of aging. Mechanosensitive thresholds were assessed using the Von Frey assay, and visceral pain sensitivity was measured via the visceromotor response to colorectal distension. Anxiety-like behavior and exploration was quantified in the elevated plus maze. In the BN strain, old rats exhibited increased mechanosensitive thresholds compared to young rats; however, age did not affect visceral sensitivity in this strain. In F344-BN rats, the number of abdominal contractions induced by the highest colonic distension pressure was significantly lower in old rats. However, following colonic sensitization, a difference was no longer apparent. In the F-344 strain, visceral hypersensitivity following afferent sensitization was evident in young rats at all distension pressures but was not observed in older animals at 20 mmHg. Aging significantly reduced maze exploration across all strains. Our data demonstrate that age- and strain-related alterations exist in pain behavior and highlight the effects of aging on exploratory behavior. These findings suggest that strain differences contribute to the controversial data on the effects of aging on pain perception.