Distinct environmental enrichment protocols reduce anxiety but differentially modulate pain sensitivity in rats

Environmental enrichment (EE) can alter anxiety as well as perception of nociceptive stimuli, suggesting a relationship between well-being and analgesia. Considering that, we aimed to evaluate the influence of different EE types on anxiety and peripheral pain sensitivity of male Wistar rats. Animals were submitted to two different types of EE: On the first one, animals were housed in standard size cages after weaning and received three different objects, one type per week, on a regular basis (simple EE - sEE). On the second one, animals were born in an already enriched environment and, after five weeks, were housed in larger cages and received five different objects, three of each type per week, on a regular basis (improved EE - iEE). Control group remained in standard cages and did not receive objects. Within 7 weeks of life, anxiety and thermal sensibility were evaluated using elevated plus maze and tail flick tests, respectively. Mechanical hyperalgesia was analyzed in the presence of acute and chronic noxious stimuli by paw pressure test. Both EE protocols tested were effective in diminish anxiety but they did not alter thermal sensibility. On the other hand, sEE protocol did not alter acute and chronic induced mechanical hypersensitivity, whereas iEE completely abolished such pain behavior, even without exercise wheel as part of the enrichment. Our results show that specific parameters (anxiety and pain sensitivity) can be differentially modulated depending on EE protocol used, making possible the implementation of welfare to experimental animals in pain research.